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Home My WebLink AboutGrading Plans - 01/16/2024DRAINAGE ANALYSIS 608 WHEDBEE STREET FORT COLLINS, CO January 15, 2024 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 RE: 608 Whedbee Street Fort Collins, Colorado Permit No. B2309556 Dear Staff: BLJRNT HILL LLC Grading/Stormwater Plan Accepted (Building Permit Released) Accepted by: sagenbroad Date: O1/16/2024 Burnt Hill LLC is pleased to submit this Drainage Analysis for the 608 Whedbee St residential property in Fort Collins, Colorado. In general, this summary serves to document the stormwater impacts associated with the proposed site improvements associated with Permit No. B2309556. SITE LOCATION AND PROJECT DESCRIPTION The 608 Whedbee Street site (referred herein as "the site") is situated in the Northwest Quarter of Section 13, Township 7 North, Range 69 West of the 6th Principal Meridian to the City of Fort Collins, Larimer County, Colorado. More specifically, the property is recognized as Lot 14, Block 166 of the Fort Collins Subdivision. Proposed improvements to the existing property consist of the following: • Demolition of an existing garage and associated concrete and landscaping. • Construction of an 816-sf detached garage with associated concrete stoops, concrete apron, and landscaping. HYDROLOGIC CRITERIA City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, provided by Figure RA-16 of the Fort Collins Stormwater Criteria Manual, are utilized for all hydrologic computations related to the site in its existing/historic and proposed conditions. Since this site is relatively small and does not have complex drainage basins, the peak flow rates for design points have been calculated based on the Rational Method as described in the USDCM and the City of Fort Collins Stormwater Criteria Manual (FCSCM) with storm duration set equal to the time of concentration for each sub-basin. This method was used to analyze the developed runoff from the 2-year (minor) and the 100-year (major) storm events. The Rational Method is widely accepted for drainage design involving small drainage areas (less than 160 acres) and short time of concentrations. Runoff coefficients are assumed based on impervious area and are given in the calculations attached. EXISTING CONDITIONS Hydrology The site, located within the Old Town Drainage Basin, is 0.22-ac in size and currently consists of roofing, concrete pavement, and grass landscaping. For the purposes of this drainage analysis, a 0.15-acre area, encompassing the improvements noted above, is analyzed hydrologically. This drainage basin area, designated within this analysis as "Basin A", has an existing imperviousness of 25.1%. Storm runoff within Basin A currently drains by way of overland flow to an alley adjacent to the eastern property boundary. Floodplain The site is not located within a FEMA or City-regulatory floodway or floodplain. IMPROVED CONDITIONS Site improvements include an increase of approximately 678-sf of hard surface area relative to existing conditions, increasing the Basin A imperviousness to 34.9%. The table on the following page summarizes the hydrologic impact associated with the proposed site improvements relative to existing conditions. Refer to the hydrology calculations and Drainage Exhibit attached for additional information. WINDSOR, CO � 303 335-5508 DRAINAGE ANALYSIS 608 WHEDBEE STREET FORT COLLINS, CO HYDROLOGIC $UMMARY TABLE Overall Area (acre) *Roof (sf) *Asphalt (sf) *Concrete (sf) *Gravel (sf) *Landscape (sf) % Imperviousness Composite Cz Composite C�oo Qz (cfs) Qioo (cfs) Basin A Existing Proposed 0.15 0.15 1, 340 1, 666 320 672 4,790 25.1% 0.43 0.54 0.14 0.59 4,112 34.9% 0.50 0.63 0.16 0.69 BURNT HILL LLC Drainage patterns and runoff flows for the improved condition will remain largely unchanged from existing conditions; runoff that does not infiltrate within Basin A is designed to be conveyed to the adjacent alley. OLD TOWN ALLEY Similar to the nature of Whedbee St and Smith St, the shared alley adjacent to the property is graded to drain northerly to East Myrtle Street. An inlet at the intersection of E Myrtle St and Smith St is designed to capture runoff related to this shared alley. The alleyway section of interest upstream of the mentioned inlet exists with positive slope from south to north and is sufficient to convey runoff from the additional impervious area associated with the 608 Whedbee St garage (refer to the attachments for additional information). Provided the negligible increase in stormwater related to minor storm events (e.g., 2-year) and a minor increase in 100-year runoff, the site improvements provide an inconsequential increase of stormwater to the alley. Due to the location of the 608 Whedbee St property relative to E Myrtle St and the nature of the existing alleyway, stormwater runoff that fails to infiltrate from the site is adequately conveyed to E Myrtle St by way of the existing alleyway. CONCLUSIONS The proposed construction ofthe 608 Whedbee St garage and associated hardscape surfaces is intended to replace an existing garage near the eastern property boundary. The existing alleyway adjacent to the eastern property boundary is sufficient to convey developed stormwater runoff from the proposed improvements to the existing design point in E Myrtle St. A minor increase in the overall imperviousness is proposed with the site improvements; however, increased runoff is conveyed to E Myrtle St by way of the existing alley without impacting downstream properties. If you should have any questions or comments as you review this drainage analysis, please feel free to contact us at your convenience. Sincerely, Burnt Hill LLC �� . Colton Beck, P.E. ATTACH M E NTS - Hydrology Calculations - Drainage Exhibit - Alleyway Hydraulic Analysis Rp,00 L/� o �•<' �p�':�9 A Nj�7"' 111 �`_ -o � 11�,2021� ; � :� ��SS/........��G�- ONAL � WINDSOR, CO � 303 335-5508 Rl1NOFF C�EFFICIENTS AND % IMPERVIDl15 6�8 Whedbee St. Fort Collins, CO � �. Areas Composite Hard 5urface cn �p �p �q Imperviousness Area Roof Asphalt Conerete Gravel Lawns Total Total Composite Runoff Coeffieients �2� Basin Design Pt. acres %I = 90% %I =100% %I =10�°� %I =40% %1=Y°� (°�I) (sf) �z �wo C=0.95 C=0.95 C=0.95 C=0.5� C=U.25 sf sf sf sf sf sf A A 0.15 6,450 1,340 320 4,790 25.1% 1660 0.43 0.54 B B 0.07 3,050 850 517 1,683 43.1% 1367 0.56 0.70 Total 0.22 9,500 2,190 - 837 - 6,473 30.9% i i i • i � Areas Composite Hard Surface Roof �� Asphalt Conerete�� Gravel�� Lawns�� Imperviousness Area Basin Oesign Pt. Total Total Composite RunoH Coeffieients �2j acres sf %I = 9�% %I = I�0% %I =100% %I =40% % 1=2% (%p (sf) �s �roo C=U.95 C=0.95 C=U.95 C=0.50 C=0.25 sf sf sf sf sf sf A A 0.15 6,450 1,666 672 4,112 34.9% 2338 0.50 0.63 B B 0.07 3,050 850 517 1,683 43.1% 1367 0.56 0.70 Total 0.22 9,500 2,516 - 1,189 - 5,795 37.6% Difference in Hard Surface Area (Basin AJ: 678 Difference in Hard Surface Area (Basin BJ: 0 Notes: (1) Recommended % Imperviousness Values per Table 4.1-3 Surface Type - Percent Impervious in Fort Collins Stormwater Criteria Manual (2) Runoff C is based Table 3.2-2. Surface Type - Runoff Coefficients and Table 3.2-3. Frequency Adjustment Factors in For[ Collins Stormwater Manual TIME �F C�NCENTRATI�N (2-YR) 6�8 Whedbee 3t, Fort Collins, C� �Z�t; _[1.87(1.1-CxCF�Ll�z]/51�3, 5= slope in %, L=length of overland flow (200' max urban, 500' max rural) �3� V=(1.49/n)RZ�35'�Z, 5= slope in ft/ft, FCSCM Equation 5-4 �4�t�=L/(V*60 sec/min) �s� Maximum t� = total length/180 + 10 �6�Minimumt�=5 min Notes: "� C=Cx*CF is less than or equal to 1.0 (Cf = 1.0) TIME �F C�NCENTRATI�N (I0�-YR) 6�8 Whedhee St, Fort Collins, CO Notes: �1� G-Cx*CF is less than or equal to 1.0 (Cf = 1.25) �Z� t; _[1.87�1.1-CxCF)L'�Z]/51�3, 5= slope in %, L=length of overland flow (200' max urban, 500' max rural) �3�V=(1.49/n�RZ�351�Z, S= slope in ft/ft, FCSCM Equation 5-4 �4ftr L/(V*60sec/min) �5� Maximum t� _ [otal length/180 + 10 �6f Minimum t�= S min RATI�NAL METH�O PEAK RLINOFF 6�8 Whedhee St, Fort Collins, CO � � � �n PROPOSED BASIN DESIGNATION 2 - YR RUNOFF COEFF. 100 - YR RUNOFF COEFF. BASIN AREA (ACRE) FLOW DIRECTION BASIN BOUNDARY EXISTING GRaUND ELEVATION — 6 a, 6 9� �� — � � � � � w � J Z J Q �J W W � � � i � l 0� I � �f� g`l� �,�, I � � � � � � � � � b I �ti��+ �;''�, ` � I � � � � � � � � ;� � 3`��`l �� t I � I AI 608 WHEDBEE STREET o� �o� Zo� 40� � DRAINAGE EXHIBIT � SCALE: 1" = 20' 0 Z DATE: January 14, 2024 PREPARED FOR: BRAD BLANK JOB NUMBER: 624001 SHEET 1 �F 1 LEGEND E MYRTLE ST I R.O. W Channel Report Hydraflow Express Extension for Autodesk0 Civil 3D0 by Autodesk, Inc. Section A-A: 100-Year Event User-defined Highlighted Invert Elev (ft) = 81.98 Depth (ft) Slope (%) = 0.60 Q (cfs) N-Value = 0.025 Area (sqft) Velocity (ft/s) Calculations Wetted Perim (ft) Compute by: Known Q Crit Depth, Yc (ft) Known Q(cfs) = 0.69 Top Width (ft) EGL (ft) (Sta, EI, n)-(Sta, EI, n)... ( 0.00, 82.15)-(5.00, 81.98, 0.025)-(10.00, 81.98, 0.025)-(15.00, 82.05, 0.025)-(20.00, 82.08, 0.025) Sunday, Jan 14 2024 = 0.10 = 0.690 = 1.05 = 0.66 = 17.94 = 0.07 = 17.94 = 0.11 Elev (ft) Section 83.00 82.50 82.00 81.50 81.00 v Depth (ft) 1.02 0.52 1 1: -0.48 _n aQ 2 0 2 4 6 8 10 12 14 Sta (ft) 16 18 20 22 v Vv