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Home My WebLink AboutDrainage Report 3,\ o L k -93- Drainage Report for Gateway Retail Center College Station, Texas February 2004 Revised March 2004 Developer: Todd Carnes College Station, Texas (979) 693 -7835 Prepared By: TEXCON General Contractors 1707 Graham Road College Station, Texas 77845 (979) 764 -7743 CERTIFICATION I, Joseph P. Schultz, Licensed Professional Engineer No. 65889, State of Texas, certify that this report for the drainage design for the Gateway Retail Center in College Station, Texas, was prepared by me in accordance with the provisions of the City of College Station Drainage Policy and Design Standards for the owners hereof, with the exception that storm water runoff detention is not being required at this site since the site discharges directly into a storm sewer system which discharges into the existing detention facility for the entire Gateway Development. �i P�� 9 S . • • s # • ▪ * JOSEPH P. SCHULTZ d r-33'. 65889 Q ` Joseph 1 / . c ltz, P.E. ,OT '4FG/S • 14 .k �. , 1 TE •• 7.rS -o 4- TABLE OF CONTENTS GATEWAY RETAIL CENTER CERTIFICATION 1 TABLE OF CONTENTS 2 LIST OF TABLES 2 INTRODUCTION 3 GENERAL LOCATION AND DESCRIPTION 3 DEVELOPMENT DRAINAGE PATTERNS 3 DRAINAGE DESIGN CRITERIA 3 STORM WATER RUNOFF DETERMINATION 4 STORM SEWER PIPE & INLET DESIGN 4 DETENTION FACILITY 5 CONCLUSIONS 5 APPENDIX A 6 Calculations EXHIBIT 11 Sheet S3 — Drainage and Grading Plan LIST OF TABLES TABLE 1 — Rainfall Intensity & Runoff Data 4 TABLE 2 — Runoff Data 4 DRAINAGE REPORT GATEWAY RETAIL CENTER INTRODUCTION The purpose of this report is to provide the hydrological effects of the construction of the infrastructure for the Gateway Retail Center project, and to show that the storm water runoff will be controlled in such a manner so as to have minimal offsite or downstream impact. GENERAL LOCATION AND DESCRIPTION The project is located on approximately 1.5 acres located in the Gateway Development in College Station, Texas. The site is a portion of Block 1, Lot 2R of the Phase 1 of the Gateway Subdivision. The project will be developed in 2 phases with approximately 0.8 acres being developed in Phase 1. This site has previously been developed with the clearing and grading completed with the Home Depot project. In addition, the site has previously constructed driveways on all four sides, some of which are on this tract. The site falls approximately 6' in elevation from the west side to east side of the site. DEVELOPMENT DRAINAGE PATTERNS The storm water runoff from the site flows into an existing storm sewer inlet at the north corner of the site. The post development drainage area boundary for both phases is shown on Sheet S3, the Drainage and Grading Plan. The grading plan is based on elevations relative to the site with the existing grate inlet top assumed to be Elevation 95.3, as shown on the plans. This inlet is located near the existing driveway east of the site. The existing storm sewer inlet was constructed with the Home Depot development located to the north of this site. The Drainage/Detention Report for the Home Depot development was prepared in August 2002 by Bury + Partners — SA, Inc. A review of this report shows that this site is part of Drainage Area 7, which had an area of 2.5 acres, a "c" value of 0.85, and a time of concentration, Tc, value of 10 minutes. This area included a portion of Lot 2R to the east of this site, which is being developed separately and will have its own storm sewer system. DRAINAGE DESIGN CRITERIA The design parameters for the storm sewer are as follows: • The Rational Method is utilized to determine peak storm water runoff rates for the storm sewer design. • Design Storm Frequency Storm sewer system 10 and 100 -year storms events • Runoff Coefficients Landscape areas C = 0.60 Impervious surfaces C = 0.90 • Rainfall Intensity equations and values for Brazos County can be found in Table 1. • Time of Concentration, t, — For smaller drainage areas, a minimum t of 10 minutes is used to determine the rainfall intensity values. TABLE 1 - Rainfall Intensity & Runoff Data Rainfall Intensity Values (in /hr) Storm t = I = b / (tc +d) Event 10 min 1 = Rainfall Intensity (in /hr) 1 7.693 110 8.635 t = U(V *60) 125 9.861 t = Time of concentration (min) 1 11.148 L = Length (ft) 1100 11.639 V = Velocity (ft/sec) Brazos County: 5 year storm 10 year storm 25 year storm 50 year storm 100 year storm b = 76 b = 80 b = 89 b = 98 b = 96 d = 8.5 d = 8.5 d = 8.5 d = 8.5 d = 8.0 e = 0.785 e = 0.763 e = 0.754 e = 0.745 e = 0.730 (Data taken from State Department of Highways and Public Transportation Hydraulic Manual, page 2 -16) STORM WATER RUNOFF DETERMINATION The peak runoff values were determined in accordance with the criteria presented in the previous section for the 10- and 100 -year storm events. The runoff coefficients are based on the development of both phases of this site. The drainage area is shown on Sheet S3, The Drainage and Grading Plan. Runoff conditions are summarized in Table 2. TABLE 2 — Runoff Data Area No Area Size Composite 1 Q10 1 100 Q100 (acres) C (in /hr) (cfs) (in /hr) (cfs) 1 1.18 0.85 8.64 8.67 11.64 11.67 STORM SEWER PIPE & INLET DESIGN The existing private storm sewer will collect the runoff from both phases of this project. A small portion of the project site will drain directly into the existing driveway along the northern boundary of the site. This runoff will eventually enter the public storm sewer system through the existing inlets located in the existing driveways. The grate for the existing storm sewer inlet is a 36" x 36" V -5736 grate manufactured by East Jordan Iron Works. The open area for the grate is 693 square inches, or 4.81 square feet (sf). Assuming the grate is 50% clogged, the available open area is 2.4 sf. The depth of water above the grate is determined by the orifice equation: Q = 4.82* A * y1 /2 4 Therefore, for the 10- and 100 -year storm events the following depths are calculated: Qio = 8.67 cfs -' y = 0.56' = 6.74" Qioo = 11.67 cfs - y = 1.02' = 12.2" The top of the grate is proposed to be at elevation 95.3, and the top of the berm around the inlet will be constructed to elevation 97.0. There will be 8" of freeboard during the 100 - year storm event. The existing 30" diameter storm sewer pipe can adequately carry the runoff. The velocity in this pipe will be 6.3 fps and 6.8 fps for the 10 -year and 100 -year storm events, respectively. DETENTION FACILITY No storm water detention is required for this site because the existing private and public storm sewer systems and the Home Depot detention pond were designed for the full development of the entire Gateway Subdivision development. The design of the facilities is provided in the aforementioned Drainage/Detention Report prepared by Bury + Partners. CONCLUSIONS The construction of this project will increase the storm water runoff from this site. However, the runoff will be carried through the proposed and existing storm sewer system and into the existing detention pond. No flood damage to downstream or adjacent landowners is expected as a result of this development. APPENDIX A Calculations 6 10- year.txt Manning Pipe Calculator Given Input Data: Shape Circular Solving for Depth of Flow Diameter 30.0000 in Flowrate 8.6700 cfs Slope 0.0100 ft /ft Manning's n 0.0140 Computed Results: Depth 9.7322 in Area 4.9087 ft2 wetted Area 1.3799 ft2 wetted Perimeter 36.3587 in Perimeter 94.2478 in velocity 6.2831 fps Hydraulic Radius 5.4652 in Percent Full 32.4406 % Full flow Flowrate 38.0873'cfs Full flow velocity 7.7591 fps Page 1 100 - year.txt Manning Pipe Calculator Given Input Data: Shape Circular Solving for Depth of Flow Diameter 30.0000 in Flowrate 11.6700 cfs Slope 0.0100 ft /ft Manning's n 0.0140 Computed Results: Depth 11.3955 in Area 4.9087 ft2 Wetted Area 1.7107 ft2 wetted Perimeter 39.8436 in Perimeter 94.2478 in velocity 6.8217 fps Hydraulic Radius 6.1828 in Percent Full 37.9850 % Full flow Flowrate 38.0873'cfs Full flow velocity 7.7591 fps Page ] EXHIBIT Sheet S3 — Drainage and Grading Plan