Design storm for erosion control

[civilman72]

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.

 

[cvg]

most roadways are designed for 10-year storm for pavement drainage and 25-year for culverts. For large cross drainage usually 50-year. In cities, the criteria for adjacent urban development is adopted which is 100-year. who is going to maintain the TRM and is repair or replacement on a more frequent basis ok with them?

 

[bpattengale]

Grouted rip-rap in a roadside ditch? That seems a little overkill. What kind of TRM are you using and what is the shear stress in your channel?

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.

 

[amb2002]

For the municipalities I've worked in, a 10 year event is typical for erosion control design (except, of course, any type of basin, which will typically look at up to the 100 year event), regardless of the design event required for the roadway/ditch/culvert/etc.

 

[civilman72]

This is a proposed road in very steep terrain - the road and adjacent ditch grade is 12%, which obviously causes high velocities. When the flows are low (i.e 10-year storm), there are TRM products that can be used. But when the flows in the ditch increase to a 100-year storm event, the program I'm using states I need grouted riprap. While I agree it seems like overkill, based on the very steep grades of the ditch, I can see how this is feasible. I've been using the North American Green ECMDS 4.3 for my calculations, but none of their products appear to have allowable shear stress anywhere near 14 psf. Any idea what TRM products have this higher allowable shear stress?

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.

 

[gbam]

civilman72;
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.

 

[civilman72]

gbam,

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?

 

[bpattengale]

civilman72,
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.

 

[gbam]

Just the opposite, flows should be less than 50-cfs.