II. FLOOD LOSS REDUCTION AND REVIEW OF PAST EXPERIENCES
C. Methodology for assessment
Non-agricultural flood damage can be categorized into two types: stock resources such as damage to infrastructure, public and private buildings; and flow resources such as interruptions to economic, social, educational and cultural activities. As per the Guidelines for Project Assessment by FPCO (1992), the first category of damage should be taken fully into account while the second category should not generally be included in the analysis because of estimation difficulties. Historic data on damage is treated as aggregate figures unless there are reasons to believe that the effects of the timing, depth and duration of floods need to be taken into account. Because development is continuing on the floodplains, the potential for the increasing value of damage over time is taken into account by increasing the value of flood damage in a given area by a percentage based on the past history of development in that area. Generally, an increase of 3 to 4 per cent annually is considered appropriate (FPCO, 1992).
In estimating the benefits of flood control projects, the damages to be considered are limited to those that fall between the lower limit of a flood causing zero damage, and the upper limit of a flood identical to the design flood on the damage-frequency curve (FPCO, 1992). Frequency analysis of observed water level data and the relationship between damage data and water levels is utilized to derive a damage-frequency curve. To take account of changed hydraulic condition as a result of interventions, simulation studies using mathematical models are carried out to derive the transformed damage-frequency curve. The mathematical expectation of annual damage is determined by the integration of the damage-frequency relationship.
In addition to the risk of the design flood being exceeded, there are also other major damage costs during the design life of a project resulting from unavoidable floodplain processes. Two major costs are: repair and rehabilitation costs due to retirement of earthen embankments necessitated by river bank erosion and shifting, and damage cost due to sudden flooding caused by breaching of embankment due to geo-technical problem associated with the complex geo-morphology of the floodplain. Planning practices followed in the country do not take into account such costs. Experience of flood control interventions in Bangladesh has proved that it is difficult to attain stated objectives of intervention without giving due consideration to the hydro-morphologic features of the floodplain and the socio-economic condition and cultural heritage of its inhabitants (Chowdhury et al., 1996).