Developing Design Curves for Rainwater Harvesting in the Coastal Region of Bangladesh

Abstract

People in Bangladesh's southwestern coastal zone confront a potable water crisis as a result of saline intrusion into surface and groundwater caused by anthropogenic and climate change. Locally available sources of water, such as ponds and wells, frequently become saline when affected by a tidal surge or embankment failure, limiting the availability of non-saline water for the population. Bangladesh's coastline region receives a large quantity of annual rainfall, making rainwater harvesting a potential water supply source for the people. Government agencies and many non-governmental organizations (NGOs) supply storage tanks ranging from 500 to 3000 liters for a normal household RWH system. The optimal storage tank required for inhabitants based on rainfall, family size, water demand, and catchment area has not been assessed, restricting the source's usability to a limited period of the year (6 to 7 months). To address this issue, the current study aims to create a comprehensive decision-making tool for a reliable rainwater harvesting system throughout the coastal belt. Design curves for the optimum storage tank have been developed for three climate conditions (wet year, dry year, and average year) taking into account available catchment area (20 – 70 m2), rainfall loss factor, drinking water demand (10 lpcd) for a range of household sizes (4 to 11) using mass curve and subsequent peak algorithm for 13 different stations along the coastal zone. A spreadsheet-based daily water balance model was developed with the goal of assessing the tank's reliability (volume and time-based) and determining the spilled volume. Slight variation has been observed in dry and average year climates, although reliability for the same tank during the wet year ranges between 82 and 85 percent. A large amount of rain water is wasted as spilled water, thus if the capacity of the tank is raised, the water may also be used for other purpose too; the economic analysis also reveals a return period of 3 – 4 years for the tank. The design curves will serve as a guideline for the development of an optimal storage tank for the inhabitants of coastal Bangladesh in order to provide drinking water for the longest period of time possible inside a year.