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April 13, 2018
Prinsco
Attn: Mr. Matt Walter
7218 S. Ridgetop Ct.
Dardenne Prairie, MO 63368
RE: Prinsco HydroStor and Sediment Row Chambers
Dear Mr. W alter:
MSD grants approval of the Prinsco HydroStor HS75 and HS180 chamber system and sediment
row for use as a standalone post construction stormwater Best Management Practice. These
chambers may be used on new development, redevelopment and highway/roadway
improvement projects of any size under the following conditions:
Properly sized sediment row chambers are provided as a measure of pretreatment at each location
where flow is injected into the HydroStor system. The sizing of the sediment rows shall be based upon
the Peak Discharge for the Water Quality Storm (Appendix D.10 of the 2000 Maryland Stormwater
Design Manual) tributary to each point of injection, and Table 1:
Table 1
Sediment Row – Flow Rates per Chamber
At a minimum, an equal number of non-sediment row chambers are required for each installation.
The HydroStor outlet shall be perched a minimum 6” and the infiltration bed area sized in
accordance with Figure 1 noted in an approval letter granted to a similar product dated
September 20, 2016. With a perched outlet, and when sized in accordance with Table 1 and
the referenced Figure 1, the sediment row and HydroStor 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” in the MSD MEP spreadsheets.
It is noted that 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” perched
outlet.
When configured with a greater than 6” but less than 18” perched outlet height, MSD will require
the following from a Professional Geotechnical Engineer licensed in Missouri: a) Onsite
geotechnical investigation to determine the actual saturated hydraulic conductivity of the
underlying soils beneath the HydroStor/sediment row system, b) assessment and
Chamber Model Max Allowable Peak Discharge for the Water
Quality Storm / (cfs) per Chamber
HS75 0.15
HS180 0.24
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recommendation for pavement structural stability, c) assessment of possible impacts to adjacent
structures due to seepage, including recommended setback distances between those features
and the HydroStor Chambers/sediment row system.
In all instances, a minimum 2 feet 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 storage, 2yr-24 hour and 100yr-24 hour detention 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, 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 HydroStor Chambers and sediment row do not satisfy
MSD rules, requirements, or construction and material specifications.
Sincerely,
Jason Peterein, P.E.
Principal Engineer (Proprietary BMP Committee Chair)
Engineering/Planning-Development Review