Section 2: Factors Affecting Floods
Anchor: #i1025589Flood 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 shape.
The response of a watershed to runoff may vary with respect to the direction in which a storm event passes. Generally, for design purposes, the orientation of the watershed may be ignored because it is usual to assume uniform rainfall distribution over the watershed.
Geographic Location. The geographic location of the watershed within the State of Texas is a significant factor for the drainage designer. Rainfall intensities and distributions, empirical hydrologic relations, and hydrologic method applications vary on the basis of geographic location. You should use hydrologic methods and parameters that are appropriate for the specific location.
Land Use. Land use significantly affects the parameters of a runoff event. Land use and human activity within most watersheds vary with respect to time. For example, a rural watershed can be developed into a commercial area in a matter of weeks. Factors subject to change with general variations in land use include the following:
- permeable and impermeable areas
- vegetation
- minor topographic features
- drainage systems.
All of these factors usually affect the rate and volume of runoff that may be expected from a watershed. Therefore, carefully consider current land use and future potential land use in the development of the parameters of any runoff hydrograph.
Land Use Changes. Diversions and area changes due to urbanization and other development inevitably occur. Try to identify or otherwise anticipate such circumstances.
Soil Type. The soil type can have considerable effect on the discharge rates of the runoff hydrograph; the soil type directly affects the permeability of the soil and thus the rate of rainfall infiltration. The Natural Resources Conservation Service (NRCS) is an excellent repository for information about soils in Texas. The hydrologic procedure used may require specific data concerning the soil type.
Topography. Topography mostly affects the rate at which runoff occurs. The rate of runoff increases with increasing slope. Furthermore, rates of runoff decrease with increasing depression storage and detention storage volumes. Many methods incorporate a watershed slope factor, but fewer methods allow the designer to consider the effects of storage on runoff.
Vegetation. In general, runoff decreases with increasing density of vegetation; vegetation helps reduce antecedent soil moisture conditions and increases interception such as to increase initial rainfall abstractions. Vegetative characteristics can vary significantly with the land use; therefore, consider them in the assessment of potential future conditions of the watershed.
Detention Storage Systems. Detention storage systems are common in urban areas mostly due to governmental requirements aimed at controlling increased runoff from developed areas. The department designer should identify any detention storage systems that might exist within the subject watershed. A detention storage facility can attenuate the runoff hydrograph, thus reducing the peak discharge. The department may design facilities that involve detained storage to conform to federal and state environmental regulations, to cooperate with local ordinances or regulations, or where you deem flood attenuation necessary.
Flow Diversions. Flow diversions within a watershed can change the runoff travel times and subsequent peak discharge rates. They can decrease discharge at some locations and increase discharge elsewhere. Flow diversions may redirect flow away from a location during light rainfall but overflow during heavy rainfall. Make an assessment of the likely effect of diversions that exist within the watershed. Also, ensure that you minimize the potential impact of necessary diversions resulting from your highway project.
Channelization. Channelization in an urban area entails the following:
- improved open channels
- curb and gutter street sections
- inverted crown street sections
- storm drain systems.
Any of these channelization types serve to make drainage more efficient. This means that flows in areas with urban channelization can be greater, and peak discharges occur much more quickly than where no significant channelization exists.
Future Conditions. Changes in watershed characteristics and climate directly affect runoff. A reasonable service life of a designed facility is expected. Therefore, base the estimate of design flooding upon runoff influences within the time of the anticipated service life of the facility.
Anchor: #i1025772Prediction Information
In general, consider estimates for future land use and watershed character within some future range. It is difficult to predict the future, but you should make an effort at such a prediction, especially with regard to watershed characteristics. Landowners, developers, realtors, local and state and federal officials, and planners can often provide information on potential future characteristics of the watershed.
In estimating future characteristics of the watershed, consider changes in vegetative cover, surface permeability, and contrived drainage systems. Climatic changes usually occur over extremely long periods of time such that it is not usually reasonable to consider potential climatic changes during the anticipated life span of the facility.