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Drainage ReportDrainage Report FOR INDIAN L4KES; PHASES N & VI COLLEGE STATION u c � tam e Phase NILUCAN Location Map NTS MAY, 2004 Prepared By: McClure do Browne Engineering /Surveying, Inc. 1008 Woodcreek Drive, Suite 103 College Station, Texas 77845 (979) 693 -3838 1n Drainage Report FOR INDIAN L4AT$ PHASES IV & VI Location Mop NTS MAY, 2004 rr Prepared By: McClure do Browne Engineering /Surveying, Inc. 1008 Woodereek Drive, Suite 103 College Station, Texas 77845 (979) 693 -3838 a CERTIFICATION I, Kent M. Laza, Registered Professional Engineer No. 65923, State of Texas, certify that the Drainage Report for the INDIAN LAKES SUBDIVISION, PHASES IV & VI, was prepared by me in accordance with the provisions of the City of College Station Drainage Policy and Design Standards for the owners thereof. Kent M. Laza, P.E. No. 65923 DRAINAGE REPORT TABLE OF CONTENTS INDIAN LAKES SUBDIVISION, PHASES IV & VI INTRODUCTION 1 GENERAL LOCATION AND DESCRIPTION 1 FLOOD HAZARD INFORMATION 1 PRIMARY DRAINAGE SYSTEM DESCRIPTION 1 DRAINAGE DESIGN CRITERIA 1 DETENTION DESIGN 2 CONCLUSION 3 EXHIBITS FEMA FLOOD INSURANCE RATE MAP EXCERPT A DRAINAGE AREA MAP - PROPOSED CONDITIONS B RATIONAL FORMULAS DRAINAGE AREA CALCULATIONS C -1 CULVERT ANALYSIS: CULVERT 1 C -2 CULVERT ANALYSIS: CULVERT 2 C -3 CULVERT ANALYSIS: CULVERT 5 C -4 DRAINAGE REPORT INDIAN LAKES SUBDIVISION, PHASES IV & VI STORM DRAINAGE REPORT INDIAN LAKES SUBDIVISION, PHASES IV & VI INTRODUCTION: This storm drainage report is intended to analyze the drainage infrastructure proposed for the development of the Indian Lakes Subdivision, Phases IV & VI. The existing and proposed conditions are included to show how the design will achieve the desired drainage objectives of Brazos County and the City of College Station. GENERAL LOCATION AND DESCRIPTION: Indian Lakes Subdivision is located in the southern portion of the extra territorial jurisdiction of College Station in Brazos County. The site is bounded by property owned by Bewley Trust to the North and Indian Lakes Subdivision, Phase 1, to the South and West. Property to the East is owned by Smiling Mallard Development, LTD. and is planned as future phases of the Indian Lakes Subdivision. FLOOD HAZARD INFORMATION: No portion of the site is shown to be within the 100 -year floodplain on the FEMA Flood Insurance Rate Maps (FIRM) for Brazos County, Texas and Incorporated Areas, Map Number 48041CO215C, effective July 2, 1992. As such, this site is not currently regulated under the National Flood Insurance Program An excerpt from the FIRM panel is shown in Exhibit A. PRIMARY DRAINAGE SYSTEM DESCRIPTION: Exhibit B is a drainage area map that shows contours and proposed drainage areas for the site. The proposed streets will have either a 24' or 30' wide paved surface with ditches. Storm water from the site will be captured in the ditches along the roads and conveyed to the natural streams on the site. Corrugated metal pipe culverts will be used at the street crossings. DRAINAGE DESIGN CRITERIA: Since this project falls within the ETJ of College Station, the drainage system for Indian Lakes was designed using the criteria in College Station's Drainage Policy and Design Standards. The design storm is a 10 -year rainfall event for ditch line culverts and a 25 -year rainfall event for all stream crossing drainage STORM DRAINAGE REPORT PAGE 1 INDIAN LAKES SUBDIVISION, PHASE IV & VI structures that are "non- trapped" and 100 -year event for "trapped" conditions. Flow calculations for culverts "1" & "2" are based on the Rational Method (Exhibit C -1). Flow for culvert "3" (emergency spillway crossing) is taken from the Engineering Report for Lake Arapaho prepared by East Texas Engineers, Inc. The 100 year flow was determined to be 100 cfs. Pipe sizes were determined by analyzing culvert capacity under inlet and outlet control. All proposed culverts meet requirements for freeboard and road overtopping. Exhibit C -2 describes the peak runoff calculations for the post - development conditions of the culverts shown in Exhibit B. A summary of these results is shown below: Flowrates (efs) from Culverts In Indian Lakes Subdivision, Phase IV & VI Culvert 1 5 year 10 year 25 year 50 year 100 year 1 31.4 35.3 40.3 45.5 51.2 2 29.6 33.2 38.0 42.9 48.2 These runoff rates were used to design the roadway crossings shown in Exhibits D -1 through D -8. The resulting culvert sizes are as follows: Culvert Sizes Culvert Culvert Size 1 2 -24" Corrugated Metal Pipes 2 ( 2 -24" Corrugated Metal Pipes 3 3 -30" Corrugated Metal Pipe DETENTION DESIGN: Detention for Phase IV & VI of Indian Lakes Subdivision is provided by Lake Arapaho, which was constructed with Phase I ofthe subdivision. The lake and its accompanying outlet works were designed to capture and store runoff from the 390 -acre drainage basin above the dam. The table that follows is an excerpt from the Engineering Report for Lake Arapaho prepared by East Texas Engineers, Inc. in September 2002. The Peak Inflow figures are based on post - development runoff rates. It shows that all rainfall events have been analyzed during the design of the lake and provisions made to store and release STORM DRAINAGE REPORT PAGE 2 INDIAN LAKES SUBDIVISION, PHASE IV & VI water at a significantly reduced rate. Flood Event j Peak Inflow Peak Outflow Max. Lake Level (CA) (cfs) (feet msl) PMF 7794 3113 264.9 48.1 %PMF 3750 958 262.2 100 -yr 1536 201 260.0 50 -yr i 1305 i 153 259.7 25 -yr ( 1100 114 259.4 10 -yr 877 113 259.0 5 -yr 660 94 258.6 2 -yr 429 57 258.3 Note that for the standard design storms (2 -yr through 100 -yr) the peak outflows are approximately 10 -15% of the peak inflow. No pre - development runoffrates were computed with the analysis ofthe lake, but the report indicates that a SCS Curve number of 82 was assumed for these post- development computations. The large, rural estate lots that are planned with this development have a very small percentage of impervious cover associated with them, so the SCS Curve number for the pre - development condition would have been only slightly less than the post - development number. This implies that only small increases in runoff rates would be expected between the pre- and post - development scenarios. The high capacity for storage that the lake offers will easily offset any small increases in runoff rates due to the development surrounding it. CONCLUSION: Indian Lakes Subdivision, Phase IV & VI is a rural subdivision for single family residences on lots that are approximately 1 acre in size or larger. The small change in impervious area from the development of this subdivision is small and has minima effects on the existing drainage system. Proposed improvements at the street crossings are designed using guidelines in the City of College Station's Drainage Policy and Design Standards. Exhibits D -1 through D -8 shows that all drainage structures in the Indian Lakes Subdivision, Phase IV & VI are capable of passing the design rainfall event with one foot of freeboard. STORM DRAINAGE REPORT PAGE 3 INDIAN LAKES SUBDIVISION, PHASE IV & VI EXHIBIT C -1 Rational Formula Drainage Area Calculations Indian Lakes Phase IV & VI 5/19/04 0408- ph4- dre.)ds Exhibit C -1 EXHIBIT C•2 CULVERT ANALYSIS INDIAN LAKES PHASE FOUR CULVERT #1 Dual 24" RCP Culvert Donlon Criteria Circular Lt. Side Rt, Side Bottom Invert Outlet Culvert Slope Top Pipe ft/ft No. 9:1 Elev. (ELI) Elev, (ELo) 1 Length Slope of Die. ft. 0.024 pipes n ft. ft. fL ft/ft Road 2.00 2 0.014 1 283.00 259.00 133.50 1 0.0300 270.45 ELM - T�offta d E Lho =W1 �_ H D r r r - - _ _ _ � TW ELI Proposed Culvert i7 R\ `- ELo ke 0.5Q Outfail Channel Design Criteria Culvert Analvals Calculations Design Storm Lt. Side Rt, Side Bottom Normal Depth ft. Slope Slope Slope Width n ft/ft 7:1 9:1 ft. I HWI 0.0100 4.00 4.00 0.00 0.024 Design Storm Total Flow cfs ) Flow per Pipe (Cfs) Critical Depth(ft.) do Normal Depth ft. HEADWATER CALCULATIONS Control HW Elev. Type of Control Outlet Velocity Me) Freeboard ft. INLET CONTROL OUTLET CONTROL HWI /D I HWI ELhI TW do do + D)/2 ho H ELho 5 31.40 15.70 1.45 0.92 1.13 1 2.26 265.26 0.95 1.45 1.73 1.73 1.32 282.05 265.26 Inlet 11.19 5.19 10 35.30 17.85 1.53 0.98 1.24 1 2.47 265.47 1.01 1.53 1.77 1.77 1.87 282.44 265.47 Inlet 11.52 4.98 25 40.30 20.15 1.60 1.06 1.33 2.85 265.65 1.09 1.60 1.80 1.80 2.18 262.98 265.85 inlet 11.90 4.80 50 45.50 22.75 1.73 1.14 1.49 2.98 265.98 1.18 1.73 1.87 1.87 2.78 263.84 265.98 1 inlet 12.24 4.47 100 1 51.201 25.80 1 1.80 1 1.24 1.70 1 3.39 288.39 1,19 1.80 1.90 1.90 3.52 264.42 266.39 1 Inlet 1 12.56 1 4.06 ELhi = Hwl + ELi H - ( 1+ ke +((2V(M2)*LYR ho - 7W or (dc + D)t2 (Whichever is Greeter) El-ho - ELo + H + ho 5/19/04 0402- cul- ph4- 01.)ds Exhibit C -2 EXHIBIT C -3 CULVERT ANALYSIS INDIAN LAKES PHASE FOUR CULVERT #2 Culvert Deslan Criteria Circular Lt. Side Rt. Side Bottom Invert Outlet Culvert Slope Top Pipe ft/ft No. 0:1 Elev. (ELI) Elev. (ELo) Length Slope 1 of Dia. ft. 0.024 Pipes n ft. ft. ft. ft/ft Road 2.00 2 0.014 1 255.00 252.00 104.00 0.0288 259.98 ELM -� Top of Road ELho � EH HWI �.��.�����- - -� ELI Proposed Culvert ELo ke 0.50 Outfall Channel Desl n Criteria Dual 24" RCP Cot wart Anal-isle Cwlrad Minns Design Storm Lt. Side Rt. Side Bottom Normal Depth (ft) Slope Slope Slope Width n ft/ft 7:1 0:1 ft. I HWI 0.0100 4.00 4.00 0.00 0.024 Design Storm Total Flow (cla) Flow per Pipe (cis Critical Depth(ft.) do Normal Depth (ft) HEADWATER CALCULATIONS Control HW Elev. Type of Control Outlet Velocity (fps) Freeboard ft. INLET CONTROL OUTLET CONTROL HWi /D I HWI Uhl TW do do + M/2 ho H ELho 5 10 25 29.60 33.20 38.00 14.80 16.60 19.00 1.40 1.50 1.55 0.90 0.96 1.04 1.08 1.18 1.26 2.16 2.36 2.52 257.16 257.36 257.52 0.95 1.00 1.06 1.40 1.50 1.55 1.70 1.75 1.78 1.70 1.75 1.78 1.03 1.30 1.70 254.73 255.05 255.47 257.16 257.36 257.52 Inlet Inlet Inlet 10.87 11.19 11.56 2.80 2.60 2.44 50 42.90 21.45 1 .85 1.12 1.41 2.81 257.81 1.12 1.85 1.83 1.83 2.18 255.99 257.81 Inlet 11.90 2.15 100 48. 20 24.10 1.75 1.20 1.59 3.17 258.17 1.18 1.75 1.88 1.88 2.73 256.81 258.17 Inlet 12.21 1.79 ELhi - Hwi + ELI H = (1 +ke +((29 (rt^2)'LyWl - 33))•((` 12 y 2 9) ho = TW or (dc + D)12 (Whichever is Greeter) ELho - ELo + H + ho 5/19/04 0402- cul- ph4.02.)1s Exhibit C -3 SPILLWAY CULVERT Triple 30" CMP EXHIBIT C-4 CULVERT ANALYSIS INDIAN LAKES PHASES VI Culvert Desian Criteria Circular Lt. Side Rt. Side Bottom Invert Outlet Culvert Slope Top Pipe ft/ft No. ?:1 Bev. (ELI) Elev. (ELo) Length Slope of Die. ft. 0.024 PI s n ft. ft. ft. ft/ft Road 2.50 3 0.020 253.85 253.38 70.00 0.0070 258.00 Culvert Anelvele Cwlculatinne El-hl Top of Road ELho �.. - -- H — HWi D ���..�.. ��� TW ELI --X Proposed Culvert �- , `-- ELo ke 0.50 Outfall Channel Desian Criteria Design Storm Lt. Side Rt. Side Bottom Normal Depth ft Slope Slope Slope Width n ft/ft ?:1 ?:1 ft. I do 0.0080 4.00 4.00 0.00 0.024 Design Storm Total Flow cfs Flow per Pipe cfs Cticl D do Normal Depth ft HEADWATER CALCULATIONS INLET CONTROL I OUTLET CONTROL Control HW Elev. Type of I Control Outlet Velocity (f D Freeboard ft. HWUD I HWI I ELM I TW I do do + D)/21 ho I H j ELho 10 1100.001 33.33 1 1.98 1 2.50 1 1.80 1 4.00 1 257.85 1 1.47 1 1.98 1 2.24 1 2.24 1 2.18 1 257.78 1 257.85 1 Inlet 8.79 1 0.15 ELhi = Hwi + ELI H = ( 1+ ke+( (29'(n "2)'LpR^1.33))•((v ho -TW or (dc + Dy2 (Whichever is Greater) ELho = ELo + H + ho 5/19/04 0408- cul- phe- 03.>1s Exhibit C-4 I it " INDIAN LAKES PHASE /V lift 1 � EX I 1 oil INDIAN LAKES ° PHASE V/ 1 == PEACH CREEK EXHIBIT A Flood Insurance Rate Map Excerpt Brazos County, Texas and Incorporated Areas Map Numbers: 48041 CO215C N.T.S. Effective Date: July 2, 1992 j ��•_ _._... �_ _.._ mot-.^_ .._.. _...... -._.. � It N Z 62 _.._ 264 — - ' � � f � — of Lot 21 — Lo #._2Q._ 266— 1,04_.._19- o —268 J ✓�' j1, r Scale: 1" 200' (Horiz.) -- 270 _ — z `'' SC - ' l � / !!� / f / ! 1 �K / , At I A i -272 � t i ay I / ! haco Canyon D ?6;9 •270 \ ! / ` � J t i t i f _ z 6k I 24" c�lY� — �J / l 'J / / O a li / f �� i l� I 'Lot 33� Lit" 35 a j Lod 6 2 48_ ! \ `8lo��fc. e ; l I 9.6 6 a C . - 252 / ` 254 -- _ ,/ / m V .._ ...... — 256 - ..� � � �,.. J N w f / I ry ,�� f , ` � ! , t \ 'ti ,� \ .. -' \ ��\ � \ \ �, 4 � � �• ,�-' �' � _._ J -- 26 - i ro CV ti ! \ \ \^ \ \J \ _ CY 256 1 ;� " ~ �p \ S\ �, \ k Legend - Drainage Area Number 0 oc - Acreage - -xxx- - - Existing Ground Contours - Drainage Flow DRAINAGE AREA MAP PROPOSED CONDITION EXHIBIT B