3.1 Summary Letter
May 16, 2017
Summary
We are proposing an 18 unit small lot subdivision in the Riley Purgatory Bluff
Creek Watershed district and the City of Chanhassen. Our primary BMP for this
project is Storm Water Harvest and Reuse for Irrigation.
We have hired Water In Motion to do the detailed Storm Water Harvest and
Reuse Plan. Their plan is attached at the back of the Storm Water Report.
Total Suspended Solids and Phosphorous Removal
We modeled our storm water system using the MIDS program written for the
MPCA by Barr Engineering. Please see the calculations in the Storm Water Quality
and Quantity Management plan prepared by Sathre Bergquist, Inc. I have
tabulated our results below.
lbs/yr TSS TP TSS%
Removal
Ph%
Removal
Existing Load 806 2.44
Proposed Load 75 0.22 91% 91%
Rate Control
We are using the storm water pond to harvest the rainfall and use it irrigation on
site. The irrigation system will draw the pond down to an elevation of 890. As the
storm water reaches the pond, the irrigation will be drawing the pond down. We
were able to reach the desired post development rate for the 2 year, 10 year, and
100 year runoff. See Below:
2-yr (cfs) 10-yr
(cfs) 100-yr (cfs) Snow Melt
(cfs)
Pre-Development Rate 4.5 11.77 29.38 0.8
Post-Development Rate 3.0 11.65 27.76 0.8
SS AATTHH RREE --BB EERRGG QQ UUII SS TT,, II NN CC ..
11 55 00 SS OO UU TT HH BB RR OO AA DD WW AA YY,, WW AA YY ZZ AA TT AA ,, MM II NN NN EE SS OO TT AA ,, 55 55 33 99 11
TEL:(952)476-6000 FAX:(952)476-0104 WEB :WWW.SATHRE.COM
– 2 – May 16, 2017
We did not model the Snow melt because it will not be able to be reached by
using Storm Water Reuse as our primary Best Management Practice. We feel that
the benefits of a Storm Water Harvest BMP far outweigh the negatives of not
meeting the Snow Melt Storm Water Event.
Rate Control
We have calculated the amount of storm water abstraction required for 1.1” of
rainfall over 6,7950 sf of hardcover to be 6,189 cf of storm water. According to
Water In Motion’s calculations, we can disperse 9,942 cf (74,380 gallons) on our
site within 48 hours. See WIM calculations.
This is just a brief summary of our calculations. Please contact me if you have any
additional questions or comments.
Thanks,
Daniel Schmidt, PE