Development of decision aid for ecohydrological consideration in flood management in deltaic floodplain

Abstract

Traditional flood management interventions adversely affect floodplain ecosystem as they do not maintain flood flow vital for preserving the floodplain ecosystem. Flood management intervention needs to be planned and implemented considering ecohydrological criteria to sustain floodplain ecosystem. Consideration of ecohydrological criteria in flood management can restore or preserve the ecosystem of a deltaic floodplain. This study has developed a decision aid framework for determination of flood management option and regulation which consider both ecological and hydrological criteria in planning flood management intervention in deltaic floodplain. The study introduces a term ‘ecohydrograph’ that combines hydrological requirement of the floodplain ecological community with respect to seasonality, and implementation of which will help sustain the floodplain ecosystem. A simple decision aid framework has been developed that gives, as an output, an eco-friendly flood management infrastructure and operation rules for flow control structures corresponding to the ecohydrograph. Implementation of the ecohydrograph will reestablish a hydrological environment in a modified or damaged floodplain, which will support living system of the biotic community of the floodplain. Flood flow characteristics that are vital for ecosystem sustenance ie hydrological indicators such as time of rise, depth, extent, duration, time of recession and frequency of flood, runoff of the floodplain have been determined from hydrologic literature review. Similarly ecological indicators such as floral and faunal diversity, water depth and time preferences of floodplain fish species for spawning migration, incubation and breeding, habitat opportunity for living and feeding, and time of out-migration have also been determined from literature review. Thus a range of hydrological and ecological indicators suitable for ecological resources have been identified and an indicator based relationship between hydrology and ecology has been developed for round the year from January to December comprising flood-cycle ie hydrograph, fish life-cycle and paddy crop-cycle. From this relationship a hydrograph suitable for ecosystem ie ecohydrograph has been determined. To determine this ecohydrograph, a decision aid framework has been developed that includes hydrological and ecological datasets, ecohydrological relationship, and a 2-D hydrodynamic model for a trial and error performance to find out the ecohydrograph and the optimum flood management option corresponding to the ecohydrograph. An intervened area with a flood control, drainage and irrigation project being implemented since 1983 has been selected for the study. Hydrological data of floodplains are not available because there is no practice of collecting floodplain hydrological data in Bangladesh. Hydrological data of the study area floodplain for preproject condition have been generated using 2-D overland flow hydrodynamic model. Ecological data have been collected using social survey techniques of key informant interview (KII), focus group discussion, and sampling and market survey. Impacts on hydrology and ecosystem have been assessed from data analysis. The decision aid framework has been applied to the study area. The ecohydrograph and the corresponding optimum flood management option have been determined using the 2-D hydrodynamic model of the decision aid framework through a trial and error process by changing time and regulating water levels of the canals. The ecohydrological status/indicators at optimum flood management option have been assessed and found close to those of the pre-project period. A comparative picture of hydrological and ecological estimates of the study area for pre-project and post-project periods and after reestablishment of the optimum flood management option corresponding to the ecohydrograph has been estimated. The connectivity between the river and the studyarea- floodplain and average water depth in the study area increase significantly. Fish habitat increases by 57% and fish production increases from 53.3 MT in the postproject period to 99.2 MT in the optimum ecohydrograph option; while only 3.34% Aman production is reduced. Biodiversity which was reduced to 19 in the post-project period from 48 in the pre-project period reaches at 35; while ‘high’ species dominance in the post-project period improves to ‘moderate’. If the decision aid framework is applied to the existing FCDI projects and the ecohydrograph is attained in every project then the floodplains of the country will be biologically more productive and diverse with many species. This ecohydrograph and decision aid framework would enable ecosystem approach of management of natural resources particularly water and aquatic biological resources.