1. 19th Engineers International Conference:
The Role of Engineering Practitioners in the Implementation of the Constitution , Nairobi , KICC, May , 2012
24-Apr-17
Flood Risk Mapping in Support of Flood Management: Case of Nzoia River, Kenya
Mr. Lawrence Omai
Department of Civil & Structural Engineering
Moi University
24-Apr-17
Flood Management through Flood Risk Mapping: Case of Nzoia River, Kenya

2. 24-Apr-17
Study Area
IEF01
Nzoia River Basin
24-Apr-17
Department of Civil and Structural Engineering, Moi University

3. Essential Characteristics of Budalang’i
24-Apr-17
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
24-Apr-17

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
24-Apr-17

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
24-Apr-17

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
24-Apr-17

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
24-Apr-17

8. Methodology
Spatial Data
River Geometry
Hydraulic Data
Hydraulic Modelling
(MIKE 11)
Quasi 2-D Flood Model
Flood Modelling & Mapping (MIKE-GIS
Scenario Analysis
Synthetic
Flood
WL,Q
WETSPro, ECQ
Breached dykes
Historical Floods
Overtopping
Department of Civil and Structural Engineering, Moi University
24-Apr-17

9. Methodology cont’d….. ADCP machine
24-Apr-17
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
24-Apr-17

10. 3. Development of Quasi 2-D flood model
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
24-Apr-17

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
24-Apr-17

12. Development of Quasi 2-D Flood model cont’d..
Link channel Implementation
Link Channel parameters specified
613500
614000
614500
615000
615500
616000
616500
617000
617500
618000
11200
11400
11600
11800
12000
12200
12400
12600
12800
13000
13200
13400
13600
13800
14000
Untitled
Nzoia River
Floodbranch_
Link Channel
1200
Department of Civil and Structural Engineering, Moi University
24-Apr-17

13. Satellite Image of 2008 flood event
Observed Flood Event
Satellite Image of 2008 flood event
Flood extent 2008
Nzoia River
Department of Civil and Structural Engineering, Moi University
24-Apr-17

14. Historical Flood Event Simulation (2008)
Overlay of Obs. & Simulated
Simulated 2008 flood event
Department of Civil and Structural Engineering, Moi University
24-Apr-17

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
24-Apr-17

16. Evaluation of Accuracy of Quasi 2D Model
Department of Civil and Structural Engineering, Moi University
24-Apr-17

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
24-Apr-17

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
24-Apr-17

19. Acknowledgement IEK VLIR-UOS- MU Project Moi University
Department of Civil and Structural Engineering, Moi University
24-Apr-17