Once a site's soil is stabilized and the project is built, it is critical to ensure landscaping and site uses stay structurally stable, drain well, use sustainable quantities of water, and do not degrade the environment into the future.
Consider soft engineering that incorporates Green Infrastructure, water retention for use and absorption into the ground on-site, add water quality protection disguised among attractive landscaping, and reduce costs over more traditional hard engineering choices. Collect rainwater from rooftops and store the water for landscape irrigation or even for treatment and consumption on-site. Implementing these complementary principles into the design of your development are the primary foci of Low Impact Development (LID). Travis County Development Regulations offer alternatives and the flexibility necessary to readily implement available LID designs.
There are various LID facilities for permanent water quality control and each type facility provides some level of water quality treatment and water flow reduction. The goal is to achieve both treatment and flow reduction. Design considerations and site constraints may restrict using any one type facility to maximize achieving both goals. A network of facilities in series could be considered instead of reliance on any one facility. As an aesthetic consideration, LID facilities with biological processes offer both passive treatment and attractive landscaping as a bonus. Examples include constructed wetlands and bioswales (UACDC, 2010).
Travis County, the Highland Lakes Watershed Ordinance (HLWO), Edwards Aquifer Protection Program, and City of Austin standards all require permanent controls to treat storm water runoff from new or modified development. These standards are necessary to maintain and protect our outstanding local water resources. Green Infrastructure storm water controls are found in the HLWO technical manual and these controls can be used on most projects in all unincorporated areas of Travis County.
- Water quality treatment required when proposed impervious cover exceeds:
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- 15% in LCRA jurisdiction (west Travis County)
- 20% in eastern Travis County
- Can be used as stand-alone or in series
- In LCRA jurisdiction, LID practices can be used as impervious cover credits:
- To reduce effective IC to less than 15% (avoid basins) or
- Serve as measures to meet ordinance goals
Possible methodologies include:
Bioretention – an attractive landscaped water quality basin that functions as a soil and plant-based infiltration device
Rain gardens – a small-scale bioretention facility that collects runoff from small drainage areas such as roof-tops or small parking areas
Infiltration/filter strips –trenches, basins, and filter strips that direct runoff into suitable soils to remove pollutants and promote recharge
Pervious pavement – materials used in parking lots, sidewalks, and roads that allow runoff to flow through the surface into water storage areas or suitable natural soils
Rainwater harvesting – collection of roof-top runoff into tanks or basins and using the stored water for beneficial purposes to reduce water supply demands
Roof-top disconnection - runoff is directed from the downspout onto vegetated surfaces to promote infiltration and filtration to reduce runoff rate and volume
Soil amendment – use of native soils at appropriate depths and native turf grasses to infiltrate and reduce runoff volume
Conservation landscaping – use of native soils at appropriate depths and native plants, shrubs, trees, and perennials to reduce runoff volume and limit chemical application
Natural area preservation – conservation of natural areas within development sites to retain pre-development runoff quantity and quality characteristics
For further information on approvable Green Infrastructure, sizing calculations, and design details, please link to the LCRA Water Quality Management Technical Manual. Travis County has adopted these technical criteria by reference for development in western watersheds of the county within its jurisdiction.
Also, Travis County will readily allow use of either City of Austin Environmental Criteria Manual (ECM) standards in our county's eastern watersheds OR will allow use of the LCRA criteria in an eastern watershed if the developer prefers.
Using the LCRA Technical Manual criteria, Impervious Cover Credits are allowed in the sizing of storm water basins. This allows for smaller and less costly infrastructure while expanding the use of LID in our community.
