The land area of the watershed upstream of the area of interest is the most crucial upslope factor in determining flood magnitude. For watersheds with a surface area of less than approximately 30 square miles or 80 square kilometers, rainfall intensity ranks second in importance. For larger watersheds, the main channel slope is the second most important factor. For small and large watersheds, channel slope and rainfall intensity rank third and fourth, respectively.
The amount of time it takes for runoff from the most distant point in the upstream drainage area to get to the point where the drainage channel that controls flooding in the area of interest is located is known as the Time of Concentration. The critical duration of the area of interest’s peak rainfall is determined by the time of concentration. Roof and parking lot drainage structures may require only a few minutes of intense rainfall, whereas river basins would require several days of cumulative rainfall.
The downstream conditions that water encounters as it moves downhill eventually slow it down. The ocean or some coastal flooding bars that form natural lakes frequently serve as the final limit on coastal flooding lands. Changes in elevation, such as tidal fluctuations, are significant factors in coastal and estuarine flooding in lowlands. Large bodies of water may also experience elevation shifts as a result of less predictable events like storm surges and tsunamis.
The geometry of the flow channel, particularly the depth of the channel, the flow speed, and the quantity of sediments in the channel are all factors that influence the elevation of the flowing water above the restriction. A closer point may control for lower water levels while a more distant point controls for higher water levels because the actual control point for any given drainage reach may change with changing water elevation.
The growth of vegetation, the accumulation of ice or debris, and the construction of bridges, buildings, or levees within the flood channel all have the potential to alter the effective geometry of the flood channel.