Stability of the New River Spit, and the Position of Oregon's Beach-Zone Line
Keywords:Beaches, beach sediments, beach-zone line, coastal, Oregon, wave runup.
AbstractThe stability of the New River Spit, Oregon, has been investigated to determine whether the State's beach-zone line should be moved. The zone line approximately followed the edge of dune vegetation when it was established in 196769, so relevant to the decision are the subsequent changes in morphology of the Spit and movement of the vegetation line. There are several factors that have produced significant changes in the morphology and stability of the New River Spit. The Coquille River to its north mainly supplies sand to the beach, while sea-cliff erosion to the south yields granules and pebbles. The result is a pronounced longshore variation in beach sediment grain sizes and morphodynamics, with beaches at the north being dissipative while the coarse-grained beaches to the south are intermediate to reflective. This longshore variation in large part controls the stability of the Spit. Also important are the low elevations of the Spit at its north end, produced by the northward migration of the mouth of the New River by 4.7 km between 1967 and 1997. There has been insufficient time for dune growth, and the low elevations permit frequent overwash events. In contrast, over most of the length of the Spit the dunes have grown vertically since 1967, and the vegetation line has shifted seaward. There is a long history of breaching of the Spit at its south end where ranchers have cut channels to drain flooded pastures, or have occurred naturally when flood waters in the river eroded the back side of the Spit, aided by storm-wave erosion of the dunes. Analyses have been undertaken of the beach and dune erosion that might occur in the future, during extreme events when unusually high tides combine with the runup of storm waves. Those analyses reaffirm that the Spit is relatively stable along its central portion where high dunes have developed, but is unstable at its north and south ends where elevations are low. It was concluded that the State's beach-zone line can be shifted oceanward along the central part of the Spit, to the present location of the vegetation line, but needs to be moved landward along portions that are now more unstable than in 1967-69 when the line was first established.