Analysis of stormwater runoff for a selected catchment of eastern Dhaka using hydrologic model

Abstract

Dhaka, the capital of Bangladesh and one of the world’s rapidly growing megacities, faces the recurring phenomena of urban flooding and waterlogging following intense rainfall nearly every year. The water level of the Brahmaputra and rainfall and runoff from the entire Brahmaputra Basin mainly influence the process of river floods in and around the city. Dhaka’s average annual rainfall is about 2,000 mm, most of it occurring during the summer monsoon. As a low-elevation city with a tropical monsoon climate, Dhaka has a long history of river flooding as a natural hazard. A growing concern is that, in a changing climate, characterized by heavier and more erratic rainfall in the Ganges-Brahmaputra-Meghna (GBM) Basin during the monsoon season, the situation may worsen. Thus, stormwater management is a challenging task in such densely populated urban area having weak water governance and a lack in the institutional arrangement. It is even more challenging as the natural drainage system experiences relatively high water levels in the peripheral rivers during the rainy season, and the system capacity is compromised due to encroachment of the runoff detention areas.

At present, only the western part of Dhaka is protected against river floods of Turag, Buriganga, and Balu rivers. The problem of eastern side of the city covers both the external river flood and internal flood from stormwater. An embankment along the right bank of Balu River can protect the Eastern Dhaka from river flood, and then the internal stormwater can be discharged into the river with a number of drainage pumps. Against this backdrop, this study focuses on the stormwater management of Begunabari canal catchment, the largest catchment of Eastern Dhaka. The general physical setting for the study is 47.67 km2 area, which has been modeled in GeoSWMM. The specific objectives of this study include proper calibration of the model, and then its application for future drainage improvements and adaptation measures considering future landuse and climate change impact.

In this study, the model is calibrated with observed water level data at Begunbari canal, Banasree and then drainage facilities have been designed for 2 day 10 year return period. The design rainfall value of 286 mm is increased by 16% considering the climate effects of A1FI 2050 scenario. The study’s key findings highlight the required pumping capacities and retention facilities after implementation of Eastern Embankment. While finding this requirement, future landuse suggested by RDP, 2035 is considered along with stated climate change effect. The suggested pumping station at Begunbari canal outlet would drain out 50 cumec stormwater to maintain internal water level not to exceed 4.5m PWD at times of design storm evacuating the excess flooding within 2 days. To do such it will require 1.5 sq. km of retention pond at the pumping station. The bottom level of retention pond is considered at -0.5 mPWD while pump start level at 3.3m PWD in the monsoon. Regulators would be functional during pre-monsoon and post monsoon period when Balu is at lower level allowing gravity drainage. Here, the designed pump has been applied for 2day rainfall of 5, 10 and 25 years return period. It was observed that the inundated area with depth greater than 1.5 m can be reduced by 15-30%, while the total inundated area can be reduced by 18-25% by pumping.

The study also explored the scope of flood reduction by storage tanks of rainwater harvesting system. There are 13 subcatchments identified where future development would occur. 30% of this future development area is considered here as rooftop area and the storage tanks are designed accordingly. The design values of storage volumes are taken from an implemented project in Gujarat, India. Total 6475 m2 storage area is considered taking the depth of tank as 10 m. The inundation depth and area both were found reduced in this case by taking measures of pumping with storage tank for rainwater harvesting. For 2 days rainfall, the area with depth greater than 1.5 m can be reduced by around 87%, while the total inundated area can be reduced by around 60% by storage tank along with the pumping. Again, the stored water can be used for household purpose and even for groundwater recharge. So, this option should be considered during future development in this area.