02 AG SUMMARYProject: Arbor Glen
Location: Chanhassen MN
Prepared for: Chanhassen
Prepared by: Sathre-Bergquist, Inc.
Date: June 4, 2015
Revised:
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 -80,389 sf
Abstraction Volume Required (AV) = 80,389 sf x 0.092 ft = 7,369 cf
0.17 Ac-Ft
ABSTRACTION ACHIEVED:
FILTRATION/INFILTRATION
AV=1 * Vol below Overflow
Storage (cf)
Underground Storage* 7,377.00
AV = 7,377.00 c.f.
0.17 Ac-Ft
Total AV from Filtration 7,377.00
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 21 home development. Drainage will be focused toward initial storage in an underground storage facility with pretreatment and infiltration, with
overflows to a pond before final discharge into the nearby wetland. Using an initially closed valve and an underdrain, the water level in the underground storage may be drawn
down faster by opening the valve if the system is not performing to standard. Tables within this report show the total impact / treatment requirements due to Arbor Glen.
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.
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.
*Volume From HydroCad. The Triton system is 4 Chambers x 42 Chambers, with a base
width of 24.04', a base length of 119.87', and a base height of 4.0'. The chambers are to be
surrounded with minimum 12" of stone for the top and sides, and 6" of stone under the
containers.
TOTAL ABSTRACTION VOLUME
Buffer Enhancement volume = 0.00
Filtration volume = 7,377.00
Total Abstracted Volume 7,377.00
Required Volume = 7,368.99
Excess Volume Provided = 8.01
lbs/yr TSS TP TSS% Removal Ph% Removal
Existing Load 385.3 1.8
Proposed Load 220.4 2.7 89.9 65.1
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)
Pre-Development Rate 2.4 6.7 19.1
Post-Development Rate 1.9 6.7 17.6
HydroCAD was used to model rate control for this project. The table below compares pre-development and post-development rates.
This site was designed to meet 90% TSS and 60% Phophorus Removal . P8 was used to model loading for this project.
Quality Control
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.