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Lined Waterway or Outlet

General

Lined 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.

Design

Lined 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.

Stone

Lined 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.

However, the diameter of the largest stone should not exceed 1.5 times the D50 size. The remaining portion of the stone should be well graded with a sufficient amount of smaller stones to fill the voids between the larger stones.

The depth of the stone should be equal to the maximum stone size plus the thickness of any bedding or filter material. However, under no circumstances should the stone placed in the channel 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)
10.81214
9.91012
8.8810
7.668
6.346
5.024

Slope Adjustment Factors for Allowable Velocity

Slope (H:V)Slope (ft/ft)Adjustment Factor
3:1.330.80
4:10.250.85
5:10.200.89
6:10.170.91
7:10.140.92
8:10.130.93
9:10.110.94
10:10.101.00
12:10.081.00
15:10.071.00

Construction

  • 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
20.030
40.033
60.036
80.037
100.039
120.040

Maintenance

  • 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

Method to Determine Practice Efficiency

Lined waterways and outlets are designed as stormwater conveyance channels and provide little treatment ability. As a result, no efficiency is given for this practice.

References