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HomeMy WebLinkAboutGrading Plans - 01/06/2020FORBES ENGINEERING Full Service Civil & Structural Engineering January 3, 2020 Dan Mogen City of Fort Collins Stormwater Engineering �oa woo� s�-P�r Fort Collins, CO 80521 RE: Grading & Drainage Memo 1124 West Maple Street Fort Collins, CO 80521 Parcel #971 C11-09-019 Mr. Mogen, Lot 4- COM 344 FT W OF SE COR OF LOT 4, TENNYSON HTS, FTC, TH N 100 FT, W 50 FT, S 100 FT, E 50 FT TPOB (SPLIT FR 97101-09-013) Tennyson Heights After-fact-drainage memo accepted 1/6/2020. DJM This letter and attached e�ibit are to serve as a drainage memorandum for the above referenced sin�le famil_y residential propert�. The 5000 square foot property is located at the Northeast corner of Pearl Street and Maple Street. Both streets have an attached sidewalk and roll-over �utter system. The lot was originally part of Parcel 97101-09-013 (Lot 4, Tennyson Heights Subdivision) and vacant until the recent site improvements. The surrounding nei�,hborhood is comprised of mature single family residential lots. Project Overview Site improvements include the canstruction of a 1754 square foot two-story home along with approximately 650 sq_uare feet of concrete walks and driveway. Existing Site & Drainage As previously mentioned, the previousl� e�sting site was a vacant lot with no improvements or utilities. The undeveloped lot had a slight slope from the high point at the southwest corner to the northeast corner (approximately 1% grade); this is a common corner for the property with two other properties (1120 Maple Street and 312 Pearl Street). A very small portion of the site flowed into the gutter system of Pearl Street and Maple Street. The estimated stormwater runoff for the un-improved lot has been calculated at 0.01 cfs (cubic feet per second) for the minor 2-year storm and 0.30 cfs for the major 100-year storm. Stormwater drainage from the exit point travels towards the east between the two mentioned nei�hboring properties, adds to the surface drainage within the block and ultimately exits onto Hanna Street which then flows into the south gutter system of Sycamore Street. Proposed Site & Drainage The site improvements include approximately 2100 square feet of roof surface and 650 sf of concrete which increases the imperviousness of the site from 0% to 72%. The home is constructed with a single slope roof system that sheds run-off towards the west and south using lower eave gutters and downspouts with extensions. This has changed the direction of the stormwater run-off and divided it up into two sub- basins. The first being on the south side that takes approximately one third of the roof and 20% of the site towards the south and the second which is approximately two thirds of the roof, most of the concrete and approximately 70% of the site and is directed towards the west. The south portion that flows into the north gutter system of West Maple Street has been estimated at 0.05 cfs for the minor 2-year storm and 0.25 cfs for the major 100-�ear storm. This travels east and into the west gutter system of Shields Street. The north portion of the site flows into the east �utter s�stem of Pearl Street and has been estimated at 0.11 cfs for the minor storm and 0.41 cfs for the major storm. This then flows north to the south gutter s�stem of Sycamore Street. A ver_y small amount of the site (662 s� continues to flow towards the northeast corner of the site and exits in the same direction as the historic flow, this generates an insi�nificant amount of stormwater run-off. The as-built grading plan and spot elevations are based on survey data provided by Stewart and Associates, Inc. dated December 12, 2019. Conclusion With the increase in impervious surfaces stormwater run-off increases but with the redirection of the run-off into the existing street gutter systems, the impact to the neighboring properties is greatly lowered. No adverse effects to the site or to the surroundin� properties are anticipated from the improvements to this site. Sincerely, f�� '� W v R 4 4 ," �, ��. F�j y . �, � / �'�.Y�-,�'� • �'�`�� ��a�D, � ' � �-- ,— r-, ,��`- - � (;�� ', � ' � � . ` ,� Forbes, �' . #35265 �1�� � :�, � � ! :�� , �, � AS�-BUILT �RADING PLAN WEST 50.00' OF LOT 4, TENNYSON HEIGHTS SUBDMSlON FORT COLLINS, COLORAQO 1124 WEST MAPLE STREET 96 �"�. � � � � � �J � .� : � 0 N � � N.P. WEST M�4PLE STREET �+—+ nres��a'isw 50.00� �--�-- � SCAL£ — i "=20' DECEMBER 12fh, 20 �9 On December 12, 2D 19, a field survey was performed by me, or under my dir�ct supervision, fhof ineasured e/evations of fhe existing site gr+oding and f/oor e/eva�ions, and thof these measurements are cccumtely shown on fhis As—Bu1/f Grodrng Plan. � +�. 7859 g•� � . �2/r2/�s,: h�`'�c ua+�� ifNN—GR0�7c/Stewarf & Assxiates, tnc.