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Section 7: Drainage Design

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This section discusses project elements typically designed by the drainage engineer. Tasks fall into categories of hydraulic design, drainage details, and storm water pollution prevention plan preparation. Drainage design requires continuous coordination with roadway design activities.

Stormwater management can be accomplished with gravity-fed open channel or closed conduit. In places where gravity drainage is impossible or not economically feasible, pump stations will be required to drain depressed sections of the roadway.

Discharge controls are often needed to mitigate the runoff quantity and/or quality impacts. Outlet controls can reduce the rate of discharge. Retention facilities are used to control the quantity and quality of runoff discharged to receiving waters and should be considered for use as a mitigation measure to reduce the runoff impact to receiving water quality.

This section includes the following groups of tasks. Tasks are listed in approximate chronological order but may be performed concurrently, unless noted otherwise. Work performed at this stage is done after preparing stream crossing hydraulics.

50270. Prepare stream crossing hydraulics

50700. Perform hydraulic design for culverts and storm drains

50710. Perform hydraulic design for pump station(s)

50720. Prepare culvert and storm drain details

50730. Prepare pump station details

50740. Design Storm Water Pollution Prevention Plan

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Hydraulic Design

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50700: Perform hydraulic design for culverts and storm drains

Description. Roadway culvert and storm drain hydraulic design includes determining culvert and storm drain sizes and grades to handle design stormwater flows. The designer should evaluate the land use to determine the best structure for the location.

Culverts carry surface water across or from the highway right of way. They also must carry traffic and earth loads; therefore, culverts require both hydraulic and structural design. Structures measuring 20 ft. or more along the roadway centerline are classified as bridges.

Since storm drains receive water through inlets and carry the water through long underground conduit, it is desirable to maintain a minimum self-cleaning velocity in storm drains to prevent deposition and loss of capacity. Repair or replacement can be very expensive.

Pertinent Project Types. Projects involving new or retrofit culvert and storm drain systems

Responsible Party. Drainage engineer


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  • Review preliminary engineering recommendations, as-built construction plans, drainage area maps, and hydrology reports.
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  • Obtain proposed typical sections, alignments, superelevation, geometric layouts, existing and proposed utilities, construction staging, natural resource agency commitments, and preliminary cross sections from the roadway design engineer.
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  • Make a site visit to inspect existing facilities, flow patterns, and erosion conditions.
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  • Review site drainage history with maintenance and local public agency personnel.
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  • Request information about existing or proposed major outfall storm drains, and review existing and proposed detention ponding by others. Determine if detention by TxDOT is appropriate.
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  • Update hydrology design to match current and proposed roadway design.
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  • Design cross drainage facilities.
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  • Design parallel ditch and culvert facilities.
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  • Finalize all computer runs for inclusion in plans.
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  • Submit bridge class culverts to Bridge Division for preliminary layout review. See Task 50610: Prepare preliminary bridge layouts.
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  • Review design results and proposed drainage changes with local agencies.

Helpful Suggestions.

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  • Perform a site visit, preferably during a major rain event. Personally inspect items such as broken or damaged culverts, culvert end treatment type, localized flooding, sedimentation, and utilities. Taking these issues into account can be critical to the design of drainage facilities. Research commitments made to natural resource agencies.
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  • Drainage design should include consideration of pedestrian facilities, utility impacts, driveway grades, outfall and ditch erosion, wildlife habitat, and retaining wall drainage.
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  • Placement of concrete traffic barrier should be evaluated for drainage impacts.
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  • Maintain ongoing communication with the roadway design engineer regarding roadway design changes affecting drainage. Make recommendations concerning geometric modifications that would result in more desirable drainage improvements.

Critical Sequencing.

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  • Roadway culvert designs can be performed after the detailed hydrology study is completed, final roadway alignments are determined, and stream crossing hydraulics are finished.

Resource Material.

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50710: Perform hydraulic design for pump station(s)

Description. Pump station design includes pump sizing, foundation design, outfall design, power/control design, and enclosure design for facilities to handle storm water in areas that cannot drain by gravity. Pumps may also be needed for wetland restoration or storm water detention facilities. The hydraulic design of pump stations includes selecting the type, capacity, and power of the pumps, determining the on/off cycling requirements, providing for trash collection, and designing a discharge chamber.

A pump station should be protected and secured with fences, gates, grates, and locks. Ample access for working areas and maintenance vehicles must be provided since pumps are mechanical, susceptible to failure, and require extensive maintenance. For this reason, pumps should be used only when absolutely necessary. Also, backup systems should be considered.

Texas Evacuation Routes should be designed to drain by gravity only, if possible, because the likelihood of a pump station failure may be greatest during the time of most critical need.

Pertinent Project Types. Projects with mild or no slope for stormwater drainage

Responsible Party. Drainage engineer


Helpful Suggestions.

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  • Avoid the need for pump stations, if possible, as they require substantial maintenance.
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  • Perform a site visit to determine a gravity flow alternative to a pump station, if one exists. Such an alternative would likely require additional right of way or easements to be initiated as early as possible.

Critical Sequencing.

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  • Pump designs should be performed simultaneously with roadway drainage design.

Resource Material.

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Drainage Details

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50720: Prepare culvert and storm drain details

Description. The primary aim of an urban storm drain design is to limit the amount of water flowing along the gutters or ponding at low points to quantities which will not interfere with the passage of traffic or incur damage to the highway and local property. This is accomplished by placing appropriately sized inlets at the proper spacing. Culverts are used to carry water underneath a roadway; storm drains typically drain sag areas.

Culvert detail sheets typically include following elements:

Storm drain detail sheets typically include the following information:

Pertinent Project Types. Projects involving proposed pipe or box culvert construction or storm drain construction

Responsible Party. Drainage engineer


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  • Prepare plan/profile sheets for inlet and outfall ditches, generally perpendicular to roadway, as required. These situations usually require a drainage easement.
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  • Prepare plan/profile sheets for large ditches, or canals, running generally parallel to the roadway, as required. These ditches do not include typical roadside ditches detailed on the roadway plan/profile sheets.
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  • Prepare culvert cross sections.
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  • Evaluate need for a geotechnical survey for trench excavation and shoring. See Task 50600: Perform final geotechnical surveys.
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  • Prepare detention/retention facility plans, cross sections, and outfall profiles.
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  • Prepare drainage area maps; these may be superimposed on the storm drain plan.
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  • Prepare storm drain plan/profiles.
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  • Identify and obtain standard details for items such as inlets, manholes, junction boxes, and end treatment.
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  • Prepare details for non-standard inlets, manholes, and junction boxes.
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  • Prepare drainage details such as outlet protection, outlet structures, and utility accommodation details.
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  • Identify pipe strength requirements.
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  • Prepare drainage facility/quantity summaries.
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  • Finalize computer runs for inclusion in plans.
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  • Obtain necessary standard detail sheets.

Helpful Suggestions.

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  • Perform a site visit during a significant event to identify contributing drainage from adjacent properties.
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  • During detailing of drainage design, utility conflicts become defined. Utility conflicts should be avoided wherever possible by redesigning proposed drainage facilities. Whether borne by utility owners or TxDOT, utility adjustments are public costs and should be minimized when possible.
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  • The drainage engineer must coordinate construction staging with the roadway design engineer. Drainage problems can be caused by not considering conditions during construction phases.
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  • Furnish preliminary storm drain plans to local agencies, if TxDOT proposes to connect to the local agency’s system.

Critical Sequencing.

Resource Material.

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50730: Prepare pump station details

Description. Pump station detail sheets typically include the following information:

Pertinent Project Types. Projects involving pump facility construction

Responsible Party. Project manager


Helpful Suggestions.

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  • Refer to the TxDOT Hydraulic Design Manual for design considerations and hydraulic calculations.
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  • The pump station should be located outside the roadway clear zone. In cases where this is not feasible, the pump station should be positioned underground or in a protected area so it does not pose a hazard to motorists.
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  • Provide space around the facility to allow access for service vehicles and workers.
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  • The control house shall include electrical outlets and lights for convenient maintenance.
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  • Consider installing a high fence around the station to discourage vandalism.
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  • The flow line of the pipe inflow to the wet well should be at or above the pump cut off elevation to prevent stormwater from backing up in the pipe system. Consider including multiple pumps and/or alternate power sources in the design for added dependability.
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  • Controls for pump stations may be linked to Intelligent Transportation System centers for monitoring or control purposes. The Traffic Operations Division may assist in designing pump controls, or an electrical consultant may be considered.

Critical Sequencing.

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  • Pump details can be prepared during, or immediately following, pump design task.

Resource Material.

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Storm Water Pollution Prevention Plan

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50740: Design Stormwater Pollution Prevention Plan

Description. Designing erosion and sediment control devices includes determining the type and size of facilities for minimizing erosion and siltation during and after project construction. Stormwater Pollution Prevention Plans (SW3Ps) are prepared to show the construction of devices that minimize erosion and siltation during construction. Various grasses, and other typically proprietary devices, are used to control long-term erosion.

For projects that do not disturb soil (traffic signals, overlays, seal coats, etc.) a standardized General Note and selected bid items or Force Account will serve as the project SW3P.

Pertinent Project Types. Projects that will disturb one or more acres

Responsible Party. Project manager


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  • Review commitments to resource agencies.
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  • Obtain drainage designs and plans.
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  • Make a site visit to inspect existing erosion conditions.
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  • Design permanent and temporary erosion control measures to be consistent with proposed construction staging. Consider roadside safety in selecting the type of devices to include.
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  • Develop SW3P to be consistent with the requirements of the current Construction General Permit (CGP) for storm water discharges (TXR150000) published by the Texas Commission on Environmental Quality (TCEQ).
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  • Prepare the MS4 Notice of Intent (NOI) if construction stormwater will be discharged into TxDOT’s MS4 system.

Helpful Suggestions.

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  • The drainage engineer should coordinate designs with the landscape architect for revegetation. This is especially important when designing items for controlling long-term erosion.
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  • SW3P details may be shown on construction phasing plans. This is especially useful for detailing erosion and sediment control by construction stage.

Critical Sequencing.

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  • SW3P designs and plans should be prepared after the roadway drainage design and traffic control plans are substantially complete.

Resource Material.

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