channel design
channel design
(OP)
I have to design a channel that intercepts water on a back lot line and moves it to a trap. The slope is 17% and will carry about 10 cfs. I havent run any velocity equations yet. Any thoughts on what type of channel (trapezoidal, triangulary) I should use and what type of liner I should use. also basic dimensions. Thanks.





RE: channel design
thanks
RE: channel design
Good luck
RE: channel design
http://www.waterengr.com/riprap/riprap.html
Russ
RE: channel design
RE: channel design
A: FHWA HEC-15 available from http://www.fhwa.dot.gov////////bridge/hydpub.htm
or B: Hydraulic Design of Flood Control Channels by USACE, Publication EM 1110-2-1601
available from www.usace.army.mil/usace-docs/
or C: State of Washington Department of Transportation Hydraulics Manual
Things to keep in mind:
1. Manning's roughness is a function of the bed material and flow depth. For large diameter material, n is typically higher than the "standard" values used for rock channels; n is likely higher than 0.035. WinXS-Pro is useful software to analyze the depth vs. n relationship. If you pick a Manning's roughness from a table, understand what it is based on.
A review of the classic Chow book on Hydraulics and the photos that correspond to the published n-values reveals that typical n values were calibrated for medium to large streams and rivers; I do not recall that any were from small streams as small as 10 cfs, such as yours.
Using the higher n-value will lead to deeper flow, which will increase the channel shear values computed using the standard HEC-15 method, which relates to the size of riprap.
2. You will have to design the filter layers beneath the riprap or else underlying fine grain soil material could tend to erode, leading to channel instability. You can use a geotextile or layers of progressively finer grained materials. The only reference I can find at the moment for filter layers is in EM 1110-2-1901 Appendix D, see the link above.
3. Consider keying in some "rock bands" so that if some displacement occurs, there is deeper section of big rock to stop or slow down migration of any "nick points" that could emerge.
Good luck,
BLT
RE: channel design
Check out appendix C from the following
http://dnr.metrokc.gov/wlr/biostabl/
BLT
RE: channel design
RE: channel design
You will need some erosion protection between the gabions to protect against erosion from the falling water, such as grouted stone, concrete, more gabions, etc.
If you are set on rip-rap, ACE publication mentioned is good...Also Bureau of Reclamation's Design of Energy Dissipators and Stilling Basins has a graph of velocity vs. rip rap diameter. Watch out though---these graphs/equations ususally specify size for rock on the bottom; rock on a 2:1 side slope will need to be larger due to gravity not being normal to the surface (and may not be stable at any size for supercritical flow)
matsalleh
RE: channel design
RE: channel design
RE: channel design
How long is the channel? If it is not very long, why not shotcrete it?
The depth of the channel for 10 cfs would be minimal. Also FHWA Hydraulic Eng Circ. No. 15 has a section in the appendix to size riprap on steep gradients. I have used the procedure to size ditch transitions for large highway interchanges. None have failed to date. The lengths were short (100-200 ft)but very steep (25%-35% slopes).
RE: channel design
RE: channel design
Municipal ordinances in PA tend to prohibit planting anything within any stormwater easement, and even if allowed, your NPDES O&M agreement will have to address perpetual maintenance of these trees (trimming, removing junk from around trucks, eventually removing (replacing?) trees).
RE: channel design
Many of the above posts have given you suggestions and help in developing a 'traditional' approach to intercepting and moving the stormwater. You may also want to consider more of an 'innovative' approach that can combine some of the above approaches and augment them with what is generally now referred to as 'low impact development' or LID approaches (see: www.lowimpactdevelopment.org).
I'm assuming that the stormwater is the only source of water, and that the 'channel' you speak of is not an existing stream that is intercepting the added runoff from the now-increased impermeable areas. If so, you could consider reducing the amount of stormwater generated by: 1) using interception techniques such as rain barrels and storm infiltration drains, 2) integrating 'rain gardens' or grassy swales into the site design to help infiltrate stormwater on-site (I'm not sure whether the 17% grade was of the channel you're designing, or is the slope from the back of the house to the property line). You could integrate the yard drain approach mentioned above by lha into the rain garden. In this way you would reduce the post-development runoff to pre-development rates (hopefully) so in essence, nothing has to be done at the property line (see the Prince George's Co. MD publications on the EPA's http://www.epa.gov/owow/nps/lid/ web site).
If there is an existing stream at the property line I have to favor the step-pool approach over the trapezoidal, rip-rap lined channel approach. You can meander the stream and put in step pools to reduce the energy and erosion potential. You can get some basic, academic information on natural step-pool systems at http://www.fsl.orst.edu/wpg/pubs/step-be... . The basic concept can be applied to smaller-scale systems. However, you may have some problems with the step-pool approach due to your relatively shallow veneer of sandy soils on top of clay.
All in all - I'd recommend reducing the runoff from the lot rather than simply developing and relying on an armored channel to convey increased runoff. I believe that approach will be better for the development, and better for the environment.