Erosion and Sediment Pollution Control Program Manual.pdf

Design Considerations
Flow transition mats should be installed on a smooth, level, side-to-side surface to avoid water
concentration. Where necessary, the surface should be graded to remove any unevenness.
Transition mats should not be used where a change in slope greater than 25% will be
encountered.
Wherever a thick sod is not present, a TRM underlayment should be installed. For slopes
greater than 10H:1V, an HPTRM should be used. The TRMs should be capable of withstanding
velocities of at least 5.5 fps.
The finish elevation of the top of the mats should be at or slightly below — 3” maximum — the
pipe outlet.
The width of the mats should be equal to the downstream channel. Where discharging directly
into a surface water, the design should allow for as much expansion of flow width as possible to
reduce velocities and promote infiltration.
The downstream slope should be as flat as possible to avoid increasing the velocity.
The length of the mat should be determined from Figure 9.6. A double layer of mats, with the
open area of the second layer offset from those of the first layer, should be used where any of
the following conditions apply:
o The installation is located on erodible soils
o The installation is located in a special protection watershed
o The anticipated discharge velocity exceeds 7 fps
o The slope on which the mat is placed is 10H:1V or steeper.
Anchor transition mats to achieve consistent contact with the underlying surface and prevent
seepage underneath the mat.
Locate the outlet and transition mat where adequate sunlight is available to promote healthy
vegetation.
Failure of a transition mat has occurred when a rill forms under the mat. Wherever that occurs,
the mat should be removed, the rill graded, and a higher form of protection provided.
FIGURE 9.6
Minimum Coverage Length vs. Exit Velocity for Flow Transition Mat
Adapted from Robeson, Colorado State University
363-2134-008 / March 31, 2012 / Page 244