II. FLOOD LOSS REDUCTION AND REVIEW OF PAST EXPERIENCES
C. Methodology for assessment
1. Assessment of hydraulic impactsExperience of flood control activities in Bangladesh shows that prevention of flooding of floodplains by embankments may increase the risk of floods in unprotected areas. The impact is transmitted far away from the project site because of very small river slope. A methodology has been established under the Flood Action Plan studies for the assessment of hydraulic impacts due to proposed projects (see FPCO, 1995b). A mathematical model of erratic flows in river-floodplain systems, based on the MIKE11 software package, developed by the Danish Hydraulic Institute, is being used as an assessment tool. The Surface Water Modelling Centre (SWMC) has been established to carry out modelling activities.
The methodology for simulation by models involves a nested approach at three levels: national, regional and local. A General Model covers the entire Ganges-Brahmaputra-Meghna (GBM) river system. Six regional models have been developed to provide finer resolution of rivers in 6 out of 7 hydrological regions, while the Chittagong region is excluded because of its hydrological independence from the GBM river system (Figure 6). The regional models receive boundary conditions from the General Model. A local model is developed for detailed representation of the river-floodplain system and it receives boundary conditions from the appropriate regional model. Model generated hydraulic data is utilized in agricultural, fisheries and environmental analyses. Model predicted changes in the hydraulic conditions serve as the basis for comparing alternative planning options and for formulating mitigation schemes for negative impacts.
Figure 6. Hydrological regions
It is recommended in FAP25 study by Kruger Consult (1992) that uncertainties
due to model error and random morphological changes should be taken into
account by adding a safety margin to the design water levels derived from
model output, i.e. the free board accounting for wind set-up, wave run-up
and other possible safety factors.