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HomeMy WebLinkAbout1ST CHOICE BANK OF FORT COLLINS (TEMPORARY FACILITY) - MINOR SUBDIVISION - 41-94 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT.40 a wind barrier. For this reason the wind erosion features at the site will consist primarily of natural features at the site, water erosion controls and the features to be constructed. The wind erosion control measures which will be implemented during construction will include the items listed below and are detailed on the Grading, Drainage and Erosion Control Plan at the end of this report. • The east/west irrigation channel berm along the south margin of the site. • The silt fence along the north and east margins of the temporary property. • The temporary building. • The paved parking areas to the south and west of the temporary building. Wind erosion control measures which will be implemented after construction will consist of mulching and seeding the undisturbed, bare area. Hay or straw mulch will be applied and crimped into the soil surface at a rate of 2 tons per acre after seeding of the site. The silt fence will be removed when the mulch is applied. INSTALLATION SCHEDULE Construction on the temporary facility is proposed to start in September 1994 and M. continue during October 1994 with the building, grading and paving completed by mid - October 1994. The approximate date of implementation of the erosion control measure during construction are shown on Standard Form C in Appendix II and on the r Drainage, Grading and Erosion Control Plan. As shown on Form C the silt fence shall ■ be constructed prior to overlot grading. Temporary vegetation will be seeded and ■ mulched after the completion of the project during the fall, planting season beginning in October prior to the next wind erosion season. EROSION CONTROL SECURITY The erosion control security requirement for the 1ST Choice Bank Temporary Facility site was estimated based on the specification of Section 2.0 of the Manual. The cost of the erosion control measures at the site as detailed above were estimated to cost $1,220 with a security requirement of $1,830. The estimated cost to revegetate the disturbed areas with a dryland grass mix according to the city's 1994 mulch and revegetation bid was $1,300 with a security requirement of $1,950. The larger amount is required prior to construction. Both cost estimates are detailed in Appendix II. REFERENCES City of Fort Collins, Colorado, Storm Drainage Design Criteria and Construction Standards, May 1984, Revised January 1991. weighted P - Factor of 1.0 was used to calculate the effectiveness. The P - Factor for the basin is presented for both the during and after construction cases are.presented on Standard Form B in Appendix II. Cover Factor Measures Cover factor measures for the site during construction will consist of the building and the parking areas and the undisturbed grass areas. The building and paved areas were assigned a C - Factor value of 0.01. The undisturbed grass cover to the east and south is estimated to contain 70 percent vegetative cover and was assigned a C-Factor value of 0.05 selected from Figure 5.1 of the Manual. Cover factor measures for the site after construction is completed will consist of the establishment of a dryland grass cover of at least 60 percent cover over the undisturbed, bare areas of the site as detailed on Sheet 4. Grass will be established with the use of hay or straw mulch applied and secured at a rate of 2 tons per acre. The mulched and revegetated areas were assigned C - Factor values of 0.06 as selected from Table 5.2 and Figure 5.1. Cover factor values for the paved areas and the undisturbed grassed areas were not changed from during construction conditions. The calculations for the weighted C - Factor for both during and after construction are presented on Standard Form B in Appendix H. "During Construction" Effectiveness Evaluation The Net Effectiveness for the erosion control measures at the site during construction was calculated as 82.5 percent and exceeds the "During Construction" Performance Standard value of 78.2 percent. Calculations of the basin effectiveness value and the Net Effectiveness for the site are presented on Standard Form B in Appendix II. "After Construction" Effectiveness Evaluation The Net Effectiveness for the erosion control measures at the site after construction was calculated as 92.6 percent and exceeds the "After Construction" Performance Standard value of 92.2 percent. Calculations of the basin effectiveness value and the Net Effectiveness for the site are presented on Standard Form B in Appendix II. WIND EROSION ANALYSIS The surface soils at the 1ST Choice Bank Temporary Facility site are rated as moderately wind erodible. Construction at the site may occur during the wind season of November to May. The disturbed area for the site is less than 1.0 acre and thus is excluded from required wind erosion controls. Although no controls are require, many of the construction features and rainfall sediment and erosion controls will retard wind born soil loss. Construction disturbance at the site as detailed above will occur only in the northeast corner of the site over an area of 0.35 acres. The limited size of the construction area Iand the necessary access in this area will restrict the available area for placement of RAINFALL EROSION ANALYSIS Site Soils The Larimer County, Soil Conservation Service Soils Report identifies the soils overlaying the site as belonging to Thedalund Loam having slight to moderate rainfall erodibility characteristics. The rainfall erosion season as detailed in the City of Fort Collins, Erosion Control Reference Manual (11 (the Manual) is between May 1 and October 31. The natural drainage of the site is from the southwest to northeast where any runoff is discharged to the southgutter flowing east along Horsetooth Road. A mixture of dryland grasses and weeds currently cover an estimated 70 percent of the site located primarily on the east and south sides. The remaining 30 percent of the 2.17 acre property having served as a pumpkin and tree lot and parking is considered to be bare ground with sparse to no vegetation. The Manual details the site as having moderate wind erodibility characteristics. The Manual states the predominant wind direction is from the west-northwest in this area. The wind erosion season as detailed in the manual is between November 1 st and May 31 st. Construction at the site is estimated to begin in mid -September 1994 and continue through October 1994, with the majority of the grading and paving work completed by mid -October 1994. This schedule places the project within the rainfall season for this area during construction and in the wind erosion season in the after construction phase. Performance Standard The estimated "During Construction" and "After Construction" Performance Standard for the 1 ST Choice Bank Temporary Facility site are 78.4 and 92.2, respectively. For the purpose of estimating the site's Performance Standard the entire site was evaluated as one, 2.17 acre basin. The Performance Standard for the site was estimated using the methods detailed in the Manual and the estimated, average slope length and slope for the basin. Flow lengths and slopes developed for calculation of the "During Construction" Performance Standard is presented on Standard Form A in Appendix II. The Performance Standard was taken from Table 5.1 of the Manual. The "After Construction" Performance Standard is calculated by dividing the "During Construction" Performance Standard by 0.85. Practice Factor Measures Sediment control practice measures for the 1ST Choice Bank Temporary Facility development will consist of the construction of a silt fence. The location and details of the practice measures are shown on the Grading, Drainage and Erosion Control Plan and Detail Sheet appended at the end of this report. Practice factor measures for the site after construction will be dismantled and thus a I 0.35 for the compacted earth and 0.10 for grassed areas. Using times of concentration taken from overland flow curves, discharges were calculated for 2-year and 100-year events under historic, developed and developed -with -improved - permeability, conditions. The curb to curb area of the intersection at Horsetooth and Landings/Stanford was measured from aerials. The rise in assumed intersection pond height was calculated for the difference in volumes produced by historic and developed with -improved - permeability runoff under a worst -case -pre -storm -conditions -scenario. The worst -case -scenario assumes that when the storm being evaluated occurs, for 2 hours the piped system draining the Marriot Hotel area is full and all line structures surcharged, that similar conditions exist at the intersection piping draining to Warren Lake and that Warren Lake is full, i.e., there is no relief available to carry off the additional runoff from the 1st Choice site. The difference between the volume of runoff produced over 2 hours between historic and developed conditions would cause a pond at the intersection to rise about 4 inches for a 2 year storm and about 12 inches for a 100 year storm. It is our opinion that this is not an unreasonable impact under the highly conservative assumptions used in the analyses and given the temporary nature of the development. Stormwater Utility concurrence is requested. i M I i Road, Landings Drive and Stanford Road to Warren Lake and the capacity of Warren Lake itself. In our opinion, an analysis of this complex system would require more effort than is warranted by the relatively minor impact of the proposed development. Hydrologic Criteria The Rational Method and the runoff from a 2-year and a 100-year storm were used to analyze the impact of the runoff from the site of interest. Calculations All calculations for this report have been made in accordance with criteria established by the City of Fort Collins. Calculations are included in Appendix I. Variances The additional runoff from the developed site may aggravate overtopping at the intersection of Horsetooth Road, Landings Drive and Stanford Road during certain concurrent events. DRAINAGE DESIGN Concept The basic approach to handling the runoff from the site when the permanent banking facility has been constructed, has been discussed with Stormwater Utilities. A detention pond will be constructed on the site containing the maximum feasible amount of runoff that can still be released by gravity to the south gutter system along Horsetooth Road and yet present an aesthetically pleasing setting for the permanent building. The flow in the gutter system will be analyzed, the flow that would most likely be diverted to the piped system draining the Marriot area will be predicted, the benefits of enlarging the inlet at the southwest corner Horsetooth and John F. Kennedy Parkway will be analyzed and the resultant impact at the Horsetooth/Landings Drive intersection from bypassed flow, will be predicted. However, no detention pond construction is envisioned for this temporary banking facility development. The additional runoff from the temporary facility will be partially offset by improving the permeability of the part of the site containing the compacted earth. A variance for the increase in overtopping that may occur at the Horsetooth/Landings Drive intersection is requested. Details Support for the following discussion will be found in the calculations in Appendix I and on the drawings furnished with this report. As stated above, the Rational Method was used for drainage calculations. The total drainable area, the area of the compacted earth, the grassed area and the new impervious area created by the development of the temporary facility, were measured. Composite runoff coefficients were calculated using base coefficients of I w w z z w w ¢ > d U d a O a U SITE EAST HORSETOOTH ROAD Imo_ f > 97 �3 �� 1 1 ❑ I \WARREN LAKE Q \ RESERVOIR tEl w z w ¢ 287 d JHARMONY ' WATER FIGURE 1 Date: JULY 1994 WASTE 1st CHOICE BANK OF FORT COLLINS & LAND SITE LOCATION MAP Project: 94412100 INC. lmu I .1 4 FINAL DRAINAGE AND EROSION CONTROL REPORT FOR THE 1 ST CHOICE BANK TEMPORARY FACILITY FORT COLLINS, COLORADO IDEVELOPMENT LOCATION AND DESCRIPTION Location A new modular building, to be used as a temporary banking facility, is planned for a site at the southeast corner of College Avenue and Horsetooth Road as shown on the enclosed Site Location Map. The site will ultimately contain the permanent banking office of the 1 st Choice Bank of Fort Collins, scheduled for completion in 1995. A site layout at a larger scale is shown on drawings furnished with this report. While the temporary facility and its parking only utilizes approximately 0.35 acres, about 2.17 acres of the permanent site requires analysis. The site lies in the Northeast Quarter of Section 36, Township 7 North, Range 69 West of the Sixth Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado. Property Description The property that both the new temporary and permanent banking facilities will occupy is undeveloped. It is presently partially covered with grasses and is partially covered with compacted earth resulting from earlier use as a fall pumpkin and Christmas tree sales location. The site slopes from the southwest to the northeast at a gradient of approximately 2%. Runoff presently enters the south gutter area of Horsetooth Road and eventually flows into Warren Lake Reservoir. DRAINAGE BASIN IDENTIFICATION IDescription The site lies in the Foothills Drainage Basin. IDRAINAGE DESIGN CRITERIA Reaulations The City of Fort Collins Storm Drainage Design Criteria [11 is being used as the basis for this report. Development Constraints Constraints include the capacity of the gutters in Horsetooth Road, the piped drainage system conveying runoff from part of Horsetooth Road and from the Marriot Hotel area to Warren Lake, the piping that conveys runoff from the intersection of Horsetooth I i 7 �J i WATEIC'* WASTE & LAND @IC. July 28, 1994 Mr.Glen D. Schlueter City of Fort Collins Utility Services Stormwater 235 Mathews Fort Collins, Colorado 80522 2629 Redwing Rd. Suite 200, Fort CL .is, Colorado 80526-2879 (303) 226-3535 fax (303) 226-6475 RE: 1 ST CHOICE BANK OF FORT COLLINS TEMPORARY FACILITY - FINAL DRAINAGE . AND EROSION CONTROL REPORT Dear Mr. Schlueter: Transmitted herewith is a copy of the subject report for your review and approval. If you have any questions or require any modifications please call me. Thank you. Sincerely, WATER, WASTE & LAND, INC Ma rice H. Lutkin, P Manager, Civil Engineering Division II Consulting Engineers and Scientists 0 FINAL DRAINAGE AND EROSION CONTROL REPORT FOR 1 ST CHOICE BANK OF FORT COLLINS TEMPORARY FACILITY FORT COLLINS, COLORADO Prepared for: 1 st Choice Bank of Greeley 2162 35th Avenue Greeley, CO 80634 Prepared by: Water, Waste & Land, Inc. 2629 Redwing Road, Suite 200 Fort Collins, Colorado 80526 July 27, 1994 p FINAL DRAINAGE AND EROSION CONTROL REPO FOR 1 ST CHOICE BANK OF FORT COLLINS TEMPORARY FACILITY FORT COLLINS, COLORADO WATER WASTE & LAND INC.