Hej,
We got feedback from our design submittal with the following technical advice:
“Rainfall intensity is normally dependent on the time of concentration. Confirm that time of concentration is approximately the same for both pre- and post-development conditions to document that the rainfall intensity does not change.”
As I interpret the feedback they want us to show if the rainfall intensity changes pre and post development? How do we do that? The area has pretty much the same ground conditions post development, just more buildings with roof instead of asphalted areas. I do not believe that it will have an impact on the weather.
Or have I interpreted it wrong?
Note our project is located in Sweden, so we cannot use existing American data.
Thanks in advance ! /Veronika
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
May 17, 2013 - 12:10 pm
Tc can vary wildly between pre and post. It can be 5 hours in pre and 5 minutes in post. Your stormwater management practice is what compensates for this.
Your rainfall intensity should not vary. You need to use the same depth in both pre and post models.
Your stormwater model and calculations should be prepared by a civil engineer or stormwater professional.
Veronika Sundberg
Environmental Engineer - CertificationSkanska Sverige AB
120 thumbs up
May 20, 2013 - 3:59 am
Okej, this is confusing to me, because in my opinion we have shown the conditions and made calculations pre and post according with the requirements. But maybe we need to clarify something. The professional level is not an issue on our project.
Thanks for your quick respond! /Veronika
Michele Riberdy
Senior Project ManagerVidaris, Inc.
5 thumbs up
May 20, 2013 - 1:25 pm
The amount of rain that falls on the site per calculated storm event does not change . What changes is the amount of run-off. The credit requires that you show a reduction in stormwater running off of impervious surfaces and leaving the project site. If your impervious surfaces are the same in your pre and post development cases then you will need to demonstrate how the project retains/infiltrates the water on site.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
May 21, 2013 - 8:44 am
Veronika, is that the entire comment?
Are the ground cover areas the same in the pre and post?
If areas were the same, I think I see where the reviewer was going with the Tc comment....the Tc for the post would need to be slower than the pre in order to generate a lesser peak-rate runoff, but it is just a really cumbersome way of stating this.
Did you submit Tc calculations?
Veronika Sundberg
Environmental Engineer - CertificationSkanska Sverige AB
120 thumbs up
May 24, 2013 - 6:28 am
Thanks for both of your replies.
We have submitted calculations called "Stormwater quantity after infiltration measures" where we calculate the runoff volumes based on rain intensity, areas and their ground conditions calculating the runoff. What we believe would be the same as a Tc calculation???
The same template for calculating the runoff has been approved on other projects.
What sprung from the results is added delaying basins for the runoff area to shoe stormwater reduction from site. These are not documented properly I believe now when revising, but the comment is still confusing.
We will go through and clarify the material and see if it helps. /Veronika
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
May 24, 2013 - 7:21 am
Are you submitting only the volume part? You need to reduce the rate as well (how fast the stormwater runs off).
Your Tc should be calculated as follows:
ftp://ftp-fc.sc.egov.usda.gov/FL/flengforms/fleng21B.pdf
Let us know how this all works out for you.