Hydraulic Design of Energy Dissipators for Culverts and Channels
Hydraulic Engineering Circular Number 14, Third Edition
Chapter 2: Erosion Hazards
This chapter discusses potential erosion hazards at culverts and countermeasures for these hazards. Section 2.1 presents the hazards associated with culvert inlets: channel alignment and approach velocity, depressed inlets, headwalls and wingwalls, and inlet and barrel failures. Section 2.2 presents the hazards associated with culvert outlets: local scour, channel degradation, and standard culvert end treatments.
2.1 Erosion Hazards At Culvert Inlets
The erosion hazard at culvert inlets from vortices, flow over wingwalls, and fill sloughing is generally minor and can be addressed by maintenance if it occurs. Designers should focus their attention on the following concerns and associated mitigation measures.
2.1.1 Channel Alignment and Approach Velocity
An erosion hazard may exist if a defined approach channel is not aligned with the culvert axis. Aligning the culvert with the approach channel axis will minimize erosion at the culvert inlet. When the culvert cannot be aligned with the channel and the channel is modified to bend into the culvert, erosion can occur at the bend in the channel. Riprap or other revetment may be needed (see Lagasse, et al., 2001).
At design discharge, water will normally pond at the culvert inlet and flow from this pool will accelerate over a relatively short distance. Significant increases in velocity only extend upstream from the culvert inlet at a distance equal to the height of the culvert. Velocity near the inlet may be approximated by dividing the flow rate by the area of the culvert opening. The risk of channel erosion should be judged on the basis of this average approach velocity. The protection provided should be adequate for flow rates that are less than the maximum design rate. Since depth of ponding at the inlet is less for smaller discharges, greater velocities may occur. This is especially true in channels with steep slopes where high velocity flow prevails.
2.1.2 Depressed Inlets
Culvert inverts are sometimes placed below existing channel grades to increase culvert capacity or to meet minimum cover requirements. Hydraulic Design Series No. 5 (HDS 5) (Normann, et al., 2001) discusses the advantages of providing a depression or fall at the culvert entrance to increase culvert capacity. However, the depression may result in progressive degradation of the upstream channel unless resistant natural materials or channel protection is provided.
Culvert invert depressions of 0.30 or 0.61 m (1 to 2 ft) are usually adequate to obtain minimum cover and may be readily provided by modification of the concrete apron. The drop may be provided in two ways. A vertical wall may be constructed at the upstream edge of the apron, from wingwall to wingwall. Where a drop is undesirable, the apron slab may be constructed on a slope to reduce or eliminate the vertical face.
Caution must be exercised in attempting to gain the advantages of a lowered inlet where placement of the outlet flow line below the channel would also be required. Locating the entire culvert flow line below channel grade may result in deposition problems.
2.1.3 Headwalls and Wingwalls
Recessing the culvert into the fill slope and retaining the fill by either a headwall parallel to the roadway or by a short headwall and wingwalls does not produce significant erosion problems. This type of design decreases the culvert length and enhances the appearance of the highway by providing culvert ends that conform to the embankment slopes. A vertical headwall parallel to the embankment shoulder line and without wingwalls should have sufficient length so that the embankment at the headwall ends remain clear of the culvert opening. Normally riprap protection of this location is not necessary if the slopes are sufficiently flat to remain stable when wet.
The inlet headwall (with or without wingwalls) does not have to extend to the maximum design headwater elevation. With the inlet and the slope above the headwall submerged, velocity of flow along the slope is low. Even with easily erodible soils, a vegetative cover is usually adequate protection in this area.
Wingwalls flared with respect to the culvert axis are commonly used and are more efficient than parallel wingwalls. The effects of various wingwall placements upon culvert capacity are discussed in HDS 5 (Normann, et al., 2001). Use of a minimum practical wingwall flare has the advantage of reducing the inlet area requiring protection against erosion. The flare angle for the given type of culvert should be consistent with recommendations of HDS 5.
If the flow velocity near the inlet indicates a possibility of scour threatening the stability of wingwall footings, erosion protection should be provided. A concrete apron between wingwalls is the most satisfactory means for providing this protection. The slab has the further advantage that it may be reinforced and used to support the wingwalls as cantilevers.
2.1.4 Inlet and Barrel Failures
Most inlet failures reported have occurred on large, flexible-type pipe culverts with projected or mitered entrances without headwalls or other entrance protection. The mitered or skewed ends of corrugated metal pipes, cut to conform to the embankment slopes, offer little resistance to bending or buckling. When soils adjacent to the inlet are eroded or become saturated, pipe inlets can be subjected to buoyant forces. Lodged drift and constricted flow conditions at culvert entrances cause buoyant and hydrostatic pressures on the culvert inlet edges that, while difficult to predict, have significant effect on the stability of culvert entrances.
To aid in preventing inlet failures of this type, protective features generally should include full or partial concrete headwalls and/or slope paving. Riprap can serve as protection for the embankment, but concrete inlet structures anchored to the pipe are the only protection against buoyant failure. Manufactured concrete or metal sections may be used in lieu of the inlet structures shown. Metal end sections for culvert pipes larger than 1350 mm (54 in) in height must be anchored to increase their resistance to failure.
Failures of inlets are of primary concern, but other types of failures have occurred. Seepage of water along the culvert barrel has caused piping or the washing out of supporting material. Hydrostatic pressure from seepage water or from flow under the culvert barrel has buckled the bottoms of large corrugated metal arch pipes. Good compaction of backfill material is essential to reduce the possibility of these types of failures. Where soils are quite erosive, special impervious bedding and backfill materials should be placed for a short distance at the culvert entrance. Further protection may be provided by cutoff collars placed at intervals along the culvert barrel or by a special subdrainage system.
2.2 Erosion Hazards At Culvert Outlets
Erosion at culvert outlets is a common condition. Determination of the local scour potential and channel erodibility should be standard procedure in the design of all highway culverts. Chapter 3 provides procedures for determining culvert outlet velocity, which will be the primary indicator of erosion potential.
2.2.1 Local Scour
Local scour is the result of high-velocity flow at the culvert outlet, but its effect extends only a limited distance downstream as the velocity transitions to outlet channel conditions. Natural channel velocities are almost always less than culvert outlet velocities because the channel cross-section, including its flood plain, is generally larger than the culvert flow area. Thus, the flow rapidly adjusts to a pattern controlled by the channel characteristics.
Long, smooth-barrel culverts on steep slopes will produce the highest velocities. These cases will no doubt require protection of the outlet channel at most sites. However, protection is also often required for culverts on mild slopes. For these culverts flowing full, the outlet velocity will be critical velocity with low tail-water and the full barrel velocity for high tail-water. Where the discharge leaves the barrel at critical depth, the velocity will usually be in the range of 3 to 6 m/s (10 to 20 ft/s). Estimating local scour at culvert outlets is an important topic discussed in more detail in Chapter 5.
A common mitigation measure for small culverts is to provide at least minimum protection (see Riprap Aprons in Chapter 10), and then inspect the outlet channel after major storms to determine if the protection must be increased or extended. Under this procedure, the initial protection against channel erosion should be sufficient to provide some assurance that extensive damage could not result from one runoff event. For larger culverts, the designer should consider estimating the size of the scour hole using the procedures in Chapter 5.
2.2.2 Channel Degradation
Culverts are generally constructed at crossings of small streams, many of which are eroding to reduce their slopes. This channel erosion or degradation may proceed in a fairly uniform manner over a long length of stream or it may occur abruptly with drops progressing upstream with every runoff event. The latter type, referred to as headcutting, can be detected by location surveys or by periodic maintenance inspections following construction. Information regarding the degree of instability of the outlet channel is an essential part of the culvert site investigation. If substantial doubt exists as to the long-term stability of the channel, measures for protection should be included in the initial construction. HEC 20 "Stream Stability at Highway Structures" (Lagasse, et al., 2001) provides procedures for evaluating horizontal and vertical channel stability.
2.2.3 Standard Culvert End Treatments
Standard practice is to use the same end treatment at the culvert entrance and exit. However, the inlet may be designed to improve culvert capacity or reduce head loss while the outlet structure should provide a smooth flow transition back to the natural channel or into an energy dissipator. Outlet transitions should provide uniform redistribution or spreading of the flow without excessive separation and turbulence. Therefore, it may not be possible to satisfy both inlet and outlet requirements with the same end treatment or design. As will be illustrated in Chapter 4, properly designed outlet transitions are essential for efficient energy dissipator design. In some cases, they may substantially reduce or eliminate the need for other end treatments.