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HomeMy WebLinkAboutGrading Plans - 09/05/2025 16911 Potts Place, Mead, CO 970.217.9148 troy@i2-consultants.com September 4, 2025 Mr. Dan Mogen Development Review Engineer City of Fort Collins Utilities 222 Laporte Avenue Fort Collins, CO 80524 RE: Drainage and Erosion Control Report for 516 Grant Avenue, Fort Collins, CO Dear Mr. Mogen: We are pleased to present this Drainage and Erosion Control Report for 516 Grant Avenue, Fort Collins, CO for your review and approval. This report has been prepared in accordance with the Fort Collins Stormwater Criteria Manual, December 2018. We look forward to your favorable review of this report. Please do not hesitate to contact our office if you have any questions. Sincerely, i2 Consultants, Inc. Troy W. Campbell, PE Senior Project Engineer ENGINEER’S CERTIFICATION I hereby certify that this report for the final drainage design for 516 Grant Avenue, Fort Collins, CO was prepared by me or under my direct supervision, in accordance with the provisions of the Fort Collins Storm Drainage Criteria Manual. I understand that the City of Fort Collins does not and will not assume liability for drainage facilities designed by others. __________________________________________ Registered Professional Engineer State of Colorado No. 41159 (Seal) 09/04/2025 Grading/Stormwater Plan Accepted (Building Permit Released) Accepted by: Date: Dan Mogen 09/05/2025 LOTS 21 AND 22, BLK 285 West Side permit B2504627 & future garage/ADU Drainage Report 516 Grant Avenue, Fort Collins, CO September 4, 2025 Page 2 i2 Consultants, Inc. Drainage Report 516 Grant Avenue, Fort Collins, CO Lots 21 and 22, Block 285, West Side Addition Located within the Northwest Quarter of Section 11, Township 7 North, Range 69 West of the 6th P.M. City of Fort Collins, County of Larimer, State of Colorado TABLE OF CONTENTS Page I. GENERAL LOCATION AND DESCRIPTION 3 A. Site Location B. Description of Property II. DRAINAGE BASINS AND SUB-BASINS 3 A. Major Basin B. Sub-basin and Site Drainage III. DRAINAGE DESIGN CRITERIA 4 A. Regulations B. Development Criteria Reference and Constraints C. Hydrological Criteria IV. DRAINAGE FACILITY DESIGN 5 A. General Concept B. Specific Details V. SEDIMENT, EROSION CONTROL AND STORMWATER QUALITY 7 A. Temporary Sediment and Erosion Control B. Permanent Water Quality BMPs VI. CONCLUSIONS 8 A. Compliance with Standards B. Drainage Concept VII. REFERENCES 9 VIII. APPENDICIES 10 Drainage Report 516 Grant Avenue, Fort Collins, CO September 4, 2025 Page 3 i2 Consultants, Inc. I. GENERAL LOCATION AND DESCRIPTION A. Site Location: The subject site is located in the northwest quarter of Section 11, Township 7 North, Range 69 West, of the 6th Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado. Furthermore, the project is north of Sycamore Street, on the east side of Grant Avenue. The subject site can be described as Lots 21 and 22, Block 285, West Side Addition to the City of Fort Collins. The subject site currently contains a single-family dwelling, out-building, concrete walks, flagstone patio and landscaping. B. Description of Property: The subject site lot consists of roughly 0.25 acres. The subject site currently contains a single-family dwelling, out-building, concrete walks, flagstone patio and landscaping. The existing improvements will be removed as part of the redevelopment of the property. The subject site generally slopes from east to west and west to east at varying slopes from 0% to 1%. The western portion of the existing site drains east to west to Grant Avenue. The remainder of the lot flows west to east and also to the north towards the adjacent property (520 Grant Avenue) and the existing alley. The existing curb and gutter for Grant Avenue slopes from south to north. Based on existing site topography, it appears the eastern flowline for Grant Avenue contains a series of low points, but runoff is directed to the north. According to the Natural Resources Conversation Service (NRCS) Web Soil Survey, the native soil in this area is Paoli Fine Sandy Loam, 0 to 1 percent slopes (Map Unit Symbol #81), which is a Type A soil. Type A soils are typically well- draining soils. Please refer to the NRCS soils information in the Appendix. Existing drainage facilities near or adjacent to the site include existing curb & gutter along Grant Avenue. II. DRAINAGE BASINS AND SUB-BASINS A. Major Basin: This site is located within the Old Town Basin as shown on the City of Fort Collins Master Drainage Plan. This basin historically generally flows from west to east. Drainage Report 516 Grant Avenue, Fort Collins, CO September 4, 2025 Page 4 i2 Consultants, Inc. The site is shown on FEMA panel 08069C0977G, effective 06/17/2008. The subject site is located with Zone X of the FEMA panel. B. Sub-basin and Site Drainage: The subject site generally slopes from east to west and west to east at varying slopes from 0% to 1%. Refer to Section I.B. above for additional information regarding the existing site. It is i2’s understanding that the subject residential site has not been studied previously in a site-specific drainage study or report. Based on existing site topography, it appears there is minimal runoff that enters the subject site from the south property (existing residence) and alley. This runoff will be conveyed through the subject site via proposed onsite swales, similar to the existing condition. The runoff will be conveyed to Grant Avenue right-of-way, proposed detention pond. The drainage patterns proposed with the subject improvements are generally consistent with the existing conditions and seek to improve the current drainage for the subject property. III. DRAINAGE DESIGN CRITERIA A. Regulations: This report has been prepared in general accordance with the Fort Collins Stormwater Criteria Manual, December 2018. B. Development Criteria Reference and Constraints: The subject site is constrained by existing conditions surrounding the site, such as the developed adjacent existing lots with flat slopes and the subject site containing flat slopes as well. Furthermore, the subject site is constrained by the existing lot lines with minimal to no slope and the elevations in the existing alley to the east. The subject site will drain to the existing Grant Avenue right-of-way and flow north and also flow to a proposed detention pond located at the low point along the northern property line. This report has been prepared in general accordance with the appropriate City criteria. Drainage Report 516 Grant Avenue, Fort Collins, CO September 4, 2025 Page 5 i2 Consultants, Inc. C. Hydrological Criteria: Fort Collins Stormwater Criteria Manual, December 2018. There are no deviations from the criteria in this report. The design storms for this project are the 2-year and the 100-year recurrence intervals. Runoff is calculated using the Rational Method since all basins are smaller than 5 acres. The Rational Method calculations are computed by the following: Q = ciA where: Q = Runoff in cubic feet per second c = Calculated runoff coefficient for design event i = Storm intensity in inches/hour A = Basin area in acres Hydrological calculations have been prepared in accordance with the Fort Collins Stormwater Criteria Manual, December 2018. IV. DRAINAGE FACILITY DESIGN A. General Concept: The drainage patterns associated with the subject single-family residence improvements are generally consistent with the existing drainage patterns and flow. The proposed site grading generally directs runoff in a pattern consistent with the current existing drainage patterns. The estimated runoff for the site was determined using the Rational Method, as detailed above. The subject site has been divided into two (2) drainage basins. The basins have been created to direct runoff towards existing conveyances or to follow existing drainage patterns. B. Specific Details: EXISTING BASINS Basin EXA Basin EXA (0.053 acres) is in the western portion of the property. The existing basin currently contains a portion of the single-family residence, concrete walks and landscaping. The existing improvements will be removed as part of the redevelopment of the property. Drainage Report 516 Grant Avenue, Fort Collins, CO September 4, 2025 Page 6 i2 Consultants, Inc. Basin EXB Basin EXB (0.204 acres) is in the eastern portion of the site and comprises the majority of the lot. This basin currently contains a portion of the single-family residence, out-building, concrete walks, flagstone patio and landscaping. The existing improvements will be removed as part of the redevelopment of the property. PROPOSED BASINS Basin A Basin A (0.094 acres) is comprised of the western portion of the subject site. This basin contains the residence, concrete walks, flagstone patio and landscaping. The basin generally drains from east to west and the runoff will be conveyed towards Grant Avenue. The downspouts for the house shall be directed towards the northern and southern lot line swales and also be directed towards Grant Avenue. Basin B Basin B (0.164 acres) is comprised of the eastern portion of the subject site. This basin contains a proposed garage, concrete and gravel driveway and landscaping. The calculations for this basin also include a 1,000 s.f. future accessory dwelling unit (ADU). The basin generally drains from the outside to inside to the proposed detention pond and low point along the northern property line. A comparison between the existing and proposed conditions is provided below: Basin ID Area (ac.) % Impervious 100- Yr Time of Concentration (min) 100-year Coefficient 100-year Runoff (cfs) EX (A & B) 0.258 15.5 10.7 0.38 0.73 Site (A & B) 0.258 44.3 6.7 0.66 1.51 Since Basin A flows to Grant Avenue, stormwater mitigation is not provided for that basin. Basin B flows to the existing low point along the northern property and will continue in the historic path, however that path is to an adjacent property. Therefore, a comparison between the existing and proposed Basin B is provided below: Basin ID Area (ac.) % Impervious 100- Yr Time of Concentration (min) 100-year Coefficient 100-year Runoff (cfs) Basin EXB 0.094 14.7 10.7 0.37 0.57 Basin B 0.164 35.0 7.0 0.57 0.82 Due to the increase in runoff, the release from Basin B will be controlled by a proposed infiltration basin with proposed volume of 420 cu.ft. An infiltration basin is proposed for the subject site within Basin B to attenuate the increase in impervious area and runoff within the basin. Drainage Report 516 Grant Avenue, Fort Collins, CO September 4, 2025 Page 7 i2 Consultants, Inc. Required volume was calculated using the following (for the 100-year event): 3.67 inches of rainfall for 100-year event Increase in roof and concrete = 1,193 sf Increase in gravel = 138 sf X40% imperviousness = 55 sf Increase in impervious area = 1,248 sf Volume required = 1,248 sf X (3.67 inches/12) = 382 cu.ft. Volume is provided in a flat bottom rectangle with dimensions of roughly 60’ x 7’ x 1’ deep. Volume provided = 420 cu.ft. Runoff will percolate into the ground and not be released to the property to the north except in the event the pond is full or there is a storm event greater than the 100-year event. The proposed runoff from this site is not anticipated to have a negative impact on the existing adjacent facilities or neighboring properties. The expected runoff from the subject site will drain to Grant Avenue and also be mitigated through an onsite infiltration basin and should not impact the adjacent property and overall basins. V. SEDIMENT, EROSION CONTROL AND STORMWATER QUALITY A. Temporary Sediment and Erosion Control: Several sediment and erosion control measures will be incorporated into the initial phase of construction. The initial sediment and erosion control measures to be installed shall include sediment control logs around the perimeter of the site. An initial control measure should be the installation of construction fence around the development perimeter. Furthermore, additional measures for sediment and erosion control shall consist of providing an area designated for a concrete washout area. The initial measures for sediment and erosion control shall be maintained and updated as necessary during the interim and final stages of the project. Additionally, any area that has been disturbed by construction activities and left exposed for more than fourteen (14) days, shall receive drilled seeding and straw mulch at 2 tons/acre, until permanent landscaping or ground cover has been provided. Four Step Process to Minimize Adverse Impacts of Urbanization Employing runoff reduction practices: There will be landscaping around the site and proposed impervious surfaces have been limited to the extent practice for the redevelopment of this residential lot. Implementing best management practices (BMPs): Onsite BMPs are not practical for the subject residential redevelopment. Drainage Report 516 Grant Avenue, Fort Collins, CO September 4, 2025 Page 8 i2 Consultants, Inc. Stabilizing Streams: The runoff from the subject site drains to existing paved surfaces and facilities. Implementing site specific and other source control BMPs: Appropriate erosion and pollution control, for residential redevelopment shall be installed around the staging area as appropriate. B. Permanent Water Quality BMPs Not required for the subject property. Water quality is provided through minimizing directly connected impervious area (MDCIA) and the proposed landscape buffering onsite. Detention will be provided for a portion of the subject property due to the increase of impervious area draining to an adjacent property. VI. CONCLUSIONS A. Compliance with Standards: The drainage design is in general compliance with the Fort Collins Stormwater Criteria Manual, December 2018. B. Drainage Concept: The runoff from this site will be conveyed by existing and proposed storm drainage improvements. The drainage design for the subject site should not negatively impact improvements downstream or the existing storm system. Drainage Report 516 Grant Avenue, Fort Collins, CO September 4, 2025 Page 9 i2 Consultants, Inc. VII.REFERENCES 1. Fort Collins Stormwater Criteria Manual, dated December 2018. 2. Urban Drainage and Flood Control District, Drainage Criteria Manual Volumes 1, Revised August 2018, Volume 2, Revised September 2017 and the Volume 3, dated November 2010 and Updated October 2019. 3. City of Fort Collins Master Drainage Basin Map, no date. Drainage Report 516 Grant Avenue, Fort Collins, CO September 4, 2025 Page 10 i2 Consultants, Inc. VIII.APPENDICIES  Vicinity Map  FEMA & Existing Geotechnical Information  Hydrologic Calculations  Hydraulic Calculations  Charts and Figures Vicinity Map © 2025 Microsoft Corporation © 2025 TomTom VICINITY MAP JLB 516 GRANT STREET FORT COLLINS, CO A-1 i2 Consultants, Inc. FEMA & Existing Geotechnical Information National Flood Hazard Layer FIRMette 0 500 1,000 1,500 2,000250 Feet Ü SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT SPECIAL FLOOD HAZARD AREAS Without Base Flood Elevation (BFE) Zone A, V, A99 With BFE or Depth Zone AE, AO, AH, VE, AR Regulatory Floodway 0.2% Annual Chance Flood Hazard, Areas of 1% annual chance flood with average depth less than one foot or with drainage areas of less than one square mile Zone X Future Conditions 1% Annual Chance Flood Hazard Zone X Area with Reduced Flood Risk due to Levee. See Notes.Zone X Area with Flood Risk due to Levee Zone D NO SCREEN Area of Minimal Flood Hazard Zone X Area of Undetermined Flood Hazard Zone D Channel, Culvert, or Storm Sewer Levee, Dike, or Floodwall Cross Sections with 1% Annual Chance 17.5 Water Surface Elevation Coastal Transect Coastal Transect Baseline Profile Baseline Hydrographic Feature Base Flood Elevation Line (BFE) Effective LOMRs Limit of Study Jurisdiction Boundary Digital Data Available No Digital Data Available Unmapped This map complies with FEMA's standards for the use of digital flood maps if it is not void as described below. The basemap shown complies with FEMA's basemap accuracy standards The flood hazard information is derived directly from the authoritative NFHL web services provided by FEMA. This map was exported on 8/15/2025 at 11:16 AM and does not reflect changes or amendments subsequent to this date and time. The NFHL and effective information may change or become superseded by new data over time. This map image is void if the one or more of the following map elements do not appear: basemap imagery, flood zone labels, legend, scale bar, map creation date, community identifiers, FIRM panel number, and FIRM effective date. Map images for unmapped and unmodernized areas cannot be used for regulatory purposes. Legend OTHER AREAS OF FLOOD HAZARD OTHER AREAS GENERAL STRUCTURES OTHER FEATURES MAP PANELS 8 B 20.2 The pin displayed on the map is an approximate point selected by the user and does not represent an authoritative property location. 1:6,000 105°5'40"W 40°35'53"N 105°5'2"W 40°35'26"N Basemap Imagery Source: USGS National Map 2023 B-1 Soil Map—Larimer County Area, Colorado (516 Grant Avenue, Ft Collins, CO) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/14/2025 Page 1 of 3 44 9 3 6 9 1 44 9 3 6 9 9 44 9 3 7 0 7 44 9 3 7 1 5 44 9 3 7 2 3 44 9 3 7 3 1 44 9 3 7 3 9 44 9 3 6 9 1 44 9 3 6 9 9 44 9 3 7 0 7 44 9 3 7 1 5 44 9 3 7 2 3 44 9 3 7 3 1 44 9 3 7 3 9 492437 492445 492453 492461 492469 492477 492485 492493 492501 492509 492437 492445 492453 492461 492469 492477 492485 492493 492501 492509 40° 35' 39'' N 10 5 ° 5 ' 2 1 ' ' W 40° 35' 39'' N 10 5 ° 5 ' 1 8 ' ' W 40° 35' 38'' N 10 5 ° 5 ' 2 1 ' ' W 40° 35' 38'' N 10 5 ° 5 ' 1 8 ' ' W N Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84 0 15 30 60 90 Feet 0 5 10 20 30 Meters Map Scale: 1:353 if printed on A landscape (11" x 8.5") sheet. Soil Map may not be valid at this scale. B-2 MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Larimer County Area, Colorado Survey Area Data: Version 19, Aug 29, 2024 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Jul 2, 2021—Aug 25, 2021 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Soil Map—Larimer County Area, Colorado (516 Grant Avenue, Ft Collins, CO) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/14/2025 Page 2 of 3 B-3 Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI 81 Paoli fine sandy loam, 0 to 1 percent slopes 0.3 100.0% Totals for Area of Interest 0.3 100.0% Soil Map—Larimer County Area, Colorado 516 Grant Avenue, Ft Collins, CO Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/14/2025 Page 3 of 3 B-4 Hydrologic Soil Group—Larimer County Area, Colorado (516 Grant Avenue, Ft Collins, CO) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/14/2025 Page 1 of 4 44 9 3 6 9 1 44 9 3 6 9 9 44 9 3 7 0 7 44 9 3 7 1 5 44 9 3 7 2 3 44 9 3 7 3 1 44 9 3 7 3 9 44 9 3 6 9 1 44 9 3 6 9 9 44 9 3 7 0 7 44 9 3 7 1 5 44 9 3 7 2 3 44 9 3 7 3 1 44 9 3 7 3 9 492437 492445 492453 492461 492469 492477 492485 492493 492501 492509 492437 492445 492453 492461 492469 492477 492485 492493 492501 492509 40° 35' 39'' N 10 5 ° 5 ' 2 1 ' ' W 40° 35' 39'' N 10 5 ° 5 ' 1 8 ' ' W 40° 35' 38'' N 10 5 ° 5 ' 2 1 ' ' W 40° 35' 38'' N 10 5 ° 5 ' 1 8 ' ' W N Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84 0 15 30 60 90 Feet 0 5 10 20 30 Meters Map Scale: 1:353 if printed on A landscape (11" x 8.5") sheet. Soil Map may not be valid at this scale. B-5 MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Rating Polygons A A/D B B/D C C/D D Not rated or not available Soil Rating Lines A A/D B B/D C C/D D Not rated or not available Soil Rating Points A A/D B B/D C C/D D Not rated or not available Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Larimer County Area, Colorado Survey Area Data: Version 19, Aug 29, 2024 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Jul 2, 2021—Aug 25, 2021 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Hydrologic Soil Group—Larimer County Area, Colorado (516 Grant Avenue, Ft Collins, CO) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/14/2025 Page 2 of 4 B-6 Hydrologic Soil Group Map unit symbol Map unit name Rating Acres in AOI Percent of AOI 81 Paoli fine sandy loam, 0 to 1 percent slopes A 0.3 100.0% Totals for Area of Interest 0.3 100.0% Description Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms. The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows: Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission. If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. Rating Options Aggregation Method: Dominant Condition Component Percent Cutoff: None Specified Hydrologic Soil Group—Larimer County Area, Colorado 516 Grant Avenue, Ft Collins, CO Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/14/2025 Page 3 of 4 B-7 Tie-break Rule: Higher Hydrologic Soil Group—Larimer County Area, Colorado 516 Grant Avenue, Ft Collins, CO Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/14/2025 Page 4 of 4 B-8 i2 Consultants, Inc. Hydrologic Calculations C-1 Basin Imperviousness i2 516 N Grant Ave Consultants, Inc. Fort Collins, CO 16911 Potts Place, Mead, CO 970.217.9148 troy@i2-consultants.com EXISTING BASINS EXISTING BASIN - Basin A Land Use Area (ft2)Area (ac)I (%)C Lawns, Clayey Soils 1,939 0.04 2 0.2 Roofs 276 0.01 100 0.95 Concrete Surfaces 110 0.00 100 0.95 Asphalt - - 100 0.95 Gravel/Flagstone - - 40 0.5 Wghtd Avg & Total Area 2,325 0.053 18.3 0.32 EXISTING BASIN - Basin B Land Use Area (ft2)Area (ac)I (%)C Lawns, Clayey Soils 7,581 0.17 2 0.2 Roofs 964 0.02 100 0.95 Concrete Surfaces 95 0.00 100 0.95 Asphalt - - 100 0.95 Gravel/Flagstone 252 0.01 40 0.5 Wghtd Avg & Total Area 8,892 0.204 14.7 0.30 EXISTING BASIN - SITE Land Use Area (ft2)Area (ac)I (%)C Lawns, Clayey Soils 9,520 0.22 2 0.2 Roofs 1,240 0.03 100 0.95 Concrete Surfaces 205 0.00 100 0.95 Asphalt - - 100 0.95 Gravel/Flagstone 252 0.01 40 0.5 Wghtd Avg & Total Area 11,217 0.258 15.5 0.30 C-2 DEVELOPED BASINS DEVELOPED BASIN - Basin A Land Use Area (ft2)Area (ac)I (%)C Lawns, Clayey Soils 1,594 0.04 2 0.2 Roofs 2,100 0.05 100 0.95 Concrete Surfaces 300 0.01 100 0.95 Asphalt - - 100 0.95 Gravel//Flagstone 96 0.00 40 0.5 Wghtd Avg & Total Area 4,090 0.094 60.4 0.65 DEVELOPED BASIN - Basin B Land Use Area (ft2)Area (ac)I (%)C Lawns, Clayey Soils 4,485 0.10 2 0.2 Roofs 1,598 0.04 100 0.95 Concrete Surfaces 654 0.02 100 0.95 Asphalt - - 100 0.95 Gravel/Flagstone 390 0.01 40 0.5 Wghtd Avg & Total Area 7,127 0.164 35.0 0.45 DEVELOPED BASIN - SITE Land Use Area (ft2)Area (ac)I (%)C Lawns, Clayey Soils 6,079 0.14 2 0.2 Roofs 3,698 0.08 100 0.95 Concrete Surfaces 954 0.02 100 0.95 Asphalt - - 100 0.95 Gravel/Flagstone 486 0.01 40 0.5 Wghtd Avg & Total Area 11,217 0.258 44.3 0.52 C-3 BASIN COMPARISON (Developed - Existing) BASIN A (Developed Basin A is 1,119 s.f. larger than Existing Basin A) Land Use Area (ft2)Area (ac)I (%) Lawns, Clayey Soils (345) (0.008) 2 Roofs 1,824 0.042 100 Concrete Surfaces 190 0.004 100 Asphalt - - 100 Gravel 96 0.002 40 BASIN B (Developed Basin B is 1,119 s.f. smaller than Existing Basin B) Land Use Area (ft2)Area (ac)I (%) Lawns, Clayey Soils (3,096) (0.071) 2 Roofs 634 0.015 100 Concrete Surfaces 559 0.013 100 Asphalt - - 100 Gravel 138 0.003 40 OVERALL SITE BASIN COMPARISON Land Use Area (ft2)Area (ac)I (%) Lawns, Clayey Soils (3,441) (0.079) 2 Roofs 2,458 0.056 100 Concrete Surfaces 749 0.017 100 Asphalt - - 100 Gravel 234 0.005 40 Basin A drains to Grant Street, therefore detention not required Calculaton for Required Detention in Basin B: 3.67 inches of rainfall for 100-year event 1193 sf (634 + 559) increase of roofs/concrete surfaces plus 138 sf X40% imperviousness 1248 sf X (3.67 in/12) = 382 cu.ft. required Net increase of 42.1% impervousness between Developed and Existing Condition due to the increase in Roofs, Flagstone and Concrete Surfaces Net increase of 17.6% impervousness between Developed and Existing Condition due to the increase in Roofs, Gravel and Concrete Surfaces Net increase of 26.5% impervousness between Developed and Existing Condition Notes Net decrease of 345 s.f. Net increase of 1,824 s.f. Net increase of 190 s.f. No change in asphalt Net decrease of 96 s.f. Notes Net decrease of 2,450 s.f. Net increase of 634 s.f. Net increase of 559 s.f. Net increase of 749 s.f. No change in asphalt Net increase of 234 s.f. No change in asphalt Net increase of 138 s.f. Notes Net decrease of 2,795 s.f. Net increase of 2,458 s.f. C-4 PROJECT:516 Grant Avenue CALCULATED BY:i2 Consultants, Inc.DATE:9/3/2025 Cf = 1.00 FINAL REMARKS tc BASIN DP AREA C C*Cf Length Slope ti Length Slope R V tt tc Total L tc ID Ac Ft % min Ft (ft/ft) Ft fps min min ft min min (0) (1) (2) (3) (4) (5) (6) (7) (8) (8a) (9) (10) (11) (12) (13) (14) 1 EX A 1 0.053 0.32 0.32 20 0.5 8.2 25 0.005 1.0 1.5 0.3 8.4 45 10.3 8.4 Use Tc = 8.4 mins 2 EX B 2 0.204 0.30 0.30 50 0.5 13.4 75 0.005 1.0 1.5 0.8 14.2 125 10.7 10.7 Use Tc = 10.7 mins 2 EX Site 0.258 0.30 0.30 50 0.5 13.3 75 0.005 1.0 1.5 0.8 14.1 125 10.7 10.7 Use Tc = 10.7 mins 3 A 1 0.094 0.65 0.65 30 2.0 3.7 60 0.005 1.0 1.5 0.7 4.3 90 10.5 5.0 Use Tc = 5.0 mins 4 B 2 0.164 0.45 0.45 40 1.0 7.6 60 0.005 1.0 1.5 0.7 8.3 100 10.6 8.3 Use Tc = 8.3 mins 5 Site 0.258 0.52 0.52 40 1.0 6.8 60 0.005 1.0 0.7 1.4 8.2 100 10.6 8.2 Use Tc = 8.2 mins Equations: Urbanized Tov tt =L tc =(L/180)+10 60V V=CvSw0.5 (1)(2) or Fig. 3-3 (3) Velocity V (1.4/n)*(R^2/3))*0.5) S in ft/ft, R = Area/Wetted Perimeter in ft, n = Roughness Coeff. RURAL OR NON URBANIZED TIME OF CONCENTRATION - 2-YEAR 1.87(1.1-CC f )*D^1/2 DATA SUB-BASIN INITIAL/OVERLAND TIME (Tov) TRAVEL TIME (tt) tc CHECK S ^1/3 Rural/Non-urban (URBANIZED) C-5 PROJECT:516 Grant Avenue CALCULATED BY:i2 Consultants, Inc.DATE:9/3/2025 Cf = 1.25 FINAL REMARKS tc BASIN DP AREA C C*Cf Length Slope ti Length Slope R V tt tc Total L tc ID Ac Ft % min Ft % Ft fps min min ft min min (0) (1) (2) (3) (4) (5) (6) (7) (8) (8a) (9) (10) (11) (12) (13) (14) 1 EX A 1 0.053 0.32 0.41 20 0.5 7.3 25 0.005 1.0 1.5 0.3 7.6 45 10.3 7.6 Use Tc = 7.6 mins 2 EX B 2 0.204 0.30 0.37 50 0.5 12.1 75 0.005 1.0 1.5 0.8 13.0 125 10.7 10.7 Use Tc = 10.7 mins 3 EX Site 0.258 0.30 0.38 50 0.5 12.0 75 0.005 1.0 1.5 0.8 12.8 125 10.7 10.7 Use Tc = 10.7 mins 4 A 1 0.094 0.65 0.81 30 2.0 2.4 60 0.005 1.0 1.5 0.7 3.0 90 10.5 5.0 Use Tc = 5.0 mins 5 B 2 0.164 0.45 0.57 40 1.0 6.3 60 0.005 1.0 1.5 0.7 7.0 100 10.6 7.0 Use Tc = 7.0 mins 6 Site 0.258 0.52 0.66 40 1.0 5.3 60 0.005 1.0 0.7 1.4 6.7 100 10.6 6.7 Use Tc = 6.7 mins Equations: Urbanized Tov tt =L tc =(L/180)+10 60V V=CvSw0.5 (1)(2) or Fig. 3-3 (3) Velocity V (1.4/n)*(R^2/3))*0.5) S in ft/ft, R = Area/Wetted Perimeter in ft, n = Roughness Coeff. TRAVEL TIME (tt) tc CHECK S ^1/3 Rural/Non-urban (URBANIZED) RURAL OR NON URBANIZED TIME OF CONCENTRATION - 100-YEAR 1.87(1.1-CC f )*D^1/2 DATA SUB-BASIN INITIAL/OVERLAND TIME (Tov) Rational Method C-6 Project: 516 Grant Avenue i2 Consultants, Inc. Location: 516 Grant Avenue 16911 Potts Place, Mead, CO Fort Collins, Colorado 970.217.9148 Project Number:1088-3 troy@i2-consultants.com Date: 8/15/2025 Rational Method:Q = CfCIA Q: Q: Runoff in cfs (peak) C: C: Runoff Coefficient Cf: Cf: Frequency Adjustment Factor I: I: Rainfall Intensity (in/hr) for design event A: A: Basin Area (acres) 2-YEAR Design Point Basin ID Area C CfC tc I Q2 1 EX A 0.053 0.32 0.32 8.4 2.36 0.04 2 EX B 0.204 0.30 0.30 10.7 2.15 0.13 2 EX Site 0.258 0.30 0.30 10.7 2.15 0.17 1 A 0.094 0.65 0.65 5.0 2.85 0.17 2 B 0.164 0.45 0.45 8.3 2.37 0.18 2 Site 0.258 0.52 0.52 8.2 2.38 0.32 C:\Users\troy\Dropbox\I2\Projects\1088-3 516 Grant\Documents\Drainage\516Grant_rational.xlsx 9/2/2025 Rational Method C-7 Project: 516 Grant Avenue i2 Consultants, Inc. Location: 516 Grant Avenue 16911 Potts Place, Mead, CO Fort Collins, Colorado 970.217.9148 Project Number:1088-3 troy@i2-consultants.com Date: 8/15/2025 Rational Method:Q = CfCIA Q: Q: Runoff in cfs (peak) C: C: Runoff Coefficient Cf: Cf: Frequency Adjustment Factor I: I: Rainfall Intensity (in/hr) for design event A: A: Basin Area (acres) 100-YEAR Design Point Basin ID Area C CfC tc I Q100 1 EX A 0.053 0.32 0.41 7.6 8.55 0.19 2 EX B 0.204 0.30 0.37 10.7 7.51 0.57 2 EX Site 0.258 0.30 0.38 10.7 7.51 0.73 1 A 0.094 0.65 0.81 5.0 9.95 0.76 2 B 0.164 0.45 0.57 7.0 8.80 0.82 2 Site 0.258 0.52 0.66 6.7 8.95 1.51 C:\Users\troy\Dropbox\I2\Projects\1088-3 516 Grant\Documents\Drainage\516Grant_rational.xlsx 9/2/2025 i2 Consultants, Inc. Hydraulic Calculations D-1 i2 Infiltration Basin Volume Consultants, Inc. 516 Grant Avenue 16911 Potts Place, Mead, CO Fort Collins, CO 970.217.9148 troy@i2-consultants.com Infiltration Basin Volume Pond Area:7'X60' Pond Depth:1' Volume:7'X60'X1' 420 cu.ft. 382 cu.ft. required, therefore volume is okay i2 Consultants, Inc. Charts and Figures FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5) 3.0 Rational Method 3.2 Runoff Coefficients Page 5 Table 3.2-2. Surface Type - Runoff Coefficients Asphalt, Concrete 0.95 Rooftop 0.95 Recycled Asphalt 0.80 Gravel 0.50 Pavers 0.50 Lawns, Sandy Soil, Flat Slope < 2% 0.10 Lawns, Sandy Soil, Avg Slope 2-7% 0.15 Lawns, Sandy Soil, Steep Slope >7% 0.20 Lawns, Clayey Soil, Flat Slope < 2% 0.20 Lawns, Clayey Soil, Avg Slope 2-7% 0.25 Lawns, Clayey Soil, Steep Slope >7% 0.35 3.2.1 Composite Runoff Coefficients Drainage sub-basins are frequently composed of land that has multiple surface types or zoning classifications. In such cases a composite runoff coefficient must be calculated for any given drainage sub-basin. The composite runoff coefficient is obtained using the following formula: ( ) t n i ii A xAC C ∑ ==1 Equation 5-2 Where: C = Composite Runoff Coefficient Ci = Runoff Coefficient for Specific Area (Ai), dimensionless Ai = Area of Surface with Runoff Coefficient of Ci, acres or square feet n = Number of different surfaces to be considered At = Total Area over which C is applicable, acres or square feet 3.2.2 Runoff Coefficient Frequency Adjustment Factor The runoff coefficients provided in Table 3.2-1 and Table 3.2-2 are appropriate for use with the 2-year storm event. For any analysis of storms with higher intensities, an adjustment of the runoff coefficient is required due to the lessening amount of infiltration, depression retention, evapotranspiration and other losses that have a proportionally smaller effect on high-intensity storm runoff. This adjustment is E-1 FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5) 3.0 Rational Method 3.3 Time of Concentration Page 6 applied to the composite runoff coefficient. These frequency adjustment factors, Cf, are found in Table 3.2-3. Table 3.2-3. Frequency Adjustment Factors Storm Return Period Frequency Adjustment 2, 5, 10 1.00 25 1.10 50 1.20 100 1.25 3.3 Time of Concentration 3.3.1 Overall Equation The next step to approximate runoff using the Rational Method is to estimate the Time of Concentration, Tc, or the time for water to flow from the most remote part of the drainage sub-basin to the design point under consideration. The Time of Concentration is represented by the following equation: 𝐓𝐓𝐜𝐜=𝐓𝐓𝐢𝐢+𝐓𝐓𝐭𝐭 Equation 5-3 Where: Tc = Total Time of Concentration, minutes Ti = Initial or Overland Flow Time of Concentration, minutes Tt = Channelized Flow in Swale, Gutter or Pipe, minutes 3.3.2 Overland Flow Time Overland flow, Ti, can be determined by the following equation: 𝐓𝐓𝐢𝐢=𝟏𝟏.𝟖𝟖𝟖𝟖(𝟏𝟏.𝟏𝟏−𝐂𝐂𝐂𝐂𝐂𝐂𝐟𝐟)√𝐋𝐋√𝐒𝐒𝟑𝟑 Equation 3.3-2 Where: C = Runoff Coefficient, dimensionless Cf = Frequency Adjustment Factor, dimensionless L = Length of Overland Flow, feet S = Slope, percent CX F PRODUCT OF CXCF CANNOT EXCEED THE VALUE OF 1 OVERLAND FLOW LENGTH L=200’ MAX IN DEVELOPED AREAS L=500’ MAX IN UNDEVELOPED AREAS E-2 FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5) 3.0 Rational Method 3.4 Intensity-Duration-Frequency Curves for Rational Method Page 8 Table 3.4-1. IDF Table for Rational Method Duration (min) 2-year 10-year 100-year Duration (min) 2-year 10-year 100-year 5 2.85 4.87 9.95 39 1.09 1.86 3.8 6 2.67 4.56 9.31 40 1.07 1.83 3.74 7 2.52 4.31 8.80 41 1.05 1.80 3.68 8 2.40 4.10 8.38 42 1.04 1.77 3.62 9 2.30 3.93 8.03 43 1.02 1.74 3.56 10 2.21 3.78 7.72 44 1.01 1.72 3.51 11 2.13 3.63 7.42 45 0.99 1.69 3.46 12 2.05 3.50 7.16 46 0.98 1.67 3.41 13 1.98 3.39 6.92 47 0.96 1.64 3.36 14 1.92 3.29 6.71 48 0.95 1.62 3.31 15 1.87 3.19 6.52 49 0.94 1.6 3.27 16 1.81 3.08 6.30 50 0.92 1.58 3.23 17 1.75 2.99 6.10 51 0.91 1.56 3.18 18 1.70 2.90 5.92 52 0.9 1.54 3.14 19 1.65 2.82 5.75 53 0.89 1.52 3.10 20 1.61 2.74 5.60 54 0.88 1.50 3.07 21 1.56 2.67 5.46 55 0.87 1.48 3.03 22 1.53 2.61 5.32 56 0.86 1.47 2.99 23 1.49 2.55 5.20 57 0.85 1.45 2.96 24 1.46 2.49 5.09 58 0.84 1.43 2.92 25 1.43 2.44 4.98 59 0.83 1.42 2.89 26 1.4 2.39 4.87 60 0.82 1.4 2.86 27 1.37 2.34 4.78 65 0.78 1.32 2.71 28 1.34 2.29 4.69 70 0.73 1.25 2.59 29 1.32 2.25 4.60 75 0.70 1.19 2.48 30 1.30 2.21 4.52 80 0.66 1.14 2.38 31 1.27 2.16 4.42 85 0.64 1.09 2.29 32 1.24 2.12 4.33 90 0.61 1.05 2.21 33 1.22 2.08 4.24 95 0.58 1.01 2.13 34 1.19 2.04 4.16 100 0.56 0.97 2.06 35 1.17 2.00 4.08 105 0.54 0.94 2.00 36 1.15 1.96 4.01 110 0.52 0.91 1.94 37 1.16 1.93 3.93 115 0.51 0.88 1.88 38 1.11 1.89 3.87 120 0.49 0.86 1.84 E-3 FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5) 3.0 Rational Method 3.4 Intensity-Duration-Frequency Curves for Rational Method Page 9 Figure 3.4-1. Rainfall IDF Curve – Fort Collins E-4 FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5) 4.0 SWMM 4.1 Input Parameters Page 11 Table 4.1-2. Land Use - Percent Impervious Urban Estate 30 Low Density 50 Medium Density 70 High Density 90 Commercial 80 Industrial 90 Open Lands, Transition 20 Greenbelts, Agriculture 2 Offsite Flow Analysis (when Land Use not defined) 45 Reference: For further guidance regarding zoning classifications, refer to the Land Use Code, Article 4. For Final Plan (FP) submittals, impervious values must be based on the proposed land surface types. Refer to Table 4.1-3 for recommended percent impervious values. Table 4.1-3. Surface Type – Percent Impervious Asphalt, Concrete 100 Rooftop 90 Recycled Asphalt 80 Gravel 40 Pavers 40 Playgrounds 25 Lawns, Sandy soil 2 Lawns, Clayey soil 2 E-5 EXISTING RESIDENCE EXISTING STRUCTURE EXISTING CONCRETE WALK EXISTING PATIO EXISTING CONCRETE WALK GR A N T S T R E E T EXI S T I N G A L L E Y EX I S T I N G S I D E W A L K 520 GRANT ST 512 GRANT ST EXISTING RESIDENCE PROPOSED 2-STORY RESIDENCE PROPOSED GARAGE PROPOSED INFILTRATION BASIN 420 CU.FT. GR A N T S T R E E T EXI S T I N G A L L E Y 500 3 . 5 0 500 3 . 5 0 50 0 4 . 0 0 50 0 4 . 0 0 5003.50 50 0 3 . 5 0 5003.50 5003.00 5003.00 50 0 3 . 0 0 EX I S T I N G S I D E W A L K 520 GRANT ST 512 GRANT ST A 0.09 60.4% B 0.16 35.0% DS DS DR A I N A G E P L A N DR-1 JLB LEGEND OF SYMBOLS Know what's below. before you dig.Call R NOTES BASIN SUMMARY EXISTING DRAINAGE PROPOSED DRAINAGE SHEET 51 6 G R A N T A V E N U E 51 6 G R A N T A V E N U E , F O R T C O L L I N S , C O L O R A D O SI N G L E - F A M I L Y R E S I D E N C E PROPOSED 2-STORY RESIDENCE DS FFE: 5004.25 FG: 5004.25 FG: 5003.50 FG: 5003.50 FG: 5004.17 FG: 5004.17FG: 5003.75 FFE: 5004.25 FFE: 5004.25 FG: 5004.25 FG: 5004.25 FFE: 5003.50 FG: 5003.17 FG: 5003.17 FG: 5003.17 FG: 5003.20 FG: 5003.75 FFE: 5004.25 FG: 5004.25 5004.19 5004.19 FL: 5003.04 FL: 5003.24 HP: 5003.40 HP: 5003.02FL: 5002.67FL: 5002.54 5003.54 5003.63 5003.85 5003.525003.165003.06 5002.82 5002.82 5002.81 5002.82 5002.82 5001.82 5001.82 5001.825001.82 5001.82 FL: 5003.15 FL: 5003.02 FL: 5003.19 FL: 5003.17 FL: 5003.31 PROPOSED GARAGE PROPOSED INFILTRATION BASIN 420 CU.FT. 3.3% 0. 4 % 0.4% 0. 6 % 0. 7 % 0.8% 0.5% 0.5 % 0.5%0.7% 6.4% 3.4% 2.6% 1. 3 % 50 0 3 . 5 0 50 0 4 . 0 0 500 4 . 0 0 500 3 . 5 0 50 0 3 . 0 0 50 0 3 . 5 0 50 0 4 . 0 0 5003.00 5003.50 GR A N T S T R E E T EXI S T I N G A L L E Y EX I S T I N G A L L E Y FG: 5004.25 DSDS DS DS EX I S T I N G S I D E W A L K 520 GRANT AVENUE 516 GRANT AVENUE DS DS 520 GRANT GR A D I N G P L A N GR-1 JLB LEGEND OF SYMBOLS Know what's below. before you dig.Call R NOTES DS EXISTING BASIN B IMPERVIOUSNESS PROPOSED BASIN B IMPERVIOUSNESS NET INCREASE OF 1,248 S.F. OF IMPERVIOUS AREA SHEET 51 6 G R A N T A V E N U E 51 6 G R A N T A V E N U E , F O R T C O L L I N S , C O L O R A D O SI N G L E - F A M I L Y R E S I D E N C E INFILTRATION BASIN DETAIL NOT TO SCALE Grading/Stormwater Plan Accepted (Building Permit Released) Accepted by: Date : Dan Mogen 09/05/2025