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Home My WebLink About4518 INNOVATION DR WIRELESS TELECOMMUNICATIONS - PDP210010 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTkimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, CO 80237 303 228 2300 DRAINAGE LETTER To: From: Date: City of Ft. Collins Brice Hammersland, P.E. Kimley-Horn and Associates, Inc August 24, 2021 Subject: City of Ft. Collins Rocky Mountain Archery Communication Tower Kimley-Horn and Associates, Inc. (Kimley-Horn) is submitting this Drainage Letter for the above referenced project. The purpose of this letter is to serve as an outline for changes to the existing development and associated drainage analysis. PROJECT DESCRIPTION The proposed Rocky Mountain Archery Communication Tower Project (the “Project”) is located at 4518 Innovation Drive, Fort Collins, Colorado (the “Site”). The Project is anticipated to consist of the construction of communication tower, ground equipment platform, chain-link fence, permeable pavers, and waveguide bridge. The communication tower is located at the southeast corner of the parcel adjacent to the existing stormwater pond. The Site is within Flood Zone “X” and lies outside of the mapped 100-year floodplain as shown on FIRM Map Panels 08069C1000F dated December 19, 2006. Please see Appendix A for the FIRM Maps and NRCS Soil Map. Below outlines the supporting calculations and documentation for the Drainage Letter. ASSUMPTIONS In coordination with City of Ft. Collins guidelines, it was determined that a limited drainage study would be performed with this project. The main objective of this project is to minimize the increase in impervious area of the parcel and impact to the existing stormwater pond. Due to the scope of this project not including a major drainage analysis, the limited analysis provided is intended to demonstrate that negative impacts to the existing storm infrastructure are not anticipated with the construction of the tower and ground improvements. kimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, CO 80237 303 228 2300 DRAINAGE ANALYSIS Impervious Area The existing development has a ±10,000 sf building, concrete parking lot, and stormwater pond. Please see Appendix B for the Original Drainage Analysis prepared by North Star Design in September 2009. The proposed development conditions of the North Star Design analysis were used as the existing conditions of the proposed analysis. The development met or exceeded all the regulations of the City of Ft. Collins Strom Drainage Design Criteria Manual at the time of approval. The proposed development includes the construction of a communication tower, ground equipment platform, chain-link fence, permeable pavers, and waveguide bridge. The increase in impervious area for the proposed development is limited to the tower, ground equipment platform foundation, chain link fence, permeable pavers, and waveguide bridge foundations. The proposed ground equipment platform will have a grated surface; therefore, will not be included in the impervious calculations. Please see Appendix C for the impervious calculations and the Proposed Development Exhibit depicting the proposed impervious area. The proposed development’s impervious area is shown on the Impervious Table in Appendix D. The “C” values depicted in the in the Impervious Table have been updated based on the latest City of Ft. Collins Storm Drainage Design Criteria Manual. The Impervious Table shows the proposed improvements will create a very small increase in the percent impervious (less than 1%). It should be noted that the total new impervious area that will be added due to the proposed improvements will be approximately 347 square feet. Per Table 5.0.1 in the Cities Stormwater Criteria Manual this is less than the 350 square feet threshold where a Drainage submittal is required. The improvements do result in a small increase in the “C” values for the 2-yr and 5-yr for the site. The “C” value for the 100-yr does remain unchanged. The discrepancy between the originally approved design and the proposed design is due to the updated City of Fort Collins Storm Drainage Design Criteria Manual. Pond Volume The original development of the site completed a Storm Drainage Certification for the pond volume. Please see Appendix E for the Storm Drainage Certification. It is believed that there is a typo in the Storm Drainage Certification in that the provided volume is in cu-ft instead of ac-ft. The As-Built condition of the original development pond provides 909 cu-ft at the water quality elevation and 1,290 cu-ft at the 100-year High Water Elevation (HWEL), which is 213 cu-ft and 334 cu-ft more than the approved design, respectively. The elevations shown on the Proposed Development Exhibit have been taken from a survey prepared by Daley Land Surveying, Inc. dated 3/18/2019. The Daley Land Surveying survey is in the NAVD 88 vertical datum, while the original analysis is in the NGVD 29 vertical datum. The conversion from NAVD 88 to NGVD 29 is NAVD 88 – 2.9’ = NGVD 29. The depicted elevations in Appendix C have been converted to the NGVD 29 vertical datum. Temporary impacts to the pond during construction will have minor impacts to the detention pond side slopes along the west side of the detention pond. These will be restored to match existing grades once construction within the detention pond is completed to ensure the existing detention pond does not reduce the current pond volume. The proposed ground equipment platform is located at the edge of the existing stormwater pond. Appendix C calls out the ground elevation at the center of piers located within the 100-yr water surface elevation Page 3 kimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, Colorado, 80237 303 228 2300 and piers adjacent to the 100-yr water surface elevation. The proposed 2 ft diameter concrete pier at the north east corner and the middle pier along the east side of the platform will encroach the volume provided in the pond. A Volume Encroachment Table has been added to Appendix C to show the volume encroachment of the existing pond. Due to the minimal impact to the pond, the proposed development condition continues to provide sufficient volume at both the Water Quality and 100-year HWEL levels. CONCLUSION The drainage analysis presented in this letter is intended to show that with the above listed assumptions, the proposed conditions are not significantly different from existing conditions and existing infrastructure is not anticipated to be negatively impacted by the proposed improvements. SIGNATURE: _________________________________ Brice Hammersland, P.E. Registered Professional Engineer State of Colorado No. 56012 Page 4 kimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, Colorado, 80237 303 228 2300 APPENDIX Appendix A – FIRM and NRCS Soil Map Appendix B – Original Drainage Analysis Appendix C – Proposed Development Exhibit Appendix D – Impervious Table Appendix E – Storm Drainage Certification Page 5 kimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, Colorado, 80237 303 228 2300 EXHIBIT A USGS The National Map: Orthoimagery. Data refreshed April 2020 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 DepthZone 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 mileZone X Future Conditions 1% Annual Chance Flood HazardZone X Area with Reduced Flood Risk due to Levee. See Notes.Zone X Area with Flood Risk due to LeveeZone D NO SCREEN Area of Minimal Flood Hazard Zone X Area of Undetermined Flood HazardZone 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 7/29/2020 at 9:30 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°3'3"W 40°31'42"N 105°2'26"W 40°31'15"N Hydrologic Soil Group—Larimer County Area, Colorado Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 7/17/2020 Page 1 of 4448595044859604485970448598044859904486000448595044859604485970448598044859904486000496080496090496100496110496120496130496140496150496160496170 496080 496090 496100 496110 496120 496130 496140 496150 496160 496170 40° 31' 29'' N 105° 2' 46'' W40° 31' 29'' N105° 2' 42'' W40° 31' 27'' N 105° 2' 46'' W40° 31' 27'' N 105° 2' 42'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84 0 20 40 80 120 Feet 0 5 10 20 30 Meters Map Scale: 1:434 if printed on A landscape (11" x 8.5") sheet. Soil Map may not be valid at this scale. 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 15, Jun 9, 2020 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Aug 11, 2018—Aug 12, 2018 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 Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 7/17/2020 Page 2 of 4 Hydrologic Soil Group Map unit symbol Map unit name Rating Acres in AOI Percent of AOI 74 Nunn clay loam, 1 to 3 percent slopes C 0.7 100.0% Totals for Area of Interest 0.7 100.0% 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 Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 7/17/2020 Page 3 of 4 Tie-break Rule: Higher Hydrologic Soil Group—Larimer County Area, Colorado Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 7/17/2020 Page 4 of 4 Page 6 kimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, Colorado, 80237 303 228 2300 EXHIBIT B Page 7 kimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, Colorado, 80237 303 228 2300 EXHIBIT C NORTHPROPOSED IMPERVIOUS PIER AREAPRELIMINARYNOT FOR CONSTRUCTION4582 S ULSTER ST. #1500DENVER, CO 80237PHONE (303) 228-2300WWW.KIMLEY-HORN.COMBoca Raton, FL 33487750 Park of Commerce DriveSuite 200Aurora, CO 8001118400 East 22nd AvenueSITE NAME:ROCKY MOUNTAIN ARCHERYT-MOBILE SITE #:DN01586CVERTICAL BRIDGE SITE #:US-CO-5082PROPOSEDDEVELOPMENTEXHIBITEXHIBITLEGENDPROPOSED IMPERVIOUS AREAPOND VOLUME ENCROACHMENTTOWER FOUNDATION = 19.6 SFPLATFORM FOUNDATION/GRATED WALKWAY = 108.8 SFFENCE POST & ICE BRIDGE FOUNDATION (1' DIA) = 3.2 SFPERMEABLE PAVERS = 215 SFTOTAL = 346.6 SFNORTH EAST PLATFORM FOUNDATION AND GRATED WALKWAYDIAMETER = 2.0'VOLUME AT 4969.86 = 2.45 CU-FT VOLUME AT 4970.1 = 3.20 CU-FTPROPOSED PERMEABLE PAVERS Page 8 kimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, Colorado, 80237 303 228 2300 EXHIBIT D Rocky Mountain ArcheryDATE:8/24/2021140064064RRJBHTYPE C SOILASPHALT/ CONCRETE ROOFTOP PAVERSLAWNS, SANDY SOIL, AVG SLOPELAND USE:AREAAREAAREAAREA2-YR RUNOFF COEFFICIENT 0.95 0.95 0.500.205-YR RUNOFF COEFFICIENT 0.95 0.95 0.500.20100-YR RUNOFF COEFFICIENT 1.19 1.19 0.630.25IMPERVIOUS % 100% 90% 40%2%ASPHALT/ CONCRETE ROOFTOP PAVERSLAWNS, SANDY SOIL, AVG SLOPETOTALDESIGN DESIGN AREAAREAAREAAREAAREABASIN POINT (AC) (AC) (AC) (AC) (AC) Cc(2) Cc(5) Cc(100) Imp %EX1 EX10.18 0.23 0.00 0.170.580.73 0.73 0.92 67.9%AA0.18 0.23 0.010.160.580.74 0.74 0.93 68.7%CHECKED BY:PROJECT NUMBER:CALCULATED BY:NOTE: C5-YR = C2-YR*1.00 PER FCSCM TABLE 3-2-3ON-SITE BASINSRUNOFF COEFFICIENTS - IMPERVIOUS CALCULATIONPROJECT NAME:COMPOSITE RUNOFF COEFFICIENTNOTE: C100-YR =C2-YR*1.25 PER FCSCM TABLE 3-2-3EXISTING BASINS Page 9 kimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, Colorado, 80237 303 228 2300 EXHIBIT E