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HomeMy WebLinkAboutDrainage Concept 1 t O'Brien 14900 Landmark,Suite 530 Dallas,Texas 75254 Ph:(972)233-2288 Fx:(972)223-2818 Hydraulics/Hydrology Civil Engineering Consulting September 5, 2005 Mr. Alan Gibbs, P.E. Sr. Asst. City Engineer Department of Public Works City of College Station College Station, Texas 77840 RE: Spring Creek Commons Preliminary Drainage Concept OEI Job #262-05-01 Dear Mr. Gibbs: This letter includes our findings of the drainage evaluation for the Spring Creek Commons Development located in College Station, Texas. The evaluation was requested by Mr. Todd McCoy of Falcon Realty Advisors, LLC to evaluate the drainage characteristics of the site under existing and future conditions. Basin Information The drainage basins for the project include approximately 75 acres contributing to the unnamed tributary into Spring Creek and a smaller area along the western edge of the proposed tract draining directly into Spring Creek. Most of the contributing drainage area to the development is located north of SH 6 and Greens Prairie Road intersection, while additional off-site drainage is located south of SH 6. Mr. Alan Gibbs, P.E. September 5, 2005 Page 2 of 6 The highest point of the watershed is south of SH 6 with an approximate elevation of 326 feet above mean sea level. Runoff flows from this point north through roadside ditches and culverts under SH 6. A total of three existing culverts drain off-site runoff into the project site where all of the off-site runoff combines and empties into the unnamed tributary to Spring Creek. Just downstream of the project site along the unnamed tributary exists the Woodland Hills Park detention pond constructed in conjunction with the Woodland Hills Subdivision. The primary outlet of the pond drains directly into Spring Creek where the elevation of the lowest point in the watershed is approximately 250 feet above mean sea level. The existing land use for the basin is mostly natural grasslands and woodlands. Some existing developments are located near the project site, which include the Woodland Hills Subdivision on the northern boundary and a commercial restaurant on the south near the previously mentioned intersection. Additional development exists off-site south of SH 6. Under the proposed conditions approximately 56 acres will be changed from the natural grasslands and woodlands to a commercial development. Hydrologic Methodology The hydrologic analysis included using the windows version of TR-55 (version 2002.00.22) created by the NRCS. TR-55 was selected over the rational method for an analysis of multiple sub-basins routed and combined at the final point before combining with Spring Creek. Using this model peak runoff rates were determined both entering and leaving the project site. Input parameters into the model for each sub-basin include area, SCS curve number, and time of concentration. Routing reach input data includes length, Manning's n roughness coefficient, slope, and channel shape. SCS curve numbers were generated for each sub-basin by splitting each basin into its respective land use and calculating a composite curve number. The method for selecting curve numbers for Mr. Alan Gibbs, P.E. September 5, 2005 Page 3 of 6 each land use follows Table III-4 of the City of College Station Drainage Policy and Design Standards. Table 1 below list the curve numbers associated with each land use used in this analysis. Table 1 -Curve Numbers for Land Uses and Cover Land Use ID Description Curve Number 20 Natural Grasslands 75 30 Landscaped Areas 77 40 Impervious Areas 98 120 Medium Density Residential 87 140 Business/Commercial 94 The time of concentration for each sub-basin was calculated using the Tc calculator found in TR- 55. This tool splits the flow type into sheet flow, shallow concentrated, and channel flow. The input parameters were determined using AutoCAD and the City topographic mapping. Routing data was compiled using AutoCAD and the previously mentioned topographic data with photographs from on-site field visits. Physical parameters such as length and slope were determined from the mapping while the Manning's n coefficient was selected using photographs and Table VII-1 of the City of College Station Drainage Policy and Design Standards as a reference for local conditions. Post-project modeling used the existing model with updates to SCS curve numbers and adjustments to the watershed schematic according to the proposed development. The SCS curve number in the project site was changed to a value of 94 for a business and commercial land use while the three existing sub-basins in the project site were merged into one as shown on Exhibits A and B,Existing Conditions Watershed Map and Proposed Conditions Watershed Map. Mr. Alan Gibbs, P.E. September 5, 2005 Page 4 of 6 Results The results for the existing and proposed conditions included peak runoff rates for the 1, 2, 5, 10, 25, 50, and 100-year storm event for each sub-basin, reach and outlet. The existing conditions has one outlet into Spring Creek, via the unnamed tributary while the proposed concept outlet would be located farther upstream of the existing tributary outfall in Spring Creek. Basin 1 in the proposed conditions will not directly contribute to the final outlet in the proposed concept, but drains directly into Spring Creek. Table 2 and Table 3 list the peak runoff results for the existing conditions and proposed concept. Table 2-Existing Peak Flow Rates 1-Year 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year BASIN 1 14 24 40 52 67 78 91 BASIN 2A 30 47 74 94 118 136 157 BASIN 2B 18 29 46 58 74 85 98 BASIN 3A 7 11 18 23 29 33 39 BASIN 3B 17 27 44 56 72 83 96 BASIN 4A 16 25 40 51 65 74 86 BASIN 4B 10 15 23 29 36 41 47 R2B 30 47 74 94 118 136 157 Down 30 47 74 93 118 135 156 R3B 7 11 18 23 29 33 39 Down 7 11 18 23 29 33 39 R4B 16 25 40 51 65 74 86 Down 16 25 40 51 65 74 86 OUTLET 105 168 272 348 444 511 594 Mr. Alan Gibbs, P.E. September 5, 2005 Page 5 of 6 Table 3 Proposed Concept Peak Flow Rates 1-Year 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year BASIN 1 24 33 46 55 66 74 84 BASIN 2A 38 57 88 111 139 159 183 BASIN 2B 150 202 281 338 408 456 516 BASIN 3 12 18 27 34 43 49 56 BASIN 4 28 41 62 77 96 108 124 OUTLET 211 293 424 517 632 711 811 Conclusions This analysis only includes a hydrologic analysis while any hydraulic computations such as detention pond sizing are not included. The detention requirements by the City of College Station are intended for the secondary drainage systems to "limit post development discharge rates to those that existed in existing conditions " The city further explains that the secondary drainage systems are sensitive to short high intensity events while the primary drainage systems are sensitive to longer duration moderate intensity rainfall events. For this analysis no detention is proposed due to the project site discharging directly into a primary drainage system. The final design will include a flow diffuser and energy dissipater to reduce outfall velocities into Spring Creek to non-erosive levels. The outfall and energy structure will require an offsite easement that the Owner will secure from the appropriate property owner. The analysis herein is based on the Architect's current site plan which includes nearly complete development of the site with commercial use and is not based on a specific site civil design. The analysis is subject to final civil design, including specific drainage feature components, sizes, alignments, slopes, drainage divides, and the like. Mr. Alan Gibbs, P.E. September 5, 2005 Page 6 of 6 It has been our pleasure to complete this analysis and we would be happy to answer any questions or respond to any comments. Feel free to contact us at the numbers above or via email at jobrien@oeidallas.com. Sincerely, O'BRIEN ENGINEERING, INC. ,:'::-'.77.,.`Z 11`4. :,P OF TFktilk4,g —4 V 1 JIM O BRIEN / "' ✓`J ��►� ,z 55861 �� �1fa.1irSS, -- ��C)`~rr Jim O'Brien, P.E., CFM President V , 0 ,r/3 .-- for Jacob S. 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