Debris Control Structures Evaluation and Countermeasures
Hydraulic Engineering Circular No. 9
Chapter 5 - Debris Countermeasures
5.1 Background
Countermeasures to mitigate and protect effects of debris depend on the type of structure. Typically, these countermeasures are grouped into structural and non-structural measures. The structural measures have many configurations and constructed from many materials. The non-structural measures typically involve long-term approaches.
This Chapter will describe debris countermeasures for culverts and bridges. As will be described in this and subsequent Chapters, while some countermeasures may have some applicability to both types of structures (as well as other not described herein), engineering judgment on use remains a key design consideration.
5.2 Countermeasures for Culverts
5.2.1 Structural Measures
There are various types of structural measures available for culverts. These measures can have many shapes and can be constructed using various materials. The measures can generally be divided into the following types:
Debris Deflectors are structures placed at the culvert inlet to deflect the major portion of the debris away from the culvert entrance. They are normally "V"-shaped in plan with the apex upstream. Examples of this type of structure measure are shown in Figure 5.1 through Figure 5.9.
Debris Racks are structures placed across the stream channel to collect the debris before it reaches the culvert entrance. Debris racks are usually vertical and at right angles to the streamflow, but they may be skewed with the flow or inclined with the vertical. Pictures of debris racks are shown in Figure 5.10 through Figure 5.22.
Debris Risers are a closed-type structure placed directly over the culvert inlet to cause deposition of flowing debris and fine detritus before it reaches the culvert inlet. Risers are usually built of metal pipe. Examples of debris risers are shown in Figure 5.23 through Figure 5.25. Risers can also be used as relief devices in the event the entrance becomes completely blocked with debris (Figure 5.25).
Debris Cribs are open crib-type structures placed vertically over the culvert inlet in log-cabin fashion to prevent inflow of coarse bed load and light floating debris. Photos of this type of structure are provided in Figure 5.26 through Figure 5.28.
Debris Fins are walls built in the stream channel upstream of the culvert. Their purpose is to align the debris with the culvert so that the debris would pass through the culvert without accumulating at the inlet. This type of measure can also be used at bridge. Examples of this type of structure measure for culverts are shown in Figure 5.29 through Figure 5.35.
Debris Dams and Basins are structures placed across well-defined channels to form basins which impede the stream flow and provide storage space for deposits of detritus and floating debris. This type of structure is shown in Figure 5.36 through Figure 5.39.
Combination Devices are a combination of two or more of the preceding debris-control structures at one site to handle more than one type of debris and to provide additional insurance against the culvert inlet from becoming clogged. Examples of combination devices are shown in Figure 5.40.
5.2.2 Non-structural Measures
The only type of non-structural measures available for culvert structures is to provide emergency and annual maintenance. Although not always feasible for remote culverts or culverts with small drainage areas, maintenance could be a viable option for larger culverts with fairly large drainage basins. Emergency maintenance could involve removing debris from the culvert entrance and/or an existing debris-control structure. Annual maintenance could involve removing debris from within the culvert, at the culvert entrance, and/or immediately upstream of the culvert, or repairing any existing structural measures.
5.3 Countermeasures for Bridges
5.3.1 Structural Measures
Various types of structural measures are also available for bridge structures. Some of the measures discussed above for the culvert structures can also be utilized at bridges. The various types include:
Debris Fins are walls built in the stream channel upstream of the bridge to align large floating trees so that their length is parallel to the flow, enabling them to pass under the bridge without incident. This type of measure is also referred to as a "pier nose extension". Examples of debris fin deflectors are provided in Figure 5.35.
In-channel Debris Basins are structures placed across well-defined channels to form basins which impede the streamflow and provide storage space for deposits of detritus and floating debris. These structures can be expensive to construct and maintain. This type of structure is shown in Figure 5.36 through Figure 5.40.
River-Training Structures are structures placed in the river flow to create counter-rotating streamwise vortices in their wakes to modify the near-bed flow pattern to redistribute flow and sediment transport within the channel cross section. Examples of this type of structure include Iowa vanes, and impermeable and permeable spurs. This type of structure is shown in Figure 5.41 and Figure 5.42.
Crib Structures are walls built between open-pile bents to prevent debris lodging between the bents. The walls are typically constructed out of timber or metal material.
Flood Relief Sections are overtopping or flow through structures that divert excess flow and floating debris away from the bridge structure and through the structure.
Debris Deflectors are structures placed upstream of the bridge piers to deflect and guide debris through the bridge opening. They are normally "V"-shaped in plan with the apex upstream. An example of this type of structure is shown in Figure 5.43. A special type of debris deflector is a hydrofoil. Hydrofoils are submerged structures placed immediately upstream of bridge piers that create counter-rotating streamwise vortices in their wakes to deflect and divert floating debris around the piers and through the bridge opening. Unfortunately, no hydrofoils have been implemented within the field. They have only been tested within a physical model study.(56)
Debris Sweeper is a polyethylene device that is attached to a vertical stainless steel cable or column affixed to the upstream side of the bridge pier. The polyethylene device travels vertically along the pier as the water surface rises and falls. It is also rotated by the flow, causing the debris to be deflected away from the pier and through the bridge opening. This type of device is shown in Figure 5.44 through Figure 5.47.
Booms are logs or timbers that float on the water surface to collect floating drift. Drift booms require guides or stays to hold them in place laterally. Booms are very limited in use and their application is not covered within this manual.
Design Features are structural features that can be implemented in the design of a proposed bridge structure. The first feature is freeboard, which is a safety precaution of providing additional space between the maximum water surface elevation and the low chord elevation of the bridge. The second feature is related to the type of piers and the location and spacing of the piers. Ideally, the piers should be a solid wall type pier that is aligned with the approaching flow. They should also be located and spaced such that the potential for debris accumulation is minimized. The third feature involves the use of special superstructure design, such as thin decks, to prevent or reduce the debris accumulation on the structure when the flood stage rises above the deck. The last feature involves providing adequate access to the structure for emergency and annual maintenance.
5.3.2 Non-Structural Measures
There are generally two types of non-structural measures available for bridge structures. The first type of non-structural measure is emergency and annual maintenance. Emergency maintenance could involve removing debris from the bridge piers and/or abutments; placing riprap near the piers, abutments, or where erosion is occurring due to flow impingement created by the debris accumulation; and/or dredging of the channel bottom. Annual maintenance could involve debris removal and repair to any existing structural measures.
The second type of non-structural measure is management of the upstream watershed. The purpose of this measure is to reduce the amount of debris delivered to the structure by reducing the sources of debris, preventing the debris from being introduced into the streams, and clearing debris from the stream channels. The type of management system implemented varies depending on the type of debris. For organic floating debris, the management system could involve removing dead and decayed trees, and/or debris jams; providing buffer zones for areas where logging practices exist; implementing a cable-assisted felling of trees system; and stabilizing hillside slopes and stream banks.
5.4 Countermeasures for Fire Damaged / Deforested Areas
5.4.1 Fire Damaged Areas
Fires can decrease the amount of floating debris introduced into the stream system. However, fires increase the magnitude of runoff from the burned area, increase the erodibility of soils, and increase the probability of catastrophic events such as debris flows and landslides, resulting in a significant increase in sediment yield from the effected area. This increase could cause an increase in fine and coarse detritus to be transported to and deposited at a culvert or bridge structure. Countermeasures that can be implemented to reduce the amount of material transported to a drainage structure include:
Surface Treatments are countermeasures that are placed directly on the burnt landscape to reduce the potential for erosion from the disturbed area. There are various types of surface treatments. One type of surface treatment is hydroseeding, which involves re-vegetation of the landscape by spraying grass or wildflower seeds. This method can be easily applied to large areas, and it is most effective when there is adequate time for the vegetation to develop. Another type of surface treatment consists of placing straw or wood fiber mulch on the landscape. A fabric mat can be used in lieu of mulch material to provide more resistance to erosive forces.
Sediment barriers are temporary structures used to help retain the soil on the site and reduce the runoff velocity across areas below it. One type of sediment barrier is a silt fence, which is a temporary structures of wood or steel fend posts, weir mesh fencing, and a suitable permeable filter fabric. Another type of a sediment barrier structure is a straw bale dike, which are constructed out of straw bales. Both of these structures should be limited to small drainage areas that have a maximum slope of 2H on 1V and flow path length of around 100 feet. Another type of sediment barrier is straw wattles. Wattles are tubes of straw or coconut fiber. Wattles help stabilize the slope by shortening the slope length and by slowing, spreading and filtering overland water flow. They are placed in trenches on the slope at selected vertical spacing and held in-place by stakes.
In-channel Debris Basins are structures placed across well-defined channels to form basins which impede the streamflow and provide storage space for deposits of detritus and floating debris.
5.4.2 Deforestation
Logging practices can cause a substantial increase in the volume of floating debris entering a channel system. Practices that reduce the quantities of floating debris include directional felling uphill with a tree-pulling system and providing a buffer strip of undisturbed vegetation along the streams. As in fires, logging can cause an increase in magnitude of runoff from the disturbed area, increase the erodibility of soils, and increase the probability of catastrophic events such as debris flows and landslides, resulting in an increase in fine and coarse detritus to be transported to and deposited at a drainage structure. The countermeasures that can be implemented to reduce the amount of material transported to a drainage structure include sediment barriers and in-channel debris basins as discussed above for fires.
Figure 5.1. Steel rail debris deflector for large rock (looking upstream of culvert).
Figure 5.2. Steel rail debris deflector (looking downstream).
Figure 5.3. Steel rail and cable debris deflector. In boulder areas, cable is more desirable for its flexibility than a rigid rail (looking towards entrance).
Figure 5.4. Steel debris deflectors installed at entrances to a battery of culverts.
Figure 5.5. Steel rail debris deflector for battery of culverts (see Figure 5.6).
Figure 5.6. Installation of Figure 5.5 during flood; functions well under heavy debris flow.
Figure 5.7. Steel rail debris deflector in area of heavy flowing debris (looking upstream).
Figure 5.8. Timber pile debris deflector for boulders and large floating debris.
Figure 5.9. Timber pile debris deflector protected culvert during heavy floods. Nearby culverts without deflectors were plugged.
Figure 5.10. Rail debris rack over sloping inlet. Heavy debris and boulders ride over rack and leave flow to culvert unimpeded.
Figure 5.11. Post and rail debris rack, in place for 35 years, for light to medium floating debris installed 100 ft upstream of culvert.
Figure 5.12. Rail debris rack.
Figure 5.13. Timber debris rack (note how suspended by cables).
Figure 5.14. Hinged steel debris rack in urban area. Due to nature of debris and possible entry by children, bar spacing is close.
Figure 5.15. Steel debris rack in urban area.
Figure 5.16. Debris rack used in State of Washington.
Figure 5.17. Rail debris rack in arid region (see Figure 5.18).
Figure 5.18. Installation in Figure 5.17 after several years of fine silt deposition at entrance.
Figure 5.19. Steel rail debris rack. Note amount of debris accumulation in upstream channel.
Figure 5.20. Steel debris rack probably saved the culvert from plugging.
Figure 5.21. Steel grill debris rack with provision for cleanout afforded by concrete paved area in foreground.
Figure 5.22. Steel grill debris rack on slope mitered culvert entrance.
Figure 5.23. Metal pipe debris riser in basin (note anti-vortex device on top).
Figure 5.24. Metal pipe debris riser placed during initial construction of culvert provides relief in case the culvert entrance becomes plugged (see Figure 5.25).
Figure 5.25. Installation shown in Figure 5.24 after flood. Riser conveyed large flows during flood. Fence partially surrounding riser was of no value for debris control.
Figure 5.26. Debris crib of precast concrete sections and metal dowels. Height increased by extending dowels and adding more sections.
Figure 5.27. Arid region debris crib of precast concrete sections and metal dowels.
Figure 5.28. Redwood debris crib with spacing to prevent passage of fine material. Basin had buildup of 30 feet.
Figure 5.29. Concrete debris fins with sloping leading edge as extension of culvert walls.
Figure 5.30. Concrete debris fin with sloping leading edge as extension of center wall.
Figure 5.31. Concrete debris fin with rounded vertical leading edge as extension of culvert center wall.
Figure 5.32. Combined installation of concrete debris fin and metal pipe debris riser with single corrugated metal pipe culvert (looking downstream).
Figure 5.33. Concrete debris fin for single culvert (Prefer more area between wingwalls and fin).
Figure 5.34. Debris fin and metal pipe debris riser in conjunction with single barrel culvert.
Figure 5.35. Timber debris fins with sloping leading edge.
Figure 5.36. Metal bin type debris dam.
Figure 5.37. Gabion debris dam
Figure 5.38. Debris dam of precast concrete sections fabricated to enable placement in interlocking fashion.
Figure 5.39. Debris dam of precast concrete sections fabricated to enable placement in interlocking fashion.
Figure 5.40. Debris dam and basin along with steel debris rack over culvert entrance in foreground. A metal pipe riser is visible over the spillway.
Figure 5.41. Bendway wi ers on outer bank of Hatchie River looking upstream (TDOT).
Figure 5.42. Kellner jacks used for redirecting the flow patterns.
Figure 5.43. Debris deflectors installed at State Route 59 south crossing of the Eel River in central Indiana.
Figure 5.44. Debris sweeper being installed on a bridge over Staunton River in Altavista, Virginia.
Figure 5.45. Close up of a debris sweeper installed on the Cedar Creek in Washington.
Figure 5.46. Close up of a debris sweeper installed on the South Fork Obion River in Tennessee.
Figure 5.47. Close up of double-stacked installation debris sweeper on Interstate 24 over the Mississippi River.
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