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 Austin
 Drainage Criteria Manual
 Section 6. OPEN CHANNELS
 Appendix 6.4.0. DESIGN REQUIREMENTS

§ 6.4.1. Grass-Lined Channels and Waterways  

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  • Key parameters in grass-lined channel or waterway design include permissible velocity, roughness coefficient, side slope, curvature, bottom width, and freeboard. The grass species selected shall conform to the City of Austin Standards and Standard Specifications and the Environmental Criteria Manual, as appropriate, and must be suitable for permanent application based upon the anticipated operation and maintenance of the channel or waterway.

    A.

    Velocity. The maximum permissible velocity for the one hundred (100) year storm is six (6) feet per second and includes all transitions to or from channels and waterways with similar or different materials. In all cases, the velocity for the one hundred (100) year storm must be non-erosive. The minimum permissible velocity for the two (2) year storm is two (2) feet per second.

    B.

    Roughness Coefficient. The roughness coefficients selected shall be based on the degree of retardance of vegetation. Table 6-2 provides minimum Manning's Coefficients for channel design. The roughness coefficient shall be adjusted to reflect the relationship between the depth of flow and the typical height of the design vegetation, especially for shallow depths of flow, as well as other factors affecting channel conveyance.

    C.

    Slope. The flow line slope of the channel shall be a minimum of two (2) percent unless the velocity for the two (2) year storm flow is greater than two (2) feet per second, in which case the channel slope may be a minimum of one (1) percent. Compliance with this requirement must take into account the variation in channel flow due to distributed inflows to the channel. A reinforced concrete pilot channel must be used if the channel slope is less than one (1) percent. The pilot channel must be at least four (4) feet wide, two (2) inches deep, and be capable of withstanding vehicular loading. Any grass-lined portion of the channel bottom must have a slope of at least two (2) percent from that portion to the concrete-lined pilot channel. However, no open channel flow line slope may be less than one-half (0.5) percent.

    D.

    Side Slopes. Side slopes shall be three (3) to one (1) or flatter.

    E.

    Curvature. The center line curvature shall have a minimum radius of twice the top width of the one hundred (100) year storm flow.

    F.

    Bottom Width. The minimum flat bottom width of the channel is three (3) feet.

    G.

    Freeboard. All grass-lined channels shall be designed to convey the one hundred (100) year storm event. The freeboard for the channel shall be the velocity head for the one hundred (100) year storm.