Testing produces design guidance

Follow this new anti-scour mat product through the testing process.

By Michael D. Robeson

Abstract

A new product with the application of culvert and stormwater outlet scour protection was tested at Colorado State University. The objective of the testing was to determine the hydraulic and sediment stability threshold conditions. This article presents the engineering design guidance offered as a result.

Introduction

Outlet erosion protection of culverts can be a serious problem due to the increased erosive potential of outlet flows. Determination of the scour potential should be common practice with the design of culverts.

Exit velocity can be considered the main factor in determining the need for erosion protection at a culvert outlet. Culverts channelize flow resulting in outlet velocities that tend to be higher than the natural stream velocity, in which case flow adjustment or energy dissipation could be necessary to mitigate the potential for downstream channel erosion. Culvert performance can be affected by the downstream water surface elevation or tailwater.

For this article, low tailwater conditions will be examined. Low tailwater conditions result when the flow exits at approximately one-third of the culvert (Stevens and Urbonas, 1996).

Two types of scour can occur in the vicinity of culvert outlets including local scour and general channel degradation. Local scour can be considered a direct result of the high-velocity flows at the culvert outlet while general channel degradation results from changes to the river regime by natural processes or human activities. This article will focus on the local scour that could occur at culvert outlets.

Test program summary

Information presented within this article is intended to provide a brief summary of the test program, present an analysis of the data collected during testing, and present design guidance. During the summer of 2005, hydraulic performance testing was conducted by Colorado State University (CSU) on the selected mat. A total of 10 tests were conducted under this program, which consisted of 4 configurations. Figure 1 is a photograph of the transition mat (TM) submitted for testing.

For this testing program, an 83.82cm (33-in.) diameter by 15.24m (50-ft) long culvert at the base of the steep gradient overtopping facility (SGOF) was utilized. An existing water-pipe network was used to deliver flow into a head box by means of an overhead diffuser to control varying levels of driving head at the invert of the culvert.

A test matrix was developed to summarize and present the details of the testing program. Tables 1 and 2 present the test matrices for the chamfered and unchamfered systems, respectively. More detailed information pertaining to the testing program can be found in the testing report (Clopper, Robeson and Thornton, 2005).

Analysis

Table 3 presents a summary matrix of the soil-loss analysis for each of the configurations tested under the described test program.

Data from the hydraulic testing of a full-scale scour countermeasure system can be used to determine the hydraulic performance threshold for each of the 4 test configurations since testing was started at reasonable performance projections based upon reliable literature regarding permissible velocities of both vegetated and high-performance turf reinforcement mat (HPTRM)-lined channels. As a basis for comparison, Table 4 presents maximum permissible velocities for common materials. The permissible velocity provided for the unvegetated HPTRM was determined from prior CSU performance testing.

The data collected for each configuration were examined by CSU and hydraulic performance limits were determined for the tests conducted under the aforementioned test program. Subsequently, the data obtained from the vegetated test, Configuration No.1, proved to indicate the critical nature of vegetation with regard to increased performance levels and lower factors of risk. Given that the sod and mat combination had exceeded industry standards for permissible velocities, a combination of HPTRM and sod with the mat was not tested; however, the utilization of such a HPTRM over the sod should lead to the same, if not increased, performance levels.

By examining each of the tested configurations, a quantitative value of relative performance can be determined from the information obtained during testing and Table 4. Table 5 presents the relative performance for each of the tested configurations.

By examining the velocity increase ratio from Table 5, it can be concluded that the TM can withstand 3.2 times more velocity than Kentucky bluegrass alone. In addition, for either Configurations No. 3 or No. 4, the TM can withstand 1.8 times more velocity than the unvegetated HPTRM alone.

It was also noted that Configurations No. 1, No. 3, and No. 4, all exceeded the permissible velocity for riprap up to 30.5 cm (12 in.), and Configuration No. 2 exceeded the permissible velocity for riprap up to 15.25 cm (6 in.). It should be noted that Configurations No. 2, No. 3, and No. 4 were tested in an unvegetated condition. Once these HPTRM and mat systems become vegetated, an increased performance threshold can be expected.

To determine the appropriate amount of mat that would be needed for a variety of hydraulic conditions, a further analysis of the erosion that was observed for each configuration were plotted as contours. Based on these contours, the minimum length of protection for each test was determined. Table 6 presents a summary of the results obtained from the contour analysis of the erosion for each test.

Design guidance

Design guidance for riprap scour protection downstream of a culvert outlet under low tailwater conditions can be found in Urbonas and Stevens, 1996. Subsequent sections provide design guidance for the TM during low tailwater conditions. For culvert pipe outlets, low tailwater can be defined when: y_t ≤ (D÷3)

where: y_t = depth of tailwater at design flow (m,ft);

D = the diameter of a circular pipe (m,ft).

The first step in design of scour protection at an outlet of a culvert regardless of the material used for protection is to find the depth and velocity at the outlet. Pipe-full flow can be found using the Manning equation and the pipe-full velocity can be found using the continuity equation as follows:

Q_full = {1.486 (U.S. Customary) or 1 (SI) A_full R_full^2/3 S_f^1/2} ÷ n

where: Q_full = Pipe-full discharge (cms,cfs);

n = Manning roughness coefficient for the pipe-full depth ();

A_full = Cross-sectional area of the pipe (m²,ft²);

R_full = Hydraulic radius for a pipe flowing full (m,ft);

S_f = Slope of the energy grade line, usually taken as the slope of the pipe (m/m,ft/ft).

V_full = Q_full / A_full

where: V_full = Cross-sectional average velocity of pipe-full flow (m/s,ft/s).

For flow conditions other than full-pipe flow, Figure 3 can be used to determine the flow depth and velocity. Using a known design discharge, Q, and the calculated pipe-full discharge, Q_full, calculate a discharge percentage as:

%Q = Q/Q_full

where: %Q = discharge percentage (%);

Q = design discharge (cms, cfs)

Utilize Figure 3 at the abscissa with the value of %Q and continue up the chart until the discharge curve is reached. Read across to the ordinate to find depth of flow as a percentage, %D. Calculate the flow depth at the end of the pipe as:

d = D(%D)

where: d = flow depth of the design discharge at the end of the culvert (m,ft);

%D = flow depth percentage (%).

Next, utilize Figure 3 at the ordinate with the value of %D, and follow across until the velocity curve is intersected. Drop down to the abscissa to determine the velocity as a percentage, %V. Calculate the design discharge flow velocity at the end of the pipe:

v = V(%V)

where: v = velocity of the design flow at the end of the pipe (m/s,ft/s);

%V = velocity percentage (%).

Once the hydraulic conditions have been calculated as outlined above, the coverage length of the TM can be determined from Figure 4 using the known exit velocity and type of installation desired. Figure 4 presents the hydraulic performance data for each installation type and conservative design curves which include each data point resulting from testing.

TM can be installed over sod (Type A), over sod covered by a TRM (Type B), over bare soil covered by a TRM (Type C), or over bare soil covered with a HPTRM (Type D¹).

These guidelines provide for the minimum recommended required coverage length to utilize. From the calculated design exit velocity, utilize Figure 4 to obtain the corresponding minimum transition mat coverage length (L_TM) for installation types A and B or C and D. Then calculate the width of the transition mat:

W_TM = 4D

where: W_TM= width of the transition mat (m,ft);

D = diameter of culvert pipe (m,ft).

It should be noted that for some installations, a smaller width of TM may be acceptable; please consult with manufacturers for guidance regarding smaller widths. It is important to note that the design curves presented in Figure 4 are dependent on culvert pipe size. For installation types A and B, the design curve can be considered valid only for a maximum pipe diameter of 152.4cm (60 in.) and a maximum exit velocity of 4.88 m/s (16 ft/s).

For installation types C and D, the design curve is considered valid for a maximum pipe diameter of 152.4cm (60 in.) and a maximum exit velocity of 4.27m/s (14 ft/s), as shown in Table 7. It should be noted that during testing at CSU, an 83.82cm (33-in.), pipe was utilized for most of the testing. During one test, a flared culvert pipe end was tested with an exit width of 152.4cm (60 in.). The data obtained from the flared end test provided the basis for allowing design up to 152.4cm (60 in.) as long as the velocity limits were maintained.

Additional design considerations

Additional design considerations are listed below as well as a design example providing a sample problem and associated calculations as a reference.

To maintain consistency with the test program it is recommended that the transition mats be placed beginning at the pipe outlet, preferably in contact with the pipe and centered laterally with the pipe, as shown in Figure 1. It is recommended that the transition mat be installed on a smooth and uniform grade.
During testing, it was observed that an additional layer of TM installed for Types C & D just at the culvert outlet, as shown in Figure 2, improved erosion protection. The double layer was observed to minimize the open area of the system and was placed such that the open area of the second layer was offset from the open area associated with the first layer.
If applicable, it is recommended that either a type A or B installation be used. If conditions do not permit the efficient establishment of vegetation, a type C or D installation utilizing a TRM which can withstand 1.67 m/s (5.5 ft/s) of velocity in unvegetated conditions should be used.
The TMs do not dissipate energy. Therefore, it has been concluded that the transition from TM to the existing channel requires further examination and proper design to adequately deal with any remaining velocity and shear forces.
Testing at Colorado State University was conducted for an average angle between the discharge slope and the downstream angle of approximately 180 degrees.

Design example

Given: Pipe Diameter

D = 81.28 cm (32 in.)Longitudinal

Pipe Slope S₀ = 0.040 m/s (ft/ft)

Pipe Manning Roughness n = 0.020

Step 1: Verify that this method is applicable.

D ≤ 152.4 cm (60 in.), Type A, B, C & D: OK

Step 2: Calculate the full-pipe discharge.

Q_full = 1.79 cms (63.34 cfs) from the Manning equation.

Step 3: Calculate the full-pipe velocity;

V_full = 3.46 m/s (11.34 ft/s) from the Continuity equation.

Step 4: The highest exit velocities for a circular pipe are associated with a depth of approximately 83% the pipe diameter. This corresponds to an exit velocity of approximately 122% the full-pipe exit velocity. Thus, the design exit velocity is:

V_design = 4.22 m/s (13.84 ft/s)

Step 5: Verify that the design velocity from Step 4 is within the acceptable range.

V_design ≤ 4.88 m/s (16 ft/s), Type A & B: OK

V_design ≤ 4.27 m/s (14 ft/s), Type C & D: OK

Step 6: Determine the necessary transition mat lengths based on installation type.

A&B L_TM = 4.88m (16 ft)

C&D L_TM = 6.10m (20 ft)

Step 7: Calculate the necessary transition mat width (not a function of installation type).

WTM = 3.66m (12 ft)

Michael D. Robeson, P.E., is the former manager of the Colorado State University Hydraulics Laboratory in Fort Collins. He is currently technical services manager at Profile Products LLC.

References

¹For installation types C and D, the minimum permissible velocity for the TRM or HPTRM is 1.67m/s (5.5 ft/s) for unvegetated conditions.

Norman, Jerome M., Houghtalen, Robert J., and Johnston, William J. (2001). Hydraulic Design of Highway Culverts. Hydraulic Design Series No. 5 (HDS-5), U.S. Federal Highway Administration, Publication No. FHWA-NHI-01-020.

Richardson, E.V., Simons, D.B, and Lagasse, P.F. (2001). River Engineering for Highway Encroachments: Highways in the River Environment. Hydraulic Design Series No. 6 (HDS-6), U.S. Federal Highway Administration, Publication No. FHWA-NHI-01-004.

Richardson, E.V., and Davis, S.R., 2001. “Evaluating Scour at Bridges – Fourth Edition,” Hydraulic Engineering Circular No. 18, U.S. Federal Highway Administration, Publication No. FHWA-NHI-01-001

Schall, James D., Richardson, E.V., and Morris, Johnny L. (2001). Introduction to Highway Hydraulics. Hydraulic Design Series No. 4 (HDS-4), U.S. Federal Highway Administration, Publication No. FHWA-NHI-01-019.

Stevens, Michael A., and Urbonas, Ben. Design of Low Tailwater Riprap Basins for Storm Sewer Pipe Outlets. Urban Drainage & Flood Control District. 1996.

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