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Single-Family Home Site Near Western and Vernon, South Los Angeles TEAM MEMBERS
Graduate Students:
PROTOTYPE This is a typical older single-family home located on a 50' x' 150' lot. SITE PROFILE The single-family home site is in the Crenshaw district of Los Angeles. This middle- to low-income area is in south central LA, near the Crenshaw High School. Convenient bus stops at Western Avenue and Slauson Avenue are within a few blocks of the property. A small park and a small commercial area are also a few blocks away. The Craftsman-style bungalow house was built in the 1920s and remains in good condition. The wood-frame house, a garage, and paved areas cover approximately 60% of the 7,500 square foot, 50 ft. x 150 ft. lot. This lot configuration is typical for most homes built in Los Angeles between 1920 and 1950. The remainder of the property consists of lawn and shrubs. A public alley provides access to the garage and public access to utilities. Before development as a residential neighborhood, this land was part of a large commercial agriculture area. The soils on the site are of the Hansford association. They are over 60 inches deep, well drained, and have moderately rapid subsoil permeability. Ornamental plants and crops require irrigation in these soils. This property now serves as the T.R.E.E.S. demonstration site and has been retrofitted based on the ideas generated by the charrette team. The home-owner, Mrs. Rozella Hall, participates in the project by keeping a journal of energy and water use, costs, and landscape maintenance practices. The information collected in the demonstration project will serve as a basis for a more complete assessment of the costs and benefits of retrofitting a typical Los Angeles-area home. SITE DESIGN PROPOSALS This design team had the benefit of working directly with the client in the knowledge that their design, or some variation of their design, would be built as the T.R.E.E.S. Demonstration Site. The owner of the home, Mrs. Hall, allowed TreePeople to retrofit her home in order to demonstrate what would happen if homes were designed to work with-not against-the city's natural cycles of water and waste. Mrs. Hall had limited ability to perform extensive yardwork, or to hire others to do it for her. Consequently, the design team produced a design that would meet all of the environmental performance requirments without requiring a burdensome amount of work for the homeowner. The design strategy is simple. A cistern is installed on the site to capture rain water. Water captured in the cistern is later used to irrigate the lawns. The lawn areas themselves are depressed to allow rain water that by-passes the cistern to collect and be absorbed in the grass and soil below. The native soils underneath these lawns are relatively free of clay particles and consequently are able to quickly absorb large amounts of water. Water from the driveway is also directed into the soil. The plan for the site produced at the charrette illustrates many of the elements that are now constructed on the site including retention grading, a vegetated swale, and a cistern. These elements are part of the final demonstration site design. A fourth element, the driveway drywell, is included as part of the post-charrette design development and is a variant on the permeable pavement strategy proposed for the driveway at the charrette. The operation of the demonstration site is illustrated in the accompanying axonometric diagram. Rain falling on the hard surfaces of the site (the roofs and pavement) is directed to depressed lawn areas (C), or cistern (A). Overflow amounts are carried by the vegetated swale (B) which also serves to accept green waste from the site. Water flowing down the driveway to the street is intercepted by the driveway drywell (D). The design strategy maximizes the storage of rain water on the site lot while minimizing the amount of earth that may require removal from the site. Low maintenance is a must at this site, so all equipment and plantings that are installed function with little or no maintenance. These constraints notwithstanding, the demonstration site successfully captures and holds all of the water falling during a two-inch storm. To accomplish this end, all water panels are designed as shallow retention basins. Safety considerations require that the maximum elevation differential be kept at six inches for the sides of these retention basins. This is accomplished by lowering the lawn panels two inches below existing grade while building up surrounding berms by four inches to make a total of six inches of water storage capacity on all lawn panels. The bordering berms are covered with shrubs, ground covers, or turf, in order to stabilize them and to prevent foot traffic over them where it might compromise the effectiveness of the retention system. Cistern Collection System A cistern collects rain water from the southeast quadrant of the roof during the wet season and then gradually uses this stored water during the dry season to irrigate the lawn and gardens. A roof wash unit collects the "first flush" water (when the first fall rains occur) and sequesters it long enough so that gravity can settle out the summer-long build-up of atmospheric dust and bird feces. The precipitate is then drained into the adjacent lawn panel while the clean water decants into the cistern. The double cistern at the demonstration site is made of Polypropylene, a plastic that is plentiful in Los Angeles' waste stream and is recycled locally by ARCO. The unit holds nearly four thousand gallons of water. The water is pumped out by an electric pump on a timer system to irrigate the yard. The cistern can also act as a flood control device. When a series of catastrophic storms occurs, cisterns can be drained and filled to regulate the flow of water into the flood control system. If implemented on a larger scale, thousands of cisterns around the Los Angeles basin can be equipped with remote control switches that will enable flood control authorities to use them as a "networked reservoir." This will create a highly effective water conservation, pollution prevention and flood control system that is able to store or release water as needed. Vegetated/Mulched Swale A swale is a low-lying or depressed stretch of land. Swales are used at the demonstration site to create an attractive and functional space that also performs a vital function in waste reduction. The mulched swale is composed of recycled greenwaste from the property. It is designed to slow the flow of stormwater and to filter pollutants so that water can be absorbed into the earth to remove toxic substances. A swale can be used in any residential setting and may be composed of grass, vegetation or organic mulch. Retention Grading The front and back yard retention grading is a "sunken garden" that holds rain water until it can be absorbed into the ground. This type of grading works best in highly permeable soils (Los Angeles type 2 and 3). At the demonstration site, the run off from the front roof panels is directed into a six-inch depression in the front lawn, while the south-east roof quadrant and half of the garage roof drains to the back yard. These mini retention structures are capable of handling a flash flood that could occur during a 100-year storm event. During a more intense storm, excess rain water would flow into the existing storm drain system. The depressed area can also be placed over coarse aggregate rock to achieve a higher infiltration rate. Driveway Dry Well This drywell system serves the dual purpose of retaining and cleansing rain water, giving the water within it time to percolate into the ground rather than carrying motor oil and other pollutants into the City storm drain system to be discharged into our beaches and bays. Rain water flowing down the driveway runs through a grate (see D) into a box containing sand and crushed rock that captures pollutants. SUMMARY More land is covered by single family homes than by any other type of land use in the Los Angeles Basin. Any program aimed at using individual sites to help alleviate environmental problems must include new strategies for the single family home and its outdoor property. The relatively small size of the typical single family home site makes it especially challenging to retrofit. Of the five sites studied, the preliminary cost/benefit assessment suggests that the single family home will be the most economically challenging. A street-by-street approach has the potential to overcome some of these challenges, especially where the residential street and lane could be used to mitigate site impacts more economically than the private residential yard. Such an exploration is beyond the scope of this project but should be considered in the near future. This notwithstanding, the demonstration site effectively proves that small sites can be constructed to hold water, reduce energy costs, prevent water pollution, and cut the consumption of potable water. If all new housing were built to do the same, the cost per household would be much lower than it is to retrofit existing homes. As the city matures and single family homes are rebuilt, the devices included at the Hall residence could be routinely required as aspects of any new construction (in the same way that fire protection sprinkler systems are now required for new single family homes in many jurisdictions). BENEFIT ANALYSIS The following benefit analysis provides a cost value per year; per thirty years; and a total value over thirty years for remediation of the entire property. This information was provided by the charrette team in 1997 dollars.
*These estimates were generated by the charrette team based on a design that was substantially different than the design eventually built. TreePeople will be using actual figures to calculate the cost/benefit ratio of the actual project. The above ratio of cost to benefit does, however, suggest that cost effectively retrofitting small single family home sites may be far more challenging than retrofitting larger commercial, institutional, multi- family housing, and industrial sites.
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