Apr 21, 2023Flood Management through Flood Risk Mapping: Case of Nzoia River, Kenya

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Flood Risk Mapping in Support of Flood Management: Case of Nzoia

River, Kenya

Mr. Lawrence OmaiDepartment of Civil & Structural Engineering

Moi University

19th Engineers International Conference:The Role of Engineering Practitioners in the

Implementation of the Constitution , Nairobi , KICC, May 9 -11, 2012

Apr 21, 2023Department of Civil and Structural Engineering, Moi University

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Study Area

Nzoia River Basin

IEF01

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 mmSoils: mainly vertisolsSettlement: concentrated mostly along the

Nzoia River.

Apr 21, 2023Department of Civil and Structural Engineering, Moi University3

Essential Characteristics of Budalang’i

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

Apr 21, 2023Department of Civil and Structural Engineering, Moi University4

Flooding Problem

Apr 21, 2023Department of Civil and Structural Engineering, Moi University5

IEK Conference

2006 Flood: Dyke Breach

2006 Flood: Displaced People

2008 Flood: Destruction of property

2006 Flood: Displacement of People

Apr 21, 2023Department of Civil and Structural Engineering, Moi University6

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 modellingFp modelled as a network of river

branches and Link channels/ spills

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

Apr 21, 2023Department of Civil and Structural Engineering, Moi University7

Why Quasi-2D Modelling Approach?

Apr 21, 2023Department of Civil and Structural Engineering, Moi University8

Methodology

Spatial Data River Geometry Hydraulic Data

Hydraulic Modelling

(MIKE 11)

Quasi 2-D Flood Model

Flood Modelling & Mapping (MIKE-GIS

Scenario Analysis

Synthetic FloodW

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Apr 21, 2023Department of Civil and Structural Engineering, Moi University9

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.

3. Development of Quasi 2-D flood modelExtension of MIKE 11 hydrodynamic ModelImplementation of fictitious flood branchExtraction of floodplain cross-sections:(DEM)Flood inundation/extent mapping: historical_2008

Apr 21, 2023Department of Civil and Structural Engineering, Moi University10

Methodology cont’d…..

•Extraction of Floodplain xns•DEM, MIKE 11-GIS used

Sample of Extracted floodplain xn

Apr 21, 2023Department of Civil and Structural Engineering, Moi University11

Development of Quasi 2-D Flood model

Link channel Implementation Link Channel parameters specified

Apr 21, 2023Department of Civil and Structural Engineering, Moi University12

Development of Quasi 2-D Flood model cont’d..

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11200

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11800

12000

12200

12400

12600

12800

13000

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Untitled

Nzoia River 0-25065.7

Floodbranch_5 0-6405.8

Link Channel 0-210.97157.48

1200

Observed Flood Event

Apr 21, 2023Department of Civil and Structural Engineering, Moi University13

Flood extent 2008

Satellite Image of 2008 flood event

Nzoia River

Historical Flood Event Simulation (2008)

Simulated 2008 flood event

Overlay of Obs. & Simulated

Apr 21, 2023Department of Civil and Structural Engineering, Moi University14

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

Apr 21, 2023Department of Civil and Structural Engineering, Moi University15

Evaluation of Accuracy of Quasi 2D Model

Apr 21, 2023Department of Civil and Structural Engineering, Moi University16

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.

Apr 21, 2023Department of Civil and Structural Engineering, Moi University17

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.

Apr 21, 2023Department of Civil and Structural Engineering, Moi University18

Acknowledgement

IEK VLIR-UOS- MU Project Moi University

Apr 21, 2023Department of Civil and Structural Engineering, Moi University19