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Debris Control Structures Evaluation and Countermeasures
The volume of large debris within a watershed can be determined using a procedure applied by Diehl and Bryan(18) for a basin of the West Harpeth River in Tennessee. The general procedure involves selecting several different reaches of the river that are representative of the conditions upstream and downstream of the selected reach. The representative reaches could be selected using aerial photographs and/or during the reconnaissance field investigation. Debris greater than a certain length is counted and measured within each of the reaches. Debris concentration is then calculated for each of the reaches as either:
The total volume of debris for each of the individual reaches is then estimated by multiplying the debris concentration by the total length that the selected reach represents. For example, a reach that has a debris concentration of 27 m3/km (i.e., 3 kilometers in length containing 81 m3 of debris) and a total representative length of 12 km would have a total volume of debris of 324 m3 (i.e., 27 m3/km times 12 km equals 324 m3).
Finally, the volumes for each of the individual reaches are summed to determine the total volume of debris. For example, a watershed that has three reaches with individual debris volumes of 324 m3, 500 m3, and 210 m3 would produce a total volume of debris of 1,034 m3 (i.e., 324 m3 plus 500 m3 plus 210 m3 equals 1,034 m3).
During the counting and measuring of the debris, additional information about the debris should be noted and documented. The information should include:
The first phase in evaluating the potential for debris accumulation at a bridge is to estimate the potential for debris delivery to the bridge site. The tasks involved for this phase include estimating the potential for delivery of floating debris, estimating the largest size of floating debris delivered to the site, and assigning location categories to all parts of the highway crossing.
The potential for debris delivery is evaluated based on the potential for the debris to be transported downstream to the bridge site and the potential for debris generation as defined by direct and indirect evidence. Observations of floating debris provide the most direct evidence for assessing the potential for debris delivery. These observations could be made of the channel system or of accumulations at bridges and/or at other sites upstream of the bridge structure or within a basin of similar characteristics. Even though present observations indicate that there is a low potential for debris delivery, infrequent catastrophic events or changes in the watershed could still result in abundant floating debris in the future. Therefore for such events, indirect evidence should be considered.
Direct evidence for high delivery potential includes the following observations:
Direct evidence of low potential for drift delivery may be indicated by the following observations:
The potential for debris delivery can also be assessed from indirect evidence of debris generation. As previously discussed, a major source of floating debris is from bank erosion. Therefore, evidence of existing or potential bank erosion can be considered as indirect evidence for high potential of debris generation. Observations of indirect evidence for abundant debris generation include:
Indirect evidence for low potential of debris generation includes the following observations:
Where indirect evidence indicates that there is a high potential for existing or potential future debris generation, the ability of the channel system to transport the debris will control the potential of the debris delivery to the site. In general, most streams are capable of transporting some of the debris, and one should assume that the stream is capable of transporting the debris unless there is evidence to the contrary. Stable, densely forested streams transport little debris and can be assumed to have a low delivery potential as long as the forest will not be cleared in the future.(17)
Applying the information above, a "High Delivery Potential" exists when there is an abundant amount of direct evidence of debris delivered to the site, or there is indirect evidence of existing or future debris generation within the watershed and the upstream channel is capable of transporting the floating debris to the site; and, a "Low Delivery Potential" exists when there is a sparse amount of direct evidence of debris delivered to the site and there is no existing indirect evidence of future debris generation within the watershed, or when the upstream channel is incapable of transporting the floating debris to the site.
The second task of the first phase is to estimate the size of the largest debris delivered to the site (Maximum Design Log Length). This debris size influences the potential size of the debris accumulation. The largest debris delivered to the site is influenced by the channel dimensions upstream of the site, particularly the channel width. These dimensions may change over the project life of a bridge as a result of future stream instabilities, and these changes should be accounted for when defining the channel dimensions.
As illustrated in Figure 3.1, the maximum design log length is estimated on the basis of the narrowest channel width immediately upstream from the site. This distance should be measured perpendicular to the banks or lines of permanent vegetation at the inflection points between bends.
The minimum channel depth required to transport large trees is estimated to be about the diameter of the butt plus the distance the root mass extends below the butt, or roughly 3 to 5 percent of the estimated tree length.
The design log length represents a length above which logs are insufficiently abundant and insufficiently strong throughout their full length to produce an accumulation equal to their length, and it does not represent the absolute maximum length of trees within the watershed upstream of the site. Diehl(17) recommends estimating the design log length at a given site as the smallest of these three values:
Width of the channel upstream from the site.
Maximum length of sturdy logs. The height and diameter of mature trees on the banks determine the maximum length of the logs that can be delivered to the bridge as floating debris and capable of withstanding hydraulic forces when lodged against the piers. The maximum sturdy-log length seems to reach about 24 m (about 80 ft) in much of the United States; however, it may be as long as about 45 m (about 150 ft) in parts of northern California and the Pacific Northwest
9 m (30 ft) plus one quarter of the width of the channel upstream from the site, in much of the United States. As indicated by Diehl(17), this third constraint reflects the rarity of long logs and their breakage during transport, and it should not be considered for sites located in northern California or the Pacific Northwest.
The last task for the first phase involves assigning location categories to all parts of the highway crossing. As previously mentioned, debris is generally transported along a relatively small portion of the channel cross section. As a result, some areas of a site may be entirely free of debris transport, whereas other areas may receive a significant amount of debris. The various categories include:
Sheltered Location. A sheltered location is defined for the section of the bridge that includes a forest area directly upstream of the bridge that traps the transported debris and prevents it from being delivered to the bridge. This category should only be applied when the gaps between trees are much narrower than the average tree height and the width of forest along the direction of flow is more than a double line of trees. Intuitively, this category should not be applied to the upstream forest area if it is potentially subject to clearing.
Bank/Floodplain Location. This category includes the slope of the bank, top of the bank, and the floodplain since piers located on the slope of banks or at the top of the bank are just as likely to accumulate debris as piers located in the floodplain. The floodplain includes any area outside of the channel that is inundated in the design flood to a depth sufficient to transport drift, and it may be either clear of trees or a forested area that is subject to future clearing. If there is evidence that debris is transported within the slope of the channel banks, then the banks should not be assigned to this category.
In the Channel Location. Debris can be transported anywhere in the channel. As expected, debris accumulations are more common for "in the channel" locations than for "bank/floodplain" locations, so the potential for debris accumulations for this category is higher than for the previous category. In humid regions, the "in the channel" location is typically defined by the base flow. In arid regions, where the base flow is relatively low, or for ephemeral streams, this location is typically defined between the toes of the banks. If there is evidence that debris is transported within the slope of the channel banks, then they should be assigned to this category.
In the Path Location. This category is defined for the portion of the cross section in which the majority of the debris is transported. As previously mentioned, floating debris is generally transported in most streams along a relatively narrow path within the channel where the secondary circulation currents converge at the surface. In a straight reach, this convergence zone typically coincides with the thalweg of the channel where the flow is the deepest and fastest. In a curved reach, this zone generally exists between the thalweg and the outside bank of the bend. The best way to identify the debris path is to observe it during bank-full or high flow conditions. The observations do not need to be of large pieces of debris since all floating material responds similarly to the flow pattern. If observations indicate that the debris path includes part of the bank or part of the flood plain, then they should be assigned to this category. If high-flow observations are not available, observations during base flow can confirm the estimates based on channel characteristics. If direct observation is impossible for all flow conditions, then the location of the debris path can be estimated based on channel characteristics and assuming that the width of the debris path is about one-third the channel width. If the location of the debris path is indefinite, then several different locations of the debris path could be considered, i.e., the left third of the channel, the middle third, or the right third, or the entire channel could be assigned to this category, which would reflect the worst case scenario.
The second phase in evaluating the potential for debris accumulation at a bridge is to estimate the debris potential on individual bridge elements. The tasks involved for this phase include assigning bridge characteristics to all immersed parts of the bridge and determining the accumulation potential for each of these parts.
There are certain characteristics of a bridge structure that influence the potential for debris accumulation. So, the bridge structure should be divided into different components and the potential for accumulation should be evaluated separately for each of the components. The different components include piers, abutments, any gaps between fixed elements of the bridge opening, and the portion of the superstructure submerged during the design flood event.
An effective width needs to be determined for both horizontal and vertical gaps in the bridge structure below the design water surface elevation. Horizontal gaps between adjacent piers, between each bank and the nearest pier in the channel, and between each abutment and the nearest pier in the channel are common locations for large accumulations. The potential for accumulation is high when the effective width of the horizontal opening is less than the length of the longest piece of debris delivered to the bridge. When this is the case, debris typically comes into contact with one of the bridge elements, and then rotates downstream until it becomes lodged against another of the bridge elements. The effective width of the horizontal gaps should be reduced to account for any skew in the bridge to the approaching flow.
When the water surface elevation is at or above the bottom elevation of the superstructure ("low chord elevation"), debris can become trapped vertically between the superstructure and the streambed below it. When floating debris hits the superstructure, most of the pieces rotate to one side and remain at the water surface, resulting in an accumulation against the superstructure at the surface. However, some debris could be lodged between the streambed and the superstructure as a result of the upstream end of the debris rotating downward until it encounters the streambed after striking the superstructure roughly endwise. The height of the vertical gap between the low chord elevation of the bridge and the elevation of the streambed beneath should be based on the minimum height since the height most likely will vary along the bridge due to the changes in the elevations of the low chord and/or the streambed.
Narrow openings of the structure elements of the bridge at the water surface elevation also determine whether debris would be deflected or trapped. Piers and superstructures with narrow openings that convey flow are significantly more likely to trap and accumulate debris. Examples of such structures include:
Also, any pier with existing accumulations should fall under this classification. Elimination of these narrow openings by using a single solid pier (wall, cylinder, or hammerhead), a superstructure with a solid parapet, and a solid beam that is connected directly to the pier would increase the likelihood of the debris being deflected and not trapped by the structure.
The first step in determining the accumulation potential for each of the bridge components is to assign a location category described in the previous section to each of the components. The selected category for a horizontal gap should be based on the most debris-prone location category occupied by the fixed elements that define the gap. For example, a horizontal gap from a bank or abutment to a pier located in the debris path should be assigned to the "in the path" location category, or a gap that has one of the fixed elements sheltered while the other element in the floodplain should be assigned to the "floodplain" location category.
|Accumulation Potential||Gap Wider than Design Log Length||Location Category||Potential for Debris Delivery|
|No||In the Channel||Low|
|Medium||No||In the Path||Low|
|High||No||In the Channel||High|
|High, Chronic||No||In the Path||High|
|Accumulation Potential||Pier type||Location Category||Potential for Debris Delivery|
|Solid Pier||In the Channel||Low|
|Piers w/ Openings||Bank/Floodplain||Low|
|Medium||Solid Pier||In the Channel||High|
|Solid Pier||In the Path||Low|
|Piers w/ Openings||Bank/Floodplain||High|
|Piers w/ Openings||In the Channel||Low|
|High||Solid Pier||In the Path||High|
|Piers w/ Openings||In the Path||Low|
|Piers w/ Openings||In the Channel||High|
|High, Chronic||Piers w/ Openings||In the Path||High|
Both of these tables were generated from the information presented by Diehl.(17) As shown in these tables, the potential for debris accumulation is based on the estimated delivery potential, which is the same for the entire site, the location category, and the effective length of the span between fixed elements relative to design log length (gap wider or narrower) for span accumulations and the presence or absence of narrow openings that carry flow for single pier accumulations.
The last phase involves calculating the hypothetical accumulation over the entire length of the bridge with a medium, high, and high, chronic potential. The hypothetical accumulation with a medium and high potential should be used to evaluate the effects that the accumulation would have on the hydraulic characteristics through the bridge and on the hydraulic loading on the structure. The hypothetical accumulation with a high, chronic potential can be used to define the potential maintenance requirements for the bridge, i.e., the location and maximum extent of debris removal.
The potential for debris accumulation estimated in the preceding section is related to the likelihood of occurrence relative to the various components of the bridge, and it does address the likely size of an accumulation. A pier with a "high potential" for accumulation indicates that there is a high potential for accumulation at the pier relative to the potential for accumulation at the other piers, and an accumulation will not necessarily form on such a pier. If an accumulation does form, it may be as wide as the design log length and extend vertically to the depth of flow, or it may be much smaller. The size of the accumulations depends mostly on the debris dimensions and delivery rates, the flow depth, and the number and proximity of gaps and piers affected. Accumulations in the channel can reach their maximum size during a single flood where delivery is high, but accumulations grow more slowly where the debris delivery is low or when the accumulation is outside of the channel.
Diehl(17) proposed that accumulations on a single pier should have a width equal to the design log length over its full flow depth, accumulations across two or more piers should extend laterally half of the design log length beyond them, and accumulations on vertical and horizontal gaps should extend across the entire width and height of the gap. Diehl based this proposal upon conservative assumptions consistent with his largest observed debris accumulations.(17)
Because of limited descriptions and observations available for debris accumulations on superstructures, Diehl could not provide a means to estimate the maximum size of accumulations on superstructures.(17) Consequently he recommended following the suggestions provided by Wellwood and Fenwick(64), which define the vertical extent of the accumulation being 1.2 m (4 ft) above the top of the bridge parapet wall and below the low chord elevation. Based on the above information, the maximum extent of debris accumulations is summarized in Table 3.4.
|Pier||Design Log Length||Flow Depth|
|Superstructure||Span Width||Vertical Height of Superstructure plus 1.2 m Above and Below the Superstructure|
|Horizontal Gap||Width of Gap||Smaller of Vertical Height of Gap or Flow Depth|
|Vertical Gap||Width of Gap||Vertical Height of Gap|
The overall potential for debris accumulation at a bridge should be defined by the highest potential estimated for the different bridge components. Therefore, a bridge should be considered as a high potential for accumulation if any of its components have been determined to have a high potential for accumulation.
Debris accumulations over the entire length of the bridge should be developed and evaluated for both medium and high potential conditions, with the second condition reflecting a condition that is more likely to occur than the first condition. As proposed by Diehl(17), an accumulation over the entire length of the bridge with a medium potential can be represented by assuming that all of the individual medium- and high-potential accumulations grow to their maximum size. Similarly, an accumulation with a high potential can be defined by assuming that all of the individual high-potential accumulations grow to their maximum size.
As previously mentioned, debris accumulations can cause significant changes in the hydraulic characteristics through the bridge and the trapping characteristics at the bridge. These changes could cause an increase in the potential for accumulation at the bridge. For example, a high-potential blockage across the channel may cause skewed flow through the bents that were initially considered to be aligned with the flow, or the superstructure could become immersed as a result of the increased backwater caused by the debris accumulation on the structure. Therefore after the initial assessment, the bridge should be re-evaluated using the bridge comprised of the debris accumulation determined from the initial assessment and the corresponding hydraulics associated with the accumulation.
Finally, the overall potential for debris accumulation at bridges depends on the probability or frequency of occurrence for the events that were used to define the potential for debris delivery and accumulation at the bridge. A high-potential assessment based on a large flood event and significant changes in the watershed and upstream channel would have different implications on the bridge design and maintenance compared to a high-potential assessment based on a 2-year flood event and existing channel conditions.
There are several different factors that can influence debris production. These factors include floods, fires, urbanization, logging, land clearing (i.e., grazing and agriculture), conservation practices, and channel improvements.
Flooding increases debris production as the associated discharges serve as a means to produce and deliver debris to a site. Higher discharges are more likely to cause erosive forces on bank and floodplains. The inundation of the flooding event affects more of the floodplain area; facilitating transport of debris into the main channel.
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.
Urbanization over time causes an opposite effect in the yield of sediment from a watershed than that of fires. Initially, sediment yield can be significantly increased during the construction phase of development due to the removal of exiting vegetation and disturbance of the soil. However over time, the sediment yield decreases as the developed land becomes restabilized and land surface area exposed to the erosive effects of rainfall and runoff is reduced as a result of the increase in impervious area, such as roads, structures, and parking lots. Hydrological effects from urbanization include an increase in runoff volume, higher peak flows, and longer durations. These effects with the decrease in the sediment yield from the watershed could result in an increase in bank erosion and scour of the streambed, which could increase the generation and delivery of floating debris to a bridge site.
Logging has been identified as a source of floating debris.(14,23,25) A study conducted by Froehlich(25) indicated that different logging practices cause substantial differences in the loads of floating debris. 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.
Land Clearing associated with logging, grazing, or agriculture practices could cause the same effects associated with fires, however the magnitude of these effects would most likely not be as severe. Also, grazing allowed near a stream can result in a significant increase in bank erosion.
Conservation practices have the opposite effects than the effects associated with clearing of the land. Implementation of a different conservation practice can reduce both the amount of erosion and runoff from the land.
Channel improvements or modifications to the channel geometry and/or vegetation clearing from the channel can influence quantities of both floating debris and fine/coarse sediment. Improper design of such improvements can cause significant instabilities to develop within the system, including increased bank erosion, increased degradation and/or aggradation of the streambed, and/or significant changes in the planform, that could increase the generation and delivery of floating debris to a structure site.
Growth of riparian forest buffer strips has been recommended and encouraged by the US Environmental Protection Agency (US EPA) for their water quality, ecological and bank erosion benefits especially in agricultural areas. These forested buffer strips adjacent to stream channels are now common with maturing trees especially in heavily agricultural areas.
Extreme events, such as ice storms, debris flows, forest fires and insect infestations can drastically increase the debris load at some point in the life of the structure.