A field based classification scheme for gravel beaches

A tripartite classification of gravel beaches, based upon morphodynamic properties, is proposed and demonstrated for 42 New Zealand beaches. The main advantage of this scheme is that it is based on simple visual classification that can be applied globally in the field and is underpinned by morphodynamic differences between the beach types. The three types identified from the results are: (1) pure gravel beach; (2) mixed sand and gravel beach; (3) composite gravel beach. Pure gravel beaches have steep slopes (tan β=0.08–0.24) and gravels extending from the storm berm to below mean low water spring tide level. Mixed sand and gravel beaches have moderate slopes (tan β=0.04–0.13) with sand and gravel entirely mixed both cross-shore and at depth. Composite gravel beaches have a steep gravel berm fronted by a low-angle intertidal terrace, with overall beach slopes of tan β=0.05–0.14. On composite beaches there is distinct hydrodynamic cross-shore sorting of the sand and gravel component. These broad types are tested using discriminant analysis and the three classes are shown to be highly significant (F=16.24, df=8, P<0.00005). The key discriminating variables are Iribarren number, beach width, average grain size and storm berm height.