Combined Sewer System About half of Portland's neighborhoods, most of them built before the 1960s, are served by a combined sewer system. Portland began a program to reduce combined sewer overflows (CSOs) in 1991 under an agreement with Oregon Department of Environmental Quality. The CSO program is scheduled for completion in 2011. This subwatershed, defined as the Taggart A, B/C, and D sewer basins, is served by combined sewers. In a small area, generally south of Division and east of 39th, some street inlets have been disconnected from the combined system and diverted to UICs. Most of the subwatershed is eligible for the Downspout Disconnection Program, which assists in diverting residential roof runoff into onsite disposal systems. Combined sewage flows into the 66-inch Southeast Interceptor and the 78-inch Southeast Relieving Interceptor through seven diversion structures and is treated at the Columbia Boulevard Wastewater Treatment Plant. CSOs from this subwatershed are discharged to the Willamette River through outfall 30 near the east end of the Ross Island Bridge.
Other Facilities Outfalls Combined sewer outfalls discharge combined sewer overflows, a mixture of stormwater and sewage. Stormwater outfalls discharge stormwater runoff from separated sewer areas. UICs (Sumps) In some areas stormwater drains into large perforated manholes formerly called sumps but now referred to as UICs. An underground injection control structure (UIC) is defined by DEQ as any system, structure, or activity that discharges fluid below the ground or subsurface. UICs are mainly used on Portland’s Eastside where porous soils provide adequate infiltration rates, and they've been used extensively to remove stormwater from the combined sewer system as part of the CSO Program. Pump Stations Pump stations pressurize sewage flows for conveyance through a force main to a point were it can continue to flow by gravity. Culverts Culverts are thin-walled pipes typically installed instead of a bridge to allow drainage to pass under a road. Culverts represent a barrier to fish passage, confine flows, and increase their velocity. Culverts can become maintenance problems if they are undersized and cannot handle the volume of stormwater flowing through them, or if they collapse or become blocked with debris. Diversions & Interceptors Until the early 1950s Portland's sewers discharged directly into the Willamette River or Columbia Slough. Diversion are small dams installed in the older sewers that divert flows into what are called interceptor sewers running parallel to the receiving stream. The interceptors, mostly built between the early 1950s and the 1960s, convey those flows to the wastewater treatment plant. The interceptors convey all of the dry weather sanitary flows but allow ,a href=http://www.portlandonline.com/bes/index.cfm?&a=47260&c=31030 Except for UICs, there are no other facilities in this subwatershed. Separate Sanitary Sewers Separated sewers serve areas of the city developed since the 1960s as well as some areas formerly served by combined sewers but now separated because of sewer capacity issues. Much of downtown Portland, for example, has been separated. Wastewater from these sewers is conveyed directly to either the Columbia Boulevard Waterwater Treatment Plant or the Tryon Creek Wastewater Treatment Plant, located in Lake Oswego but part of Portland's sewer system. On average, 95% of sanitary wastewater receives treatment, and the remaining 5% is discharged untreated through combined sewer overflows or sewer leakages (CSO Characterization Report, 1992)). There is no separate sanitary sewer system in this subwatershed. Stormwater Conveyance Stormwater flows through combined sewers, separate storm sewers, and open channels. A 25-year storm event is the design standard for capacity, and many older parts of the stormwater conveyance system are inadequate. Stormwater runoff in this subwatershed is generally conveyed through the combined sewer system. In a small area, generally south of Division and east of 39th, some street inlets have been disconnected from the combined system and diverted to UICs. A small storm sewer sytem collects runoff from the roadways in Mt Tabor Park at the eastern edge of the subwatershed, but these flows eventually enter the combined system. Most of the subwatershed is eligible for the Downspout Disconnection Program, which assists in diverting residential roof runoff into onsite disposal systems.
System Deficiencies System deficiencies described in the characterization are primarily from BES's 1999 Public Facilities Plan. Other more recent issues have been documented by BES Maintenance staff and is included when available. BES is currently updating its Public Facilities Plan, which will provide an updated list of systems deficiencies for each subwatershed. The wastewater collection infrastructure in this subwatershed has a long history of basement flooding, pipe capacity problems, and structural condition problems. Substantial surcharging and major flooding occur throughout the subwatershed. The area west of SE 16th experiences localized flooding. The area along the Holgate trunkline is prone to widespread flooding due to insufficient branch sewer capacity caused by high backwater at the intersection of Holgate and 26th Ave. The area west of SE 24th has localized basement flooding and surcharging as a result of undersized and locally damaged pipes. Surcharging in the rest of the subwatershed occurs on all the major collectors. Since 1994, basement flooding events have increased, partially due to a single storm event in 1996, but largely due to local capacity deficiencies in the smaller feeder pipes draining to the trunk system. The most significant pipe failure occurs along the 30-inch monolithic sewer on 37th Ave south of Gladstone. Television inspections of the line show that the Holgate trunkline has localized areas of small openings, cracks, fractures, and significant infiltration (BES, 1999). Every major conveyance system in the subwatershed exhibits significant surcharging or basement flooding during the 25 year design storm event. Nearly all of the collectors surcharge and 70% of the total modeled pipe system shows surcharging. |