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Home My WebLink AboutReports - Drainage - 11/12/2025 DRAINAGE REPORT 250630 Fort Collins, CO BDR SUBMITTAL: 1 1 /12 /202 5 FOR 205 SOUTH WHITCOMB STREET ADU November 10, 2025 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 RE: Preliminary Drainage Report for ADU at 205 S. Whitcomb Street Dear Staff: Sanbell is pleased to submit this Preliminary Drainage Report for your review. This report accompanies the Basic Development Plan submittal for the proposed ADU at 205 S. Whitcomb Street. This report has been prepared in accordance with the Fort Collins Stormwater Criteria Manual (FCSCM) and serves to document the stormwater impacts associated with the proposed project. We understand that review by the City is to assure general compliance with standardized criteria contained in the FCSCM. If you should have any questions as you review this report, please feel free to contact us. Sincerely, SANBELL Charles Sonnier, PE Senior Engineer/Project Manager Whitcomb Accessory ADU November 2025 Page 1 Project No. 250630 November 2025 Project No. 250630 PRELIMINARY STORMWATER MANAGEMENT REPORT FOR 205 S. WHITCOMB STREET ADU FORT COLLINS, COLORADO I. Overview Narrative This project proposes an Accessory Dwelling Unit (ADU) associated with the single-family property located at 205 S. Whitcomb Street, Fort Collins, Colorado. The purpose of this report is to identify the potential drainage impacts to the site due to the proposed improvements. The design standards governing this project are the Fort Collins Stormwater Criteria Manual (FCSCM), Dec 2018. II. General Project Location The proposed development is approximately 0.2 acres located at the southwest corner of West Oak Street and South Whitcomb Street intersection (Parcel No. 9711314004). More specifically, the project site lies within the Loomis Addition Annex Subdivision, Lot 4, Block 269 in the southwest corner of Section 11, Township 7 North, Range 69 West. The site is generally bounded by an alley on the west, single-family lots (Parcel Nos. 9711314001 & 9711314002) on the north, South Whitcomb Street on the east and a single-family lot (Parcel No. 9711314005) on the south. The existing project site is a single- family residence with a garage/workshop and two sheds in the rear of the lot. Refer to Appendix A for a Vicinity map. III. Existing Site Information Major Basin Description This property is located within the Old Town Drainage Basin, which generally drains from west to east and receives some runoff water from the Canal Importation Basin directly west of Old Town. Most of the basin outfalls to the Poudre River drainage basin which is a major tributary to the South Platte River. Sub-Basin Description Historically, a majority of the site’s stormwater has sheet flowed to the north and south lot lines and then to the east to the South Whitcomb Street right-of-way. A minor northwest portion of the lot has historically flowed west to the existing alley and ultimately south to West Olive Street. See Appendix B for the existing drainage basin exhibit and stormwater flow patterns. This project site sits within the FEMA Zone X, Area of Minimal flood Hazard, per FEMA FIRM Maps 08069C0979H, dated May 2, 2012. The subject parcel does, however, lie within the City of Fort Collins Floodplain and is identified as Moderate-Risk within the Old Town floodplain at equal to- or less than 1- foot of ponding depth. See Appendix A for the City Floodplains Map and FEMA FIRMette Map. According to the NRCS website, the site consists of Fort Collins loam, 0 to 3 percent slopes classified as Type C soils. The soils have an erodibility factor of 0.28 to 0.43 which suggests a moderate susceptibility to sheet and rill erosion by runoff. Refer to Appendix A for soil data. Whitcomb Accessory ADU November 2025 Page 2 Project No. 250630 IV. Project Description The proposed project will consist of expanding the existing garage structure (that sits just west of the main house) 20 feet to the west and constructing an assessor dwelling unit (ADU) on top of the modified garage structure. The overall new building dimensions will be 20-feet x 24-feet with a new sidewalk, concrete apron for parking, and stairs and landing for access to the ADU. Refer to Appendix A for Site Plan. V. Drainage Design Criteria A. Regulations/Development Criteria The design standards governing this project are the Fort Collins Stormwater Criteria Manual (FCSCM), Dec 2018. B. Four-Step Process The overall stormwater management strategy to be employed with the proposed project will utilize the “Four Step Process” to minimize adverse impacts of urbanization on receiving waters. The following is a description of how the proposed project has incorporated each step. Step 1 – Employ Runoff Reduction Practices Certain techniques will be evaluated with the proposed development to facilitate the reduction of runoff peaks, volumes, and pollutant loads. The overall improvements will amount to less than 1,000 square feet of new impervious area and will not trigger Low Impact Development (LID) requirements. Stormwater is proposed to sheet flow across grassed buffers to lot line drainage before conveying to the street right-of-way MS4 regulated drainage facilities. Step 2 – Implement BMPs That Provide a Water Quality Capture Volume (WQCV) with Slow Release LID requirements will not apply to this project due to the limited increase and/or modification of impervious areas. Step 3 – Stabilize Streams There are no major wet drainage conveyances within the subject property. Step 4 – Implement Site Specific and Other Source Control BMPs Due to limited lot space, grassed buffers and swales will be the only practical method for BMPs. The proposed project will result in a minimal increase to peak flows and intends to match the historic runoff patterns to the extent possible. C. Hydrological Criteria 1. The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in Figure 3.4-1 of the FCSCM, serve as the source for all hydrologic computations associated with the proposed development. Tabulated data contained in Table 3.4-1 in the FCSCM has been utilized for Rational Method runoff calculations. Refer to Appendix B for criteria used. Whitcomb Accessory ADU November 2025 Page 3 Project No. 250630 2. The Rational Method will be utilized to compute stormwater runoff utilizing coefficients and calculation methods contained in Chapter 5, Section 3.2 of the FCSCM. 3. Two separate design storms will be utilized to address distinct drainage scenarios. The first design storm event analyzed was the “Minor,” or “Initial” Storm, which has a 2-year recurrence interval. The second event considered is the “Major Storm,” which has a 100-year recurrence interval. 4. The project site falls within the Old Town Basin Master Drainage Plan. The four step Stormwater Basin Master Planning Process and peak flowrates, flow depths and impacts to the city floodplain have been evaluated ensure conformance. 5. No other assumptions or calculation methods have been used with this development that are not referenced by current City of Fort Collins criteria. D. Hydraulic Criteria 1. The major storm event was analyzed to verify that it can safely convey through the site without inundating adjacent structures. No stormwater control facilities (surface or sub-surface) are proposed for this project as the proposed improvements will have an insignificant impact on the existing site and will continue to drain as it has historically. 2. All stormwater calculations for the site are in accordance with criteria outlined in the FCSCM. Refer to Appendix B for Rational Method calculation. E. Variances and Modifications of Criteria 1. The proposed development is not requesting any variances or modifications to criteria at this time. VI. Proposed Drainage Facilities A. General Concept The intent of the proposed design is to safely and adequately convey the developed minor and major design storm events given the site’s existing constraints. 1. The project proposes minimal increases to impervious surfaces and therefore will result in insignificant increases to peak runoff rates. Proposed downspouts will be located on both the north and south sides of the new building, as they currently are in existing conditions, and discharge to the surface via splash blocks. Proposed grading will convey runoff away from the building foundation and sheet flow across grassed areas to common lot line drainage at the north and south of the property before conveying east to the South Whitcomb right-of-way. The intent is to match existing condition drainage patterns to the extent possible and provide a safe conveyance for runoff while providing basic water quality through grassed buffer treatments. Whitcomb Accessory ADU November 2025 Page 4 Project No. 250630 B. Specific Basin Details The developed storm water flows will travel via sheet flow to the north and south lot line drainage and then to the South Whitcomb right-of-way as it has historically. The four (4) sub- basins contributing to the site and roadway runoff are described as follows: Sub-Basin No. 1 (SB-1): This basin totals approximately 870 square feet (0.02 acres) and is located at the back of the lot, which includes a small portion of the existing Arthur Ditch (Irrigation concrete box culvert crossing) and existing gravel surface drive alley. No proposed improvements are proposed within this basin, and runoff from this basin will flow as it has historically to the alley and south toward West Olive Street. Sub-Basin No. 2 (SB-2): This basin totals approximately 3,485 square feet (0.08 acres) and is comprised of mainly grassed yard area with a portion that includes Arthur Ditch (irrigation concrete box culvert crossing). A new sidewalk and landing to serve the proposed ADU and roof drainage from the north half of the ADU also lies within this basin. Proposed lot grading will convey runoff via sheet flow from this basin towards the north lot line and into existing lot line drainage that will convey runoff toward and into South Whitcomb Street right-of-way as it has under existing conditions. Sub-Basin No. 3 (SB-3): This basin totals approximately 3,050 square feet (0.07 acres) and is located on the northeast portion of the lot, which includes the north roof drainage of the existing single-family residence along with the existing concrete pavers and trench drain to the rear the existing house. No improvements are proposed within this basin, and runoff from this basin will flow to the north lot line and east to the South Whitcomb Street right-of-way, as it has under existing conditions. Sub-Basin No. 4 (SB-4): This basin totals approximately 3,920 square feet (0.09 acres) and is located on the southeast portion of the lot, which includes the south roof drainage and existing concrete patio, sidewalk, and driveway associated with the existing single-family residence. The proposed gravel and concrete pads and north half of the roof drainage associated with the proposed ADU also lie within this basin. Runoff from this basin will flow to the south lot line and east to South Whitcomb Street right-of- way. C. Basin Runoff Results Table 1 below summarizes the peak runoff rates for the minor and major design storm events under both developed and existing conditions. The table also shows the addition and reduction of the peak runoff rates associated with each design point. Refer to the Proposed Basin Map in Appendix B for Design Point locations. Table 1. Existing Basin Developed Basin Change in Flowrates for 100-yr Event Basin Design Points Q 2-year (cfs) Q 100-year (cfs) Q 2-year (cfs) Q 100-year (cfs) Q 100-Year (cfs) Design Pnt. 1 0.03 0.13 0.03 0.13 0.0 Design Pnt. 2 0.09 0.40 0.12 0.53 +0.13 Design Pnt. 3 0.15 0.66 0.13 0.55 -0.11 Design Pnt. 4 0.13 0.55 0.17 0.76 +0.21 Total: 0.23 Whitcomb Accessory ADU November 2025 Page 5 Project No. 250630 As the table above shows, the increases to the 100-year peak flowrates within each sub-basin vary from no change to up to 0.21 cfs. The main conveyance within sub-basin SB-2 is anticipated to sufficiently carry the 100-year developed flowrate of 0.53 cfs to Design Point 2 (DP-2) which equates to approximately 0.13 feet (or approx. 1.6”) of surcharge depth given the proposed grading (See Appendix B for SB-2 conveyance calculation). When adding DP-2 and DP-3 together, there is essentially no change in flowrates along the north lot line and to the South Whitcomb right-of-way as the flowrate was reduced in sub-basin SB-3 due to a reduction in basin area. The largest increase in flowrate, 0.21 cfs, takes place in sub-basin SB-4 that flows to DP-4 resulting in a flow depth increase of approximately 0.02’ (or approx. a quarter of an inch). Final developed flowrates to the South Whitcomb Street right-of-way are the summation of DP-3 and DP-4 with an increased flow depth of 0.01’ (or approx. one eighth inch). The above results show there will be a minimal increase in flowrates and depths and therefore will have an insignificant impact to the onsite and adjacent conveyances as well as to the city floodplain. Refer to Appendix B for rational calculations. D. Low Impact Development (LID) LID requirements will not be required for this project as the newly added and/or modified impervious area did not exceed 1,000 square-feet (See Appendix B for Existing and Proposed Impervious Areas exhibit and calculations). VII. Erosion and Sediment Control Erosion and sediment control will be achieved through temporary construction control measures to include various BMPs selected from the FCSCM to include such items as construction fencing, sediment control logs, vehicle tracking devices, rock socks, inlet/outlet protection, concrete washout area, stockpile management and stabilized staging area to prevent pollutants from leaving the construction site. Permanent seeding/sodding and installation of hardscapes will be used to stabilize the site. VIII. Conclusion The proposed drainage design is intended to provide a safe conveyance of the minor and major storm events while providing basic water quality treatment prior to entering adjacent street conveyances and MS4 regulated systems. The proposed project will add minimal impervious areas that will result in insignificant increases to peak runoff rates and flow depths. The Old Town Basin Master Drainage Plan was also considered and determined that the project is in conformance regarding release rates, ponding depths, and basic water quality standards. The City’s floodplain will also not be negatively impacted. A. Compliance with Standards 1. The drainage design proposed with the proposed project complies with the City of Fort Collins’ Stormwater Criteria Manual. 2. The drainage design proposed with this project conforms to the design of the Old Town Basin Master Drainage Plan. Whitcomb Accessory ADU November 2025 Page 6 Project No. 250630 3. The drainage design proposed with this development is compliant with all applicable State and Federal regulations governing stormwater discharge. 4. Any erosion control materials that will be used will comply with the requirements set forth by the City of Fort Collins’ Stormwater Criteria Manual. B. Drainage Design 1. The drainage design proposed with this project will safely convey developed runoff away from existing and proposed structures with insignificant impacts to surrounding conveyances. Whitcomb Accessory ADU November 2025 Page 7 Project No. 250630 REFERENCES 1. City of Fort Collins, December 2018, Fort Collins Stormwater Criteria Manual. 2. United States Department of Agriculture Natural Resources Conservation Service; Web Soil Survey; accessed on-line October 2025. 3. Federal Emergency Management Agency; FEMA Flood Map Service Center; accessed on-line October 2025. 4. Stormwater Basin and Floodplain Master Planning; City of Fort Collins; accessed on-line October 2025. APPENDICES 1. Appendix A - Site Information a. Vicinity Map b. Site Plan c. NRCS Soils Data d. FIRMette Map e. Fort Collins Floodplain Map 2. Appendix B – Criteria, Calculations, and Maps a. Storm Criteria b. Existing and Proposed Drainage Basin Maps c. Rational Calculations d. Conveyance Calculations e. Impervious Areas Exhibit and Calculations 205 SOUTH WHITCOMB STREET ADU – FORT COLLINS , CO 250630 A P P E N D I X A S I T E I N F O R M A T I O N VICINIT Y M AP 205 S W HITCOMB ACCESSO RY ADU N ➤➤ N Image © 2025 A ir busImage © 2025 A irbus SITE PR E L I M I N A R Y - F O R R E V I E W FI L E : PR O J E C T N O : CA D : QU A L I T Y A S S U R A N C E : DR A W I N G H I S T O R Y DA T E D E S C R I P T I O N SI T E P L A N SI T E P L A N S F O R 2 0 5 S . W H I T C O M B S T . A D U LO T 4 , B L O C K 2 6 9 , L O O M I S A D D I T I O N FO R T C O L L I N S , C O 8 0 5 2 1 C2.2 CS 25 0 6 3 0 _ S I T E P L A N . 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" +", -./012345678539:.;<:==3>?-3@01A23B3-C1D3@C==.;E3 F F 010 SCALE:1" = 20' 2010 WHITCOMB ACCESSORY ADU C5 C BASIN # AREA P: \ 2 5 0 6 3 0 _ W h i t c o m b _ S t _ A D U \ S T O R M W A T E R \ D W G \ 2 5 0 6 3 0 _ D R A I N A G E _ B A S E . d w g , 1 1 x 1 7 L A N D S C A P E , 1 1 / 5 / 2 0 2 5 1 : 5 4 : 5 5 P M , R s t r a u s , 1 : 1 WHITCOMB ACCESSORY ADU 010 SCALE:1" = 20' 2010 C5 C BASIN # AREA P: \ 2 5 0 6 3 0 _ W h i t c o m b _ S t _ A D U \ S T O R M W A T E R \ D W G \ 2 5 0 6 3 0 _ P R O _ D R A I N A G E _ B A S E . d w g , 1 1 x 1 7 L A N D S C A P E , 1 1 / 5 / 2 0 2 5 1 : 5 2 : 5 1 P M , R s t r a u s , 1 : 1 DEVELOPED BASIN COMPOSITE RUNOFF COEFFICIENT & PERCENT IMPERVIOUSNESS SB-1 ALLEY & DITCH BOX C 0.02 0.01 0.02 0.04 0.57 0.57 0.72 0.57 SB-2 BACKYARD & NORTH ADU C 0.02 - 0.04 0.06 0.47 0.47 0.58 0.47 SB-3 NORTH SIDE YARD C 0.04 - 0.03 0.07 0.63 0.63 0.78 0.63 SB-4 SOUTH SIDE YARD, FRONTAGE, & SOUTH ADU C 0.05 0.00 0.04 0.10 0.64 0.64 0.80 0.64 0.27 0.59 0.59 0.73 0.59 EXISTING BASIN COMPOSITE RUNOFF COEFFICIENT & PERCENT IMPERVIOUSNESS EX-1 ALLEY & DITCH BOX C 0.02 0.01 0.02 0.04 0.57 0.57 0.72 0.57 EX-2 BACKYARD C 0.01 - 0.05 0.05 0.32 0.32 0.40 0.32 EX-3 NORTH SIDE YARD C 0.04 - 0.04 0.08 0.62 0.62 0.78 0.62 EX-4 SOUTH SIDE YARD & FRONTAGE C 0.04 - 0.06 0.09 0.52 0.52 0.65 0.52 0.27 0.52 0.52 0.65 0.52 Paved/Concrete/ Roofs 0.95 (acres) Total Area (acres) Composite Runoff CoefficientC100 Basin Name Basin Name Soil Type C10 C10 C2 Gravel 0.50 (acres) Landscape 0.25 (acres)Soil Type TOTAL TOTAL Basin Description Basin Description C2 C100 Paved/Concrete/ Roofs 0.95 (acres) Total Area (acres) Landscape 0.25 (acres) Gravel 0.50 (acres) Composite Runoff Coefficient 10/27/2025 DEVELOPED BASIN RUNOFF CALCULATIONS SB-1 ALLEY & DITCH BOX 0.04 0.57 0.72 5.00 2.85 9.95 0.07 0.32 SB-2 BACKYARD & NORTH ADU 0.06 0.47 0.58 5.00 2.85 9.95 0.08 0.34 SB-3 NORTH SIDE YARD 0.07 0.63 0.78 5.00 2.85 9.95 0.12 0.51 SB-4 SOUTH SIDE YARD, FRONTAGE, & SOUTH ADU 0.10 0.64 0.80 5.00 2.85 9.95 0.18 0.80 EXISTING BASIN RUNOFF CALCULATIONS EX-1 ALLEY & DITCH BOX 0.04 0.57 0.72 5.00 2.85 9.95 0.07 0.32 EX-2 BACKYARD 0.05 0.32 0.40 5.00 2.85 9.95 0.05 0.21 EX-3 NORTH SIDE YARD 0.08 0.62 0.78 5.00 2.85 9.95 0.14 0.62 EX-4 SOUTH SIDE YARD & FRONTAGE 0.09 0.52 0.65 5.00 2.85 9.95 0.13 0.59 C100 Sub-basin I 2 (in/hr) Q 100-yr (cfs) Area (acres) Intensities Basin Name Sub-basin Basin Name Description Area (acres) I 100 (in/hr) Tc* (min) Basin Characteristics C2 * If time of concentration was less than 5 minutes, 5 minutes was used. Description C2 Q 2-yr (cfs) I 100 (in/hr) Q 2-yr (cfs) Q 100-yr (cfs) Basin Characteristics Intensities C100 Tc* (min) I 2 (in/hr) Storm Rational Calcs.xls Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Monday, Nov 10 2025 Main Conveyance within SB-2 Trapezoidal Bottom Width (ft) = 3.00 Side Slopes (z:1) = 66.00, 66.00 Total Depth (ft) = 0.20 Invert Elev (ft) = 1.00 Slope (%) = 0.28 N-Value = 0.035 Calculations Compute by: Known Q Known Q (cfs) = 0.53 Highlighted Depth (ft) = 0.13 Q (cfs) = 0.530 Area (sqft) = 1.51 Velocity (ft/s) = 0.35 Wetted Perim (ft) = 20.16 Crit Depth, Yc (ft) = 0.07 Top Width (ft) = 20.16 EGL (ft) = 0.13 0 5 10 15 20 25 30 35 40 Elev (ft) Depth (ft)Section 0.75 -0.25 1.00 0.00 1.25 0.25 1.50 0.50 1.75 0.75 2.00 1.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Tuesday, Nov 11 2025 North Lot Line SB-2 + SB-3 Triangular Side Slopes (z:1) = 50.00, 50.00 Total Depth (ft) = 0.25 Invert Elev (ft) = 1.00 Slope (%) = 0.85 N-Value = 0.035 Calculations Compute by: Known Q Known Q (cfs) = 1.08 Highlighted Depth (ft) = 0.17 Q (cfs) = 1.080 Area (sqft) = 1.45 Velocity (ft/s) = 0.75 Wetted Perim (ft) = 17.00 Crit Depth, Yc (ft) = 0.13 Top Width (ft) = 17.00 EGL (ft) = 0.18 0 5 10 15 20 25 30 35 Elev (ft) Depth (ft)Section 0.75 -0.25 1.00 0.00 1.25 0.25 1.50 0.50 1.75 0.75 2.00 1.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Tuesday, Nov 11 2025 South Lot Line SB-4 Triangular Side Slopes (z:1) = 50.00, 50.00 Total Depth (ft) = 0.25 Invert Elev (ft) = 1.00 Slope (%) = 0.85 N-Value = 0.035 Calculations Compute by: Known Q Known Q (cfs) = 0.76 Highlighted Depth (ft) = 0.15 Q (cfs) = 0.760 Area (sqft) = 1.13 Velocity (ft/s) = 0.68 Wetted Perim (ft) = 15.00 Crit Depth, Yc (ft) = 0.11 Top Width (ft) = 15.00 EGL (ft) = 0.16 0 5 10 15 20 25 30 35 Elev (ft) Depth (ft)Section 0.75 -0.25 1.00 0.00 1.25 0.25 1.50 0.50 1.75 0.75 2.00 1.00 Reach (ft) 205 S. WHITCOMB STREET - EXISTING IMPERVIOUS AREAS 010 SCALE:1" = 20' 2010 P: \ 2 5 0 6 3 0 _ W h i t c o m b _ S t _ A D U \ C A D D _ C 3 D \ R E F _ D W G \ W O R K I N G \ 2 5 0 6 3 0 _ S I T E _ B A S E _ R F S . d w g , 1 1 x 1 7 L A N D S C A P E , 1 1 / 1 1 / 2 0 2 5 2 : 4 5 : 2 3 P M , R s t r a u s , 1 : 1 205 S. WHITCOMB STREET - PROPOSED IMPERVIOUS AREAS 010 SCALE:1" = 20' 2010 P: \ 2 5 0 6 3 0 _ W h i t c o m b _ S t _ A D U \ C A D D _ C 3 D \ R E F _ D W G \ W O R K I N G \ 2 5 0 6 3 0 _ S I T E _ B A S E _ R F S . d w g , 1 1 x 1 7 L A N D S C A P E ( 2 ) , 1 1 / 1 1 / 2 0 2 5 2 : 4 5 : 2 4 P M , R s t r a u s , 1 : 1