International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064 Index Copernicus Value (2016): 79.57 | Impact Factor (2017): 7.296 Volume 7 Issue 6, June 2018 www.ijsr.net Licensed Under Creative Commons Attribution CC BY Integrated Urban Pluvial Flooding Analysis and Modelling for Nairobi West and South C in Nairobi City Simon O. Okoth 1 , Christian T. Omuto 2 , John P. Obiero 3 1, 2, 3 Department of Environmental & Biosystems Engineering, School of Engineering, University of Nairobi, P. O. Box 30197 – 00100 Nairobi Abstract: Urban pluvial flooding cases have increased due to urban densification, fast changing urban hydrology as well as inadequate urban drainage design especially the combine storm-sewer systems. In Nairobi city, residents are at greater urban pluvial flooding risk as has been witnessed with a number of flood damages having already been experienced. However, there are a number of tools that have been developed to help in analysing urban pluvial flooding risks and support sound planning to avert such catastrophes. This purpose of the study was to analyse and model urban pluvial flooding in Nairobi’s South C and Nairobi West areas using Storm Water Management Model version 5.1 (SWMM5.1) and demonstrate to city planners among other key stakeholders the applicability of SWMM 5.1 in analysing urban pluvial flooding to support urban planning. Three main datasets were used in the study including Geographic Information Systems (GIS) data, rainfall data disaggregated into 15-minutes events and the sewer network data. Other key parameters were drawn from existing literature. The response of two delineated sub-catchments to the rainfall event of 26 th December 2012 was then modelled and sensitivity analysis conducted to identify the relative influence of some model input parameters on the peak runoff. The results from the model showed significant flooding of 20.134 ha-m surface runoff and 81% of sewer system surcharging. The peak runoff was found to be significantly responsive to variations in Impervious N and % imperviousness parameters. The study demonstrated that SWMM5.1 model is a useful tool for simulating urban pluvial flooding and improving urban stormwater management. Keywords: Pluvial Flooding; Runoff; Sewer Surcharge; Disaggregation; and urbanisation. 1. Introduction Flooding in urban areas is occurring with increasing frequency all over the world and is causing repeated damage that calls for improved management of floods [1]. In addition, the extent, magnitude and frequency of urban pluvial flooding are likely to increase in the near future, given the increasing effects of climate change, the increased urbanisation and population growth [2]. As a result, the significance of modelling urban flooding is continuously growing as a result of the ever-changing urban hydrological regime resulting from urbanization, changing climate and population growth. Urbanization and the resulting land-use change strongly affect the water cycle and runoff-processes in watersheds [3]. Many authors have attempted to advance the definition of urban pluvial flooding. The dictionary definition of a flood is “An overflowing or irruption of a great body of water over land in a built up area not usually submerged [4].” Urban flooding may be due to various causes: overland flows on streets, flooding flows from rivers and overflows or surcharges from sewer networks [5]. Distinct from flooding, the term ‘surcharge’ is defined as a ‘condition in which wastewater and/or surface water is held under pressure within a gravity drain or sewer system, but does not escape to the surface to cause flooding [6]. Picture 1a and 1b: Sewer surcharges on the streets and neighbourhoods after the conduit capacity is overwhelmed The definition of flooding given by European Standard EN 752 is “a condition where wastewater and/or surface water escapes from or cannot enter a drain or sewer system and either remains on the surface or enters buildings” [7]. Falconer defines urban pluvial flooding as “The result of rainfall-generated overland flow and ponding before the runoff enters any watercourse, drainage system or sewer, or cannot enter it because the network is full to capacity” [8] and [9]. During heavy storms pluvial flooding can take place even if flow in the sewer network capacity is with a free surface, i.e. the sewer network is not exceeded, if inlet capacity is insufficient to capture surface run-off [10]. The extreme events with a duration of up to several hours cause the urban storm water drainage system to be overloaded, either due to the intensity of the rain or due to runoff from urban green space which normally infiltrates into the ground [11]. In other words, the potential for evapotranspiration, water infiltration and recharge in urban catchments which have consistently attenuated due the continued removal of vegetation, forestry and top soil cover and their subsequent replacement with impermeable surfaces have significantly Paper ID: ART20182721 DOI: 10.21275/ART20182721 334