Debris Control Structures Evaluation and Countermeasures
Hydraulic Engineering Circular No. 9
Chapter 7 - Maintenance
Maintenance is a vital component in how a debris-control countermeasure would function at a drainage structure. The lack of maintenance can cause improper functioning of the structure, resulting in possible damage to or failure of the structure. Although no specific guidelines for highway maintenance practices have been established, the general maintenance practice of these structures should involve regular inspections and cleaning, coupled with emergency removal of debris. Regular periodic inspections allow minor problems to be discovered and corrected before they become serious. The procedure and documentation of the inspection should be based on the guidelines provided in FHWA's Culvert Inspection Manual(4) and FHWA's Bridge Inspector's Training Manual(27). Debris accumulated on these structures should be removed in a timely manner since the presence of debris can have several severe consequences as discussed in Chapter 2 of this manual. In addition to the adverse effects associated with debris accumulations, structures with an existing debris accumulation have a higher potential for trapping additional debris than if they were cleared of debris. Emergency debris removal during a flood event can be critical to the survival of a structure laden with debris and to the flooding impacts upstream of the structure.
The frequency of maintenance must be considered in the design of these structures. Structures located on a primary highway may have a higher frequency of maintenance than those on a secondary highway. If a low standard of maintenance is to be provided at a debris-control countermeasure or structure, it may be desirable to use a debris-control countermeasure that requires less attention, a combination of debris-control countermeasures, and/or a larger structure. This consideration may also determine the choice of alternatives when two or more debris-control countermeasures are being considered for a site.
As mentioned in Section 6.4 of this manual, provisions should be made for maintenance access to the debris-control countermeasure or culvert/bridge structure. Unfortunately, access is often difficult to provide for a debris-control countermeasure and structure, particularly when it is near an existing high embankment. Such installations, however, usually require less maintenance because of the additional storage available for the debris. If access roads are impractical and the risk associated with flooding is high, it may be necessary to provide an area near the structure that mechanical equipment, such as a crane, could be located for removing debris from the structure without disrupting highway traffic. Access should be provided to the substructure of bridges to ensure prompt and complete removal of debris accumulations on the structure. Tracked vehicles can usually be used to remove the debris during low flows at bridges over small streams. For bridges over large rivers, a barge might be required to remove the debris, so a launching site for the barge may be necessary at such a site.
Debris removed from a culvert or bridge should be disposed in an acceptable fashion. It should not be disposed directly downstream of the structure, ignoring the consequences to any structure further downstream. Debris should not be placed where it may be carried away by the stream flow or where it may block the drainage of an area. Potential disposal options include burning, burial, using it as firewood, using it as chipped wood for horticultural purposes if low grade wood, or using it for structural purposes if high grade wood. The latter two options are preferable as they may create some financial return for the operator, while burial is costly and burning is environmentally undesirable, especially if the wood has become contaminated in the river by toxins, hydrocarbons, or heavy metals. In addition, this process seems to have limited success in reducing the amount of debris.
Ideally, a maintenance plan that clearly defines the maintenance activities and the responsibility of the maintenance crew should be developed for a structure susceptible to debris problems. As a minimum, this plan should contain information about the various items discussed above, i.e., inspections, regular and emergency debris maintenance activities, frequency of maintenance, access, and disposal of the debris. In addition, the general location and maximum extent of debris removal as estimated using the procedures and recommendations provided in Chapter 3 of this manual can be provided for bridges.
Maintenance associated with debris removal should be considered in the design of a new or replacement bridge. As discussed in Section 6.4 of this manual, there are certain features that can be incorporated into the design to simplify debris removal. Solid wall piers that extend a short distance upstream of the bridge deck edge are easier to remove debris from and less likely to cause damage to the bridge structure during the removal operations than other pier types. Superstructures that allow access to the pier nose from directly above also ease debris removal, such as, a wide deck with a simple parapet and adequate load-bearing capacity for heavy equipment.