Gravel Road through woods stormwater help.

[RobLEED]

I need some ideas on how to design and provide stormwater peak rate backup calcs and mitigation for gravel roads as part of a windfarm project. The roads have a 30' wide total R/W consisting of a 15' gravel road and 15' vegetated filter strip (used for walking a huge track crane during turbine erection). The project is located in the middle of nowhere with hundreds/thousands of acres of undisturbed woods on all sides.

On other projects we have been able to successfully argue that the gravel and vegetated filter strips, combined with the 50+ acres of downhill woodlands would buffer and have minimal impact on the peak rate from the project.

My issues:
1. The review agency is asking for peak rate backup calcs for all storm events. We would like to minimize our disturbance footprint and not half to build a large network of swales and basins to mitigate. We were contemplating using retentive grading and/or small berms to pond water at varous locations along the road but are finding it difficult to model, site, and regulate the outflow from all of them. Most of the roads are along mountain ridges where there is very little uphill drainage.
2. The pre and post CN only changes in tenths, and most programs will not calculate a CN that is not a whole number.

I also thing that the gravel road base could store the runoff increase, but cant get my head around how to prove this in calculations.

Any suggestions on a proposition to the reviewing agency or design approaches would be greatly appreciated.

 

[psmart]

Without commenting on the significance of fractional CN values, HydroCAD can be configured for additional decimal places, if you wish to do so.

Just open the Setting|Units screen, click Custom, select the CN parameter, and increase the number of decimal places from 0 to 1.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Ryb01]

Hi RobLEED,

I've been involved in similar projects and the additional runoff produced from a gravel road is insignificant in the grand schem of things. However, you'd be surprised to see the extent of the erosion caused by such a small change in topography. This would be further magnified in your case since you are in a mountainous region.

I'm not sure that the road base would be a good option for storage as with the road grade and compaction from vehicular traffic would essentially render the gravel surface impermeable(Unless you could direct the runoff into an filtration system beneath the surface).

An option may be to consider shallow swales on either side of the road with check dams for attenuation.

Another option to consider, maybe the CN method is not the best method for modelling this particular project as the curve number only varies by tenths pre to post.

Hope this helps.

Ryb01

 

[tsgrue]

.

Your situation is commonly faced by forest engineers...

For analysis, I suggest checking out FSWEPP, the forestry focused version of WEPP: "

http://forest.moscowfsl.wsu.edu/fswepp/".

For various references on the subject, I suggest you start at the USDA Forest Service Rocky Mountain Research Station W3 site listing for such: "

http://forest.moscowfsl.wsu.edu/cgi-bin/engr/library/searchpub.pl?category=road erosion".

Oregon State University is probably the leading forest engineering program in the US and would have faculty/staff that could point you in the right direction: "

http://ferm.forestry.oregonstate.edu/".

I also suggest that you contact the local, state, and federal forest service personnel in your area as well as state cooperative extension service staff. They should both have staff to assist you in this at not cost - likely with graduate education and years of experience in the matter.

Ryb01 is very much correct about the erosion impact from such roads.

Hope that helps!

.



tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com