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FHWA > Engineering > Hydraulics > HEC 15 
Design of Roadside Channels with Flexible Linings

Type  Description 

OpenWeave Textile  A temporary degradable RECP composed of processed natural or polymer yarns woven into a matrix, used to provide erosion control and facilitate vegetation establishment. Examples: jute net, woven paper net, straw with net. 
Erosion Control Blankey  A temporary degradable RECP composed of processed natural or polymer fibers mechanically, structurally or chemically bound together to form a continuous matrix to provide erosion control and facilitate vegetation establishment. Example: curled wood mat. 
Turfreinforcement Mat (TRM)  A nondegradable RECP composed of UV stabilized synthetic fibers, filaments, netting and/or wire mesh processed into a threedimensional matrix. TRMs provide sufficient thickness, strength and void space to permit soil filling and establishment of grass roots within the matrix. Example: synthetic mat. 
The density, stiffness and thickness of lightweight manufactured linings known as rolled erosion control products (RECPs) are the main properties that relate to flow resistance and erosion control performance. There are a series of standard tests referred to as index tests that measure these physical properties. The AASHTO National Transportation Product Evaluation Program (NTPEP) (AASHTO/NTPEP, 2002) has identified a set of test methods applicable to RECPs. Research on RECPs has not resulted in a relationship between these index tests and hydraulic properties. Hydraulic properties must be determined by full scale testing in laboratory flumes using defined testing protocols (ASTM D 6460). Table 5.2 summarizes index tests that relate to the physical properties of density, stiffness and thickness.
Qualitatively, denser linings prevent soil from entering into the highervelocity flow above the liner (Gharabaghi, et al., 2002; Cotton, 1993). Linings with higher tensile strength and flexural rigidity have less deformation due to shear and uplift forces of the flow and remain in closer contact with the soil. Linings with more thickness have a larger moment of inertia, which further reduces the deformation of the lining.
NTPEP also includes two bench tests developed by the Erosion Control Technology Council (ECTC) that relate to channel erosion. Table 5.3 briefly describes the bench scale test methods applicable to RECP channel linings. The values generated from benchscale tests are intended for qualitative comparison of products and product quality verification. These values should not be used to design a channel lining.Because of their small scale, these tests do not reflect larger scale currents that are generated in full scale testing in laboratory flumes using defined testing protocols (AASHTO/NTPEP, 2002; Robeson, et al., 2003).
Property  Index Test  Description 

Density  ASTM D 6475  Standard Test Method for Mass per Unit Area for Erosion Control Blankets 
ASTM D 6566  Standard Test Method for Measuring Mass per Unit Area of Turf Reinforcement Mats  
ASTM D 6567  Standard Test Method for Measuring the Light Penetration of Turf Reinforcement Mats  
Stiffness  ASTM D 4595  Test Method for Tensile Properties of Geotextile by the WideWidth Strip Method 
Thickness  ASTM D 6525  Standard Test Method for Measuring Nominal Thickness of Erosion Control Products 
Bench Test  Description 

ECTC  Draft Method 3 Channel Erosion  Standard test method for determination of RECP ability to protect soil from hydraulically induced shear stresses under benchscale conditions. 
ECTC  Draft Method 4 Germination and Plant Growth  Standard test method for determination of RECP performance in encouraging seed germination and plant growth. 
Proper installation of RECPs is critical to their performance. This includes the stapling of the lining to the channel perimeter, the lapping of adjacent fabric edges and the frequency of cutoff trenches. Each manufacturer provides guidelines on installation, which should be reviewed and incorporated into installation specifications. Construction inspection should verify that all installation specifications have been met prior to acceptance.
There is no single n value formula for RECPs. The roughness of these linings must be determined by fullscale testing in laboratory flumes using defined testing protocols. As with vegetated linings, the n value varies significantly with the applied shear due to the displacement of the lining by shear and uplift forces.
The designer will need to obtain from the RECP manufacturer a table of n value versus applied shear. Three n values, with the corresponding applied shear values need to be provided by the manufacturer as shown in Table 5.4. The upper shear stress should equal or exceed the lining shear, τ_{l}. The upper and lower shear stress values must equal twice and onehalf of the middle value, respectively.
Applied Shear, N/m^{2} (lb/ft^{2})  n value 

τ_{lower} = τ_{mid}/2  n_{lower} 
τ_{mid}  n_{mid} 
τ_{upper} = 2 τ_{mid}  n_{upper} 
This information is used to determine the following n value relationship:
(5.1) 
where,
n  = Manning's roughness value for the specific RECP 
a  = coefficient based on Equation 5.2 
b  = exponent based on Equation 5.3 
τ_{o}  = mean boundary shear stress, N/m^{2} (lb/ft^{2}) 
The coefficient "a" is based on the n value at the midrange of applied shear.
(5.2) 
The exponent "b" is computed by the following equation:
(5.3) 
Note that exponent "b" should be a negative value.
The permissible shear stress of an RECP lining is determined both by the underlying soil properties as well as those of the RECP. In the case of TRMs, the presence of vegetation also influences erosion resistance properties.
RECPs dissipate shear stress before it reaches the soil surface. When the shear stress at the soil surface is less than the permissible shear for the soil surface, then erosion of the soil surface will be controlled. RECPs provide shear reduction primarily by providing cover for the soil surface. As the hydraulic forces on the RECP lining increase, the lining is detached from the soil, which permits a current to establish between the lining and the soil surface. Turbulent fluctuations within this current eventually erode the soil surface. This process model for RECP shear on the soil surface is given by the following (See Appendix F for derivation):
(5.4) 
where,
τ_{e}  = effective shear stress on the soil, N/m^{2} (lb/ft^{2}) 
τ_{d }  = design shear stress, N/m^{2} (lb/ft^{2}) 
τ_{l}  = shear stress on the RECP that results in 12.5 mm (0.5 in) of erosion 
α  = unit conversion constant, 6.5 (SI), 0.14 (CU) 
The value of τ_{l} is determined based on a standard soil specified in the testing protocol. Permissible shear stress for the underlying soil has been presented in Section 4.3.2. The reader is referred to that section for that discussion.
The combined effects of the soil permissible shear stress and the effective shear stress transferred through the RECP lining results in a permissible shear stress for the RECP lining. Taking Equation 5.4 and substituting the permissible shear stress for the soil for the effective shear stress on the soil, τ_{e}, gives the following equation for permissible shear stress for the RECP lining:
(5.5) 
where,
τ_{p}  = permissible shear stress on the RECP lining, N/m^{2} (lb/ft^{2}) 
τ_{l}  = shear stress on the RECP that results in 12.5 mm (0.5 in) of erosion 
τ_{p,soil}  = permissible soil shear stress, N/m^{2} (lb/ft^{2}) 
α  = unit conversion constant, 6.5 (SI), 0.14 (CU) 
Evaluate a temporary channel lining for a roadside channel. Two alternative RECPs are available. Alternative A costs less.
Given:
Shape: Trapezoidal, B = 0.9 m, Z = 3
Soil: Clayey sand (SC classification), PI = 16, e = 0.5
Grade: 3.0 percent
Flow: 0.30 m^{3}/s
RECP Product A:
Erosion Control Blanket, ECB, Manufacturers performance data
τ_{l} = 60 N/m^{2} (Shear on lining at 12.5 mm soil loss)
Roughness rating:
Applied Shear, N/m^{2} n value 35 0.038 70 0.034 140 0.031
RECP Product B:
Erosion Control Blanket, ECB, Manufacturers performance data
τ_{l} = 100 N/m^{2} (Shear on lining at 12.5 mm soil loss)
Roughness rating:
Applied Shear, N/m^{2} n value 50 0.040 100 0.036 200 0.033
First, try the less expensive "Product A." The solution is accomplished using procedure given in Section 3.1 for a straight channel.
Step 1. Channel slope, shape, and discharge have been given.
Step 2. Select erosion ECB A.
Step 3. Initial depth is estimated at 0.30 m
From the geometric relationship of a trapezoid (see Appendix B):
R = A/P = (0.540)/(2.80) = 0.193 m
Step 4. To estimate n, the applied shear stress on the lining is given by Equation 2.3
Determine a Manning's n value from Equation 5.1 with support from Equations 5.2 and 5.3.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5. Since this value is more than 5 percent different from the design flow, we need to go back to step 3 to estimate a new flow depth.
Step 3 (2^{nd} iteration). Estimate a new depth solving Equation 2.2 or other appropriate method iteratively to find the next estimate for depth:
d = 0.18 m
Revised hydraulic radius.
R = A/P = (0.259)/(2.04) = 0.127 m
Step 4 (2^{nd} iteration). To estimate n, the applied shear stress on the lining is given by Equation 2.3
Determine a Manning's n value from Equation 5.1. The exponent, b, and the coefficient, a, are unchanged from the earlier calculation.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5 (2^{nd} iteration). Since this value is within 5 percent of the design flow, we can proceed to step 6.
Step 6. The maximum shear on the lining of the channel bottom is.
Determine the permissible soil shear stress from Equation 4.6 and Table 4.6.
Equation 5.5 gives the permissible shear on the RECP.
Safety factor for this channel is selected to be equal to 1.0.
Step 7. Product A (ECB lining) is not acceptable since the maximum shear on the RECP surface is greater than the permissible shear of the RECP.
Now try the alternative "Product B." The flow and channel configuration as well as the permissible shear stress are the same. Also, it is reasonable to assume an initial depth equal to the last depth we calculated for Product A. Therefore, using the area and hydraulic radius from that calculation, we can start with Step 4.
Step 4. To estimate n, the applied shear stress on the lining is given by Equation 2.3
Determine a Manning's n value from Equation 5.1 with support from Equations 5.2 and 5.3.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5. Since this value is more than 5 percent different from the design flow, we need to go back to step 3 to estimate a new flow depth.
Step 3 (2^{nd} iteration). Estimate a new depth solving Equation 2.2 or other appropriate method iteratively to find the next estimate for depth:
d = 0.19 m
Revise hydraulic radius.
R = A/P = (0.279)/(2.10) = 0.132 m
Step 4 (2^{nd} iteration). To estimate n, the applied shear stress on the lining is given by Equation 2.3
Determine a Manning's n value from Equation 5.1. The exponent, b, and the coefficient, a, are unchanged from the earlier calculation.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5 (2^{nd} iteration). Since this value is within 5 percent of the design flow, we can proceed to step 6.
Step 6. The maximum shear on the lining of the channel bottom is.
Equation 5.5 gives the permissible shear on the RECP.
Step 8. Product B (ECB lining) is an acceptable temporary lining since the maximum shear on the RECP surface is less than the permissible shear of the RECP.Choose Product B. (Remember the permanent vegetative lining must also be evaluated.)
Evaluate a temporary channel lining for a roadside channel. Two alternative RECPs are available. Alternative A costs less.
Given:
Shape: Trapezoidal, B = 3.0 ft, Z = 3
Soil: Clayey sand (SC classification), PI = 16, e = 0.5
Grade: 3.0 percent
Flow: 10 ft^{3}/s
RECP Product A:
Erosion Control Blanket, ECB, Manufacturers performance data
τ_{l} = 1.25 lb/ft^{2} (Shear on lining at 0.5 in soil loss)
Roughness rating:
Applied Shear, lb/ft^{2} n value 0.75 0.038 1.5 0.034 3.0 0.031
RECP Product B:
Erosion Control Blanket, ECB, Manufacturers performance data
τ_{l} = 2.0 lb/ft^{2} (Shear on lining at 0.5 in soil loss)
Roughness rating:
Applied Shear, lb/ft^{2} n value 1.0 0.038 2.0 0.034 4.0 0.031
First, try the less expensive "Product A." The solution is accomplished using procedure given in Section 3.1 for a straight channel.
Step 1. Channel slope, shape, and discharge have been given.
Step 2. Try ECB Product A.
Step 3. Initial depth is estimated at 1.0 ft
From the geometric relationship of a trapezoid (see Appendix B):
R = A/P = (6.00)/(9.32) = 0.644 ft
Step 4. To estimate n, the applied shear stress on the lining is given by Equation 2.3
Determine a Manning's n value from Equation 5.1 with support from Equations 5.2 and 5.3.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5. Since this value is more than 5 percent different from the design flow, we need to go back to step 3 to estimate a new flow depth.
Step 3 (2^{nd} iteration). Estimate a new depth solving Equation 2.2 or other appropriate method iteratively to find the next estimate for depth:
d = 0.57 ft
Revise hydraulic radius.
R = A/P = (2.68)/(6.60) = 0.406 ft
Step 4 (2^{nd} iteration). To estimate n, the applied shear stress on the lining is given by Equation 2.3
Determine a Manning's n value from Equation 5.1. The exponent, b, and the coefficient, a, are unchanged from the earlier calculation.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5 (2^{nd} iteration). Since this value is within 5 percent of the design flow, we can proceed to step 6.
Step 6. The maximum shear on the channel bottom is:
Determine the permissible soil shear stress from Equation 4.6 and Table 4.6.
Equation 5.5 gives the permissible shear on the RECP.
Safety factor for this channel is selected to be equal to 1.0.
Step 7. Product A (ECB lining) is not acceptable since the maximum shear on the RECP surface is greater than the permissible shear of the RECP.
Now try the alternative "Product B." The flow and channel configuration as well as the permissible shear stress are the same. Also, it is reasonable to assume an initial depth equal to the last depth we calculated for Product A. Therefore, using the area and hydraulic radius from that calculation, we can start with Step 4.
Step 4. To estimate n, the applied shear stress on the lining is given by Equation 2.3
Determine a Manning's n value from Equation 5.1 with support from Equations 5.2 and 5.3.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5. Since this value is more than 5 percent different from the design flow, we need to go back to step 3 to estimate a new flow depth.
Step 3 (2^{nd} iteration). Estimate a new depth solving Equation 2.2 or other appropriate method iteratively to find the next estimate for depth:
d = 0.59 ft
Revise hydraulic radius.
R = A/P = (2.81)/(6.73) = 0.418 ft
Step 4 (2^{nd} iteration). To estimate n, the applied shear stress on the lining is given by Equation 2.3
Determine a Manning's n value from Equation 5.1. The exponent, b, and the coefficient, a, are unchanged from the earlier calculation.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5 (2^{nd} iteration). Since this value is within 5 percent of the design flow, we can proceed to step 6.
Step 6. The maximum shear on the lining of the channel bottom is.
Equation 5.5 gives the permissible shear on the RECP.
Step 7. Product B (ECB lining) is an acceptable temporary lining since the maximum shear on the RECP is less than the permissible shear of the RECP. Choose Product B. (Remember the permanent vegetative lining must also be evaluated.)
Turf reinforcement integrates soil, lining material and grass/stems roots within a single matrix (Santha and Santha, 1995). Since turf reinforcement is a longterm solution, the lining consists of nondegradable materials. Turf Reinforcement Mats (TRMs), a subset of RECPs, are integrated with soil, and subsequently vegetation, by either covering the mat with soil or through surface application (no soil filling) allowing the vegetation to grow up through the TRM.
In the initial unvegetated state, the linings respond according to Equation 5.4. However, stability of the TRMs is achieved through proper installation per the manufacturer's recommended methods and use of proper length and quantity of fasteners.
As grass roots/stems develop within or through the TRM matrix, the lining becomes more integrated with the vegetation and soil. In the case of TRM linings, the plant roots/stems bind the mat, which prevents the detachment of the mat from the soil surface  significantly reducing the formation of under currents between the mat and soil. Grass growth further deflects turbulence away from the soil surface, establishing a positive relationship between lining and grass growth.
Where lining material is placed on top of the seed bed, the plant stem will grow through the lining (Lancaster, 1996). In this type of placement, the lining material offers more initial protection for the seed bed and provides stem reinforcement of the vegetation. However, the grass stem may be less effective at securing the lining to the soil surface than the plant roots, permitting the lining to be displaced by hydraulic forces. Specific system performance is determined by the interaction of the vegetation and the soilfilled or surface applied TRM.
When lining material remains in place for the longterm, roadside maintenance activities need to be considered, particularly mowing. Use of proper installation methods, including a sufficient quantity and size of fasteners is necessary to prevent potential problems with mowing and other vegetation maintenance equipment. Additionally, proper maintenance techniques are required to avoid damage to the installation and ensure the integrity of the system over time.
Unlike other RECPs there is no broadly accepted protocol for the testing of TRMs. This places an additional burden on the designer of a TRM lining to review and understand how each manufacturer has tested its products. The following checklist (Table 5.5) is based on ASTM D 6460 with the addition of requirements for TRM testing (Lipscomb, et al., 2003). It can be used as minimum standard to evaluate manufacturer's testing protocols. Products that are based on a testing protocol for TRMs that do not meet these minimum elements should only be used cautiously.
Test data consisting of the following is recommended:
Items (1) and (2) will vary depending on the soil and vegetation used in the testing. However, this is mitigated by their use in a relative, not absolute, manner as will be described in the next section. In addition, the definition of instability provided in Table 5.5 is qualitative and may be expected to vary from researcher to researcher. As long as a researcher maintains consistency within a set of tests for a particular TRM, the results should be acceptable.
A TRM modifies the cover factor for vegetated linings (Equation 4.3). The adjusted cover factor is determined by the following equation.
(5.6) 
where,
τ_{p,VEGtest}  = permissible shear stress on the vegetative lining, N/m^{2} (lb/ft^{2}) as reported by Manufacturer's testing 
τ_{p},_{TRMtest}  = permissible shear stress on the turfreinforced vegetative lining, N/m^{2} (lb/ft^{2}) as reported by manufacturer's testing 
C_{f,VEG }  = grass cover factor (see Table 4.5) 
C_{f,TRM }  = TRM cover factor 
If the manufacturer notes that the TRM affects plant cover density, this information should be used in the selection of C_{f,veg} from Table 4.5.
Element Protocol  Description  Check 

Test Channel Preparation  In accordance with ASTM D 6460, except that soil type may vary.


Calibration  In accordance with ASTM D 6460.  
PreTest Documentation  In accordance with ASTM D 6460. In addition, the vegetation type and density should be determined.


Test Setup  In accordance with ASTM D 6460. In addition:


Test Operation and Data Collection  In accordance with ASTM D 6460, except that the test should not be conducted to catastrophic failure.

Evaluate the following proposed lining design for a vegetated channel reinforced with turf reinforcement mat (TRM). The TRM will be placed into the soil and secured to channel boundary following manufacturer's recommendations. The permissible shear stress values for the TRM were developed from testing that meets the minimum requirements of Table 5.5.
Given:
Shape: Trapezoidal, B = 0.6 m, Z = 3
Soil: Silty sand (SM classification), PI = 17, e= 0.6
Grass: Sod, good condition, h = 0.150 m
Grade: 10.0 percent
Flow: 0.25 m^{3}/s
TRM Product Information from manufacturer:
τ_{p,TRMtest}= 550 N/m^{3}
τ_{p,VEGtest}= 425 N/m^{3}
Effect on plant density is negligible.
First, we will check to see if the channel is stable with a grass lining alone.
The solution is accomplished using procedure given in Section 3.1 for a straight channel.
Step 1. Channel slope, shape, and discharge have been given.
Step 2. A vegetative lining on silty sand soil will be evaluated
Step 3. Initial depth is estimated at 0.30 m
From the geometric relationship of a trapezoid (see Appendix B):
R = A/P = (0.540)/(2.80) = 0.180 m
Step 4. Estimate the Manning's n value appropriate for the lining type from Equation 4.2, first calculating the mean boundary shear.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5. Since this value is more than 5 percent different from the design flow, we need to go back to step 3 to estimate a new flow depth.
Step 3 (2^{nd} iteration). Estimate a new depth solving Equation 2.2 or other appropriate method iteratively to find the next estimate for depth:
d = 0.13 m
Revise the hydraulic radius
R = A/P = (0.129)/(1.42) = 0.091 m
Step 4 (2^{nd} iteration). Estimate the Manning's n value appropriate for the lining type from Equation 4.2, first calculating the mean boundary shear.
Determine a Manning's n value from Equation 4.2. From Table 4.3, C_{n} = 0.205
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5 (2^{nd} iteration). Since this value is within 5 percent of the design flow, we can proceed to step 6.
Step 6. The maximum shear on the channel bottom is.
Determine the permissible soil shear stress from Equation 4.6.
Equation 4.7 gives the permissible shear stress on the vegetation. The value of C_{f} is found in Table 4.5.
The safety factor for this channel is taken as 1.0.
Step 7. The grass lining is not acceptable since the maximum shear on the vegetation, 128 N/m^{2} is more than the permissible shear of grass lining, 107 N/m^{2}.
Now try the same grass lining with turf reinforcement. The flow and channel configuration are the same. Therefore, we begin at step 6.
Step 6. The maximum shear on the channel bottom and the permissible soil shear are the same as in the previous iteration. A new cover factor is computed based on the TRM properties. Equation 4.7 gives the permissible shear stress on the vegetation. The value of C_{f} is computed using Equation 5.6
The safety factor for this channel is taken as 1.0.
Step 7. The turf reinforced grass lining is acceptable since the maximum shear on the vegetation, 128 N/m^{2} is less than the permissible shear of the reinforced grass lining, 138 N/m^{2}.
Evaluate the following proposed lining design for a vegetated channel reinforced with turf reinforcement mat (TRM). The TRM will be placed into the soil and secured to channel boundary following manufacturer's recommendations. The permissible shear stress values for the TRM were developed from testing that meets the minimum requirements of Table 5.5.
Given:
Shape: Trapezoidal, B = 2.0 ft, Z = 3
Soil: Silty sand (SM classification), PI = 16, e= 0.6
Grass: Sod, good condition, h = 0.5 ft
Grade: 10.0 percent
Flow: 8.8 ft^{3}/s
TRM Product Information from Manufacturer:
τ_{p,TRMtest}= 11.5 lb/ft^{3}
τ_{p,VEGtest}= 8.9 lb/ft^{3}
Effect on plant density is negligible.
First, we will check to see if the channel is stable with a grass lining alone.
The solution is accomplished using procedure given in Section 3.1 for a straight channel.
Step 1. Channel slope, shape, and discharge have been given.
Step 2. A vegetative lining on silty sand soil will be evaluated
Step 3. Initial depth is estimated at 1.0 ft
From the geometric relationship of a trapezoid (see Appendix B):
R = A/P = (5.00)/(8.32) = 0.601 ft
Step 4. Estimate the Manning's n value appropriate for the lining type from Equation 4.2, first calculating the mean boundary shear.
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5. Since this value is more than 5 percent different from the design flow, we need to go back to step 3 to estimate a new flow depth.
Step 3 (2^{nd} iteration). Estimate a new depth solving Equation 2.2 or other appropriate method iteratively to find the next estimate for depth:
d = 0.43 ft
Revise the hydraulic radius
R = A/P = (1.41)/(4.72) = 0.299 ft
Step 4 (2^{nd} iteration). To estimate n, the applied shear stress on the grass lining is given by Equation 2.3
The discharge is calculated using Manning's equation (Equation 2.1):
Step 5 (2^{nd} iteration). Since this value is within 5 percent of the design flow, we can proceed to step 6.
Step 6. The maximum shear on the channel bottom is.
Determine the permissible soil shear stress from Equation 4.6.
Equation 4.7 gives the permissible shear stress on the vegetation. The value of C_{f} is found in Table 4.5.
The safety factor for this channel is taken as 1.0.
Step 7. The grass lining is not acceptable since the maximum shear on the vegetation, 2.7 lb/ft^{2} is more than the permissible shear of grass lining, 2.2 lb/ft^{2}.
Now try the same grass lining with turf reinforcement. The flow and channel configuration are the same. Therefore, we begin at step 6.
Step 6. The maximum shear on the channel bottom and the permissible soil shear are the same as in the previous iteration. A new cover factor is computed based on the TRM properties. Equation 4.7 gives the permissible shear stress on the vegetation. The value of C_{f} is found in Table 4.5.
The safety factor for this channel is taken as 1.0.
Step 7. The turf reinforced grass lining is acceptable since the maximum shear on the vegetation, 2.7 lb/ft^{2} is less than the permissible shear of the reinforced grass lining, 2.9 lb/ft^{2}.