Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Design Guideline 18 Riprap Protection for Bottomless Culverts

18.1 INTRODUCTION

Bottomless (or three-sided) culverts are structures that have natural channel materials as the bottom. Figure 18.1 shows a common type of bottomless culvert that is over 10 feet (3 m) high and over 40 feet (12 m) wide. These cast-in-place, precast, or prefabricated structures may be rectangular in shape or may have a more rounded top. They are typically founded on spread footings although pile foundations and pedestal walls are also used. Regardless of the foundation type, the structure may be highly susceptible to scour. Bottomless culverts on spread footings are best suited for non-erodible rock but with caution and with scour protection can be used for other soils. Bottomless culverts with pile foundations may still require riprap protection because scour below the pile cap can cause the approach embankment to fail into the scour hole.

Scour is greatest at the upstream corners of the culvert entrance. Pressure flow can greatly increase scour potential. This design guideline is only applicable for free-surface flow conditions (i.e., no pressure flow up to and including the 500-year flood event) so the shape of the culvert (rectangular or curved) does not significantly affect the hydraulic conditions, scour potential, or riprap size. Because bridges are checked for stability for the superflood condition, they are typically designed to withstand scour up through the 500-year event. Therefore, the riprap size should be determined for the worst-case condition, which may be the 500-year event or a lower flow. This design guideline is also only applicable for culverts that include flared wing walls at the upstream and downstream ends. The riprap size and layout presented in this guideline are intended to protect the culvert foundations that act as abutments. If dual bottomless culverts (side-by-side) are used then the center foundation acts as a pier and must be designed to be stable for the total scour depth (degradation, contraction and pier scour) without a countermeasure.

Figure 18.1 Bottomless Culvert on Whitehall Road over Euclid Creek in Cuyahoga County, OH.

The following determinations must be made in the design of rock protection for bottomless culverts:

• Size of stone
• Foundation depth
• Layer thickness (sufficient to accommodate the largest stones and to account for contraction scour and long-term degradation)
• Horizontal extent of riprap (to account for contraction scour and long-term degradation)
• Filter type and extent (geotextile filter fabric to prevent substrate material from being washed out through the voids of the stone)

Bottomless culverts have several advantages over other crossing structures. The natural bottom material is more environmentally attractive than a traditional closed culvert, particularly where fish passage is a concern. They are also considered by many highway agencies to be economical alternatives to short bridges. They are more easily constructed than conventional bridges because they are commonly prefabricated.

18.2 LABORATORY INVESTIGATIONS OF SCOUR AND RIPRAP AT BOTTOMLESS CULVERTS

FHWA sponsored two laboratory studies of scour and riprap at bottomless culverts (Kerenyi, Jones and Stein 2003, 2007, Kerenyi and Pagan-Ortiz, 2007). The studies concluded that the scour is analogous to contraction scour caused by concentration of flow (primary flow) and to abutment scour caused by vortices and strong turbulence (secondary flow) (Figure 18.2). The studies included rectangular and arched shapes with and without wing walls (Figures 18.3 - 18.5). These figures show that scour is usually greatest at the upstream corners of the culvert entrance.

Figure 18.2. Flow concentration and separation zone (Kerenyi et al. 2007).

Figure 18.3. Rectangular model with vertical face (Kerenyi et al. 2003).

Figure 18.4. Rectangular model with wing walls (Kerenyi et al. 2003).

Figure 18.5. Arched model with wing walls (Kerenyi et al. 2003).

Riprap scour protection was also investigated in the two laboratory studies. Figure 18.6 shows a physical model of a bottomless culvert in a flume with riprap protection across the bottom. Another riprap placement alternative is the MDSHA (Maryland State Highway Agency 2005) standard plan (Figure 18.7). This alternative includes riprap placed along the wing walls and at the base of the vertical sides of the culvert.

Figure 18.6. Riprap scour protection with a rectangular model (Kerenyi et al. 2003).

Figure 18.7. Riprap protection at a bottomless culvert with the MDSHA Standard Plan (Kerenyi et al. 2007).

18.3 RIPRAP SIZE

The results obtained from the Phase II laboratory study (Kerenyi et al. 2007) were used to develop a riprap size equation that accounts for the local velocity at the corner of the culvert entrance and the vorticity and turbulence of the flow. The equation is:

(18.1)

where:

18.4 LAYOUT DETAILS

The MDSHA (Maryland State Highway Agency 2005) standard plan for riprap was tested as a countermeasure (Figure 18.7). When the plan was tested, riprap launched into the scour hole and then stabilized. Figure 18.8 shows the riprap condition after the test. Based on the results of the FHWA riprap experiments for bottomless culverts, the MDSHA standard plan tests, and on other design criteria found in HEC-18 (Richardson and Davis 2001) and other HEC-23 design guides, the following guidance is recommended for protecting the foundations of bottomless culverts with riprap. It should be noted that these layout details have not been tested in the laboratory or in the field. The designer is ultimately responsible for adapting these recommendations to a particular site installation.

Figure 18.8. MDSHA standard plan after test (Kerenyi et al. 2007).

Riprap Extent: Figure 18.9 shows the riprap layout based, in part, on the MDSHA standard plan. Riprap should extend along the entire length of the culvert wall and wing walls (upstream and downstream). The recommended wing wall flare is 45° for the entrance and 8° for the exit. The riprap should extend from the end of the wing wall along the toe of the embankment at least 10 feet (3 m) but not less than two times the local water depth on the upstream end and at least 20 feet (6 m) but not less than four times the local water depth on the downstream end. Riprap should also be placed up the embankment slopes. If a greater flare is used at the downstream end the flow may separate resulting in a vortex along the downstream wing wall.

Figure 18.9. Riprap layout.

Elevation of Riprap Protection: The top of the riprap should be placed flush with the channel bed surface. This elevation is required for inspecting the riprap. The bottom of the riprap should be at least 1 ft below the top of footing. The thickness of the riprap layer should not be less than three times d₅₀ of the riprap (3xd₅₀).

Footing Elevations: HEC-18 (Richardson and Davis 2001) provides guidance on the elevations of spread footings. The guidance indicates that for soil the top of the footing should be below the sum of long-term degradation, contraction scour and lateral migration. Erodible rock may require the same treatment as soil. For soils the guidance indicates that the bottom of the footing should be below the total scour (including local scour). In the case where riprap is used to protect spread footings that act as abutments at bottomless culverts, the HEC-18 bottom of footing recommendation does not apply.

Riprap Cross Section: MDSHA (2005) has a requirement to "Design the width and thickness of the riprap wall protection to keep contraction scour away from the wall footings..." This is the approach adopted in this design guide. Figure 18.10 shows riprap constructed with a horizontal bench adjacent to the culvert wall and a sloping surface down to the lower riprap elevation. The sloping riprap is constructed at a 1V:3H slope to provide a stable riprap mass that is not intended to launch. Regulatory requirements may also dictate the allowable extent of riprap. If environmental and regulatory requirements permit, armoring the entire bottom is an option. The riprap size determined from Equation 18.1 would also apply to a full armor.

Design Evaluation: If the width or thickness of the riprap is excessive either from the standpoint of construction or permitting, then a wider culvert or deeper foundation should be considered. A wider culvert will reduce velocity and flow concentration, which results in less contraction scour and smaller riprap. The designer could also consider using a pile foundation for culvert walls and wing walls. Erosion of material from under the pile-supported footing (pile cap) would remain as a concern because this could result in the failure of the approach embankment. The riprap design should be reviewed by structural and geotechnical engineers to determine whether the culvert foundation design is affected by the loading of the riprap.

18.5 FILTER REQUIREMENTS

There are two kinds of filters used in conjunction with riprap; granular filters and geotextile filters. For this application only geotextile filters are recommended. The geotextile filter should extend a distance of W_B out from the culvert walls and wing walls. Detailed guidance for filter design is provided in Design Guideline 16 of this document.

18.6 Design Example

A bottomless culvert is being installed on erodible materials with a spread footing foundation. The design discharge is 1000 cfs (28 m³/s). The culvert width is 30 ft (9.1 m) and the flow depth for the design discharge is 6.7 feet (2.0 m). The computed contraction scour is 2.6 ft (0.79 m), the anticipated long-term degradation is 1.0 ft (0.30 m) and the channel thalweg elevation is 100.7 ft (30.7 m), which is 1.5 ft (0.46 m) lower than the channel bed elevation along the culvert walls. The riprap specific gravity is 2.65. Calculate the riprap size, the top of footing elevation, Y_Tot, W_T and W_B for the sloping rock protection.

Figure 18.10. Cross Section for Sloping Rock.

Step 1. Calculate the average velocity in the culvert:

V_AC = Q/(Wc x y₀) = 1000/(30 x 6.7) = 5.0 ft/s (1.52 m/s)

Step 2. Calculate the median stone diameter d₅₀:

Step 3. Calculate the top of footing elevation.

The top of footing is at or below the contraction scour plus long-term degradation relative to the channel thalweg.

Y_SC = contraction scour + long-term degradation = 2.6 + 1.0 = 3.6 ft (1.1 m)
Top of Footing Elevation = Invert elevation - Y_SC = 100.7 - 3.6 = 97.1 ft (29.6 m)

Step 4. Calculate the riprap layer thickness, Y_Tot:

Y_Tot is the riprap layer thickness. The top of riprap is the channel bed level at the culvert wall and the bottom of riprap is at least 1 ft (0.3 m) below the top of footing.
Y_Tot = Y_SC + difference between invert and bed at the culvert wall + 1 = 3.6 + 1.5 +1= 6.1 ft (1.86 m) (which is greater than 3 x d₅₀, therefore use 6.1 ft).

Step 5. Calculate the riprap top width, W_T:

W_T is 3 x d₅₀ or 5 ft, whichever is greater.
W_T = 3 x 1.4 = 4.2 ft, therefore W_T = 5.0 ft (1.5 m)

Step 6. Calculate W_B:

W_B = W_T + 3.0Y_Tot = 5.0 + (3.0 x 6.1) = 23.3 ft (7.1 m)

Note: In this case the riprap will extend the full width of the opening and may not be acceptable from the standpoint of environmental permitting or fish passage. However, the natural bottom is expected to persist except during floods when the contraction scour would erode down to the riprap surface. The contraction scour hole is expected to refill after a flood if live-bed conditions exist. Increasing the culvert width will reduce the contraction scour and the riprap size. This reduces Y_Tot and the extent of riprap.

18.7 RIPRAP AS A RETROFIT SCOUR COUNTERMEASURE

If an existing culvert has a history of scour problems or is scour critical based on an analysis of flood conditions, then riprap can be considered as a scour countermeasure as part of a Plan of Action (see HEC-23, Volume 1, Chapter 2). A common approach to a retrofit includes:

1. Dewater the length of culvert and staging area. The upstream side could be dammed and the water pumped or piped through a pipe placed along one footing on the inside of the culvert span.
2. A small skid-steer loader can be used to remove stream bottom material. The bottom elevation of removal to the top-of-footing is preferable, but may have to be lower to accommodate the loader height.
3. Place the appropriately selected geotextile filter fabric under the specified location of the riprap.
4. Place riprap with the loader according to the construction plans and specifications. A bedding layer of clean granular material may be necessary to protect the filter fabric. The bedding layer should be more permeable than the filter fabric.
5. As the riprap is placed, backfill with natural streambed material on top of the riprap up to the stream invert elevation. This step can be omitted if a "natural" bed is not required in the culvert section.

18.8 REFERENCES

Kerenyi, K., Jones, J.S., and Stein, S., 2003. "Bottomless Culvert Scour Study: Phase I Laboratory Report," Federal Highway Administration, Report No. FHWA-RD-02-078.

Kerenyi, K., Jones, J.S., and Stein, S., 2007. "Bottomless Culvert Scour Study: Phase II Laboratory Report," Federal Highway Administration, Report No. FHWA-HRT-07-026.

Kerenyi, K. and Pagán-Ortiz, J., 2007. "Testing Bottomless Culverts," Public Roads, Vol. 70, No. 6, May/June 2007.

MDSHA, (Maryland State Highway Agency) 2005. "Office of Bridge Development Manual for Hydrologic and Hydraulic Design, Chapter 11, Evaluating Scour at Bridges, Appendix C, Estimating Scour at Bottomless Arch Culverts, September 2005.

Richardson, E.V. and Davis, S.R., 2001. "Evaluating Scour at Bridges," Fourth Edition, Hydraulic Engineering Circular No. 18, Federal Highways Administration Publication No. FHWA NHI 01-001, Washington, D.C.