Mathematical modelling of urban watersheds for drainage and environment improvement

Abstract

Due to the increase in population in the urban area more and more land is necessary for living. Therefore reclamation of marshy land in urban areas has taken place over the years without giving due consideration for stormwater drainage. It was observed that the flooding prolongs mainly due to the insufficient drainage sizes together with reduction in detention / retention areas. As a result there is uncontrolled flooding causing enormous problems. The disruption of business and community activities accelerate deterioration of the environment and increases health risks etc. In the planning process it is important, therefore, to be able to assess the probable impact of urbanization upon the magnitude of flood peaks. I. the Basic Model HEC-l that has been used to estimate Runoff and 2. HEC-RAS model that has been used to establish the Rating Curve. In Sri Lanka especially in urban areas basic statistical data such as runoff data, land use data and important tested parameters are not available to assist runoff values for the designing of a drainage system. However, availability of statistical data and literature to assess peak discharge is limited and hence, parameters are assumed based on available information and judgements. The objective of this study is to provide solutions to the drainage issue due to urbanization by studying urban hydrology and identifying the necessary parameters to model urban watersheds. Based on this the objectives are identified as follows: 1. Study the behavior of rainfall and runoff of urbanized area. 2. Develop a mathematical model for the drainage system. III 3. Identify the important parameters and their values for mathematical modeling of watersheds to calculate peak runoff. Two urban watersheds were gauged mathematically and modelled using the HEC model to identify the behaviour for urban watersheds in Colombo and also to identify parameters applicable to these. The study area consists of two locations independent of each other but quite close within the Greater Colombo Area. They are: 1. Torrington watershed consisting of 290 ha located within the heart of Colombo & 2. Badowita - Attidiya watershed consisting of 270 ha located within the periphery of the Greater Colombo. The distinct difference between the two areas is in the land use, which is a vital factor that contributes to the run-off. The stream network and storage area identified in both watersheds were linked to establish the model. Required data such as rainfall, land use, events of flood records, tide level data were collected and checked before using in the model. Land cover of the watersheds showed that in the urbanized areas the pervious extent was approximately 62% while in less urbanized areas the previous extent was about 76%. Average Curve Numbers for the urbanized areas were 95 and 70 for impervious and pervious areas respectively. The HEC rainfall-runoff model developed for both watersheds produced ve~y good peak discharge matching and hence these models could be used for drainage environment improvement projects in urban areas. It was also found that Curve Number values, so obtained appear to be realistic and could be easily used for similar watersheds instead of general val use. Canal network of both watersheds were modelled and rating curves were developed and verified for both gauging stations. The established model for urban areas can be used for rehabilitation of existing drainage systems as well as for designing of new drainage systems.