HomeMy WebLinkAboutSP-24-20 - Supplemental - 0200 da Vinci Drive (30)The Stormwater Division has reviewed the BSD Aviation Technical Center Site Plan Application provided
by EIV Technical Services. Please note that several issues were identified that, once remedied, may have
resulting implications for the stormwater design’s compliance with the City’s LDRs and will likely require
additional review. We would like to offer the following comments:
1. General Application Comments:
a. Please provide a stormwater maintenance plan.
b. If the applicant is proposing the use of site balancing to meet the requirements of the
LDRs, please provide a narrative description of said site balancing.
2. HydroCAD Modeling:
a. 1-year storm: Peak discharge at CB#2 under the proposed conditions exceeds the
existing conditions peak discharge at CB#2. As per the LDRs Section 13.05.E.2, “The post-
construction peak runoff rate for the one-year, twenty-four hour (rainfall amounts to be
determined using NOAA, Atlas 14 data and a type II rainfall distribution) rain event shall
not exceed the existing peak runoff rate for the same storm event from the site under
conditions existing prior to submittal of an application.” Please revise design to comply
with the LDRs.
b. 25-year storm: Model indicates surcharging occurring at CB#2 under the proposed
conditions. As per the LDRs Section 13.05.F.3, please revise design to ensure
downstream structures retain capacity to convey the 25-year storm.
c. The biorentention calls out a 1.0 in/hr discarded exfiltration, but also calls out 2.0 in/hr
exfiltration routed to the outlet structure through an underdrain. Please provide
justification for the discarded exfiltration modeling decision.
d. Pre-treatment forebay: please provide sizing information for the forebay, ensuring that
it meets the sizing requirements laid out in the Vermont Stormwater Management
Manual (VSMM).
3. Sheet GR-01:
a. Grading in the pervious (grassed) areas is inadequate to determine flow path of
stormwater. Please provide details of how applicant will ensure all stormwater draining
from the impervious area in Sub-catchment #2 will enter the bioretention via the pre-
treatment forebay.
b. Please provide information on how the applicant will prevent stormwater from creating
a ponded area in the vicinity, both on and off the pavement, of the 312’ contour on the
section of pavement connecting the two parking wings. Please note that there is an
existing light pole to remain directly adjacent to this area. Grading information is
inadequate to determine the extent of the ponding that may occur.
c. Please indicate the elevation at the edge of the bioretention area on the plan.
d. Staff recommends the applicant add curbing along the parking area to ensure all
stormwater flows to the treatment area and enters via the pre-treatment forebay.
4. Sheet P-01:
a. The delineation for Sub-catchment Area #2 does not appear to match the routing in the
HydroCAD model. Please explain how runoff from the new impervious in the western
corner of Sub-catchment Area #2 is being conveyed to the proposed bioretention area.
b. Is site balancing being proposed to meet treatment requirements for the section of new
impervious in Sub-catchment Area #1? If so, please indicate on the plan the size of the
new area and the corresponding area in Sub-catchment Area #2 that will be used for site
balancing.
5. Sheet MD-03:
a. Please clarify the following in the bioretention basin detail:
i. There is an underdrain shown, but the basin is called out as an “infiltration
basin”. Please clarify whether or not this is an infiltrating bioretention practice.
ii. The “Temporary/Permanent Sediment Forebay/Basin” callout. Please clarify
which of these options is shown here.
iii. Please clarify whether or not the outlet structure is meant to comply with the
“Drainage Structure” detail on this same sheet. If not, please provide details of
the outlet structure.
6. EPSC: Please consider using the footprint of the bioretention area as a temporary sediment
basin during construction.