Design storm for erosion control
Design storm for erosion control
(OP)
What is a typical design storm event for design of erosion control, specifically for turf-reinforcement blanket in a roadside ditch? Obviously the conservative design would include a design to accommodate a 100-year storm event, but this seems like overkill to me. The municipality I'm dealing with has no set standards, so I was hoping to determine what was typical - I was hoping that a 10-year storm event would be acceptable.
Based on my prelim calcs, I can use a (fairly) standard TRM in the ditch and the factor of safety is acceptable for a 10-year storm event. However the 100-year storm event will require the installation of grouted rip-rap along the entire roadside ditch (which is definitely much more expensive and therefore much less desirable).
Any thoughts would be appreciated.
Based on my prelim calcs, I can use a (fairly) standard TRM in the ditch and the factor of safety is acceptable for a 10-year storm event. However the 100-year storm event will require the installation of grouted rip-rap along the entire roadside ditch (which is definitely much more expensive and therefore much less desirable).
Any thoughts would be appreciated.





RE: Design storm for erosion control
RE: Design storm for erosion control
The high end TRM with full vegatative cover have allowable shear stress of 14.0lbs/ft^2. Granted that is the manufacture's test data, but the installations I've seen haven't expeceinced any significant erosion.
RE: Design storm for erosion control
RE: Design storm for erosion control
I'm leaning towards using the 10-year design storm, it just seems to make the most sense... I know that water quality calculations are only generally required up to 10-year storm events (sometimes I've seen a 25-year storm event required). I believe the thought is that when you have larger storm events (> 10-year), runoff conveyance and flood control become the controling factors for drainage design, and water quality is a secondary thought. This train of thought when applied to erosion control in a roadside ditch would also seem to dictate that a 10-year storm event would be applicable.
Thank you for all your input.
RE: Design storm for erosion control
It depends on the flow rate. Is the 100-Year event </= 50cfs? If so FHWA has a roadside design manual (HEC-15, I Think) that you can use to evaluate a number of alternatives. There is even a steep slope design segment. Regarding the Erosion control mat, I have seen these fail many times. As a result I typically default to rock.
Good Luck.
RE: Design storm for erosion control
Thanks for the response. The 10-year max flow in ditch is about 5 cfs. 100-year is 12 cfs. Based on what you stated, I'm assuming the FHWA manual is only applicable if flows are >50 cfs?
RE: Design storm for erosion control
The product I was refering to is actually a North American Green Product. It was the VMAX P550 mat.
Have you tried directly calculating the shear stress?
Sheer Stress = Unit wight of water x depth x slope.
gbam is saying that the FHWA manual is applicable for flows LESS THAN 50 cfs. Its generally what I refer to for small low flow open channel linings.
RE: Design storm for erosion control