Modelling Hydrological Response to Urbanization: Case Study Syokimau – Katani Area, Machakos County, Kenya

Abstract

The Syokimau – Katani peri-urban catchment has over time been affected by adverse urban development related flooding. The encroachment of the expanding Nairobi city on this metropolitan space has spurred the mushrooming of impervious surfaces on an already poorly drained gently sloping clay-filled landscape. This has, in turn, reduced rainfall losses by interception and infiltration and increased net rainfall-runoff amounts and flood frequency, thus posing a livelihood safety challenge in the area. From the analysis done of the problem-solving approach countries such as Belgium, Rwanda, Slovakia and Iran have assimilated, a physically-based hydrological modelling approach using the WetSpa tool has been chosen and adopted in this project for this catchment. This framework has made provision for the use of geologically and geophysically validated readily-available datasets on the topography, soils and land-use of the Syokimau – Katani region in a rapid-prototyping approach to determine the area’s hydrological response. Herein, a delineation of the study area’s watershed was achieved alongside an analysis of its land-use (over the years 1999, 2000, 2010 and 2016), soils, stratigraphy and geology. Also, an added analysis of locally acquired ERT and drillhole data was used to aid in the interpretation and justification of the study area’s soils and geology coverage. From the validated topography, soils and land-use datasets, spatially indexed potential runoff coefficient values were generated using the WetSpa tool upon which hotspot runoff areas within the study area were mapped. A time series of the rainfall-runoff depth estimates was also generated based on both the LULC-only and unique scenarios to isolate the effects of rainfall intensity and land-use/land cover to these estimates. Even though weather and climate play a significant role in generating runoff depths in this study area, land use and land cover have also significantly contributed to runoff depth yields shown from the LULC-only and unique scenarios. This has been exemplified by the 1999-2000 drought case and the 2016 increase in urbanization discussed in this project. The highlighted impact of urbanization to flooding in this research can therefore be used to inform urban development policies and settlement procedures in the Syokimau – Katani area, as well as in urban/peri-urban centres and metropolitan spaces within the country. The potential runoff coefficient maps generated herein are viable in aiding the placement and design of storm management systems in MSc Dissertation: I56/82940/2015

more effective ways that can help control this urban-flooding problem. It is also recommended to use an advanced form of the Wetspa tool known as WetSpa-Python for the increased spatial and temporal resolution flexibility for a varied distribution of urban catchments.