There are 46 dwelling units in the Pacifica development with a total of 94 parking spaces, at least 24 of which were designated for compact vehicles. The LID measures implemented, like any LID, can be divided into two basic categories: source control and stormwater treatment. The primary purpose of the source control measures is to reduce runoff by enhancing infiltration and/or retaining rainfall runoff water at its point of origin. The purpose of the stormwater treatment devices is to store and/or treat stormwater after it leaves its point of origin and before it leaves the site. The combination of these measures is intended to make the post-development hydrology mimic that of the pre-development.
Source Control Measures:
Source control measures employed include minimizing impervious road, parking, and roof area, installing pervious parking and sidewalk, and a roof runoff collection system. Impervious road and parking areas were minimized by installing 24 ft wide roads with no curb and gutter along with pervious concrete for most of the parking spaces. The length of the road was limited by having it not extend to every housing unit, which may not be possible for all residential subdivisions (figure 1). A 20 ft. wide fire access lane was built for emergency vehicle access to housing uints that were not next to the road. The lane was part of the 8 ft. wide private walking path (dark gray with light brown along in figure 1) except that it had an added 4 ft. wide shoulder on both sides that was stabilized with grassroad pavers (plastic grid).
Figure 1. Map of site with light gray indicating roads and
darker gray impervious path.
All sidewalks and walkways (figure 2) were made of pervious concrete or pervious pavers (Figures 3 and 4 of sidewalk). Building roof area (34,670 ft2) was minimized by making most homes 2 or 3 stories thereby reducing the building footprint. In spite of these measures, impervious surfaces still covered 23.8% of the overall 8.28 acres of the site; however, few, if any, of the impervious areas were directly connected to the storm drain system. The 23.8% imperviousness is much less than the 53% coverage by impervious surfaces in a conventionally-developed residential subdivision near Cary, NC studied previously (Line and White, 2007). Further, impervious surfaces in five residential subdivisions in Olympia, WA averaged 40% of the overall area, while for four multifamily developments the impervious surfaces averaged 48% (City of Olympia, 1995).
Figure 2. Map of site with tan shading showing areas of
permeable pavement.
Figure 3. Pervious pavers sidewalk leading away from
pervious concrete sidewalk.
Figure 4. Close-up of pervious concrete sidewalk.
The area of impervious surfaces may be further reduced considering that rainwater runoff from the roof of the 3,300 ft2 common house was routed to a storage and reuse system. Roof runoff is stored in a 12,500 gallon tank (figure 12) for use in washing machines and a toilet in the common house. The collection, storage and use of the roof water essentially removes this roof area from the impervious surfaces reduces the percent imperviousness, which would reduce the percent imperviousness to 22.9%.
At Pacifica, the use of native vegetation is an integral part of source reduction as these plants generally require less fertilizer; however, they also often require more time to become established. In order to stabilize the site from erosion and allow time for native vegetation to become established, wood chip mulch was applied to almost all of the pervious areas of the site. When the trees were removed at the start of the project, the stumps were ground (figure 5) and the resulting mulch stored on-site for later application on the exposed soil areas of the site (figure 5). This helped reduce sediment export until plants could be established to provide long term stabilization of the site. Several other erosion and sediment control measures including a large basin with a floating outlet were installed on site during construction as part of the approved erosion and sediment control plan. In addition to those in the plan, the developer installed a temporary level spreader (figure 6) at the outlet of the basin to distribute outflow across the wooded riparian buffer to further reduce the turbidity and sediment load of the outflow.
Figure 5. Stump grinding (left) and mulch spreading (right).
Figure 6. Temporary level spreader for sediment and turbidity reduction.
Stormwater Treatment Measures:
Treatment measures at Pacifica included grassed swales, bioretention areas, level spreaders, and a detention/irrigation pond. Due to the drought during this past summer (2007), vegetation in the development was severely limited and therefore the grassed swales were basically nonexistent. Most of the swales have been stabilized with erosion control matting, which prevents erosion, but provides little, if any, treatment (figure 7).
Figure 7. Stabilized swale.
Bioretention areas or rain gardens are expected to provide the bulk of the stormwater treatment at Pacifica. Three bioretention areas were constructed primarily to treat runoff from the impervious road surface, while two others were constructed for general stormwater treatment from the site (Figure 8). All bioretention areas were equipped with underdrains as the subsoils were relatively impermeable (Figure 9). Sand and soil layers were added above the stone and the surface area was stabilized (Figure 10).
Figure 8. Locations of the five bioretention areas.
Figure 9. Bioretention underdrain with stone layer above.
Figure 10. Finished bioretention area with sides stabilized.
The level spreaders were designed to distribute the outflow from two storm drains across the wooded riparian buffer at the downslope end of the development (Figure 11). The spreaders were installed above ground in order to minimize their effect on tree roots (Figures 12 and 13). The above ground installation has caused some problems with erosion at the ground surface due to concentrated flow just downslope of the spreaders.
Figure 11. Locations of the level spreaders.
Figure 12. Level spreader during construction.
Figure 13. Completed level spreader.
A stormwater detention/irrigation pond (figure 14) was constructed at the outlets of several storm drains as the last piece of the treatment system. The purpose of the detention/irrigation pond was to store runoff for use on the community gardens. Water from the pond will be pumped via a solar pump to a holding tank near the garden area where it will be used as needed. This water use along with the roof runoff collection system for the community house (figure 15) further reduces runoff from the site.
Figure 14. Locations of detention pond and roof runoff collection tank.
Figure 15. Large community house roof runoff collection cistern.
References:
City of Olympia, 1995. Impervious Surface Reduction Study: Final Report. City of Olympia Public Works Department, Water Resources Program. Olympia, WA.
Line, D.E. and N.M. White. 2007. Effects of Development on Runoff and Pollutant Export. Water Environment Research 79(2):185-190.
