Change of Halda river flow due to different water control structures and its impact on Halda ecosystem

Abstract

Halda river is one of the most important natural carp spawning grounds in Bangladesh and has long been the major source of naturally produced carp fry for pond culture in much of the country. There are several factors that are thought to have been responsible for continuous reduction in spawning and consequently in egg collection. These include flow reduction in the river during spawning season because of a number of water control structures implemented across different tributaries, straightening of ox-bow bends, industrial waste discharge, salinity intrusion etc. This study was carried out to investigate the status of different water control structures and their functions across different tributaries, to analyze the impacts of major water control structures on fresh water flow to Halda river and to gather the perception of the egg collectors about the impact of the modified runoff from the streams on the Halda river ecosystem. A socio-technical approach was followed in the study. The status of major water control structures was assessed by direct observation during field visits, purposive interviews and a number of Participatory Rural Appraisal (PRA)tools, including Key Informant Interviews and Focus Group Discussions (FGDs). Rainfall-runoff analysis was conducted for the basin using an off-the-shelf hydrologic model, HEC-HMS, to estimate relative flow contributions from different sub-catchment areas of hill stream tributaries. Perceptions of the fishermen or egg collectors about the impact of reduced flows from different tributaries were gathered through several FGDs and Timeline Analysis conducted with them. It is found that the major contributors to the main Halda river were Dhurang (15.1% ~ 6.9%), Kutubchari (11.7% ~ 7.8%), Lelang (10.8% ~ 7.5%), Sarta (10.8% ~ 7.4%) and Boalia (11.9% ~ 6.1%). Except these individual streams, the streams that were obstructed under Karnafuli Irrigation Project – Halda Unit (22.9% ~ 16.7%) and Bhujpur Rubber Dam (20.7% ~ 17.4%) contribute a considerable amount of flow to the river. The water control structures that were constructed under different projects to store water for dry season irrigation have no specified operational rules. Most of the structures are non-functional or blocked that aggravates drainage congestion during monsoon. The functional structures are maintained manually. Complete flow obstruction during dry season for irrigation (December to May), which overlaps the carp spawning time (April to June), means that sufficient flow is not available in Halda river during pre-monsoon thunderstorms, a favorable condition for Halda spawning. Comparison of timing of construction of water control structures (e.g. Dhurang weir, WCSs under Karnafuli Irrigation project, Boalia regulator, Mondakini WCS etc.) with reduction of carp spawning clearly revealed that carp spawning has reduced substantially due to flow reduction in the river, and this is the most important factor. The popular perception that straightening of oxbow bends was the one most significant factors behind reduction in carp spawning was found questionable; straightening did have negative impact, however, to a lesser extent compared to the impacts caused by the reduced flow contributions from the tributaries. In order for carp spawning to be improved, the functionalities of different water control structures need to be restored and operational rules need to be established to operate the sluice gates and regulators for the major tributary streams (e.g. Dhurang, Kutubchari, Lelang, Boalia etc.) as well as the rubber dam at Bhujpur. In sum, there needs to be a balance between irrigation water use and release of fresh water from important tributaries to the main Halda River.