Design of Cascades in Steep Channels
Design of Cascades in Steep Channels
(OP)
What are the accepted procedures for designing cascades to improve a steep, natural ephemeral channel (flow only during storms) on a residential site? I am trying to improve a channel with a slope of 30-40% (100-year design flow of 28.5 cfs). Following the steep section, the channel becomes an earthen swale of 3-8% slope, then flows into an 18-in culvert at the street. I want to design cascades to dissipate energy during storms before the water flows into the earthen swale (which I will probably reinforce with matting), which is next to the residence.





RE: Design of Cascades in Steep Channels
RE: Design of Cascades in Steep Channels
That said, typical designs use material large enough to dissipate the energy, but graded so as to prevent excess percolation into underlying soils. You will likely need a filter layer or geotextile. Also be aware that with large bed materials in small channels the roughness factor is higher than those typically used for analysis of larger and less steep systems.
I have worked with these types of designs before, and the tasks generally consists of site and channel geometry analyses, hydraulic and channel shear stress/velocity analyses, and channel stability design. If fish passage is an issue the design is more complicated. Geotechnical assessment is highly recommended as well.
Here is a starting list of references (Google search as needed to find some of these):
HEC-RAS (software for analyzing hydraulics)
WinXS Pro (software to calibrate roughness to streambed material)
EM 1110-2-1601 Hydraulic Design of Flood Control Channels (guidance on riprap design)
ERDC/CHL TR-01-28 Hydraulic Design of Stream Restoration Projects (covers all aspects)
http://www.wa.gov/wdfw/hab/ahg/ispgdoc.htm (check out appendices)
and http://www.usda.gov/stream_restoration/ (excellent resource result of joint federal agency effort)
Good luck!
BLTseattle
RE: Design of Cascades in Steep Channels
Check out the Denver Urban Flood control District. They have extensive information on grade control, drops, checks and riprap design.
http://www.udfcd.org/index.html
RE: Design of Cascades in Steep Channels
Three drops of 2.5 to 3.5 feet each would do it, with 7 foot max spacing between the drops based on the site conditions. Standard broad crested weir calcs indicate a 6 foot width would work with critical depth of about 1 foot over the weir. I have found some references for nappe length calculation, which indicates the 7 foot spacing is a little tight. Standard design practice indicates the use of 30-in or bigger stone for construction.
I read about the use of timber crib weirs in streams in Austrailia. Has anyone had experience with these? how about segmental walls in these conditions
RE: Design of Cascades in Steep Channels
RE: Design of Cascades in Steep Channels
RE: Design of Cascades in Steep Channels
RE: Design of Cascades in Steep Channels