Debris Control Structures Evaluation and Countermeasures
Hydraulic Engineering Circular No. 9
Chapter 2 - Debris Characterization
2.1 Debris Classification
Flood flow reaching a culvert or bridge structure typically carries floating as well as submerged debris. As discussed in further detail in the next section of this manual, debris should be a concern to highway engineers because it can accumulate at and obstruct the waterway entrance of culverts or bridges, adversely affect the operation of the structure or cause failure of the structure.(14,17,40,47,49,50) A thorough study of the supply of debris originating in the drainage basin is essential for proper design of a drainage structure.
2.1.1 Types of Debris
The selection of a debris countermeasure depends on the type of debris transported to the site; therefore, the various types of debris should be defined and classified to assist in the selection of an effective debris countermeasure. This current edition retains, but slightly modifies, the classification system used in earlier editions. This classification is presented as follows:
Very Small Buoyant Debris or No Debris.
Small Floating Debris. Small floating debris includes small limbs or sticks, orchard prunings, tules, leaves, and refuse. This material can be easily transported by the stream and overland flow. Therefore, this type of debris can be introduced into the stream from the local runoff from a watershed, and then easily transported downstream by the stream flows. This type of debris also comes from tree and vegetation that are introduced into the stream due to bank erosion, landmass failures, wind action or collapsing due to biological factors such as decay and old age, or from the loss of foliage during the changing of seasons. There are usually no significant problems associated with this type of debris at bridge structures; however, it is an important component in the development of mature debris jams of large floating debris, and it could accumulate at and clog culvert structures.
Medium Floating Debris. Medium floating debris consists of tree limbs or large sticks. The source of this material comes from trees introduced into the stream by bank erosion, mass wasting, windthrow, or collapsing of trees due to ice loading, beaver activities, or biological factors such as old age and diseases; or from erosion of emergent and riparian trees within the streams. Vegetation within the floodplain could also be a source of this type of debris. This type of debris could accumulate at both culvert and bridge structures.
Large Floating Debris. Large floating debris consists of logs or trees (such large floating debris is also known as "drift"). The sources of this type of debris are the same sources discussed for "Medium Floating Debris". Transport and storage of this material depends on discharge, channel characteristics, the size of the drift pieces relative to the channel dimensions, and the hydraulic characteristics (depth and slope) of the system.(17) In small and intermediate size streams, this material is not easily transported, and it is usually transported during larger floods or prolonged periods of high water.(17) Once introduced into the main channel of these small and intermediate size streams, this material can form into a jam, which is a collection of debris formed around large, whole trees that may be anchored to the bed or banks at one or both ends.(17, 61) Larger streams and rivers do not store much of this material within the channel. During most flood events, a larger stream will transport nearly all large floating debris entering the reach.(17) The size of the jam depends on the type of vegetation existing within the watershed and the channel characteristics transporting the material. This type of debris causes a significant problem at bridge structures because of its size, shape, and facility for entrapment on bridge piers.
Fine Detritus. Fine detritus consists of silt, sand, and fine gravel more or less devoid of floating debris. The size of this material ranges from 0.004 to 8 mm (0.00016 to 0.31 inches). This type of debris is transported along the bed and in the water column above the bed, i.e., as bed and suspended load. The source of this material is from sheet and rill erosion, gully erosion, landmass movement, and channel and bank erosion. Sediment yield rates for this material can be significantly influenced by the conditions of and changes within the watershed (e.g., urbanization, fire, etc.). Deposition of fine sediment could possibly block a culvert structure or significantly reduce the waterway opening through a bridge structure.
Coarse Detritus. Coarse detritus consists of coarse gravel or rock ranging in size from 16 to 256 mm (0.6 to 10 inches). The source of this material is from bed and/or bank erosion, gully erosion, or landmass movement. This material is usually transported as bed load, however it can be transported as both bed and suspended load within high gradient streams or gullies. Course detritus deposition can easily block a culvert entrance or significantly reduce a bridge waterway opening.
Boulders. Material comprised of large rock ranging in size from 256 to 2048 mm (0.84 to 6.7 feet). This type of material is usually associated with steep mountain streams or gullies, and it is transported as bed load. The source of the boulders is from bed and/or bank erosion or landmass movements. This material can easily block the entrance to a culvert and/or cause damage to the bridge piers from the impact forces.
Flowing Debris. Flowing debris is a heterogeneous fluid mass of clay, silt, sand, gravel, rock, refuse, trees, and/or branches. In general, it is a combination of the different types of debris mentioned above.
Ice Debris. Ice debris is accumulation or transport of ice floes in the waterway. This edition of the document does not describe or characterize this type of debris in any detail. In future editions, FHWA intends to add supplementary information based on results of on-going research efforts.
2.2 Flow Behavior of Floating Debris
A brief discussion on the flow behavior of large floating debris is provided in the previous section. A more detailed discussion of the subject is provided in this section because it is an important concept to consider for evaluating the potential for debris accumulations at bridges and/or for developing watershed management plans for debris reduction. The potential for transport, mode of transport, formation of debris rafts, potential locations for debris accumulations, and general characteristics of debris accumulations at bridge structures are discussed in the remaining portions of this section. The information for these various topics was obtained from several different sources(14,17,18,43,59,63), but most of the information was obtained from the report prepared by Diehl.(17)
2.2.1 Debris Transport and Transport Mechanisms
The potential for transport depends on the discharge, channel characteristics, debris source availability, size of the floating debris relative to the channel dimensions, orientation of the debris relative to the channel alignment, and type of anchorage.(17,18,43) The potential for transport increases with increasing discharge due to the increase in the flow velocity, depth, and energy slope of the river.(17) Unfortunately at the present time, in most cases, there are no relationships available that define the minimum velocity and/or slope necessary to initiate transport of large floating debris. However, with respect to ice debris, the AASHTO LRFD Bridge Design Specifications describe several relationships regarding ice pressures and loads.(5) Ongoing research is investigating this and other ice debris issues.
The width, depth, and slope of the channel are important channel characteristics that influence the potential for transport.(17) In general, this potential increases with increasing widths. Consequently the abundance of floating debris stored in the channel typically decreases with increasing widths. The length of the large floating debris transported increases with increasing channel widths.(17,63) Narrow streams rarely transport large floating debris, except for steep streams subject to debris torrents.(14,17) For narrow streams, trees and logs, (i.e., large floating debris) are usually longer than the width of the channel, so they typically become lodged across the channel, and rarely move without being broken into smaller pieces.(17) For most intermediate size streams, only some of the large floating debris is transported during large floods since most of it accumulates within the channel to form sizable debris jams.(17) Furthermore, for rivers and wide streams with adequate flow depth, nearly all of the large floating debris introduced into the main channel is transported by frequent flood events.(14,17) The depth of flow within the channel has to be deep enough to buoy up large floating debris. The depth sufficient to float logs and large trees is about the diameter of the tree butt plus the distance the roots extend below the butt.(17) The potential for transport is higher for high-gradient streams than it would be for low-gradient streams with the same channel dimensions, since the forces of flowing water on stored debris in low-gradient streams are less.(17) Stored debris can be abundant in large, low-gradient channels.(17)
The ratio of the effective length of the tree to the width of the channel is an important factor in a waterway's capacity to transport a particular size of debris.(17) The relation of the length of the debris pieces to the channel width is a primary indicator in defining the transport rate and the type and amount of debris stored in the channel. The potential for debris transport increases as the ratio of the debris length over the channel width decreases. For example, more debris would be transported in an intermediate size stream if the debris length is one-half of the channel width, than would be transported if the debris length equals the width of the channel. The maximum size of the debris is limited by the channel width; however, the amount of debris is limited by the supply of debris and the capability of the channel to transport the debris.
The potential for transport is influenced by the orientation of the debris relative to the channel alignment and type of anchorage.(18) Isolated pieces are more likely to be transported than pieces within debris jams or clumps. Also, trees with the root mass oriented upstream are more likely to be transported than trees with the root mass downstream or near the stream bank since it is easier for the piece of debris to be rotated by the flow.(18) Debris that is anchored to the side and perpendicular to the flow would most likely remain in place and not be transported downstream until the debris is dislodged from the bank due to bank erosion.
After mobilization has occurred, large floating debris is transported either by floating along the water surface or dragging along the bed. Observations noted by Diehl indicate that debris is typically transported on the surface as individual pieces aligned with the flow and traveling at about the same velocity as the average water velocity at the surface.(17) The results of a physical model study performed by Ng(43) agrees with Diehl's observation. Floating debris can occasionally be transported in short-lived clumps that eventually get broken apart due to turbulence.(14,17) The debris typically concentrates in a path occupying only a small fraction of the channel width. This path is defined by the zone of convergence that exists in some channels near the thalweg of the streams where the flow is the deepest and fastest under some flow conditions.(17,43) The zone of surface convergence for a straight and curved channel is illustrated in Figure 2.1.(17)The flow patterns reflected in this figure are hypothetical flow patterns at a particular location in a single bend during bankfull flow conditions, and the flow patterns would be entirely different for larger flood flows, different radii of curvature, or different channel conditions.
Observations noted by Chang(14) indicate that floating debris within straight reaches tends to move inward to the thalweg at the rising stage of a flood and outward to the banks at the receding stage. The reasons are unclear; however, it could possibly be related to changes in the direction of the secondary flow patterns within the channel. As noted by Ng(43), the opposite pattern can occur, i.e., outward to the banks during the rising and inward to the thalweg during the receding, if water leaves the channel and flows into the floodplains. Diehl's(17) observations in curved channel reaches indicate that floating debris may be transported on the outside of the curve during both rising and falling conditions. He also observed that debris typically travels between the center of the channel and the outside bank rather than in contact with the bank vegetation.
Figure 2.1. Hypothetical Patterns of secondary flow in straight and curved channels.
2.2.2 Debris Jams and Debris Dams
Floating debris introduced into the channel can form into debris jams or dams.(17, 63) Debris jams are usually formed when large, whole trees are introduced into the channel and are anchored to the bed or banks at one or both ends. The large trees act as a filter by trapping smaller floating debris and possibly sediment. The size and location of a jam depend on the size of the stream and the size of the trees. In small streams, a fallen tree may not be readily transported. However, this tree may trap and accumulate smaller debris from upstream and form a debris jam. Conversely, larger rivers can readily transport the debris downstream so it may not be able to accumulate into a large jam. Most of the accumulations in large rivers occur on the channel margins or outside the channel on islands, in floodplain forests, and in sloughs.(17, 63)
As noted by Wallerstein and Thorne(63), debris jams influence the geomorphology of rivers by influencing the overall channel form (i.e., they distort pool-riffle sequence and gravel bar formation); by changing the channel topography (i.e., they influence the erosional and depositional processes and widen the channel through bank erosion); and by increasing the channel apparent roughness through increased energy dissipation and eddy formation. (Note energy dissipation cannot be increased in a river, it can only be redistributed. Instead of the energy being dissipated along the channel, it is dissipated in intense drops over and around the debris. Upstream, the damming effect reduces the dissipation along the backwater-affected reach).
Wallerstein and Thorne(63) classified debris jams according to what they called "engineering-geomorphic impacts" as follows:
Underflow Jams occur in small watersheds where the fallen trees span the channel at bankfull level (define bankfull level in glossary). The in-channel geomorphic impact associated with this type of debris jams is minimal; however, local bed scour could occur under the jam during high flows.
Dam Jams usually occur when the tree height is approximately equal to the channel width. These types of jams can cause significant localized bank erosion and bed scour due to the constriction in flow, and backwater effects upstream that could cause sediment deposition upstream of the jam.
Deflector Jams usually occur when the channel width is slightly greater than the average tree height. They usually redirect the flow to one or both of the banks causing bed and bank erosion that could result in more trees being introduced into the river. They can also create backwater sediment wedges and downstream bars depending on the level of dissipation caused by the jam.
Parallel Jams exist when the channel width is significantly greater than the debris length, and the flow is capable of rotating the debris so that it is parallel with the flow. Bank erosion and bed scour associated with these jams are usually minimal. Parallel jams could actually stabilize the bank toe and protect it from erosion, and they may also initiate or accelerate the formation of mid-channel and lateral bars.
2.2.3 Debris Accumulation
Floating debris can accumulate at various locations and at obstructions within the river such as bridge piers and abutments, mid-channel bars, point bars, island heads, the streambed, or in pools along the base on the outside bank of bends. Debris accumulations typically grow in the upstream direction through the accretion of additional floating debris and fine and coarse sediment.(17) The rate of accumulation depends on the concentration, defined as number of debris per length of channel, of floating debris that is being transported and the magnitude of the flood.(14) In general, debris accumulations occur most frequently and in the largest sizes where the path of floating debris encounters obstructions.(17) The potential for trapping of debris at a bridge structure can be aggravated by the location and type of bridge piers.(14,17) Multiple columns can act as a sieve unless exactly aligned with flow; however, alignment of piers to flow at all flood levels for which debris is transported is unlikely. The gaps between columns are narrow relative to length of the floating debris, resulting in a high potential for accumulating debris. Floating debris can become entangled in a group of columns in ways that are not possible for a single-column pier. Floating debris accumulations at bridges generally fall into two classes: single-pier accumulations and span-blockage accumulations.(17)
Single-pier accumulation occurs when the maximum effective length of the floating debris is less than the effective opening between the bridge piers. The effective length of debris is the length of the debris element that can support the load of the debris accumulation. The effective opening corresponds to the distance between the piers normal to the approaching flow. The width of the opening can be determined by extending lines parallel to the approaching flow upstream from the nose of each pier and measuring the perpendicular distance between the two lines. As noted by Diehl, single-pier accumulations typically contain one or more trees extending the full width of the accumulation perpendicular to the approaching flow.(17) The full-width tree can be either at the surface or submerged and concealed beneath smaller floating debris. Pier placement is extremely important for this type of accumulation. Even if the span length is significantly greater than the maximum length of the floating debris, a pier located within the path of floating debris (Figure 2.2) can result in a high potential for accumulation at the pier.(17)
Figure 2.2. Single pier debris accumulation (led to pier scour failure).
Span-blockage accumulations occur when the length of floating debris exceeds the effective opening between piers, resulting in the floating debris resting against two piers (Figure 2.3). This type of accumulation can also exist between a pier and an abutment. A similar type of accumulation can occur between a pier and a bank or other large fixed object, such as boulders and trees that can support one end of the floating debris. Like the potential for single-pier accumulations, the potential for span blockages is influenced by pier placement.(17)
Figure 2.3. Span blockage accumulation bridge failure. (Louisiana)
As noted by Diehl, most large debris accumulations are similar in shape. Floating debris is initially trapped on the pier perpendicular to the approaching flow, but as the accumulation increases in size, debris accumulates parallel to the upstream edge of the accumulation. This process results in an accumulation with a curved upstream edge, and with the upstream nose of the accumulation raft near the thalweg, where most of the debris is transported. The accumulation is typically deepest at the piers that support them, and widest at the surface. The potential to achieve a roughly rectangular cross section from the bed to the water surface depends on the abundance of debris, prolonged periods of high water, or multiple floods without removal of the accumulation.(17)
Debris accumulations initially form at the water surface, grow toward and eventually become part of the streambed. As the water surface increases during a flood, floating debris already existing on the bridge usually remains in place as additional floating debris accumulates at the water surface. When the flood subsides, the new accumulated debris usually slides downward until it rests on the bed or on the previous debris accumulation to form a solid mass with irregular protrusions around the base of the pier.
2.3 Problems Associated with Debris
There are various potential problems associated with debris accumulations. In general, debris accumulations can adversely impact the conveyance through a culvert or bridge structure, exacerbate the contraction and local scour at a bridge structure, increase the hydraulic loading on a bridge structure, and cause upstream flooding. Several failures of highway bridges, roadway embankments and highway culverts have been attributed at least in part to debris accumulation. Figure 2.4and Figure 2.5 shows two bridge failures attributed to debris accumulation during flood events.(49, 53)
Figure 2.4. Missouri Highway 113 bridge over Florida Creek near Skidmore, Missouri.
Figure 2.5. Debris accumulation failure at bridge located in Oklahoma
Debris accumulation can partially or totally block the waterway opening for a culvert or bridge structure.(14,17,40,47,49,50) A massive accumulation of woody debris at a bridge is shown in Figure 2.6. Blockage of large portions of the waterway opening will increase backwater elevations upstream, increase flow velocity through the contracted opening under the structure, and modify flow patterns.(14,17,49,50) The increase in backwater upstream significantly increases the upstream inundation boundaries. High velocity contracted flow and large water surface elevation differences from the upstream to downstream side of the bridge can cause high drag and hydrostatic forces on the structure that can cause structural failure and collapse. Flows increased in velocity by the obstruction of the waterway and deflected away from the main channel can cause severe erosion near abutments or stream banks. Reduction in bridge waterway opening by debris can also cause a reduction in the flow rate required to overtop and potentially damage bridge approach roadways and embankments. Large accumulations could adversely affect the flow patterns near the structure by creating a strong lateral flow across the river towards the adjacent piers or embankment fill at unanticipated and potentially severe angles of attack, resulting in deep local scour at piers or abutment embankment fill.(17,50,51)
Figure 2.6. Debris accumulation at a bridge structure.(17)
Debris accumulations can also exacerbate the scour near the culvert or bridge structure. Figure 2.7 depicts streambank failure associated with a span blockage (already shown in Figure 2.3). As stated above, the blockage of flow area from debris accumulations can cause a significant increase in the flow velocities through the bridge structure. This increase in flow velocities and boundary shear stresses may cause an increase in the contraction scour through the bridge if the entire bridge opening is affected.
Figure 2.7. Debris induced bridge and bank failure structure. (Louisiana)
A laboratory study performed by Dongol(20) showed that debris accumulations (simulated using cylindrical shaped, PVC disks) cause larger and deeper scour holes to develop as a result of the significant increase in the downward velocity below the debris and the increase in both the horseshoe vortex size and the contact area of the vortex. The increase in both the contraction and local scour near the bridge structure could possibly damage or cause failure of the structure due to undermining of the pier footing or the abutment toe. Unfortunately, there has been only limited research conducted on local scour at piers with debris accumulation. Therefore, the scour associated with debris accumulation is extremely difficult to assess with any reliability.
Damage and failure of several bridges has been related to the increase in the hydraulic loading on the structures caused by debris accumulations.(14,17,40,47,49,50) Highway bridges partially or fully submerged during a flood event are subjected to various types of forces. These forces include hydrodynamic drag and side forces, hydrostatic forces, buoyant forces, hydrodynamic-lift forces, and impact forces.(49)
Hydrodynamic drag forces result from the reaction of the water as it flows around an object, and it acts parallel to the direction of flow.
Side forces are similar to drag forces, but act perpendicular to the flow direction.
Hydrostatic forces on the bridge elements are related to the differential in water surface elevations at the upstream and downstream sides of the structure caused by the flow constriction through the bridge.
Buoyant forces result from the displacement of water by the bridge or by the debris lodged under the bridge.
Hydrodynamic-lift forces are related to the total dynamic pressure force acting in the vertical direction perpendicular to the flow direction and the side force.
Impact forces are related to the moving debris colliding with the bridge structure.
Debris accumulations cause an increase in these forces due to the increase in upstream water surface elevations, increase in the flow velocities through the bridge, and increase in projected area of these forces on the structure.(49) Increases of these forces may cause the bridge structure to collapse either by buckling of the bridge substructures, shearing of roadway deck supports, or overturning of the structure (see Figure 2.8).(49,50)
Figure 2.8. Effects of debris accumulation at a bridge structure. (New York)
Miscellaneous problems associated with debris accumulation include difficult and expensive maintenance programs required for debris removal, an increase in fire potential near the structure, and minor damage to the structure.(14)