Browsing by Author "Odongo, Malachi"

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    HEC-RAS Modeling for the Determination of Highest Water Mark as the Reference Surface for Delineation of Riparian Reserve: Case Study of Nairobi River-Kenya
    (African Journal on Land Policy and Geospatial Sciences, 2025-05) Odongo, Malachi; Mireri, Caled
    Context and background: Highest Water Marks (HWM) has often been used as the points of reference for measurement of the width of the riparian reserve. Its determination is usually a challenge and, in most cases, its often approximated manually by physically going to the river banks during heavy rainfall seasons. This has often caused a huge challenge when delineating riparian reserves for purposes of protecting and conserving it. Hydrological Engineering Centreline River Analysis System (HEC-RAS) has proven to provide accurate simulation of the highest water mark over a long period of time using both geomorphological features and climatic data. Goal and objectives: The main aim of this study was to apply HEC-RAS model to determine the highest water mark (HWM) for Nairobi River for a flood return period of 30 years. Methodology: HEC-RAS version 6.3 was used to simulate the HWM with inputs of the Digital Terran Model (DTM), River Discharge and rainfall gauge station data. The model was found to very suitable for determining the HWM after conducting validations during the high rain fall periods of April/May 2024 which caused a heavy flood in Nairobi City. The HWM was then used to offset the different set back distances of 2m, 6m and 30m as per the legal framework and laws governing riparian delineation. Levels of encroachment was then established within those delineated riparian reserves. Results: The upper river segment, the highest water mark was found to range between 2-3m above the river bed on a well V-shaped river slope hence not susceptible for flooding while the middle river segment the highest water level was above 5m on a U-shaped river slope rendering it a flood plain where as the lower river segment the highest water mark was found be 1.5m above the river bed on a widened U-shaped river slope making it very prone to huge floods.