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HomeMy WebLinkAbout37 Development Permit 393 Westminster SubdivisionA S tor 11n wa.ter J:::>r3-irt..a.ge Stu..dy for Westmi.T1ster S u..·bdi.v-i.si.o:n.: a 108. 781 acre tract in Brazos Co u nty, Texas Prepared For Bert Wheel e r, lnr:. by Municipal Development Group 203 Holleman East College Station, Texas 77840 25 October 1993 Approved by North Bardell, P.E. TABLE OF CONTENTS I. Introduction 11. Analysis III. Calculations IV. Drainage Facility Design v. Drainage Design LIST OF FIGURES 1. Figure 1: Drainage Area Map INTRODUCTION The proposed Westminster Subdivision is a 108.781 acre tract lying on the North and South sides of existing Barron Rd. immediately East of its intersection with Wellborn Rd.(F.M. 2154). It is planned to contain 33 single family residential lots ranging in size from 2.5 to 5.61 acres. These lots will be covered by deed restrictions which among other constraints include a prohibition of any further s ubdivision; therefore the maximum number of residential units projected for the project is 33. 1 ANALYSIS The roadway ditches on Barron Road drain the part of the subject subdivision which lies North of Barron Rd. This area can be divided into three specific areas (see the "Drainage Study" figure 1) with drainage area Dl draining through a 36" corrugated metal culvert under Barron Road and through drainage area D2 on the south side of barron road. Drainage areas A,B,H drain to the West and Drainage Areas G and F drain to the East, both in the Barron Rd. ditches. Drainage area E which contains Picadilly Circle will be drained in the proposed 20' easement (section B-B, Drainage area map) shown on and along the joint property line of lot 12 and lot 13 Block 8. Drainage area D which contains Wimbledon circle also has a depression and a small earthen tank through which the discharge flows. Figure 1 and the Master Preliminary Plat show a forty foot wide easement (section A-A, Drainage Area Map) to contain this flow. The existing tank will be filled and the channel straightened and confined to the proposed easement. Drainage area C which contains Buckingham Circle will be drained in the proposed 20 ' easement (section B-B, Drainage Area Map) shown on and along the joint property line of lot 4 and lot 5 , Block 8. All other areas sheet drain to existing road way ditches and existing culverts under Barron Cut Off and Wellborn Road. The three cul de sac streets will utilize rural drainage design ditches to collect water and discharge it into the three easements provided. The grades on these three streets will be set to accommodate curb and gutter construction in the future. 2 The general existing conditions on the 108. 781 tract are cattle grazing pasture, sparsely wooded with little underbrush and exhibiting gentle grades of 0.5% to 2.0 % generally f rom North to South. The soil is an erosible silty mixture with some exposed clay lenses. 3 CALCULATIONS A time of concentration of ten minutes was chosen for rainfall frequency calculations due to maxi.mum existing grades running from the culvert under Barron Road to the culvert under Barron Cut Off. An assumption of 6.8% of the developmental area to be converted to impervious conditions when fully developed allows the following: Freguenc;y Area Cpre Cpost Intensit;y ~ 5 108. 781 .30 .341 7.7 251.3 10 108.781 .30 .341 8.6 280.2 25 108. 781 .30 .341 9.9 323.0 50 108. 781 .30 .341 11.1 362.2 100 108.781 .30 .341 12.6 411.1 (108. 781 x .068 x .90) + (108. 781 x .932 x .30) = 108.781 6.657 4 + 30.4152 = 108. 781 Qpost 285.6 318.9 367.2 411.7 467.3 .341 These post development runoff calculations do not include the desirable effects of any landscaping on lawn and planting improvements done by the future residents. The runoff from Drainage Area E for a 100 year storm will be: 7 .1 5 acres x .341 x 12.6 = 30. 7 c.f .s. The runoff from Drainage Area D for a 100 year storm will be: 37.5 acres x .341 x 12.6 = 161.1 c.f .s. The runoff from Drainage Area C for a 100 year storm will be: 6.8 acres x .341 x 12.6 = 29.2 c.f .s. 4 DRAINAGE DESIGN The calculated Q 111 of 30. 7 c.f .s. for Drainage Area E demands a minimum cross section of 12.5 square feet. The calculated Qm of 161.1 c.f .s. for Drainage Area D demands a minimum cross section of 60 square feet. The calculated Q 111 of 29.2 c.f .s. from Drainage Area C demands a minimum cross section of 12.5 square feet. Typical Section for Drainage Easements c g. E (B-B) ·•; Typical Section for Drainage Easement D (A-A) 5 FIGURE 1 -- DRAINAGE AR.EA MAP -~· City of Colleg · tation fS. POST OFFICE BOX 9960 11 1 T EXAS AVENUE SI~ COLLEGE STATION , TEXAS 77840 P lanning Division TO---r-._ --'-"-_ __,,_0 _______ £:f I MESSAGE----------------------------------~ v Spencer J . Buchanan. P .E. (1903-1982) /March 16, 1992 Buchanan/Soil Mechanics, Inc. Consulting Engineers Bryan, Texas 206 North Sims. 77803/P.O. Box 672, 77806/(409) 8'Z2-3767 Veronica J.B. Morgan, P.E. Assistant to the City Engineer City of College Station P.O. Box 9960 College Station, Texas 77842 Reference: Drainage Report for ETJ Subdivision, particularly at Barron Road, Barron Cutoff Road, and Wellborn Road Dear Veronica: Edsel J . Buikhart. P.E. Thank you for your offer to mark the outline of the "Final Drainage Report" contained in College Station's Drainage Policy and Design Standards to identify the items required for the proposed subdivision in College Station's Extraterritorial Jurisdiction. A copy from Policy and Standards is enclosed .. Please mark it as it should be and return to me. Thank you for your assistance in this matter. Sincerely, BUCHANAN/SOIL MECHANICS, INC. Robert /ds Enclosure: As stated / 2. Upon completion of the preliminary drainage conference, the Administrator may require the submission of a preliminary drainage report and plans. These shall be prepared along the lines of the requirements of the Final Drainage Report and Drawings. D. Final Drainage Report and Drawings j 1. The purpose of the Final Drainage Report is to identify and define conceptual solutions to the problems which may occur 2. j on site and off site as a result of the development. All reports shall be typed on 8-1/2" x 11" paper and bound together. The report shall include a cover letter presenting the proposed desi gn for review and shall be prepared by a Registered Professional Engineer licensed in Texas; The report shall contain a certification sheet as follows: "I hereb y certify that this report (plan) for the drainage desi gn of (Name of Development) was prepared by me (or under my supe rvis io n) in accordance with the provisions of the City of College Station Drainage Policy and Desi gn Standards for the owners there o f." Registered Professional Enginee r State of Texas No . (Affix Seal) Report Contents The Final Drainage Report shall be in accordance with following outline and contain the applicable information listed: kz;\0\ ~ ~O a. General Location and Description ~ Ji~a,<f'\ ~ ~ ~ :. ,\J_ .J\vl! ( 1) ;acation / 'vi-¢' lt) ffrZD~ /a .) Local streets within and adjacent to the 0-~ 0 development \ \ ~f\ /~.) the Primary Drainage System draina geways and /c.) facilities Names of surrounding developments 91 ~2) Description of Property /a.) Acreage V'b.) Land cover ~.) Primary and Secondary System within the property drainageways v1' d.) General project description b. Drainage Basins and Sub-Basins (1) Primary Basin Description a.) Reference to Primary Drainage System planning studies such as flood hazard delineation reports, and flood insurance rate maps. b.) Primary Basin drainage characteristics. (2) Secondary Drainage Basin Description -Discussion /of off site drainage flow patterns, and impact of ./ development on existing and proposed pathways to Primary Drainage System. c. Drainage Design Criteria (1) Regulations -Discussion of the optional criteria ~ selected or the deviation from the Standards, if any. (2) Development Criteria Reference and Constraints / a.) Discussion of previous drainage studies for the site in question that influence or are influenced by the drainage design and how the plan will affect draina ge design for the site. b.) Discussion of the drainage impact of site constraints such as streets, e xis ti ng structures, and development or site plan. ~) Hydrological Criteria ~) Identify design rainfall ~ Identify runoff calculation method /c.) /d.) Identify detention discharge and storage calculation method,~~ Identify design storm recurrence intervals 92 e.) Discussion and justification of other criteria or calculation methods used that are not presented in or referenced by the Standards. ~) Hydraulic Criteria _,/a.) Identify various capacity references entify detention/retention outlet / c. r -biscussion of other drainage facility design criteria used that are not presented in the Standards. d. Drainage Facility Design (1) General Concept ~.) Discussion of concept and typical drainage patterns · b.) Discussion of compliance with off site runoff ../ considerations /c.) Discussion of the content of tables, charts, figures, or drawin gs presented in the report ~.) Discussion of anticipated and proposed drainage patterns. (2) ;»ecific Det ails ../"a.) Discussion o f draina ge problems encountered and solutions at specific design points /h ·) Discussi~n detention design, if any ~ storage and outlet Discussio maintenance 'I f:-.) of the design \//e. Conclusions access and aspects _,/(l) Compliance with Standards of the City of College /(2 ) ~ Station Drainage Pol icy and Design Standards Effectiveness of draina ge design to control damage from stormwater flows Explanation of the effectiveness of existing and proposed improvements with regard to controlling the dischar ges of the 100 year storm with ultimate development conditions of the secondary system basins affected by the proposed development. 93 . f ;/Ref erenc~s -Reference all criteria and "\JI" information used. technical g. Appendices (Where Applicable) (1) H~rologic Computations ~.) Land use assumptions regarding properties adjacent b.) Minor and major storm runoff at specific design points c.) Historic and f ully developed computations at specific design points d.) Hydrographs at critical (2) HJ<iraulic Computations ~.) Culvert capacities t,,/)Y .) Storm sewer capac i ty design points runoff / c.) Street capacity ~·I~ O,;'v"'-j . /d.) Storm inlet capa city including inlet control / rating at connection to storm sewer -l J ll./vO .,/' e.) Open channel desi gn /t .) Detention area/vol ume capacity capacity calculations --\~ i\.t\t\.'-6 and outlet E . F i nal Drainage Report Drawin g Cont ents i/ Sheet #1 -General Location Ma p t hat V a . Depicts draina ge flows e ntering and lea ving th e development site; ~ b. Identifies major construct ion alon g path of drainage; c. / d. Illustrates general drainag e flow within entire basin; and Is drawn at a scale of 1"=50 0' to 1"=2000'. 2.~Sheet #2 -Floodplain Informat ion ~ a. Copies of existing City of College Station 100-year floodplain maps showing the location of the subject development. 94 3. Sheet #3 Drainage Plan Map(s) of the proposed development at a scale of 1"=20 ' to 1"=200' on a 24" x 36" drawing shall be included. following: The plan shall show the .,/'a . Existing and proposed contours at 2-feet maximum intervals. // b. Property lines, and easements with purposes noted. / c. Streets. d. Existing drainage facilities, roadside ditches, v , / / e. f. g. drainageways, gutter flow directions , and culverts. All pertinent information such as material, size, shape, slope~ and location shall also be included. Overall draina ge area boundary and drainage sub-area boundaries. Proposed type of street flow, r oadside ditch, and/ or gutter flow directions. Proposed storm sewers and open drainageways, including inlets, manholes, culverts, and other appurtenances . ./'/ h. Proposed outfall point for runoff from the developed area and facilities to convey flows along the proposed pathways to the final outfall points in the primary draina g e system without damage to downstream properties, or exceed ing the capacities of the receiving facilities as de fi ned in these standards . /. i. Routing and accumulation of flows at various cri ti cal points for the minor storm runoff. 1..-/ j. Path(s) chosen for computation of time-of- ,/ m. V n. concentration. Details of detention/retention storage facilities and outlet works. ~) l-r::v"'--f,.._ Location and elevations of all defined floodplains affecting the property. ;~0/1).._A.,K Location and elevations of all existing and proposed . I ~ . utilities affected by or affecting the draina ge design. !rf U..1 ) Routing of off site drainage flow through the development. 95 F. Construction Drawings and Specifications G. /'- 1. Where drainage improvements are to be constructed in accordance with the approved Final Drainage Report, the construction plans (on 24" x 36" sheets) and specifications shall be submitted for review and approval prior to construction. The plans and specifications for the drainage improvements will include: /a. Storm sewers, inlets, and outlets . ..........-b. Culverts, end sections, and inlet/outlet protection. /' c. Channels, ditches, and swales . .....--d . Erosion control facilities. /e . Detention pond grading , pilot channels, outlets, and landscaping. /f . Other drainage related structures and facilities. )\g. Maintenance access considerations . .. /h. Finished floor elevations of adjacent buildings. ~· Existing and Ultimate 100-year water surface elevations. _/'j. Engineer certification ~k. Approval by the City. 2. The infor;:;ia tion required for the drawings and specifications shall be in accordance with sound engineering principles, these Standards and the City requirements for subdivision designs. Constructio n documents shall include geometric, dimensional, structural, foundation, beddin g , hydraulic, landscaping, and other details facility. included as as needed to The approved part of the construct the Final Drainage storm draina ge Plan shall be construction documents for all facilities affected by the drainage plan. "As-Built Drawin gs " As constructed finished plans ("As-Builts") for all public improvements shall be attested to by a professional engineer registered in Texas and submit ted to the City before the City will accept the improvements. The construction drawings are acceptable, if they remain true after construction and are attested to by a re g istered engineer to represent "As-Built" conditions consistent with the intent of the approved plan. 96 XII. Development Permitting Process The City of College Station Storrnwater Management Ordinance requires the acquisition of a development permit prior to beginning "development" activities defined within the Ordinance. A blank Development Permit Application Form (for areas outside established flood hazard areas) is included in this section as Figure XII -1. A blank Development Permit Application Form (for areas within established flood hazard areas) is included in this section as Figure XII-2. 97 U-~t\~ n~~ ~~~~L6v\ ...