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 follows:
- rural, urban, or a mix
- unimproved to improved
- narrow to wide-wooded
- rapid flow to sluggish
Profile. Extend the stream profile sufficiently upstream and downstream of the facility to determine the average slope and to encompass any channel changes or aberrations. USGS recommends a minimum distance of 500 ft. (150 m) both upstream and downstream for a total of 1000 ft. (300 m) or a distance equal to twice the width of the floodplain, whichever is greater. Topographic maps published by USGS are useful in determining overall channel slopes.
Channel Location. Note the location of the main channel and any subchannels, creeks, and sloughs within the profile section.
Cross Sections. Cross sections must represent the stream geometry and contain the highest expected water-surface elevation to be considered. For hydraulic computations, use cross sections that are perpendicular or normal to the anticipated direction of flow. In some instances, particularly in wide floodplains where a single straight line across is not adequate, break the cross section into segments for a dogleg effect as shown in Figure 4-2. Adjacent cross sections should not cross each other.
Figure 4-2. Dog-legged Cross Section
The minimum number of cross sections is four, located as follows:
- At the beginning of the profile stretch
- At the downstream face of the structure (or where the downstream face will be)
- At the upstream face of the structure (or where the upstream face will be)
- At the end of the profile stretch
Additional cross sections are necessary at each change in roughness, slope, shape, or floodplain width. Take enough cross sections to analyze fully the stream flow.
Do not leave the choice of the typical cross section entirely to the field survey party. Carefully consider the location and orientation of the cross section used in the channel analysis without regard to surveyor convenience or expedience.
Locate sections as follows:
- Sections along the right-of-way line can be misleading hydraulically because they may represent only local, cleared conditions that do not reflect the stream reach. For similar reasons, avoid cross sections along utility easements and other narrow cleared areas.
- Avoid local depressions or crests that are not typical of a whole stream reach.
- Generally try to space sections about 1.5 to 2 times the approximate floodplain width. A notable exception to this is at structures where more definition is needed.
Roughness Characteristics. The Manning’s equation for uniform flow is the most commonly used conveyance relation in highway drainage design. Note and record the physical details of the streambed and floodplain; you will use them later to determine the Manning’s roughness coefficients (n values). Details include vegetation type and density, material (rock type, clay soil, gravel), trash, streambed shape, cross section geometry, and any item that may affect streamflow during normal and flood conditions.
Flow Controls. Note anything upstream and downstream within the profile section, including the following:
- Any downstream confluences
- Significant choking sections
- Bridges and low water crossings
- Abrupt meanders
- Heavily vegetated areas
- Material borrow pits in the floodplain
Include all observations about size, type, location, and flow over or through. Bridge data should include span lengths and types and dimensions of piers.
Reservoirs. Note any reservoirs and ponds along with their spillway elevations and operations or other control operations. Dams with hydroelectric generators may raise water levels significantly during generator operations.
The following organizations may have complete reports concerning the operation, capacity, and design of proposed or existing conservation and flood-control reservoirs:
- Natural Resources Conservation Service (NRCS)
- Corps of Engineers (USACE)
- Bureau of Reclamation
- Texas Natural Resource Conservation Commission (TNRCC)
- Municipalities
Flood Stages. Obtain information on historic flood stages from TxDOT personnel, city and county officials, and local residents. If possible, observe the structure under flood conditions to learn about the stream behavior. When possible, take videos and photographs of the flood action at or near the structure for use in future studies. Determine the direction of stream lines with relation to the low flow channel, estimated velocity, estimated drifting material (amount and size), natural tendency for erosion in the channel, the drop in water surface elevation from the upstream side to the downstream side of the structure, and the highest stage with the date of occurrence.
Anchor: #i1000516Geotechnical Information
Soil Properties. A geotechnical report provides information about the soils in the area and soils used on highway projects. The detail of such reports can vary greatly but usually will include the following:
- Soil type, soil density (blow count), and depth for each soil type
- Soil properties such as acidity/alkalinity, resistivity, and other significant constituents
- Presence, depth, and type of bedrock
- Sieve analyses (D50 and D90 values)
Scour Observations. Note the presence of scour around pilings and abutments. Record size, depth, and location of each scour hole. Also record any deposition of material including type (rock, gravel, dirt, etc.), location, and depth.
Stream Stability. Erosion problems may occur in a stream system even without the presence of a bridge. Record the following data:
- Any occurrence or possibility of streambed degradation (head cutting). Head cutting may be caused by dredging or mining downstream or channel modifications such as straightening.
- Signs of bank slippage and erosion such as buildings located closer to the bank than seem reasonable, trees growing at odd angles from the bank, exposed tree roots, and trees with trunks curved near the ground.
- The location and likely direction of lateral migration (meanders).
For more information, see the discussion on stream stability in Chapter 7.
Anchor: #i1000579Adjacent Properties
Note the location of any driveways, utilities, and structures adjacent to the project site that will be affected by construction. Note the elevations of any improvements or insurable structures near the proposed site that may be affected by a rise in water surface elevations up through and including the 100-year event.