Editing Flood (section)

===Upslope factors===
[[File:Ein Avdat Flood 1.JPG|thumb|Flash flood in Ein Avdat, Negev, Israel]]
River flooding is often caused by heavy rain, sometimes increased by melting snow. A flood that rises rapidly, with little or no warning, is called a [[flash flood]]. Flash floods usually result from intense rainfall over a relatively small area, or if the area was already saturated from previous precipitation.

The amount, location, and timing of water reaching a drainage channel from natural precipitation and controlled or uncontrolled reservoir releases determines the flow at downstream locations. Some precipitation evaporates, some slowly percolates through soil, some may be temporarily sequestered as snow or ice, and some may produce rapid runoff from surfaces including rock, pavement, roofs, and saturated or frozen ground. The fraction of incident precipitation promptly reaching a drainage channel has been observed from nil for light rain on dry, level ground to as high as 170 percent for warm rain on accumulated snow.<ref>Babbitt, Harold E. & Doland, James J., ''Water Supply Engineering'', McGraw-Hill Book Company, 1949</ref>

Most precipitation records are based on a measured depth of water received within a fixed time interval. ''Frequency'' of a precipitation threshold of interest may be determined from the number of measurements exceeding that threshold value within the total time period for which observations are available. Individual data points are converted to ''intensity'' by dividing each measured depth by the period of time between observations. This intensity will be less than the actual peak intensity if the ''duration'' of the rainfall event was less than the fixed time interval for which measurements are reported. Convective precipitation events (thunderstorms) tend to produce shorter duration storm events than orographic precipitation. Duration, intensity, and frequency of rainfall events are important to flood prediction. Short duration precipitation is more significant to flooding within small drainage basins.<ref>Simon, Andrew L., ''Basic Hydraulics'', John Wiley & Sons, 1981, {{ISBN|0-471-07965-0}}</ref>

The most important upslope factor in determining flood magnitude is the land area of the watershed upstream of the area of interest. Rainfall intensity is the second most important factor for watersheds of less than approximately {{convert|30|sqmi|sigfig=1|disp=or}}. The main channel slope is the second most important factor for larger watersheds. Channel slope and rainfall intensity become the third most important factors for small and large watersheds, respectively.<ref name="Simon">Simon, Andrew L., ''Practical Hydraulics'', John Wiley & Sons, 1981, {{ISBN|0-471-05381-3}}</ref>

[[Time of Concentration]] is the time required for runoff from the most distant point of the upstream drainage area to reach the point of the drainage channel controlling flooding of the area of interest. The time of concentration defines the critical duration of peak rainfall for the area of interest.<ref name="Urquhart">Urquhart, Leonard Church, ''Civil Engineering Handbook'', McGraw-Hill Book Company, 1959</ref> The critical duration of intense rainfall might be only a few minutes for roof and parking lot drainage structures, while cumulative rainfall over several days would be critical for river basins.