4.1 Summary SpreadsheetProject: Arbor Glen
Location: Chanhassen MN
Prepared for: Chanhassen
Prepared by: Sathre-Bergquist, Inc.
Date: June 4, 2015
Revised: September 13, 2016
Revised: 05/16/2017
Stormwater Management Rules
3.1a Rate Control:
3.1b Volume Control:
3.1c Phosphorus and TSS Control
3.5 Buffer Credit
3.6 Low-floor elevation
ABSTRACTION REQUIRED:
Required Infiltration - 1.1 inch = 0.092 ft
Hardcover -67,518 sf
Abstraction Volume Required (AV) = 67,518 sf x 0.092 ft = 6,189 cf
0.14 Ac-Ft
ABSTRACTION ACHIEVED:
Water Harvesting
AV = 0.16 Ac-Ft
6,800.00 cf within 48 hours
9,942.00 cf within 48 hours from WIM Calcs
lbs/yr TSS TP TSS% Removal Ph% Removal
Existing Load 806 2.44
Proposed Load 75 0.22 91% 91%
Rate Control from the pond was achieved by using a wier wall in the outlet structure.
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
SUMMARY
Riley Purgatory Bluff Creek Watershed District
Limit peak runoff flow rates to that from existing conditions for the two-, 10- and 100-year frequency storm events using a nested 24- hour
rainfall distribution, and a 100-year frequency, 10-day snowmelt event, for all points where stormwater discharge leaves the site;
Provide for the abstraction onsite of 1.1 inches of runoff from impervious surface of the parcel; i Where infiltration or filtration facilities,
practices or systems are proposed, pretreatment of runoff must be provided. ii The bottom of infiltration facilities must be at least three feet
above the seasonal high water table.
No structure may be constructed or reconstructed such that its lowest floor elevation is less than 2 feet above the 100-year event flood
elevation and no stormwater management system may be constructed or reconstructed in a manner that brings the low floor elevation of an
adjacent structure into noncompliance with this standard.
Stormwater management capacity of buffer area created in compliance with Rule D or otherwise will be credited toward compliance with
the criteria in this rule.
Arbor Glen is a proposed 18 home development. Drainage will be focused rear yard swales, a stormwater pond, wetland buffers, and a final discharge into the nearby wetland.
Tables within this report show the total impact / treatment requirements due to Arbor Glen.
HydroCAD was used to model rate control for this project. The table below compares pre-development and post-development rates.
Volume Control
Volume Control requirements state that the stormwater management plan must provide for the abstraction of 1.1" of rainfall for the site's impervious surface.
Quality Control
Provide for at least sixty percent (60 percent) annual removal efficiency for total phosphorus, and at least ninety percent (90%) annual
removal efficiency for total suspended solids from site runoff. The onsite abstraction of runoff may be included in demonstrating
compliance with the total suspended solids and total phosphorus removal requirements.
Rate Control
Rate control design standards are to maintain pre-development peak runoff rates for the 2-yr, 10-yr, 100-yr 24-hour storm events.
This site was designed to meet 90% TSS and 60% Phophorus Removal . MIDS was used to model loading for this project; note that the SUMP has not been modeled in MIDS
We have met the abstraction volume by providing a water harvesting system. We plan to irrigate the individual lots with 0.5" of water and
the open space with 1.5" of water over a 48 hour period as shown on the attached plan by Water in Motion.