The preliminary drainage design is performed to a sufficient level of detail to determine the placement, size and performance of proposed drainage structures on a project. The proposed drainage structures are placed on the geometric schematic.

The design of these structures should provide sufficient information to determine ROW needs and/or temporary or permanent drainage easements.

A hydrologic study is performed to estimate flood magnitudes (i.e., Annual Exceedance Probability (AEP)) caused by precipitation. An analysis provides the designer with fundamental data necessary to perform preliminary sizing of drainage facilities and bridges. Data compiled includes peak runoff (discharge) and discharge hydrographs. Refer to TxDOT’s

Hydraulic Design Manual

to determine what projects require a hydrologic study to be performed and methods to use.

Other valuable reports to obtain include soil surveys, U.S. Natural Resources Conservation Service (NRCS) reports, as well as USGS and FEMA flood maps.

Coordinate with LGs for any type of zoning and land use maps, or specific local requirements to incorporate when performing the hydrologic design.

The purpose of the hydraulic design is to determine the approximate elevations and sizes of cross drainage structures and to establish their effects on the roadway profile. The analysis conducted must result in an estimate of the most efficient, cost-effective drainage facilities that can accommodate the design storm.

A hydraulic analysis is required to create the roadway's preliminary profiles for all projects affecting existing drainage, including those that add a concrete median barrier to an existing facility.

The type of drainage facilities provided in the hydraulic design will be determined by the highway classification, ROW, geometry and other considerations. The primary focus at this stage is to balance traffic safety and hydraulic efficiency and to comply with any regulations such as FEMA.

Other major considerations must be determined, such as the need for large storm drain structures, detention ponds, pump stations, and other hydraulic facilities; these must be assessed for cost and ROW requirements. Refer to TxDOT’s

Hydraulic Design Manual

for specific details in performing the hydraulic design.

A preliminary drainage report is developed and submitted for review by the District and DES - H&H Section. The preliminary drainage report details all of the hydrologic and hydraulic analysis and design including the scour analysis for bridge structures over streams.