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
1
Flood Management
through Flood Risk Mapping:
Case of Nzoia River, Kenya
13-Apr-15
Study Area
IEF01
Nzoia River Basin
2
Department of Civil and Structural Engineering, Moi University
13-Apr-15
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.
3
Department of Civil and Structural Engineering, Moi University
13-Apr-15
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

4
Department of Civil and Structural Engineering, Moi University
13-Apr-15
IEK Conference
2006 Flood: Dyke Breach
2006 Flood: Displaced People
2008 Flood: Destruction of property
2006 Flood: Displacement of People
5
Department of Civil and Structural Engineering, Moi University
13-Apr-15
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
6
Department of Civil and Structural Engineering,
Moi University
13-Apr-15
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
7
Department of Civil and Structural Engineering, Moi University
13-Apr-15
Methodology
River Geometry
Hydraulic Data
WETSPro, ECQ
Spatial Data
Hydraulic Modelling
(MIKE 11)
WL,Q
Overtopping
Synthetic
Flood
Flood Modelling & Mapping
(MIKE-GIS
Scenario Analysis
8
Historical Floods
Breached dykes
Quasi 2-D Flood Model
Department of Civil and Structural Engineering, Moi University
13-Apr-15
Methodology cont’d…..
1. Analysis of spatial & hydraulic data
• Toposheets
• LiDAR survey data
• Flow and Lake levels data
• River geometry data
ADCP machin
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.
9
Department of Civil and Structural Engineering, Moi University
13-Apr-15
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
10
Department of Civil and Structural Engineering,
Moi University
13-Apr-15
Development of Quasi 2-D Flood model
•Extraction
of Floodplain xns
•DEM, MIKE 11-GIS used
Sample of Extracted floodplain xn
11
Department of Civil and Structural Engineering,
Moi University
13-Apr-15
Development of Quasi 2-D Flood model
cont’d..
Link channel Implementation
 Link Channel parameters specified
Untitled
14000
1200
13800
13600
13400
13200
13000
12800
12600
12400
7157.48
12200
12000
11800
11600
11400
11200
613500
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614000
614500
615000
615500
616000
616500
617000
617500
Department of Civil and Structural Engineering,
Moi University
618000
13-Apr-15
Observed Flood Event
Satellite Image of 2008 flood event
Flood extent 2008
Nzoia River
13
Department of Civil and Structural Engineering, Moi University
13-Apr-15
Historical Flood Event Simulation (2008)
Simulated 2008 flood event
14
Overlay of Obs. & Simulated
Department of Civil and Structural Engineering, Moi University
13-Apr-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
15
Department of Civil and Structural Engineering, Moi University
13-Apr-15
Evaluation of Accuracy of Quasi 2D Model
1140.2
Water Level Simulated(m)
1140.0
y = 1.006x - 7.351
R² = 0.80
1139.8
1139.6
1139.4
1139.2
1139.0
1138.8
1138.6
1138.4
1138.2
1138.20 1138.40 1138.60 1138.80 1139.00 1139.20 1139.40 1139.60 1139.80 1140.00
Water Level Observed(m)
16
Department of Civil and Structural Engineering,
Moi University
13-Apr-15
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.
17
Department of Civil and Structural Engineering, Moi University
13-Apr-15
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.
18
Department of Civil and Structural Engineering, Moi University
13-Apr-15
Acknowledgement
 IEK
 VLIR-UOS- MU Project
 Moi University
19
Department of Civil and Structural Engineering, Moi University
13-Apr-15