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Debris Control Structures Evaluation and Countermeasures
Hydraulic Engineering Circular No. 9

Chapter 6 - Design Procedures for Debris Countermeasures

6.1 General Procedures and Considerations

6.1.1 Field Investigations

Field investigations should be conducted prior to the design of a debris-control countermeasure or culvert/bridge structure. The purpose of the investigations is obtaining a general understanding of the debris problem at the site; acquiring data required for estimating the quantities of debris transported to the site; performing hydrologic, hydraulic, and sedimentation analyses; and attaining other miscellaneous design data. Several field investigations may be required for obtaining all of the data.

The type of debris transported to the site will influence the selection of the debris-control countermeasure, and define the type of data and analyses required to estimate the quantity of debris transported to the site. The estimated quantity of debris is needed by the designer to provide adequate debris storage immediately upstream of the site or to evaluate the potential impacts associated with debris accumulating on the culvert/bridge structure. The most useful and desired source of information on the types and quantities of debris delivered to the site would be from past floods. Such information could be secured from maintenance personnel, from inhabitants of the immediate area, or by personal observation. Unfortunately, this type of information rarely exists. Therefore, information to assist the designer in estimating the types and quantities of debris transported to the site must be obtained from field investigations. This information may include soil, land use, and topographic mapping; additional survey data; stream and watershed characteristics upstream of the site; aerial photographs; observations of the flow characteristics near the site and any direct and indirect evidence of high delivery potential for floating debris upstream of the site; sediment and discharge data; and future changes in the watershed.

Land use and soil maps are useful in estimating sediment yields of fine and coarse sediment. Land use maps can also provide an indication of future changes in the watershed that might influence the quantities of debris delivered to the site. There are many uses for topographic mapping and survey data (surveyed cross sections or digital terrain models, DTM). Some of these uses include developing a hydraulic model to evaluate the hydraulic characteristics upstream and downstream of the structure, defining the flow path of floating debris, defining the maximum allowable headwater elevation for a culvert structure, estimating the amount of debris storage available at the site, and defining potential access to the site. Sediment yield rates from gully, channel bank erosion, and mass wasting can be estimated by making a comparison between existing and historical topographic maps or survey data.

Information on the stream and watershed characteristics would include the locations and approximate extent of any lateral channel instabilities, aggradation or degradation trends within the watershed, roughness coefficients of the main channel and floodplain, type of sediment in the streambed and banks, location of any hydraulic controls, high water marks, channel dimensions, locations of existing debris accumulations, vegetation characteristics, and any potential damage locations if the debris-control countermeasure and/or culvert/bridge structure becomes clogged. Most of this information could be obtained when applying the stream reconnaissance technique documented in Chapter 4 of HEC-20 (34).

A well documented methodology for predicting bank erosion associated with stream meander migration using aerial photograph and maps has been developed by Lagasse, Spitz, Zevenbergen, Zachmann, and Thorne(36,37). The research for the development of this methodology was conducted as part of the National Cooperative Highway Research Program (NCHRP Project 24-16) for the Transportation Research Board, and the principal product of the research is a stand-alone Handbook(37). Information covered by the Handbook(37) includes:

  • Screening and classification of meander sites. A morphological classification scheme for alluvial rivers developed by Brice(11) was selected as the most appropriate system for the methodology developed. The original classification, however, was modified since not all of the original classes are commonly encountered. Based on the modified classification system, a screening system was established to define if the methodology would be applicable to a study reach. The methodology is not applicable for the classifications that possess either considerable stability or excessive instability.

  • Sources of mapping and aerial photographic data. The Handbook provides guidance on obtaining contemporary and historical aerial photographs and maps.

  • Basic principles and theory of aerial photograph comparison. The Handbook includes a brief discussion on the types of photogrammetry, photogrammetric products, and the application of photogrammetry to meander migration.

  • Manual overlay and computer assisted techniques. The procedures for the manual overlay techniques documented in the Handbook are briefly presented as follows: (1) obtain aerial photograph and maps; (2) convert aerials/maps to a common scale; (3) define common points for all aerial/maps; (4) trace banklines and registration points onto a transparent overlay for each set of aerials/maps; (5) define the average bankline arc, the radius of curvature of the bend, and the bend centroid position for best-fitted circles of the outer bank of each bend; and (6) define the future position of the bend by simple extrapolation based on the assumption that the bend will continue to move at the same rate and in approximately the same direction as it has in the past. The same general procedure for the manual overlay techniques can be accomplished more easily and efficiently using common computer software with drawing capabilities, such as most word processing and presentation applications or more powerful and versatile computer aided drawing (CAD) programs.

  • GIS-based measurement and extrapolation techniques. Another product of the research was the development of Geographic Information System (GIS) software based menu-driven extensions to assist in applying the developed methodology. Two extensions were developed: (1) Data Logger, and (2) Channel Migration Predictor. The Data Logger extension was developed to streamline the measurement and analysis of bend migration data and aid in predicting channel migration. This extension provides the user a quick and easy way to gather and archive river planform data. This extension records various river characteristics that are arranged by the river reach for each river bend and historical record. The data archived by this extension is then utilized by the Channel Migration Predictor extension to predict the probable magnitude and direction of the bend migration at some specified time in the future. Both of these extensions are provided on a CD in Appendix G of the Handbook.

  • Statistical analysis. A statistical analysis of an extensive data set (nearly 2,500 measurements) was conducted to determine relationship for channel migration to the morphological classification scheme for alluvial rivers. This analysis showed that equiwidth meander reaches are fairly stable and will not change significantly over time, applying standard regression techniques to directly predict meander migration did not yield statistically significant relationships, and the results from the frequency approach should be used primarily as a supplement to the comparative analysis using aerial photography.

  • Sources of error and limitations. Information related to the sources of errors and limitations for using the developed methodology is provided in Chapter 6 of the Handbook.

  • Illustrated examples and application using manual overlay techniques. An illustrated example for applying the developed methodology is provided in Chapter 8 of the Handbook.

Aerial photographs are useful in defining the roughness coefficients of the stream and floodplain, defining the representative reaches for estimating the volume of floating debris within a watershed as discussed in Chapter 3 of this manual, defining locations of bank erosion, and showing the present conditions of the watershed. Sequences of historical aerial photographs can be used to estimate channel bank or gully erosion by measuring the aerial differences between the sets of photos. The volume of bank erosion can be estimated for long term erosion and bank migration rates or for single hydrologic events, if photos are available before and after the event.

Observations should be made of the flow conditions near the site during low and high flow conditions. These observations could be useful in defining the flow path of floating debris and the flow patterns near the site. For an existing structure, the observations could be useful in defining the region where the flow is affected by the structure, i.e., contraction and expansion lengths. Also, any direct and indirect evidence related to the delivery potential of floating debris as mentioned in Chapter 3 of this manual should be noted. For example, abundant floating debris stored in the channel is direct evidence for high delivery potential. There is considerable direct and indirect evidence of debris generation that can be collected and used to evaluate the potential for debris accumulation at a site.

Measured sediment and discharge data are useful in estimating sediment yields of fine and coarse detritus. Unfortunately, this type of data is seldom available. Therefore, hydrological and sedimentation analyses are required to estimate the quantities of this type of debris. The only additional types of data needed for these analyses that have not already been mentioned are sediment gradation curves of the streambed and banks.

Among the factors to be considered are possible future changes in the type and quantity of debris that might result from changes in land use within the drainage basin. Some of these potential changes include floods, fires, urbanization, logging, grazing, agriculture, channel improvements, and conservation practices. As an example, logging in a previously virgin area could increase the quantity of floating debris introduced into the stream and change the nature of the debris problem from one of "medium floating" to "large floating" debris.

Lastly, any data needed for the debris structure should be obtained during the field investigations. An example would be the maximum allowable headwater and embankment height for culvert or debris-control countermeasure. This information might also be necessary in selecting the type of debris-control countermeasure best suited to the particular problem.

In summary, the following information should be obtained during the field investigations:

  • Classification (as to the type) of the debris transported to the site.

  • Information for estimating the quantity of debris.

  • Land use and soil maps.

  • Existing and historical survey data.

  • Stream and watershed characteristics upstream of the site.

  • Aerial photographs.

  • Observations of the flow conditions near the site during low and high flow conditions.

  • Direct and indirect evidence related to the delivery potential of floating debris.

  • Information about future changes that could influence the quantity of debris.

  • Data required for design, i.e., maximum allowable headwater elevation for a culvert structure.

6.1.2 Selecting the Type of Countermeasures

As noted in the previous chapter of this manual, there are a wide variety of countermeasures available for debris-control. A debris-control countermeasures matrix, presented in Table 6.1, has been developed to provide guidance in the selection of countermeasures suitable for various types of the debris. The matrix is organized to highlight the various groups of countermeasures and to identify their individual characteristics. The left side of the matrix lists the type of countermeasures available for the three general groups of structural countermeasures for culverts and bridges, and non-structural countermeasures for both. In each row of the matrix, the countermeasures suitable for the various types of debris are identified. The matrix also identifies the States that have used the countermeasure and the general level of maintenance resources required for the countermeasure. Finally, a resource for design guidelines is noted, where available.

Table 6.1. Debris-Control Countermeasures Matrix.
Countermeasure Countermeasure Characteristics
Debris Classification Maintenance Aesthetics4 Env. Impact5 Installation Experience by State Design Guideline6
Floating Debris Flowing Debris Bed Material Estimated Allocation of Resources
Light Medium Large Fine Detritus Coarse Detritus Boulders H = High
M = Moderate
L = Low
Group 1. Structural Countermeasure
Group 1.A. Culvert Structures
Deflectors   x x       x H A L CA 6.2.1
Racks x x           H A L CT, CA 6.2.2
Risers       x x x   L A L CA 6.2.3
Cribs x         x   M A L CA 6.2.4
Fin     x         M A L SD, TN, CA 6.2.5
Dams and Basins       x x x   H A H Widely Used 6.2.6
Group 1.B. Bridge Structures
Deflectors   x x       x H - M U L CA, MS, IN, OR, WI, LA 6.3.1
Fins   x x         M A L CA 6.3.2
Crib Structure   x x           U L MS, CA, KS 6.3.3
R.T.S. - Iowa Vanes         x     M A M IA HEC-23(35)
R.T.S. - Permeable Spurs       x x x   M A M AZ, CA, IA, MS, NE, OK, SD, TX HEC-23(35)
R.T.S. - Impermeable Spurs       x x x   H A H Widely Used HEC-23(35)
In-channel Debris Basins       x x x   H A H Widely Used -
Flood Relief Sections   x x         L A L Widely Used 6.3.6
Debris Sweeper (Bridgeshark)   x x         L A L OK, VA, TN, OR 6.3.7
Booms x x           K U M ID -
D.F. - Freeboard   x x         L D L Widely Used 6.3.8
D.F. - Pier Type, Location, and Spacing   x x         L D L Widely Used 6.3.8
D.F. - Special Superstructure   x x         L D L TX, MS 6.3.8
Group 2. Non-Structural Countermeasure
Emergency and Annual Maintenance   x x x       H U M Widely Used -
Debris Management Plan   x x x       H D L   6.4

Notes:

  1. "x" corresponds to a suitable device.
  2. R.T.S. corresponds to River Training Structures.
  3. D.F. corresponds to Design Features.
  4. Classification for Aesthetics is: (1) U for Undesirable, (2) A for Acceptable, and (3) D for Desirable.
  5. Classification for Environmental Impact is: (1) L for Low, (2) M for Medium, and (3) H for High.
  6. Reference made above for the Design Guidance is related to the section indicated in this manual, i.e., information on deflectors for culverts is provided in Section 6.2.1.

In addition to the information contained within the matrix, the selection of the countermeasure should be based on the construction and maintenance costs, risk of failure, risk of property damage, and environmental and aesthetic considerations. The safety of highway traffic should also be considered in the selection of the countermeasure. The culvert end and the countermeasure should be located beyond the usual recovery area for errant vehicles or the countermeasure should be designed to enhance the drivers' chance of recovery. At existing sites where modifications cannot be made to meet this objective, an appropriate vehicle restraining device or an impact attenuating device should be provided on the roadside.

The countermeasure matrix (Table 6.1) was developed to identify distinctive characteristics for each type of countermeasure. Four categories of countermeasure characteristics were defined to aid in the selection and implementation of the countermeasures:

  • Debris Classification
  • Maintenance
  • Installation/Experience by State
  • Design Guideline References

These categories were used to answer the following questions:

  • For what type of debris is the countermeasure applicable?
  • What level of resources will need to be allocated for maintenance of the countermeasure?
  • What States or regions in the U.S. have experience with this countermeasure?
  • Where do I obtain design guidance reference material?

The Debris Classification Category describes the type of debris for which a given countermeasure is best suited or under which there would be a reasonable expectation of success. Conversely, this category could indicate the type of debris under which experience has shown that a countermeasure may not perform well or was not intended.

The type of debris considered for this category is based on the debris classification provided in Chapter 2 of this manual:

  • Light Floating Debris
  • Medium Floating Debris
  • Large Floating Debris
  • Flowing Debris
  • Fine Detritus
  • Coarse Detritus
  • Boulders

The suitable countermeasure for each debris classification is indicated by "x". When the debris is comprised of more than one type of debris, the information provided in this category can be used as guidance in selecting a combination of countermeasures to address the debris problems.

The Maintenance Category identifies the estimated level of maintenance that may need to be allocated to service the countermeasure. The rating for this category is subjective, and it ranges from "Low" to "High." The ratings represent the relative amount of resources required for maintenance with respect to other countermeasures provided within the matrix (Table 6.1). A low rating indicates that the countermeasure is relatively maintenance free; a moderate rating indicates that some maintenance is required; and a high rating indicates that the countermeasure requires more maintenance than most of the countermeasures in the matrix.

The Aesthetics Category identifies the estimated level of appearance associated with the countermeasure with respect to other countermeasures provided within the matrix (Table 6.1). The rating for this category is subjective, and it ranges from "Undesirable" to "Desirable." An undesirable rating indicates that the countermeasure is noticeably unpleasing to the sight; an acceptable rating indicates that majority of the structure is pleasing to the sight; and a desired rating indicates that the countermeasure is noticeably pleasing to the sight.

The Environmental Impact Category identifies the estimated level of impact the countermeasure would have on the environment with respect to other countermeasures provided within the matrix (Table 6.1). The rating for this category is also subjective, and it ranges from "Low" to "High." A low rating indicates that the countermeasure does not adversely impact the environment or the impacts are considered short term; a moderate rating indicates that some adverse impacts could occur with implementation of the countermeasure; and a high rating indicates that the countermeasure would adversely impact on the environment.

The Installation/Experience by State category identifies DOTs that have used the countermeasure. This information was obtained from three sources: response of the DOTs to a debris-related questionnaire documented in "Debris Problems in the River Environment" (1979)(14); Brice and Blodgett, "Countermeasures for Hydraulic Problems at Bridges, Volumes 1 and 2" (1978)(12, 13); and correspondence between FHWA and DOT staff. It is expected that additional information on state use will be obtained as this matrix is distributed and revised. Countermeasures that have been used by many States are given a listing of "Widely Used." The listing reflects both successful and unsuccessful experiences.

The countermeasures matrix (Table 6.1) is a convenient reference guide on a wide range of countermeasures applicable to addressing debris problems at culvert and bridge structures. A comprehensive plan of action would be to provide conceptual design and cost information on several alternative countermeasures, with a recommended alternative being selected based on a variety of engineering, environmental, and cost factors. The countermeasures matrix is a good way to begin identifying and prioritizing possible alternatives. The information provided in the matrix related to the suitable applications for the various types of debris and maintenance issues should facilitate preliminary selection of feasible alternatives prior to more detailed investigation.

6.1.3 Design for Bridges versus Culverts

The countermeasures provided in Table 6.1 have been divided into two groups: Structural countermeasures and Non-structural countermeasures. As seen in the table, the structural countermeasures have been further divided into structures available for culvert and bridge structures. Culverts, as distinguished from bridges, are usually covered with embankment and have structural material around the entire perimeter, although some are supported on spread footings with the streambed serving as the bottom of the culvert. Culverts are typically provided for small drainage basins and watercourses, and they are considered minor structures compared to bridges. Bridges, on the other hand, are usually used where the discharge of the watercourse is significant (larger drainage basins) or where the stream to be crossed is large in extent.

Because of the significant difference in size and function of these structures, some of the countermeasures available for culvert structures cannot be used for bridge structures, and vice a versa. Some countermeasures available for culvert structures are also available for bridge structures even though the intended purpose is different for the two structures. The debris deflector is an example of such a countermeasure. Debris deflectors are used at culverts to prevent debris from going through the culvert by deflecting it to the side of the structure where it is stored (debris retention), whereas debris deflectors at bridges are used to deflect the debris away from the pier and through the bridge opening (debris passage).

At many locations, either a culvert or bridge structure will satisfy both the structural and hydraulic requirements of the stream crossing. Structure choice at these locations should be based on construction and maintenance costs, risk of failure, risk of property damage, traffic safety, environmental, and aesthetic considerations. An additional deciding factor at these locations may be related to debris passage. Instead of providing a debris-control countermeasure at a proposed site, it may be desired to design the structure for debris passage. However, there are some obvious limitations in the case of culverts. There is no real assurance that doubling the size of a culvert will eliminate the threat of the culvert becoming plugged if debris poses a problem at the site. It is obvious that the probability of this occurring does decrease to some degree with increases in the size of the culvert. However, it is extremely difficult to demonstrate what level of protection would be obtained by such increases. Therefore, it may be necessary to use a bridge structure that is designed for debris passage with a higher degree of certainty in lieu of a culvert structure even though it can adequately convey the anticipated flows.

Both types of non-structural countermeasures can be considered for culvert and bridge structures. However, a debris management plan would more likely be implemented at a bridge than a culvert due to the high cost and allocation of resources required to develop and execute such a plan making it infeasible for small drainage structures.

6.1.4 Existing Structures Versus Proposed Structures

The selection and design of the countermeasures presented in Table 6.1 could depend on if the countermeasure is for an existing or proposed structure. Constraints at an existing structure can prevent the use of certain countermeasures or influence the design of the countermeasure. The constraints could be related to the physical conditions at the site; the structure itself; monetary reasons; environmental or maintenance requirements; limited or no access to the culvert or bridge; or other reasons. Recent development adjacent to the watercourse upstream of an existing structure might prevent the use of an in-channel debris basin or dam because of potential flooding impacts or political pressure from the residents. The geometry of the existing structure could influence the configuration and dimensions of the proposed countermeasure. Certain countermeasures might require that part of the existing structure be demolished and significantly modified, making it too expensive to implement. It is also possible that environmental restrictions due to fish passage or vegetation removal could limit the type of countermeasure that could be selected and constructed at a particular site.

All of the countermeasures presented in Table 6.1 could be used at proposed structures. However, this is not the case for existing structures. Unfortunately, the most common type of countermeasures used for bridge structures are usually infeasible to implement at existing bridge structures. These measures are identified in Table 6.1 as design features, i.e., "D.F.", and they include adequate freeboard, the use of special superstructure, and considerations to the type, location, and spacing of piers for reducing the potential of accumulation. These countermeasures can easily be incorporated into the design of a proposed structure, whereas they are difficult to implement at an existing structure. The anticipated debris accumulation on a proposed bridge can be considered in the: (1) design of the hydraulic opening through the bridge to safely convey the design flood without overtopping the structure, (2) structural design of the bridge components to withstand the increase in lateral and overturning forces associated with the debris accumulations, and (3) design of the pier and abutment foundations to prevent undermining of the structure by the significant scour associated with debris accumulation.

Proposed structures also have the benefit over existing structures in that access for maintenance can be included in the design of the structure. Access to a proposed structure can be incorporated into the design of the highway embankment where this might not be a viable option for an existing highway embankment.

For existing structures, the problems associated with debris are usually more easily understood. There is generally sufficient information on the type of debris, quantity of debris transported to the site, and the associated problems with the debris available to select and design countermeasures to address the problem. In some instances where the investment is relatively small and there is little chance of interruption to current operations, it may be more desirable at a proposed site to select and design the countermeasure after the problems with debris have developed and are fully understood.

6.1.5 Maintenance Accessibility of the Countermeasure

Maintenance is an important factor to consider in selecting a debris-control countermeasure or designing a bridge structure. This should entail both regular and emergency maintenance activities. Considerations should be made as to the ease and cost of maintaining the countermeasure and accessibility to the countermeasure for performing the maintenance work. A countermeasure that is more expensive to construct may be more desirable if it is easier and less expensive to maintain.

Provisions should be made for access to the countermeasure for maintenance purposes. Maintenance personnel should be consulted with when designing the access to the site. Unfortunately, access is often difficult to provide, and it may not be provided for countermeasures designed for secondary highways or lower class roads. If access roads to the countermeasure are impractical and the risk associated with flooding is high, it may be necessary to provide an area near the countermeasure where mechanical equipment, such as a crane, could perform maintenance activities, such as, debris and sediment removal.

Maintenance accessibility for debris removal should be considered in the design of a new or replacement bridge. There are certain features that can be incorporated into the design to simplify debris removal. For instance, the use of solid wall piers that extend slightly upstream of the edge of the bridge deck. This type of pier configuration provides for easier removal of debris than other pier types. Debris not only accumulates more readily on multiple-column piers, but also may become entangled between the columns along the full width of the underside of the bridge, making it extremely difficult to remove the debris and/or causing access problems for the debris-removal crew. Debris trapped on trusses and piers with multiple columns can be entangled among multiple structural elements. The entanglement makes debris removal more difficult and increases the possibility that the bridge could be damaged during the removal operations. Hammerhead piers are an alternative to multiple columns. This type of pier eliminates the potential for entanglement. However, debris removal is still difficult since the pier nose is well beneath the bridge deck and it is extremely difficult to access and lift the debris from the bridge deck. Superstructures that allow access to the pier nose from directly above also ease debris removal. A wide deck with a simple parapet and adequate load-bearing capacity for heavy equipment at the upstream edge provides the best opportunity for debris removal from the bridge deck.

Access should be provided to the substructure of bridges to ensure prompt and complete removal of debris that accumulates on the structure. Debris removal can usually be accomplished during low flows with tracked vehicles for bridges over small streams. However 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.

6.2 Design Guidelines for Culverts

6.2.1 Debris Deflectors for Culverts
  1. FUNCTION - The function of a debris deflector (Figure 5.1 through Figure 5.9) is to divert medium and large floating debris and large rocks from the culvert inlet for accumulation in a storage area where it can be removed after the flood subsides. Their structural stability and orientation with the flow make deflectors particularly suitable for large culverts, high velocity flow, and debris consisting of heavy logs, stumps, or large boulders.

  2. STORAGE AREA - The storage area provided must be adequate to retain the anticipated type and quantity of debris expected to accumulate during any one storm or between cleanouts.

  3. TYPE OF MATERIAL - Debris deflectors are usually built of heavy rail or steel sections (Figure 5.1 through Figure 5.9), although timber (Figure 5.8 and Figure 5.9) and steel pipe are sometimes used when the type of debris consists of light floating debris and/or fine detritus. The decision to use timber in lieu of steel could also be based on the availability of the material within the region and construction costs. Wire and post debris deflectors (Figure 5.3) have also been used for light floating debris. For economy, salvaged railroad rails may be used if available. Figure 5.3 shows a deflector that uses a cable as its lower longitudinal member. This modification has proved to be superior in locations where heavy boulders damage rigid members.

  4. LOCATION AND ORIENTATION - The deflector should be built at the culvert entrance and aligned with the stream rather than the culvert so that the accumulated debris will not block the channel. Individual deflectors can be built over each pipe (Figure 5.4) or a single deflector can be built over multiple pipe culverts (Figure 5.5). The deflector may be placed at the culvert entrance or a distance of 1 culvert dimension upstream. The apex of the deflector will "point" upstream.

  5. DIMENSIONS - The angle at the apex of the deflector should be between 15° and 25°, and the total area of the two sides of the deflector should be at least 10 times the cross-sectional area of the culvert. The base width and height of the deflector should be at least 1.1 times the respective dimensions of the culvert. The upstream member is vertical on most installations. However, a sloping member at the apex (sloping downstream from bottom of member) would reduce the impact of large floating debris and boulders, and probably prevent debris from gathering at that point. Therefore, deflectors with a sloping member at the apex are recommended over a vertical upstream member.

  6. BAR SPACING - Spacing between vertical members should not be greater than the minimum culvert span dimension nor less than 1/2 the minimum dimension. A spacing of 2/3 the minimum dimension is commonly used. In addition to what is required for structural support, spacing of the horizontal bars along the sides of the deflector follow similar characteristics. Where headwater from the design flood is expected to be above the top elevation of the deflector and floating debris is anticipated, horizontal members should be placed across the top. The spacing of horizontal members on the top should be no greater than 1/2 the smallest dimension of the culvert opening.

6.2.1.1 Debris Deflector Example (Culvert)

Given:

Circular Culvert, Diameter = 1.8 m
Sediment material comprised of coarse detritus and medium floating debris

Determine:

Determine the dimensions of a triangular shaped debris deflector.

Solution:

Step 1: Determine the height and minimum width of the debris deflector

Height, H = 1.1(Culvert diameter, D) = 1.1(1.8) = 1.98 m (use 2.0 m)

Minimum width, Wmin = 1.1D = 1.1(1.8) = 1.98 m (use 2.0 m)

Step 2: Select desired apex angle, bar spacing and thickness

Apex angle, α, can range from 15° to 25°, use 20°

Bar spacing, s, can range from 1/2(D) to D. 2/3(D) is common spacing, so use 1.2 m.

Deflector will be constructed out of 76-millimeters-thick, t, steel rails. The thickness of the rail was selected taking into account the type of debris, availability of the material, cost, and structural stability.

Step 3: Determine the side length of the debris deflector

A trial and success procedure is required to determine side length of the debris deflector:

  1. determine the minimum side area of deflector,
  2. assume a length of the deflector,
  3. determine the number of vertical and horizontal bars,
  4. compute the gross area of the deflector,
  5. compute the total area of the steel rails,
  6. compute the net area of the deflector, and
  7. complete substeps "b" through "f" until the net area of the deflector is slightly greater than the minimum side area of the deflector.

Implementing these substeps "a" through "g"

Substep "a"

Minimum side area of deflector on each side = 5(Area of culvert)
Minimum side area of deflector on each side = 5(πD2/4) = (5)((3.14)(1.8)2)/4) = 12.72 m2

As this is a trial and success solution, this example will only show the values in the first and final iteration. The "final" iteration will be a bold, italicized value provided within parenthesis.

Substep "b"

Assume a length, L, of 6.7 m   (Final: 7.9 m)

Substep "c"

Number of vertical bars = (L + s)/(s + t) = (6.7 + 1.2)/(1.2 + 0.076) = 6.19 -> use 7   (Final: 8 bars)

Number of horizontal bars = (H + s)/(s + t) = (2.0 + 1.2)/(1.2 + 0.076) = 2.51 -> use 3   (Final: 3 bars)

Substep "d"

Gross area per side = (L)(H) = (6.7)(2.0) = 13.4 m2   (Final: 15.80 m2)

Substep "e"

Area of bars = (number of horizontal bars)(t)(L) + (number of vertical bars)(t)(H)
Area of bars = (3)(0.076)(6.7) + (7)(0.076)(2.0) = 2.59 m2   (Final: 3.02 m2)

Substep "f"

Net Area = Gross area - Area of bars = 13.40 - 2.59 = 10.81 m2   (Final: 12.78 m2)

Substep "g"

Compare Net Area and Side Area: 10.81 m2 less than 12.72 m2. Therefore increase length (substep "b") and try another iteration.

Step 4: Determine the distance to the apex of the deflector and the width of deflector

Distance to the apex of the deflector = (L)(cos(0.5α)) = (7.9)(cos(10)) = 7.78 m
Width of the deflector = (2)(L)(sin(0.5α)) = (2)(7.9)(sin(10)) = 2.74 m

Figure 6.1 depicts the resulting culvert debris deflector developed in the example. The figure is for illustration purposes only and should not be used as a specification or detail.

A schematic of the debris deflector designed in the example. The plan view shows the v-shaped portion of the deflector. The apex extends upstream. The side view shows the vertical bars and top and bottom horizontal bars. The leading and trailing vertical bars extend below the channel bed and are set in concrete to hold them in place.
Figure 6.1. Debris deflector designed in example.

6.2.2 Debris Racks for Culverts
  1. FUNCTION - The function of a debris rack (Figure 5.10 through Figure 5.22) is to essentially create a barrier across the stream channel to trap light and medium floating debris that is too large to pass through the culvert.

  2. STORAGE AREA - The storage area provided must be adequate to retain the anticipated type and quantity of debris expected to be accumulated during any one storm or between cleanouts. If a large debris storage area is provided upstream of the rack location, the frequency of maintenance can significantly be reduced and added safety is provided against overtopping of the installation during a single storm.

  3. TYPE OF MATERIAL - Debris racks are usually built of heavy rail or steel sections, although they can be constructed out of various types of material. Inclined racks and rubber tires have been used to help reduce the impact of heavy debris striking at high velocity. Chain-link fence has also been used to remove light floating debris from low velocity streams. This type of barrier is particularly advantageous in tidal areas where the function of flap or check gates can be hampered by light debris collecting on the gate seats and thereby blocking complete closure of the gates. Since vertical racks receive the full impact of floating debris and boulders, their structural design should incorporate brace members set in concrete.

  4. LOCATION AND ORIENTATION - Debris racks may be vertical or inclined and may be placed over the culvert inlet (Figure 5.10, Figure 5.14, Figure 5.15, Figure 5.21, and Figure 5.22) or upstream from the culvert (Figure 5.11, Figure 5.12, Figure 5.13, Figure 5.16, Figure 5.17, Figure 5.18, Figure 5.19, and Figure 5.20). Racks should not be placed in the plane of the culvert entrance, since they become easily plugged. Where a well-defined channel exists upstream of the culvert, the debris rack should be placed upstream from the culvert entrance a minimum distance of two times the culvert diameter. However, it should not be placed so far upstream that debris can enter the channel between the structure and the culvert entrance, or accessibility to and maintenance of the structure becomes difficult and/or costly. In addition, right-of-way constraints are important considerations in locating debris racks. Debris racks generally do not have top or horizontal members that extend from the rack to the culvert headwall, although there are exceptions.

  5. DIMENSIONS - The total straining area of a rack should be at least ten times the cross-sectional area of the culvert being protected. The overall dimensions of the rack should be a function of the amount of debris expected per storm, the frequency of storms, and the schedule of expected cleanouts. When a rack is installed at the upstream end of the wingwalls, it should be at least as high as the culvert. Also, the height of racks should allow some freeboard above the expected depth of flow in the upstream channel for the design flood. Racks 10 to 20 feet high have been constructed.

  6. BAR SPACING - Spacing between vertical members should not be greater than 2/3 the minimum culvert dimension nor less than 1/2 the minimum culvert dimension. This spacing permits the lighter debris to pass through the rack and the culvert. In urban areas, bar spacing of racks should be a maximum of 6 inches and tied to the culvert headwall by top bars to prevent children from entering the culvert. Unfortunately, the close spacing of the bars creates a debris trap and increases the maintenance required. To reduce the amount of debris becoming trapped, it is preferable to have the lowest edge of rack about six inches above the flow line of the ditch, permitting some debris to pass under the rack during low flows.

6.2.2.1 Debris Rack Example (Culvert)

Given:

Culvert Diameter = 1.8 m
Design Discharge = 9.9 m3/s
Design Headwater Depth = 2.7 m
Upstream channel width = 7.3 m
Flow carries light to medium debris
Rack constructed out of 76-millimeter-thick steel rails

Determine:

Determine the dimensions of a vertical debris rack comprised of two horizontal members.

Solution:

Step 1: Determine the minimum area of the debris rack

Minimum area of debris rack, Arack = 10(Area of culvert)
Minimum area of debris rack, Arack = 10(πD2/4) = 10(3.14)(1.8)2)/4 = 25.45 m2

Step 2: Determine the number of vertical bars and spacing

Minimum spacing = 1/2(D) = 1/2(1.8) = 0.9 m

Maximum spacing = 2/3(D) = 2/3(1.8) = 1.2 m

Number of vertical bars for min. spacing equals (Rack width, w minus Spacing, s) divided by (Bar thickness, t plus Spacing, s) equals (7.3 minus 0.9) divided by (0.076 plus 0.9) equals 6.6 use 7

Number of vertical bars for max. spacing equals quantity (7.3 minus 1.2) divided by (0.076 plus 1.2) equals 4.8 use 5

Try 5 vertical bars for debris rack (assume this is the smallest material and fabrication cost).

Spacing equals (w minus (t times number of vertical bars)) divided by (number of vertical bars plus 1) equals (7.3 minus (0.076 times 5)) divided by (5 plus 1) equals 1.15 m

The spacing of 1.15 m falls between the minimum (0.9 m) and maximum (1.2) spacing values. Therefore the spacing is adequate.

Step 3: Determine the height of the debris rack

Approximate height, H equals A Sub rack divided by ((n plus 1) times s) plus (t times number of horizontal bars) equals 25.45 divided by (6 times 1.15) plus (0.076 times 2) equals 3.84 m

(note: variable "n" is the number of vertical bars in the rack - in this example "5").

Use a height of 4 m to account for additional loss in area from the horizontal bars. No adjustment required to satisfy freeboard since the design headwater elevation is about 1.3 m below the top of the rack.

Gross area = (w)(H) = (7.3)(4) = 29.2 m2

Area of bars = (number of horizontal bars)(t)(w) + (number of vertical bars)(t)(H)
Area of bars = (2)(0.076)(7.3) + (5)(0.076)(4) = 2.63 m2

Net Area = Gross area - Area of bars = 29.2 - 2.63 = 26.57 m2

Compare Net Area and Minimum Rack Area: 26.57 m2 is greater than 25.45 m2 . Therefore, the design is adequate.

6.2.3 Debris Risers for Culverts
  1. FUNCTION - Debris risers (Figure 5.23 through Figure 5.25, and Figure 5.32) generally consist of vertical culvert pipes that are commonly used as relief structures, either independent of the main culvert or in conjunction with it. Risers are used where considerable height of embankment is available and the type of debris consists of flowing masses of clay, silt, sand, sticks, or medium floating debris without boulders. They are seldom structurally stable under high-velocity flow conditions because of their vulnerability to damage by impact, and they are usually suitable for culvert installations of less than 54-inch diameter.

  2. STORAGE AREA - Storage area must be provided to adequately retain the anticipated type and quantity of debris expected to be accumulated during any one storm or between cleanouts. The use of risers induces deposition of the sediment material upstream of the riser as a result of the ponding created by the riser.

  3. TYPE OF MATERIAL - Debris risers are usually built out of corrugated metal pipe, although they can be constructed out of steel pipe. A corrugated metal pipe reducing elbow can be used to connect risers to an existing culvert inlet, although damage to the metal elbow from falling rocks may occur. Occasionally, concrete is placed inside the elbow to prevent the metal from wearing through by this abrasive action. A solution for extremely severe conditions is to connect riser and culvert by a concrete junction box having the inside shaped as an elbow.

  4. LOCATION AND ORIENTATION - Debris risers are typically vertical, however they have been built at an angle between vertical and the stream grade. This reduces the impact of debris at the elbow and assists in moving debris through the culvert. Risers can be either an independent structure or connected to an existing culvert. If connected to the existing culvert, the riser should be located a minimum of 3 feet or one half of the culvert diameter, whichever is greater, from the existing culvert headwall.

  5. DIMENSIONS - Good practice will build riser pipes at least 36 inches in diameter to provide an area large enough for maintenance access. To avoid vibration of the riser pipe and unstable flow conditions, the riser diameter should be about 1 foot larger than the culvert diameter. If the embankment is of sufficient height, provisions should be made to extend the riser vertically, if necessary. In the case of corrugated metal pipe risers, this can be accomplished by means of standard coupling bands.

  6. GRATE AND SLOT FEATURES - Debris risers should be covered by a grate or cage to prevent clogging of the culvert. The grate bars can be reinforcing steel or other such material with vertical spacing no greater than 1/2 the culvert diameter. Slots or holes are placed in the sides of the riser to carry low flow. It is preferable to have these holes punched before galvanizing to avoid deterioration by rust. The holes are considered to have no hydraulic capacity under peak flow conditions because of the likelihood of their becoming plugged by light floating debris and silt. It is also desirable to connect the grate bars to a coupling band, rather than directly to the riser pipe, so the grate can be removed should cleaning be required.

6.2.4 Debris Cribs for Culverts
  1. FUNCTION - A debris crib (Figure 5.26 through Figure 5.28) is particularly adapted to small-size culverts where a sharp change in stream grade or constriction of the channel causes deposition of coarse detritus at the culvert inlet. Debris can almost envelop a crib without completely blocking the flow and plugging the culvert. Cribs are somewhat similar to risers, however cribs are more appropriate than risers where the culvert has little cover and the debris is comprised of coarse detritus. Due to the debris type and site conditions associated with debris cribs and also risers, these two types of countermeasures have shown to be the most consistently successful in producing an efficient, maintenance-free installation for culverts.

  2. TYPE OF MATERIAL - Debris cribs are usually constructed of precast concrete or wood, and precast concrete should be used when the debris consists of medium to large cobbles.

  3. LOCATION AND ORIENTATION - The crib is usually placed directly over the culvert inlet and is generally built in log-cabin fashion although other designs have been used. Debris cribs may be open (Figure 5.26 and Figure 5.27) or covered (Figure 5.28).

  4. DIMENSIONS - The spacing between the bars should be about 6 inches. This spacing also applies to the horizontal top members of a covered crib. The height of an open crib should be higher than the depth of debris deposited at the structure. When an open crib is used as a riser and an accumulation of detritus is expected to build up, provision should be made for the height of the structure to be increased as needed (Figure 5.26 and Figure 5.27). Cribs have been built as high as 50 feet above a pipe invert with little change in the efficiency of the facility.

6.2.5 Debris Fins for Culverts
  1. FUNCTION - Debris fins are thin walls installed upstream of the culvert parallel with the flow (Figure 5.29 through Figure 5.35). They have been used successfully with large culverts or for multiple box culverts where the debris consists mostly of floating material that can pass through the culvert if oriented parallel with the culvert barrel. Debris that is not aligned by the fin to pass through the culvert is retained at the front of the fin for later removal by maintenance personnel. If the fin is sloped upward toward the culvert, the debris that does not pass through the culvert can float upward and prevent debris from blocking the culvert inlet. Fins are generally not used on culverts with a minimum dimension less than 4 feet.

  2. TYPE OF MATERIAL - Debris fins are usually concrete, although they have been constructed of steel and timber.

  3. LOCATION AND ORIENTATION - Debris fins can extend from the interior walls of culverts (Figure 5.29 through Figure 5.31) or located on the centerline of a single culvert (Figure 5.32 through Figure 5.34). The upstream end of the fin should be rounded and sloped upward toward the culvert (Figure 5.29 and Figure 5.30) to reduce impact, turbulence, and the probability of gathering debris, rather than vertical as shown in Figure 5.31 through Figure 5.34. If the upstream end of the fin is vertical, rounding that edge would be preferable to a square edge (Figure 5.31).

  4. DIMENSIONS - A debris fin is usually constructed to the height of the culvert; thus, its effectiveness is limited after the inlet becomes submerged. Field experience indicates the fin length should be 1 1/2 to 2 times the culvert height. The leading edge would thus have a slope from 1-1/2:1 to 2:1 (from 33.7 to 26.6 degrees). The thickness of the fin should be the minimum needed to satisfy structural requirements in order to minimize disturbance to the flow.

6.2.6 Debris Dams / Basins for Culverts
  1. FUNCTION - Debris dams are structures placed across a well-defined channel to form a barrier that impedes the stream flow. The dams also form a basin that provides storage for deposits of detritus and floating debris (Figure 5.36 through Figure 5.39). Debris dams and basins are used at sites that convey heavy debris loads where it is economically impracticable to provide a culvert large enough to convey the surges of debris. They are also used to trap heavy boulders or coarse gravel that would clog culverts, especially on low fills. In some locations, debris dams have been built to provide the added benefit of ground water recharge resulting from ponded water.

  2. TYPE OF MATERIAL - Debris dams are usually earth or rock filled structures. Debris dams, however, can be built out of metal (Figure 5.36), rock held in place by wire (Figure 5.37), (i.e., gabions), or precast concrete beams placed in crisscross or log-cabin fashion with rock dumped between the members (Figure 5.38 and Figure 5.39).

  3. LOCATION AND ORIENTATION - Debris dams and basins are usually placed some distance upstream from a culvert. However at some locations, the highway embankment can serve as the embankment for the debris dam.

  4. DESIGN FEATURES -There are several features that must be considered in the design of the debris dam and basin. Some of the important features are the embankment, inlet protection, outlet structure, and emergency spillway structure. Information on the design of these features and sedimentation basins in general is provided in "Design of Sedimentation Basins".(22) Prior to initiating the design of the debris dam, state agencies should be contacted to ensure that state regulations are met in the design of the structure. Also, hazards created by the failure of the debris dam need to be considered and evaluated during the design of the structure.

    Various items must be considered in the design of the embankment. One of the more important items is the height of the embankment. The top of the embankment should be set at an elevation sufficiently above the maximum ponding elevation associated with the design volume of runoff and debris to assure that the runoff and debris are contained with a high level of certainty, i.e., embankment has adequate freeboard. When defining this elevation, the debris storage volume should be based on the assumption that the deposition slope of the debris is horizontal and not one half of the natural valley slope that has commonly been used. This assumption eliminates the potential of the embankment being overtopped due to the momentum of the flow, which has occurred for some of the debris dams and basins in the Los Angeles area designed assuming a deposition slope equal to one half of the natural valley slope. Stability of the embankment is also a major concern. The embankment should be designed to withstand the total forces from soil and hydrostatic pressure, seepage uplift, and earthquake on the structure. Special considerations for slope stability should be made for earth-fill embankments. The upstream and downstream slopes of the earthen structure depends on the soil material used to construct the embankment; however, the slope typically ranges between 2.5 to 1 and 3 to 1 for the upstream face and between 2 to 1 and 3 to 1 for the downstream face. Another important item for earthen structures is slope protection. Both the upstream and downstream face of the embankment should be protected with some type of slope protection measures, i.e., vegetation, riprap, matting, or mulch, to prevent erosion of the embankment.

    Occasionally, extensive excavation below the natural streambed is necessary to provide the required storage for the debris. For this type of basin design, the upper end of the basin should be protected with revetment to prevent any upstream erosion of the streambed due to headcutting.

    An outlet structure should be provided to drain the floodwater temporarily stored behind the structure. The structure could be either a closed conduit consisting of a culvert with a riser set above the expected level of the debris deposit or an open channel acting as a weir structure. The design of the structure will have an influence on the design volume of the basin and embankment height. In general, an outlet structure designed to convey more of the runoff volume will reduce the design volume of the basin and lower the embankment height, but the cost of the structure will increase. Therefore, several different types and sizes of the outlet structure should be considered in the design of the structure to optimize the total cost of the debris dam. Significant scour can develop downstream of the outlet structure due to the high velocity, turbulent flow leaving the structure and the significant reduction in the sediment load resulting from the upstream deposition. Therefore, protection measures must be provided at the downstream end of the structure to protect the structure and embankment from failure due to undermining. Access for maintenance and repair work should be provided to the upstream and downstream ends of the structure.

    The debris dam must have an emergency spillway to safely convey flows greater than the design event. The spillway should be located off to one side of the embankment and excavated into an adjoining hillside since this location is more stable against breaching than a spillway over a fill or over the embankment structure. Outlet structures designed as open channels can also be designed to serve as the emergency spillway. Protection measures must be provided at the downstream end of the spillway to protect the structure and embankment from failure caused by significant scour.

6.2.7 Combined Debris Controls for Culverts

Each drainage basin presents its own debris problem. Often more than one problem exists at a site and two or more types of debris-control countermeasures are required to adequately address the problems. Combined measures can also be used at locations where it may be preferable to remove the larger debris at a location upstream from the culvert and to remove the smaller material nearer the culvert inlet. Combined measures can also be used at locations where it may be advisable to install two types of devices so that one will function if the other fails. An example of this is shown in Figure 5.24 where a debris riser was installed over the entrance of a culvert to assure water is conveyed through the culvert in the event that the culvert entrance becomes plugged. A photograph of this installation after a flood event is shown in Figure 5.25. Other examples of combined countermeasures are shown in Figure 5.32, Figure 5.34, and Figure 5.40. In these cases, Figure 5.32 and Figure 5.34 show a culvert protected by both a debris fin and a debris riser. Figure 5.40 shows an installation consisting of a debris dam and settling basin with a debris deflector at the inlet and a debris riser.

6.3 Design Guidelines For Bridges

6.3.1 Debris Deflectors for Bridges
  1. FUNCTION - Debris deflectors are placed upstream of bridge piers to divert and guide debris through the bridge opening. Deflectors are used where the debris consists of medium to large floating debris.

  2. TYPE OF MATERIAL - Debris deflectors attached to the pier are usually constructed of steel rails, whereas steel piles filled with concrete are used for deflectors located some distance upstream.

  3. LOCATION AND ORIENTATION - Debris deflectors can be attached to the pier or located at some distance upstream from the pier. The effectiveness of deflectors is largely controlled by the direction of stream flow. Changes in flow direction can cause the deflector to be ineffective and in some cases actually worsen the situation. Therefore, deflectors can be greatly improved if the flow direction in the stream can be stabilized by auxiliary structures such as guide banks which confine and stabilize the flow in a certain direction. The flow patterns around the deflector are complex and cannot be easily predicted. The effectiveness of the structure is difficult to assess. Therefore, in the determination of proper location and configuration of the deflector, physical modeling is encouraged to assure proper functioning of deflector for various discharges.

6.3.2 Debris Fins for Bridges
  1. FUNCTION - Debris fins are thin walls installed upstream of the bridge parallel with the flow (Figure 5.35). Debris fins have been successfully used to align debris with the waterway opening and to avoid the accumulation of debris on bridge piers. They are used when the debris consists mostly of floating material. Fins have also been successful in reducing ice clogging by displacing ice sheets upward along the sloping top surface.

  2. TYPE OF MATERIAL - Debris fins are usually concrete, although they have been constructed of steel and treated timber piling and bracing.

  3. LOCATION AND ORIENTATION - Debris fins are usually located on the centerline of the bridge piers, and they should be carefully aligned with the flow in order to avoid increasing the projected pier width and a corresponding greater depth of pier scour. The upstream end of the fin should be rounded and sloped upward toward the bridge to reduce impact, turbulence, and the probability of gathering debris.

  4. DIMENSIONS - The debris fin consists of a vertical and sloped section. The vertical section exists from the upstream face of the pier to 1.8 m (6 ft) upstream and has a minimum height of 0.76 m (2.5 ft) above the maximum water surface elevation for the design flood event. The sloped section has a height equal to the maximum water depth at the upstream end of the vertical section. The sloped section extends upstream a distance of twice the maximum water depth. The profile of the sloped section consists of a 3:1 sloped segment and a curved segment with a point of intersection located one-half of the maximum water depth above and downstream of the upstream end of the fin. The overall width of the debris fin transitions from the width of the bridge pier to a width of 0.3 m (1 ft) at the upstream end of the fin. The debris fin foundation must be sufficient to withstand the predicted scour depth.

6.3.3 Crib Structures for Bridges
  1. FUNCTION - Debris cribs are used for open-pile bents to prevent debris from trapping and accumulating between the piles. The crib structure is constructed around the existing pier structures by doweling the sheathing members directly into the existing piers or to vertical columns that are tied into the foundation of the existing piers.

  2. TYPE OF MATERIAL - Debris cribs are usually built of timber or metal sheathing, although concrete sheathing has been used.

  3. SPACING - The effectiveness of debris cribs is largely dependent on the spacing between the sheathing members. Unfortunately, there are no guidelines available for defining the spacing of the crib structure. In general, large spacing should be avoided since it creates a favorable condition for entrapping and accumulating debris.

  4. FLOW DIRECTION - Special considerations should be made when the pile bents are skewed to the approaching flow. The narrow openings created by the structure increase the potential for debris trapping, and debris that would normally pass through the pile bents could accumulate on the structure.

6.3.4 River Training Structures for Bridges

River training structures are structures placed in the river flow to create counter-rotating streamwise vortices in its wakes to modify the near-bed flow pattern to redistribute flow and sediment transport within the channel cross section. Design guidelines for this type of structure are provided in HEC-23(35) as Design Guideline 9. The guidelines provided in HEC-23 cover the longitudinal extent of spur field, spur length, spur orientation, spur permeability, spur height and crest profile, bed and bank contact, spur spacing, shape and size of spurs, and rock sizes.

6.3.5 In-Channel Debris Basin for Bridges

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. Unfortunately, no design guidelines exist for these types of structures. The flow patterns around these structures are complex and cannot be easily predicted. Therefore, a physical model should be used to design and analyze the structure to assure that it functions properly for various discharges.

In-channel debris basins for floating debris have been used in parts of Europe. Two such structures are the "Arzbach Treibholzfang" and the "Lainbach Treibholzfang". Both of these structures were designed from physical model testing conducted at the Hydraulics Laboratory of the Technical University of Munich(61). The two structures have similar configurations, however there are some noticeable differences between them. The "Lainbach" structure was built with a double row of posts that was later found to be unnecessary, so the "Arzbach" structure was built only with a single row of posts. The posts within the two structures are the same. They have a diameter of 0.66 m (2.2 ft) and a height of 4 m (13.1 ft) above the channel bed. They are comprised of a steel sleeve with a concrete core with each post set into a concrete foundation that is supported on piles extending 4.4 m (14.4 ft) below the channel bed. Both of the structures have riprap revetment along the bed and side slopes of the channel upstream and downstream of the posts to protect against erosion. Another difference between the two structures is that a performed scour hole downstream of the post was incorporated into the design of the "Arzbach" structure. The maintenance requirements for these structures are high with debris having to be removed periodically and possibly on an annual basis.

6.3.6 Flood Relief Structures for Bridges

Flood relief structures are flow through or overtopping structures that divert excess flow and floating debris through the structure and away from the bridge structure. These structures can significantly reduce the risk of significant damage or failure of the structure by reducing the pressure of the flowing water on the increased width of a pier resulting from the lodged debris and the amount of debris conveyed to the bridge. These structures were determined to be very effective in preventing failure of several bridge structures with debris accumulations during severe flooding in Pennsylvania and New York from Hurricane Agnes(47). Therefore, a flood relief structure should be considered for bridges that have a high potential for debris accumulation and where there is space available and no physical constraints that would otherwise preclude their use.

Flood relief structures should be located near the ends of the bridge. These structures can be incorporated into the design of a bridge, where the anticipated debris accumulation is included in the design of the structure, to function as an emergency structure for conveying flows greater than the design discharge. They can also be utilized at existing bridges where debris accumulated on the structure has significantly reduced the discharge conveyed through the bridge and has caused significant increases in the upstream water surface elevation.

The discharge that a relief bridge would need to convey can be estimated using the following procedure:

  1. Compute the water surface profile through the bridge for the design discharge, assuming no debris accumulation on the structure.

  2. Estimate the location and extent of the debris accumulation using the procedures provided in Chapter 3 of this manual.

  3. Reflect the accumulated debris and re-compute the water surface profile through the bridge for the design discharge to determine the effect the debris accumulations has on the upstream water surface elevation.

  4. Compute a rating curve of discharge versus upstream water surface elevation for the bridge structure with debris accumulations.

  5. Determine the maximum allowable water surface elevation upstream of the bridge structure using topographic mapping, historical flood information, and information from the field investigation. This elevation could also be defined as the elevation associated with potential failure of the bridge caused by the increase in hydraulic loading on the structure due to the debris accumulation.

  6. Determine the flow through the bridge structure for the maximum allowable water surface elevation using the rating curve computed in the fourth step.

  7. Determine the design discharge for the relief structure by subtracting the discharge computed in the previous step from the design discharge.

Relief structures should be protected with revetment where significant damage to the structure is undesired or when the anticipated difference between the upstream and downstream water surface elevations is large and there is a potential of catastrophic flooding downstream of the structure. Revetment should also be provided for the downstream slopes of highway embankments that are designed for overflow, or that are subject to overtopping, and the anticipated drop between the upstream and downstream water surface elevations is large.

6.3.7 Debris Sweepers for Bridges

A debris sweeper is a device, generally made of polyethylene, which is attached to a vertical stainless steel cable or column affixed to the upstream side of the bridge pier. The debris sweeper travels vertically along the cable or column as the water surface rises and falls. The devices are also rotated by the flow, causing floating debris to be deflected away from the pier and through the bridge opening. Two devices could be placed on the same track with one of the devices being completely submerged while the other device is near the water surface. The devices could be aligned with the pier or offset from the pier, and special considerations on the placement of the devices are required for skewed flow conditions. If access to the substructure from the bridge deck is a problem, then a column application can be utilized or the devices can be installed using a boat. Debris sweepers can be used for most types of floating debris with the larger, heavier debris requiring a stronger bracket design. Several States are still assessing the use of such sweepers.

Maintenance and inspection of these devices is recommended after a high-water event. All cable and anchors for the bracketing system should be inspected for proper tension, and any debris near the device and/or bracket system should be removed immediately, so that the performance of the device is not compromised during subsequent events.

An important design guideline appears to be carefully checking the suitability of the site to the sweeper application. For example, the device would not be an appropriate measure if the design log length is greater than the effective opening between the piers.

6.3.8 Design Features for Bridges

The most commonly used countermeasures for bridge structures are features incorporated into the design of the structure to reduce the potential for trapping and accumulating debris. Unfortunately, specific guidance or guidelines do not presently exist for these design features. However, general guidance is presented below.

  1. FREEBOARD. Freeboard is a safety precaution of providing additional space between the design water surface elevation and the low chord elevation of the bridge. Considerations to the delivery potential of floating debris should be made in defining the amount of freeboard for a proposed bridge structure. When the potential for floating debris is remote or relatively low, freeboard is less important, whereas a careful selection of the freeboard is required for a bridge over a stream with a high potential for floating debris. The minimum freeboard of a bridge structure should be 0.6 meters (2.0 feet) where there is a high potential for floating debris. The freeboard should be increased to 1 to 1.2 meters (3.3 to 3.9 ft) where debris is abundant and known debris problems exist. Unfortunately, freeboard alone cannot guarantee the complete elimination of damage because the degree of protection is limited by the ever-present chance that a flood will occur that exceeds the level of protection provided by the freeboard.

    Increasing freeboard will decrease the probability of debris hazards to a certain degree; however, the cost of construction may increase significantly depending on the geometry of the river crossing and the bridge. For such locations, a cost-risk analysis should be performed to establish the recommended freeboard at the site.

  2. PIER TYPE, LOCATION AND SPACING. As indicated in Chapters 2 and 3 of this manual, the potential for debris accumulation at a bridge structure is significantly influenced by the pier type, location and spacing. Therefore, these features should be evaluated during the design of proposed and replacement bridges where there is a high potential for debris delivery to the site. Piers located within the path of floating debris can have a high potential for accumulation even if the span length between the piers is significantly greater than the maximum length of the floating debris; or, piers that have adequate spacing and are out of the debris path can still have a high potential to accumulate debris if the piers have narrow openings that can easily trap debris.

    The type of pier can influence the potential for debris to become trapped rather than deflected. Piers with narrow openings that convey flow are significantly more likely to trap and accumulate debris than piers without openings. Therefore to minimize the potential for entrapment, the bridge piers should be solid, round-nosed piers that are aligned with the approaching flow. If multiple columns are used, then considerations should be made to reduce the potential entrapment of debris between the columns by providing a solid web wall between the columns.

    As previously stated, debris accumulations exist most frequently and in the greatest amount where the path of floating drift encounters fixed objects that divide the flow. Therefore, bridge piers should be placed outside of the debris path, which can be estimated using the information provided in Chapter 3 of this manual. In general, for a curved channel reach, piers should not be located near the bank toe on the outside bend, and in a straight reach, piers should not be located near the thalweg of the channel where the flow is the deepest and fastest. For critical locations, the piers should not be placed within the main channel, if this can be avoided.

    The span length can influence the type of debris accumulation occurring at the bridge and the overall width of the accumulation. If the span length is less than the design log length, debris could become lodged between two piers or between a pier and the adjacent abutment and potentially block the entire span opening, i.e., span-blockage accumulation. Debris for this type of accumulations can extend beyond the piers, so the total width of the accumulation could be greater than the design log length. On the other hand if the span length is greater than the design log length, debris only accumulates on the piers at a width approximately equal to the design log length, i.e., single-pier accumulations. As a minimum, the span length should be slightly greater than the design log length, which can be determined using the recommendations by Diehl(17) provided on page 3.6 of this manual. Pier spacing should be even greater for streams with a high potential for debris delivery to the site since longer spans are less prone to debris blockage. Since the total cost of the bridge generally rises with increasing pier spacing and span length, the total cost of the bridge in relation to the pier spacing should be carefully evaluated.

  3. SUPERSTRUCTURE. Where debris hazards persist and there is a high chance of the bridge being overtopped, the design of the superstructure should take into account the consequence of overtopping. The superstructure should be designed to withstand extreme floods even in a submerged condition. A thin deck and low railings could be incorporated into the design of the superstructure to minimize the lateral hydraulic forces on the structure. Also, the superstructure should be designed to minimize the potential for debris accumulation on the structure by eliminating any unnecessary narrow openings in the structure, i.e., solid parapet walls in lieu of open railings, and at the connection with the pier, i.e., a solid beam that is connected directly to the pier that would entrap and accumulate debris.

6.4 Non-Structural Guidelines (Debris Management)

The implementation of a debris management plan might be a cost effective method for structures on small watersheds. The purpose of this plan is to reduce excessive debris input into the stream network by clearing trash, debris jams and downed trees from the channel and floodplain of a stream and/or through multipurpose channel stabilization schemes. Large woody debris within a channel is a beneficial and vital geomorphologic and ecologic component of a river system (59,60,61) and the plan should recognize these benefits. Wallerstein and Thorne have developed such a plan(61) by taking into account the relationship between the large woody debris jam formations and channel processes discussed in Section 2.2 of this manual. This plan is summarized in Table 6.2.

Based on information of a given reach, the management plan provides information on the type of debris jam formation most likely to be present within the reach, impacts on the channel morphology associated with the type of debris jam formation, and an appropriate management strategy for the reach. The information required for the given reach includes the vegetation type, average riparian tree height, average channel width, and the type of sediment within the reach. The vegetation type is defined as either forest, agricultural, or open water with forest being the only type where substantial jams can form. The ratio of tree height to channel width is used to define the type of debris jams most likely to be present within the reach. The precise limits used to define the type of debris jams were determined from empirical relationships developed from field studies. The type of sediment, either fine (sand) or coarse (gravel) detritus, is used to distinguish if backwater sediment wedges or downstream bars will occur at the jams.

Table 6.2. Management Plan for the Large Woody Debris Formations.
Vegetation Type Vegetation Height/ Channel Width Sediment Management Strategy(61)
Agricultural or Open Space n.a. n.a. Substantial debris jams are unlikely to form within the reach since the immediate riparian zone is agricultural land or open water. Therefore, debris removal is unnecessary. Artificial debris input may be desirable for habitat enhancement, stabilization of sand bed channels through backwater sediment retention, or to reduce bank velocities on the outside of meanders.
Forest 1.3W ≤ H n.a. UNDERFLOW jams exist within the reach. Debris clearance is unnecessary since there would be minimal adverse geomorphic impacts associated with the jams (local scour may occur under the debris at high flows) and a significant quantity of heavy floating debris would unlikely be transported downstream. Therefore, bridge and other structures in the reach should not be affected by persistent debris accumulations.
Forest 0.95W < H < 1.3W Coarse Detritus DAM jams exist within the reach. Jams may cause significant local bed scour and bank erosion due to flow constriction. Backwater sediment wedges and bars may form upstream of the jams since the sediment consist of coarse detritus. The jams may also increase the duration of overbank flooding. A limited amount of floating debris may be transported downstream from the reach. Debris clearance may be necessary if the local bed and bank scour results in a significant increase of large woody debris being introduced into the stream.
Forest 0.95W < H < 1.3W Fine Detritus DAM jams exist within the reach. Jams may cause significant local bed scour and bank erosion due to flow constriction. Backwater sediment wedges and bars are unlikely to form upstream of the jams since the sediment consist of fine detritus. The jams may also increase the duration of overbank flooding. A limited amount of floating debris may be transported downstream from the reach. Debris clearance may be necessary if the local bed and bank scour results in a significant increase of large woody debris being introduced into the stream.
Forest 0.60W ≤ H ≤ 0.95W Coarse Detritus DEFLECTOR jams exist within the reach. Jams may cause significant bank erosion of one or both banks that could result in a significant increase of large woody debris being introduced into the stream. Since the sediment consist of coarse detritus, local bed scour induced by the jams will most likely be negligible and backwater sediment wedges may form upstream of the jams. Debris clearance unnecessary except where localized bank erosion results in a significant increase of large woody debris being introduced into the stream.
Forest 0.60W ≤ H ≤ 0.95W Fine Detritus DEFLECTOR jams exist within the reach. Jams may cause significant bank erosion of one or both banks that could result in a significant increase of large woody debris being introduced into the stream. Since the sediment consist of fine detritus, local bed scour induced by the jams might be significant and backwater sediment wedges and bars would most likely not form upstream of the jams. Debris clearance necessary to prevent local bank erosion.
Forest H < 0.60W n.a. FLOW PARALLEL jams exist within the reach. Large woody debris will be transported downstream in high flows from this reach and deposited at bank base in meanders and at run-of-river structures. Adverse geomorphic impacts associated with the jams are minimal. Banks may be stabilized due to debris build-up, and debris may also accelerate formation of mid-channel bars. Debris clearance from channel unnecessary if it is keyed into place at bank toes and bars.

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Contact:

Brian Beucler
Office of Bridges and Structures
202-366-4598
brian.beucler@dot.gov


Joe Krolak
Office of Bridges and Structures
202-366-4611
joe.krolak@dot.gov

Updated: 04/07/2011
 

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United States Department of Transportation - Federal Highway Administration