Estimation of Floods Using Channel Geometry

Knowledge of high flood discharges is essential in management and operation of watershed streams. Floods of high discharges often cause flooding of floodplain areas with associated large channel or flow width. Measurements of high flood discharges under these conditions are often difficult to be made. Satellite data images, air photographs and flood marks provide valuable information about the extent of floods and their associated flow width while difficulty arises in estimating the magnitude of the flood. An estimation of the magnitude of high flood discharges is possible based on information about channel geometry. The data needed are flood channel width caused by the flood and channel conditions before the flood. This information is substituted into analytical formula developed based on extremal equilibrium concepts. The extremal concepts include extremal energy dissipation, extremal sediment efficiency and extremal friction factor. It is shown that when the discharge is correlated only to channel width, the three concepts are equivalent. The flood estimate assists in implementing emergency and evacuation plans for downstream residents and structures across flooded channels utilising observations of flooded upstream watershed reaches. The estimation also improves knowledge on flood records and channel hydrology when the approach is applied to historical data.