Flood Risk Mapping in Support of Flood Management: Case of Nzoia River, Keny a Mr. Lawrence Omai

1. 19th Engineers International Conference: The Role of Engineering Practitioners in the Implementation of the Constitution , Nairobi , KICC, May 9 -11, 2012 Flood Risk Mapping in Support of Flood Management: Case of Nzoia River, Kenya Mr. Lawrence Omai Department of Civil & Structural Engineering Moi University Flood Management through Flood Risk Mapping: Case of Nzoia River, Kenya

2. Study Area IEF01 Nzoia River Basin Department of Civil and Structural Engineering, Moi University

3. Essential Characteristics of Budalang’i • Location: Busia county • Coverage: 128,000 hectares • Floodplain topography: flat to very gently undulating, slope < 2 %. • Extensive meandering of river: extensive floodplain with wetlands. • Mean annual rainfall: 750– 1000 mm • Soils: mainly vertisols • Settlement: concentrated mostly along the Nzoia River. Department of Civil and Structural Engineering, Moi University

4. Flooding Problem • Frequent flooding • Dyke protection failure • Destruction of residences • Loss of property & Lives • Possible outbreak of diseases • Food Insecurity • Continued Encroachment • Inadequate coping mechanisms during flood events Department of Civil and Structural Engineering, Moi University

5. IEK Conference 2006 Flood: Dyke Breach 2006 Flood: Displaced People 2008 Flood: Destruction of property 2006 Flood: Displacement of People Department of Civil and Structural Engineering, Moi University

6. Types of Floodplain Modelllng Floodplain as Part of the River Assumed that water level in the river & Fp is same Floodplain as Storage Reservoir(s) E-V relationship derived (DEM) Quasi 2D flood modelling Fp modelled as a network of river branches and Link channels/ spills Department of Civil and Structural Engineering, Moi University

7. Why Quasi-2D Modelling Approach? • 1-D approach can but modelling floodplain flows where the ‘1-D’ assumption is in question, then the accuracy and appropriateness of the model decreases. • 2-D models: effective for floodplain modelling, but have high hardware requirement, data & computational time. • Quasi 2-D approach is a compromise Department of Civil and Structural Engineering, Moi University

8. Spatial Data River Geometry Hydraulic Data Hydraulic Modelling (MIKE 11) Quasi 2-D Flood Model Synthetic Flood Flood Modelling & Mapping (MIKE-GIS Scenario Analysis Methodology WETSPro, ECQ WL,Q Breached dykes Historical Floods Overtopping Department of Civil and Structural Engineering, Moi University

9. Methodology cont’d….. ADCP machine 1. Analysis of spatial & hydraulic data • Toposheets • LiDAR survey data • Flow and Lake levels data • River geometry data 2. River Hydrodynamic Modelling • MIKE 11-construction and set-up • Set up of MIKE-11 files and achieve its stability. • Water level and discharge simulation • Considered: intact, breached and overtopping of dykes. Department of Civil and Structural Engineering, Moi University

10. Methodology cont’d….. 3. Development of Quasi 2-D flood model • Extension of MIKE 11 hydrodynamic Model • Implementation of fictitious flood branch • Extraction of floodplain cross-sections:(DEM) • Flood inundation/extent mapping: historical_2008 Department of Civil and Structural Engineering, Moi University

11. Development of Quasi 2-D Flood model Extraction of Floodplain xns DEM, MIKE 11-GIS used Sample of Extracted floodplain xn Department of Civil and Structural Engineering, Moi University

12. Untitled 14000 1200 13800 13600 13400 Nzoia River 0-25065.7 13200 13000 Floodbranch_5 0-6405.8 12800 12600 12400 Link Channel 0-210.9 7157.48 12200 12000 11800 11600 11400 11200 613500 614000 614500 615000 615500 616000 616500 617000 617500 618000 Development of Quasi 2-D Flood model cont’d.. Link channel Implementation • Link Channel parameters specified Department of Civil and Structural Engineering, Moi University

13. Observed Flood Event Satellite Image of 2008 flood event Flood extent 2008 Nzoia River Department of Civil and Structural Engineering, Moi University

14. Historical Flood Event Simulation (2008) Simulated 2008 flood event Overlay of Obs. & Simulated Department of Civil and Structural Engineering, Moi University

15. Evaluation of Accuracy of Quasi 2D Model • Quantitative evaluation of accuracy of the model was done. • 85 pts along the border of obs & sim flood extent chosen • Water level data of the pts was derived using DEM/ MIKE 11-GIS. • Scatter plot prepared Department of Civil and Structural Engineering, Moi University

16. Evaluation of Accuracy of Quasi 2D Model Department of Civil and Structural Engineering, Moi University

17. Conclusion • With R² of 0.8 the Quasi 2D model simulated reasonably well the 2008 flood event. • The slight over and underestimations mainly can be attributed to lack of capture of peak moments of the flood event by satellite. Department of Civil and Structural Engineering, Moi University

18. Recommendations • With further improvement the Model can be useful as a flood management tool e.g. towards floodplain zoning. • Flood menace in the Budalangi and in the entire country can only be alleviated through integrated approach where multiple options are taken into consideration. • The county government to harness local expertise for disaster management. Department of Civil and Structural Engineering, Moi University

19. Acknowledgement • IEK • VLIR-UOS- MU Project • Moi University Department of Civil and Structural Engineering, Moi University