LEVEE DESIGN GENERAL
Levees are not just a pile of rubbish molded to stop water, nor are they mounds of debris pushed together. They are engineered structures using the generally accepted US Army Corps of Engineers principles for safety and efficiency
The Columbia River levees were first built between 1915 and 1920 when farmers of the area wanted to farm year-round. They began to build the levees and were later joined by the US Army Corps of Engineers and the City of Portland to complete them in 1939. In 1950 to 1961 the levees were raised and stengthened, and levee revetments completed between 1962 and 1997. In 2006 resurfacing of the levee slopes began and will continue for several years.
All Columbia river levees are made of sand. They are designed to absorb water, to become saturated and to weep out the landward side into levee toe drains in which water is carried away into the ditch systems. It is the levee backside where district staff watches to see how the levees are doing. During high water, if the water seeps as designed, it comes through clear and is taken away by the toe drains. If it comes through dirty, staff knows that sand and soil is being removed and the area needs to be protected through standard engineering practices to equalize water pressure and eliminate the soil movement.
There is a difference from one drainage district to the other in the style of levees built. In some the width is significant, where in others the height differences are noticeable. PEN 1 has a flood wall as part of its levee. PEN 2 has the Bridgeton neighborhood built on part of its federal levee. MCDD has high and wide levees with some residential structures built on the river into the overbuild. SDIC has high levees built the closest to federal standards throughout and with a greater amount of land between the levee and the river than the other districts.
There are also varibles in design. Overbuild is the amount of sand/soil provided in addition to the bare minimum required by federal levee standards. In some cases there is much overbuild, while in others there is very little. Levee height is also different. Levees in SDIC are 44 feet (NGVD) high providing eleven feet of freeboard during a twenty-nine foot high, 100-year high water event. In PEN 2 the levee is 35 feet high providing six feet of freeboard during the same 100 year high water event. Another variable is levee width. In part of PEN 2, levee width is at the minimum to create the federal standard critical levee section is buried deep inside a levee wide enough to accomadate a full roadway, a bike/pedestrian path and the sloped riprap area.
Finally, all levees are inspected annually by the US Army Corps of Enginnerings to assure they meet the federal levee standards necessary for certification. Three of the four districts have no or just a few minor dificiencies and have been certified. The fourth, PEN 2, has a levee with trees that need to be removed before the levee will be certified. That work is scheduled for summer of 2008. PEN 1:
River levee type: Earthern levee at 35 feet in height.
River Flood Wall: Concrete constructed with concrete inserts into pilings at 35 feet height.
Marine Drive Flood Wall: Concrete inserts into pilings placed across the road just North of Graphic Packaging.
Graphic Packaging Flood Wall: Concrete inserts into pilings placed across the road West of GP under the Railroad levee.
Columbia Slough Levee: Earthen levee at 35 feet.
Cross levee between PEN 1 and PEN 2: I-5 acts as the cross levee and is somewhat vulnerable at Marine Drive, and at the Schmeer Road underpass.
Cross levee between PEN 1 and Rivergate: The railroad levee serves that purpose at 35 feet.
Work necessary: The City of Portland is responsible for installing the flood walls and did a training exercise in 2007 to do so. |
