channel design

[johnhan76]

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.

 

[johnhan76]

I have done a preliminary design. Trapezoidal channel. rip-rap sides @ 2:1. depth of 2'. I get a velocity of 9 fps. My design reference only pertains to froude numbers of 1.2 and less. The froude number on this channel is greater than 2. Does anyone have a design reference for this type of channel. I need something to size the rip-rap.

thanks

 

[RWF7437]

Very steep. High velocity. You might consider concrete lining for this channel if it is not too long. The Corps of Engineers is probably your best bet for sizing the riprap. I can send you a reference for their design manual if it would help.

Good luck

 

[RWF7437]

Here's some software that might help"

http://www.waterengr.com/riprap/riprap.html

Russ

 

[johnhan76]

ok i am thinking of buying the software - is it user friendly?

 

[bltseattle]

You need to consider both the velocity and channel shear stresses. The slope of your channel is steep, gabions may be an appropriate choice if there are not fish in the channel. I generally start with the following references:

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

http://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

 

[bltseattle]

Here is one more reference I found, fairly clear and concise and includes info for filter design.

Check out appendix C from the following

http://dnr.metrokc.gov/wlr/biostabl/

BLT

 

[cvg]

can you flatten the channel? Maybe construct some drop structures and design for subcritical flow? A supercritical, riprap lined channel is not an ideal design.

 

[matsalleh]

Drop structures are the way to go. Using large rip rap on a steep slope is sketchy...the equations may work out but you have to be concerned about a large stone breaking loose and the ensuing destruction. I am working on a similar situation, and have found gabions with 2 to 2.5 foot drops to be the best alternative, both from engineering and economic viewpoints. They are easy-to-construct gravity walls, although not the most aesthetically pleasing structure. Put mortar caps on each of the gabions after they have settled and filter fabric behind the gabions. I treat each drop as a broad crested weir. The lengths and water surface profile between the drops can be estimated using equations found in Chanson 1999...I can get you the full reference if you need it.

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

 

[johnhan76]

I revisited the site yesterday. Here is my situation: The channel is located on the back property line of a residential subdivision. The land slopes from the front of the houses to the back property line. The distance from the back of the houses to the back property line is 200 feet. There will not be any concentrated flow. The proposed channel is designed to prevent increased post development flows from entering the neighbors property. The site is very sandy (40 inches to clay). after reading the above replies I am beginning to rethink the channel. Perhaps I would have less problems letting the flow go onto the neighbors property. Most of the water from the houses and driveway must go a long distance over sandy soil to reach the property line and I dont need this for detention. any insights would be most appreciated.

 

[johnhan76]

For those of you still following this thread: The county engineer told me I must channel the water and I can not let the water go to the next property. I am considering using cable concrete - does anyone have any experience using this product.

 

[gbam]

Johnhan,
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).

 

[dicksewerrat]

Cable concrete is a good erosion control item. Place the Cable Conc. on a good fabric and staple it down. Use the heaviest you can find. I have used a willow tree for additional stablility on the slopes. They root in a few weeks and grow very well even in colder climates. MNDOT uses the same willow for bank stabilization on hillside adjacent to roads. Find a willow that spreads from the roots. Plant it on the slopes of the channel.

 

[LHA]

Many great ideas here, but at this slope and small flow, I can't believe a deep, small diameter yard drain, small diameter pipe and some beefy outlet end treatment wouldn't be cheaper and less headache. Both with installation and maintenance. ADS and their subsidiary, Nyloplast have some very practical products.

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).

 

[RStanford]

johnhan76 -

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:

http://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-bed.pdf

. 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.