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HomeMy WebLinkAboutGrading Plans - 07/16/2021��� ENGINEERiNG a, � � . � �, � - � f���'Sf. . � ' t '! R � .. ' 1 •. - �o _ _ June 14, 2021 Mr. Wes Lamarque City of Fort Collins Stormwater Department 700 Wood Street Fort Collins, CO 80521 (970) 416-2418 Re: Drainage Memorandum for 119 Shields Street Dear Mr. Lamarque, Grading/Stormwater Plan Accepted (Building Permit Released) Accepted by: Matt Simpson Date: 07/16/2021 119 N Shields Street Grandview Lot 9 Block A B2102847 - addition B2101300 - garage The project site is located 1 l9 Shields Street, Fort Collins, Colorado, and shown as Lot 9, Block "A" Grandview Addition to the City of Fort Collins, consisting of approximately 032 +/- acres. The proposed improvements for the 119 Shields Street project will occur over approximately 0.11 acres of the 0.32 acre site and will entail a new addition to the e�isting residence, a detached garage, minor re-grading of the existing gravel driveway area, a new concrete trickle pan added along tlle south fence to convey rtuloff east and out to Shields Street, as historical(y occurs, and a new gravel driveway extension west and to the proposed detached garage. The proposed improvements will add approximately: 2,017 square feet of new roof area (for home addition and detaclled garage); 1,716 square feet of new gt•avel area (1,340 sf for driveway area, 376 sf for patio area); and 372 square feet of concrete tricl:le pan area. The proposed improvements will ceplace e�isting grass, gt•avel, and dirt areas. The existing drainage pattern foc the majority of the lot is from the northwest to southeast, with runoff flowing east and out to Shields St�•eet. A portion of the lot, along the northern boundary of the property, flows east along the north propei-ty line and out to Shields Street. The eYisting lot is relatively flat with grades ranging from approxiinately 0.2% to 3%. Proposed grading improvements, internal to the site and within the improvement area, will provide for grades ranging from 0.5% to 2.7%. Historic drainage patterns and flows along the northern and southern property boundaries will continue to be discharged out to Shields Street, as historically occurs. The proposed grading nnprovements and proposed concrete trickle pan along the so��th boundary will improve eYistilig conditions by providing for a continuous positive conveyance of attributable runoff east and out to Shields St�-eet. Grading along and near the northein property line will be veiy minin7al, «�ith the exception of grading neYt to the new addition's foundation and against the north side of the detached garage to provide ior positive conveyance of iunoff away from the new structures, and then east and out to Shields Street. Aspen has modeled the existing and proposed conditions of the site, with rational calculations attached to this memorandum. Please refer to the attached rational calculations for impervious calculations and flow data. The existing imperviousness of the 0.32 acre lot is approaimately 17%, with e�isting raiional 2-yeac and 100- vear flows of 0.11 and 0.47 cfs, respectively. Tlie proposed improvements will provide for the 032 ac�•e lot having an imperviousness of approximately 48%, with rational2-year and 100-year tlows being 0.37 and 1.79 cfis. respectively. The rational `c' value for the existing and proposed conditions are 0.20 and 0.56, respectively. It's important to note that the proposed rational calculations provide for the entire elisting gravel area and new gravel area for the driveway exteiision and patio area bellig modeled as eonerete, to allow the o��ners to pave theu- driveway and patio in concrete ar other like sut•face, now or in the future. Aspen understands the owners plan to only install gravel for tl�ese areas at this time. Roof drain downspouts will discharge to green/landscape areas, thus pcoviding LID treatment for the project. Runoff from majoc stonn events, that could exceed the landscaping a»d green areas, will follow historic runoff patterns to the north and south property boundaries and then east and out to Shields Street. Considering the draina;e masterplan for this area, it appears that even with the proposed improvements, the impervjousness of 48% is less than that allowed for the lot. Therefore, the proposed improvements appear to be in conformance with the maste�plan and Aspen does not anticipate any adverse impacts to the esisting sh-eet's capacity or to other adjacent oi• downstream properties, due to the �ninor flow increase fi�om the lot improvements. Thank you for your time and consideration in reviewing this addendum. If you have any further questions, please feel free to contact me. subbasin HW B150 Sincerely, /�.,' �t � 8 � ssc•tti. .�r� JoHry GoocH, PE ASPEN ETIGINEERING O\;o�p,�0 ��� . FtYAN ,�s C.� ,�\' �p :, �� .� o � n : .�: 2 � 2'� o,�• �, �- ��I' " �I •' �v ��S��NAL E���� Old Town Drainage Basin Imperviousness = 50 % ao o� a o m � � H �+ i a� .� � d o� U W w � O � C� � � Q O y LLI� L � QI •� � � P � i :� � .% w � 0 0 r r rn rn M M � 6� M (�] � � 0 0 N N � n � � � � M M � �� N N M M � � .� � W N F 7 � ' X � Z � � W .�9 � O G. 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