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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. ����"�` � �_ �-= �. I`� �� � �=- �., � -� _,.' �- _� I � � �--E �'�- �`=E � �._�i` �, ��" ;= . _ � �O {-Li' ��L_l_t t�3 S �� �-_.� . �V � i �- �H-h`s;�ass � ta�s���'t"jbr,a c�FT s_�rs sHaw�� 4� o p�Es.1 bi�-�t-a la� '1���.c��rct�•t� i+.> c���r- c�.��k� ���� {z�cese.c�,� �_�d-c-s -a„s..... 1��c A._ �'�,d. 1..s2��.�,c�:::__ Fe,.s.._�'�'-v �e-e.��t<<- X ' ' r X . i i ,� , i . 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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