Section 6: Flood Damage PreventionAnchor: #i1009571
Extent of Flood Damage Prevention Measures
The response of alluvial streams to floods is often unpredictable. Knowledge of the history of a stream and its response to floods is the best guide for determining the extent of flood damage prevention measures. When protection is needed, whether at the time of construction or at a later date, the cost of providing the control measures should be compared to the potential costs associated with flood damage without the prevention measures.
Flood-related damage results from a variety of factors including the following:
- scour around piers and abutments
- erosion along toe of highway embankment due to along-embankment flow
- erosion of embankment due to overtopping flow
- long term vertical degradation of stream bed
- horizontal migration of stream banks
- debris impact on structure
- clogging due to debris causing redirection of flow.
The designer should assess the potential for these and other conditions to occur and consider measures that reduce the potential for damage from flooding.Anchor: #i1009626
The primary flood-related concern at piers is the potential for scour. Two typical approaches are to design deep enough foundations to accommodate scour and to protect the streambed around the foundation to prevent or reduce the potential for scour.
Primary protection measures at piers include concrete riprap, stone protection, gabions, and grout-filled or sand/cement-filled bags. See FHWA IH-97-030, “Bridge Scour and Stream Instability Countermeasures” ( HEC-23) for discussion on selection of measures.
The following should be considered the following to reduce the potential for pier scour:
- Reduce numbers of piers by increasing span lengths, especially where you expect large debris loads.
- Use bullet-nosed or circular-shaped piers.
- Use drilled shaft foundations.
- Align bents with flood flow to degree practicable.
- Increase bridge length to reduce through-bridge velocities.
Where there is a chance of submergence, a superstructure that is as slender as possible with open rails and no curb should be used.
Because of uncertainties in scour predictions, use extreme conservatism in foundation design. In other words, deeper foundations may be cheaper. The capital costs of providing a foundation secure against scour are usually small when compared to the risk costs of scour-related failure.Anchor: #i1009681
Embankments that encroach on floodplains are most commonly subjected to scour and erosion damage by overflow and by flow directed along the embankment to the waterway openings. Erosion can also occur on the downstream embankment due to turbulence and eddying as flow expands from the openings to the floodplain and due to overtopping flow.
The incidence of damage from flow along an approach embankment is probably highest in wooded floodplains where the right-of-way is cleared of all trees and where borrow areas are established upstream of the embankment. Damage to approach embankment is usually not severe, but scour at the abutments from the flow contraction may be significant if the abutment is not protected.
The potential for erosion along the toe of approach embankment can be minimized by avoiding extensive clearing of vegetation and avoiding the use of borrow areas in the adjacent floodplain. Embankment protection such as stone protection can be used, but stable vegetation on the embankment may suffice. Other measures that may be used are riprap, pervious dikes of timber, or finger dikes of earthen material spaced along and normal to the approach fill to impede flow along the embankment.
The embankment may need to be protected if significant overtopping of the approach embankment is anticipated during the life of the crossing. The embankment can be protected with soil cement or revetments, rock, wire-enclosed rock, or concrete.
Preventive measures are also needed at some crossings to protect the embankment against wave action, especially at reservoirs. Riprap of durable, hard rock should be used at such locations. The top elevation of the rock required depends on storage and flood elevations in the reservoir and wave height computed using wind velocities and the reservoir fetch.Anchor: #i1009713
Protective measures used at abutments consist of the following:
- riprap header slopes and deep toe walls (stone protection is generally preferred to concrete)
- vertical abutment walls
- sheet pile toe walls
- deep foundations of piles or drilled shafts.
Vertical abutment walls will protect bridge ends and the embankment if the walls are extended around the fill slopes to below the depth of anticipated scour. Sheet pile toe walls are usually installed to repair scour damage after a flood. They are commonly used where rock is not available or access for placing rock is difficult. Sheet pile may be used only under guidance from the Bridge Division’s Geotechnical Branch.
Revetment is usually placed at the abutment on the slopes under the bridge end and around the corners of the embankment to guard against progressive embankment erosion. Revetment on the fill slope may be susceptible to contraction scour. To prevent embankment failure from undermining by contraction scour, a toewall must be extended below the level of expected scour.
Two common types of revetments used to protect abutments are rigid (i.e. concrete riprap) and flexible (i.e. stone protection, articulated concrete blocks, and gabion mattresses). A unique feature of stone protection is can be designed to be self launching. That is, the rocks will shift to fill any area that scours and inhibit any further scour.Anchor: #i1009769
Guide Banks (Spur Dikes)
The twofold purpose of guide banks is to align flow from the floodplain with the waterway opening and minimize scour at the abutment by moving the scour-causing turbulence to the upstream end of the guide bank. Where the floodwater must flow along the embankment for more than 800 feet, guide banks should be considered. Figure 9-20 shows a typical plan form.
Figure 9-20. Typical Guide Bank
Guide banks are usually constructed of earthen embankment but are sometimes constructed from rock. The dike should be protected by revetment where scour is expected to occur, although a failure at the upstream end of a spur dike usually does not immediately threaten the bridge end.
Clearing around the end of the dike in wooded floodplains should be minimized to enhance the effectiveness. A drainage channel around the end of the dike for local drainage may induce turbulence from mixed flows. Instead, a small culvert through the dike will help minimize the turbulence of mixed flows from different directions.
The suggested shape of guide banks is elliptical with a major-to-minor axis ratio of 2.5:1. The suggested length varies with the ratio of flow diverted from the floodplain to flow in the first 100 feet of waterway under the bridge. The suggested shape is based on laboratory experiments, and the length is based on modeling and field data. The optimum shape and length may differ for each site and possibly for each flood at a site. However, field experience shows, however, that the recommended elliptical shape is usually quite effective in reducing turbulence. Should practical reasons require the use of another shape such as a straight dike, more scour may be expected at the upstream end of the guide banks. Guide banks can also be used at the downstream side of the bridge to help direct flow back into the overbanks.Anchor: #i1009805
Bank Stabilization and River Training Devices
Bank stabilization and river training devices are intended to inhibit the erosion and movement of stream banks. They may be needed either to defend against actions of the stream that threaten the highway crossing or to protect the stream banks and the highway from an anticipated response to highway construction.
Various materials and devices designers use include the following:
- stone protection
- concrete lining
- wood, steel, or rock jetties
- steel or concrete jack fields
- wire fences
- timber bulkheads
- articulated concrete mattresses
- guide banks, dikes, and spurs (usually constructed of earth and rock).
The choice of the appropriate device or devices for use depends on the geomorphology of the river. Futile attempts at localized control can be avoided where the river is in the midst of changes by studying long reaches. Regardless of the size of the stream and the control measures used, stream response to the measure must be considered. For instance, bank stabilization at a crossing may cause scour in the bed of the channel or redirect the current toward an otherwise stable bank downstream.
Bank stabilization and river training is a specialized field requiring familiarity with the stream and its propensity to change, knowledge of the bed load and debris carrying characteristics of the stream, and experience and experimentation at similar sites on the same or a similar stream.
The following are general principles for the design and construction of bank protection and training works:
- The cost of the protective measures should not exceed the cost of the consequences of the anticipated stream action.
- Base designs on studies of channel morphology and processes and on experience with compatible situations. Consider the ultimate effects of the work on the natural channel (both upstream and downstream).
- Inspect the work periodically after construction with the aid of surveys to check results and to modify the design, if necessary.
- Understand that the objective of installing bank stabilization and river training measures is to protect the highway. The protective measures themselves are expendable.
Refer to the FHWA publication Stream Stability at Highway Structures ( HEC 20) for more detailed information regarding bank stabilization and stream training facilities.
The effectiveness of protective and training measures in many alluvial streams and the need for the measures may be short-term because of the dynamic nature of streams. The stream will move to attack another location or outflank the installation.
A cost comparison of viable options should be made. Alternatives to stream protection measures include the following:
- a continuing effort to protect the highway by successive installations intended to counter the most recent actions of the stream
- relocation of the roadway away from the river hazard
- a larger opening designed to accommodate the hazard
- abutment foundations designed sufficiently to allow them to become interior bents at a later date.
Minimization of Hydraulic Forces and Debris Impact on the Superstructure
The most obvious design guideline is to avoid the imposition of hydraulic forces on a bridge superstructure by placing the bridge at an elevation above which the probability of submergence is small. Obviously, this is not always economically or physically practical.
One design alternative is to make the superstructure as shallow as possible. Box girders that would displace great volumes of water and have a relatively small weight compared to the weight of water displaced are not a good design alternative unless the probability of submergence is very small. Solid parapets and curbs that increase the effective depth of the superstructure can give increased buoyancy over that of open rail designs. If submerged, the increased effective depth of the superstructure causes increased general scour, and drag forces on the superstructure are much greater than with open rails.
Another consideration is to provide a roadway approach profile that will be overtopped prior to the submergence of the bridge superstructure. This will reduce the probability of submergence of the bridge and help to reduce the potential for scour at the bridge . The consequence may be the need for repairs to the roadway approach.
Where large volumes of debris are likely to occur, longer spans and high freeboards may be warranted. In extreme situations, debris racks may be installed to stop the debris before it reaches the structure. Bridge designers should consult with Design Division Hydraulics prior to specifying or installing debris racks.
For even a small probability of total or partial submergence, see the Bridge Division Design Manual for guidance. If the dead load of the structure is not sufficient to resist buoyant, drag, and debris impact forces, the superstructure may need to be anchored to the substructure. Air holes may also be provided through each span and between each girder to reduce the uplift pressure.