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m <br /> _- SUBDIVISIONS <br /> i <br /> ARTICLE 9: STORMWATER COLLECTION AN6 This responsibility includes the storm water flowing onto <br /> DRAINAGE SYSTEMS the property by any other developed property as well as the <br /> drainage naturally flowing through the property by reason of <br /> topography. <br /> DIVISION 1: IN 6ENERAL D. Design Based on Maximum Build-Out Configuration. Drainage <br /> Section 3.9.1.1 Flood Control Requirements improvements shall accommodate runoff from the upstream <br /> drainage area in its anticipated maximum"build-out"condition, <br /> A. Flood Damage Prevention Ordinance. Developments and shall be designed to prevent overloading the capacity of <br /> and improvements in or near a FEMA floodplain shall meet the downstream drainage system.The City may require the <br /> the requirements of the Chapter 39 of the City's Code of phasing of development,the use of control methods such as <br /> Ordinances. retention or detention,or the construction of off-site drainage <br /> B. Site Stormwater Management.The following two items should improvements in order to mitigate the impact of the proposeddevelopment. <br /> be considered during the design process: <br /> E. Design Storm Event.All drainage facilities (including streets, <br /> 1. Diversion of storm water away from the natural curbs,gutters,storm drains,ditches,creeks,detention ponds, <br /> watercourse will not be allowed, except within the property etc.) shall be designed to intercept and transport runoff from <br /> boundaries controlled by the developer under the following a twenty five (25)year frequency storm.The drainage system <br /> conditions: shall be designed to convey those flows greater than a twenty <br /> a. The storm water is returned to its natural flowing five (25)year frequency, up to and including a one hundred <br /> watercourse prior to leaving the developer's property, (100)year frequency storm within defined rights-of-way or <br /> drainage easements. <br /> b. For watersheds greater than twenty (20) acres, <br /> a timing analysis of the existing and diverted F. Detention or Retention Required. Drainage facilities shall be <br /> hydrograph must be performed to confirm that the designed and constructed so that the rate of runoff from a site <br /> peak flow rate has not been increased at the point after construction shall be equal to or less than the runoff prior <br /> that it reenters the watercourse, as a result of the to construction for the two(2),ten (10),twenty five(25),and <br /> diversion. one hundred (100) year storm frequencies. <br /> 2. All developments shall provide adequate drainage outfall 1. The timing of the hydrograph released from the detention <br /> at the lower end of the site into an existing street, alley, facility must be checked against the timing of the flow rate <br /> drainage, easements or right-of-way,or to the centerline of in the first open watercourse to prevent any increase in the <br /> an existing natural drain.Where a proposed street,storm peak flow rate in the receiving watercourse. For detention <br /> drain, or open channel does not discharge into a natural basins constructed in-line on an existing watercourse,the <br /> low or into an existing adequate drainage easement,then creation of the basin shall not increase flood elevations in <br /> facilities and drainage easements of adequate width—to the channel upstream of the new development boundaries. <br /> contain the design discharge—shall be constructed and 2. Computation of the rate of runoff shall be based on an <br /> dedicated. assumption of a contributing drainage area or watershed <br /> 3. Developments cannot increase the water surface elevation fully developed in accordance with the Stormwater <br /> off-site unless contained within a dedicated drainage Technical Manual. <br /> easement or right-of-way. 3. Low impact development practices can be used to <br /> C. Responsibility to Accept Storm Water.The owner or developer reduce peak flow rates to reduce or eliminate detention <br /> of property to be developed shall be responsible for the requirements when designed in accordance with the <br /> conveyance of all storm water flowing through the property. Stormwater Technical Manual criteria. <br /> 3:58 San Marcos Development Code Amended:September 1, 2020 <br />