3 Waters Modelling Project - Stormwater - · PDF fileDocument 3 Waters Modelling Project - Stormwater Date 29 October 2012 Prepared by Mike Summerhays Reviewed by Craig Davidson Revision

Hamilton City Council 29 October 2012

3 Waters Modelling Project - Stormwater

AECOM 3 Waters Modelling Project - Stormwater

29 October 2012

3 Waters Modelling Project - Stormwater

Prepared for

Hamilton City Council

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29 October 2012

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29 October 2012

Quality Information Document 3 Waters Modelling Project - Stormwater

Date 29 October 2012

Prepared by Mike Summerhays

Reviewed by Craig Davidson

Revision History

Revision Revision Date Details Authorised

Name/Position Signature

29 Oct 2012 Final Craig Davidson Regional Manager


29 October 2012

Table of Contents Executive Summary i 1.0 Introduction 1

1.1 Background 1 1.2 Purpose of this report 1

2.0 Stormwater Modelling Overview 1 3.0 Flood Hazard Design Inputs 2

3.1 Industry Practice Review 2 3.2 Flood Hazard Design Inputs 2

3.2.1 General 2 3.2.2 Climate Change 2 3.2.3 Design Storm 3 3.2.4 Flooding Threshold Depths 3 3.2.5 Flooding Hazard Matrix 3 3.2.6 Summary of Design Flood Hazard Inputs 4

4.0 Flood Hazard Assessments 4 4.1 General 4 4.2 Rapid Flood Assessment (RFA) 4 4.3 Detailed Flood Hazard Modelling 5 4.4 Catchment Management Plan modelling 5 4.5 Comparison of RFA and Detailed Flood Hazard Modelling Outputs 5 4.6 Waikato River Flood Hazard Modelling 6 4.7 General 6

4.7.1 Option 1 – Rapid Flood Assessment 6 4.7.2 Option 2 – Current Detailed Modelling Information 7 4.7.3 Option 3 – Extend the Detailed Modelling to cover the city 7 4.7.4 Option 4 – Complete CMP programme 7 4.7.5 Preferred Option 7

4.8 District Plan Flood Hazard Maps 7 4.8.1 Cleansing of data 7 4.8.2 Smoothing 8

5.0 Recommendations for District Plan Flood Hazard Maps 8 6.0 Limitation 8 Appendix A

Flood Hazard Matrices A Appendix B

Flood Hazard Classifications B Appendix C

RFA Assessment Areas C Appendix D

Detailed Modelling Areas D Appendix E

Comparison of RFA and Intermediate Assessment Outputs E


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Executive Summary Hamilton City Council (HCC) has a programme to develop computer models to represent the behaviour and performance of its wastewater, water supply and stormwater networks. The programme was divided into three phases and commenced in 2010, with Phase 1 substantially completed in June 2012.

The long term goal for HCC is to complete Catchment Management Plans (CMPs) across the entire city. HCC have allocated $3.4M over the next 10 years to assist in completion of these CMPs.

HCC are in the process of reviewing the District Plan and propose to include Flood Hazard Mapping in the Proposed District Plan. The purpose of the Flood Hazard Mapping is to identify areas that may be subject to flooding in a 100yr event, and to manage subdivision, use and development in light of the perceived risk. HCC propose to use the results of the stormwater modelling work undertaken in Phase 1 to inform the Proposed District Plan.

The first step in Phase 1 of the modelling programme was to undertake a Rapid Flood Assessment (RFA) across the entire city. The purpose of the RFA phase was to identify and prioritise areas for more detailed modelling assessment. Detailed modelling has currently been completed for approximately 11% of the city.

The RFA outputs are not considered sufficiently accurate to identify flood hazard areas in the Proposed District Plan. The outputs of the detailed modelling however are considered suitable for this process.

It is recommended that the detailed modelling information be augmented with historical HCC flooding information.

The Waikato Regional Council flood hazard information for the Waikato River has been utilised for the Proposed District Plan.

It is also recommended that the District Plan be regularly updated as CMP or if further detailed modelling is completed.


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1.0 Introduction 1.1 Background Hamilton City Council (HCC) has a five year strategy to develop computer models to represent the behaviour and performance of its wastewater, water supply and stormwater networks. AECOM was engaged by HCC in 2010 to help deliver the HCC 3 Waters Modelling programme which is a key component of the five year strategy. The programme was split into the following three phases:

Phase 1

- Develop trunk models for the water supply and wastewater systems

- High level identification of flooding issues followed by more detailed assessments in key flooding areas

- Support stormwater catchment management planning (e.g. Peacocke Stage 1).

This phase was substantially completed in June 2012.

Phase 2

- Develop detailed models for the water supply and wastewater systems

- Support stormwater catchment management planning

Phase 2 is currently underway and is scheduled to be completed by June 2014 at the latest.

Phase 3

- Development of a web based portal for operating the water supply, stormwater and wastewater models.

1.2 Purpose of this report HCC are in the process of reviewing their District Plan and propose to include a Flood Hazard Mapping in the Proposed District Plan. The purpose of the Flood Hazard Mapping is to identify areas that may be subject to flooding in a 100yr event, and to manage subdivision, use and development in light of the perceived risk. HCC propose to use the results of the stormwater modelling work undertaken in Phase 1 to inform the District Plan Flood hazard map.

The purpose of this report is to provide:

- An overview of the stormwater modelling work undertaken in Phase 1,

- Engineering recommendations for development of the Proposed District Plan Flood Hazard Mapping.

2.0 Stormwater Modelling Overview A three phase approach was adopted for the stormwater modelling programme with each phase building on the work of the previous phase. The phases adopted were:

Flood Hazard Scoping

Detailed Flood Hazard Modelling

Catchment Management Planning.

A Rapid Flood Assessment (RFA) was undertaken in the Flood Hazard Scoping. The purpose of the RFA assessment was to provide a high level understanding of areas that may flood in a significant storm event to enable prioritisation of areas for Detailed Flood Hazard Modelling. The RFA assessment did not include the primary stormwater network in the model (apart from road culverts) and assumed that all ground acts like an impervious surface i.e. all rainfall becomes surface runoff.


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The information from the RFA was used to prioritise areas of the city for Detailed Flood Hazard modelling based on the number of properties affected and the cost for completing the modelling. Detailed Flood Hazard modelling was undertaken over approximately 11% of the total Hamilton City area.

The final phase of the stormwater modelling journey is the development of Catchment Management Plans (CMPs) for the entire city. HCC have allocated around $3.4M over the next 10 years in the Long Term Plan 2012 - 2022 (LTP) programme for completion of the CMPs.

While development of the CMPs is outside the scope of the HCC 3 Waters Modelling programme, it is intended that the stormwater model will be used to support the development of the CMPs.

3.0 Flood Hazard Design Inputs

3.1 Industry Practice Review There is currently no national guidance on what criteria and methodology should be adopted for citywide stormwater hazard mapping for extreme events. A review of what other councils have done in the area of stormwater hazard mapping was therefore undertaken as part of this assessment to assist HCC in their decision making process. The following councils were included in the review:

- Tauranga City Council (TCC)

- Auckland Council (AC)

- Dunedin City Council (DCC)

- Christchurch City Council (CCC)

The results of the review are presented in Table 1 below. The information indicates that all four Councils have adopted broadly similar criteria for their flood hazard assessments. Table 1 Flood Hazard Assessment criteria for Extreme Storm Events

Assessment Criteria TCC AC DCC CCC

Climate change temperature increase (refer Section 3.2.2) Yes Yes Yes Yes

Extreme Event Design Storm (refer Section 3.2.3) 100 year 100 year 100 year 50 year

Flooding threshold depth level (refer Section 3.2.4) 100mm 100mm1 50mm 400mm Notes:

1. The Auckland Council flooding threshold depth criteria are applied differently to the others. They still use a depth of 100mm to identify a risk but for ponded areas greater than 300mm depth, the water surface extent is mapped to the edge of the ponded area. This means that properties can be identified as being prone to flooding with a ponding depth less than 50mm where the associated flooding exceeds 300mm elsewhere.

The Proposed Waikato Regional Policy Statement requires an extreme event deign storm of 100years return period.

3.2 Flood Hazard Design Inputs 3.2.1 General

As noted above, there is currently no national guidance on the design criteria for establishing Flood Hazard Maps for inclusion in the District Plan. The Flood Hazard criteria outlined in the following sections are based on a review of industry practice augmented with our experience in stormwater management and Flood Hazard mapping.

3.2.2 Climate Change

Inclusion of climate change forecasts in the analyses increases the likelihood and magnitude of the predicted flooding. Climate change guidance in the HCC Development Manual is based on a report prepared by NIWA (NIWA Client Report WLG2008-010). The report recommends the use of a median temperature increase of 2.08 degrees Celsius from 1990 to 2090.


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A review of industry practice has highlighted that most Councils have adopted the climate change recommendations in the NIWA report in their flood hazard assessment. The NIWA recommendations have been adopted for the HCC flood hazard assessment.

3.2.3 Design Storm

The Building Act requires the 50 year design storm as a minimum. The difference in terms of flood levels between the 50 year and 100 year design storms is typically minor and therefore Councils typically adopt the 100 year design storm to assess flooding risks as recommended by NZS 4404:2010. This approach provides some degree of future proofing for system upgrade works aimed at flood alleviation where a 100 year level of service is required for floor flood protection.

The flood analyses were based on a long duration storm event that contained rainfall information from shorter duration storms within it (known as a nested storm). This approach provides the critical storm parameters (intensity and duration) that yield the worst case rainfall scenario in all parts of the catchment.

A six-hour duration, 100 year nested storm was adopted as the design storm for flood hazard assessment.

3.2.4 Flooding Threshold Depths

The flooding threshold depth level can have a significant impact on the number of properties determined to be affected by flooding i.e. for a lower threshold depth, a greater number of properties are likely to be classified as being at risk from flooding.

The sensitivity of changing the flood threshold depth for the Detailed Flood Hazard modelling areas is presented in Table 2 below. The table includes the number of properties where flood coverage is greater than 10% of the property area. Table 2 Impact of Flood Threshold depth on Properties Flooding

Flood Threshold Depth Number of Properties Predicted to be affected by flooding

100mm 5,953

150mm 5,506

200mm 4,956

300mm 3,843

A flood threshold depth of 100mm was adopted for the flood hazard assessment. This was considered to be a practical depth whereby flooding below this depth was unlikely to cause significant damage to residential dwellings as the Building Code requires a minimum of 150mm between floor levels and adjacent outside surfaces.

We note that the proposed Waikato Regional Policy Statement has the effect of creating a minimum ‘flooding threshold depth level’ of 0mm (refer Appendix A). This was considered too low for practical assessment purposes. The proposed flood threshold depth of 100mm was discussed with Waikato Regional Council staff prior to implementation.

3.2.5 Flooding Hazard Matrix

In addition to identifying the flooding threshold level discussed above, the Waikato Regional Council (WRC) and other councils have adopted a hazard matrix for quantifying flooding risks. The hazard matrix defines the level of risk i.e. low, medium or high in terms of flood depth and velocity. The various flood hazard matrices adopted by WRC and a number of other councils are presented in Appendix A.

The adopted flood hazard matrix is presented below as Figure 1. The rationale for the selection and definition of various flood hazard classifications are presented in Appendix B.

It is noted that the flood hazard matrix is similar to that adopted by the WRC. The key difference between the WRC matrix and the adopted flood hazard matrix is the inclusion of an additional category in the adopted flood hazard matrix for floods between 0.1m and 0.5m depth with velocities less than 1m/s. The additional risk category was included in recognition that the step between 0m and 1m flood depth in the WRC matrix was too coarse for practical urban flood management purposes within the Proposed District Plan.


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Figure 1: HCC Flood Hazard Matrix

3.2.6 Summary of Design Flood Hazard Inputs

The following summarises the flood hazard inputs adopted for the flood hazard assessment:

NIWA climate change recommendations

100 year nested storm event

Six hour storm duration

Flood threshold depth of 100mm

HCC Flood Hazard Matrix (Figure 1).

4.0 Flood Hazard Assessments

4.1 General As noted earlier, a phased approach was adopted for the stormwater modelling programme with each phase building on the work of the previous phase. The following sections outline the scope of work undertaken for each phase of the assessment.

4.2 Rapid Flood Assessment (RFA) The RFA was undertaken using HCC LiDAR survey data to develop a three dimensional ground surface profile. Generally LiDAR data is understood to have an accuracy level of +/- 250mm.

The RFA was completed across the entire city using a 5m x 5m grid. The grid models the ground surface as a single level by averaging the surveyed ground levels within the grid i.e. the smaller the grid spacing, the more accurate the model surface. A second RFA assessment using a smaller 2m x 2m grid was undertaken on part of the Rototuna and Kirikiriroa catchments to assess the sensitivity of grid size on the model outputs.

The purpose of the RFA was to provide a high level understanding of areas that may flood in a significant storm event in order to enable prioritisation of areas for Detailed Flood Hazard Modelling. The RFA assessment did not include the primary stormwater i.e. pipes, watercourses, culverts, drains, ponds and catchpits network in the model and assumed that the ground acts like an impervious surface i.e. all rainfall becomes surface runoff.


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For the 5m x 5m grid spacing RFA, road culverts greater than 900mm in diameter were included in the model to more accurately reflect surface water flows. These pipes were included in the model as the likelihood of pipes of this size blocking in extreme events (100yr + climate change) was considered lower than for smaller culverts (>900mm).

For the 2m x 2m grid spacing RFA, road culverts less than 900mm diameter were also added to the model. Inclusion of the smaller culverts more accurately reflect surface water flows from upper parts of the catchment that may have been held back in the 5m x 5m grid spacing RFA to more accurately model the catchment behaviour in a significant storm event.

The range of uncertainty in flood water level from the RFA process as a result of the combined effect of LiDAR and other inaccuracies could in some situations be in excess of 0.5 metres. The data is therefore not considered sufficiently accurate for use as a formal mechanism for assessing flood risk. HCC and other users of the data should take appropriate care in using the results taking into account these uncertainties.

A map showing areas where the RFA assessments were undertaken are presented in Appendix C.

4.3 Detailed Flood Hazard Modelling Detailed flood hazard modelling was undertaken on the following sub-catchment areas identified in the RFA as having the highest number of properties at risk of flooding within the budget:

Hamilton East

Mangakotukutuku

Waitawhiriwhiri catchments

City CBD

Callum Brae

The above sub-catchments represent approximately 11% of the total Hamilton City area. A map showing areas where the detailed flood hazard modelling was undertaken is presented in Appendix D.

The detailed flood hazard modelling adopted a 2m by 2m grid (industry standard) and included all primary drainage infrastructure in the model i.e. pipes, culverts, watercourses, drains, ponds and catchpits.

The outputs from the detailed modelling were reviewed by representatives from AECOM and HCC to confirm the validity of the data. In particular, HCC field operations staff confirmed that the model results aligned with their experience of where flooding generally occurred during large storm events.

4.4 Catchment Management Plan modelling CMP assessments are the final and most detailed level of stormwater assessment and include the work undertaken in the ‘Detailed Flood Hazard Modelling’ outlined above. CMP modelling normally covers the whole catchment being considered and incorporates the entire stormwater pipe network as well as secondary flow paths. The CMP process also considers the effects of proposed future development as well as range of rainfall events.

CMPs are currently being developed for Peacocke Stage 1 and the proposed Ruakura development. As noted previously, HCC have allocated around $3.4M over the next 10 years in their LTP to complete CMPs across the entire city.

4.5 Comparison of RFA and Detailed Flood Hazard Modelling Outputs A comparison of model outputs from the 5m by 5m grid RFA and the detailed flood hazard modelling for Callum Brae was undertaken in order to assess the accuracy of the RFA. Callum Brae was selected for the comparison as the RFA indicated extensive flooding which was considered unlikely given that it was a recently developed area, designed and constructed in accordance with the HCC Development Manual. Results of the comparison are presented in Appendix E.

The Callum Brae detailed flood hazard assessment indicates that flooding is primarily contained within road reserves which was the design philosophy adopted for secondary flow path management for the subdivision.


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While the Callum Brae example is likely to be an extreme example, it does highlight the conservatism and inaccuracies associated with the RFA modelling.

4.6 Waikato River Flood Hazard Modelling The Waikato River flood hazard modelling was undertaken by the WRC. The methodology used by the WRC to develop the Waikato River flood hazard mapping differed to that used by AECOM in the urban environment. The WRC methodology used to determine the 100yr ARI water level profile and hazard classification was based on:

Developing a river model derived using the software Mike 11 and three approaches to obtain the water level:

1. Using the water level records for various stations along the Waikato River.

2. Hydraulic investigation looking at the effects of land-use changes in the upper catchment.

3. the river flow is based on the expected 100 year outflow from the Karapiro Dam (approximately 1,100m3/s) as assessed by Mighty River Power.

WRC then spatially mapped the inundation extent using specialist software (WaterRide) and the 2007/2008 LiDAR survey. The resulting inundation information was provided to AECOM in the form of water depth raster files.

AECOM converted the raster files into 1m x 1m square polygons with depth attributes. At this stage some gaps were filled where no data existed within the extent of Waikato River.

The water depth was used for each square polygon and the hazard classified accordingly. Note that the data for the Waikato River is derived from a 1D hydraulic model and water speed information is not available. This means a full hazard analysis based on depth and velocity cannot be carried out. The Flood Hazard Matrix for the Waikato River is therefore different as it is only based on depth.

In terms of the 100yr ARI event level, it should be noted that the Waikato River catchment is large and complex (includes the Waikato Hydro Scheme). This introduces a high variability regarding how the river will respond during periods of heavy rainfall. The flood flow in the Waikato River through Hamilton City depends on the operation of the Waikato Hydro Scheme and the resulting outflow from the Karapiro Dam. The flood flow may also be the result of greater storm durations than used to develop the urban flood hazard maps.

The WRC water levels through the Hamilton City area are considered to be appropriate to use for the Proposed District Plan and flooding hazard maps.

4.7 General As noted HCC propose to include Flood Hazard Maps to identify areas that may be at risk of flooding in the Proposed District Plan. HCC propose to use stormwater hazard information obtained in Phase 1 of HCC 3 Waters Modelling programme as the basis for the Flood Hazard Mapping. It is noted however that HCC is currently part way through the proposed modelling programme and that detailed modelling information is not available for all parts of the city. This section provides options and engineering recommendations for development of the Proposed District Plan Flood Hazard Mapping.

The following four options were considered for development of the District Plan Flood Hazard Maps:

1. Use the RFA citywide results (2m and 5m grids) with some data cleansing and engineering assessment

2. Use the detailed modelling results augmented with the WRC Waikato River Hazard Modelling and historic HCC flood hazard information for parts of the city not covered by the detailed modelling work

3. Extend the detailed modelling work across the city

4. Complete the CMP work across the city

Further discussion on the four options is presented in the following sections.

4.7.1 Option 1 – Rapid Flood Assessment

The purpose of the RFA phase was to identify and prioritise areas for detailed modelling. The RFA outputs are not considered to be sufficiently accurate for the purposes of defining flood hazard areas in the Proposed District


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Plan. We therefore we do not recommend that the RFA information be used for this purpose. We note also that other Councils do not use RFA outputs to identify flood hazard areas in their District Plans.

Notwithstanding the above, a desk top assessment using data cleansing and engineering experience to refine the outputs from the RFA assessment was considered. This approach would provide a low cost method to provide preliminary citywide flood hazard information. However, this approach would be difficult to defend as the model assumptions are too simplistic. The results may predict flooding in areas where it may not occur. Conversely flooding may not be predicted in areas where it may occur.

4.7.2 Option 2 – Current Detailed Modelling Information

The outputs from the detailed modelling assessments are considered to be sufficiently accurate to be used to identify flood hazard areas in the Proposed District Plan. However, it is noted that to date, detailed modelling has only been completed for 11% of the city.

Historical HCC flood assessment maps could be utilised for areas outside of the detailed modelling assessments until such time as more detailed information becomes available. It is noted that this data has not been tested against current best practice. However, this information is currently used in the existing District Plan and therefore the risks associated with using this information remain the same.

The Waikato River Flood hazard modelling undertaken by the WRC would be used to augment the results of the detailed modelling assessments.

4.7.3 Option 3 – Extend the Detailed Modelling to cover the city

This option will provide citywide information that will be sufficiently accurate to be used to identify flood hazard risks in the District Plan. However, extending the detailed modelling assessment across the city will require a significant level of investment both in terms of time and cost. For preliminary budgeting purposes we estimate that that it would cost approximately $1.5 Million to extend the detailed modelling to cover the city and take approximately 12 months to complete.

4.7.4 Option 4 – Complete CMP programme

CMP assessments are the final and most detailed level of stormwater assessment and include the work undertaken in the Detailed Flood Hazard Modelling. HCC have allocated approximately $3.4M over the next 10 years for CMPs in their LTP programme. HCC have the option to accelerate this programme. However, acceleration of the programme may mean that HCC will have to fully fund the CMPs rather than requiring developers to complete CMPs as part of their subdivision design process e.g. the developer is currently completing a CMP for the proposed Ruakura Structure Plan.

4.7.5 Preferred Option

In consultation with HCC, Option 2 was adopted as the preferred option.

4.8 District Plan Flood Hazard Maps The outputs of the detailed flood hazard assessment were “cleansed” and “smoothed” to simplify and improve the visual appearance of the District Plan Flood Hazard Maps. Details of the “cleansing” and “smoothing” process are described below:

4.8.1 Cleansing of data

The outputs of the detailed flood hazard assessment contained areas of small isolated flood hazards which were likely to be related to localised depressions or inaccuracies in the LiDAR data (e.g. through tree cover, building coverage). These isolated areas were reviewed to confirm their accuracy and determine whether they should be included on the District Plan Flood Hazard Maps. The process adopted is summarised below.

Isolated flood hazard squares

Isolated flood hazard squares were defined as groups of flood hazard squares not touching other flood hazard squares. The following criteria were adopted to determine whether the isolated flood hazard squares were removed for the District Plan Flood Hazard Maps:

0 to 3 isolated flood hazard squares (less than 12m2 area) were automatically removed.


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4 to 6 isolated flood hazard squares (less than 24m2 area) were reviewed to assess the risk associated with the flood hazard. Isolated flood hazard squares considered to pose a low flood risk to the property were removed.

Greater than 6 isolated flood hazard squares, the hazard rating remained unchanged.

Touching flood hazard squares

‘Touching’ flood hazard squares were defined as small groups of flood hazard squares adjacent to or surrounded by a lower hazard classification i.e. a small group of high hazard squares surrounded by a medium hazard squares. It is noted that no changes were made in the case where a lower hazard group was surrounded by a higher hazard i.e. there were no increases in hazard classifications. The following criteria were adopted to determine whether the hazard classification of the ‘touching’ squares was reduced to that of the surrounding squares for the purposes of the District Plan Flood Hazard Maps:

0 to 3 ‘touching’ flood hazard squares (less than 12m2 area) were automatically reduced to the lower hazard classification of the surrounding squares.

4 to 6 ‘touching’ flood hazard squares (less than 24m2 area) were reviewed to assess the risk associated with the flood hazard from the ‘touching’ flood hazard squares. Cases where the ‘touching’ flood hazard squares were considered to pose a similar flood risk to the surrounding squares were adjusted i.e. the touching squares were reduced to the lower hazard classification of the surrounding squares.

Greater than 6 isolated hazard cells, the hazard rating remained unchanged.

4.8.2 Smoothing

After the cleansing process was completed the raster (square cell) results were smoothed to provide an improved visual output. This was applied to both the detailed flood hazard assessment areas and the Waikato River data.

5.0 Recommendations for District Plan Flood Hazard Maps Councils are required to make available flood hazard information either through the District Plan or on property LIMs if the information is considered to be reliable. The outputs of the detailed modelling completed to date are considered to be reliable and therefore we support the earlier decision made in consultation with HCC staff that the Proposed District Plan Flood Hazard Maps be based on this information i.e. Option 2 along with cleansing and smoothing of the GIS outputs.

It is noted that the detailed modelling information currently available only covers 11% of the city. We therefore recommend that this information be augmented by the historical HCC flood assessment information and the WRC flood hazard information for the Waikato River.

We recommend that the District Plan maps be updated in the future as CMPs are completed or if detailed modelling is completed on other parts of the city.

6.0 Limitation This report has been prepared for the particular project described in the brief to us and no responsibility is accepted for the use of any part of this report in other contexts or for any other purposes.

For further information regarding the modelling process and limitations the final AECOM Flood Hazard Report dated 29 October 2012 should be referred to.


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AAppendix A

Flood Hazard Matrices


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Waikato Regional Council Flood Hazard Matrix

Tauranga City Flood Hazard Matrix

00.10.20.30.40.50.60.70.80.9

11.11.21.31.41.5

0 0.5 1 2 3

Dep

th (m

)

Velocity (m/s)

Extreme Significant Hazard

00.10.20.30.40.50.60.70.80.9

11.11.21.31.41.5

0 0.5 1 2 3

Dep

th (m

)

Velocity (m/s)

High Medium


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Auckland City Flood Hazard Matrix

Dunedin City Flood Hazard Matrix


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Flood Hazard Classifications

BAppendix B


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Classification Depth Criteria Velocity Criteria Depth x Velocity Criteria

Justification

Insignificant 0-0.1m Any velocity N/A Subjective value. At this depth surface water is unlikely to be a hazard to people and unlikely to cause damage to property. NZBC E1 specifies 150mm freeboard to floors, therefore, theoretically habitable floors should be protected from significant damage. Also takes into account inaccuracies found in lidar and modelling.

Low 0.1 to 0.5m <1.0m/s N/A At this depth and velocity flood hazards are normally traversable by emergency vehicles and damage to property is minor to moderate. Humans can usually stand. Scour is unlikely to occur.

Medium 0.5 to 1.0m <2.0m/s N/A At this depth and velocity human stability in water is at risk. According to the Federal Emergency Management Agency (1979), a moderate sized person begins to lose stability in 0.91 m (3 ft) deep water flowing 0.61 m/s (2 ft/s). Damage to property can be financially significant (Auckland Council Flood Damage Analysis)

High >1.0m >2.0m/s >1 At velocities in excess of 2m/s the stability of foundations can be significantly affected by scour (NSW Flood Plain Development Manual). At depths in excess of 1m significant damage to building and risk to life and limb is very likely


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CAppendix C

RFA Assessment Areas


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DAppendix D

Detailed Modelling Areas


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EAppendix E

Comparison of RFA and Intermediate Assessment Outputs


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E-1

Rapid Flood Assessment Results

Intermediate Modelling Results

Green low hazard (0.05 to 0.1m deep)

Blue medium hazard (0.1 to 1m deep)

Red high hazard (greater than 1m deep)

Green low hazard (0.05 to 0.1m deep)

Blue medium hazard (0.1 to 1m deep)

Red high hazard (greater than 1m deep)

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3 Waters Modelling Project - Stormwater - · PDF fileDocument 3 Waters Modelling Project - Stormwater Date 29 October 2012 Prepared by Mike Summerhays Reviewed by Craig Davidson Revision