HomeMy WebLinkAboutGrading Plans - 12/21/2023October 28, 2023
Harrington Construction, LLC
1006 W. Mulberry Street
Fort Collins, Colorado 80521
RE: DRAINAGE MEMO FOR 318 N. SHERWOOD
FORT COLLWS, COLORADO
NE PROJECT NUMBER: 1079-005
To Whom It May Concern:
Grading/Stormwater Plan Accepted
(Building Permit Released)
Accepted by: sagenbroad
Date: 12/21/2023
Northern Engineering Services has performed a grading design and imperviousness evaluation for the expansion
of the home at 318 N. Sherwood Street in Fort Collins. This memo accompanies the grading pfans and provides
additional details regarding the requirements of the design.
EXISTING CONDITIONS:
The lot contains an existing single-family home, a storage shed, an asphalt driveway, and a garage structure. The
total existing impervious area of the lot was calculated to be 2,925 SF using 100% impervious for the above-
mentioned structures. The lot is oriented with the front of the home at the west and the rear afley at the back.
The entire lot currently drains to the alley at onfy approximately 0.50% grade, with the high elevation of the (ot
being the public sidewalk along N. Sherwood Street.
The builder has provided a test hole indicating that there are 2' of sandy loam above a 4.5' layer of sandy clay
soil that is underlain by sandy gravel with cobble from 6.5' below the existing grade. Groundwater was observed
by the builder at 7.5' below grade, within the layer of sandy gravel with cobble. This soil would exhibit
reasonabfy high hydraulic conductivity.
PROPOSED CONDITIONS:
It is proposed that the existing garage, asphalt driveway, storage shed and rear portion of the existing home be
demoed. A 1,650 SF addition with a paved patio is proposed behind the existing structure with a new concrete
front driveway. A new carriage-house-style garage is proposed toward the back of the lot with a concrete
driveway and a crusher fines path leading to the structure from the front.
The proposed impervious area of the lot is 5,237 SF including the new improvements. The change in
imperviousness is proposed to be 2,312 SF. In order to ensure that there is no increase in runoff to the rear alley,
it is proposed that two infiltration galleries be constructed towards the rear of the lot, near the alley. The
galleries have been sized to accommodate the runoff from a 100-year storm of 3.67" depth per the City of Fort
Collins stormwater criteria.
The northeast infiltration gallery will accept the entire expansion of the original structure, half of the new
carriage house garage, and the driveway. All downspouts from these improvements shall be directed into the
gallery. The required volume was calculated to be 941 CF. This gallery is expected to be excavated 5' below the
proposed finished grade and backfilled with 4' of 1" rock. The bed shall be underlain by woven geotextile fabric.
There will be 12" of ponding over top of the rock bed in the 100-year storm.
The southeast infiltration gallery will accept the southerly half of the proposed carriage house garage and all
downspouts from that structure shall be directed into the facility. The required volume was calculated to be 183
CF. The gallery is expected to be excavated 4' below the proposed finished grade and backfilled with 4' of 1"
NORTHERNENGINEERING.COM � 970.221.4158 FINAL DRAINAGE MEM0:318 N SHERWOOD ST
FORT COLLINS � GREELEY 1 � 2
rock, up to the finished grade and underlain by woven geotextile fabric. All stormwater storage for the 100-year
storm is expected to be contained within the rock section.
Due to the expected high hydraulic conductivity and separation of the galleries from the groundwater surface, it
is expected that this gallery will effectively infiltrate stormwater, however it is recommended that the infiltration
rate be verified prior to construction.
CONCLUSIONS:
The drainage design proposed with this project will effectivefy comply with the City of Fort Collins Drainage
Criteria. The calculated impervious area proposed for this project remains below the typical Fort Collins
Stormwater Criteria Manual, Tabfe 4.1.2 imperviousness of 50% for Low Density and will have negligible impact
on the historic drainage basin. The grading and storm conveyance design for this site will effectively prevent
additional impervious areas from discharging runoff into the alley.
If you have any questions, please feel free to contact me at your earfiest convenience.
Si ncerely,
NORTHERN ENGINEERING
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Ryan Banning, P.E, vJ��NA
Project Manager
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NORTHERNENGINEERING.COM � 970.221.4158 FINAL DRAINAGE MEM0:318 N SHERWOOD ST
FORT COLLINS � GREELEY 2 � 2
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December 20, 2023
Jason Harrington
Harrington Construction, LLC
1006 West Mulberry Street
Fort Collins, Colorado, 80521
Subject: Test Pit Observation
Binns - Accessory Dwelling Unit
318 North Sherwood Street
Fort Collins, Colorado
CTL�T Project Number: FC10923.000_L1
Per your request, we conducted a site observation to the above referenced site on June 26, 2023.
The purpose of this site visit was to observe a test pit that was just behind the existing house in the
middle of the back yard. The pit was dug to an approximate depth of 8'/2 feet.
Soils observed in the test pit consisted of 6" of topsoil over 6 feet of sandy clay over gravel and
cobble to the bottom of the test pit. Ground water was observed at a depth of 8 feet at the initial
observation. After 72 hours, the ground water was measured at 7'/2 feet.
We appreciate the opportunity to work with you on this project. If you have any questions regarding
the information provided in this letter, please do not hesitate to contact us.
Respectfully submitted,
CTL�THOMPSON, INC.
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Alfonso J.� vera, P.E.
Project Engineer
arivera(a�ctlthompson.com
Reviewed by:
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Wyatt M. Knutson, P.E.
Vice President � Regional Manager
wknutson(c�ctlthompson.com
CTL�Thompson, Inc.
Denver, Fort Collins, Colorado Springs, Glenwood Sprinqs, Pueblo, Summit Countv — Colorado
Chevenne, Wyoming and Bozeman, Montana