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HomeMy WebLinkAbout1721 S. COLLEGE TOWNHOMES - PDP - PDP160042 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE RELATED DOCUMENTkimley-horn.com 1001 Warrenville Road, Suite 350, Lisle, IL 60532 630 487 5550 PRELIMINARY DRAINAGE MEMORANDUM To: City of Fort Collins From: Lesley Netzer Kimley-Horn and Associates, Inc Date: December 6, 2016 Re: College Avenue Town Homes 1751 S College Avenue Fort Collins, CO Introduction Kimley-Horn and Associates, Inc., serves as the engineering consultant for CORE Fort Collins 1751 College, LLC. They are proposing to construct a 10-unit town house development at 1751 S College Avenue in Fort Collins, Colorado (Larimer County). The site work on the approximate 0.35 acre parcel includes demolition, grading, stormwater management, water, sanitary sewer, paving installation, and landscape improvements. The “Urban Drainage and Flood Control District Manual” Volumes 1, 2, and 3 (UDFCD Manual) and the Fort Collins Amendments to the UDFCD, with latest revisions, were used to prepare the stormwater approach and calculations. Existing Conditions The approximate 0.35-acre parcel was previously developed as commercial/retail building with a parking lot and associated utilities. The building was demolished and left as a gravel pad, and the existing parking lot remains. The site is bound to the west and south by multi-family residential buildings (Choice Center) and to the north by an existing commercial/retail center. The site has frontage along S College Avenue on the east side of the site. Stormwater drainage for the site discharges to a concrete channel along the west side of the site, which is routed to a storm sewer in Choice Center Drive. The site is part of the Spring Creek Drainage Basin, stormwater runoff flows to Spring Creek, ultimately tributary to the Poudre River. Based on the NRCS soils map provided in Appendix A, the expected soils on site are mainly Fort Collins Loam having a hydrologic soil group rating C. Currently developed, the site is approximately 98% impervious, which includes the gravel pad where the building was demolished. The site is located within City of Fort Collins Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map Number 08069C0979H, revised May 2, 2012 and is designated as an area outside the 500-year floodplain. The FEMA Firmette is included in Appendix B. Page 2 kimley-horn.com 1001 Warrenville Road, Suite 350, Lisle, IL 60532 630 487 5550 Proposed Conditions The proposed project includes 10-unit town house development with a new drive connection to College Avenue and the adjacent parcel to the north. The proposed impervious percentage is 77%, a net decrease in impervious area of approximately +/-3,600 square feet. Reference the existing and proposed area exhibit in Appendix C. Runoff for the site will primarily be directed to the proposed pervious paver system located between the east and west town home buildings. Roof drains for the proposed buildings will discharge on the surface, directing flow to the pervious pavers. An underdrain is proposed for the pavers system, routing flow to a 10” storm sewer along the south end of the site. This storm sewer will discharge flow into the drainage channel along the west property line. The storm sewers are properly sized for a minor storm event (2-10 year storm) per Volume 1, Chapter 4 of the Fort Collins Amendments to the UDFCD. Adequate provisions will also be made to pass a major storm event (100-year storm) through the development. Since there is a net decrease in percent impervious from existing conditions to proposed, detention will not be required for the development with respect to stormwater release. Water Quality and BMP Selection The proposed development will be required to meet all applicable water quality design criteria referenced in Volume 3, Chapter 3.0 of the UDFCD Manual. The manual recommends a four-step process for treating the required water quality volume as shown below. 1. Employ runoff reduction practices - The redevelopment on an urban site provides limited opportunities to employ runoff reduction practices. The site has been developed to install landscaping wherever pavement (or building) is not required for the functionality of the site. 2. Implement BMPs that provide a water quality capture volume with a slow release – The majority of site will be treated by “Permeable Interlocking Concrete Pavement” (Pervious Pavers) per Volume 3, Chapter 4, Section T-10.1 of the UDFCD Manual. 3. Stabilize Drainage Ways – Not applicable. This development is not adjacent to a stream, nor does it directly impact the downstream water body (Spring Creek). 4. Implement site specific and other source control BMPs –Trash and recycling containers will be provided on site. Vehicular parking will be located in garages to reduce potential for contamination discharges. The water quality volume required in permanent best management practices is calculated below: 𝑊𝑄𝐶𝑉 (𝑊𝑎𝑡𝑒𝑟𝑠ℎ𝑒𝑑 𝐼𝑛𝑐ℎ𝑒𝑠) = 𝑎(0.91𝐼 3 − 1.19𝐼 2 + 0.78𝐼) I = Composite Tributary Imperviousness [%/100] = 0.77 a = Coefficient of drain time (a = 0.8 for 12-hour drain time) 𝑊𝑄𝐶𝑉 (𝑊𝑎𝑡𝑒𝑟𝑠ℎ𝑒𝑑 𝐼𝑛𝑐ℎ𝑒𝑠) = 0.8[0.91(. 77) 3 − 1.19(. 77) 2 + 0.78(. 77)] = 0.31 𝑖𝑛𝑐ℎ𝑒𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 = (𝑊𝑄𝐶𝑉) 12 𝐴 Page 3 kimley-horn.com 1001 Warrenville Road, Suite 350, Lisle, IL 60532 630 487 5550 WQCV = Watershed Inches (calculated above) A = Tributary Area (acres) 𝑉𝑜𝑙𝑢𝑚𝑒 = (𝑊𝑄𝐶𝑉) 12 𝐴 = 0.31 12 (0.4) = 0.01 𝑎𝑐 − 𝑓𝑡 = 450 𝐶𝐹 In total, there will be approximately 2,900 SF of pervious pavers with a 6” stone base and 4” perforated underdrain. Assuming 40% porosity, this equates to 580 CF of water quality storage in the stone voids, thus meeting the 450 CF requirement. Per Volume 3, Chapter 3 of Fort Collins Amendments to the UDFCD;  50% of the newly added or modified impervious area must be treated by LID techniques and  25% of the new paved areas must be pervious. The pervious pavers will consist of 33% of the proposed pavement areas, thus meeting the second requirement. Approximately 0.25 acres of the site (62% of site runoff) will be tributary to the pavers, thus more than 50% of the site will be treated by the water quality BMP. Attachments  NRCS Soils Map  FEMA Floodplain Map  Existing and Proposed Area Exhibit Hydrologic Soil Group—Larimer County Area, Colorado Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 12/6/2016 Page 1 of 4 4490330 4490340 4490350 4490360 4490370 4490380 4490390 4490400 4490410 4490420 4490430 4490440 4490330 4490340 4490350 4490360 4490370 4490380 4490390 4490400 4490410 4490420 4490430 4490440 493400 493410 493420 493430 493440 493450 493460 493470 493480 493400 493410 493420 493430 493440 493450 493460 493470 493480 40° 33' 53'' N 105° 4' 40'' W 40° 33' 53'' N 105° 4' 37'' W 40° 33' 49'' N 105° 4' 40'' W 40° 33' 49'' N 105° 4' 37'' W N Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84 0 25 50 100 150 Feet 0 5 10 20 30 Meters Map Scale: 1:563 if printed on A portrait (8.5" x 11") sheet. Warning: 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: http://websoilsurvey.nrcs.usda.gov 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 Hydrologic Soil Group Hydrologic Soil Group— Summary by Map Unit — Larimer County Area, Colorado (CO644) Map unit symbol Map unit name Rating Acres in AOI Percent of AOI 4 Altvan-Satanta loams, 3 to 9 percent slopes B 0.4 24.5% 35 Fort Collins loam, 0 to 3 percent slopes C 1.2 75.5% Totals for Area of Interest 1.6 100.0% 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 Hydrologic Soil Group—Larimer County Area, Colorado Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 12/6/2016 Page 3 of 4 Component Percent Cutoff: None Specified Tie-break Rule: Higher Hydrologic Soil Group—Larimer County Area, Colorado Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 12/6/2016 Page 4 of 4 CHOICE CENTER DRIVE COLLEGE AVENUE CHOICE CENTER DRIVE COLLEGE AVENUE © TOWNHOUSES COLORADO STATE 1751 S. COLLEGE AVENUE FORT COLLINS, CO 80525 AREA LEGEND EXISTING CONDITIONS 1" = 20' NORTH PROPOSED CONDITIONS 1" = 20' EX 1.0 EXISTING AND PROPOSED CONDITIONS EXISTING CONDITIONS PROPOSED CONDITIONS Survey Area Data: Version 11, Sep 23, 2016 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Apr 22, 2011—Apr 28, 2011 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 12/6/2016 Page 2 of 4