Design of Cascades in Steep Channels 1

[matsalleh]

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.

 

[cvg]

what are you trying to improve? I mean, this is a natural channel - does it overtop? Have you changed the discharge? Is it transporting sediment into your culvert? Is it meandering or is there significant scour or bank erosion occuring? Are you trying to move it or straighten it?

 

[bltseattle]

IMO this is not a simple or quick design endeavor, as there is high risk of the stream unraveling if the design and construction is not done correctly. There are courses available in this sort of thing and consultants who can assist you.

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

 

[cvg]

BLT is correct, messing with an existing stream is risky. And you need to ask a specific question that can be answered.

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

 

[matsalleh]

To answer the question of necessity...the flow from annual storms results in water that essentially runs into a dwelling, as the house was built 80 years ago where it would never be built today. The problem will be exacerbated by the really big storms. The owners sandbag each winter at the onset of Pacific storms, guiding the water around the house to prevent damage. The idea is to create a series of drops to shorten the stream length upstream of the dwelling, which would allow a suitable channel to be built around the dwelling at a (relatively)moderate slope of 4%.

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

 

[cvg]

I have done similar designs (albeit, much larger flows). Designs I have analyzed generally use either some form of concrete or soil cement, or gabions to form the structures. 30 inch boulders might work, just make sure that the foundation is treated properly for the rock. As stated before, filter fabric might work. Or you might prefer a graded granular filter layer. I would keep your drops to 2.5' or less for safety. Make sure you provide an adequate toe-down to prevent scour from undermining the drops and causing headcutting. Also, protect the banks so that flow over the drops doesn't cause lateral migration of the channel.

 

[matsalleh]

cvg--thanks for your help. One last question, have you used gabions for this app?

 

[cvg]

yes, gabions work well. The gabions are more stable than dumped riprap and you can use much smaller size rocks.

 

[johnhan76]

Have you considered cable concrete?