Stability of the New River Spit, and the position of Oregon's beach-zone line

The stability of the New River Spit, Oregon, has been investigated to deter mine whether the State's beach-zone line should be moved. The zone line app roximately followed the edge of dune vegetation when it was established in 1967-69, so relevant to the decision are the subsequent changes in morpholo gy of the Spit and movement of the vegetation are several factors that have produced significant changes in the morphology and stability of the New Ri ver 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 res ult is a pronounced longshore variation in beach sediment grain sizes and m orphodynamics, with beaches at the north being dissipative while the coarse -grained beaches to the south are intermediate to reflective. This longshor e variation in large part controls the stability of the Spit. Also importan t are the low elevations of the Spit at its north end, produced by the nort hward migration of the mouth of the New River by 4.7 kin between 1967 and 1 997. There has been insufficient time for dune growth, and the low elevatio ns permit frequent overwash events. In contrast, over most of the length of the Spit the dunes have grown vertically since 1967, and the vegetation li ne 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 s ide of the Spit, aided by storm-wave erosion of the dunes. Analyses have be en 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 alo ng 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 th e State's beach-zone line can be shifted oceanward along the central part o f 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 w hen the line was first established.