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Section 7: Drainage Design

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Overview

This section discusses project elements typically designed by the drainage engineer. For information on designing bridge-class culverts, refer to Section 6, Bridge Design. 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.

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. (See 5150: Prepare stream crossing hydraulics).

  • Hydraulic Design

5540. Perform hydraulic design for culverts and storm drains

5560. Perform hydraulic design for pump station(s)

  • Drainage Details

5570. Prepare culvert and storm drain details

5590. Prepare pump station details

  • Storm Water Pollution Prevention Plan

5600. Design Storm Water Pollution Prevention Plan

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Hydraulic Design

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5540: 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 storm water flows.

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

Responsible Party. Drainage engineer

Sub-tasks.

  • Review preliminary engineering recommendations, as-built construction plans, drainage area maps, and hydrology reports.
  • Obtain proposed typical sections, alignments, superelevation, geometric layouts, existing and proposed utilities, construction staging, resource agency commitments, and preliminary cross sections from the roadway design engineer.
  • Make a site visit to inspect existing facilities, flow patterns, and erosion conditions.
  • Review site drainage history with maintenance and local public agency personnel. 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.
  • Update hydrology design (see the Hydraulic Design Manual) to match current, proposed roadway design.
  • Design cross drainage facilities (see the Hydraulic Design Manual).
  • Design parallel ditch and culvert facilities (see the Hydraulic Design Manual).
  • Finalize all computer runs (see the Hydraulic Design Manual) for inclusion in plans.
  • Data must be submitted to the Design Division for review and approval if an individual storm drain has an outfall flow exceeding 200 cfs (5.66 m3/s) for the design year storm.
  • Review design results and proposed drainage changes with local agencies.

Helpful Suggestions.

  • 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 resource agencies.
  • Drainage design should include consideration of pedestrian facilities, utility impacts, driveway grades, outfall and ditch erosion, and retaining wall drainage.
  • Placement of concrete traffic barrier should be evaluated for drainage impacts.
  • Maintain on-going 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. 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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5560: 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 flows in sump areas. Pumps may also be needed for wetland restoration or storm water detention facilities. Generally, pump stations are used when the existing outfall is too high above the elevation required to drain a roadway with a gravity drain. The hydraulic design of pump stations includes selecting the type, capacity, and power of the pumps, determining the on- and off- cycling requirements, providing for trash collection, and designing a discharge chamber. Pumps require extensive maintenance, since they are mechanical and susceptible to failure. For this reason, pumps should be used only when absolutely necessary. Also, backup systems should be considered.

Pertinent Project Types. Projects involving sump storm water conditions.

Responsible Party. Drainage engineer

Sub-tasks.

  • Coordinate with Design Division before designing pump stations.
  • Review preliminary engineering recommendations, drainage areas, and hydrology report.
  • Make a site visit to inspect existing flow patterns near proposed sump and outfall areas. Consider the effect of the pump effluent on the outfall recipient.
  • Order additional survey data if necessary for outfall design.
  • Modify design storm water flow rates as necessary.
  • Design outfall (see the Hydraulic Design Manual).
  • Select pump type and size (see the Hydraulic Design Manual).
  • Design control house.
  • Analyze for, and design to avoid, flotation of pump wet well.
  • Request input from TxDOT or local public agency personnel responsible for maintenance of pump systems.

Helpful Suggestions.

  • Avoid the need for pump stations if possible as they require substantial maintenance.
  • 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. Pump designs should be performed simultaneously with roadway drainage design.

Resource Material.

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Drainage Details

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5570: 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.

Culvert detail sheets typically include following elements:

  • culvert cross sections
  • ditch plan/profiles
  • miscellaneous drainage details
  • culvert hydraulics (table or computer output)
  • drainage facility/quantity summaries
  • standard details

Storm drain detail sheets typically include the following information:

  • drainage areas
  • storm drain plan/profiles
  • ditch plan/profiles
  • detention/retention facility details
  • storm drain hydraulics (table or computer output)
  • drainage facility/quantity summaries
  • outfall details
  • inlet details
  • manhole and junction box standards
  • location of underground utilities.

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

Responsible Party. Drainage engineer

Sub-tasks.

  • Prepare plan/profile sheets for inlet and outfall ditches (generally perpendicular to roadway), as required. These situations usually require a drainage easement.
  • 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.
  • Prepare culvert cross sections. (See the example in the PS&E Preparation Manual Chapter 2, Section 2, Culvert Cross Sections, Layout and Detail Sheets).
  • Evaluate need for a geotechnical survey (see 5460: Perform final geotechnical surveys) for trench excavation and shoring.
  • Prepare detention/retention facility plans, cross sections, and outfall profiles.
  • Prepare drainage area maps; these may be superimposed on the storm drain plan.
  • Prepare storm drain plan/profiles.
  • Identify and obtain standard details for items such as inlets, manholes, junction boxes, and end treatment.
  • Prepare details for non-standard inlets, manholes, and junction boxes.
  • Prepare drainage details such as outlet protection, outlet structures, and utility accommodation details.
  • Identify pipe strength requirements.
  • Prepare drainage facility/quantity summaries.
  • Finalize computer runs for inclusion in plans.
  • Obtain necessary standard detail sheets.

Helpful Suggestions.

  • Perform a site visit during a significant event to identify contributing drainage from adjacent properties.
  • 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.
  • The drainage engineer must coordinate construction staging with the roadway design engineer. Drainage problems can be caused by not considering conditions during construction phases.
  • Furnish preliminary storm drain plans to local agencies if TxDOT proposes to connect to the local agency’s system.

Critical Sequencing. Storm drain and culvert drainage details can be prepared during, or immediately following, design of storm drain and culvert drainage. (See 5540: Perform hydraulic design for culverts and storm drains).

Resource Material.

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5590: Prepare pump station details

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

  • wet well details
  • control house structural details
  • outfall plan/profiles
  • control panel details
  • electrical service/motor control center details
  • electrical wiring diagrams
  • miscellaneous drainage details
  • pump hydraulics (table or computer output)
  • quantity summaries.

Pertinent Project Types. Projects involving pump facility construction.

Responsible Party. Project manager

Sub-tasks.

  • Prepare drainage area maps for areas draining to the pump station
  • Prepare pump outfall plan/profiles
  • Prepare control house details
  • Prepare electrical service/motor control center details
  • Prepare control panel details
  • Prepare backup generator details and specifications
  • Prepare wiring diagrams
  • Prepare structural details for pump house wet wells and site plans
  • Prepare pump details and specifications
  • Prepare miscellaneous drainage details
  • Prepare miscellaneous pump station details
  • Prepare quantity summaries
  • Finalize hydraulic analysis or computer runs for inclusion in plans
  • Determine a suitable source of energy for the backup generator (gasoline, diesel, gas).

Helpful Suggestions.

  • Refer to the TxDOT Hydraulic Design Manual for design considerations and hydraulic calculations.
  • 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.
  • Provide space around the facility to allow access for service vehicles. The pump housing should include electrical outlets and lights for convenient maintenance.
  • The control house shall include electrical outlets and lights for convenient maintenance.
  • Consider installing a high fence around the station to discourage vandalism.
  • The flow line of the pipe inflow to the wet well should be at or above the pump cut-off elevation to prevent storm water from backing up in the pipe system. Consider including multiple pumps and/or alternate power sources in the design for added dependability.
  • 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. 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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5600: Design Storm Water 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. Storm Water 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.

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

Responsible Party. Project manager

Sub-tasks.

  • Review commitments to resource agencies.
  • Obtain drainage designs and plans.
  • Make a site visit to inspect existing erosion conditions.
  • 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.
  • 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).

Prepare the Notice of Intent (see the Environmental Manual).

Helpful Suggestions.

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

Critical Sequencing. 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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