Magnitudes, spatial extent, time scales and causes of shoreline change adjacent to an ebb tidal delta, Katikati inlet, New Zealand

The ebb delta and adjacent beaches at a mixed energy (tide dominated) inlet were monitored to identify the influence of the inlet/ebb-delta system on erosion and accretion of the adjacent beaches. Sub-aerial beach profile dat a were collected at 25 locations at monthly intervals over four years. Beac h excursion distances and volumes, dispersion diagrams, principal component s analysis, and time-series correlations with wave parameters were used to extract the magnitudes, time scales, and causes of the beach changes. The d ivergence induced in the regional longshore transport regime by wave refrac tion over the ebb delta was also investigated. Changes on the adjacent beac hes, beyond the wave shadow of the ebb delta and more than 3-4 km from the inlet centreline, were dominated by a quasi-annual signal which reflected c ross-shore sand transport forced by storm waves. Lesser changes were linked with longshore transport, which reversed its prevailing direction at inter -annual time scales. The longshore transport caused sand oscillation betwee n the small headlands bounding the western beach, sand inputs to the inlet/ ebb-delta system from the beaches either side, and a standing pattern of er osion and accretion about the inlet due to refraction-induced transport div ergence. Immediately behind the ebb delta the quasi-annual storm-wave signa l was either small or non-discernible against much larger, multiyear change s. The multi-year changes correlated with the longshore transport potential and were interpreted as being associated with scouring by the marginal flo od-tidal flows, accretion of sand bars migrating shoreward from the ebb del ta platform and flanks, and longshore transport divergence. Most of these c hanges appeared to be the on-shore signature of part of a cycle of sand cir culation between the beaches and the ebb delta bars.