Section 4: Required Hydraulic Analysis
Anchor: #i1002402Function and Scope of Hydraulic Analysis
Flood frequency for design and checks must be considered for a new location, replacement, or modification of a facility. Hydrologic and hydraulic analyses are required to determine, justify, and document the need for and size of a hydraulic facility. Each District must maintain complete hydrologic and hydraulic design data for all waterway crossings. The same hydraulic analysis is required for new locations, proposed facility replacements, and widening of existing facilities.
The intent of a design flood is to establish conditions under which the highway facility will provide uninterrupted service with minimal damage to the highway. The design flood must not overtop the highway.
A check flood must be applied on proposed highway or stream crossing facilities to determine whether a proposed crossing will cause significant damage to the highway or to any other property, in excess of damage that is likely to occur without the proposed facility. For TxDOT design, the 100-year event is the primary check condition. The check flood may be conveyed both over the highway and through the hydraulic facilities. An additional check flood is the incipient highway overtopping condition.
Analysis should include a comparison of existing conditions with proposed conditions for interim and estimated future watershed characteristics. Its extent should correspond with the importance of the highway and its environment. The goal should be to achieve an adequate balance between incurred capital costs and potential risks.
Usually watershed characteristics will have changed since the placement of the existing facilities. Most often, runoff rates are higher due to increased impervious cover and more efficient drainage. In such cases, larger facilities may be needed to replace the existing ones.
Occasionally, flood control systems may have been constructed that significantly reduce runoff rates at the highway site. In such instances, verify the permanence and effect of the flood control facility, and consider the possibility of designing smaller hydraulic facilities than those to be replaced.
Anchor: #i1002437Widening Existing Facilities
If available, valid, hydraulic data exist, simplify the process. Changed watershed conditions or outdated hydrologic and hydraulic methods warrant reappraisal using updated methods and field information.
Expend additional effort to show any impact of the widening and to justify why replacement is not necessary or practicable for all bridge class structures and culverts. However, a hydraulic adequacy estimate based on past performance may be reasonable for culverts on existing rural locations where all of the following are true:
- Minor modifications only are planned (e.g., safety-end treatment and short extensions).
- Traffic volumes are low.
- Surrounding properties are not sensitive to damage due to backwater or high velocities.
- There is no adjustment of the roadway profile or addition of a roadway safety barrier.
- Sufficient information on past performance is available.
The determination of hydraulic adequacy refers to an estimate of design frequency based on a review and appraisal of historical high water, overtopping frequency, duration and depth, and maintenance history.
In addition to bridges and culverts, roadway widening often involves the relocation of inlets and extension of storm drain conduit. Design storm drain inlets and conduit systems in accordance with practices outlined in Chapter 10.
When an existing structure is discovered to be inadequate or oversized, either adjust the size of the facility as appropriate or assign a new capacity rating with a corresponding increase or decrease in the hydraulic standards that were previously established.
Because highway rehabilitation, modification, or maintenance work is not intended to include physical adjustment to hydraulic facilities, it does not preclude the need for considering the hydraulic-related impact. The following instances should include verification of continued adequate hydraulic performance:
- Roadway surface overlays or regrading projects that reduce the effective opening area or allowable ponding depth of storm drain inlets or reduce gutter capacities.
- Roadway widenings or addition of roadway barrier resulting in a higher overtopping elevation where the 100-year flood would previously have overtopped the roadway. If such modifications are made in a designated floodway, coordination with FEMA is required, as discussed in Section 5.
- Replacement of inlet grates with lower effective openings than existing.
- Removal of any hydraulic feature including flumes and energy dissipaters.