Detailed Design Process

Proposed Redevelopment Of Athlone Power Station:

Stormwater Network: Detailed Design Process

Kosikee Emma-Iwuoha

Prepared for:

Cape Town Civic Centre

Buitengracht St, Cape Town, 8001

South Africa

29th August, 2011

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Table of Contents

Table of contents i

List of Figures iii

List of Tables iv

1. Introduction 1-1

2. General considerations 2-1

2.1 Slope of Site 2-1

2.2 Outflow options 2-1

2.3 Railway 2-1

2.4 Minimal Roadspace 2-1

2.5 SuDS 2-1

2.6 Notable land-uses 2-1

3. Minor design 3-1

3.1 Design considerations 3-1

3.1.1 Important requirements 3-1

3.1.2 Other considerations 3-1

3.1.3 Design Assumptions: 3-2

3.2 Layout Choice 3-2

3.2.1 Layout one 3-2

3.2.2 Layout two 3-2

3.2.3 Layout three 3-2

3.2.4 Motivation for choice 3-2

3.3 Design challenges 3-3

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3.4 Solutions 3-3

3.5 Network Design Features 3-3

3.6 Pipe Design 3-3

4. SuDS design 4-1

4.1 Requirements 4-1

4.2 Assumptions 4-1

4.3 Design Process 4-1

4.3.1 Swale Design 4-1

4.3.2 Swale vegetation 4-2

4.3.3 Retention pond Design 4-2

4.3.4 Retention pond vegetation 4-3

5. Major design 5-1

5.1 Assumptions: 5-1

5.2 Design Challenges: 5-1

5.3 Solutions: 5-1

5.4 Procedure: 5-1

6. Recommendations 6-1

6.1 Revise conceptual design 6-1

6.2 Extend use of SuDS 6-1

6.3 Determine quantities for SuDS 6-1

7. References 7-1

iii

List of Figures

Figure 1-1 Standard design of a vegetated swale 4-2

Figure 1-2 Standard design of a detention pond 4-3

iv

List of Tables

Table 3-1 Desirable and absolute minimum slopes for stormwater pipes 3-1

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1. Introduction

This report details the design of the stormwater network. All considerations and assumptions

which significantly influenced the design are mentioned. Challenges encountered, and the

solutions applied are discussed in this report.

Emma-Iwuoha: Proposed Redevelopment of Athlone Power Station

Chapter 1: Introduction

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2. General considerations

2.1 Slope of SiteGiven the expected development of the site it is expected that the site will end up being mostly flat, thus the sloping of the site need not be considered. However in order to design for a flat site some assumptions will have to be made about the level at which the different land uses will be filled to. It is important to remember that the sight varies from 10m to 4 m. this will deter -mine what level different areas on site will be filled to eventually.

2.2 Outflow optionsBecause the existing storm water network passes very close to the site, and also due to the presence of existing outflow infrastructure on the site there are various options for locating outflow(s). Thus the locations chosen will probably be determined by other factors. It will be important to choose the most efficient out flow option(s).

2.3 RailwayThe active railway running through the site may create issues. Because of the constraints surrounding railways (as mentioned in section 3.3) it will be difficult to plan the network in the area of the sight cut off by the railway. All possible options will have to be considered for network design and outlet placement.

2.4 Minimal RoadspaceThe road layout from the conceptual design is limiting in that there is very little roadspace. Thus options such as green open space and non-conventional drainage applications will have to be explored to ensure that sufficient drainage infrastructure is provided.

2.5 SuDSThere is an increasing move towards the use of sustainable services. City of Cape Town regulations, specifically prescribe the use of SuDS to restore quality of runoff to pre-development levels. Beyond meeting requirements, SuDS can be used very successfully to store and attenuate flood runoff. SuDS should be used as extensively as constraints allow.


Chapter 4: Considerations

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2.6 Notable land-usesAreas on the site are designated for land uses which carry the potential for high levels of runoff contamination (e.g. ARTS and Light Industrial), as well as land-use types which typically resulting a high percentage of hard surfaces, thus decreasing runoff infiltration at the source. These issues will have to be mitigated by the design


Chapter 4: Considerations

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3. Minor design

3.1 Design considerations

3.1.1 Important requirements

The minimum allowable manhole depth is 1.4m, and the maximum desirable depth is 4m. Each pipe diameter has a corresponding minimum slope at which it can be laid. This is shown in table 3-2 below (CSIR, 2000):

Table 3-3: Desirable and absolute minimum slopes for stormwater pipes

Pipe Diameter (mm)Desirable Minimum

GradientAbsolute Minimum

Gradient

300 80 230

375 110 300

450 140 400

525 170 500

600 200 600

675 240 700

750 280 800

825 320 900

900 350 1000

1050 440 1250

1200 520 1500

The minimum desirable velocity in pipes is 0.9m/s – 1.5m/s. Also, the minimum allowable pipe diameter is 300mm.


Chapter 2: Minor Network

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3.1.2 Other considerations

The network is a gravity system, but the site is very flat so absolute minimum slopes will be probably be necessary.

3.1.3 Design Assumptions:

Different land-use areas were assumed to have been to be filled to the highest level present. Except light industrial, which e=was filled to the lower level present. The difference in levels was only2m; therefore it was not thought to be a big concern.

3.2 Layout ChoiceBefore a design was finalized three layouts were developed. They are compared below. Layout three is chosen at the end of the comparison. All three layouts can be found in Appendices C-E.

3.2.1 Layout one

Based on assumption that ground levels are not changed by construction (not likely).

Relies heavily on on-site controls e.g. filter drains and permeable pavements.

Does not make use of all roads for catchpit placement.

Uses a swale in green open space.

A large number of pipe sections running under buildings.

Drains into existing infrastructure, but not river or wetland

3.2.2 Layout two

Assumes site has been filled and is now flat.

Makes efficient use of roads for catchment placement.

Uses two Bioretention ponds to attenuate flows and treat runoff.

No attenuating or treatment with SuDS for runoff from the majority of the site.

New outlet pipe must be made, will require digging under highway.



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Pipe ends at swale and another pipe begins after swale

Pipes cross railway

3.2.3 Layout three

Assumes site has been filled and is now flat.

Same as 2 except no bioretention pond, instead 2 detention ponds added to accommodate runoff from the residential area and another for the whole sight.

Industrial and waste depot runoff treated before reaching wetland.

Makes efficient use of roads for catchpit placement.

Pipes from the residential area ends at a swale (for treatment of the water) and another pipe begins after the swale.

New outlet pipes must be made, which will require digging under highways at two points.

3.2.4 Motivation for choice

2 and 3 have more rational pipe placement, and easier maintained than 1.

2 and 3 also make more extensive and efficient use of SuDS (specifically retention ponds) to attenuate flows.

2 and 3 have an outlet into a wetland for further treating and localized disposal of flows.

Layout 3 is most desirable because of added treatment/attenuation of industrial and ARTS depot runoff. Also, it doesn’t require a pipe to pass below the railway.

3.3 Design challenges The pipe from the commercial area would possibly have to pass below the railway. Pipe section in retail area is very long, thus may end up going deeper than 4m. Similarly, pipes at the outflow to the wetland may come in too low and go under wetland,

and pipes ending at the swale could pass beneath it.



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3.4 Solutions Instead of going under railway, have an outlet at the end of the commercial pipe section go-

ing across Jan Smuts Drive to connect to existing stormwater network. Placing a retention pond in the swale such that pipe sections may meet at depth at inflow.

The outflow is level with swale bottom thereby recovering height as the pipe section from the end of the swale can begin 1.4m below the base of the swale.

Placing a retention pond before the outflow to wetland. This means the inflow pipe can go in at depth and the outflow comes out at 1.4m below ground level. Then slope the outflow pipe such that its end comes out level with the wetland.

3.5 Network Design Features The use of swales and retention ponds for attenuation and treatment of residential flows. Retention ponds used to treat and further attenuate flows from the majority of site (except

the commercial area). Note that flows from ARTS and Light Industrial areas have not been treated and/or attenuated before this point.

All runoff except that from commercial goes into the wetland for bioprocessing and then to the river. But this is not of great concern as runoff from the area is likely to contain low to moderate contamination, though that cannot be stated with certainty.

3.6 Pipe DesignPipe sizing, gradients and levels were calculating using a model developed in Microsoft Excel. This model utilized hydraulic slope and velocity equations.

Certain assumptions were made about ground levels and the interaction of the conven-tional pipe system with the SuDS applications. The model also checked that pipe velocities were above the required minimum range.

Output from the model detailing ground levels, crown levels, cover levels, and invert levels can be found in Appendix C.



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4. SuDS design

4.1 Requirements For improved runoff quality: design for ½ year return peak flow. For stability of downstream channels - 24 hour extended detention of the1-year return

flood, 24 hour storm event. For fairly frequent floods: Up to 10-year return flood peak flow reduced to pre-develop-

ment levels. Extreme Flood Events: Up to 50-year return peak flow reduced to existing development

levels (City of Cape Town, 2009).

4.2 Assumptions 10 year flood event design requirements are less than 1 year, 24 hour flood requirements.

Thus quality design results in specifications which are sufficient for the 10 year event. Swales simply channel flow (not designed for storage of large volumes) therefore not re-

quired to store for 1 year 24hour design. Instead storage requirements are met by retention ponds (which are designed for storage).

4.3 Design Process Determine Q for 1 year according to rational method. C is a weighted average of c for all

the land-use types, weighted by area of land use. MAP determined by consulting Figure 2.8 in Flood Hydrology (Haarof and Cassa).

Draw simplified hydrographs. Extended for 24 hour flood event for retention ponds, but use a normal hydrograph for swales (hydrographs can be found in Appendix H).

Area of the hydrograph gives the volume of runoff to be stored. Design dimensions of SuDS accordingly by consulting design guidelines. SuDS also form part of the major, therefore check with rest of major system for adequacy

in a 50 year return storm situation, and adjust dimensions if necessary.

4.3.1 Swale Design

Swales are used to channel all residential and part of industrial and cultural runoff The swales are standard vegetated swales, as shown in figure 4-1 below:

Armitage & Carden: General format for reports and dissertations

Chapter 3: SuDS Network

Figure 4-3: Standard design of a vegetated swale


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There are three swales: s1, s2, s3. S1 is swale from north-east to south west through resid-ential area. S2 is swale directly below s1. S3 is swale section after s1 and s2 meet.

Swales: s1=234.36m3, s2=346.68m3, s3=2624.4m3. Manning is then used to determine design dimensions of swales. Chose S1 and 2: base width = 0.5m and S3 base width = 1m The final design specifications can be found in Appendix H

4.3.2 Swale vegetation

The City of Cape Town prescribes grassy vegetation which can withstand mostly dry condi-tions (City of Cape Town, 2006). Ficinia lateralis (sedge) and Ficinia nigrenscens (sedge) were chosen vegetation from the avail-able indigenous plant types prescribed. Both are Indigenous, do not impede flow, and are suit-able to mostly dry conditions. Nigrenscens does not have a soil preference. Both are commer-cially availa

4.3.3 Retention pond Design

The retention pond is to follow the general design as shown below:




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Two retention ponds were designed. R1 is located at the end of the swale. R2 is located just before the outflow to the wetland

Retention pond specifications can be found in Appendix H.

4.3.4 Retention pond vegetation

The vegetation recommended is shrub-types which can withstand mostly dry conditions. The chosen vegetation is ficinia bulbosa (sedge) and muraltia mitior. Neither impedes flow. Bulbosa has good growth height and is commercially available. Mitior is not commercially available, but is only shrub which can withstand mostly dry conditions and grows to 1m tall thus providing good cover if needed.



Figure 4-6: Standard design of a detention pond

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5. Major design

5.1 Assumptions: Standard dimensions for local roads: all roads 7.4m wide (two 3.7m wide lanes), and

110mm deep. Assume the critical case where retention ponds fill quickly, thus assume their capacity is

negligible in this situation. Runoff coefficient C = 1, as it is assumed the ground is saturated so all precipitation will

run off. Assume all pipes are blocked and so roads and swales act as river channels. Assume when a catchment drains into more than one swale of different lengths and areas,

they can be modelled as one equivalent swale of an area and length equal to the sum of areas and lengths of the individual swales.

5.2 Design Challenges: Runoff from commercial area is trapped between the highway and railway which it is not

to pass over or along. Swales are sending runoff in one direction and roads are sending runoff in another – prob-

lem for routing. Also keeps runoff on site for longer period – undesirable.

5.3 Solutions: Design for storm drain passing under the railway. Route runoff from swale to roads to get it going in the desired direction. Use the first retention pond to store swale flows. It is already assumed it not part of major

network, so roads and swales can move flows without it. Thus it is free for use as tempor-ary storage.

5.4 Procedure: Route flows conceptually through road and open space network, more specifically through

swales, since it is assumed that the retention ponds have negligible capacity. Selected a return period of 50 years. The Rational Method is used to calculate the discharge. Catchment areas are identified and measured in ArcGIS. The longest ‘watercourse’ (road or swale) is identified and its length is measured in Arc-

GIS. The channel profile is obtained using ArcGIS and its average slope is calculated using the


Chapter 4: Major Network

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10-85 method. The time of concentration was obtained using the equation 5.1:

(5.1)

Tc was then used to obtain the precipitation intensity i from table the discharges from each catchment were calculated using the Rational Method Using the calculated discharges, Manning’s equation was used to check if the roads and

swales were sufficient to carry runoff from catchments. The results are shown in Appendix I.

The design was found to be adequate for 50yr return flood.


Chapter 4: Major Network

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6. Recommendations

6.1 Revise conceptual designMuch of the design challenges encountered were a direct result of the chosen land-use layout, as set out in the conceptual design. Therefore it is recommended that the conceptual design be reworked, especially the road layout.

6.2 Extend use of SuDSFurther SuDS should be implemented on the site. Source controls, specifically permeable paving is recommended, especially since most of the land-use types will result in a lot of hard ground surfaces or large roof areas.

6.3 Determine quantities for SuDSIt must be recommended that specialists be contacted for determining the requisite quantities for use in costing.

The simple vegetated swale requires mostly earthworks and landscaping. The same applies to retention ponds, but some specialist construction may be required to ensure proper function and procurement of materials esp. clay/plastic liners and retaining structures. Thus determining quantities for costing of retention ponds requires a higher level of expertise and experience


Chapter 7: Recommendations

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Armitage & Carden: General Report format

Chapter 4: Error: Reference source not found

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7. ReferencesCity of Cape Town (2009). Management of Urban Stormwater Impacts Policy. Cape Town:

City of Cape Town.

CSIR (2000). Guidelines for Development Planning and Design. Pretoria: CSIR.

Armitage & Carden: General Report format

Chapter 4: Error: Reference source not found

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Detailed Design Process