San Marcos Development Code 9.1.2020

3 C H A P T E R

Subdivisions

Article 9: STORMWATER Collection AND DRAINAGE systems

This responsibility includes the storm water flowing onto the property by any other developed property as well as the drainage naturally flowing through the property by reason of topography. D. Design Based on Maximum Build-Out Configuration. Drainage improvements shall accommodate runoff from the upstream drainage area in its anticipated maximum “build-out” condition, and shall be designed to prevent overloading the capacity of the downstream drainage system. The City may require the phasing of development, the use of control methods such as retention or detention, or the construction of off-site drainage improvements in order to mitigate the impact of the proposed development. E. Design Storm Event. All drainage facilities (including streets, curbs, gutters, storm drains, ditches, creeks, detention ponds, etc.) shall be designed to intercept and transport runoff from a twenty five (25) year frequency storm. The drainage system shall be designed to convey those flows greater than a twenty five (25) year frequency, up to and including a one hundred (100) year frequency storm within defined rights-of-way or drainage easements. F. Detention or Retention Required. Drainage facilities shall be designed and constructed so that the rate of runoff from a site after construction shall be equal to or less than the runoff prior to construction for the two (2), ten (10), twenty five (25), and one hundred (100) year storm frequencies. 1. The timing of the hydrograph released from the detention facility must be checked against the timing of the flow rate in the first open watercourse to prevent any increase in the peak flow rate in the receiving watercourse. For detention basins constructed in-line on an existing watercourse, the creation of the basin shall not increase flood elevations in the channel upstream of the new development boundaries. 2. Computation of the rate of runoff shall be based on an assumption of a contributing drainage area or watershed fully developed in accordance with the Stormwater Technical Manual. 3. Low impact development practices can be used to reduce peak flow rates to reduce or eliminate detention requirements when designed in accordance with the Stormwater Technical Manual criteria.

Division 1: IN GENERAL

Section 3.9.1.1Flood Control Requirements

A. Flood Damage Prevention Ordinance. Developments and improvements in or near a FEMA floodplain shall meet the requirements of the Chapter 39 of the City’s Code of Ordinances. B. Site Stormwater Management. The following two items should be considered during the design process: 1. Diversion of storm water away from the natural watercourse will not be allowed, except within the property boundaries controlled by the developer under the following conditions: a. The storm water is returned to its natural flowing watercourse prior to leaving the developer’s property, b. For watersheds greater than twenty (20) acres, a timing analysis of the existing and diverted hydrograph must be performed to confirm that the peak flow rate has not been increased at the point that it reenters the watercourse, as a result of the diversion. 2. All developments shall provide adequate drainage outfall at the lower end of the site into an existing street, alley, drainage, easements or right-of-way, or to the centerline of an existing natural drain. Where a proposed street, storm drain, or open channel does not discharge into a natural low or into an existing adequate drainage easement, then facilities and drainage easements of adequate width — to contain the design discharge — shall be constructed and dedicated. 3. Developments cannot increase the water surface elevation off-site unless contained within a dedicated drainage easement or right-of-way. C. Responsibility to Accept Storm Water. The owner or developer of property to be developed shall be responsible for the conveyance of all storm water flowing through the property.

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San Marcos Development Code Amended: September 1, 2020