HomeMy WebLinkAboutGrading Plans - 05/17/2021May 16, 2021
Wes Lamarque
City of Fort Collins
Water Utilities Engineering Department
281 North College Avenue
Fort Collins, CO 80524
wlamarque�fc�ov.com
RE: Lot Grading Plan and Drainage Analysis
108 Harvard Avenue
Dear Mr. Lamarque,
City of Fort Collins Approved Plans
Approved by: Wes Lamarque
Date: 05/17/2021
The purpose of this letter is to provide a proposed grading and drainage plan for 108 Harvard Avenue,
Fort Collins, CO, 80525. The homeowner is proposing a 926 square feet (sf) addition including a
basement floor area. The total roof area witl add 636 sf of roof area to the lot. Below we will discuss
how the proposed design is not impacting neighboring properties prior to the stormwater entering a
public street in the City of Fort Collins.
Lot Grading Conditions
Looking at the lot relative to the subdivision, it appears that the existing lot drains across neighboring
properties before entering a public street and an existing inlet located in Yale Avenue. Please see the
attached Overall Drainage Basin Exhibit.
The existing elevations for the lot above were surveyed on May 8, 2021. The street appears to drain
towards the west; however, a majority of the lot behind the existing sidewalk drains towards the
backyard fence line along the north property boundary. In general, the property appears to drain
towards the northeast corner of the lot. Please see the attached Existing Lot Grading exhibit.
In the proposed conditions, the drainage flow patterns will remain relatively the same as the existing
conditions. The lot will continue to drain towards the north boundary and then east towards the
northeast corner of the lot. The addition will add two downspouts approximately 22 feet to the north of
the existing house, and the remaining downspouts will not be altered from the existing drainage
patterns. Please see the attached Proposed Lot Grading exhibit.
Draina�e Analvsis
Since this addition will add greater than 350 square feet of roof area, a drainage analysis was performed
to understand possible stormwater impacts to neighboring properties prior to the stormwater entering
a public street. The 2-yr, 10-yr, and 100-yr storm events were analyzed for the existing and proposed
conditions using the rational runoff methods as described in the City of Fort Collins Stormwater Criteria
Manual adopted in 2018.
It appears that the amount of runoff would slightly increase stormwater from the property for the 2-
year and 100-year runoff by 0.03 and 0.10 cfs, respectively. Please see attached runoff calculations.
With the small increase of runoff above, it appears that it would be insignificant to impact neighboring
structures and properties before the runoff entered the Yale Avenue. However, we analyzed the
storage requirements using the Modified FAA method in accordance with-the City of Fort Collins
Stormwater Criteria Manual for lots less than 5.0 acres. Please see attached stormwater storage
calculations.
Thus, a gravel storage area for stormwater runoff is proposed for this lot. This will consist of a 22 feet
long by 3.5 feet wide by 2 feet deep gravel storage area for the increase of runoff in the 100-year storm
event. This 3/4-gravel bed will have a porosity of 0.4, stormwater storage volume of 60 cubic feet, and a
total volume of 154 cubic feet including gravel storage. The downspouts from the roof drainage will
direct stormwater into this storage area to ensure the 100-year stormwater runoff from the lot will
remain the same as existing conditions.
Conclusion
The proposed grading and drainage plan will not alter existing drainage patterns and will not affect
neighboring properties after the final construction of the proposed addition and gravel storage area for
the slight increase of storm water in a 100-year storm event.
Thank you for your time and efforts on this building application. If you have any questions about the
proposed grading and stormwater plan, please call me at 307-710-6217.
Sincerely,
/ ��
Seth Tourney, P.E. ����"�`
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STORMWATER STORAGE; MODIFIED FAA METHOD w/ Ft Collins IDF
Project Number : 21-001
Project Name : 108 Harvard Avenue
Project Location : Fort Collins, CO
Pond No : 1
Input Variables Results
Design Point 1
Design Storm 100-yr Required Detention Volume
C = 0.39
Tc = 5.00 min 60 ft3
A= 0.26 acres 451 gallons
Mau Release Rate = 0.90 cfs 0.00 ac-ft
Ft Collins Inflow Outflow Storage
Time (min) 100-yr Volume Adjustment Q�� Outflow Volume Volume
Intensity �fts� Factor (cfs) (ft ) �fts�
in/hr
5 9.950 300 1.00 0.90 270 30
10 7.720 465 0.75 0.68 405 60
15 6.520 590 0.67 0.60 540 50
20 5.600 675 0.63 0.56 675 0
25 4.980 750 0.60 0.54 810 -60
30 4.520 817 0.58 0.53 945 -128
35 4.080 861 0.57 0.51 1080 -219
40 3.740 902 0.56 0.51 1215 -313
45 3.460 939 0.56 0.50 1350 -411
50 3.230 973 0.55 0.50 1485 -512
55 3.030 1005 0.55 0.49 1620 -615
60 2.860 1034 0.54 0.49 1755 -721
65 2.720 1066 0.54 0.48 1890 -824
70 2.590 1093 0.54 0.48 2025 -932
75 2.480 1121 0.53 0.48 2160 -1039
80 2.380 1148 0.53 0.48 2295 -1147
85 2.290 1173 0.53 0.48 2430 -1257
90 2.210 1199 0.53 0.48 2565 -1366
95 2.130 1220 0.53 0.47 2700 -1480
100 2.060 1242 0.53 0.47 2835 -'1593
105 2.000 1266 0.52 0.47 2970 -1704
110 1.940 1286 0.52 0.47 3105 -1819
115 1.890 1310 0.52 0.47 3240 -1930
120 1.840 1331 0.52 0.47 3375 -2044
GRAVEL BED STORAGE CALCULATIONS
Required Gravel Volume Available Gravel Volume
Desired Stora e 60 ft3 Len th 22 ft
Gravel Porosity 0.4 Wide 3.5 ft
Gravel Volume Required 151 ft3 Depth 2 ft
Volume 154 ft3
Volume 5.7 cy
Densi 1.4 ard/ton
Weight 8.0 ton