Lined Waterway or Outlet
GeneralLined waterways or outlets are channels lined with stone and provide for non-erosive conveyance of runoff or concentrated flow in areas where grassed waterways or unlined channels are inadequate or not permissible.
These practices are applicable on sites where vegetation cannot be established due to shading; where a lining is required to control erosion; where high velocities, steep grades, seepage, prolonged base flow, or wetness would cause erosion; or where use by people or animals precludes the use of vegetated channels. Lined waterways or outlets are designed as conveyance channels and do not enhance water quality or reduce peak flows. As a result, they are best used in conjunction with other management practices.
DesignLined waterways or outlets should be designed to convey the maximum designed outflow from the structure or structures that it serves. The maximum flow velocities for stone linings are determined from the median diameter of the stone (D50) placed in the channel. However, on slopes steeper than 10%, a slope adjustment factor must be used.
The channel should be designed with cross sections that are triangular, parabolic, or trapezoidal. Side slopes should be 2:1 or flatter with a minimum freeboard of 0.25 feet above the designed flow depth.
StoneLined waterways or outlets should consist of clean, angular stone that is resistant to weathering. The stone should be sized by using the median stone size (D50). Once the D50 has been selected, 50% of the stone, by weight, should be larger than the D50.
The riprap should be well graded, with the diameter of the largest stone not exceeding 1.5 times the D50 size. The depth of the stone should be at least twice the D50 size and may not reduce the designed cross section of the channel.
Maximum Velocities for Various D50 Sizes and Shapes
|Maximum Velocity (ft/sec)||D50 Cubical (in)||D50 Spherical (in)|
Slope Adjustment Factors for Allowable Velocity
|Slope (H:V)||Slope (ft/ft)||Adjustment Factor|
- Construction should be completed before any water is allowed through the outlet
- In applications where damage is possible to the outlet structure, the stone should be hand placed
Manning's "n" Values for Various Rock Sizes
|Diameter (in)||"n" Value|
- Lined waterways and outlets should be inspected after all storm events for displaced stones – all necessary repairs should be made immediately
- Accumulated sediments should be removed periodically