Section 2: Site Investigation Data
Anchor: #i1000225Introduction
TxDOT policy requires a hydrologic and hydraulic analysis for projects that involve:
- new locations
- replacing facilities
- widening existing locations
Drainage Area Characteristics
Refer to linked “File 3a” for a Documentation Checklist for Hydraulic Design Project References based on the following paragraphs.
Size. Drainage area size is usually important for estimating runoff characteristics. Determine the size of the drainage by one of the following methods:
- Conduct direct field surveys with conventional surveying instruments.
- Use topographic maps together with field checks for artificial barriers such as terraces and ponds. (USGS topographic maps are available for many areas of the state through retail outlets for maps and surveying supplies. Many municipal and county entities as well as some developers have developed topographic maps of their own. Determine the suitability and usefulness of all these maps.)
- Use any other available resources.
Topography. Estimate relief and slope characteristics of the watershed by one or more of the methods listed above for drainage area sizes. Most hydrologic procedures used by TxDOT depend on watershed slopes and other physical characteristics.
Soil Type. Watershed soil type(s) and associated characteristics correlate with infiltration, interception, depression storage, and detention storage. Use Natural Resources Conservation Service publications, including maps, reports, and work plans, to identify and quantify soil parameters in the watershed. See U.S. Department of Agriculture for contact information.
Vegetation. Present and future vegetation characteristics influence the amount and rate of watershed runoff as well as the streamflow patterns expected in and around the drainage facility. Look at surveys or obtain data from a site visit.
Anchor: #i1000301Land Use
There are several forms of land use data and many sources from which to obtain them.
Development Prediction Source. Ordinarily, the drainage facility design includes a reasonable anticipation of service life. Because the facility must accommodate potential flows during that service life, consider possible future development of the watershed. Predicting future development of a watershed is difficult. However, you can estimate future development by interviewing landowners, developers, officials, planners, local and regional planning organizations, realtors, and local residents.
Watershed Characteristic Sources. Look at master plans for development from city planning departments. Land use data are available in different forms, including topographic maps, aerial photographs, zoning maps, satellite images, and geographic information systems. Municipalities have records and maps of storm drain systems and channel improvements.
Anchor: #i1000321Stream Course Data
Streams are classified as fol