Planning & Zoning Commission - 1108 (07/22/2025)

July 14, 2025 
 
 
 
Mr.  Ryan McEvoy, PE 
Principal Civil Engineer 
SLR International Corporation  
99 Realty Drive 
Cheshire, CT  06410 
 
RE:
 
Peer Review – Site Plan & Stormwater Management Applications 
51 Daniels Avenue 
Waterford, Connecticut 
Langan Project No.: 140310301 
 
Enclosed please find our responses to your letter dated June 25, 2025.  Below please find each 
comment followed by our response in bold. 
 
Review Comments  
 
1. 
The applicant's engineer should confirm the amount of cover over the proposed 
underground Retain-It® systems. Specifically for System P1A near CCB-112, there is 
potential for less than 1 foot of cover over the system. For System P2B, the top of the 
concrete chambers is close to proposed finished grade at the southern end of the system. 
 
COMMENT RESPONSE:  The systems have been designed to meet the minimum 
cover requirements allowed for a Retain-It chamber. Per Retain-It manufacturer 
guidance, the precast concrete systems allow for a minimum cover of 6”. The 
systems will be reviewed and approved by the Retain-It manufacturer prior to 
construction. At the lowest point of finished grade, Systems P1A and P2B have 
approximately 9” of cover.  
 
2. 
Based on a site visit by SLR, there is an existing catch basin in a low point in the northwest 
corner of the property near Daniels Avenue that captures a majority of the runoff from 
Existing Watershed 2. This existing catch basin on site drains to the existing catch basin 
(TF=96.36) in Daniels Avenue. We recommend the location of the existing catch basin on 
site is added to the site plans along with the inverts for this catch basin and existing catch 
basin within Daniels Avenue. Coordination with Waterford Public Works might be needed 
to remove inserts from the catch basin within Daniels Avenue to obtain pipe inverts and 
sizes accurately. 
 
COMMENT RESPONSE:  A callout has been added to sheet CG101 noting the 
location of this existing catch basin. Prior to construction the contractor will be 
required report the location and invert information to Langan. The structure will be 
maintained and protected during construction.  
 

Peer Review – Site Plan & Stormwater Management Applications 
99 Realty Drive 
Cheshire, CT  06410 
Langan Project No.: 140310301 
July 14, 2025
Page 2 of 5 
 
 
 
3. 
In order to mimic existing site hydrology, consideration should be given to directing the 
discharge from Stormwater Basin P2A to the existing swale/depression noted in 
Comment #2.  Presently, Existing Watershed-2 (EXWS-2) ultimately discharges to the 
depression and inlet structure, connecting to CCB (TF=96.36).  The proposed condition 
stormwater design connects to existing storm drainage upgradient of the CCB noted 
above. 
 
COMMENT RESPONSE:  This option was considered; however, the existing 
structure is located outside of the proposed property limits. The design intent is to 
avoid the installation of any stormwater structures, pipes and grading disturbance 
on the portion of the existing property that is to be retained by the Town.  
 
4. 
The hydrology analysis in the Stormwater Management Report has included the entirety 
of the school roof area with Proposed Watershed 2A and directed to Stormwater 
Detention Basin P2A.  Please note that Sheets CG101 and CG102 contain a note that 
states, "Connect all roof leaders to nearest storm structure or nearest drainage pipe at 
minimum 0.5%."  Absent are any specifically routed pipes on the east of the building that 
connect to Basin P2A, and roof leaders could be directed to storm drainage routed to 
underground system Basin P2B.  We recommend that either 1) storm drainage is provided 
along the east side of the building, directed to Basin P2A, or 2) modify the hydrology 
model to direct a portion of the east side of the building to Basin P2B. 
 
COMMENT RESPONSE:  The note on sheets CG101 and CG102 will be removed and 
was an error. The entirety of the roof area runoff will be collected and conveyed 
through the four roof drains located on the western side of the building. The 
stormwater model reflects this routing, and the routing has been confirmed with 
the project MEP. 
 
5. 
For Existing Watershed 1, the stormwater runoff from this watershed is dispersed over a 
large area with no apparent concentration to a single swale, watercourse, storm structure 
etc. Under proposed conditions, peak runoff rates are being decreased compared to 
existing conditions, but a concentration of runoff is being proposed via the proposed level 
spreader/scour hole. For example, for the 100-year storm under existing conditions there 
is a peak runoff rate of 35.44 cubic feet per second (cfs) for Watershed 1. Under proposed 
conditions there is a peak runoff rate of 28.67 cfs with 17.40 cfs being discharged by the 
proposed level spreader/scour hole. There is potential for properties downgradient of the 
level spreader/scour hole to receive an increase in peak runoff rates under proposed 
conditions compared to existing conditions due to the proposed concentration of 
stormwater discharge. The applicant's engineer should consider increasing the length of 
the level spreader and/or disperse the discharge in more than one area to attempt to 
better mimic existing runoff conditions.   
 
COMMENT RESPONSE:  The outfall location has been selected during the Wetland 
Commission review and approval, in consultation with the commission, to reduce 
the potential for downgradient properties to receive stormwater runoff by ensuring 
discharge from HW-100 will reach the existing off-site wetland to the west. The 
discharge from system P1A will be routed through a drop manhole structure, scour 

Peer Review – Site Plan & Stormwater Management Applications 
99 Realty Drive 
Cheshire, CT  06410 
Langan Project No.: 140310301 
July 14, 2025
Page 3 of 5 
 
 
 
hole and 50-foot level spreader to unconcentrated flows and provide additional 
energy dissipation. The flow rate over the level spreader for the 100-year storm is 
approximately 1.57 fps (see Appendix B). 
 
6. 
Computations for the Level Spreader/Scour Hole at HW-100 should be provided to 
demonstrate non-erosive velocities over the curb. 
 
COMMENT RESPONSE:  Flow over the 50-foot level spreader for the 100-year storm 
is approximately 1.57 fps (see Appendix B), which is below the allowable velocity 
threshold of 2.5 fps for sandy soils per the 2024 Connecticut Guidelines for Soil 
Erosion & Sediment Control Manual. Additionally, the Manual states the general 
rule of thumb for level spreaders is to provide 4 feet of length for every 1 cfs for the 
10-year storm. The P1A 10-year peak flow rate = 7.26 cfs which requires a minimum 
level spreader length of 29 feet. As noted above and in Appendix B, the velocity 
exiting the level spreader is below the guideline level. 
 
7. 
We recommend that the invert elevation of HW-100 is lowered to be lower than the top 
of curb elevation of the level spreader. 
 
COMMENT RESPONSE:  Noted – the invert elevation for HW-100 has been lowered 
below the top of curb elevation. See revised HW-100 invert on sheet CG101. 
 
8. 
The applicant's engineer should consider isolator rows for the proposed underground 
systems P1A and P2B. 
 
COMMENT RESPONSE:  Isolator rows were considered; however, due to the 
configuration of system P1A, ease of maintenance, and the use of prefabricated 
concrete chambers rather than plastic arch chambers, it was determined that water 
quality units are a more effective pretreatment device. Water quality units are 
provided at all inlet locations for systems P1A and P2B. 
 
9. 
Specifications for the Bioretention Soil for Stormwater Detention Basin P2A should be 
provided. 
 
COMMENT RESPONSE:  Refer to bioretention soil specifications provided under 
detail 3 on sheet CG503. This specification calls for a mixture of 60% sand and 40% 
horticultural soil. This blend will promote infiltration, support plant growth and 
stabilization within the basin. 
 
10. 
The planting schedule or seed mixes for Stormwater Detention Basin P2A should be 
provided. 
 
COMMENT RESPONSE:  Refer to sheet L04.03 for the location of Basin P2A seeding 
information. The Turf and Grasses specification calls for New England Erosion 
Control/Restoration Mix. This mix is appropriate for detention basins not holding 
standing water, and the mix is formulated for ecologically sensitive restorations 
that will do well in recently disturbed sites to stabilize the soil surface. 
 

Peer Review – Site Plan & Stormwater Management Applications 
99 Realty Drive 
Cheshire, CT  06410 
Langan Project No.: 140310301 
July 14, 2025
Page 4 of 5 
 
 
 
11. 
According to Section 25.6.5.8 Conveyance Criteria of the Stormwater Management and 
Low Impact Development Standards in the Town of Waterford Zoning Regulations, 
“Emergency outlets must safely pass the post-development peak runoff from the 100-
year storm event in a controlled manner without erosion of the outlet works or 
downstream drainage system and provide a freeboard of at least one (1) foot.”. Less than 
1 foot of freeboard is provided for Stormwater Basin P2A for the 10-year storm through 
100-year storm. 
 
COMMENT RESPONSE:  The proposed infiltration basin berm elevation has been 
raised to elevation 105. The peak elevation in Basin P2A during the 100-year storm 
is approximately ±103.56. As a result, the revised plans provide about 1.44 feet of 
freeboard. 
 
12. 
The top of frame elevation specified for Trench Drain 114 does not appear to match with 
nearby proposed grading. 
 
COMMENT RESPONSE:  The previously called out TD-114 frame elevation was an 
error. The revised frame elevation has been revised (see sheet CG101). 
 
13. 
Based on the Test Pit Log for TP-01, there is potential for groundwater to be within a foot 
or two of the bottom of System P1A. The applicant's engineer should consider a 
dewatering underdrain set within the stone below the Retain-It chambers to ensure the 
system drains out in between rain events. 
 
COMMENT RESPONSE:  TP-01 indicates groundwater approximately 9 feet below 
existing grade. This relative groundwater elevation was assumed to be consistent 
at the proposed location of System P1A (i.e., assumed GW elevation 97.5 feet). The 
bottom of chamber elevation for system P1A was set at elevation 101.33 feet to 
provide the minimum 3-foot clearance to groundwater. If groundwater is 
encountered above the anticipated elevation during excavation, modifications – 
such as a shallower infiltration system or alternative improvements – will be 
incorporated if requested by the Commission. 
 
 
14. 
Chapter 10 - General Design Guidance for Stormwater Infiltration Systems: General 
Design Guidance - Vertical Separation to Groundwater and Bedrock of the 2024 
Connecticut Stormwater Quality Manual states, "The 3-foot vertical separation distance 
from the bottom of the infiltration system to the SHGT and bedrock may be reduced to 2 
feet in the following situations: ...For Stormwater Retrofits where the minimum 3-foot 
separation cannot be met due to existing site constraints and there is little risk to 
groundwater quality from the infiltrated stormwater". System P1A should be modeled 
without infiltration due to the presence of groundwater less than 2 feet below the bottom 
of the system. 
 
COMMENT RESPONSE:  As noted in comment response 13, based on the Test Pit 
Log data, the 3-foot clearance has been provided. If groundwater is encountered 
above the anticipated elevation during excavation, modifications – such as a 

Peer Review – Site Plan & Stormwater Management Applications 
99 Realty Drive 
Cheshire, CT  06410 
Langan Project No.: 140310301 
July 14, 2025
Page 5 of 5 
 
 
 
shallower infiltration system or alternative improvements – will be incorporated if 
requested by the Commission.  
 
15. 
The applicant's engineer should consider using high strength, high-density polyethylene 
(HDPE) pipe or Class V reinforced concrete pipe (RCP) for the proposed piping upgradient 
of CLCB 401 and in between MH 400 and CLCB 401. 
 
COMMENT RESPONSE:  The pipe between MH-400 and CLCB-401 has been 
changed to Class V RCP. The pipe upgradient of CLCB-401 is ADS N-12 HDPE. This 
pipe is rated for a minimum of 12” of cover and will have no vehicular loading.  
 
16. 
Due to the addition of a berm for the vegetated drainage swale, the time of concentration 
flow path for Proposed Watershed 2C is no longer valid. We recommend the applicant's 
engineer update the HydroCAD model to account for the likely revised time of 
concentration path. 
 
COMMENT RESPONSE:  The time of concentration and watershed boundary for 
PRWS-2C has been revised to reflect the accurate time of concentration. The 
HydroCAD model has been updated accordingly (see Appendix B). 
 
17. 
The existing condition hydrology analysis uses a ‘fair’ condition for grassed areas (50 to 
75% coverage), while under proposed conditions, much of the grassed areas classified 
as ‘good’ condition (>75% coverage) or meadow.  ‘Good’ grass and meadow areas have 
a lower curve number, which helps to reduce the flow rates under proposed conditions. 
Therefore, we would ask the applicant’s engineer to provide the justification for the use 
of the ‘fair’ grass under existing conditions, as opposed to ‘good’ grass or meadow under 
proposed conditions. 
 
COMMENT RESPONSE:  All existing grass in the existing HydroCAD model has 
been changed from ‘fair’ to ‘good’  as requested (see Appendix B). 
 
18. 
The applicant’s engineer should clarify if the proposed stormwater detention basin will be 
utilized as a sediment trap during construction. If yes, notes on the conversion of the 
basin from sediment trap to bioretention basin should be provided.  
 
COMMENT RESPONSE:  The location of the proposed stormwater basin P2A will 
not be utilized as a sediment trap. Clear instructions have been included on the plan 
sheets to keep this area clear of material stockpiles, heavy machinery, and any 
compaction of existing soils (see notes on sheets CS003 and CE102). 
 
Sincerely, 
 
Langan CT, Inc. 
 
Bill Willsey, PE 
Senior Staff Engineer 
 
\\langan.com\data\NHV\data3\140310301\Project Data\Correspondence\Comment Response Letter\2025-07-10 SLR Review - Comment Response Letter.docx