Appeal 25079

OPSC 307.1, CoP Title 24.10.070(K)

Section 312.0 The drainage system of each new building and of new work installed in any existing building shall be separate and independent from that of any other building and, when available, every building shall have an independent connection with a public or private sewer.

Section 308.1 Except as otherwise provided in this code, no plumbing system, drainage system, building sewer, private sewage disposal system, or parts thereof shall be located in any lot other than the lot that is the site of the building, structure, or premises served by such facilities. The stormwater management facility for each lot shall be entirely independent

Propose shared stormwater drywell for three homes on three lots in the shared private street tract for infiltration of on-site storm water.

The project is located at 1037-1041 SE 111th Ave. Portland, OR 97216, Lot 48-51 TL 3300. A land division will be requested to create a resource area tract, a private street tract, and three home site lots. The project will develop three (3) single family dwellings with ADUs on three (3) separate lots. A total of one (1) infiltration facility will be located on the private street tract south of the three (3) lots. All drainpipes enter the private street tract for the respective lots without crossing adjacent lots. Both the new and existing impervious areas will be directing runoff to the stormwater management system.

This project proposes to manage stormwater with a single PSORB StormFilter for water quality and then a two chamber StormTech SC-740 system for complete infiltration of the runoff on-site. The StormFilter and StormTech combined system have been sized to handle 7,258 sq. ft. of runoff. The total of new and retained impervious areas on site from three separate 1,248 sq. ft. single-family residences, the shared private road (2,080 sq. ft.), and parking areas (1,134 sq. ft) servicing the residences. The StormFilter and StormTech combined system is shown on the drawings of the attached “May 2021 Redtail Stormwater Report” prepared by Deborah Beck, PE.

A shared drywell is the most practical alternative for ensuring adequate drainage provided to each lot while maintaining maximum distance from the site’s steepest slopes, distance from property lines, and structures.

The site is 0.59 acres (25,860 sq. ft.) entirely located within Multnomah County and zoned R5. Minimum density at the site is 2 houses—the maximum is 4. The northern 45% of the property is in an environmental conservation area. The site steps down in elevation from south to north. The steepest portion of the site is in the far north in an area within the conservation area. A second portion of the site with steep slopes is approximately halfway through the property from north to south around 70 feet from the south property line. This slope was created 50 or 60 years ago when a driveway was cut into the slope. A survey of the site is included for reference: “Redtail Nest Survey R20011.” The existing conditions do not include on-site storm water management.

Infiltration rates throughout the site are very good and exceed 20 inches per hour. See “GeoTech Infiltration Report” by Mia Mahedy, PE.

Onsite infiltration of stormwater is required because tested infiltration rates on a property are greater than or equal to 2 inches per hour. SWMM Section 1.3. The preferred place to infiltrate storm water is far from steep slopes. See Standard Setbacks Table 2-1 of the SWMM.

Most of the site’s trees (shown on the survey with size and species information) must be protected in accordance with Portland City Code Title 11.

On this site the area furthest from a slope, and outside the conservation area, is at the southern portion in the private street tract. The location is also furthest from the most trees. The proposed infiltration location will not impair the site’s slope’s stability. See the enclosed “GeoTech Slope drainage Field memo” by Mia Mahedy, PE.

Each lot is required to have separate connections to public water and sewer utilities. These utilities will enter the site from SE 111th onto the private street tract and then on to each lot from its frontage on the private street. Water, sanitary, and storm sewer infrastructure must be adequately separated for health, safety, and maintenance reasons. The City of Portland Sewer design manual recommends 5 feet of separation between sanitary or storm sewer and a water line. In order to accommodate separation of water, sanitary, and storm utilities, the available space for drywells is further constrained. The proposed design meets the separation of water, sanitary sewer, and storm sewer criteria.

After satisfying setback requirements from slopes and buildings, utility separation requirements, and avoiding conflicts with the trees and landscaping, it is not practical to construct a separate drywell on each individual lot. Alternatively, the more feasible solution is to construct a shared drywell that provides the same level of service as separate drywells. This will allow onsite infiltration of all on-site stormwater, preserve the greatest number of trees, and maintain the greatest possible clearance from buildings and slopes.