Profile Picture BrandyRiche

created Mar 15 2018

updated Dec 11 2018

Description

As part of a Channel Migration Zone Study, GeoEngineers, Inc. was contracted by Pierce County, Public Works Surface Water Management formerly “Water Programs” Division to create a series of shapefiles including the SPC_Migration_Potential_Areas.shp. Pierce_Migration_Potential_Areas.shp combines the include severe, moderate or low migration potential areas. GeoEngineers, Inc. completed migration potential studies of the White, Puyallup and Carbon Rivers (completed 2003 adopted 2005), South Prairie Creek (completed 2005 adopted 2017) and Upper Nisqually River (completed 2007 adopted 2017). These were accepted by SWM and Adopted by County Council.The MPA delineation involved identifying severe, moderate and low migration potential areas within the delineated CMZ. The MPA delineation approach is similar to that employed in our CMZ analysis; that future rates and character of migration will be similar to those of the past for similar water discharges, sediment influx, and debris entrainment conditions. This analysis was also based on the absence of levees, revetments and other confining structures. The width of each MPA was measured, based on delineation criteria developed specifically for this project, and then adjusted to accommodate geomorphic conditions not accounted for in the maximum migration rates. Criteria developed for mapping severe, moderate and low MPA are provided in the following paragraphs:Severe MPA includes the area lying inside the HCOT, and an area immediately outside the HCOT boundary equivalent to a distance the channel could travel in a specified period. The extent of the Severe Migration Potential Area outside the HCOT boundary is determined by two criteria. The first criterion is the distance the outside channel edge could travel in 10 years of steady lateral migration away from the HCOT boundary (Maximum lateral migration rates multiplied by a ten- year period). The second is defined by the distance the outside channel edge could travel in storm single event (i.e. maximum overnight rate) from the current channel position (2002). The landward most boundary of the two criteria defines the Severe Migration Potential Area.Moderate MPA includes areas adjacent to the outside edge of the severe migration potential area. The width of the moderate migration potential area is determined by the distance the outside channel edge could travel in five years (for South Prairie Creek 10 years) of steady lateral migration beyond the outside edge of the severe migration potential area. The CMZ boundary will serve as the outside edge of the moderate migration potential boundary at sites where the distance between the severe migration potential boundary and the CMZ boundary represents less the five years (for South Prairie Creek 10 years)of steady lateral migration. Moderate migration potential areas are not included at sites where the outside edge of the severe migration potential area is determined by the location of the CMZ boundary. The rate of migration used in the calculation is the maximum average rate of migration for each geomorphic reach (measured as described above). In some places the width of the Moderate Migration Potential Area may be modified based on geologic interpretation, professional judgment.Low MPA includes areas adjacent to the outside edge of the moderate migration potential area. The extent of the Low Migration Potential Area beyond the moderate migration potential boundary will be determined by CMZ boundary, as determined by our CMZ evaluation. Low migration potential areas will not be included at sites where the outside edge of either a severe or moderate migration potential area is determined by the location of the CMZ boundary.The most common adjustments typically involved widening the moderate MPA to include ancient abandoned channel deemed capable of arresting main stem flow in an avulsion event. Other common Moderate MPA adjustments involved increasing or decreasing the base width to accommodate the following conditions; The presence of native erosion resistant bank materials, such as the Osceola Mudflow or local downstream or oblique direction of meander bend migration.

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