Demonstrate Off-Site Stability

[mweng]

In NJ Standards for Soil Erosion and Sediment Control, the reliability of the model for demonstrating off-site stability (non-erosive velocities for point discharges) is limited to 10 cfs for the 25-year storm. Does anyone know the basis for this limitation?

We have a project where our discharge is 17 cfs for the 25-year storm and have no means for splitting into two discharge points. We meet every other standard for demonstrating off-site stability.

 

[LHA]

You mention a limiting Q, but the goal parameter is stated to be velocity. Q by itself does not lead to erosion, v and shear do.

Spread out the discharge via level spreader, and show permissible v and shear for the liner type.

Discuss this with the reviewer first; nothing works until it clears the reviewer.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[mweng]

Steve,
That was the point we made. The Standards, however, indicate that the model is unreliable if Q25 is greater than 10 cfs. Additionally, "level spreaders are not considered to be an acceptable design".

I am looking for what "model" predicates the standard that 10 cfs is the maximum Q25.

 

[LHA]

I have no idea. Here in neighboring PA, we have no such "standard". I've been doing E&S here for 5 years.

Why are level spreaders out? In PA, they are recommended in the E&S Manual. Go here, page 97:

http://164.156.71.80/VWRQ.asp?docid...aspx?fs=0442d740780d00008000084d0000084d&ft=1

Also, if post <= pre, you can't legally be prohibited from releasing runoff which currently goes there. So, unless you are changing watersheds (putting more water than before to that point), you have to be allowed to release any amount...10 cfs, 17 cfs, 1000 cfs...the amount is not relevant. If you have added to the watershed, then you have to cut it back to pre.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[queque]

I agree with IHA.
You state:
The Standards, however, indicate that the model is unreliable if Q25 is greater than 10 cfs.

How was that determined and under what assumptions?

If the model is unreliable under those conditions, why were the results accepted?

 

[mweng]

queque,

That is MY question. I am questioning how the State Standards were developed and why they say the model is unreliable. We meet permissible velocities, etc. but our Q25 exceeds 10 cfs and therefore, they have stated that we don't meet off-site stability.

I wanted to know if anyone else in NJ has run into similar situation and has any info on the "model".

Thanks

 

[queque]

The answer lies with the agency that accepted the standards. They obviously did not develop the standards.
"They" have the background material ( technical meetings, documents, etc) that led to the approval. In most cases , many of these folks are long gone, but there may be someone who uses these models regularly within the agency.