Study on surface water availability for future water demand for Dhaka city

Abstract

Dhaka, the capital of Bangladesh, is one of the fastest growing cities of the world. It remains a great challenge to ensure uninterrupted water supply in the city with adequate quantity and quality round the year. Necessary measures are undertaken to meet the growing demand of water supply which is presently dependent on abstraction of groundwater. It appears that no further abstraction is feasible as the groundwater level is declining very fast. To reduce the overwhelming dependence on groundwater resources, surface water in the vicinity of the Dhaka city can be utilized.

This study deals with the surface water availability to meet the growing demand of Dhaka city water supply. Primarily, the existing water supply system of the city has been reviewed to ascertain the possible reasons of water supply crisis. Review illustrated that rapid groundwater depletion caused by excessive extraction, extreme surface water pollution through industrial waste and sewage disposal are the major reasons of water crisis of the city.

Realizing the necessity to explore options of water supply system, potentials of all available sources were critically examined through a detail analysis involving tools such as survey, investigation, test of water quality parameter, preparation of flow and water level hydrographs, determination of environmental flow and hydrodynamic HEC-RAS model analysis. The study includes water demand and population projection upto 2035.

The city is surrounded by six rivers i.e., Balu, Buriganga, Sitalakhya, Dhaleswari, Turag and Tongi khal and more so connected with two nearby large rivers such as Padma and Meghna. The quality of water of all these surface water sources has been studied. Analyses showed that the Balu, Buriganga, Turag and Tongi khal contain more pollutant in dry season. However, for Dhaleswari, Sitalakhya, Padma and Meghna contain lesser pollution. The situation worsens in dry season due to lack of precipitation and reduced upstream flow resulting in low DO and high concentration BOD, COD, ammonium and phosphate. The assessment revealed that the water of Dhaleswari, Sitalakhya, Padma and Meghna remain usable after treatment throughout the year. The water quality from the rivers Balu, Buriganga, Turag and Tongi khal found to be improved for the wet period from May to November. To determine the water availability, flow and water level hydrographs of 10 years from 2006 to 2016 have been used for the analyses. The flow exceedance curve was prepared for the analysis of determination of environmental flow. The environmental flow was calculated by Tenant, Q50 and Q90 methods. The aspects of navigability of the rivers have also been considered to assess the impact of water withdrawal from the selected sources. Thus, the absrtactable water was determined from the available flow. Hydrodynamic of the river network using HEC-RAS was simulated for various abstraction scenarios. It was observed that water velocity, depth and water level of the selected rivers also decreased from the base flow condition after withdrawal of required water. In terms of availability, it was observed that Buriganga, Balu, Sitalakhya, Dhaleswari, Padma and Meghna are good source of surface water. These sources can provide total amount of estimated future demand subjected to proper treatment upto year 2035. An evaluation has also been made for the surface water sources considering their available quantity, required quality and cost effectiveness. In terms of cost effectiveness, it was found that peripheral rivers are more economical compared to large rivers due to nearby location from the city. Thus, from both data analysis and model simulation, it is evident that surface water from rivers can solve the water crisis of the Dhaka City. The operation plan for future water demand as proposed in this study will be able to provide water requirements till the year 2035. It can be opined that results and suggestions put forward in this study can be considered as an initial step towards successful attainment of sustainable development goals for water management and sanitation.