Mathematical modelling for analysis of drainage congestion problem of polder 29 in Khulna district

Abstract

The coastal belt of Bay of Bengal lies in the southern part of Bangladesh faces severity of natural catastrophes such as drainage congestion, inundation of land etc at monsoon. To protect this area, polders are constructed during sixties. But waterlogging problem is getting intensified with time in southwest coastal region of Bangladesh due to gradual riverbed siltation, inadequate drainage channels and improper operation and maintainance of regulators and flapgates. There is a growing concern that these effects will exacerbate in future due to effects of climate change. In this study, Polder 29 has been selected as the study area because condition of most of the internal drainage channels is almost disagreeable and some gates at eastern and western periphery remain non-operational which results prolonged waterlogging. So, this study is formulated to analyze the existing drainage congestion problem of the study area and an adaptive measure is suggested to alleviate this problem. GeoSWMM was used in this study to carry out stormwater analysis of the study area comprising 79.33 km2 area. The model was calibrated and validated using the observed water level data (both inside and outside) of Kanchannagar sluice. Frequency analysis of rainfall and water level data was carried out for different return periods to estimate the design rainfall and water level. Adequacy of existing drainage channels was investigated by simulating the model at design storm scenario (1in 10 year period). The result of the analysis reveals that some drainage channels are inadequate for conveying the design flow volume. To solve this problem, increasing cross-sectional area is suggested. Flood inundation map was prepared to assess the waterlogging problem of the study area. The analysis reveals that 52.33% of the total polder area is waterlogged under the existing condition and application of the ideal pump facility can reduce this to 37.33%. Largest Kanchannagar catchment was modeled separately to understand the actual extent of reduction in inundation depth. The analysis shows that 38.70% of the total Kanchannagar catchment area is flooded at the existing scenario and re-excavation and addition of new drainage channels help reduce the waterlogged area to 11.26 %. The flooding loss was about 139860 m3 with this adaptive measure and after utilizing pump with capacities of 1, 2 and 3 cumec this loss is reduced to 136900 m3, 119400 m3 and 110280 m3 respectively