HomeMy Public PortalAboutADS_IsolatorRowPlus_ApprovalLetter_020723Metropolitan
St. Louis Sewer
District
2350 Market Street
St. Louis, MO 63103-2555
(314) 768-6200
February 7, 2023
Advanced Drainage Systems, Inc.
Mr. Craig Dahlgren
127 Windcliffe Dr.
Ballwin, MO 63021
RE: StormTech and Isolator Row PLUS General Use Level (GUL) Approval with the
District
Dear Sir,
The Metropolitan St. Louis Sewer District has reviewed the information you provided on October
27, 2022, regarding the Isolator Row PLUS treatment system. MSD hereby grants GUL approval
for the StormTech/Isolator Row PLUS system as a standalone post-construction BMP for use on
new development, redevelopment, and highway/roadway improvement projects of any size,
subject to the following provisions:
Properly sized Isolator Row PLUS chambers are provided as a measure of pretreatment at each
location where flow is injected into the StormTech system. The sizing of the Isolator Rows shall
be based upon the Peak Discharge for the Water Quality Storm for the area tributary to each point
of injection. Appendix D.10 of the 2000 Maryland Stormwater Design Manual provides a
procedure to calculate the Water Quality Storm peak discharge.
Isolator Row PLUS chambers are hereby approved for treatment at a rate of 0.009 cfs/sq ft of
Isolator Row PLUS chamber bottom, up to a maximum of 0.501 cfs/chamber. These amounts are
independent of the chamber model. The Isolator Row PLUS requires specific geotextile properties
to meet the approved treatment flow rate. The geotextile used on Isolator Row PLUS systems
must be that specified by Advanced Drainage Systems, or as otherwise approved by MSD, meeting
the following minimum requirements:
Table 1: Minimum Geotextile Properties for Isolator Row PLUS
PROPERTY TEST
METHOD
ENGLISH MARY
(MARV2)
METRIC MARY
(MARY2)
Weight ASTM D5261 8.0 oz/sq yd 271.25 g/sq m
Grab Tensile Strength ASTM D4632 325 lbs.1.45 kN
Grab Elongation ASTM D4632 15%15%
Trapezoidal Tear Strength ASTM D4533 200 lbs.0.89 kN
GBR Puncture Resistance ASTM D6241 1124 lbs.5.0 kN
At a minimum, an equal number of non-Isolator Row PLUS chambers are required for each
installation.
The StormTech outlet shall be perched a minimum of 6 inches and the infiltration bed area sized
in accordance with the attached Figure 1. With a perched outlet, and when sized in accordance
with the Isolator Row PLUS treatment flow rate guidance above and Figure 1, the Isolator Row
PLUS and StormTech chambers provide sufficient water quality treatment for its tributary area.
A runoff volume reduction credit of 0.45 may be entered into “Item 11, Site Specific BMP” on the
MSD MEP spreadsheet.
Figure 1 was derived using an assumed underlying subgrade saturated hydraulic conductivity of
0.05 in/hr. This may be the default assumption for all sites utilizing a 6-inch perched outlet. The
maximum depth the underdrain may be perched is 18 inches.
When configured with a perched outlet height greater than 6 inches, MSD will require the
following from a Professional Geotechnical Engineer licensed in Missouri:
1) Onsite geotechnical investigation to determine the actual saturated hydraulic conductivity
of the underlying soils beneath the proposed system
2) Assessment and recommendations for pavement structural stability
3) Assessment of possible impacts to adjacent structures due to seepage, including
recommended setback distances between those features and the StormTech/Isolator Row
PLUS system.
In all instances, a minimum of 2 feet of separation is required between the bottom of the infiltration
bed and the seasonal high water table.
Integration with Channel Protection and Detention Design
Channel Protection (CPV) storage and flood detention (QPz^QPioo) storage may be managed
within the same system, with a properly configured, sized, maintenance-accessible, reinforced
concrete control structure. The footprint of the infiltration bed/chamber system shall be sized at a
minimum to manage Water Quality (when applicable), and expanded as necessary to manage these
additional volumes as required on sites.
MSD reserves the ability to withdraw or modify this approval based on subsequent information,
including information indicating that the StormTech and Isolator Row PLUS system does not
satisfy MSD rules, requirements, or construction and material specifications.
Curtis Zurliene, PE
Civil Engineer (BMP Committee Chairman)
Engineering Department / Development Review
314-768-6377
Metropolitan St. Louis Sewer District
Attachment: Figure 1 - StormTech underground infiltration bed area sizing graph
InfiltrationBedAreaasPercentrageofTributaryArea (%)
50% 40% 30% 20% 10% 0%
Figure
1 -
StormTech
Underground
Infiltration
Bed
Area
Sizing
Graph
0
rtm6"
Underdrain
Height
t12"
Underdrain
Height
41•
8•
018"
Underdrain
Height
10
20
30
40
50
60
Contributing
Watershed
Percent
Impervious
70
80
90100