About This Project
With an eye toward the last half of its name, the District recently initiated a $2.5M construction contract that will not only update its parking lot and grounds, but will do so utilizing state-of-the-art water conservation and low impact development techniques and practices. The Project was partly funded by a $475,000 grant issued by the State Water Resource Control Board and administered by the Santa Ana Watershed Project Authority. The major features of the project include:
- Conversion of approximately 8,400 square feet of existing asphalt pavement & base with new porous asphalt pavement and new porous concrete pavement and sub drain systems;
- Construction of two raised flow-through planters and one landscape filter basin (“rain gardens");
- Revision of the parking circulation layout to reduce impervious asphalt and eliminate over 600 lineal feet of concrete curb, gutter, and storm drain in favor of a vegetated infiltration swale;
- Replacement of two-thirds of the site’s turfed area with drought tolerant landscaping and efficient irrigation systems designed to meet the County’s Water Efficient Landscape Ordinance.
- Deepening of an existing infiltration basin to facilitate positive drainage for the LID features; and
- Construction of 10 monitoring vaults with flow and water quality monitoring equipment.
In addition to conserving water, the project will also be a laboratory for testing the water quality and water conservation benefits of these LID features. The long term effectiveness and durability of the LID features will also be tracked. The data will be used to improve the design and deployment of these features in the future.
Flow-through planter boxes are filled with vegetation planted in an engineered soil mix. The soil acts as a filter that captures pollutants while the plants and bacterial activity in the soil metabolize the trapped pollutants. This process is called biofiltration. The plant roots also help to keep the basin bottom from becoming clogged.
The soil mix in the planter box also acts as a sponge during rain events, helping to reduce runoff. Raised planter boxes adjacent to buildings are usually lined; therefore underdrains are used to discharge the treated runoff. In this installation, the subdrain collection system will carry the water not captured in the reservoir/filter layer “sponge” to a monitoring station. The quality and quantity of the runoff will be measured and compared to untreated rooftop runoff from the same building.
Permeable Paver Systems
Permeable modular blocks allows for a reduction in the volume of stormwater runoff as well as a reduction in pollutants. The blocks, which are impervious, provide a decorative hard surface. The blocks are placed in such a way that gaps, filled with porous aggregate, are created. Stormwater passes freely through the gaps in the blocks to a gravel reservoir layer beneath the blocks. The stormwater is allowed to infiltrate into the sub-soils over time. Porous pavement surfaces work best when they are designed to be flat or with gentle slopes.
Most Asphalt Concrete is made from 3 constituents: a binder (asphalt), gravel and sand. The sand typically fills all the void space in the asphalt mix, making it impervious. “Porous Asphalt Concrete” uses a special gravel mix that nearly eliminates the need for sand. The binder “pastes” the special gravel mix together in a way that creates porous interconnected voids without weakening the asphalt. Stormwater passes freely through these voids to a gravel reservoir/filter layer beneath. In most applications of this LID feature, the stormwater is allowed to slowly infiltrate into the sub-soils. In this installation, we have an impermeable liner to prevent infiltration and a subdrain collection system that will instead carry the water from a reservoir/filter layer to the central monitoring station. The quality and quantity of the runoff will be measured and compared to the runoff from a standard impervious asphalt parking lot.
Most concrete is made from 4 constituents: cement, water, gravel and sand. The sand typically fills all the void space in the concrete, making it impervious. “Porous Concrete” uses a special gravel mix with very little sand. Carefully controlled amounts of water and cement make a paste that binds the gravel aggregate particles together in a way that creates porous interconnected voids without weakening the concrete. Stormwater passes freely through these voids to a gravel reservoir layer beneath. Usually, the stormwater is allowed to infiltrate into the sub-soils. In this installation, we have an impermeable liner to prevent infiltration and a subdrain collection system that will instead carry the water from the reservoir/filter layer to the central monitoring station. The quality and quantity of the runoff will be measured and compared to the runoff from a standard impervious parking lot.
A bio-swale is landscaped area designed to capture and treat runoff from impervious areas. Swales are small channels landscaped with vegetation planted in an engineered planter mix soil. The soil acts as a filter that traps pollutants in runoff while the plants and biological activity in the soil metabolize the pollutants. This process is called biofiltration.
The soil mix in the swale also acts as a sponge during rain events, helping to reduce runoff. Swales differ from basins in that they are not designed to pond water but are free draining to an outlet point. During small storms the stormwater will infiltrate into the sub-soils beneath the swale. In larger storms, flows pass through the swales and pollutants settle out over its length. The plant roots help to keep the swale bottom from becoming clogged.
Landscaped filter basins are simply depressed landscaped areas designed to capture and treat runoff from impervious areas. The landscaped filter basins are filled with vegetation planted in an engineered soil. The soil acts as a filter that captures pollutants while the plants and bacterial activity in the soil metabolize the trapped pollutants. This process is called biofiltration.
The soil mix in the landscaped filter basin acts as a sponge during rain events, helping to reduce runoff. The plant roots help to keep the basin from becoming clogged. The basin also includes an overflow outlet to address any storm flows that exceed the design capacity of the BMP.
Typically, the remaining stormwater that isn’t captured by the sponge is allowed to infiltrate into the sub-soils. However, in this installation, we have an impermeable liner to prevent infiltration into the sub-soils and a subdrain collection system that will instead carry the filtered water to a central monitoring station. The quality and quantity of the runoff will be compared to the runoff from a standard impervious parking lot.
A large portion of the District Grounds drains to an existing infiltration basin, which allows the captured runoff to seep into the ground. This basin will be deepened and its landscaping converted from irrigated turf grass to a more water efficient mix of shrubs, ground cover and trees. The deepening is required to assure that runoff from ordinary rain events will not impact the subdrainage system serving the monitored Low Impact Development features.
Because this is demonstration project, flows from the onsite LID features will be monitored for water quality improvements as well as stormwater volume reductions. The flows will then be discharged into an infiltration basin located adjacent to the monitoring building. Monitoring also includes visual observations and record keeping regarding the construction of the facility and the long term operation and maintenance.
The monitoring data will help answer the following questions regarding the design, construction and operation of LID features:
- What lessons did the construction process teach us about how to better design these types of features?
- How much volume reduction results from stormwater captured within the engineered soil/media layers of these features?
- What is the pollutant removal effectiveness of these features?
- Do the features perform as expected over time?
- Do the operations and maintenance regimes specified in our LID Manuals need to be adjusted based on real-world performance of these features?
- How can we improve the design of these features based on lessons learned from long-term operation and maintenance?
California Friendly Garden
Plantings in this installation make use of California native and Mediterranean plants that have evolved to live easily with our soils, wildlife and climate. Many natives, as well as many Mediterranean species, tolerate dry summers with little or no water once they are established. This greatly reduces the need for irrigation.
Approximately 210 plant markers are distributed throughout the site highlighting approximately 100 species of plants.
Appropriately sited native or Mediterranean type plants require less soil preparation, watering, mowing, fertilizing and spraying, all of which promotes healthy soil that stores water and nutrients.
Open Mon – Fri 8:00am to 5:00pm
The following links contain additional information and resources regarding LID:
California Stormwater Quality Association
United States Environmental Protection Agency LID Portal
State Water Resource Control Board LID Portal
Project Specific Information:
What is Low Impact Development (LID)?
Low Impact Development is an ecosystem-based approach to development that allows the development to function as part of the natural environment as opposed to apart from it. LID techniques, including planter boxes, bio-swales, porous paving, and landscape filter basins are designed to capture excess rainwater runoff that is typically associated with new impervious areas from developments. These techniques help to restore the natural water balance and prevent pollution. LID developments are also designed to drain impervious areas to pervious areas to allow more water to infiltrate onto the site and to capture pollutants that would otherwise have been washed off the site.
What are pervious and impervious surfaces?
Developments, such as government offices, residential communities and business districts often have combinations of pervious and impervious surfaces. Pervious surfaces typically include landscaped areas and open space areas, while impervious areas include rooftops, parking lots and other structural features of a development. The pervious portion of a development allows stormwater to infiltrate into the ground as it might have naturally. Impervious areas do not allow stormwater to infiltrate. Rainfall that hits impervious surfaces becomes surface runoff like that seen in street curbs during rain events.
Why are impervious areas important?
When land is developed, the percentage of impervious area typically increases from the natural state. Higher impervious areas increase the volume and flow rate of storm water runoff. Increased impervious areas can also prevent rainfall from infiltrating the ground and replenishing groundwater basins used for drinking water supplies. Increased runoff caused by impervious areas can also pick up pollutants such as trash and pesticide residue and increase the transfer of these pollutants from urban areas to lakes and streams.