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Ord 64 Estahblishing a Forumla for determining Flood DischargesORDINANCE NO. 64 AN ORDINANCE ESTABLISHING A FORMULA FOR DETERMIN- ING FLOOD DISCHARGES IN THE TOWN OF WdESTLAKE, ADOPTING A RUN --OFF COEFFICIENT; PROVIDING FREQUENCY CURVES FOR DETERMINING RAINFALL INTENSITY: DEFINING "SPREAD OF WATER"; PROVIDING A STANDARD FOR DESIGN OF STORM SEWERS; PROVIDING A STANDARD FOR DESIGN OF OPEN DRAIN- AGE CHANNELS;, AND PROVIDING A STANDARD FOR DESIGN OF ROADWAY CULVERTS BE IT ORDAINED BY THE CITY COUNCIL OF THE TOWN OF WESTLAKE, TEXAS, that: I. The terms and provisions of Exhibit "A", attached hereto, incorporated herein and made a part hereof, are hereby approved and adopted as the Storm Water Drainage Design Standards of the Town of Westlake, Texas. II. The terms of Exhibit "A" shall be binding upon all developers, land planners and all other persons, firms and corporations who shall hereafter develop housing projects, shopping centers, business establishments or industrial areas. III. All construction designed to cope with Storm Water Drainage in the Town of Westlake shall conform to the terms of said Exhibit "A". this da PASSED AND APPROVED y of f c, L<.c. , 197 A. ATTEST:. City Secretary ' APPROVED: Mayor I TOWN OF WESTLAKE, TEXAS EXHIBIT "A" STORM DRAINAGE CRITERIA A. General - The design of storm drainage improvements in the Town of Westlake shall be based on flood discharges determined from the Rational Formula. The formula for calculating storm flows in this manner is: Q = CIA, where Q is the storm flow at a given point C is a coefficient equal to the ratio that the peak rate of runoff bears to the average rate of rainfall I is the average intensity of rainfall for a period equal to the time of flow from the farthest point of the drainage area to the first inlet point on the storm sewer A is the area tributary to the design point B. Runoff Coefficient - The runoff coefficient which considers the slope of the terrain, the character of the land use, the length of overlsnd flow, and the imperviousness of the drainage area shall be determined from the ultimate land development as shown on the Future Land Use Map, Town of Westlake Master Plan.. The runoff coefficients for the appropriate land uses shall be as follows: Business Areas 0.90 Industrial Areas 0.70 Residential Areas 0.40 Park Areas 0.30 C. Rainfall Intensity - Frequency - The rainfall intensity - frequency curves iwhich are shown on Plate 1 are plotted from data compiled by the U. S. Depart- ment of Commerce We::ther Bureau in Technical Paper No. 40 (modified). The intensity, I, in the formula Q = CIA is determined f)-om these curves by arriv- ing at a time of concentration and adapting a storm frequency upon which to base the drainage improvements. 1. Time of Concentration - The time of concentration, which is the time of -low from the farthest point of the drainage area to the first inlet in the system, consists of the time required to flow overland plus the time required to flow in the gutter to the inlet. A minimum time of concen- tration of ten (10) minutes shall be used for all areas except business areas and a minimum time of concentration of five (5) minutes shall be used in business areas. A nomograph, shown on Plate 2, is attached for estimating the time of concentration. - 2 - Ordinance No. 64 2. Storm Frequency improvements in Type of Facility - Recommended design the Town of Westlake DESIGN STORM Description of Area to be Drained storm frequencies for are shown in a table FREQUENCY Maximum Time of Concentration (Minutes) the storm drainage as follows: ... Recommended Design Frequency -(Years) *Storm Residential, 30 5 Sewers Commercial and Manuf8cturing *Culverts, Any type of area 30 5 Bridges, less than 100 Channels acres and Creeks "Culverts, Any type of area 45 10 Bridges, greater than 100 Channels acres but less than and Creeks 1,000 acres ***Culverts, Any type of area 60 25 Bridges, greater than Channels 1,000 acres and Creeks Vfi en the maximum time of concentration or area to be drained is exceeded, the design shall be based on a ten (10) year frequency. **When the maximum time of concentration or area to be drained is exceeded, the design shall be based on a twenty-five (25) year frequency. ***Whenever, in a storm sewer system, an inlet is located at a low point so that flow in excess of the storm sewer capacity would be directed onto private property, the design frequency shall be increased beyond five (5) years. If the inlet loca- tion is such that overflow could cause damage or serious inconvenience, it may be desirable to increase the design frequency to as much as twenty-five (25) years. D. Area - The area used in determining flows by the "Rational Formula" shall be calculated by subdividing a map into drainage areas within the basin contri- buting storm water runoff to the system. E. Spread of Water - During the design storm, the Quantity of storm water that is allowed to collect in the streets before being intercepted by a storm drainage system is referred to as the 'spread of water". In determining the limitations for carrying storm water in the street, the ultimate development of the street shall be considered. The use of the street for carrying storm water shall be limited to the following: - 3 - ORDINANCE NO. 64 SLREAD OF WATER Expressways - Eight feet (8') from face of curb. Major thoroughfares (divided) - One traffic lane on each side to remain clear. Thoroughfares (not divided) - Two traffic lanes to remain clear. Secondary streets -- One traffic lane to remain clear. Residential streets - Six inch (6") depth of flow at curb or no lanes completely clear. Curves are provided on Plate 3 for determining the spread of water for certain gutter slopes, gutter discharge and pavement crown. F. Storm Sewer Design - Storm water in excess of that allowed to collect in the streets shall be intercepted in inlets and carried a+,ay in a storm sewer system. Storm sewer capacity shall be calculated by Mannings Formula Q = 1.486 AR 2/3 S 1/2 were n Q is the discharge in cubic feet per second A is the cross-sectional area of flow in square feet R is the hydraulic radius in feet S is the slope of the hydraulic gradient in feet per foot n is the coefficient of roughness (n= .013 for new concrete pipe) Pipe Flow Charts for determining required pipe sizes based on mannings formula and computation sheets 1, 2 and 3 are provided for inlet and storm sewer design cal- culations. In the design of the storm sewer system, the elevation of the hydraulic.gradient of the storm sewer shall be a minimum of 1.5 feet below the elevation of the adja- cent street ,gutter. Storm seiner pipe sizes shall be so selected that the average velocity in the pipe will not exceed 15 feet per second. ´┐ŻOpen Channel Design - Stor.. water runoff in excess of that allowed to collect in the streets in developed areas and runoff in undeveloped area may be carried in open channels (not in the street rights of way). Open channel capacity shall be calculated by Mannings Formula and roughness coefficients shall be as follows: Type_ of Lining Earth (Bermuda Grass) Earth (Non Vegetated) Concrete Lines Rou hness Coefficient"n" 0.035 0.030 0.015 MW Maximum Permissab le Mean Velocity 8ft. per sec 5 ft. per sec 15 ft. per sec ZiI"Cinance i,qo, ce, Open channels may be constructed with various cross-sectional shapes but are usually V-shaped or trapezoidal. The trapezoidal ditch which has a more natural shape and has greater capacity shall have side slopes no steeper than 2:1 in earth and 1:1 to 1.5:1 when lined with concrete. H. Culvert Design - At locations of creels cro8sings with proposed roadway improve- ments, it is sometimes necessary to receive and transport storm water under the roadway in culverts. The quantity of flow shall be determined by the Rational Formula and the capacity of the culvert shall be calculated by Manning's Foxmula. Design of culverts shall include the determination of upstream bacicivater condi- tions as well as downstream velocities and flooding conditions. Consideration shall be given to the discharge velocity from culverts and the following limita- tions are allowed: CULVERT DISCHARGE - VELOCITY LIMITATIONS Culvert Discharging On To Earth Sod earth Paved or riprap apron Shale Rock Maximum Allowable Velocity (f.p,s.) 6 3 15 10 15 Generally, all culverts shall be designed with a free outfall and the following head losses shall govern the design of the culvert: 1. Frictional Head Loss hf = sfL when e sf = Slope of frictional gradient in feet_ per foot L = Length of culvert in feet 2. Head Loss clue to change in Velocity 2 by = v2 - v12 where 2g 2g v2 = Velocity in culvert vi - Velocity in channel above culvert g = Acceleration clue to gravity. 3. Head Luss at Upstream Entrance to Culvert Due to Entrance and Change in Section lie = V22 where vl is equal to or less than 2g six ft. (6') per sec. he = v2 2 - 0.5 vi 2 where vl is greater than six ft. 2g 2g (6') per sec. 5 - TOWN OF WESTLARE CURB AND CUTTER REQUIREMENTS I N D E X ORDINANCE NUMBER 65 PACE SECTION l...DEQ'INITIONS.......................................... I SECTION 2...COUNCIL TO DETERMINE NATURE & EXTENT OF IMPROVEMENT.. 1 SECTION 3...PERMI.T.............................................. 1 SECTION 4...PERMIT FEES .......................................... 1 & 2 SECTION 5 -BOND REQUIRED ........................................ 2 SECTION 6 ... HOMEOWNERS EXCLUDED-- ............................ 2 & 3 SECTION 7...SPECIFICATTONS....................................... 3 SECTION 8...SUPERVISION & APPROVAL OF WORK ....................... 3 SECTION 9...WARNING DEVICES REQUIRED FOR PROTECTION OF TRAFFIC... 3 SECTION 10..EMERGENCY CLAUSE ..................................... 3 EXHIBIT A. - SPECIFICATIONS FOR CONSTRUCTION OR REPAIR OF CURB AND CUTTER I...........DEFINITION........................................... 4 II.......... PLANS ................................................ 4 III ......... MATERIALS ............................................ 4 IV .......... STANDARD DETAILS ..................................... 4 V ........... EXCAVATION ........................................... 5 .FORMS . 5 VII ......... REINFORCEMENT ........................................ 5 VIII........ CONCRETING & INSPECTION ..............................6 IX .......... CONSTRUCTION ......................................... 6 X........... FINISH ............................................... 7 XI.......... CURING ............................................... 7 & 8 XII......... TESTING ..............................................8 XIII ........ BACKFILL, .................. .............................8 XIV......... FINAL ACCEPTANCE ..................................... 8