LONGSHORE TRANSPORT AND SAND SPIT CONSTRUCTION: HAVE WE GOTTEN IT WRONG ALL THESE YEARS?

Textbook sandspit construction involves the growth of the spit in the dominant down drift direction. One might expect to deduce the dominant direction of longshore transport from the orientation of a sand spit across a bay mouth, rivermouth, or other coastal reentrant. One would expect, in the case of coastal California, to find sand spits pointing south across many coastal reentrants due to the dominant southerly flow of longshore currents. This is, however, not the case. While many spits point south, few in this orientation are found in a natural state. Especially in Southern California those southward pointing spits are anchored with rip-rap or some other artifice. Whereas along the Northern California coast there are numerous spits in a natural state, the majority of which point north in defiance of the deductively reasoned textbook explanation. Several examples from the north coast provide clues to a possible explanation, with the Mad River providing key evidence suggesting that spits do indeed grow up drift. Does this mean that spits do not grow down drift? Of course not, but their growth in the updrift direction is clearly related to interaction or competition between river and wave actions. The simplest explantion is found where the sole aggregate along a stretch of beach is clearly derived from the river associated with said spit. In these cases wave action promotes aggradation on the down drift bank of the river at its mouth and consequently the channel migrates up drift and the spit grows up drift trapping the river between spit and seacliff or other backshore anchor such as a dune field. The spit may grow as far as a nearby headland as was the case for the Mad River before its progress was halted by our own. A groin was built to prevent its northward migration, but a swale marks its former path well north of the groin that now turns its mouth prematurely to the sea. A model will be presented in demonstration of this interpretation.