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Section 2: Factors Affecting Floods

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Flood Factors

The following factors affect floods in the watershed: runoff, watershed area information, geographic location, land use, soil type, topography, vegetation, detention storage systems, flow diversions, channelization, and future conditions.

Runoff. Two main factors influence runoff from a watershed: precipitation and abstractions. Precipitation in the State of Texas is represented most significantly by rainfall, although snow, sleet, and hail can influence runoff. Rainfall rate distributions within a watershed vary both temporally and spatially. For most determinations of peak flow for use in department design and analysis efforts, assume rainfall rates not to vary within the watershed during the rainfall event.

Generally, the entire volume of rainfall occurring on a watershed does not appear as runoff. Losses, known as abstractions, tend to reduce the volume of water appearing as runoff. Abstractions of precipitation in its evolution into runoff are numerous. However, for the typical highway drainage design problem, only six abstractions are commonly considered. They are shown in the order of their significance to the runoff.

  • Infiltration—The amount of the precipitation that percolates into the ground in the watershed. This abstraction is a function of soil type and characteristics, terrain slopes, and ground cover.
  • Depression storage—The precipitation stored permanently in inescapable depressions within the watershed. It is a function of land use, ground cover, and general topography.
  • Detention storage—The precipitation stored temporarily in the flow of streams, channels, and reservoirs in the watershed. It is a function of the general drainage network of streams, channels, ponds, etc. in the watershed.
  • Interception—The precipitation that serves to first “wet” the physical features of the watershed (e.g., leaves, rooftops, pavements). It is a function of most watershed characteristics.
  • Evaporation—The precipitation that returns to the atmosphere as water vapor by the process of evaporation from water concentrations. It is mostly a function of climate factors, but it is associated with exposed areas of water surface.
  • Transpiration—The precipitation that returns to the atmosphere as water vapor and that is generated by a natural process of vegetation foliage. It is a function of ground cover and vegetation.

The specific consideration of each of these abstractions is not usually explicit in the many hydrologic methods available.

Watershed Area Information. Most runoff estimation techniques use the size of the contributing watershed as a principal factor. Generally, runoff rates and volumes increase with increasing drainage area. The size of a watershed will not usually change over the service life. However, agricultural activity and land development may cause the watershed area to change. Diversions and area changes due to urbanization and other development inevitably occur. Try to identify or otherwise anticipate such circumstances.

The watershed shape usually will affect runoff rates. For example, a long, narrow watershed is likely to experience lower runoff rates than a short, wide watershed of the same size and other characteristics. Some hydrologic methods accommodate watershed shape explicitly or implicitly; others may not. If a drainage area is unusually bulbous in shape or extremely narrow, the designer should consider using a hydrologic method that explicitly accommodates watershed s