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Design of Roadside Channels with Flexible Linings
Hydraulic Engineering Circular Number 15, Third Edition

Appendix E: Fall-Board Test for Grass Density-Stiffness Parameter, Cs

The fall-board test is a simple way of estimating the combined effect of grass density and stiffness. The test should be conducted at random locations within an area with a homogeneous grass species mix and grass height. A standard board size and length is required. As shown in Figure E.1, the board is stood on end and allowed to freely rotate to the grass cover. When the board hits the grass, it slides length wise in the direction of rotation, imparting a friction force, which along with the weight of the board, deflects the grass in a manner that is similar to flowing water. The test should be repeated a sufficient number of times within an area to produce a reliable average value for the fall height, hb.

schematic showing the board in its original position and after the fall
Figure E.1. Schematic of the Fall-board Test (after Kouwen, 1988)

Materials:

Wooden board = 455 kg/m3 (28.5 lb/ft3) (i.e., ponderosa pine, redwood, spruce)

Board dimensions = 1.829 m x 0.305 m (6 ft x 1 ft)

Board weight = 4.85 kg (10.7 lb)

Test Procedure:

  1. Select an area with homogeneous mix of grass species and grass height.
  2. Randomly select locations within this area to conduct the fall-board test.
  3. At each location record the height of grass stems, h.
  4. Stand the board vertically on one end and freely allow it to fall over. As the board falls, it will rotate about the end in contact with the ground and slide forward.
  5. Record fall height, hb, as the distance between the ground and the bottom edge of the fallen end of the board.
  6. Repeat the test at the other locations.

Calculations:

  1. Compute the average of the grass height measurements and the fall height measurements.
  2. Compute the grass density-stiffness coefficient using the following equation:
C sub s equals alpha times (h sub b divided by h) raised to the 2.82 power (E.1)

where,

Cs= density-stiffness coefficient
α= unit conversion constant 3120 (SI), 265 (CU)
Design Example (SI)

A 100 m length of roadside ditch has a mature grass lining that was planted from a standard grass seed mix that is considered to be typical of that region of the state. Six (6) fall-board tests were conducted at random intervals along the ditch. Results of these tests are summarized in the following table.

Location (station) 0+018 0+025 0+037 0+053 0+075 0+093 Mean
Grass height, h (m) 0.159 0.158 0.153 0.154 0.143 0.147 0.152
Fall-board height, hb (m) 0.043 0.034 0.049 0.037 0.030 0.036 0.038

C sub s equals alpha times (h sub b divided by h) to the 2.82 power equals 3120 times (0.038 divided by 0.147) to the 2.82 power equals 69

Design Example (CU)

A 300 ft length of roadside ditch has a mature grass lining that was planted from a standard grass seed mix that is considered to be typical of that region of the state. Six (6) fall-board tests were conducted at random intervals along the ditch. Results of these tests are summarized in the following table.

Location (station) 0+48 0+75 1+11 1+59 2+25 2+79 Mean
Grass height, h (ft) 0.52 0.52 0.50 0.51 0.47 0.48 0.50
Fall-board height, hb (ft) 0.14 0.11 0.16 0.12 0.10 0.12 0.13

C sub s equals alpha times (h sub b divided by h) to the 2.82 power equals 265 times (0.12 divided by 0.48) to the 2.82 power equals 5.3

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Dan Ghere
Resource Center (Matteson)
708-283-3557
dan.ghere@dot.gov

Updated: 04/07/2011
 

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