7.7 Drainage Design
Drainage design includes hydrologic and hydraulic (H&H) analysis. Hydrologic analysis provides estimates of flood magnitudes (i.e., Annual Exceedance Probability (AEP)) as a result of precipitation. These estimates consider processes in a watershed that transform precipitation into runoff and that transport water through the system to a project’s location. A hydraulic analysis determines the water levels and how fast the water moves based on the calculated AEP for open channels, bridge structures, culverts, storm drains and stormwater management.
7.7.1 Final Hydrologic Design
Hydrologic analysis is the most important step prior to hydraulic design to establish stormwater flow rates, flow volumes, and locations of inflow and outflow to the highway facility for significant drainage areas. Recommendations from these analytical studies can affect such major items as roadway alignments, bridge lengths, bridge lateral restraints, bridge foundations, and channel design.
The initial hydrologic study is typically prepared during preliminary engineering (see
). During final design, a refinement of the original study may be all that is needed. This refinement is usually needed to reflect detailed field survey data or a change in a basic design condition, an assumption, or to reflect revised methodology, or if there has been a significant delay between preliminary engineering and final design development.
If the project is in a tidal area, consider storm surge (tide) analysis.
Specific tasks to be performed include:
- Identify any new, relevant data;
- Verify validity of previous hydrologic study and determine if the study method used is still appropriate;
- Evaluate any existing hydrologic data/results from previous studies and update as appropriate or perform new hydrologic analysis for proposed hydraulic structure locations;
- Based on the model, determine whether watershed revisions change stream water surface elevations; and
- Coordinate with the local FEMA floodplain administrator (FPA) for changes to water surface elevations and flood maps.
7.7.2 Final Hydraulic Design
The initial hydraulic design is performed during preliminary engineering (see
). During final design, refinement of the hydraulic design is needed to incorporate final roadway alignments and profiles into the design. The design storm frequency should be verified and all allowable velocities, allowable backwater/headwater, FEMA rules and regulations, National Flood Insurance Program (NFIP) requirements along with any other local or state agency requirements should be confirmed.
Coordinate with the local FEMA FPA early if applicable. The FPA can advise which base model to use and how much, if any, increase in backwater will be allowed. The FPA may also provide a copy of the original model or help locate one. It is advised to work closely with the FPA throughout the project.
Drainage design requires continuous coordination with roadway design activities.
Hydraulic design may result in the need for drainage easements in areas not already owned by or classified as waters of the State. In such instances, this task will have direct input into reviewing and obtaining additional ROW, access control, and easement requirements.
7.7.3 Final Drainage Report
A final drainage report must be prepared and submitted along with finalized hydraulic calculation sheets to the District for review and then submitted to DES – H&H section for review and approval before the Final (90%) milestone submittal. The final drainage report should be
signed and sealed
by the drainage engineer of record.The drainage report and H&H calculations should be retained in the District project File of Record for permanent reference (refer to the HDM for more information on specific documentation retention requirements). Retaining these records provides many benefits, including:
- Ease of reference for future alteration or rehabilitation of the subject drainage structure;
- Justification of design decisions in case of future challenges or litigation;
- Valuable reference information for the design of other structures that cross the same stream or are in the same watershed; and
- Proof of intended compliance with regulations such as NFIP rules.
7.7.4 Channels
A roadside drainage channel is an open channel usually paralleling the highway embankment and within limits of the ROW. The primary function of a drainage channel is to collect surface runoff from the roadway and areas that drain to the ROW and to convey the accumulated runoff to acceptable outlet points.
Drainage channels must be designed to carry the design runoff and to accommodate excessive storm water with minimal roadway flooding or damage. The design frequency should correspond with the storm drain frequency. Refer to the RDM for details on ditch shape design and side ditches. When the RDM requirements can’t be met, the channel must be enclosed in a storm drain system (see
).
Refer to the HDM for more guidance on the design of channels.
7.7.5 Culverts
A culvert conveys surface water through the roadway embankment, away from the highway ROW, or into a channel along the ROW. In addition to the hydraulic function, the culvert must also support construction and highway traffic and earth loads; therefore, culvert design involves both hydraulic and structural design.
Culvert hydraulic design includes determining culvert sizes and grades to handle design stormwater flows. The designer must evaluate the land use to determine the best structure for the location.
Culverts with a total span greater than or equal to 20 feet are considered bridge-class culverts and are analyzed as a bridge (see
).
7.7.6 Bridges
Bridges enable streams to maintain flow conveyance and to sustain aquatic life. Bridges include bridge structures and bridge-class culverts.
Stream crossings at highways often involve a constricted flow opening. The designer establishes a design storm frequency and other criteria and determines the size and type of opening. The design storm frequency is established considering factors such as functional classification of highway, size of the stream or by performing a risk assessment. Other criteria include allowable velocities, allowable backwater/headwater, FEMA rules and regulations, the NFIP, and any other local or state agency requirements.
FHWA requires a bridge scour evaluation as part of the hydraulic design process for span bridges across streams. The results of such an analysis may highlight the need for design adjustments such as increasing opening size, deeper foundations, pier and abutment protection, or other mitigation measures. Scour countermeasure design is approved by the Geotechnical Section of BRG. Include the bridge scour evaluation in the final drainage report (see
).
Bridge class culverts may require inlet and outlet protection for scour and debris control. Inlet scour results from the flow contraction as the culvert barrel constricts the natural channel. Scour at culvert outlets is a common occurrence; sediment and debris in a turbulent flow can be erosive; however, a formal scour design is not required for bridge class culverts.
7.7.7 Storm Drains
The primary aim of a 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.
Since storm drains receive water through inlets and carry the water through long underground conduits, it is desirable to maintain a minimum self-cleaning velocity in storm drains to prevent deposition and loss of capacity. Repair or replacement of storm drains can be expensive.
Considerations in the hydraulic design of storm drains include:
- Storm drains which drain sag points where runoff can only be removed through the storm drainage system must be designed for a minimum 50-year frequency storm;
- 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; and
- Perform clash detection to eliminate conflicts with existing or proposed utilities.
7.7.8 Stormwater Management
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 must be considered for use as a mitigation measure to reduce the runoff impact to receiving water quality. Refer to the HDM for additional discussion on the design of stormwater management drainage design.
7.7.8.1 Pump Station Hydraulic Design
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 must 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 must be used only when absolutely necessary. Also, backup systems must be considered.
Texas Evacuation Routes must 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.
Considerations in the hydraulic design of pump stations include:
- Avoid the need for pump stations, if possible, as they require substantial maintenance; and
- Perform a site visit to determine a gravity flow alternative to a pump station if one exists. Such an alternative would likely require additional ROW or easements to be initiated as early as possible.
7.7.9 Drainage Plan Sheets
Refer to the
PS&E Preparation Manual
and the PS&E QC Milestone Checklist for information on specific elements to include on the drainage plan sheets.7.7.9.1 Culvert and Storm Drain Plan Sheets
Culvert and storm drain plan sheets typically include:
- Drainage area maps;
- Hydraulic calculations;
- Culvert layouts (plan and cross section views);
- Special ditch plan and profile sheets (can be shown on roadway P&P sheets);
- Storm drain plan and profile sheets;
- Junction box details;
- Detention/retention facility details; and
- Miscellaneous drainage details (e.g., Special headwall/wingwall details, inlet modifications, outfall details, pipe bedding details, concrete collar details, and RC pipe connections).
Considerations in the preparation of culvert and storm drain layouts and details include:
- Evaluate the need for trench excavation and protection;
- Identify and resolve utility conflicts due to drainage elements;
- Evaluate the need for interim drainage elements due to construction phasing; and
- Coordinate with local agencies if connecting to local agency’s system.
7.7.9.2 Pump Station Plan Sheets
Pump station detail sheets typically include:
- Pump outfall plan and profile sheets;
- Control house details
- Electrical service/motor control center details;
- Control panel details;
- Backup generator details and specifications;
- Wiring diagrams:
- Structural details for pump house wet wells and site plans;
- Pump station details and specifications; and
- Miscellaneous drainage details.
Considerations in the preparation of pump station plan sheets and details include:
- A suitable source of energy for the backup generator (e.g., gasoline, diesel, gas) must be determined;
- The pump station must be located outside the roadway clear zone. In cases where this is not feasible, the pump station must 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 and workers; and
- The control house must include electrical outlets and lights for convenient maintenance.
7.7.9.3 Bridge and Bridge-Class Culvert Hydraulic Data Sheets
Each bridge and bridge class culvert must have its own Hydraulic Data Sheet to convey delineated drainage areas, floodplain cross sections used in hydraulic modeling, run-off calculations used, and other elements to document the hydrology and hydraulic design for these structures.
Refer to TxDOT’s
PS&E Preparation Manual
and the PS&E QC Milestone checklist for specific elements to include on the hydraulic data sheets.Considerations in the preparation of hydraulic data sheets include:
- Design improvements in coordination with the roadway design and structural engineers as applicable;
- Contact local FEMA FPA early. The FPA can advise which base model to use and how much, if any, increase in backwater will be allowed. The FPA may also provide a copy of the original model or help locate one. It is advised to work closely with the FPA throughout the project;
- The backwater profile program used in original development of the study may have been one of several types and from one of several sources. In Texas, the model most commonly used is the Hydrologic Engineering Center’s River Analysis System model (HECRAS) from the USACE. Typically, FEMA encourages that the project model be updated to the most current acceptable model. If the stream was originally modeled using HEC-2, subsequent models should be done using HEC-RAS; and
- Floodplain encroachments must be explained in the environmental document.
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