Ord 2018-002/replacement of Subpart B (Land Development Code) and Subpart C (SmartCode) with a new Subpart B (San Marcos Development Code “Code SMTX”) including Appendix A - E (Design Manual) and consider amendments to the City’s Preferred Scenario Map an

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s a SUBDIVISIONS r ARTICLE 9: STORMWATER COLLECTION AND This responsibility includes the storm water flowing onto the property by any other developed property as well as the DRAINAGE SYSTEMS drainage naturally flowing through the property by reason of topography. DIVISION 1: IN 6ENERAI D. Design Based on Maximum Build-Out Configuration.Drainage Section 3.9.1.1 Flood Control Requirements improvements shall accommodate runoff from the upstream drainage area in its anticipated maximum"build-out"condition, A. Flood Damage Prevention Ordinance. Developments and shall be designed to prevent overloading the capacity of and improvements in or near a FEMA floodplain shall meet the downstream drainage system.The City may require the the requirements of the Chapter 39 of the City's Code of phasing of development,the use of control methods such as Ordinances. retention or detention, or the construction of off-site drainage improvements in order to mitigate the impact of the proposed B. Site Stormwater Management.The following two items should development. be considered during the design process: E. Design Storm Event.All drainage facilities(including streets, 1. Diversion of storm water away from the natural curbs,gutters, storm drains,ditches, creeks, detention ponds, watercourse will not be allowed, except within the property etc.) shall be designed to intercept and transport runoff from boundaries controlled by the developer under the following a twenty five(25)year frequency storm.The drainage system conditions: shall be designed to convey those flows greater than a twenty a. The storm water is returned to its natural flowing five (25)year frequency, up to and including a one hundred watercourse prior to leaving the developer's property, (100)year frequency storm within defined rights-of-way or drainage easements. b. For watersheds greater than twenty(20) acres, a timing analysis of the existing and diverted F. Detention or Retention Required.Drainage facilities shall be hydrograph must be performed to confirm that the designed and constructed so that the rate of runoff from a site peak flow rate has not been increased at the point after construction shall be equal to or less than the runoff prior that it reenters the watercourse,as a result of the to construction for the two(2),ten (10),twenty five(25),and diversion. one hundred (100)year storm frequencies. 2. All developments shall provide adequate drainage outfall 1. The timing of the hydrograph released from the detention at the lower end of the site into an existing street,alley, facility must be checked against the timing of the flow rate drainage, easements or right-of-way, or to the centerline of in the first open watercourse to prevent any increase in the an existing natural drain.Where a proposed street, storm peak flow rate in the receiving watercourse. For detention drain,or open channel does not discharge into a natural basins constructed in-line on an existing watercourse,the low or into an existing adequate drainage easement,then creation of the basin shall not increase flood elevations in facilities and drainage easements of adequate width—to the channel upstream of the new development boundaries. contain the design discharge—shall be constructed and 2. Computation of the rate of runoff shall be based on an dedicated. assumption of a contributing drainage area or watershed 3. Developments cannot increase the water surface elevation fully developed in accordance with the Stormwater off-site unless contained within a dedicated drainage Technical Manual. easement or right-of-way. 3. Low impact development practices can be used to C. Responsibility to Accept Storm Water.The owner or developer reduce peak flow rates to reduce or eliminate detention of property to be developed shall be responsible for the requirements when designed in accordance with the conveyance of all storm water flowing through the property. Stormwater Technical Manual criteria. Adopted April 17,2018 San Marcos Development Code 3:51