Geological Unit Description Aquifer Status · PDF fileGeological Unit Description Aquifer Status ... The ReFH Method has been run using the winter ... WSH-1255 1.3m diameter circular

University of Cambridge Level 3 Flood Risk Assessment

D127313 September 2011 15

Geological Unit Description Aquifer Status

Topsoil Encountered across the majority of the Application Site

Brown, locally clayey/silty, sandy ranging in thickness between 0.2m and 1.3m

Non Aquifer

Made Ground In the eastern part of the Application Site towards the southern corner and at the Cambridge University Farm to the north. Related to previous and current development in the area

Brown silty sand, sand and gravel, stiff brown and white friable clay, yellow brown silty sand and gravel. Thickness is highly variable ranging from 0.25m to 3.2m.

Non Aquifer

Head Deposits Associated with reworking of the underlying strata by natural geomorphological processes. Encountered above the Gault Clay in the south western half of the Application Site.

Firm to stiff (locally very stiff) orange/brown/grey locally sandy, gravely (flint, chalk) clay and range in thickness from approximately 0.20m to 3.80m.

Non Aquifer

Head Gravels and Observatory Gravels Only encountered in the central northern to north eastern portion of the Application Site.

Dense (locally loose, medium dense and very dense) orange/brown locally silty gravelly sand, clayey sandy gravel or clayey/silty sand and gravel (gravel of chalk, flint). The thickness ranges from 0.10m to 4.00m

Secondary Undifferentiated Aquifer

Chalk Marl Identified in the far eastern portion of the Application Site. Naturally present in this area but likely to have been reworked following Corprolite extraction as it is directly underlain by Gault Clay and not by Cambridge Greensand.

Light grey locally clayey and sandy weathered Chalk. Deposits encountered at depths ranging from 0.40m to 0.50m. They range in thickness from 0.70m to 3.25m with the deposits increasing in thickness with distance east.

Principal Aquifer

Gault Formation (Grey Mudstone) Encountered in most boreholes. It forms the slopes to the Washpit Brook and underlies head gravels and chalk mark to the centre and east of the Application Site, respectively.

Stiff to very stiff (becoming hard) grey/brown occasionally mottled orange brown desiccated clay with occasional calcareous nodules and locally occasional shell fragments. Encountered at depths ranging from 0.20m to 6.50m and not fully penetrated in any of the 25m long exploratory holes.

Non Aquifer

Lower Greensand Underlying Gault Clay

Not encountered during geotechnical investigation, which extended to a depth of 25m.

Principal Aquifer



Station Distance Years of

data QMED AM L-CV L-SKEW Discord-

ancy

41020 (Bevern Stream @ Clappers Bridge)

2.989 39 13.490 0.225 0.222 0.429

33054 (Babingley @ Castle Rising) 3.022 32 1.129 0.222 0.071 0.182

As no recorded flow data was available, QMED was estimated using the donor station method. The selection of a suitable donor station takes into account the catchment centroid distance and whether the station is suitable for estimating QMED. The most suitable donor station for estimating QMED was deemed to be Station 33052 (Swaffham Lode@Swaffham). The Urban Adjustment Factor (UAF) (Kjeldsen, 2009) has been applied to the flood frequency curve as the catchment is considered to be slightly urbanised when using this method.

5.2.4 ReFH Method

The ReFH Method has been run using the winter design rainfall parameters as the catchment urbanisation descriptor (URBEXT1990) is less than 0.125. The urban expansion equation (Section 6.5.8, Volume 5 of FEH) was used to consider how peak flows have been affected by urbanisation within the catchment since 1990. When the URBEXT1990 adjusted catchment descriptor was used within the ReFH model, the impact on peak flows was negligible.

5.2.5 Comparison of flows

The estimated flow rates using the FEH Statistical Method and the ReFH model for the 1 in 20 year, 1 in 100 year and 1 in 1000 year events are provided in Table 5-3. Audit sheets are included in Annex H.

Table 5-3: Peak Flow Estimates for Washpit Brook Sub-Catchments using ReFH and WINFAP-FEH 3 Methods

ReFH Method Winter Event Peak Flows

(m3/s)

FEH Statistical Method Event Peak Flows (m3/s) Catchment

Number Catchment Area km2

20 100 1000 20 100 1000

Sub-Catchment 1 1.11 0.52 0.76 1.41 0.27 0.38 0.60

Sub-Catchment 2 0.53 0.25 0.36 0.67 0.13 0.18 0.29

Sub-Catchment 3 1.38 0.64 0.94 1.75 0.34 0.48 0.75

Sub-Catchment 4 0.67 0.31 0.46 0.86 0.16 0.23 0.37

Sub-Catchment 5 0.70 0.33 0.48 0.89 0.17 0.24 0.38

Sub-Catchment 6 0.14 0.06 0.09 0.17 0.03 0.05 0.07

Sub-Catchment 7 0.81 0.38 0.55 1.03 0.20 0.28 0.44

Sub-Catchment 8 1.11 0.52 0.75 1.41 0.27 0.38 0.60

Sub-Catchment 9 0.73 0.34 0.50 0.93 0.18 0.25 0.40

Total Catchment 7.16 3.34 4.88 9.11 1.75 2.48 3.89



The results displayed in Table 5-3 indicate that the peak flow estimates for the Washpit Brook are considerably lower when using the FEH Statistical Method compared to the ReFH Method. A number of stations had to be removed from the initial pooling group as they were deemed unsuitable for pooling (potentially due to the Application Site’s location in the catchment headwaters).

The revised pooling group stations were a greater distance from the subject catchment, which may result in a less reliable peak flow estimate.

For the purposes of this Level 3 FRA, the ReFH Method peak flow estimates (and associated hydrographs) have been used to ensure a conservative approach to the model outputs and development design.

5.3 Model Construction

5.3.1 Model Software and Approach

ISIS is an industry standard computational modelling package that provides a comprehensive range of methods for simulating flows and levels in open channels, structures, floodplains, reservoirs and estuaries. It can provide the 1D component of the water level within the channel and on the floodplain.

The Washpit Brook is largely located within the valley bottom and therefore the hydraulic modelling approach has incorporated extended cross sections to represent the channel and floodplain.

5.3.2 Topographic Information

A topographic survey of the Washpit Brook was commissioned by URS/Scott Wilson following a site visit. The survey was undertaken by Greenhatch Group in August 2010.

URS/Scott Wilson undertook a site visit to identify specific cross section locations of the Washpit Brook and its tributaries to allow an accurate representation of the watercourse and floodplain geometry. This included information at all key structures in the model reach. These cross sections were used within the hydraulic model at the specified locations to represent the Washpit Brook and the adjacent floodplain and are presented within Annex I.

Within the Application Site, cross sections were extended through the floodplain where necessary using the site topographic survey. Beyond the Application Site, cross sections were extended using 1m resolution LiDAR data (airborne topographic information).

5.3.3 Channel Geometry Node Locations

A cross section or node location plan is included within Annex E. Cross section interpolations were included within the hydraulic model to improve the stability.

5.3.4 Hydraulic Model Extent

The upstream extent of the hydraulic model was formed adjacent to the east site boundary, north of the park and ride. The downstream extent is located at Washpit Road, approximately 1km downstream of the M11/A14 Interchange. The reach of the Washpit Brook incorporated into the hydraulic model is illustrated in Figure 3-2 and consists of a 3.1km river length. Annex E



illustrates the location of the key cross sections in the hydraulic model, as well as various other features within the Application Site and downstream.



5.3.5 Hydrological Representation

Runoff generated by the Application Site itself was represented within the model using lateral inflow units, to apportion inflow along the length of the channel. Inflows from the remaining sub-catchments were incorporated as a point source hydrograph, at the position of the particular culvert, where it joins with the Washpit Brook. This is illustrated in Annex E.

5.3.6 Structures

All key structures were identified and included in the hydraulic model, and they are summarised in Table 5-4. This incorporated various bridges and culverts, many of which were anticipated to have an impact on local water levels, especially during flood flows. Spill units have been incorporated at each structure, where appropriate, to allow flow to spill over the structure and back into the watercourse downstream.

Table 5-4: Description of in-line structures

Location Cross Section

Details

Opposite park and ride WSH-2922 1m diameter pipe with concrete access structure

Upstream of plantation WSH-2657 0.6m diameter pipe with brick face

Mid to lower end of Application Site

WSH-1579 1.3m diameter circular culvert with brick face

Lower end of Application Site

WSH-1395 1.3m diameter circular culvert forming main vehicular access

Lower end of Application Site

WSH-1255 1.3m diameter circular culvert with brick face – poor condition

Downstream site extent WSH-1014 1.3m diameter circular culvert beneath A428

Downstream of A428 WSH-0939 1m diameter circular farm access culvert

Beneath A1317 slip road WSH-0808 2m wide bridge

Beneath A14 WSH-0699 2.2m wide bridge

Beneath Washpit Road WSH-0000 Twin 1.3m pipe culvert with brick headwall

The structures identified above are inline structures only. There are a number of other structures, such as those that discharge flow into the Washpit Brook from beneath the M11. However, it was not necessary to include these within the hydraulic model and therefore they have not been identified above.



5.4 1 in 100 Year Event Model Conditions

The model conditions consist of various parameters that will exert a control upon the calculated water levels within the hydraulic model. The 1 in 100 year event model forms the key scenario which is tested as part of the sensitivity analysis, as discussed in Section 5.7, to ensure the model is robust and reliable.

5.4.1 Upstream Boundary Conditions (Inflows)

As discussed in Section 5.2.3, the upstream boundary conditions have been based upon the 1 in 100 year event, using the ReFH Method.

5.4.2 Downstream Boundary Conditions

The downstream boundary is not affected by any influence (such as tidal conditions or another watercourse) and therefore a normal depth boundary matching the slope of the lower reaches of the Washpit Brook was used.

Sensitivity testing has confirmed the acceptability and position of the downstream boundary condition, which is discussed in further detail in Annex L.

5.4.3 Roughness Coefficient (Manning’s ‘n’ Value)

The roughness coefficients used in the hydraulic model are represented by Manning’s ‘n’ values. Values of Manning’s ‘n’ for the channel and floodplain were estimated from visual inspection of the channel and floodplain and with reference to Open Channel Hydraulics (Chow, 1973 Table 5-6).

Further to this, the Conveyance Estimation System (CES) produced by HR Wallingford was used to confirm the Manning’s ‘n’ values. A channel roughness value of 0.05 was identified due to the extent of the overhanging vegetation observed in the summer months. The same value was conservatively used for the short to medium length grassed floodplain.

5.4.4 Model Run Parameters

Model run parameters were retained for all modelled scenarios. Table 5-5 lists the model run parameters applied.

Table 5-5: Model run parameters

Parameter

Run time 24.5 hours

Timestep 2 seconds

dflood 5

Maximum iterations 49

Preissmann slot �

Spill threshold 0.001

All other parameters Default



5.5 Model Results

5.5.1 Flood Mapping

Flood levels from each of the cross-sections were exported to Mapinfo. The flood levels at each of the cross-sections were projected onto a 2 dimensional digital terrain model. Mapinfo allows the automation of the flood mapping process, by adjoining maximum flood levels from adjacent cross sections, following the contours of the digital terrain model.

5.5.2 Flood Outline

The 1 in 100 year event is equivalent to Flood Zone 3a. Hydraulic modelling of the baseline event has shown that Flood Zone 3 is generally confined within the banks of the Washpit Brook in the upper half of the Application Site. Downstream, flooding is generally confined to the watercourse corridor, until the reach of the Washpit Brook flows away from the M11. At this location floodwater inundates a relatively wide area of floodplain due to the presence of shallow topography adjacent to the watercourse. This is exacerbated by a number of existing culverts that cause the capacity of the existing watercourse to be locally reduced. Towards the downstream extent of the Application Site the topography is more pronounced and hence the flood extent is more limited. The flood map is included within Annex J. A full table of output results for some of the storm events discussed in this chapter is provided within Annex K.

The flood extent identified is relatively consistent with the South Cambridgeshire District Council SFRA flood map included in Annex C, as discussed in Section 2.3.

5.6 Other Model Results

5.6.1 1 in 20 year event

The 1 in 20 year event is equivalent to with Flood Zone 3b, otherwise known as the ‘Functional Floodplain’. The extent of Flood Zone 3b is very limited within the Application Site and is generally confined to the river corridor, except at the reach of the watercourse where it flows away from the M11. This is a result of the topography in this area, as discussed in paragraph 5.5.2. The flood map is included within Annex J.

5.6.2 1 in 100 year event +20% allowance for climate change

PPS25 requires that the anticipated effects of climate change are considered by the addition of 20% to the peak river flows. The 1 in 100 year event inflows were increased to account for the impact of climate change to allow hydraulic modelling of this scenario.

With the inclusion of climate change upon the 1 in 100 year event, flooding within the Application Site is more significant. However, the general description discussed in paragraph 5.5.2 is still appropriate.

The flood map for this scenario can be found within Annex J.

5.6.3 1 in 1000 year event

The 1 in 1000 year event is equivalent to Flood Zone 2. Under this event, the culvert located beneath the A428 exerts a significant control upon flood levels within the Application Site. Floodwater is unable to flow over or around the structure due to the large road embankment



located at the upstream face of the culvert. Therefore, floodwater accumulates within the Application Site and a significant backwater affect is observed. Consequently, more elevated flood levels are experienced and a comparatively larger flood extent is observed.

The hydraulic modelling has also identified the presence of a flow path in a westerly direction upstream of Pheasant Plantation, as the Washpit Brook marginally overtops its left bank. The Washpit Brook is slightly perched above the valley bottom in the location and therefore flow paths will direct floodwater down gradient and into a ditch alongside the M11. This will eventually feed back into the Washpit Brook. This flow path has only been found to occur on the 1 in 1000 year event. The flood extent associated with this flow path has not been identified on the flood map because the limitations of the 1D hydraulic model will not allow an accurate representation of this. However, flow a path arrow has been included to represent this flooding mechanism.

The flood map for this scenario can be found within Annex J.

5.6.4 Tributary

The ditch located downstream of sub-catchment 6 (see Figure 3-2) flows between the Application Site boundary and the A428. The ditch potentially represents a source of flood risk. However, the estimated flows associated with the sub-catchment were minimal (i.e. 0.17m3/s for the in 1000 year event, see Table 5-3). A flow capacity analysis suggested that the ditch and culvert beneath Huntingdon Road have sufficient capacity to contain the 1 in 1000 year flows and therefore the flood risk associated with this ditch is considered to be negligible.

A number of other minor tributaries or ditches are located within the upper reaches of the Application Site, which were not included within the hydraulic model. The extent of the Washpit Brook included in the hydraulic model is illustrated in Figure 3-2. No flooding has been identified in the upper reaches of the Washpit Brook and therefore it was not considered necessary to extend the hydraulic model to account for these minor ditches.

5.7 Sensitivity Analysis

Sensitivity testing has been undertaken on the hydraulic model to assess the impact of altering key parameters within the model and observing the change in output. This provides an indication of the robustness of the hydraulic model. The key parameters adjusted were:

• Upstream (i.e. inflows);

• Manning’s ‘n’ values (roughness coefficient); and

• Downstream boundary condition.

Blockage scenarios of the culverts/bridges have also been undertaken to test the impact upon the likely flood extents.

The sensitivity analysis has suggested that the hydraulic model is relatively insensitive and has demonstrated a relatively typical response across the scenarios considered. The sensitivity analysis is discussed further within Annex L.

The blockage scenarios have identified a relatively localised impact, except under the blockage of cross section WSH-1014 (i.e. located beneath A428 / M11 slip road). A backwater effect in excess of 1km is observed due to the presence of the road embankment, which will promote the accumulation of floodwater within the Application Site. However, maximum flood levels



experienced under all the blockage scenarios are less than those observed under the 1 in 1000 year event.

5.8 Summary

An hydraulic modelling study has been carried out to identify fluvial flooding conditions in relation to the Application Site. The results of this hydraulic analysis of this pre-development baseline are as follows:

� The 1 in 20 year event is equivalent to with Flood Zone 3b. The extent of Flood Zone 3b is very limited within the Application Site and is generally confined to the river corridor, except at the reach of the watercourse where it flows away from the M11.

� The 1 in 100 year event is equivalent to Flood Zone 3a. and flooding is generally confined within the banks of the Washpit Brook in the upper half of the Application Site.

� The 1 in 1000 year event is equivalent to Flood Zone 2. The culvert located beneath the A428 exerts a significant control upon flood levels within the Application Site as a result floodwater accumulates within the Application Site and a significant backwater affect is observed. Consequently, more elevated flood levels are experienced and a comparatively larger flood extent is observed



6 Fluvial Flood Hydraulic Modelling of the Proposed Flow Control Structure and Two Stage Channel

The Environment Agency Flood Map identifies the potential for significant flooding within Girton at the confluence of the Washpit Brook and Beck Brook, which is situated approximately 2km downstream of the Application Site, and nine properties on Dodford Lane were believed to have flooded on 21 October 2001.

As a precautionary measure, to reduce flood risk from the Application Site, the Proposed Development includes an online flow control structure and a two stage channel that are constructed along the section of Washpit Brook that extends through the Application Site to enable the peak flows downstream of the Application Site to be reduced for a range of return periods and for the excess flow to be stored within the landscaped areas of the Application Site. A hydraulic model has been constructed to quantify the reduction in flood risk and to establish amended flood maps for the Application Site, which is presented within Section 6.3 of this document.

6.1 Constraints

The proposed modifications to the watercourse have been designed to provide compatibility with the following three constraints:-

1. Impact upon peak flows and the flood hydrograph downstream of the Application Site;

2. Impact upstream of the Application Site, via the M11 culverts and,

3. Impact upon flood extent within the Application Site.

The existing developed areas located downstream of the Proposed Development are vulnerable to flooding resulting from a range of rainfall return periods. The proposed scheme has therefore been designed to reduce the peak flow downstream for rainfall events with a return period of 1 in 20, 1 in 100 and 1 in 1000 years, rather than focusing purely on extreme events.

As discussed in Section 3, the culverts that pass below the M11 allow passage of water into the Application Site from land to the west. Two of these culverts discharge directly into the Washpit Brook within the Application Site and therefore any increase in flood levels within the Application Site adjacent to the M11 culverts, could increase flood levels upstream of the Application Site. The proposed flood alleviation scheme has been designed to ensure that water levels will not be increased in the vicinity of these culverts.

The storage of floodwater within the Application Site will cause water levels to be elevated downstream of the M11 culverts. As discussed in Section 4, earthwork bunds are proposed on the western edge of the Proposed Development to attenuate noise generated from the adjacent M11 and to complement the landscape design. These bunds have been designed geometrically to assist in the storage of floodwater, whilst also protecting the Proposed Development from flooding occurring as a result of the elevated water levels.



6.2 Proposed Flow Control and Storage

The flow control structure, which consists of a 1.3m diameter low flow pipe with a raised weir level of 12.87m AOD, will be positioned towards the downstream extent of the Application Site in the location shown in Figure 6-1.

A flood storage area will be constructed upstream of the flow control structure through the excavation of material adjacent to the existing channel in order to effectively form a two stage channel. The existing channel will be maintained to a depth of 0.5m in order to form the low flow channel, whilst the width of the second stage channel will vary as it will typically have a width of 45m at its southern extent and 20m closer to the flow control structure. The extent of the proposed two stage channel is shown in Figure 6-2.

The proposed earthworks bunds have been design geometrically to function in combination with the two stage channel in order to contain floodwater and ensure that flood risk to the Proposed Development will not be increased by the flood alleviation measures.

An existing culvert, which is located at the coordinates 542,030mE, 260,731mN, will be removed as part of the scheme (shown in Figure 6-3). The culvert removal is necessary due to the impact upon peak water levels upstream and its position within the proposed reach of a flood storage area. The culvert removal also provides opportunities for the bed of the Washpit Brook to be locally re-profiled to form a more uniform gradient and thereby enable the storage volume to be maximised. The downstream half of the Washpit Brook will be reprofiled only, as the reach upstream of this was more constrained due to the presence of the culverts located beneath the M11.

Reproduced from Ordnance Survey digital map data © Crown copyright 2010. All rights reserved. Licence number 0100031673

Figure 6-1: Proposed Location of Flow Control Structure



Groundwater has been recorded at a depth of between 0.9m and 3.8m below ground level within the Head Gravels and Observatory Gravels, often in the lower part. Groundwater was also encountered at a depth of 2m in the Chalk Marl and only on one occasion within the Gault Clay at a depth of 19.45m rising to 17.95m.

The inconsistent presence of shallow groundwater and variation in relative levels across the Application Site, suggest that encountered groundwater is largely indicative of perched water above the Gault Clay, and strongly influenced by seasonal fluctuations in rainfall and in the shorter term, can be affected by antecedent weather conditions. Groundwater contained in the secondary aquifer is likely to support flow to the surface water ditches that run across the Application Site, including the Washpit Brook, and there is therefore potential for pollutants to be transmitted to the groundwater.

The Application Site geology shows that the natural ground conditions comprise Head Deposits and Head Gravels within the centre of the Application Site overlying the Gault Clay Formation, which forms the slopes to the Washpit Brook. The Gault Clay has a low permeability; therefore soakaways are unlikely to be feasible on the western portion of the Application Site. The Head Gravels are permeable and will generally be suitable for soakaways, except where shallow groundwater or Gault Clay is encountered.

On the eastern part of the Application Site, reworked Chalk Marl is locally present overlying the Gault Clay in the area of the historic landfill. Limited depths of Made Ground are present in the eastern portion of the Application Site, towards the south side, with areas of disturbed ground associated with historic Coprolite workings. The reworked chalk may be susceptible to solution if groundwater discharge is locally intensified; therefore the use of porous paving is likely to be preferable to soakaways.

The eastern and central portion of the Application Site is classified as a Principal and Secondary aquifer, respectively.

3.11 Source Protection Zones

There are no Source Protection Zones on site or within the surrounding area.




Figure 6-2: Proposed Extent of the Two Stage Channel

Figure 6-3: Culvert Proposed for Removal



The proposed bank on the eastern side of the second stage channel has also been raised to ensure floodwater is contained within the channel adjacent to the proposed development. A 5m wide access strip will be provided on either side of the Washpit Brook in order to ensure that maintenance vehicles may gain access to the low flow and second stage channel.

A drawing is enclosed within Annex M, which presents the proposed works to the existing watercourse.

6.3 Model Construction

The baseline hydraulic model has been amended to include additional cross sections to enable the proposed bunds, flow control device and two stage channel to be accurately represented. A drawing included in Annex M illustrates the location of each of these changes.

The changes made to the baseline model are summarised below:

• Landscape bunds have been included between node WSH-2657 and WSH-1189. A culvert has also been included at node WSH-2631, which is intended to allow unrestricted passage of flow through the landscaping bunds;

• The flow control structure, which comprises a culvert and raised spill level, has been included at WSH-1406 to locally restrict the hydraulic capacity of the watercourse and promote the storage of floodwater upstream;

• The culvert at node WSH-1579 has been removed to allow for construction of the flood storage area and to remove impact upon water level;

• The Washpit Brook channel has been re-profiled between nodes WSH-1887 and WSH-1395 to maximise the storage volume;

• Ground levels on banks of Washpit Brook have been lowered to increase storage volume between nodes WSH-2437 and WSH-1395;

• The level of the eastern bank has been raised to 13m AOD between node WSH-1783 and WSH-1406 and the level of the eastern bank has been raised to 12.8m AOD at node WAS-1400 and WAS-1395 to ensure the floodwater does not encroach upon built areas within the Proposed Development.

6.4 Hydrology

The hydrology from the baseline modelling has been retained for the proposed flood modelling. This applied the ReFH method, which was a conservative assumption. .



6.5 Model Results

6.5.1 Reduction in Downstream Peak Flows

Table 6-1 provides a comparison of the peak flows downstream of the proposed Proposed Development by considering the baseline and proposed scenario and it defines the percentage reduction in peak flow that may be obtained through the implementation of the flood alleviation scheme.

Table 6-1: Impact upon Peak Flows Downstream of the Application Site 20 Year Return Period

Node BASELINE PROPOSED IMPACT WSH-2060 1.85 1.33 -28%

100 year Return Period


100 year Return Period + Climate Change


1000 Year Return Period


Table 6-1 indicates that the proposed scheme will provide a significant reduction in peak flows downstream for a range of rainfall return periods. For example, under the 1 in 20 year event (i.e.Q20) a reduction in flow of 28% is observed. The percentage reduction in peak flow is reduced as the rainfall return period increases because the positive impact is diluted by the higher flows. However, even under the 1 in 1000 year event, the peak flow is reduced by 7%.

Figure 6-4 is provided to graphically illustrate the effect that the proposed scheme will have on the downstream hydrograph. This figure indicates that these measures will permit the peak flows to be reduced from 3.00 to 2.69m3/s for a 1 in 100 year event including a 20% allowance for climate change. The volume of floodwater discharged will be the same; however, it will be discharged over a longer period of time.

Downstream of the Application Site the Washpit Brooks meets with Beck Brook. Appropriate checks have been undertaken which confirm that the flood alleviation scheme will also benefit flood risk downstream of this confluence.



Figure 6-4: 1 in 100 year event+20% allowance for climate change peak flow at WSH-0939

6.5.2 Water Levels at face of the M11 Culverts

Table 6-2 shows the impact upon flood levels within the Application Site, adjacent to the M11 culverts. The fourth column on the table reflects the difference between the peak flood level from the proposed scenario, compared to the baseline scenario.

Table 6-2: Impact upon Peak Flood Level at the M11 Culverts (all levels expressed as AOD) 20 Year Return Period

Node BASELINE PROPOSED IMPACT WSH-2060 12.54 12.31 -0.23m WSH-2195 12.56 12.31 -0.25m



100 Year return period + Climate Change






Table 6-2 indicates that the flood alleviation scheme will not cause the peak water level adjacent to the M11 culverts to be increased. The proposed flood alleviation scheme causes the peak flood level to be reduced adjacent to the culverts to be reduced by 0.23m for a rainfall event with a return period of 20 years. The reduction is peak water level is reduced as the rainfall return period increases because the positive impact is diluted by the higher flows. However, even under the 1 in 1000 year event, the peak water level adjacent to the culverts is reduced by 0.08m.

6.5.3 Flood Maps

The hydraulic modelling has also resulted in the identification of new flood zones within the Application Site. Updated flood maps have been provided within Annex N in order to define the extent of flooding that would occur within the Application Site as a result of the implementation of the flood alleviation scheme.

The revised flood maps indicate that the proposed two stage channel and earthwork bunds will be capable of containing excess floodwater that is generated by the attenuation of flows for events up to and including a rainfall return period of 100 years including climate change. Consequently no development blocks are affected by this event.

The flood maps also indicate that the entire built development will be located outside Flood Zone 1, which is defined by the 1 in 1000 year floodplain, with the exception of a single block in the northwest corner of the Application Site, which is partially located in Flood Zone 2.

6.5.4 Sensitivity Testing

A sensitivity analysis is included within Annex L, which considers the impact of blockage upon the flow control structure.

The sensitivity analysis also investigates the impact of the application of a different set of hydrographs, which was purchased from the Environment Agency, prepared for the Cottenham Load Flood Alleviation Scheme modelling by Halcrow in 2003. It is understood that the modelling has also been used to define the Environment Agency Flood Zone Maps for the Washpit Brook, downstream of the Application Site, which are included within Annex A. The sensitivity test has not identified any issues or concerns and the conclusions drawn above as the benefits of the proposed flood alleviation scheme are retained if the Halcrow hydrology is used.

A full table of output results for some of the storm events discussed in this chapter is provided within Annex K.

The proposed flood alleviation scheme is subject to detailed design.



7 Flood Risk – to the Proposed Development

PPS25 requires that all potential sources of flooding that could affect the Proposed Development are considered. The section considers all potential sources as listed in Annex C (Forms of Flooding) of PPS25.

A review of the South Cambridgeshire District Council and Cambridge City SFRA identified no flood history within the Application Site, from any flood source.

Hydraulic models have been prepared to assess the fluvial flood risk to the Application Site and to define the Flood Zones for the existing baseline and future developed scenarios. A risk based approach for the assessment of the other potential flood sources is presented below.

7.1 Fluvial Flooding

Fluvial sources include rivers, streams and ditches. As outlined in Section 5, the Washpit Brook is the only major source of fluvial flood risk within the Application Site. Hydraulic models have been constructed to assess the fluvial flood risk to the Application Site and to define the flood extents for the existing baseline and future developed scenarios.

The baseline hydraulic modelling demonstrates that the majority of the Application Site is considered to be located within Flood Zone 1 but that a small portion of the Application Site situated adjacent to the Washpit Brook could be located within Flood Zone 2 and 3, if the watercourse was unaltered.

As a precautionary measure, to reduce flood risk to, and from, the Application Site, the Proposed Development includes an online flow control structure and a two stage channel to enable peak flows downstream of the Application Site to be reduced for a range of return periods and for the excess flow to be stored within the landscaped areas of the Application Site. The hydraulic modelling of the proposed scheme has resulted in the identification of new flood zones within the Application Site. This has enabled the entire built development to be located in Flood Zone 1, with the exception of one development block, which is partially located in Flood Zone 2. Under this storm event, the extent of the floodwater affecting the block is minimal.

The assessment of fluvial flooding from the Washpit Brook has considered the impact of a number of ditches that discharge into it. These are all headwater features and associated flood risk to the Application Site is considered to be negligible.

Flood Maps for the existing baseline and future developed scenarios are presented in Annex J and N, respectively. Section 10 identifies the proposed flood risk management intended to protect the Proposed Development from fluvial flooding.

A small ditch located to the north of the Application Site, flows in a northerly direction away from the Washpit Brook and into a neighbouring catchment. This is also a headwater feature and flood risk associated with this is considered to be negligible.



7.2 Tidal Flooding

Tidal flood sources include the sea and estuaries. The Application Site is inland and away from tidal sources so therefore the risk of tidal flooding is negligible.

7.3 Overland Flow

Flooding from the land or surface water flooding can occur when water fails to infiltrate into the ground and travels over the ground surface as ‘overland flow’. This can occur in both urban and rural situations. Impermeable ground conditions often found in the urban environment exacerbate this type of flooding.

The Application Site is situated within the headwaters of the Washpit Brook; therefore there are very limited areas beyond the Application Site boundary that may contribute to flooding from overland flow, as water from beyond the Application Site will tend to flow into a neighbouring catchment.

Whilst the areas surrounding the Application Site are unlikely to form major sources of overland flow, the Application Site itself is likely to experience some overland flow due to the impermeable nature of the underlying clay soils. The majority of the Application Site is located on a hillside that slopes towards the Washpit Brook. Therefore, any overland flow is only likely to accumulate in the valley bottom adjacent to the Washpit Brook, away from the Proposed Development. A number of drains cross the Application Site that may intercept a proportion of overland flow, which discharge into the Washpit Brook. This has been confirmed through inspection of the South Cambridgeshire and Cambridge City Level 1 SFRA surface water flood risk map.

The risk associated with overland flow sources is considered to be low.

Development of the Application Site will modify the runoff response of the Application Site. Instead of running freely from the Application Site, overland flow will be directed to a surface water drainage system, which will be designed to control the rate of runoff discharged from the Application Site and into the Washpit Brook. This will provide a control upon flooding from overland flow and is discussed in detail within Section 8.

7.4 Sewer Flooding

In urban areas rainwater is often collected and discharged into combined sewers which contain both surface water and foul water. Flooding can result when the sewer is overwhelmed by heavy rainfall, potentially due to blockage or insufficient capacity. This is often a problem in heavily urbanised areas or where ageing surface water sewer systems exist.

No public sewers are located within the Application Site, however various public sewers can be found within the surrounding roads as identified within Section 3.7. All of these sewers are minor, with a maximum diameter of 300mm. The South Cambridgeshire and Cambridge City Level 1 SFRA identifies historical sewer flooding incidents, on Huntingdon Road, to the north of the Application Site. No further information is available on these incidents, however, the sewers are minor and any flooding from these sources is likely to be limited.

No other incidents have been identified within the proximity of the Application Site.



The risk associated with sewer flooding sources is considered to be negligible.

7.5 Groundwater

Groundwater flooding can occur when groundwater levels rise above ground levels. A site investigation was undertaken by URS/Scott Wilson during the summer of 2010, which recorded groundwater levels across the Application Site. Groundwater or damp conditions were observed in seven exploratory holes during drilling. In five of these instances, this was between approximately 0.90m below ground level (bgl) and 3.80m bgl within the Head Gravels and Observatory Gravels (see Section 3.10). Groundwater was only encountered within the Gault Clay on one occasion at 19.45m bgl, rising to 17.95 m bgl, in BH101, whilst it was recorded at approximately 2.00m bgl in the Chalk Marl in WS220.

The proposed surface water drainage system will incorporate filter drains where the interface between permeable and impermeable strata is located close to the finished ground level in order to enable groundwater to be intercepted and directed away from the Proposed Development.

In the unlikely event of groundwater levels reaching ground level, they would behave similarly to overland flows and would quickly drain away towards the Washpit Brook following the general topography.

The South Cambridgeshire and Cambridge City Level 1 SFRA did not identify any historical groundwater flood incidents at the Application Site.

The risk associated with groundwater flooding is considered to be negligible.

7.6 Artificial Sources

Artificial flood sources include raised channels such as canals or storage features (i.e. large ponds and reservoirs). There are no major artificial sources of flooding upstream or uphill of the Application Site with the potential to present a flood risk. There are a number of small balancing ponds located adjacent to the Application Site, as identified in Section 3. However, these are only minor features and are not impounded. Therefore the Application Site is considered to be at negligible flood risk from artificial sources.



8 Flood Risk – From the Proposed Development

8.1 Surface Water Management

8.1.1 General

A suitable surface water drainage strategy has been conceived to mitigate against the effects of increased surface water runoff caused by an increase in impermeable surface area. The strategy has been developed following liaison with the statutory drainage authority for the area with advice and further information obtained from the Environment Agency and is based upon the parameter plans.

The basis of this drainage strategy is the requirements of PPS25, in particular, Annex F. This Annex refers to the use of Sustainable Drainage Systems (SuDS) and outlines the importance as a planning consideration of both regional and local planning authorities in the management of development run-off and surface water drainage. The resulting strategy represents a major opportunity to demonstrate best practice in the collection, treatment and reuse of surface water.

8.1.2 Proposed Drainage Strategy

The topographical survey for the existing Application Site indicates that the majority of surface water runoff will flow in a south-westerly direction, where it is naturally intercepted by the Washpit Brook, which flows in a north-westerly direction. However, a plateau extends along the northern edge of the Application Site where the ground falls gently in a northerly direction away from the Washpit Brook. Surface runoff from this area generally infiltrates into the existing ground, due to the presence of permeable Head Gravels and reworked Chalk, which support the flow of surface water to the ditches that run across the Application Site, including the Washpit Brook. Drawing Reference D127313-500-101, which is contained in Annex O, defines the location of existing surface water drainage features and indicates the locations of the individual catchments.

The drainage strategy for the Proposed Development has been designed to mimic the existing drainage regime, whist taking cognisance of constraints imposed by the underlying ground conditions. Surface water run off from the Proposed Development in the central and western areas of the Application Site will be discharged to the Washpit Brook, which is the natural conduit for current drainage from the Application Site, given the existing topography. Attenuation features and flow control structures will be provided to ensure that the developed runoff rates and volumes do not exceed the existing greenfield runoff rates and thereby ensure that flood risk is not increased downstream of the Proposed Development. Infiltration devices will be provided to permit surface runoff to be returned to the ground wherever the development blocks overlie significant depths of permeable Head Gravels.

Development blocks constructed in the north eastern portion of the Application Site are underlain by reworked chalk. Large soakaways will not be provided in this area as they could cause groundwater discharge to be locally intensified and thereby potentially increase the risk of solution features being formed that could adversely affect the Proposed Development. The surface runoff from development plots on the north eastern area of the Application Site will either be intercepted and returned to ground over a large area by porous paving situated greater than 20m away from proposed buildings in order to mimic the existing regime, or discharged to the Washpit Brook at a rate of QBar or 2l/s/ha.



Surface water will be attenuated on site within feasible SuDS features constructed as an integral part of the Proposed Development. It is proposed that a two stage SuDS ‘train’ will be implemented within the Proposed Development to both attenuate peak flow rates into the downstream system and also provide water improvements to water quality. This will entail the implementation of source control features within development blocks, discharging to swales or sewers located within the surrounding green corridors or streets, respectively. The swales and sewers will then in turn discharge under gravity to site control features in the form of retention ponds within the proposed open space of the Proposed Development. A multi stage outlet will also be incorporated to ensure discharge rates will not exceed pre-development rates up to and including the 1 in 100 year design storm with an additional allowance of 30% to allow for the predicted affects of climate change over the lifetime of the Proposed Development.

As a minimum it is proposed that surface water drains, sewers and SuDS devices located below ground will be designed to withhold the runoff for rainfall events with a return period of 1 in 30 years without overflow or flooding. Runoff generated above this return period up to a 1 in 100 year event, with an additional 30% allowance on rainfall intensities for climate change, is generally to be directed via overland flow routes to retention ponds situated in open space located on the low ground to ensure that the runoff is retained within the Application Site. Finished levels within the Proposed Development will generally be engineered to ensure flows are directed safely away from dwellings and other building towards the retention ponds to prevent flooding to property. Where finished levels are constrained in the north-eastern corner of the Application Site, the surface water drainage network will be designed to be capable of conveying the attenuated runoff from a rainfall event with a 1 in 100 return period and 30% allowance for climate change to ensure that flood risk to existing properties adjacent to the northern boundary of the Application Site will not be increased by the Proposed Development.

8.1.3 Greenfield Runoff Rate

It is proposed to drain the Application Site at rates equivalent to the greenfield runoff for the 1, 30 and 100 year key return period events. The greenfield runoff rate will be dependant upon the permeability of the underlying strata. Ground Investigation works indicate that the geology will vary across the Application Site and that the Proposed Development will be underlain by areas of Gault Clay, Head Gravels and Chalk Marl, as described within Section 3.10.

Greenfield runoff rates have been calculated for each underlying strata, using the MicroDrainage computer software suite, by considering the 1 in 1, 1 in 30 and 1 in 100 year events. The greenfield runoff rates have been tabulated in Table 8-1, together with the mean annual runoff rate, QBar. These results have been verified using the procedure defined in the document entitled "Preliminary Rainfall Runoff Management for Developments" Revision D, which is published jointly by the Environment Agency and Department of Environment, Food and Rural Affairs (DEFRA).

Table 8-1: Calculated greenfield runoff rates

Return Period (years)

Permitted Discharge for area of Application Site

overlying Clay (l/sec/ha)


overlying Gravel (l/sec/ha)


overlying Chalk (l/sec/ha)

QBar 3.29 1.37 1.37

1 2.87 1.20 1.20

30 7.92 3.30 3.30



4 Proposed Development

The Proposed Development of the existing greenfield site will incorporate residential and employment use. The main elements of the proposed scheme comprise:

• Approximately 3,000 market and key worker dwellings;

• Collegiate housing;

• Academic and commercial research;

• School;

• Nursery;

• Hotel;

• Local Centre, incorporating retail uses;

• Spine roads and local roads providing vehicle access to and around the Proposed Development; and,

• Large areas of open land.

The Land Use Parameter Plan for the Proposed Development is shown in Figure 4-1. It identifies a series of development blocks, with specified land uses, such as residential, collegiate housing etc, akin to the general main elements of the proposed scheme as identified above. Each development block may incorporate buildings, landscaping and car parking; a block does not represent a single building. This allows the sequential allocation of land use within each development block with consideration of flood risk, where necessary, as discussed in Section 10.

The Proposed Development will be designed to meet with the requirements of the Code for Sustainable Homes (CSH) in order to comply with the North West Cambridge Area Action Plan. Initially, the development will seek to achieve a Code Level 4 for the first 50 dwellings approved on or after 1st April 2013. Subsequently, all other dwellings will comply with Code Level 5 or above. Non residential buildings will comply with BREEAM Excellent Standard.

4.1 Proposed Topography

The development of the Application Site proposes to retain the current general ground levels as far as possible whilst allowing for building foundations, roads, and ponds to be formed. The natural slopes situated on the western edge of the Proposed Development will be utilised to accommodate semi basements for car parking.



100 11.74 4.90 4.90

Drawing Reference D127313-500-108, which is contained within Annex O, has been prepared to define the variation in ground conditions and overall greenfield runoff rate for each catchment area. This drawing indicates that the north-eastern portion of the Application Site has been excluded from the runoff calculation as the topographical survey indicates that the existing ground falls away from the Washpit Brook.

8.1.4 Surface Water Drainage Design, Adoption and Maintenance

The surface drainage system will be split, each having separate criteria in terms of design, adoption and maintenance. These systems can be described as follows:

Private Drainage: This describes the piped network draining houses, commercial or community buildings. Private drainage is designed in accordance with Building Regulations and is owned and maintained by the building owner.

Adopted Public Surface Sewers: The Statutory Drainage Authority, in this case Anglian Water, can adopt and maintain sewers in the surface water network. These sewers must be designed in accordance with ‘Sewers for Adoption 6th Edition’. They must only drain areas of hard paving and roofs. No groundwater or water courses can enter adopted systems as this is classified as land drainage.

Land Drainage: Groundwater and water running in watercourses is classified as land drainage and is the responsibility of Cambridge City Council, South Cambridgeshire District Council and the Environment Agency.

Highway Drainage: Drains which only serve the public highway are the responsibility of the Highway Authority and Cambridgeshire County Council. They are designed in accordance with the Highway Authority Specification and Guidelines which are based upon Design Manual for Roads and Bridges published by the Highway Agency.

SUDS Features: The Flood and Water Management Act 2010 encourages the use of sustainable drainage in new developments and requires drainage systems to be approved by a SuDS Approving Body (SAB), which will be the unitary or County Council for the area, in this case Cambridgeshire County Council. The Act also places a duty on the local authority to adopt and maintain SuDS, providing that they serve more than one property and have been completed to their satisfaction.

Anglian Water will not, at the time of writing, adopt SuDS features located in open space because they can attract groundwater which is not covered under the terms of their responsibility. The SuDS features could be included within the ‘open space’ and maintained by Cambridge City Council.

8.1.5 SuDS Features

Surface water will be attenuated on site within SUDS features, constructed as an integral part of the Proposed Development. Features will be implemented with due regard to CIRIA publication C697 – The SuDS Manual, ‘Sustainable Drainage, Cambridge Design and Adoption Guide dated’ October 2009 and the Environment Agency SuDS hierarchy, which is duplicated in Table 8-2. The selection of appropriate SuDS features has been undertaken by considering the proposed density of the Proposed Development, land use and development plan as required by the local planning authority.



Table 8-2: Environment Agency SuDS Hierarchy

SUDS Feature Rank

Living/Green Roofs 1

Constructed Wetlands/Retention Pond 2

Detention Basins 3

Filter strips and Swales 4

Soakaways 5

Infiltration Trenches 6

Gravelled Areas 7

Porous Paving 8

Over-sized pipes 9

Storage Tanks/Cells 10

The Proposed Development has been divided into various catchments, which replicate as far as possible, the natural topography of the Application Site. These catchments discharge into the Washpit Brook by a range of SuDS features located in areas predominantly to the south and east of the Application Site.

Features likely to be implemented include:

• Green roofs

• Constructed wetlands and/or retention ponds;

• Filter strips and swales;

• Soakaways and infiltration features;

• Porous paving for level surface parking areas;

• Storage tanks for parking areas constructed on sloping topography.

It is intended to use a tiered approach to the surface water drainage and SuDS provision. In the first instance, green roofs, swales, porous paving and filter strips will be used at source to collect surface water run off from paved areas and improve its quality to reduce the concentration of hydrocarbons and other contaminants. Cellular storage tanks will be provided below porous paving for development blocks constructed on steeply sloping topography as attenuation within surface features will be impractical. Where porous paving or filter strips overlie significant depths of gravels or chalk situated within the centre or east of the Application Site, runoff will be permitted to discharge to ground directly. All infiltration devices will be positioned in spaces within the housing blocks and will be situated a minimum of distance of 5m away from all buildings or other SuDS feature if they are constructed within gravel, or 20m if they are constructed within chalk.



The second form of control will consist of site control measures, including retention ponds and linear wetlands, which are considered the best option at the lower end of the system as they feature high on the SuDS hierarchy due to their ability to improve water quality. Retention ponds will be used in each catchment to store surface water and discharge an attenuated flow into the Washpit Brook to mimic the existing greenfield rate. In most cases, retention ponds will be located close to the Washpit Brook to ensure that overflow occurring during extreme events may be safely routed away from existing and proposed dwellings. All retention ponds and wetlands will be steered outside of the 1 in 100 year event + 20% climate change event flood. This will ensure that there is no informal pathway of surface water runoff from the Application Site to contribute to flooding downstream. Above ground drainage systems will be utilised, wherever possible, and swales will generally be used to convey runoff from source control devices to the site control features situated adjacent to the Washpit Brook.

Adopted roads are likely to be surfaced using conventional bituminous materials, which will not provide a cleansing benefit; therefore swales will be introduced within the system where necessary to prevent high level contaminants from large adopted road areas from entering the retention ponds.

Not all areas within the Proposed Development will be able to accommodate SuDS features to provide site control and there will therefore be a requirement for source control features to discharge directly into retention ponds situated on the western edge of the Application Site. This approach will ensure that the surface runoff from all areas of the Application Site is attenuated and that the proposed discharge into the Washpit Brook will mimic existing greenfield runoff rates.

8.1.6 Attenuation Volumes

Both Attenuation Storage and Long Term Storage (LTS) will be incorporated within the Proposed Development, in accordance with the requirements of PPS25 and the mandatory requirements of the Code for Sustainable Homes (CSH).

Attenuation Storage is provided to allow the discharge from a development to be limited to existing greenfield runoff rates. LTS is provided to mitigate the additional volume of runoff generated by an increase in impermeable areas within a development site. LTS will either be allowed to soak into the ground or, if sufficient infiltration is not available, it will be stored and discharged at a rate not exceeding QBar or 2l/s/ha, whichever is the lesser. All surface runoff that it intercepted from roof areas that is accommodated within long term storage devices will be made available for reuse within the building, through water harvesting, to minimise potable water demand. The geology underlying the Proposed Development will not be suitable for infiltration in all locations, as discussed within section 8.1.2; therefore long term storage devices with flow controls capable of restricting the discharge to QBar or 2l/s/ha, whichever is the lesser are proposed to ensure that the volume of runoff entering the watercourse will not be increased.

A computer model of the strategic surface water drainage for the Proposed Development has been constructed using MicroDrainage software. The model has been subjected to simulated rainfall events for 1 in 1, 1 in 30, 1 in 100 and 1 in 100 year events with a 30% allowance for climate change, to calculate a sufficient attenuation storage volume for the Proposed Development, based upon the restriction to the equivalent existing greenfield rate for the storm return period simulated.



The development site is likely to cover an impermeable area of approximately 56ha, excluding green areas. The Proposed Development is likely to increase the percentage impermeable area by approximately 50% due to the construction of buildings, streets and hard-paved areas. Calculations have been produced to establish the increase in surface run-off from these areas resulting in a total attenuation storage volume of approximately 23,700m3 and an additional long-term storage requirement in the region of 20,900m3.

Calculation output results from MicroDrainage are contained within Annex O to support the attenuation figures stated above. A spreadsheet is also included in Annex O to calculate LTS required to retain the additional volume of run off generated by the Proposed Development. The sheet is based on “Preliminary Rainfall Runoff Management for Developments” (Revision D) published by the Environment Agency.

Drawing Reference D127313-500-102, which is contained in Annex O, shows the volume of attenuation and long term storage that is required within each catchment. Attenuation volumes have been stated for the 1 in 1, 1 in 30 and 1 in 100 year + 30% climate change rainfall events to demonstrate that equivalent greenfield runoff rates can be achieved within the stated volumes.

A preliminary drainage strategy scheme for the Proposed Development and typical details of preferred SuDS features are shown on Drawing References D127313-500-103 and 104, respectively, which are contained in Annex O. The drainage strategy will be subject to consultation with stakeholders, agreement with the relevant authorities and further detailed design, but is presented as a broad indication of the features proposed for the Proposed Development site. The drawings also demonstrate that there are sufficient open space areas located throughout the site to accommodate the desired SuDS features.

8.2 Flood Water Displacement

The Proposed Development will extend marginally into the floodplain associated with the 1 in 1 in 1000 year event. Earthwork bunds are also proposed within the existing floodplain. These have noise attenutation characteristics and complement the landscape design. They are an integral part of the Proposed Development landform and would also have flood storage and attenutuation characteristics.

Section 6 includes hydraulic calculations to demonstrate that the alterations to the existing watercourse will reduce the downstream peak flow rate in order to ensure that third parties will not be adversely affected by the displacement of floodwater.



9 Planning Policy Statement 25 Context

9.1 Vulnerability Classification and PPS25 Sequential Testing

PPS25, paragraph E9 (pg32) states that:

“Planning application for development proposals of 1 hectare or greater in Flood Zone 1 should be accompanied by a FRA”.

In addition, for sites greater than 1ha the Environment Agency is a statutory consultee and will comment on flood risk and drainage issues pertaining to development of sites.

The type of Proposed Development is classified in general terms of Flood Vulnerability as ‘More Vulnerable’ under Table D2 of PPS25, which is shown as Table 9-1. Table D3 of PPS25, which is shown as Table 9-2, defines the relationship between Flood Vulnerability and Flood Zone Compatibility.

The main elements of proposed scheme were identified in Section 4. These have been replicated below with an indication of the Development Vulnerability, as illustrated in Table D2 of PPS25 and Table 9-1.

• Market and key worker dwellings – More Vulnerable;

• Student Housing – More Vulnerable ;

• Academic and Commercial Research – More Vulnerable;

• Primary School – More Vulnerable;

• Nursery – More Vulnerable;

• Hotel – More Vulnerable;

• A local centre, incorporating retail uses – Less Vulnerable;

• Large areas designated as open space – Water Compatible: and,

• Earth bunds acting as flood storage area – Water Compatible.

As discussed in Section 5.8 previously, built elements of the Proposed Development are almost entirely located with Flood Zone 1: Low Probability, i.e. land being assessed as having less than 1 in 1000 annual probability of river flooding in any year, as defined through the hydraulic modelling outlined in Section 5 and 6. No development blocks are proposed within Flood Zone 3a or Flood Zone 3b. However, the western extremity of one development block situated in the north-western corner of the Application Site will be marginally affected by the 1 in 1000 year event (Flood Zone 2), as shown in Annex N.



Table D3 of PPS25 states that all uses of land are appropriate in Flood Zone 1, and all but ‘Highly Vulnerable’ land use is appropriate in Flood Zone 2. As none of the Proposed Development is classified as ‘Highly Vulnerable’, the Proposed Development is considered to be in compliance with the requirement of PPS25 and an Exception Test will not be required for the development proposals. More importantly, land use within each development block will be sequentially located to steer all ‘More Vulnerable’ and ‘Less Vulnerable’ development (i.e. buildings) into Flood Zone 1, to ensure that the site is safe, as discussed in Section 10.3.

Table 9-1: Flood Risk Vulnerability Classification (From Table D2, PPS25)

Essential Infrastructure

• Essential transport infrastructure (including mass evacuation routes), which has to cross the area at risk, and strategic utility infrastructure, including electricity generating power stations and grid and primary substations.

Highly Vulnerable

• Police stations, Ambulance stations and Fire stations and Command Centres and telecommunications installations required to be operational during flooding.

• Emergency dispersal points. • Basement dwellings. • Caravans, mobile homes and park homes intended for permanent residential

use. • Installations requiring hazardous substances consent.

More Vulnerable

• Hospitals. • Residential institutions such as residential care homes, children’s homes, social

services homes, prisons and hostels. • Buildings used for: dwelling houses; student halls of residence; drinking

establishments; nightclubs; and hotels. • Non–residential uses for health services, nurseries and educational

establishments. • Landfill and sites used for waste management facilities for hazardous waste. • Sites used for holiday or short-let caravans and camping, subject to a specific

warning and evacuation plan.

Less Vulnerable

• Buildings used for: shops; financial, professional and other services; restaurants and cafes; hot food takeaways; offices; general industry; storage and distribution; non–residential institutions not included in ‘more vulnerable’; and assembly and leisure.

• Land and buildings used for agriculture and forestry. • Waste treatment (except landfill and hazardous waste facilities). • Minerals working and processing (except for sand and gravel working). • Water treatment plants. • Sewage treatment plants (if adequate pollution control measures are in place).

Water-compatible Development

• Flood control infrastructure. • Water transmission infrastructure and pumping stations. • Sewage transmission infrastructure and pumping stations. • Sand and gravel workings. • Docks, marinas and wharves. • Navigation facilities. • MOD defence installations. • Ship building, repairing and dismantling, dockside fish processing and

refrigeration and compatible activities requiring a waterside location. • Water-based recreation (excluding sleeping accommodation). • Lifeguard and coastguard stations. • Amenity open space, nature conservation and biodiversity, outdoor sports and

recreation and essential facilities such as changing rooms. • Essential ancillary sleeping or residential accommodation for staff required by

uses in this category, subject to a specific warning and evacuation plan.



Table 9.2: Flood Risk Vulnerability and Flood Zone Compatibility (from Table D3, PPS25)

Flood Risk Vulnerability Classification

Essential Infrastructure

Water Compatible

Highly Vulnerable

More Vulnerable

Less Vulnerable

Zone 1 � � � � �

Zone 2 � � Exception Test Required

� �

Zone 3a Exception Test Required

� � Exception Test Required

�

Flo

od

Zo

ne

Zone 3b ‘Functional Floodplain’

Exception Test Required

� � � �



10 Proposed Flood Risk Management

10.1 Mitigation of Drainage Flood Risk

The Proposed Development will potentially alter the drainage regime of the site by increasing impermeable surfaces. Mitigation measures will be provided to ensure that the rate and volume of surface runoff entering the watercourse will not be increased.

Section 8.1 presents a preliminary design for the surface water drainage system on the site. Source control and site control features will be provided to intercept, attenuate and improve the quality of runoff before it is discharged to the existing watercourse. Flow control structures will be provided to ensure that the rate of discharge to the watercourse will not exceed the greenfield runoff rate. Section 8 of this report has identified that approximately 23,700m3 of attenuation storage will be required to store the excess runoff generated through the attenuation of a rainfall event with a return period of 1 in 100 years, including an allowance of 30% for climate change. This storage will be provided through a combination of green roofs, porous paving and underground storage tanks on plot; swales adjacent to streets and linear wetlands or retention ponds situated adjacent to the existing watercourse within areas of open space. Long Term Storage with a volume of 20,900m3 will be provided in the form of infiltration devices that discharge to ground or storage devices with flow control devices to restrict the discharge to the receiving watercourse to QBar or 2l/s/ha, whichever is the lesser. All surface runoff that it intercepted from roof areas that is accommodated within long term storage devices will be made available for reuse within the building, through water harvesting, to minimise potable water demand The Long Term Storage devices will ensure that the volume of water discharged to the existing watercourse will not be increased by the Proposed Development.

Flows exceeding the capacity of any below-ground drainage structures will be directed towards attenuation features prior to discharge to the receiving watercourse in order to ensure that runoff from rainfall events with a return period of up to the 1 in 100 years will be retained on site. Flow control structures will be provided to ensure that the discharge from the attenuation features will not exceed pre-development rates for rainfall events with a rainfall return period of up to and including 1 in 100 years with an additional allowance of 30% on rainfall intensity to allow for the predicted effects of climate change over the lifetime of the Proposed Development.

10.2 Proposed Amendments to the Washpit Brook

The Environment Agency Flood Map also identifies the potential for significant flooding within Girton at the confluence of the Washpit Brook and Beck Brook, which is situated approximately 2km downstream of the site, and nine properties on Dodford Lane were believed to have flooded on 21 October 2001.

As a precautionary measure to reduce flood risk to, and from, the Application Site, the Proposed Development includes an online flow control structure and a two stage channel that are constructed along the section of Washpit Brook that extends through the site to enable the peak flows downstream of the site to be reduced for a range of return periods and for the excess flow to be stored within the landscaped areas of the site. Earthwork bunds are proposed on the western edge of the development to complement the landscape design. These bunds have been designed geometrically to assist in the storage of floodwater, whilst also protecting the Proposed



Development from flooding. A hydraulic model has been constructed to quantify the reduction in flood risk, which is presented within Section 6 of this document.

The hydraulic modelling of the proposed scheme indicates that the peak flows downstream of the site may be reduced by up to 28% and 10% for rainfall events with a return period of 20 and 100 years, respectively. The hydraulic modelling has also resulted in the identification of new flood zones within the Application Site. This has enabled the built elements of the Proposed Development almost entirely to be located in Flood Zone 1, with the exception of one development block, which is partially located in Flood Zone 2.

10.3 Sequential Allocation of Development

Hydraulic models have been prepared to determine the extent of Flood Zone 1, 2 and 3a and 3b for the baseline and future developed scenarios. The parameter plans for the Proposed Development, including the proposed flood alleviation scheme have generally been designed to ensure that built development within the Proposed Development will almost entirely be situated within Flood Zone 1, with the exception of one development block, which is located in the northwest corner of the site and is partially located in Flood Zone 2. However, at detailed design, buildings will be sequentially located so that the land use within this block is determined based upon the extent of flood risk. This will be achieved by allocating open spaces, landscaped areas or car parking into the region of the block affected by flooding. This process will ensure that all buildings will be located within Flood Zone 1. Safe and dry access is available from the site by foot or by vehicle.

10.4 Finished Floor Levels (FFL)

All main buildings will be steered into Flood Zone 1. The culvert blockage scenarios undertaken as part of the hydraulic model sensitivity test (Section 5.7) did not result in flood levels any greater than the 1 in 1000 year event. Therefore, there is no requirement to define FFL based upon flood risk and should instead be informed by the requirements of Building Regulations.

10.5 Flood Flow Paths

Under the 1 in 1000 year event a minor flow path was identified from the Washpit Brook upstream of the Pheasant Plantation and across the southern portion of the site towards the M11. The approximate location of this flow path is identified on the flood maps included within Annex N. Allotments and sports pitches are proposed in this area as they will not be vulnerable to flooding from this flow path under the 1 in 1000 year event.

10.6 Reservoirs Legislation

Reservoir legislation can apply to combinations of landforms and flow control structures with a floodwater storage and attenuation function. Under the Reservoirs Act 1975 a reservoir is defined as retaining more than 25,000 m3.

The Flood and Water Management Act 2010 brings up to date ageing reservoir safety legislation and reflects a more risk-based approach to reservoir regulation through:

• Reducing the capacity at which a reservoir will be regulated from 25,000m³ to 10,000m³;



• Ensuring that only those reservoirs assessed as a higher risk are subject to regulation;

• All undertakers with reservoirs over 10,000m³ must register their reservoirs with the Environment Agency;

• Inspecting engineers must provide a report on their inspection within 6 months;

• All undertakers must prepare a reservoir flood plan;

• All incidents at reservoirs must be reported5;

Despite becoming law immediately, the new legal requirements within the Flood and Water Management Act cannot be applied straight away. In the meantime, the Reservoirs Act 1975 continues to apply.

The Proposed Development will introduce additional storage of floodwater within the Application Site. The proposed flow control structure will create a positive head gradient, by retaining floodwater upstream. This difference in head has been used to calculate the volume stored because it is this volume that would spill downstream if the flow control structure were to fail. The total volume of water upstream has not been calculated because a large volume of floodwater would be stored on site under the baseline conditions.

All of the model simulations were investigated (including the URS Scott Wilson and Halcrow hydrology) to determine the maximum difference in head across the flow control structure. This was found to occur under the 1 in 1000 year event on the basis of the URS Scott Wilson hydrology, where a head difference of only 0.37m was observed (see Figure 10-1). This equates to a total volume of approximately 19,000m2, over the area of stored floodwater upstream of the flow control structure. This is less than that defined as a reservoir under the Reservoirs Act 1975 and would not therefore be regulated under the Reservoirs Act 1975 but would be within the limits for regulation under the Flood and Water Management Act..

With respect to the risk-based approach identified in the Flood and Water Management Act 2010, the proposed flood storage area is not considered to introduce any significant risk, with only a small difference in head across the flow control structure. Furthermore, in the unlikely event that this were to fail, the M11 slip road embankment, located at the downstream extent of the site will contain floodwater. Floodwater will be released from the site slowly, throttled by the culverts beneath the highway infrastructure.

10.7 Ditches

Built development is proposed on a number of existing minor headwater ditches. The ditches are considered to have been constructed to assist with field drainage and they will therefore be retained or diverted, wherever possible. The existing ditches will not be culverted unless it is essential to extend an access across the ditch. Proposed culverts will be constructed using perforated pipes in order to permit them to intercept groundwater from permeable strata. A plan is included within Annex P, which illustrates the diversion and culverting proposals.

5 http://www.environment-agency.gov.uk/business/sectors/118432.aspx

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10.8 Watercourse Maintenance

All watercourses within the Application Site must be maintained to an acceptable standard, which includes clearance of debris in the channel and at the upstream face of all culverts. No built development will be constructed within 5m of either the banks of the Washpit Brook or any connecting ditches to ensure that access is available for maintenance requirements.

The proposed scheme involves the construction of a two stage channel adjacent to the existing Washpit Brook in order to store excess flow generated by provision of online flow control structures that will facilitate a reduction in downstream peak flow. However, it is not intended that the development proposals will alter the maintenance responsibility of South Cambridgeshire District County Council, which will still include land located within a distance of 5m from the top of the bank of the existing watercourse. The remainder of the two stage channel will be maintained by the landowner, the University of Cambridge, as part of the wider landscape management strategy.

10.9 Access and Egress

All buildings will be placed in Flood Zone 1. Various safe and dry access routes to the building by foot or by vehicle are available from or to Huntingdon Road. These access routes will be suitable if residents etc wish to evade the Application Site when under flood conditions.

10.10 Construction Phases

The bunds on the western edge of the Proposed Development will be constructed using earthwork material arising from the development construction in order to remove the requirement for earthworks material to be imported or exported.

The entire southern bund will be completed by 2014, in line with Phase 1 of the Proposed Development. The remaining bunds will be partially constructed, to a height required to contain floodwater. The other elements of the will also be constructed as part of the Phase 1 development, to enable downstream flood risk to be reduced at the earliest possible stage.

Under subsequent construction phases of the Proposed Development, the height of the central and northern bunds will be increased as more material is generated on site, with completion anticipated in 2026.



11 Conclusions

Following the completion of this Level 3 Flood Risk Assessment, in line with the recommendations of PPS25, the following conclusions can be made:

11.1 Flood Risk – To the Proposed Development

• Hydraulic modelling has identified a small portion of the site, along the Washpit Brook to be situated within Flood Zone 3. A larger flood extent has been identified as Flood Zone 2. However, the majority of the site is considered to be located within Flood Zone 1.

• The Washpit Brook will be amended as part of the Proposed Development to incorporate an online flow control structure and a two stage channel, which will enable downstream peak flows to be reduced and excess flood water to be stored within landscaped areas of the Application Site. This proposal will result in an adjustment to the existing or baseline flood zones identified by hydraulic modelling and Will thereby enable the entire built development to be located in Flood Zone 1, with the exception of one development block, which is partially located in Flood Zone 2.

• Flood risk associated with the various headwater ditches that dissect the site is considered to be negligible.

• The site is not considered to be at risk of flooding from any other flood sources.

11.2 Flood Risk - From the Proposed Development

• The existing Washpit Brook watercourse will be modified to include an online flow control structure and a two stage channel to permit peak flows discharged from the Application Site to be reduced and for the excess floodwater to be stored in order to assist in alleviating the existing flooding observed downstream in Girton.

11.3 Flood Risk Management

• Built elements of the Proposed Development will almost entirely be located within Flood Zone 1 save for parts of a building block situated in the northwest corner of the Application Site. Thus the least vulnerable parts of the Proposed Development will be steered into Flood Zone 2, such as landscaping, which is classified in PPS25 as Water Compatible and is therefore compatible. More Vulnerable and Less Vulnerable development (i.e. buildings) will be steered into Flood Zone 1.

• All main buildings will be steered into Flood Zone 1 therefore there is no requirement to define finished floor levels based upon flood risk;

• A flood flow path identified in the 1 in 1000 year event (i.e. Flood Zone 2) in the proximity of Pheasant Plantation will have no impact as there is no proposed built development at this location.



• No built development will be undertaken within 5m of either the banks of the Washpit Brook or any connecting ditches to allow for maintenance of all watercourses that will remain post-development.

• All watercourses within the Application Site must be maintained to an acceptable standard, which includes clearance of debris in the channel and at the upstream face of all culverts.

• Various safe and dry access routes to the building by foot or by vehicle are available from or to Huntingdon Road, which will be suitable if residents etc wish to evade the Application Site when under flood conditions.

• Finished levels within development will be engineered to ensure flows are directed safely away from dwellings and other buildings towards the open space in the lower parts of the Proposed Development to prevent flooding to property.


• The Proposed Development will incorporate a new surface water drainage system. The system will be designed to attenuate surface water runoff to greenfield runoff rates up to and including the 1 in 100 year event with a 30% additional allowance on rainfall intensity for climate change. This will ensure that flooding downstream of the Application Site will not be exacerbated by an increase in surface water runoff from the Proposed Development.

• Approximately 23,700m3 of attenuation storage is required to ensure that the discharge from the Proposed Development into the Washpit Brook does not exceed existing greenfield runoff rates. This will be provided using porous paving, geo-cellular storage, swales and retention ponds.

• Approximately 20,900m3 of Long Term Storage (LTS) will be required to ensure that the volume of water discharged into the Washpit Brook is not increased. This will be provided using infiltration devices. LTS will either be allowed to soak into the ground or, if sufficient infiltration is not available, it will be stored and discharged at a rate not exceeding QBar or 2l/s/ha, whichever is the lesser. All surface runoff that it intercepted from roof areas that is accommodated within long term storage devices will be made available for reuse within the building, through water harvesting, to minimise potable water demand.

• A preliminary drainage strategy has demonstrated that there is sufficient open space throughout the Proposed Development to accommodate the range of preferred SuDS features. By controlling surface water run off from the Proposed Development, and it will therefore ensure the risk of flooding to other sites downstream will not be increased by the Proposed Development.



12 Annexes


D127313 September 2011 A

Annex A – Environment Agency Flood Map

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D127313 September 2011 B

Annex B – Correspondence

Brampton Office, Bromholme Lane, Brampton, Huntingdon PE28 4NE *Daytime calls cost up to 8p Customer Services Line: 08708 506 506 * plus 6ppm from BT lines.Email:[email protected] Mobile and other providers’

www.environment-agency.gov. Page 1 of 2

1

Date 4 June 2010

Please quote on all correspondence

Scott Wilson Ltd Our ref CCC/2010/16380Nick Bosanko Your Ref D127313 The Crescent Centre Temple Back BristolBS1 6EZ

Dear Mr Bosanko

NW Cambridge

We are pleased to forward our response to your recent enquiry.

As Washpit Brook is not designated as Main River we do not have any hydraulic models or survey information for this reach. We also do not hold any record of historic flooding within or near to the site. The Flood Zones are based on a model that begins downstream of the A14. This model only looks at Main River and was developed to investigate the main area of flood risk at Girton.

The section of Washpit Brook within the vicinity of the site as detailed is an Awarded Watercourse to South Cambridgeshire District Council. Our Hydrometry and Telemetry team therefore have no data for this watercourse and we do not hold any watercourse survey information for this stretch. The Main River only runs to the North of the A14 (Huntingdon Road) into Beck Brook which is part of the Cottenham Lode system.

We do not hold any details on the balancing ponds within the vicinity of the site.

We have no flood defences or structures in the vicinity of the site. We recommend that prior to commencing a Flood Risk Assessment (FRA) you discuss the proposal with the Local Planning Authority to request that they undertake the flood risk Sequential Test. New development should be steered to areas at the lowest probability of flooding.

Any proposed development must demonstrate that it will not be at risk from flooding and that it will not cause nor exacerbate flooding elsewhere. All flood risks should be looked at, inclusive of surface water and groundwater.

Washpit Brook is classified as an ordinary watercourse, however it is awarded to South Cambridgeshire District Council for maintenance purposes. In addition they also have their own byelaws under which consent must be sought for any works adjacent to and in the brook.

The site is partially located in Flood Zone 2, which is the medium to low risk zone and is defined for mapping purposes by our Flood Zone Maps.

This is land where the indicative annual probability of flooding is between 1 in 100 and 1 in 1000 years from river sources (I.e. between 1% and 0.1% chance in any given year.

A FRA (Flood Risk Assessment) should be submitted with your application to the Local Planning Authority. Guidance on FRA requirements for this type of development in Flood Zone 2 can be found in Annex E of PPS25 – Development and Flood Risk and on the Environment Agency’s Flood Risk Standard Advice website Version 2 February 2009 http://www.environment-agency.gov.uk/research/planning/33098.aspx.

Hydraulic modelling may be needed to determine the 1 in 100 year flood level which should also consider climate change in line with PPS 25.

For any new development, the implementation of sustainable drainage systems (SuDS) should be carefully considered and this approach is encouraged in Planning Policy Statement 25 (Development and Flood Risk). These techniques can be used to minimise pollution impacts and provide a method for attenuating run-off that could otherwise lead to flooding. The Agency encourages the use of SuDS that reflect the local ground drainage characteristics by distributing water at source.

Such sustainable systems should be designed in accordance with the principles given in CIRIA C522 and C523. Guidance may be found within CIRIA Report 156 "Infiltration Drainage - Manual of good practice".

We have assessed the general catchment characteristics for this area and consider the Greenfield Runoff rate to be 2l/sec/developed hectare. However, you may wish to undertake site specific calculations. Attenuation will be required to accommodate a 1 in 100 year critical storm with climate change allowance. The applicant must demonstrate that there is sufficient capacity within the receiving watercourse for the maximum rate of discharge.

The information is provided subject to the enclosed Standard Notice Commercial that details how you may use the supplied information.

Yours sincerely

Trisha Harewood Corporate Services

Direct Dial: 01480 483917 Direct Fax: 01480 413381

[email protected]

Brampton Office, Bromholme Lane, Brampton, Huntingdon PE28 4NE *Daytime calls cost up to 8p Customer Services Line: 08708 506 506 * plus 6ppm from BT lines.Email:[email protected] Mobile and other providers’

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2



Earthwork bunds are proposed on the western edge of the Proposed Development. These will have noise attenuation properties and complement the landscape design. The position of the bunds is shown in Figure 4-2 below. The bunds will align the Washpit Brook and the offset between the toe of the bund and the nearest riverbank will vary from 5m to 90m in order to permit the riparian zone to be retained. The bunds will be constructed in two phases, which will coincide with the construction of the wider Application Site to permit earthworks generated by the construction works to be used to form the bunds, and thereby avoid the requirement for earthworks material to be imported.


Figure 4-2: Bund Location Plan (shown in green)

4.2 Drainage

As an essential part of the Proposed Development, a new surface water drainage system will be designed and installed across the Application Site to allow runoff to be intercepted and attenuated via a cascading system of SuDS before being discharged to the Washpit Brook. The SuDS will be designed to ensure that the flow and volume of water that is discharged to the Washpit Brook will not exceed greenfield runoff rates for events up to and including the 1 in 100 year event with an additional 30% allowance for the effects of climate change.

Surface runoff generated by events with a return period of up to 100 years will be directed away from all buildings, retained on site and discharged at greenfield runoff rates to ensure that the flow and volume of water discharged to the Washpit Brook is not increased.

The surface water management strategy is discussed in more detail in Section 8.

1

Kumaresan, Bhuvaneshwari

From: Empson, Katie [[email protected]]Sent: 29 June 2010 20:06To: Kumaresan, BhuvaneshwariSubject: 14384/BKU- Site at M11, Huntingdon Road & Madingley Road, Cambridge

(CCN/2010/24437)Attachments: Copyright statement and disclaimer.doc

Dear Buhvaneshwari

New Roads and Street Works ActScheme Ref No: 14384/BKUSite at M11, Huntingdon Road & Madingley Road, Cambridge

Thank you for your email below.

Please be aware the written consent of the Environment Agency is required for any proposed works which are in, over or within 9m of a main river. Please contact our Development & Flood Risk Team on 01480 483915 or [email protected] to discuss your proposals. The team will then be able to advisewill need consent for your works.

We also have some land ownership in the area, please contact Pat Neal, Estates Database Officer on 01733464email [email protected] to see if you will need permission for works to take place on our land.

I hope this information is of use to you. Should you have any further queires, please do not hesitate to contact me ag

The information is subject to the attached notice, which we advise you read.

Yours sincerely

Katie Empson (Miss) External Relations Officer

Extension: 7 50 5031Direct Dial: 01522 785031Fax: 01522 512927

Waterside House, Waterside North, Lincoln, LN2 5HA

From: Kumaresan, Bhuvaneshwari [mailto:[email protected]] Sent: 28 June 2010 12:10 To: Es-Pipelines; National Plant Enquiry's; Easynet; Enquiries, Unit; Fibrespan; Maygurney(Fijuitsu); Gammatelecom; GTC; Global Crossing; [email protected]; Networkrail; Mcnicholas; Telenttelia; Verizonbusiness; Vtesse Network; [email protected]; [email protected]; [email protected]; [email protected]; [email protected] Subject: PLANT ENQUIRY: 14384/BKU- Site at M11, Huntingdon Road & Madingley Road, Cambridge

Click here to report this email as spam.

2

McNicholas - Please reply for COLT, KPN & TATA.

14384/BKU- Site at M11, Huntingdon Road & Madingley Road, Cambridge, 541918,261097 to 542733,260094 to 543237,259266, NA

Works Description- Feasibility Study

Reply required by date – 09/07/2010

Please find attached a plant enquiry with corresponding map for your attention. Please could you send us plans showing the location of your company's affected plant to the below address quoting our reference number and the name of the scheme shown above.

Please cover the entire area shown in the boundary on the attached map not just the arrow, this is just an indicator of the middle of the site

If you do not have any apparatus in this area, please could you send written confirmation to declare that no apparatus is affected.

Many thanks,

Please don't hesitate to contact me with any questions.

Many Thanks,Bhuvana.K

Bhuvaneshwari.KumaresanStatistics Research Co-Ordinate, CommunicationsËTKINSThe�Official�Engineering�design�service�providerfor�the�LONDON�2012�Olympic�and�Paralympic�Games�

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Project Data

Title NW Cambridge Reference Number D127313

Call Information

To Pat Matthews (Drainage Engineer) Date 14/9/2010

Company South Cambridgeshire District Council Time 12:00

From Nick Bosanko

Messages

The proposed development was discussed and the key issues identified in a flood risk and drainage perspective were:

1. 5m maintenance strip on either side of the Washpit Brook, where no development is allowed

2. A maintenance contribution for increased maintenance requirements of the Washpit Brook after development of the site

3. Development proposals should seek a betterment in flood risk from the site, if possible 4. Consider the impact of the A14 widening

A more detailed summary of our discussion is included below. Some other issues are also identified. Washpit Brook Maintenance South Cambridgeshire District Council undertakes annual maintenance of the Washpit Brook. They require a 5m maintenance from both banks of the watercourse to allow access. South Cambridgeshire District Council is likely to require a maintenance contribution towards increased maintenance of the Washpit Brook after development of the site. They believe that more maintenance will be required and an annual schedule is likely to be increased to 2-3 visits each year. The watercourses within the site must be maintained to an adequate standard. Surface Water Maintenance The FRA must confirm who will maintain the surface water features etc. He suggested that SCDC may take this on, which should be considered in the first instance. However, alternatives should also be considered if the Council are unwilling to provide maintenance. If so, perhaps this can be included in the general ground maintenance requirements. Washpit Brook Flooding and Surface Water Runoff South Cambridgeshire District Council hold no records of flooding within the site. This is because the site consists of agricultural land and no houses etc have been affected by flooding that may or may not have occurred. Therefore, they have no requirement to record it. Significant flooding occurred at Dodford Lane, Girton on 21 October 2001. This is located where the Washpit Brook meets with another watercourse, downstream of the site. Local residents may scrutinise any development proposals upstream in terms of the impacts upon surface water runoff and increased flood risk at Girton.

Page 1 of 5

Meeting Notes

Project Name: NW Cambridge Project No.: D127313

Subject: Flood Risk and Surface Water Management Meeting No.: 1

Venue: Environment Agency Brampton Office Date and Time: 5/7/11 13:30

Attendees Apologies

Jennifer Gough – Environment Agency

Owen Pitt – Environment Agency

Pat Mathews – South Cambridgeshire District Council

Nick Bosanko – URS Scott Wilson

James Rose – URS Scott Wilson

Distribution – As above plus:

Roger Taylor – University of Cambridge

Adrian Howell – Turner and Townsend

Heather Topel – AECOM

Steve Pickles – Gardiner and Theobald

David Smith – URS Scott Wilson

ITEM NOTES ACTION

1.0 Current Development Proposals

URS SW tabled the June 2011 masterplan and advised that it had been updated to address comments supplied by consultees following the pre-application submission. The main changes to the masterplan have occurred at the western edge, where the scale of landscaped earthworks has been adjusted to accommodate the volume of material predicted to be generated by the proposed construction works. These bunds have been designed geometrically; firstly, to form noise attenuation features that will reduce noise levels at ground floor level within landscaped areas and proposed buildings on the western edge; and secondly, to contain surface runoff generated by the proposed development on site before it is discharged to the Washpit Brook at an attenuated rate. They also assist in containing fluvial floodwater from the Washpit Brook and thereby complement proposed flood storage areas.

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2.0 Fluvial Flood Modelling

URS SW advised that they organised a hydrographic survey of the section of the Washpit Brook that extends through the site and used this survey to construct a fluvial flood model.

The fluvial flood modelling work for the existing watercourse indicated that the outer edge of two development plots would extend slightly into the 1 in 100 year + climate change flood extent but that buildings would be positioned to avoid the flooding. The Environment Agency (EA) provided comments on the pre-application submission, which suggested that a two stage channel could potentially be provided to accommodate floodwater and thereby ensure that these plots no longer extend into the flood plain generated by the 1 in 100 year event, including an allowance for climate change.

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Page 2 of 5

Meeting Notes

ITEM NOTES ACTION

2.0

Cont

Fluvial Flood Modelling Continued

URS SW have constructed an additional fluvial flood model in order to determine the benefits that may be gained by adjusting the geometry of the Washpit Brook to incorporate a two stage channel, in combination with earthworks bunds and two flow control structures. This model has been designed to ensure that water levels will not be increased at the culverts that pass below the M11 and thereby ensure no impact upstream of the site.

A comparison of the flood maps and downstream hydrographs for the existing and proposed models indicated that the proposed works could enable the two development plots to be taken out of the 1 in 100 year + CC flood extent and that the peak discharge from the site could be potentially reduced for the 1 in 20 and 1 in 100 + CC events. The flood map for the proposed watercourse indicated that two development plots still extend slightly into the 1 in 1000 flood extent, one being situated adjacent to the second flow control device and the other being occupied by the Energy Centre.

The Environment Agency advised that this approach to reducing flood risk downstream would be acceptable and suggested that the two stage channel is extended to the proposed bund in order to bring the development plot situated close to the second flow control structure out of the 1 in 1000 flood extent and to specify that the future Energy Centre building should ideally be situated outside the 1 in 1000 flood extent or for it to be flood resilient.

URS SW indicated that the proposed earthworks bunds extend into the Flood Zone 3b, which is classified as the functional floodplain but that this approach complies with the requirements of PPS25 as the bunds will form flood management infrastructure, which are considered to be Water Compatible and are therefore compatible with Flood Zone 3b. The EA indicated that it would be acceptable to extend the bunds into the Flood Zone 3b as the proposed adjustments to the watercourse would essentially redefine the flood extent.

The EA advised that the Reservoirs Act could apply if the volume of water stored in a single location exceeds 25,000m3 and SCDC indicated that changes in legislation would introduce a requirement for water bodies to be classified as reservoirs, or otherwise, through risk assessment.

The EA requested URS SW to supply an electronic copy of the fluvial model for the existing and proposed watercourse. A brief commentary should accompany the models to define the purpose of each file and the extent of sensitivity testing that has been undertaken.

The EA indicated that the proposed works to the Washpit Brook, including the excavation of the two stage channel and installation of flow control structures, should be constructed and landscaped during the initial phases of the development to ensure the flood risk benefits are obtained early.

The EA requested URS SW to quantify the benefit that the proposed measures would provide by defining the percentage reduction in peak flow downstream or by defining the increase in return period and to prepare a standard detail showing the two stage channel for inclusion within the FRA.

The EA suggested a clear audit trail is required with respect to the proposed scheme to ensure no accountability falls upon the Universities Consultant Team if flooding were to occur downstream and residents blame the new development.

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Page 3 of 5

Meeting Notes

ITEM NOTES ACTION

3.0 Maintenance of Washpit Brook

SCDC confirmed that a riparian zone of 5m would be required along either edge of the watercourse to permit maintenance works to be undertaken as the Washpit Brook is an award watercourse.

SCDC advised that the frequency of maintenance activities are likely to increase as a result of the development, but that the extent of the award cannot be extended further than the existing riparian zone; therefore the University of Cambridge would be responsible for the maintenance of the two stage channel, which is likely to be landscaped using mown grass.

SCDC indicated that they will require a firm area to be maintained adjacent to the Washpit Brook that will be capable of supporting maintenance vehicles. The EA suggested that the two stage channel could be dished slightly to ensure any localised ponding would occur towards the centre of the two stage channel rather than adjacent to the riparian zone.

SCDC requested that URS SW add notes to the drawings stating that no works should be undertaken within 5m of the Washpit Brook without prior consent from SCDC to highlight this obligation to future Contractors. Drawings should also be provided to confirm areas of maintenance responsibility for each relevant party.

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URS SW

4.0 Pollution

The EA indicated that contaminants and sediment would need to be carefully controlled during the construction and operational phase of the development to avoid pollution of groundwater, as the northern parts of the site are classified as a secondary aquifer, which is likely to support flow to the surface water ditches that run across the site, including the Washpit Brook. URS SW advised that Chapter 15 of the Environmental Statement identifies pollution control measures.

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5.0 Infiltration in Foul Sewers

The EA stated that they had observed instances of groundwater infiltration within foul sewers situated within areas of permeable head deposits overlying impermeable clay, and suggested that it would be appropriate to consider the use of clay plugs in sewer runs to minimise groundwater infiltration into the sewers.

URS SW


URS SW described the proposed surface water management system, which will include rainwater harvesting systems to enable runoff from roof areas to be reused within dwellings to minimise potable water demand and act as long term storage to prevent there being an increase in volume of water discharged to the Washpit Brook. A cascading system of Sustainable Drainage Systems (SuDS) will be used to attenuate and improve the quality of runoff from roads and other paved areas within the development. Swales will be provided to intercept runoff from adopted highways and these swales will connect to retention ponds situated adjacent to the western edge of the development, via green corridors that extend through the development. Porous paving or filter strips will be provided to intercept and improve the quality of runoff from private areas before either being discharged to ground, or being conveyed to the retention ponds via the green corridors. Underground storage tanks will be provided below areas of porous paving situated on areas of sloping ground to enable storage volumes to be maximised. Filter drains will be provided at the interface between the proposed development and the curtilage of existing dwellings to ensure that water does not pond in this area due to a severance of existing flow paths. Bunds will be provided adjacent to the retention ponds to enable the runoff from events with a return period of up to 1 in 100 years to be retained on site.

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Page 4 of 5

Meeting Notes

ITEM NOTES ACTION

6.0

Cont

Surface Water Management Cont

Calculations have generally been prepared in accordance with the document entitled ‘Preliminary rainfall runoff management for developments’, which is jointly published by the Environment Agency and DEFRA. WinDES has been used where the development extends into areas of the site that fall away from the Washpit Brook as the equations provided within the aforementioned document do not consider this situation. Attenuation storage has been designed to restrict discharge to equivalent greenfield runoff rates (i.e. runoff from the post developed site for the 1, 30 and 100+CC year events will be restricted to greenfield runoff rates for the pre-developed site for the 1, 30 and 100 year events). Long term storage has been designed to accommodate the additional volume of water generated by the development during a 1 in 100 year rainfall event with a duration of 6 hours.

URS SW confirmed that permissible discharge rates have been calculated based upon greenfield runoff rates applied to impermeable areas, rather than to the areas of the entire catchment. URS SW also advised that the permeable areas on the northern and eastern edges of the site have not been considered to contribute surface runoff to the Washpit Brook as the existing ground levels fall away from the watercourse.

URS SW stated that the surface water management strategy has been prepared using parameter plans to support an outline planning application and that the impermeable areas could change subsequently. The Environment Agency indicated that notes should be added to the drawings to state that permissible discharge rates and storage volumes may be prorated in the event that the estimated impermeable areas change.

The EA advised that erosion control would be required where carrier drains discharge an attenuated flow to the Washpit Brook as the retention ponds serve a large catchment and requested URS SW supply a typical detail.

The EA also suggested that outfall from the central pond should be situated downstream of the second flow control structure to mimic the existing drainage regime.

The EA requested URS SW quantify the benefit that the proposed surface water management system would provide in terms of flood risk and to check if it would be feasible to attenuate runoff generated by the 1 in 100 year rainfall event, including an allowance for climate change, to greenfield runoff rates for a 1 in 30 year event or to the mean annual flood, Qbar. URS SW indicated that they considered it would not be practical to attenuate the 1 in 100 + CC event to Qbar, but would investigate whether it would be practical to attenuate back to the greenfield runoff rates for a 1 in 30 year event.

The EA stated that the FRA should define the body that will be responsible for maintaining SuDS features to ensure that are no features proposed that cannot be maintained. The EA suggested consultation with Simon Bunn at Cambridge City Council and a review of the adoption information on the Anglian Water website.

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Page 5 of 5

Meeting Notes

ITEM NOTES ACTION

7.0 Existing Drainage Ditches and Ponds

The Environment Agency advised that existing drainage ditches should be retained or diverted, wherever possible and that infilling should be avoided. The EA suggested that a plan should be prepared for inclusion within the planning application showing the location of existing drainage ditches, the extent and route of potential diversions and location of culverts required where the ditches pass below proposed roads. Culverts should generally be perforated to permit them to receive groundwater and thereby permit the existing drainage regime to be replicated. The EA indicated that this would be particularly important as they will not retrospectively grant consent for ditches to be culverted if this requirement is not highlighted in the planning application.

The Environment Agency indicated that it will be necessary to replicate the existing drainage regime as closely as possible to ensure that existing ponds will not dry up. Infiltration features should therefore be used where ground conditions permit to enable groundwater to be recharged, particularly where there is hydrological connectivity with existing ponds.

URS SW AECOM

URS SW

8.0 Foul Drainage

URS SW indicated that a Predevelopment Enquiry Report had been obtained from Anglian Water, which indicated that the Milton Sewage Treatment Works had sufficient capacity to receive the foul discharge from the proposed development and that the trunk sewer situated to the southeast of the site has a capacity of 42.4l/s. URS SW advised that a gravity sewer network would be provided on site to convey foul water to pumping stations that will be used to balance foul flows and ensure that foul water is discharged to the sewer at a rate not exceeding 42.4l/s.

The EA indicated that high level discussions were ongoing with Anglian Water to determine the most appropriate method of accommodating the additional volume of foul water that will be generated by all of the developments that are identified within the Stage 1 Water Cycle Study.

Info

Info

9.0 Conclusion

The Environment Agency indicated that they are generally content with the methodology that has been used to assess flood risk to and from the development and have suggested that minor amendments and additions are made to the application to clarify specific issues, such as:- amending the proposed two stage channel design to ensure all development is situated out with the 1 in 1000 year flood zone, with the exception of the land reserved for the future Energy Centre; quantifying the reduction in flood risk downstream; defining maintenance responsibilities; clarifying the extent of existing drainage ditches that are to be realigned to accommodate development and defining the location of culverts that will be required to enable flow to be maintained below proposed roads.

URS SW

10.0 Date of Next Meeting

To be confirmed

Notes By: James Rose



4.3 Rainwater Harvesting

The Code for Sustainable Homes (CSH) introduces a requirement for potable water demand to be reduced to 80 l/h/day for dwellings constructed to Code Level 5 or 6. Rainwater Harvesting will be used throughout the Application Site to collect and store rainwater runoff from roof areas and permit it to be reused for non-potable uses such as irrigation, toilet and urinal flushing. Rainwater falling onto hardstanding areas will be allowed to flow directly to swales or the underground drainage system as it is more likely to be contaminated and would need further treatment before it can be used.

The CSH also states that if the volume of rainwater runoff generated by the developed site is greater than for the pre-developed runoff, due to an increase in impermeable area, then it will be necessary to reduce the additional predicted volume to zero by retaining the additional water generated by a 1 in 100 year 6 hour event on site. The additional predicted volume of water should be stored at source and either discharged to ground via infiltration devices, or discharged to the receiving watercourse at a rate of QBar or 2l/s/ha, whichever is the lesser. Surface runoff from roof areas that is accommodated within long term storage devices will be made available for reuse within the proposed buildings to minimise the potable water demand and ultimately remove the requirement for this water to be discharged to the receiving watercourse.


D127313 September 2011 C

Annex C – South Cambridgeshire DC SFRA Flood Map

GIRTON

HISTON

MILTON

SAWSTON

HARSTON

CAMBRIDGE

MADINGLEY

IMPINGTON

OAKINGTON

WATERBEACH

TRUMPINGTON

GREAT SHELFORD

LITTLE SHELFORD

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Reproduced from Ordnance Survey data bypermission of the Controller of Her Majesty'sStationary Office © Crown copyright (2008).All rights reserved. License No. 0100022500.© 2008 NAVTEQ

FIGURE No:

TITLE:

SOUTH CAMBRIDGESHIRE DCAND CAMBRIDGE CITY COUNCIL

LEVEL 1 SFRA

FLOOD RISK CONSTRAINTS1402 - D - 1.50 0.5 1 1.5 2

Kilometres

KEY

South Cambridgeshire DC& Cambridge City Council Boundaries

Hydraulically Modelled Flood Risk Return Periods

EA Flood Zone Mapping

EA Flood Zone 3

EA Flood Zone 2

EA Flood Zone 1

Notes1. Where detailed hydraulic modelling in unavailable for Flood Zones 2 and 3, the E.A.'s flood outlinesas shown on their website have been provided.

2. In the absence of hydraulic modelling showing 1 in 100 year climate change extents, the E.A.'s Flood Zone 2 should be taken as the 1 in 100year climate change outline.

3. Please refer to section 4.10 of the SFRA forfurther details of the modelled flood outlines.

SCALE: 1:45,000

Flood Zone 3b1 in 20 Year Flood Outline (including defences)

Flood Zone 3a1 in 100 Year Flood Outline (including defences)Flood Zone 3a1 in 100 Year Flood Outline (undefended)Flood Zone 3a + Climate Change1 in 100 Year +CC Flood Outline (including defences)Flood Zone 3a + Climate Change1 in 100 Year +CC Flood Outline (undefended)Flood Zone 21 in 1000 Year Flood Outline (including defences)Flood Zone 21 in 1000 Year Flood Outline (undefended)


D127313 September 2011 D

Annex D – Application Site Topographic Survey



5 Baseline Fluvial Flood Hydraulic Modelling

This Chapter identifies the baseline fluvial flooding conditions experienced on the existing Application Site identified through hydraulic modelling.

5.1 Background

As part of this Level 3 FRA, fluvial flood modelling was undertaken to assess the flood regime of the Washpit Brook and its tributaries that flow through the Application Site. The rationale behind this was to provide an indication of how the watercourses represent a constraint to development opportunities at the Application Site. This information was then used to inform the parameter plans.

To inform the understanding of fluvial flood risk at the Application Site a 1-dimensional (1D) hydrodynamic hydraulic model was developed using the ISIS (version 3.4) computational modelling package.

The ISIS hydraulic modelling has been undertaken in the following five stages, which are described within the following sections of the report:

1. Hydrology (Section 5.2);

2. Model construction (Section 5.3);

3. Baseline model conditions (Section 5.4);

4. Review of the baseline results (Section 5.5 and 5.6); and

5. Sensitivity analysis (Section 5.7).

5.2 Hydrology

Hydrological estimates are required for inclusion as upstream boundary conditions in the hydraulic modelling study being undertaken for the Washpit Brook.

The Application Site is located in the headwaters of the Washpit Brook catchment. The hydrology across the Application Site is heavily influenced by existing transport infrastructure (i.e. M11, A14, A428 and A1307), which includes a number of culverts that control the location of discharge from offsite sources into the reach of the Washpit Brook within the site. These are previously discussed in Section 3.4.

To ensure an accurate representation of the hydrological regime, the Washpit Brook catchment has been split into a number of sub-catchments based upon the sources of flow evident on the site. A flow has been calculated for each sub-catchment and has been entered into the hydraulic model at the appropriate location, as point or lateral inflows.

The hydrological assessment has been undertaken for the total catchment using both the Revitalised Flood Hydrograph (ReFH) Method and WINFAP-FEH 3 (2009) Statistical Method to allow comparison of flow estimates.



A total of nine sub-catchments are presented. Catchments 2, 3 and 4 discharge flow through a culvert from beneath the M11, as described in Section 3.4. Catchment 6 also discharges flow through a culvert and into the Application Site. However, the catchment size is minor and the flow contribution is insignificant.

Two further culverts were identified on site, which represent point sources of flow into the Application Site. The first is located beneath Madingley Road and the second from near Lansdowne Road. A surface water sewer discharges into the Application Site at each of these locations, as identified in Section 3.7. However, the catchment areas associated with these two sources are insignificant and accounted for within the wider sub-catchment, as discussed in Section 3.4 and illustrated in Annex F.

5.2.1 Catchment Analysis

The catchment descriptors, central to the flow estimation process, have been extracted from the FEH CD-ROM v3 (CEH, 2009). The catchment descriptors are based on the downstream boundary of the model (National Grid Reference: TL418620). The total catchment area at this point is 7.13 km2. A full list of the catchment descriptors used within the flow estimation process is shown in Table 5-1.

Table 5-1: Catchment Descriptors Downstream Model Boundary (NGR: TL418620) AREA (km2) 7.13 RMED-2D 35.1 D2 0.26031

ALTBAR 23 SAAR 552 D3 0.291

ASPBAR 17 SAAR4170 554 E 0.31712

ASPVAR 0.48 SPRHOST 55.25 F 2.44283

BFIHOST 0.321 URBCONC1990 0.448 C(1 km) -0.026

DPLBAR 2.35 URBEXT1990 0.0181 D1(1 km) 0.32

DPSBAR 16.7 URBLOC1990 0.976 D2(1 km) 0.242

FARL 1 URBCONC2000 0.586 D3(1 km) 0.28

LDP 4.84 URBEXT2000 0.0321 E(1 km) 0.317

PROPWET 0.26 URBLOC2000 0.799 F(1 km) 2.446

RMED-1H 10.8 C -0.02615 - -

RMED-1D 28.6 D1 0.3141 - -

The catchment boundary was also extracted from the FEH CD-ROM v3 and overlain with a 1:10,000 scale Ordnance Survey (OS) map using the Geographical Information System (GIS) software MapInfo. A catchment review was undertaken to confirm the extent of the total catchment area boundary and to identify the extent of each sub-catchment associated with each point source of flow. The review was based on the following resources:

• Observations made during the site walkover;

• Contour lines and ditch positions shown on OS map;

• Topographic survey; and

• Aerial photography.



This review identified some minor alterations to the total catchment area, which are identified in Annex F. This resulted in an increase in the catchment area from 7.13km2 to 7.16km2.

5.2.2 Flow Estimation

The hydrological assessment was undertaken using both the ReFH Method and the FEH Statistical Method to allow a flow estimate comparison. These methods have been chosen due to the catchment being ungauged and no appropriate donor sites (in adjacent catchments) with similar hydrological characteristics. The original downstream model catchment area (i.e. 7.13km2) has been used within each flow estimation method. These flows have then been proportionally increased, based on the revised catchment area (7.16km2). Flow for each individual sub-catchment area, as shown in Annex F, has been calculated on a proportion basis using the ratio between sub-catchment and total catchment size.

Flows associated with the M11 balancing ponds and public sewers that discharge into the Washpit Brook through the Application Site have not been directly considered in the hydrological flow analysis. However, urban extent is represented by catchment descriptors (i.e. Table 5-1), which is used within the flow estimation. Therefore, it is anticipated that the urban influence has been adequately considered in the flow estimation.

5.2.3 FEH Statistical Method

An initial pooling group was selected and reviewed to remove unsuitable stations. The subsequent revised pooling group was used to estimate a growth curve using the generalised logistic distribution. The revised pooling group is shown below in Table 5-2.

Table 5-2: WINFAP-FEH 3 Revised Pooling Group Stations

Station Distance Years of

data QMED AM L-CV L-SKEW Discord-

ancy

27073 (Brompton Beck @ Snainton Ings) 0.540 28 0.739 0.210 0.017 0.439

33045 (Wittle @ Quidenham) 2.005 40 1.081 0.345 0.178 0.571

29009 (Ancholme @ Toft Newton) 2.019 34 1.919 0.390 0.367 1.731

20002 (West Peffer Burn @ Luffness) 2.181 41 3.299 0.292 0.015 1.609

36009 (Brett @ Cockfield) 2.242 38 3.661 0.263 -0.103 1.573

26802 (Gypsey Race @ Kirby Grindalythe)

2.596 9 0.142 0.236 0.134 0.629

27051 (Crimple @ Burn Bridge) 2.603 36 4.610 0.219 0.122 1.106

44009 (Wey @ Broadwey) 2.608 31 1.679 0.345 0.259 0.346

203046 (Rathmore Burn @ Rathmore Bridge)

2.698 26 10.996 0.126 0.115 1.753

25019 (Leven @ Easby) 2.722 30 5.538 0.361 0.411 0.987

44006 (Sydling Water @ Sydling st Nicholas)

2.823 34 0.861 0.231 0.087 0.141

36010 (Bumpstead Brook @ Broad Green)

2.833 41 6.759 0.433 0.239 1.777

45816 (Haddeo @ Upton) 2.898 14 3.427 0.318 0.449 1.424

72014 (Conder @ Galgate) 2.973 41 16.957 0.189 0.054 1.302

Documents

Geological Unit Description Aquifer Status · PDF fileGeological Unit Description Aquifer Status ... The ReFH Method has been run using the winter ... WSH-1255 1.3m diameter circular