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FLOOD HYDROLOGY
The problem of flood hydrology can be stated as follows:
Given storm precipitation and watershed [geometric, hydrologic, and hydraulic] properties, what is the flood hydrograph at the basin outlet?
This is the question that RAINFLO seeks to answer. In order to understand how it works, let us examine a few basic principles of flood hydrology.
The flood hydrograph is characterized by its peak, its volume, and its shape. The peak flood is a function of the following properties:
- rainfall distribution
- runoff curve number
- choice of unit hydrograph
- primarily the slope and, secondarily, the shape of channel reaches.
The hydrograph volume is a function of the runoff curve number. The larger the runoff curve number, the larger the hydrograph volume.
The hydrograph shape is a function of the following properties:
- rainfall distribution
- primarily the slope and, secondarily, the shape of channel reaches.
The steeper the slope and wider [rectangular] the channel reach, the less runoff diffusion and the more the tendency for the hydrograph to steepen. Conversely, the shallower the slope and the narrower [and deeper] the channel reach, the more runoff diffusion and the more the tendency for the hydrograph to flatten. Hydrograph steepening or flattening can only be calculated with nonlinear [variable] routing parameters.
An effective way to reduce flood risk is through either: (1) soil storage, or (2) surface storage. By suitable watershed management, soil storage seeks to reduce the applicable curve number. Surface storage seeks to attenuatie the flood peak by storing flood hydrographs in reservoirs.
A decrease in curve number will lead to a decrease in flood risk downstream. An increase in surface storage will lead to a decrease in flood risk downstream.