The flood hazard assessment is related to the estimation of probable future discharge and associated parameters. Some of the important parameters are the flood peak discharges and their frequency of occurrence at the given river cross-sections. It is also possible to develop a hydrograph of flooding for the derived peak discharge. A hydrograph is a graphical representation of the variation of the height of water surface above some fixed datum (stage) with time. If a significant number of years of river level or discharge data are available for a single river cross section , it is possible to determine the flood frequency by which a given rivers peak discharge will be equal or exceeded. The return period is the average interval of the time in years that occur between flood events having peak discharge exceeding some assigned value. But these analyses are rather tentative and could be modified accordingly whenever more data are available.

One of the most effective methods of determining extreme flood levels is through the detailed survey of the river basin and questioning of local inhabitants. The old channel shape, boulder impact marks on the stems and branch of trees, the amount of debris deposited on the flood plains and the sedimentary structures may provide indirect information on the discharge. Old inhabitants can very often recall catastrophic conditions especially if they occurred during their earlier years of life. Frequency of flood peak discharge, probable maximum peak flows and associated hydrographs can also be estimate from the metrological data. The method is based on the relationship between the precipitation and the resulting flood discharges. Another important aspect of flood hazard assessment is the critical time of rainfall. It depends on factors such as geologic condition of soil and rock, slope of area, the infiltration and the length of path of flow. The increase in gradient enhances the speed of run off the length of path increases the time required by the rainwater to reach the area under hazard. The geological conditions of the catchments may strongly influence the flood. They contribute to the river channel morphology and the erosion and sedimentation rate. If the fragile rocks and soil are present in river gorges, the landslide damming may occur and as a result, the peak discharge may be altered considerably. Photo credit: sciencedirect.com