7.9 Special Culvert Modeling Issues
The modeler may encounter a variety of special problems when analyzing culverts. The following sections describe some of the most common situations.
Flow Attenuation
A significant issue that may be overlooked by modelers is the flow attenuation that may occur due to large floodplain storage upstream of a culvert location. A culvert through a large roadway embankment often resembles a dam with a low-flow opening, as shown in Figure 7.17. Culvert designs are often based on discharges calculated from a regional frequency equation, as described in Chapter 5. The calculation of peak discharge is based on drainage area, stream slope, and other definable variables, but not on the local site characteristics. With a high embankment in place, there is usually significant storage for the reach upstream of the culvert. To properly analyze the peak flow through the culvert requires either a hydrologic or hydraulic routing, thereby taking into account the flow attenuation caused by the floodplain storage upstream of the embankment. This type of hydrologic routing operation is similar to that for a reservoir and takes place outside of HEC-RAS, using a hydrologic program with input from HEC-RAS. This type of analysis is further discussed in the following paragraphs as well as in Chapters 8 and 14. Hydraulic routing is performed using HEC-RAS in an unsteady flow mode and is further described in Chapter 14.
Figure 7.17 Culvert through a high embankment.
Cross sections and/or topographic maps are used to determine the storage for the reach upstream of the embankment. Surface areas are calculated for selected elevations or contour intervals, ranging from the culvert invert to the top of the roadway embankment (see Figure 7.18). The surface area and elevation information is converted to an accumulated storage versus elevation relationship. In a separate analysis, various discharges are used to compute water surface elevations just upstream of the culvert with a multiprofile backwater analysis using HEC-RAS. This latter operation gives a discharge versus elevation relationship just upstream of the culvert. The elevation-accumulated storage and discharge-elevation relationships are then linked to form an accumulated storage versus outflow relationship for the culvert and the upstream storage reach, shown in Figure 7.19. This relationship is used to route the selected hydrograph(s) through the constriction caused by the culvert, typically using the Modified Puls (or level pool routing or the storage indication method) routing procedure found in many hydrologic programs, such as HEC-HMS and Haestad Methods' PondPack.
Figure 7.18 Elevation contours upstream of a culvert crossing.
Figure 7.19 Schematic to develop storage-outflow relationship for a hydrologic routing.
Routing determines the actual peak outflow from the culvert, which is often a much smaller value than first computed with a regional equation. Figure 7.20 shows an inflow and outflow hydrograph representing the culvert and upstream storage of Figure 7.18. As shown in Figure 7.20, the routing operation results in a significant reduction in the peak discharge passed by the culvert at location B (Figure 7.18) when compared to the peak at location A. Location A reflects the peak discharge at the beginning of the storage reach created by the culvert embankment, prior to the routing operation. In developing final water surface profiles, the attenuated peak discharge is used in the HEC-RAS model to compute the profile through and upstream of the culvert, as depicted in Figure 7.21. The reduced flow often lowers profiles for some distance downstream of the culvert structure, depending on the stream topology. The peak discharge at A and for some distance upstream is the peak discharge from the inflow hydrograph, prior to routing. A reduced discharge for the sections in the storage reach (between locations A and B) in Figure 7.21 would be interpolated.
Figure 7.20 Typical inflow hydrograph at A and outflow (routed) hydrograph at B for a culvert through a high embankment.
Figure 7.21 Profile of culvert reach. Applicable peak discharges are shown at the cross sections indicated.
The modeler should always be on the alert for flow attenuation when designing or analyzing culverts through significant embankment fills. For this situation, regional frequency equations, which yield only a peak discharge, are usually inadequate to properly analyze or design the culvert. Hydrograph routing (or hydraulic routing) should be incorporated to develop the best and most defensible solutions for the design discharge at a culvert. The data development and mechanics of the hydrologic routing process for culvert analysis are nearly identical to that for reach routing and are further discussed and demonstrated in Chapter 8.
Minimum Energy Loss Culverts.
Minimum energy loss (MEL) culverts have been successfully used in Australia. They feature very streamlined entrance and exit conditions to minimize the losses, while maximizing the discharge through the culvert structure. Figure 7.22 provides a typical plan and profile view. A MEL culvert is more costly than a standard culvert design but may be appropriate for a replacement structure that is limited in width and must pass a higher design discharge than was required of the older structure. A MEL culvert passes the design flow through the structure at or near critical depth, thus maximizing the capacity. Because the highly streamlined entrance and exit conditions minimize energy losses, outlet control is the expected culvert regime. The MEL structure may be most applicable for straight rectangular channel sections where the velocity distribution is as uniform as possible. Design guidance and additional detailed information on the use of the MEL method for culverts and bridges is given by Apelt (Apelt, 1983) and by Chanson (Chanson, 1999).
Figure 7.22 Minimum energy loss culvert plan and profile views.
Sediment and Debris
Restrictions caused by culverts can result in unfavorable flow conditions resulting from sediment deposition upstream of the structure, debris blockages at the culvert mouth, or both. The stream conditions giving rise to these occurrences should be evaluated as part of the design process. If the stream carries considerable sediment or debris, culvert features should be incorporated as part of the culvert design to prevent a blockage of the culvert entrance.
Sedimentation.
A culvert's opening usually represents a small percentage of the upstream flow area that would exist under natural conditions. This often causes lower velocities in the upstream approach reach to the culvert than before the culvert's construction. These lower velocities result in deposition of sediment carried by the flow, especially in the channel section for some distance upstream of the culvert. Where the stream carries a significant sediment load, considerable deposition may be encountered following every runoff event. Figure 7.23 shows sediment deposition in a concrete-lined channel following one moderate thunderstorm event.
Figure 7.23 Upstream deposition in concrete-lined channel following a moderate runoff event.
At locations where significant sediment deposition is expected, provisions must be made to periodically remove the sediment to maintain the full, unobstructed culvert capacity. If multiple culverts are proposed, locating the invert of one culvert lower than the rest will often ensure that most sediment stays in suspension for low flows and passes through the culvert. The inclusion of debris or sediment basins to catch and settle out most of the floating material may be necessary for channels located within steep, mountainous terrain. More information can be found on sediment/debris basin design in "Sedimentation Investigations of Rivers and Reservoirs" (USACE, 1995).
Sediment deposition may also occur within the barrel of the culvert. Sensitivity tests using HEC-RAS can evaluate the performance of the culvert with a specified depth of sediment for the full length of the culvert. The field labeled Depth Blocked in the Culvert Data Editor (Figure 7.16) is for specification of this information. When a value is entered into this field, the culvert is completely blocked up to the depth specified. This blocked-out area persists the entire length of the culvert. For example, if a value of 2 were entered into the Depth Blocked field, the first 2 feet of the culvert area would be removed from the culvert flow computations. The n values corresponding to the deposited material and to the balance of the culvert surface would also be specified in the Culvert Data Editor.
Debris.
Debris can be a major consideration in maintaining unobstructed flow through the culvert. Debris may consist of large cobbles, boulders, leaves, brush, trash, tree limbs, and/or full trees. In urban areas, tires, old appliances, and other trash dumped in the stream can further increase the debris level. Automobiles swept into the stream can also be part of the debris load carried by a stream during a large flood. Figure 7.24 shows a total blockage of four large corrugated metal pipe culverts through a levee following a large runoff event. Upstream land clearing left a great quantity of felled trees and other debris available for flood flows to carry and deposit at the culverts.
Figure 7.24 Total blockage of four 84 in. (2.13 m) corrugated metal pipes by debris.
The likelihood that significant debris will be carried during a flood event at the culvert site should be available from any previous flood history, or from observing the amount of potential debris in and adjacent to the stream. Where debris potential is known or is believed to be significant, the integrity of the culvert opening should be ensured through use of a debris basin or other means. Debris basins are essentially detention ponds for the settlement of debris. These structures provide more cross-sectional area than the channel and slow the velocity, thereby causing the debris to settle out of the flow. These basins are common in mountainous terrain at the point where a milder sloping reach is encountered, such as an alluvial fan at the mouth of a canyon. More information on debris basin design can be found in "Hydraulic Design of Flood Control Channels" (USACE, 1991).
A more common solution for milder sloping terrain is the presence of a debris barrier a few feet upstream of the culvert mouth, which can help prevent the debris from sealing the culvert opening. This solution could require the removal of accumulated debris after every significant runoff event. Figure 7.25 and Figure 7.26 show two debris barriers employed for culvert crossings. Bar spacing should be about one-third to one-half of the least culvert dimension (Linsley et al., 1992). The debris barrier should never be directly on the culvert entrance because that location allows debris to collect in the culvert entrance and ensures that the full capacity will not be available for the runoff event. Additional information on debris barriers can be found in HEC-9, "Debris Control Structures" (Reihsen and Harrison, 1971).
American Iron and Steel Institute
Figure 7.25 Debris barrier at a culvert.
Figure 7.26 Debris rack located near culvert mouth.
Scour at Culvert Outlets
Velocities at the culvert outlet can be very high and may therefore result in erosion of channel materials. Figure 7.27 shows supercritical flow exiting a multiple-barrel culvert with a low-grade hydraulic jump just downstream of the exit. HEC-RAS can compute the depths and velocities for the geometry entered in this example; however, the design of an energy dissipation structure at the culvert outlet would be performed outside of the program. If such a structure is not provided, the erosion can undermine the culvert exit and cause the structure to fail.
USACE
Figure 7.27 High-velocity flow exiting a multiple box culvert.
Exit velocities during the culvert design should be checked and appropriate scour protection included. Protection is most often graded riprap but could include more complex designs, such as concrete energy dissipaters. Several references, including FHWA's "Hydraulic Design of Energy Dissipators for Culverts and Channels," HEC No. 14 (Corry et al., 1983); "Hydraulic Design of Reservoir Outlet Works" (USACE, 1980); and "Hydraulic Design of Flood Control Channels" (USACE, 1991) are available to aide in the design of outlet scour protection.
Horizontal Bends in Culverts.
Although most culverts are uniform in shape, size, and slope from the upstream to downstream end, there are exceptions.
A culvert can have one or more horizontal bends between its entrance and exit to bypass utility lines or other in-place items. Bends can result in additional losses for culverts operating under outlet control. If the bends are less than 15 degrees and are at least 50 ft (15 m) apart, the additional losses are considered insignificant and can be neglected (FHWA, 1985). When bends do not meet these criteria, there are additional culvert losses. Bend losses (vertical or horizontal) can be estimated using
| where |
hb |
= |
the loss in each bend (ft, m) |
| Kb |
= |
the bend loss coefficient (dimensionless) |
|
| V |
= |
culvert velocity under full conduit flow (ft/s, m/s) |
(7.8)
The bend loss coefficient is a function of the angle of the bend and the size of the culvert. Table 7.4 lists estimates of Kb for culverts flowing full. The cross-sectional area of noncircular shapes may be used to compute an "equivalent diameter" for that shape.
Example 7.3 Bend losses in a culvert.
A 5-ft diameter culvert is 250 ft long and has a 30-degree bend at a point along its length. For a discharge of 150 ft3/s, compute the bend loss, assuming that the culvert is flowing full.
Table 7.4 is used to linearly interpolate a bend loss coefficient of 0.117, rounded to 0.12, for the culvert diameter and bend angle. The velocity in the pipe, assuming full pipe flow, is V = Q/A = 150/19.63 = 7.64 ft/s. The bend loss is then determined with Equation 7.8 to be
.
In HEC-RAS, bend losses are not part of the information entered into the Culvert Data Editor. Therefore, to include this bend loss in the culvert data, 0.12 could be added to the entrance loss coefficient.
Vertical Bends in Culverts
Vertical bends are most often employed to avoid excavating into rock or for designing a culvert as an inverted siphon, or sag culvert (see Figure 7.28a). These situations may require additional analysis outside of most hydraulic programs to determine the control and the headwater elevation. Sedimentation in the inverted siphon is a potential problem that should be addressed by the designer, with these structures mainly used to carry irrigation flows under an existing stream or roadway. For the inverted siphon of Figure 7.28a, the structure is designed to flow under pressure for outlet control. Bends in an inverted siphon would be modeled by adding a bend loss using Equation 7.8, if necessary.
With the additional vertical bends within the "broken back" culvert of Figure 7.28b, there can be a control at one of these bends rather than at the entrance or exit, as is normally the case. The upstream bend may transition the flow from subcritical upstream to supercritical downstream, with the flow control occurring at the bend. Control by the downstream bend is unlikely, but the modeler can include bend losses at this point (for outlet control), if deemed necessary. In addition to the application of the normal culvert routines within HEC-RAS, a flow profile through a culvert with vertical bends should also be computed with the culvert operating in open channel flow. In this method, the culvert routine is not used. Cross sections upstream and downstream of the culvert and at close intervals throughout the culvert, especially at each bend, model the culvert as a series of normal cross sections. Using HEC-RAS in mixed-flow mode (discussed in Chapter 8) and modeling the culvert as a prismatic channel with a lid provides the best estimate of conditions through the culvert, as well as a determination of the control location. The solution from this analysis is compared to the standard headwater-tailwater computations in the culvert analysis to determine the controlling criteria.
Figure 7.28 Culverts with vertical bends.
Changing Culvert Shape
Occasionally, the modeler may encounter a long culvert that has a shape change. If the culvert is flowing full throughout its length under outlet control, separate friction losses can be computed for each segment and added together. A separate expansion or contraction loss from the upstream to the downstream culvert segment may also be needed. If the culvert flows less than full, it could be modeled by a series of cross sections under open channel flow, with closely spaced sections at and near the change in shape, as described in the preceding subsection.
If the culvert is operating under inlet control, the effect of the downstream shape change may be negligible, unless the culvert size decreases. If the downstream culvert segment has open channel flow, water surface profiles should be computed with HEC-RAS in mixed-flow mode (discussed in Chapter 8) and with closely spaced cross sections. However, if the downstream culvert segment is pressurized, causing outlet control in this segment, but the upstream culvert segment is not pressurized, separate computations may be necessary to first establish the headwater elevation at the upstream end of the smaller segment, and then a similar analysis to compute a water surface profile for the upstream culvert segment.
Changing Discharge within a Culvert
A lateral pipe carrying storm sewer flow may enter the culvert along its length. For culverts operating under outlet control, the additional losses at the junction are computed with the following equations, developed by FHWA (1979), and added to the other losses. Head loss is given by
(7.9)
The change in hydraulic grade line, y', is given by
(7.10)
If the culvert is operating under inlet control, no additional losses are associated with the junction. However, proper hydraulic design should be incorporated at the junction to minimize the effects of added flow in a supercritical flow situation and to avoid potentially significant roll waves created by the flow addition. Guidance for designing junctions for supercritical flow is given in "Hydraulic Design of Flood Control Channels" (USACE, 1991).
Changing Materials within a Culvert
The culvert material may change along the culvert's length or, more typically, the culvert section may be composed of different materials around the perimeter. For corrugated culverts carrying acidic runoff, it is common for the lower portion to be coated with asphalt and the upper portion to be left as corrugated steel. For a culvert consisting of two lengths of different materials, friction losses are computed separately in each and combined, similar to the method described in the section, "Changing Culvert Shape." For a culvert section composed of different materials around its perimeter, a weighted n value should be computed. The suggested method within HEC-RAS for computing a weighted n is
(7.11)
In Open Channel Hydraulics (Chow, 1959), Chow describes other procedures for computing a weighted n. Other references may also be consulted to estimate n for mixed culvert materials. In HEC-RAS, input for multiple n values is included in the fields "Manning's n for Top," "Manning's n for Bottom," and "Depth to Use Bottom n," as shown in Figure 7.16. This feature would be used in the previous example where the lowest 2 ft (0.6 m) of a corrugated-metal culvert is asphalt coated (n = 0.013 or higher) and the remainder of the culvert is bare metal (n = 0.021 or higher).
Example 7.4 Culvert consisting of different materials.
An 8-by-8 ft concrete box culvert has a uniform layer of 1 ft of gravel (n = 0.032) along its length to encourage fish passage. If the culvert carries water at a depth of 5 ft, compute a composite n value.
Solution
Assume that the value of n for concrete is 0.013, and then use Equation 7.11 to compute a composite n value:
In HEC-RAS, a depth of 1 ft and the two n values would be specified in the Culvert Editor, as described in the previous subsection titled "Sedimentation." The area corresponding to the 1-ft depth is removed from the culvert cross section and the weighted n is computed by HEC-RAS.
Drop Culvert
When a culvert is required to operate under inlet control at a site having little available cover between the top of the culvert and the roadway, a drop culvert, or drop inlet at the upstream culvert entrance, is often employed, as illustrated in Figure 7.29. For the culverts operating under inlet control, a design headwater depth is computed from Equation 7.1, 7.2, or 7.3. If the culvert entrance is lower than the upstream channel invert, the computed headwater depth is still valid at the culvert entrance. Thus, additional headwater depth for passage of the design flow can be obtained by lowering the culvert and its upstream inlet, without increasing the allowable water surface elevation upstream of the culvert. The site topography must be such as to allow lowering the culvert and inlet and still maintain inlet control through the culvert. Design of a drop, or sump, inlet is presented in HDS 5, "Hydraulic Design of Highway Culverts" (FHWA, 1985).
Figure 7.29 Drop culvert under inlet control. Note the streamlined wingwalls to improve headwater flow efficiency.
Fish Passage
In the past, culverts were designed for the single purpose of passing a flood discharge. With modern environmental concerns, incorporating a fish passage as part of culvert design is becoming equally important.
Culverts can be oversized and partially filled with stream material (see Figure 7.30) to facilitate fish migration (State of Washington, Department of Fish and Wildlife, 1999). The culvert must be considerably oversized for the design discharge so that bed material can be placed along the full length of the culvert and fill one-third to one-half of the culvert depth. For high-velocity flow, the lower portion of the culvert may be lined with revetment to prevent scour of the bed material. The irregular surface of the revetment also provides holes and depressions, serving as temporary resting places for migrating fish. The scenarios depicted in Figure 7.30 suggest that the culvert width at the surface of the bed material should be 20 percent wider than the upstream channel width plus an additional 2 ft. In addition, slots or narrow grates between the top of the culvert and the roadway surface can be added to allow daylight into the culvert, if needed, to encourage fish passage. For box culverts, baffles may be inserted in the culvert bottom in lieu of stream bed material, again requiring an oversized culvert, to provide resting places for fish traveling through high-velocity culvert flows (see Figure 7.31).
Figure 7.30 Fish passage designs.
Figure 7.31 Plan view showing a baffle arrangement for fish passage.
If the culvert is made with mixed materials, a corresponding n is required and can be computed with HEC-RAS. The cross-sectional area of the bed material is removed from the culvert by the program, the baffles can be incorporated with a user-supplied estimate of a weighted n, or the modeler can remove the height of the baffle from the culvert cross-sectional area with HEC-RAS. Design procedures for fish baffles are described in Chang and Normann (1976) and several other manuals have been written on the design considerations of fish passage. Where fish passage is required, the modeler should consult with local fisheries' biologists to determine the most amenable solution for the types of fish in the stream where the culvert will be located.
Replacing Bridges with Culverts
Where roads are to be widened to provide additional lane and shoulder width, bridge replacement is an expensive solution. An alternative to total replacement of an existing bridge is to build multiple culverts through the bridge opening, with upstream and downstream headwalls, and grout or other porous material filling the spaces between the bridge and culvert. Figure 7.32 shows such a replacement. This may be a practical solution for situations where the original bridge design discharge is greater than the design discharge applicable today or where any upstream lands adversely affected by the replacement culverts are in public ownership, or owned by the party constructing the culvert.
Although this solution is usually economical, the engineer should carefully analyze the flood profile with the culverts compared to the bridge. Flow capacity with culverts is typically less than that of the original bridge and the losses are greater. Upstream flood heights may be significantly higher with the culvert installation than for the same flows with the bridge in place. Flood profiles for a full range of events should compare profiles for both the bridge and the replacement culvert structure. To avoid litigation, the replacement culvert design should result in little or no increase in upstream water surface elevations.
Figure 7.32 Replacing bridge piers with multiple culverts.
