FLOODING DUE TO RUNOFF

The most common type of flood results from the runoff of rain or of melted snow and ice, or a combination of these, towards the site. Runoff occurs when the amount of water from precipitation falling or melting in a given period exceeds the losses of water by evaporation, transpiration, interception (such as by the leaves of trees), infiltration into the ground and storage in depressions in the ground. 

Design basis precipitation is the estimated depth of precipitation for a given duration, drainage area and time of year, for which there is a specified probability of exceedance. The design basis flood for a given duration and drainage area should approach and approximate the maximum value that is considered to be physically possible. 

The positions of the storms over the basin should be selected in such a way that the maximum runoff (in terms of volume or peak water level, whichever is critical) will occur. 

In basins where snow melt can contribute significantly to the flood hazard, special consideration should be given to the maximization of a combined event of rain plus snow melt. To compute the maximized contribution of snow melt to the flood in such basins, the seasonal accumulation of snow should be maximized and a critical melt sequence should be selected. A design basis precipitation event appropriate to the time of year should then be added to the maximized snow melt event, and the additional snow melt due to the precipitation (if it is rain) should be included. 

Losses of water should be estimated by comparing the incremental precipitation with the runoff for the recorded storms. Usually losses are expressed as an initial loss followed by a contmuing constant loss over a period 

---