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Section 5: Velocity Protection and Control Devices

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Excess Velocity

Several possible solutions are available for both protection and control to minimize the negative effects of excessive velocity. Solutions are categorized as either velocity protection devices or velocity control devices.

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Velocity Protection Devices

A velocity protection device does not necessarily reduce excessive velocity but does protect threatened features from damage. Such devices are usually economical and effective in that they serve to provide a physical interim for the flow to return to a more natural velocity. The protection devices discussed here include the following:

  • Channel liner -- Most of the various types of channel liner have proven effective for erosion protection. Some types of channel liner include low quality concrete (lightly reinforced), rock, soil retention blankets, articulated concrete blocks, and revetment mattresses. Channel liners, when used as an outlet velocity protection measure, should be applied to the channel area immediately downstream of the culvert outlet for some distance, possibly to the right of way line and beyond (with appropriate easement). (See Chapter 6 for types and guidelines.) These liners, however, are viewed as creating environment problems, including decreased habitat and increased water temperature. They also are viewed to increase impervious cover, decrease time of concentration, and change the hydrograph timing downstream. In many instances, the liner may stabilize the area in question, only to have the problem shift downstream to where the channel is not lined.
  • Pre-formed outlets - Pre-formed outlets approximate a natural scour hole but protect the stream bed while dissipating energy. These have been shown to be effective protection in areas threatened by excessive outlet velocities. Such appurtenances should be lined with some type of riprap. (A velocity appurtenance for a culvert may be classified broadly as either a protection device or a control device.)
  • Channel recovery reach -- Similar to a pre-formed outlet, a channel recovery reach provides a means for the flow to return to an equilibrium state with the natural, unconstricted stream flow. The recovery reach should be well protected against the threat of scour or other damage.
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Velocity Control Devices

A velocity control device serves to effectively reduce an excessive culvert outlet velocity to an acceptable level. The design of some control devices is based analytically while, for others, the specific control may be unpredictable. Some velocity control devices are as follows:

  • Natural hydraulic jumps (most control devices are intended to force a hydraulic jump) -- Most velocity control devices rely on the establishment of a hydraulic jump. Because a culvert being on a relatively steep slope usually results in excessive outlet velocity from the culvert, the depth downstream of the culvert exit is usually not great enough to induce a hydraulic jump. However, some mechanisms may be available to provide a simulation of a greater depth necessary to create a natural hydraulic jump.
  • Broken-back culvert configuration -- One mechanism for creating a hydraulic jump is the broken back configuration, two types of which are depicted in Figure 8‑26 and Figure 8‑27. When used appropriately, a broken back culvert configuration can influence and contain a hydraulic jump. However, there must be sufficient tailwater, and there should be sufficient friction and length in unit 3 (see Figure 8‑26 and Figure 8‑27) of the culvert. In ordinary circumstances for broken back culverts, you may need to employ one or more devices such as roughness baffles to create a high enough tailwater.
  • Sills -- The use of the sill is effective in forcing a hydraulic jump in culverts. One disadvantage of sills is the possible susceptibility for silting. Sills must usually be maintained frequently to keep it free of sediment deposition. Another disadvantage is the waterfall effect that they usually cause. Riprap should be installed immediately downstream of the sill for a minimum distance of 10 feet to protect features from the turbulence of the waterfall effect.
  • Sills (click in image to see full-size image) Anchor: #i1006046grtop

    Figure 8-24. Sills

    Roughness baffles -- Roughness baffles, sometimes referred to as 'dragon's teeth', can be effective in inducing turbulence, dissipating energy, and reducing culvert outlet velocity (see Figure 8-25). Care must be taken in the design and placement of the baffles; if the baffles are too small or placed too far apart, they are ineffective. In addition, they may interfere with mowing operations around the culvert outlet. If these become damaged or broken from being hit by a mower, they are ineffective. To limit the amount of potential damage, baffles must be reinforced with rebar.

    Roughness Baffles (click in image to see full-size image)

    Figure 8-25. Roughness Baffles

  • Energy dissipators -- An efficient but usually expensive countermeasure is an energy dissipator. Some energy dissipators have an analytical basis for design while others are intended to cause turbulence in unpredictable ways. With turbulence in flow, energy is dissipated and velocity can be reduced.

Other controls are described in the FHWA publication Hydraulic Design of Energy Dissipators for Culverts and Channels, HEC-14.

Three Unit Broken Back Culvert (click in image to see full-size image)

Figure 8-26. Three Unit Broken Back Culvert

Two Unit Broken Back Culvert (click in image to see full-size image)

Figure 8-27. Two Unit Broken Back Culvert

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Broken Back Design and Provisions Procedure

The design of a broken back culvert is not particularly difficult, but it requires reducing velocity at the outlet. Use the following procedure:

  1. With design discharge and an associated tailwater, establish the flow line profile using the following considerations:
    • With reference to Figure 8‑26 and Figure 8‑27, unit 3 should be as long enough to ensure that the hydraulic jump occurs within the culvert.
    • For a given total drop, the resulting length of unit 2 is short, but this may cause the slope of unit 2 to be very steep.
    • Provided that unit 1 is on a mild slope, its length has no effect on the outlet velocity of any downstream hydraulic function. It is recommended that unit 1 either not be used or be very short; the result is additional latitude for adjustment in the profiles of units 2 and 3.
    • A longer unit 3 and a milder (but still steep) slope in unit 2 together enhance the possibility of a hydraulic jump within the culvert. However, these two conditions are contradictory and usually not feasible for a given culvert location. Make some compromise between the length of unit 3 and the slope of unit 2. Unit 3 must be on a mild slope (du > dc). This slope should be no greater than necessary to prevent ponding of water in the unit. Do not use an adverse (negative) slope.
  2. Size the culvert initially according to the directions outlined in step 1 under Design Guidelines and Procedure for Culverts.
    • If a unit 1 is used, the headwater will most likely result from the backwater effect of critical depth between units 1 and 2.
    • If a unit 1 is not used, the headwater will most likely result from inlet control.
  3. Starting at the upstream end of unit 2, calculate a supercritical profile, beginning at critical depth and working downstream through unit 3. The Direct Step Backwater Method is appropriate. Note the following:
    • Critical depth will not change from one unit to the next, but uniform depth will vary with the slope of the unit.
    • The increment, d, should be such that the change in adjacent velocities is not more than 10%.
    • The depth in unit 2 should tend to decrease towards uniform depth, so d should be negative. The resulting profile is termed an S2 curve.
    • Also, d should be small enough when approaching unit 3 such that the cumulative length does not far exceed the beginning of unit 3.
    • For hand computations, an acceptable expedient is to omit the profile calculation in unit 2 and assume that the exit depth from unit 2 is equal to uniform depth in unit 2.
  4. When you reach unit 3, complete the profile computations with the following considerations.
    • Because uniform depth is now greater than critical depth (mild slope), and flow depth is lower than critical depth, the flow depth tends to increase towards critical depth. Therefore, in unit 3, d should be positive.
    • The starting depth for unit 3 is the calculated depth at the end of unit 2.
    • Reset the cumulative length, L, to zero.
    • The resulting water surface profile is termed an M3 curve.

    As the profile is calculated, perform the checks outlined below:

    • As each depth is calculated along unit 3, calculate the sequent depth, ds. For more information, see the Direct Step Backwater Method, Hydraulic Jump in Culverts, and Sequent Depth subsections in Section 3.
    • Calculate the elevation of sequent depth (ds + flow line elevation) and compare it with the tailwater elevation. Tailwater elevation may be a natural stream flow elevation, or may produced artificially by installing a sill on the downstream apron between wingwalls. Design Division Hydraulics does not recommend the use of sills. (see Velocity Control Devices). If sills are used, the total vertical dimension of the artificial tailwater is determined by adding the elevation at the top of the sill and the critical depth of design discharge flow over the sill. Base this critical depth on the rectangular section formed by the top of the sill and the two vertical wingwalls. If the elevation of sequent depth is lower than the tailwater elevation, the following points apply; go to Step 5:
      • Hydraulic jump is likely to occur within the culvert.
      • Outlet velocity is based on the lower of tailwater depth, TW, and barrel height, D.
      • Profile calculations may cease even though the end of the barrel has not been reached.
    • If the computed profile tends towards critical depth before reaching the end of the culvert, the following apply and you should go to Step 5:
      • Hydraulic jump is likely to occur within the culvert.
      • Outlet depth will be equal to critical depth and outlet velocity is based on critical depth.
      • Profile calculations may cease even though the end of the barrel has not been reached.
    • Compare the cumulative length, L, to unit 3 length. If L length of unit 3, the following apply:
      • Hydraulic jump does not form within the length of unit 3.
      • Exit depth is the present value of d.
      • Exit velocity is based on exit depth.
      • The broken-back culvert configuration is ineffective as a velocity control device and should be changed in some manner. Alternatives include rearrangement of the culvert profile, addition of a sill, and investigation of another device. If the profile is reconfigured, go back to step 3. Otherwise, skip step 5 and seek alternative measures.
  5. Consider hydraulic jump cautions. The hydraulic jump is likely to occur within the culvert for the design conditions. However, it is prudent to consider the following cautions:
    • If tailwater is very sensitive to varying downstream conditions, it may be appropriate to check the occurrence of the hydraulic jump based on the lowest tailwater that is likely to occur.
    • The hydraulic jump may not occur within the barrel under other flow conditions. It is wise to check the sensitivity of the hydraulic jump to varying flow conditions to help assess the risk of excessive velocities.
    • If a sill has been employed to force an artificial tailwater, and the hydraulic jump has formed, the outlet velocity calculated represents the velocity of water as it exits the barrel. However, the velocity at which water re-enters the channel is the crucial velocity. This velocity would be the critical velocity of sill overflow.
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Energy Dissipators

Impact basins are effective energy dissipators but are relatively expensive structures (see Figure 8-28).

Impact Basins (click in image to see full-size image) Anchor: #i1006356grtop

Figure 8-28. Impact Basins

Stilling basins are hydraulically similar to sills (Figure 8-29). However, they are more expensive in construction and could present serious silting problems. A chief advantage in stilling basins is the lack of a waterfall effect.

Stilling Basins (click in image to see full-size image)

Figure 8-29. Stilling Basins

Radial energy dissipators are quite effective but extremely expensive to construct and, therefore, not ordinarily justified (Figure 8-30). They function on the principle of a circular hydraulic jump. For a detailed discussion on dissipator types, along with a variety of design methods for velocity control devices, refer to HEC-14.

Radial Flow Energy Dissipator (click in image to see full-size image)

Figure 8-30. Radial Flow Energy Dissipator

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