River-beach interaction on mixed sand and gravel coasts: a geomorphic model for water resource planning

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A distinctive sequence of river mouth offsetting and freshwater lagoon development is described from mixed sand and gravel coasts on the east coast of the South Island, New Zealand, particularly the Rakaia river. Though total annual sediment load exceeds 4 × 10 6t, the Rakaia is a ‘small’ river in that the coarse fraction nourishing beaches and the regional longshore drift is insufficient to maintain either the river mouth or the adjacent coasts against long-term erosion by the sea. Dominance of marine over fluvial processes is an important overall control of the lagoon and spit development sequence. Long, narrow spits grow generally, but not exclusively northward for up to 3 km at the Rakaia under the influence of longshore drift in low to moderate stages of river flow. Under higher river flows the spits are breached, the lagoon circulation alters and a ‘slug’ of the regional drift sediments temporarily stored in the spits is bypassed to the downdrift coast. River-injected materials contained in flood formed subtidal deltas also contribute to the drift and to marked modulation of the regional net longshore sediment transport which displays a ‘pulsational’ character at downdrift sites. The river mouth lagoons are non-tidal and non-saline owing to generally steep bed gradients in the braided rivers and to low tidal range, though backwater effects are observable and some salt enters by storm overwash and spray. Permeability variations within enclosing spits are important in both the process of mouth offsetting and in flood breaching. A range of potential and actual environmental impacts on such river mouths relating to use of freshwater resources for activities such as hydroelectric power generation and irrigation is discussed in the light of the fact that the coastal lagoons present important concentrations of recreational, wildlife and fisheries values. Irrigation drawoff and hydroelectric power generation can affect the magnitudes of river flow and its temporal pattern at the coast, as well as delivery of coarse sediments to the spits enclosing the mouth, thus markedly altering the interaction with marine processes in the river mouth region. Water resource development on the river has the potential to increase freshwater flooding adjacent to the mouth, to cause protracted closure of the mouths and to accentuate coastal erosion.