HomeMy WebLinkAboutLot Certifications - 08/09/2019
August 1, 2019
FORT COLLINS UTILITIES
700 Wood Street
Fort Collins, Colorado 80521
Re: 122 3rd Street Certification of Lot Grading – 2ND ADDENDUM to May 8, 2019
The as-built grading and drainage patterns for 122 3rd Street are in general
conformance with the Grading Plan for 122 3rd Street. A field change was made to raise
the Finished Floor 1.0 feet. This allowed most of the slopes to nominally increase while
maintaining all of the drainage patterns and drainage area boundaries. The area of
critical concern remains the north lot line which is shared with the existing home to the
north (116 3rd Street). The existing house and entire yard of that adjacent home MUST
drain east along the common lot line towards the rear lot line where it discharges to an
existing drainage channel on the abutting O’Dell property. The contractor did maintain
this drainage pattern very well.
UPDATE
In re-examining the downspout locations, they all are correctly installed and direct flow
away from the foundation. However, small field corrections to more favorable discharge
locations have caused a change in the direction of impervious roof runoff and a related
change in drainage basin boundary. This is entirely related to the swale on the north
property line which drains to the east or rear of the lot. With the 5-foot set back largely
consumed by the eastbound swale, a second swale or piped discharge draining to the
west towards 3rd street is impractical as the eastbound swale is below 3rd street.
The eastbound swale is controlled by the elevation of the existing house abutting the lot
to the north (128 3rd Street). That house sits at or below the sidewalk level and its front
yard is BELOW the 3rd street sidewalk and it will NOT drain to 3rd street. The contractor
has invested in constructing a well defined, mutually beneficial riprapped swale along
the lot line, maintaining the historical drainage of both lots toward the existing drainage
ditch along the rear lot line.
The drainage ditch on O’Dell property but along the rear lot line begins at the rear of
128 3rd Street and drains south along 122 & 116 3rd Street and 300 Lincoln Avenue
towards Lincoln Avenue where it joins a piped drainage system. This drainage ditch
system begins at a high point in 3rd Street’s vertical design and will not likely see any
future additional residential flows from 3rd Street.
REAR DRAINAGE BASIN
(THAT DRAINS EAST TO O’DELL)
The Existing Basin and Basin B are from the Approved/signed Construction Documents, sheet C5 of C9.
Asbuilt info is from field inspections.
The 100-year Intensity of 7.72 in/hour is from the Fort Collins Storm Water Manual – Rational Formula
with an assumed Tc of 10 minutes.
No contributions from offsite sources are included in this calculation.
The 0.30 cfs asbuilt Q is greater than the Basin ‘B’ Q of 0.21.
However, the 0.30 cfs asbuilt Q is still less than the 0.49 cfs Existing (or Historic) Basin.
The additional 0.09 cfs in runoff generated by this change in Drainage Basin Area and C
coefficient with the additional impervious area (roof and patio) while significant in an individual
lot calculation, is negligible in the peak runoff tributary to the drainage ditch and to the
downstream system.
122 3RD ST - aka Lot 1
100 yr i =7.72 Tc=10 min
Sheet C5 of C9
Buckingham
Place 2ND Filing
0.44
0.145
0.0638
Basin 'B' 0.41
0.065
0.0267 0.21
Rear Basin C Acres CxA Q (cfs)
Impervious 0.9
0.033
0.0301 0.23
Landscape 0.2
0.041
0.0082 0.06
TTL Asbuilt
0.51
0.074
0.0382 0.30
ASBUILT Q < EXISTING Q
This unofficial copy was downloaded on May-13-2019 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact Engineering Office 281 North College Fort Collins, CO 80521 USA