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Home My WebLink About4235 S MASON ST - MASON CENTER SOUTH - FDP200010 - SUBMITTAL DOCUMENTS - ROUND 4 - DRAINAGE REPORTDRAINAGE REPORT Blue Ribbon Auto Body Site Improvements 4235 & 4221 S. Mason Street (NORTHWEST CORNER OF MASON STREET & TROUTMAN PARKWAY) FORT COLLINS, COLORADO April 30, 2021 Revised July 21, 2021 Revised Sept 8, 2021 Prepared for: Blue Ribbon Auto Body Contact: Marc James Shen Maliakai Architectural Design 636 Fairfield Lane, Louisville, CO 80027 Ph: 303.478.1875 www.maliakaidesign.com Prepared By: Armfield Engineering, LLC Kenneth N. Armfield, P.E. 904 Alta Street, Longmont, CO 80501 (720) 363-7125 Compliance statement: “I hereby attest that this report for the [preliminary or final] drainage design for the 4235 S. Mason Street - Mason Center South project was prepared by me or under my direct supervision, in accordance with the provisions of the Fort Collins Stormwater Criteria Manual. I understand that the City of Fort Collins does not and will not assume liability for drainage facilities designed by others.” ________________________ Kenneth N. Armfield Registered Professional Engineer State of Colorado No. 32527 Armfield Engineering armfieldengineering.com (720) 363‐7125 FINAL DRAINAGE REPORT BLUE RIBBON AUTO BODY SITE IMPROVEMENTS 4235 & 4221 S. MASON STREET, LONGMONT, COLORADO I. INTRODUCTION This report describes the storm drainage design and conformance to applicable drainage criteria for the proposed improvements for the site. Please also refer to the drawings in the appendix of this report. In general, the proposed improvements for the site will include fence, landscaping, and the placement of stabilized gravel for a vehicle staging area located the west of the existing building at 4235 S. Mason Street and southeast of the existing Blue Ribbon Auto Body building at 4221 S. Mason Street. In addition, the driveway entrance onto Mason Street at the 4235 S. Mason Street building will be re-worked to bring it up to ADA standards. (Approx. 700 sf.) The amount of disturbed area on the site includes the grading of approximately 4,000 square feet (0.09 acres) to the west of the building at 4235 S. Mason Street to level out the land in preparation for the placement of the stabilized gravel. In addition, stabilized gravel will also be placed over another 6,600 sf (0.15 acres) of land behind 4235 S. Mason and between this area and the south end of 4221 S. Mason. This area is currently a gravel storage area and will require relatively little grading prior to the addition of the stabilized gravel layer on top of it. Total disturbed area for all improvements is expected to be approximately 5,000 SF, well below the City’s 10,000 SF threshold for a separate erosion control plan. The proposed stabilized gravel will be a layer of gravel placed in a TrueGrid Geogrid (or approved equal) geotextile mat. This gravel stabilization method has previously been used in Fort Collins at 3325 S. Timberline Road. To provide Water Quality treatment, ¼ of the gravel area will have an 8” thick storage/treatment section with an underdrain. II. GENERAL LOCATION AND DESCRIPTION The site is located at 4235 S. Mason Street, on the northwest corner of Mason Street and Troutman Parkway in Fort Collins, Colorado. It also includes an area between the back half of 4235 S. Mason and the southwest corner of the existing Blue Ribbon Auto Body building at 4221 S. Mason Street. Drainage Report 4235 & 4221 S. Mason Street Blue Ribbon Auto Body Sept 8, 2021 Page 2 of 4 Armfield Engineering, LLC armfieldengineering.com Ph: 720‐363‐7125 A Trans-Fort Bus stop borders the site on the west, a commercial building is to the north of 4235 S. Mason, and the streets border the east and south sides. III. CALCULATION METHODS, PROGRAMS, AND CRITERIA The property is located within the boundaries of the Mail Creek Master Drainage Plan, however based upon conversation with the City of Fort Collins engineering department there are no special drainage requirements pertaining to that master plan for this site. The proposed project will increase the imperviousness of the site by 0.2%. Please refer to the imperviousness spreadsheet and the Existing and Proposed Conditions basin maps in the appendix. The net change in impervious area is approximately negative 300 sf. The modified paved area is approximately 905 sf including the addition of the landscape area at the SE corner of the building. (MIA = impervious surfaces changing type. eg. paving to building or regrading a paved area). The sum of these areas is approximately 600 sf, which is less than 1000 sf. Because approximately 11,000 sf of existing area will be regraded or modified by placing the stabilized gravel, stormwater water quality (WQ) treatment will be provided. Per agreement with the City of Fort Collins stormwater department, this WQ treatment may be achieved by increasing the gravel base below the geo-grid to an 8” thickness with an underdrain. IV. TOPOGRAPHY The site of the proposed vehicle staging area is an existing gravel storage area on the west side of the existing building at 4235 S. Mason, plus an existing gravel storage area to the north of the back half of 4235 S. Mason. Behind (west of) the building at 4235 S. Mason street the site slopes generally to the northeast with 10% slopes along the west and southwest side of the proposed paved area which flatten out to flatter slopes in the center of the parking area. The existing parking area drains to the northeast and then eastward to Mason Street through an existing gate and paved driveway on the north side of the 4235 building. The area to the north of the back half of 4235 S. Mason drains to the south and east with slopes of approximately 1 to 2%. This area also drains eastward to Mason Street through the existing gate and paved driveway on the north side of the 4235 building, and by sheet flow through the paved parking area of 4229 S. Mason Street. Drainage Report 4235 & 4221 S. Mason Street Blue Ribbon Auto Body Sept 8, 2021 Page 3 of 4 Armfield Engineering, LLC armfieldengineering.com Ph: 720‐363‐7125 V. EXISTING DRAINAGE CONDITIONS The site is not in a FEMA floodplain. Drainage Basins: (Please refer to the Existing Basins drainage map exhibit.) Runoff from the 4235 S. Mason Street property currently drains either eastward into Mason Street or southward into Troutman Parkway. From these two streets, the runoff flows to the existing public storm inlet located at the NW corner of these two streets. Basin E: Basin E on the Existing Basins map contains 0.26 acres and consists of the area west of and northwest of the existing building at 4235 S. Mason Street that is proposed to have grading and stabilized gravel improvements. Runoff from the existing west rear storage area of 4235 S. Mason (the south portion of Basin E on the Existing Basins map) currently drains to and through the gate and paved driveway at the northeast corner of the parking area at Design Point 1 (DP1). Once the runoff passes through the gate at DP1, it flows eastward into Mason Street. Runoff from the area to the north of the back half of 4235 S. Mason Street Mason (the north portion of Basin E on the Existing Basins map) drains to the south and east with slopes of approximately 1 to 2%. This area also drains eastward to Mason Street through the existing gate and paved driveway on the north side of the 4235 building at Design Point 1, and by sheet flow through the paved parking area of 4229 S. Mason Street. Basin B: Basin B on the Existing Basins map consists of 0.06 acres of an existing asphalt paved area on the west side of the 4235 S. Mason Street building. This basin is not proposed for any reconstruction and drains to DP1 at the NW corner of the 4235 S. Mason Street building. No change in imperviousness is proposed for this basin. Basin C: Basin C on the Existing Basins map consists of 0.33 acres on the eastern half of the 4235 S. Mason Street property and contains the roof of the existing building and the existing parking, driveway, and landscape areas. This basin is not proposed for any improvement (except for the reconstruction of the Mason Street driveway) and drains both eastward to Mason Street and southward into the Troutman Parkway cul de sac. No change in imperviousness is proposed for this basin. VI. DRAINAGE FACILITY DESIGN In the following paragraphs will be discussed the proposed drainage concepts for the developed conditions. Supporting calculations and drawings are contained in the appendix of this report. Runoff Patterns: Runoff patterns will remain the same for all basins. Basins B and C will remain essentially unchanged from existing conditions (except for the reconstruction of the Drainage Report 4235 & 4221 S. Mason Street Blue Ribbon Auto Body Sept 8, 2021 Page 4 of 4 Armfield Engineering, LLC armfieldengineering.com Ph: 720‐363‐7125 Mason Street driveway). Basin P will have the same runoff pattern as the existing conditions of Basin E, except that an interceptor swale will be added along the east side of the northern half of the staging area to direct runoff away from the neighboring property. A small area (aprox 400 sf) at the SE corner of the building will be modified to add some landscaping and to install a sidewalk between the building and the Troutman Parkway sidewalk. The amount of pavement being removed is slightly more than the amount of concrete being added, but still considered to be a negligible amount. Basin P: Basin P consists of the 0.26 acres contain in Basin E. Improvements in Basin P will include grading, placement of stabilized gravel, and the addition of a landscaping buffer on the south side of the basin. Work in Basin P will include the removal of approximately 540 SF of existing concrete pavement and the addition of approximately 230 SF of concrete at the north end of Basin P at the proposed entrance gate, for a net removal of approximately 310 SF of concrete. To provide Water Quality treatment, ¼ of the gravel area in Basin P will have an 8” thick storage/treatment section with an underdrain. The underdrain will route flows to a location north of the the NE corner of the existing building, where it will daylight into an existing landscape area. This is the historic flow direction and destination for runoff from Basin P. Although there will be a net reduction of impervious concrete areas in Basin P, there will also be a net reduction in the amount of grass area (which will be replaced by gravel/riprap). Because the imperviousness of grass is 0% but the imperviousness of gravel/riprap is 40%, this results in a net increase of 0.5% imperviousness for Basin P compared to Basin E. When the rest of the site’s imperviousness (adding in Basins B and C) is taken into account, the total site’s imperviousness changes from 64.5% to 64.7%, for a net increase of 0.2%. This is considered to be a negligible change regarding potential stormwater runoff. VIII. CONCLUSION: The drainage plan for this site is designed to provide for the safe conveyance of stormwater runoff away from the proposed development and to follow the criteria of the City of Fort Collins as well as standard engineering practices. Runoff from the site will be released downstream in its historical drainage patterns and with negligible increase in runoff amount, and the proposed improvements are not expected to cause damage to downstream property owners. APPENDIX Chemical Soil Properties This table shows estimates of some chemical characteristics and features that affect soil behavior. These estimates are given for the layers of each soil in the survey area. The estimates are based on field observations and on test data for these and similar soils. Depth to the upper and lower boundaries of each layer is indicated. Cation-exchange capacity is the total amount of extractable cations that can be held by the soil, expressed in terms of milliequivalents per 100 grams of soil at neutrality (pH 7.0) or at some other stated pH value. Soils having a low cation- exchange capacity hold fewer cations and may require more frequent applications of fertilizer than soils having a high cation-exchange capacity. The ability to retain cations reduces the hazard of ground-water pollution. Effective cation-exchange capacity refers to the sum of extractable cations plus aluminum expressed in terms of milliequivalents per 100 grams of soil. It is determined for soils that have pH of less than 5.5. Soil reaction is a measure of acidity or alkalinity. It is important in selecting crops and other plants, in evaluating soil amendments for fertility and stabilization, and in determining the risk of corrosion. Calcium carbonate equivalent is the percent of carbonates, by weight, in the fraction of the soil less than 2 millimeters in size. The availability of plant nutrients is influenced by the amount of carbonates in the soil. Gypsum is expressed as a percent, by weight, of hydrated calcium sulfates in the fraction of the soil less than 20 millimeters in size. Gypsum is partially soluble in water. Soils that have a high content of gypsum may collapse if the gypsum is removed by percolating water. Salinity is a measure of soluble salts in the soil at saturation. It is expressed as the electrical conductivity of the saturation extract, in millimhos per centimeter at 25 degrees C. Estimates are based on field and laboratory measurements at representative sites of nonirrigated soils. The salinity of irrigated soils is affected by the quality of the irrigation water and by the frequency of water application. Hence, the salinity of soils in individual fields can differ greatly from the value given in the table. Salinity affects the suitability of a soil for crop production, the stability of soil if used as construction material, and the potential of the soil to corrode metal and concrete. Sodium adsorption ratio (SAR) is a measure of the amount of sodium (Na) relative to calcium (Ca) and magnesium (Mg) in the water extract from saturated soil paste. It is the ratio of the Na concentration divided by the square root of one-half of the Ca + Mg concentration. Soils that have SAR values of 13 or more may be characterized by an increased dispersion of organic matter and clay particles, reduced saturated hydraulic conductivity and aeration, and a general degradation of soil structure. Chemical Soil Properties---Larimer County Area, Colorado 4235 S Mason SOIL PH Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/29/2021 Page 1 of 2 Report—Chemical Soil Properties Chemical Soil Properties–Larimer County Area, Colorado Map symbol and soil name Depth Cation- exchange capacity Effective cation- exchange capacity Soil reaction Calcium carbonate Gypsum Salinity Sodium adsorption ratio In meq/100g meq/100g pH Pct Pct mmhos/cm 4—Altvan-Satanta loams, 3 to 9 percent slopes Altvan 0-9 10-20 —6.1-7.8 0 0 0 0 9-16 10-25 —6.6-8.4 0-5 0 0 0 16-31 5.0-10 —7.4-9.0 1-10 0 0 0 31-60 0.0-5.0 —7.4-9.0 1-10 0 0 0 Satanta 0-9 5.0-20 —6.1-7.8 0 0 0 0 9-14 10-30 —6.6-8.4 0-5 0 0 0 14-60 5.0-20 —7.4-8.4 1-10 0 0 0 74—Nunn clay loam, 1 to 3 percent slopes Nunn 0-9 22-29 —6.6-7.8 0-1 0 0.1-2.0 0 9-13 27-33 —6.6-7.8 0-1 0 0.1-2.0 0 13-25 26-31 —7.4-8.5 0-5 0 0.1-2.0 0 25-38 14-24 —7.9-8.6 2-5 0 0.1-2.0 0-1 38-80 11-24 —7.4-8.6 2-7 0 0.1-2.0 0-1 Data Source Information Soil Survey Area: Larimer County Area, Colorado Survey Area Data: Version 15, Jun 9, 2020 Chemical Soil Properties---Larimer County Area, Colorado 4235 S Mason SOIL PH Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/29/2021 Page 2 of 2 ImperviousnessEPBC IMPERVIOUSNESS CALCULATIONS EXISTING & DEVELOPED AREA Blue Ribbon Auto Body KNA 7/21/21 Basin B (same for existing & proposed) AREA % OF IMPERV (SF.) TOTAL AREA % gravel 0.00 0.0 40 pavement 2,620.00 100.0 100 grass/weeds 0.00 0.0 0 TOTAL 2,620.00 100.0 100.00 Basin C (same for existing & proposed) AREA % OF IMPERV (SF.) TOTAL AREA % gravel 0.00 0.0 40 pavement/roof 11,100.00 77.4 100 grass/landscaping 3,250.00 22.6 0 TOTAL 14,350.00 100.0 77.352 Basin E (existing west side) AREA % OF IMPERV (SF.) TOTAL AREA % gravel 9,860.00 86.3 40 pavement 660.00 5.8 100 grass/weeds 910.00 8.0 0 TOTAL 11,430.00 100.0 40.28 Basin P (proposed west side) AREA % OF IMPERV (SF.) TOTAL AREA % gravel 10,780.00 94.3 40 pavement (P1) 350.00 3.1 100 grass/landscaping 300.00 2.6 0 TOTAL 11,430.00 100.0 40.79 EXISTING CONDITIONS (entire site) AREA % OF IMPERV (SF.) TOTAL AREA % BASIN E 11,430.00 40.2 40.28 BASIN B 2,620.00 9.2 100 BASIN C 14,350.00 50.5 77.35 TOTAL 28,400.00 100.0 64.52 EXISTING CONDITIONS (entire site by surface) AREA % OF IMPERV (SF.) TOTAL AREA % gravel 9,860.00 34.7 40.28 pavement/roof 14,380.00 50.6 100 grass/landscaping 4,160.00 14.6 77.35 TOTAL 28,400.00 100.0 75.95 PROPOSED CONDITIONS (entire site) AREA % OF IMPERV (SF.) TOTAL AREA % BASIN P 11,430.00 40.2 40.79 BASIN B 2,620.00 9.2 100 BASIN C 14,350.00 50.5 77.35 TOTAL 28,400.00 100.0 64.73 PROP. CONDITIONS (entire site by surface) AREA % OF IMPERV (SF.) TOTAL AREA % gravel 10,780.00 38.0 40.79 pavement/roof 14,070.00 49.5 100 grass/landscaping 3,550.00 12.5 77.35 TOTAL 28,400.00 100.0 74.69 CHANGE FROM EXIST TO PROP. CONDITION* AREA (SF.) gravel 920.00 pavement/roof -310.00 grass/landscaping -610.00 *Assumes negligible change in Basins B&C