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Frequently Asked Questions

Using physical and biological treatment mechanisms, biofiltration uses an organic filtration media with vegetation to remove pollutants. As with sedimentation/filtration systems, runoff is first diverted into a sedimentation basin, where particulate pollutants are removed via gravity settling. This is followed by filtration through an 18" layer of vegetated media.

Biofiltration systems are considered to provide a level of treatment equivalent to sedimentation/filtration, and also provide extended detention that enhances baseflow and reduces stream erosion. Biofiltration systems are not allowed in Barton Springs Zone (BSZ) watersheds as a stand-alone water quality control, as they are not capable of achieving a non-degradation level of treatment.

Because of the vegetation, biofiltration systems can be aesthetic amenities and may be eligible for landscape credit (unlike sedimentation/filtration systems). To ensure proper management of the pond system, filtration media, and vegetation, an Integrated Pest Management (IPM) Plan is required.

The current design criteria is similar to that for sedimentation/filtration systems, and two design alternatives are available. In “full” sedimentation/filtration systems the entire water quality volume is held in the sedimentation basin, which then slowly discharges runoff to the filtration basin via a perforated riser pipe. The alternative “partial” sedimentation/filtration design foregoes the perforated riser pipe, and distributes the water quality volume between the filtration basin and a sediment chamber, the latter separated from the filtration bed by a vegetated hedgerow. The “full” design is required when the City of Austin is responsible for maintenance.

Design guidelines for biofiltration can be found in Section 1.6.7.C of the Environmental Criteria Manual. For information on the biofiltration media, go to biofiltration media guidance. Also available is a list of potential suppliers.

Integrated pest management (IPM) is an environmentally-sound method of controlling pests (weeds, diseases, insects or others). Pests are identified, action thresholds are considered, all possible control options are evaluated, and selected control(s) are implemented. Control options used to prevent or remedy unacceptable pest activity or damage include:

• Biological - recognize, encourage, and/or introduce beneficial predators in your landscape • Cultural - plant native, pest-resistant plant varieties, and give them proper light, water and nutrients • Mechanical - hand-pick insects, or use traps, barriers, or water blasts to infected areas • Chemical - use botanical, mineral, and insecticidal soap or synthetic chemicals

Choice of control option(s) is based on effectiveness, environmental impact, site characteristics, worker/public health and safety, and economics. IPM takes advantage of all appropriate pest management options.

Learn more about Integrated Pest Management.

Click here to download a Green Stormwater Infrastructure Maintenance Manual.

The City of Austin requires landscaping for development. Additional non-required vegetation, especially trees, can help reduce stormwater runoff and enhance groundwater recharge by breaking the impact of raindrops and improving soil structure. A tree's effectiveness in this capacity is correlated with the size of the crown and root zone area.

There are numerous environmental and stormwater benefits to additional vegetation. Non-required vegetation can act as a natural stormwater management area by filtering particulate matter, including pollutants, some nutrients, sediments, and pesticides, and by absorbing water. A study done by the U.S. Department of Agriculture's Center for Urban Forest Research found that a medium-sized tree can intercept 2,380 gallons of rain per year (Center for Urban Forest Research 2002).

Non-required vegetation is eligible for water quality credit, in terms of pervious area (impervious area reduction), if it meets the criteria described in Section 1.6.7.G of the Environmental Criteria Manual. However, it is not eligible for credit in the Barton Springs Zone (BSZ)watersheds.

Unlike conventional centralized stormwater management systems, rain gardens may employ multiple controls dispersed across a development, and may be incorporated into the landscape to provide aesthetic as well as ecological benefits.

Rain gardens can be designed to provide a level of treatment equivalent to sedimentation/filtration, and also provide extended detention that enhances baseflow and reduces stream erosion.

Non-required vegetation requires an Integrated Pest Management (IPM) Plan.

  • Non-Structural Controls are Best Management Practices (BMPs) that do not involve a structured, or engineered solution. They include such measures as education, site planning, and stormwater management regulations. Because it is usually easier and more effective to prevent pollution before it occurs, non structural BMPs are very cost-effective. These measures limit or eliminate pollutants before they end up in the stormwater.
  • Non-structural controls include:  non-required vegetation, vegetated filter disconnect, integrated pest management, and regulations.

Porous Pavement includes a load-bearing, durable concrete surface together with an underlying layered structure that temporarily stores water prior to infiltration. Porous Pavement is a water quality control best management practice (BMP) using the storage within the underlying structure or sub-base to provide groundwater recharge and to reduce pollutants in stormwater runoff.

To ensure proper functioning of porous pavement, no off-site runoff is allowed and proper subgrade conditions must exist.

Porous pavement is currently only allowed for pedestrian use and not for parking lots, stormwater hot spots, or areas where land use or activities generate highly contaminated runoff. Since porous pavement is an infiltration practice, it should not be applied at stormwater hot spots due to the potential for ground water contamination.
Environmental Criteria Manual 1.6.7.E of the Environmental Criteria Manual

A rain garden is a vegetated filtration and/or infiltration system that has a contributing drainage area not to exceed two acres, and a ponding depth not to exceed twelve inches.

Unlike conventional centralized stormwater management systems, rain gardens may employ multiple controls dispersed across a development, and may be incorporated into the landscape to provide aesthetic as well as ecological benefits.

Rain gardens can be designed to provide a level of treatment equivalent to sedimentation/filtration, and also provide extended detention that enhances baseflow and reduces stream erosion.

Rain Garden FAQs
Can rain gardens be used in the Barton Springs zone as a stand-alone water quality control?
No – Rain Gardens are not allowed in Barton Springs Zone (BSZ)watersheds as a stand-alone water quality control, as they are not capable of achieving a non-degradation level of treatment. The use of a Rain Garden as a water quality control is limited to Commercial and Multi-Family developments only.

Are rain gardens eligible for landscape credit?
Yes - Because of the vegetation, rain gardens can be aesthetic amenities and may be eligible for landscape credit (unlike sedimentation/filtration systems).

What is an IPM plan?
IPM stands for Integrated Pest Management.  To ensure proper management of the pond system, filtration media, and vegetation, an Integrated Pest Management (IPM) Plan is required.  www.austintexas.gov/ipm

Where can I find design guidelines?
Design guidelines for rain gardens can be found in Section 1.6.7.H of the Environmental Criteria Manual.  Click here for guidelines and resources for small scale green stormwater infrastructure.

Where can I learn more about biofiltration media?
The rain garden filtration media is the same as that used for biofiltration systems. For information on the biofiltration media go to the Biofiltration Media guidance document.

Where can I find a list of potential biofiltration media suppliers?
Click here for a list of potential biofiltration media suppliers

Rooftops can generate large volumes of runoff which, when discharged to paved surfaces and landscaped areas, can generate large pollutant loads. Rainwater harvesting systems can capture this runoff before it is discharged, thus preventing pollution while also putting the captured water to beneficial use, such as landscape irrigation or cooling water.

Rainwater harvesting is eligible for water quality credit only for commercial development. The amount of credit will depend on the size (water quality volume) and drawdown time of the system. Rainwater harvesting systems can provide equivalent treatment to a sedimentation/filtration system, or be designed to meet a non-degradation level of treatment required in Barton Springs Zone watersheds. An Integrated Pest Management (IPM) Plan is required if the captured rainwater is applied to vegetation.

Design guidelines for rainwater harvesting can be found in Section 1.6.7.D of the Environmental Criteria Manual.

The Water Conservation staff of the City of Austin Water Utility is available to provide input on how to achieve cost-efficient design and equipment selection that will also help reduce water and wastewater costs.  

Under the SOS regulations, certain watersheds in Austin allow no increase in pollutant load to receiving streams. Retention irrigation ponds capture stormwater in a holding pond and use the captured volume for irrigation of the surrounding landscaped areas rather than allowing direct release to receiving streams. There is virtually no discharge of runoff off-site and it mimics the undeveloped watershed conditions by allowing infiltration of smaller rainfalls. Retention irrigation systems have excellent pollutant removal efficiency.

Environmental Criteria Manual 1.6.9 (Guidance for Compliance with Technical Requirements of the SOS Ordinance)
 

 

Sedimentation/Filtration systems are the primary stormwater treatment device used in Austin. Runoff is first diverted into a sedimentation basin, where particulate pollutants are removed via gravity settling, followed by filtration through an 18” layer of sand. These systems can achieve removal rates of 40-90% for suspended solids, heavy metals, and organics. Properly operating systems will typically capture 90% or more of all runoff from the contributing drainage area, and release it at a slow rate that enhances baseflow and reduces stream erosion.

Sedimentation/filtration systems are not allowed in Barton Springs Zone (BSZ)watersheds as a stand-alone water quality control, as they are not capable of achieving a non-degradation level of treatment.

Two design variations are allowed in Austin. In “full” sedimentation/filtration systems the entire water quality volume is held in the sedimentation basin, which then slowly discharges runoff to the filtration basin via a perforated riser pipe. The alternative “partial” sedimentation/filtration design foregoes the perforated riser pipe, and distributes the water quality volume between the filtration basin and a sediment chamber. The latter is then separated from the filtration bed by a gabion wall or other porous structure. The “full” design is required when the City of Austin is responsible for maintenance.

Design guidelines for full and partial sedimentation/filtration ponds are provided in Section 1.6.5.of the Environmental Criteria Manual (see 1.6.5.A for “full” systems and 1.6.5.B for “partial” systems).

  • Structural water quality controls may consist of engineered and constructed filters, chambers, basins, or ponds which are designed to treat stormwater runoff by settling, filtration, flotation, absorption, and/or biological processes. The City of Austin Land Development Code establishes the need for structural controls to enhance water quality and the Environmental Criteria Manual provides guidelines for both the design and long-term maintenance of these facilities.
  • Structural controls include: biofiltration, porous pavement, rain garden, rainwater harvesting, retention irrigation ponds, sedimentation filtration ponds, vegetated filter strips, and wet ponds.

A low impact development (LID) technique for reducing the impact of stormwater is to "disconnect" impervious areas by routing runoff to a vegetated filter strip. This will promote infiltration, sediment deposition, and filtration of pollutants.

This water quality control is similar to vegetative filter strips described in Section 1.6.7.B of the Environmental Criteria Manual . It will however typically be smaller in order to fit into spaces too small for a full-sized water quality control, but still large enough to provide some treatment. The amount of water quality credit will vary with the size of the filter strip and its drainage area characteristics (size and impervious cover). Vegetative filter strips for treatment of disconnected impervious cover can provide partial treatment equivalent to a sedimentation/filtration system but are not acceptable as a primary method in Barton Springs Zone (BSZ) watersheds, where a non-degradation treatment level is required.

As with other vegetative water quality controls an Integrated Pest Management (IPM) Plan is required.

Design guidelines for this control can be found in Section 1.6.7.F of the Environmental Criteria Manual.

A vegetative filter strip is an innovative water quality control in which runoff is routed as sheet flow through a mildly sloped, well-vegetated area, thus promoting infiltration, sediment deposition, and filtration of pollutants. Because of the need to maintain sheet flow, filter strips are typically used to treat small drainage areas, or areas with low impervious cover. These treatment systems can be used in both Barton Springs Zone (BSZ) and non-BSZ watersheds, but those in BSZ watersheds must be larger. To maintain the proper functioning of these systems the vegetation must not be cut too short (minimum 3” for turfgrass and 18” for bunchgrass), grass clippings must be removed out of the filter strip, and an Integrated Pest Management (IPM) plan is required. Design guidelines for vegetated filter strips are provided in the Environmental Criteria Manual (Section 1.6.7 Alternative Water Quality Controls)