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City of Austin Water Quality Regulations Overview

- In the City of Austin outside the Barton Springs Zone, the annual pollutant load of the runoff must be equal to the outgoing water quality of the filtration system after treating the required minimum water quality volume (WQV)

Stormwater control systems (SCMs) or Best Management Practices (BMPs) must have a minimum WQV of 0.5 inches of runoff and add additional 0.1 inches for every 10% impervious cover (once over 20%) on the site


  • Impervious cover is defined as an surface that can't absorb rainfall

  • Example: if a property has 40% impervious cover the SCM must hold 0.7 inches of runoff

- Maintenance and Construction Requirements


  • Routine inspections and maintenance is required for each kind of SCM, the frequency of the maintenance depends on the type and number of major rain events

    • Trash should be removed every 6 months

    • Annually inspect all structural parts

  • Repairs/maintenance necessary if: draining takes more than 96 hrs; to maintain plant height between 4-18 inches tall in SCMs; inlets/outlets are obstructed in any way

Sedimentation/Filtration Systems

- Design Guidelines for Sedimentation/Filtration Systems


  • Full sedimentation with Filtration must drain within 48 hours and not expel more cubic feet per second than the pre-developed conditions

    • This system allows to both control the speed of the water leaving the site and to separate out the pollutant

    • Storage capacity of sedimentation is greater than or equal to WQV

    • Storage capacity of filtration is a minimum 20% of the WQV

    • Outlet structure – allows the water to go from sedimentation area to filtration area within the 48 hour time period (riser pipe)

    • Underdrain piping – allows water to exit the filtration system and flow off the property


  • Partial sedimentation with Filtration is used for higher pollutant loads to prevent clogging,

    • The sedimentation chamber is shallower (than for a full) with a flow spreader that water can flow over or through depending on WQV

    • May require a splitter/diversion structure to bypass flow when in excess of the WQV

    • Volume of sedimentation chamber has a minimum of 20% of WQV

Sedimentation/Filtration Systems are best used when space is limited, or in arid/desert climates.

Biofiltration Systems

- Design Guidelines for Biofiltration Systems


  • Type of SCM that uses plants, microbes and soils to remove pollutants

  • Biofiltration systems can also follow the sedimentation/filtration systems as they can be classified as full or partial systems

  • Functions similarly to a sedimentation/filtration system but is generally more effective in its filtration

  • Requires a sedimentation basin first to provide pre-treatment to runoff to prevent large pollutants from clogging the filtration medium and to slow the velocity of the water

  • In a biofiltration bed with underdrain, medium must be at least 18 inches thick before the piping (which can be surrounded with a filter and gravel)

Biofiltration systems are generally cheaper than other BMPs and require very little maintenance.


- General Design Guidelines for Raingardens


  • Vegetated, depressed landscape area designed to capture stormwater runoff and provide removal of pollutants

    • They have smaller drainage areas and shallower ponding depths so to utilize one must disperse many raingardens throughout the site

    • Creates less landscape irrigation requirements

  • Site selection – best for small sites, dispersed across site

  • Land use – only for commercial, multi-family, civil uses, public right of way or single family projects

    • Must be located in dedicated common area or drainage easement

  • Rain gardens are restricted to a drainage area less than 2 acres and cannot exceed a ponding depth of 12 inches, the slope must be ≤15%

  • The native soil must have an infiltration rate that will allow the ponding to be drained within 48 hrs

  • Growing Medium must be able to sustain both plant growth and a healthy microorganism population

  • High plant diversity is best, choose plants that can withstand water inundation and dry periods

- Types of Raingardens


  • Full infiltration

    • Not allowed in areas with high contamination

    • Has no underdrain, not suitable for high flooding areas as all WQV must be absorbed by ground


  • Partial infiltration (filtration system with raised outlet or partial underdrain)

    • Not allowed in areas with high contamination

    • Allows for some WQV to be absorbed by ground but overflow can be taken to separate site

    • Uses infiltration and a raised outlet pipe for excess WQV


  • Full Filtration

    • Allowed in areas with higher contamination

    • Use an filtration bed over a underdrain system

One advantage of raingardens is that they can be incorporated into a site's landscaping.

Retention/Irrigation Systems

- Details on Retention/Irrigation Systems


  • This SCM captures runoff in a holding pond, then uses the captured water for the irrigation of nearby landscape areas.

  • There are vey high maintenance requirements associated with this BMP

  • This SCM should not be used for small drainage areas

Retention/Irrigation Systems remove 100% of the pollutants for the water quality capture volume when properly maintained.

Wet Basins

- Details on Wet Basins


  • This SCM cannot be placed on steep slopes.

  • Wet Basins are usually not used in arid/desert climates

  • Algae, or another form of aquatic weed control, is used to filter out sediment from the water

  • A clay liner or geomembrane will be necessary in order to keep the groundwater from becoming contaminated

Wet Basins are ideal for large site areas.

AquaLogic Cartridge Filter System

- Details on AquaLogic Cartridge Filter System


  • This SCM can be used in places where space is limited

  • A permeable media in the form of a cartridge is utilized to remove particles from the stormwater 

  • The cartridge filters will need to be removed and replaced over time

AquaLogic Cartridge Filters are appropriate for covered installations and have fairly high suspended solid removal efficiency.

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