Design of Urban Stormwater Harvesting and Application System (Case Study Nairobi CBD)

ABSTRACT 

The city of Nairobi was initially engineered to keep stormwater out of sight and out of mind. Unfortunately, that approach has turned a potential resource into a waste product and created new environmental hazards. Once the rain falls onto the impervious surfaces of the streets, sidewalk, parking lots and rooftops, it picks up any and all pollutants that are  in  its  path,  gathering  volume  and  speed  into  the  storm  drain  where  it  is  piped underground into a stream. Although this is happening, the city has a scarcity of water.  This project studies the issue of stormwater management in the main business part of the Nairobi Central Business District and will develop a set of recommendations on how best to harness this wasted resource. Before providing the technical options, the first design phase  will  begin  by  determining  the  runoff  potential  of  the  study  area  using  GIS techniques.  This  will  be  done  by  capturing  the  potential  catchment  surfaces  and subsequently calculating the amount of runoff that can be harvested from these surfaces. From the results so acquired, the conveyance channels for the stormwater collection and the storage systems will be design. Screens will be installed for pollutant removal. This project is the starting point for delving into a future of our cities and towns that has been often ignored, but is now recognized for its capacity to be both a major source of pollution or a major resource

Table of Contents 

1  INTRODUCTION 

1.1  BACKGROUND INFORMATION 

1.2  PROBLEM STATEMENT 

1.3  JUSTIFICATION 

1.4  SITE ANALYSIS AND INVENTORY 

1.4.1  Geographical location 

1.4.2  Climate 

1.4.3 SOILS

1.5  OBJECTIVES 

1.5.1  Overall objective 

1.5.2  Specific objectives 

1.6  STATEMENT OF SCOPE 

2  LITERATURE REVIEW 

2.1  The Current water crisis in urban areas 

2.2  Introduction of Urban Stormwater Harvesting and Application System Design 

2.2.1  Natural Hydrology System 

2.2.2  Urbanization 

2.3  Harvesting stormwater for reuse 

2.3.1  Elements of Stormwater harvesting and application system 

2.3.2  Potential applications of harvested stormwater 

2.4  Stormwater Management Polices and Institutional Arrangements 

2.4.1  RWH legislation in other Countries

2.5  Current Existing Urban Stormwater Harvesting and Application System Design Cases Study  

2.5.1  The Water System in Berlin’s Potsdamer Platz 

2.5.2  Summary 

2.6  Background on Water Sector Reforms in Kenya 

2.6.1  The Water Act 2002  

3  THEORETICAL CONSIDERATIONS 

3.1  Determination of the effective impervious area of the selected catchment area. ....... 27 3.2  Determination of Rainfall reliability 

3.3  Estimating potential runoff volume and peak runoff rate 

3.3.1  Runoff Coefficient 

3.3.2  Estimated Potential Runoff Volume 

3.3.3  Peak Runoff Rate 

3.3.4  Time of concentration

3.4  Water Conveyance Channel designs 

3.5  Storage System Design 

3.5.1  Approximation of storage capacity 

3.5.2  Depth of the reservoir 

3.6  Design of a spillway 

3.7  Removal of Gross Pollutants (Treatment) 

3.7.1  Screening, Sedimentation and filtration 

3.8  Distribution (Application) system designs 

3.8.1  Setting up of a pump 

4  METHODOLOGY 

4.1  Data Acquisition 

4.1.1  Topographical map 

4.1.2  Land Use map 

4.1.3  Rainfall data 

4.2  Data Analysis 

4.2.1  Determination of the effective impervious area of the selected catchment area.

4.2.2  Determination of Rainfall reliability using Gumbel’s distribution method

4.2.3  Determination of Expected runoff volume and peak runoff rate 

4.2.4  Determination of Time of Concentration (Tc) 

4.2.5  Design of a trapezoidal conveyance channel

4.2.6  Determination of reservoir capacity using Mass curve of inflow method 

4.2.7  Design of a Spillway 

4.2.8  Design of an Application Mechanism 

5  RESULTS AND DATA ANALYSIS 

5.1  Determination of the effective impervious area of the selected catchment area

5.1.1  Topographical map 

5.1.2  Land-Use map 

5.2  Determination of Rainfall reliability using Gumbel’s distribution method 

5.3  Determination of Expected runoff volume and peak runoff rate using Runoff Coefficient & Rational Method 

5.3.1  Frequency Factors for Rational Formula 

5.3.2  Runoff coefficients 

5.3.3  Rainfall Amount 

5.3.4  Expected Utilizable Runoff Volume 

5.3.5  Peak Runoff Rate 

5.3.6  Time of Concentration (Tc) 

5.3.7  Designing a Trapezoidal water conveyance channel 

5.4  Determination of reservoir capacity 

5.4.1  Reservoir Volume approximation using mass curve of inflow method

5.4.2  Design of an Underground storage system 

5.5  Design of a spillway along the conveyance channel 

5.5.1  Determination of Normal Runoff Rate 

5.5.2  Channel Parameters for the normal runoff rate 

5.5.3  Spillway Design 

5.6  Design of an application System 

5.6.1  Selection of the Pump 

5.6.2  Sizing of the Supply Tank

5.6.3  Application system layout 

5.6.4  Application Network system 

6  DISCUSSIONS OF RESULTS 

6.1  Overview 

6.2  Non Potable Uses of Stormwater 

6.2.1  Fire fighting  

6.2.2  Landscaping 

6.2.3  Flushing Toilet 

6.2.4  Recreation purposes 

6.2.5  Irrigation of parks, Gardens and roadside grass 

6.3  BILL OF QUANTITIES 

6.4  COST BENEFIT ANALYSIS 

7  CONCLUSIONS AND RECOMMENDATIONS 

7.1  Conclusions drawn from the design study. 

7.2  Recommendations proposed from the design project 

8  REFERENCES 

9  APPENDICES 

9.1  Appendix A 

9.2  Appendix B 

9.3  Appendix C 

9.4  Appendix E 

9.5  APPENDIX F 

9.6  APPENDIX G 

9.7  APPENDIX H