Sedimentation in a river dominated estuary

Abstract

The Mgeni Estuary on the wave dominated east coast of South Africa occupies a narrow, bedrock confined, alluvial valley and is partially blocked at the coast by an elongate sandy barrier. Fluvial sediment extends to the barrier and marine deposition is restricted to a small flood tidal delta. Sequential aerial photography, sediment sampling and topographical surveys reveal a cyclical pattern of sedimentation that is mediated by severe fluvial floods which exceed normal energy thresholds. During severe floods (up to 10x 103 m3 s−1), lateral channel confinement promotes vertical erosion ofbed material. Eroded material is deposited as an ephemeral delta in the sea. After floods the river gradient is restored within a few months through rapid fluvial deposition and formation of a shallow, braided channel. Over an extended period (approximately 70 years) the estuary banks and bars are stabilised by vegetation and mud deposition. Subsequent downcutting in marginal areas transforms the channel to an anastomosing pattern which represents a stable morphology which adjusts to the normal range of hydrodynamic conditions. This cyclical pattern of deposition produces multiple fill sequences in such estuaries under conditions of stable sea level. The barrier and adjacent coastline prograde temporarily after major floods as the eroded barrier is reformed by wave action, but excess sediment is ultimately eroded as waves adjust the barrier to an equilibrium plan form morphology. Deltaic progradation is prevented by a steep nearshore slope, and rapid sediment dispersal by wave action and shelf currents. During transgression, estuarine sedimentation patterns are controlled by the balance between sedimentation rates and receiving basin volume. If fluvial sedimentation keeps pace with the volume increase of a basin an estuary may remain shallow and river dominated throughout its evolution and excess fluvial sediments pass through the estuary into the sea. Only if the rate of volume increase of the drowned river valley exceeds the volume of sediment supply are deep water environments formed. Under such conditions an estuary becomes a sediment sink and infills by deltaic progradation and lateral accretion as predicted by evolutionary models for microtidal estuaries. Bedrock valley geometry may exert an important control on this rate of volume increase independently of variations in the rate of relative sea level change. If estuarine morphology is viewed as a function of the balance of wave, tidal and fluvial processes, the Mgeni Estuary may be defined as a river dominated estuary in which deltaic progradation at the coast is limited by high wave energy. It is broadly representative of other river dominated estuaries along the Natal coast and a conceptual regional depositional model is proposed. Refinement of a globally applicable model will require further comparative studies of river dominated estuaries in this and other settings, but it is proposed that river dominated estuaries represent a distinct type of estuarine morphol...

River-Dominated Coasts

“…Figure 5 shows a broader application of this scheme to the coastal waterways of Australia distinguish ing estuaries that are still infilling from deltas where sediment reaches the coast and is redistributed along the adjacent shore line causing it to prograde (Harris et al, 2002). Cooper has suggested that it is possible also to distinguish river-dominated estuaries (Cooper, 1993). The broad classification of coastal depositional environ ments proposed by Boyd et al (1992) in the context of sequence stratigraphy, and the relative balance of river, wave, and tide processes, has also been used as a basis for inferring changes through time.…”

Geomorphology and dynamics of the Mfolozi River floodplain, KwaZulu-Natal, South Africa

“…The result of aggradation at the coast is further lowering of the river's longitudinal gradient, which in turn causes feedback that further encourages sedimentation. Unless the estuary and lower floodplain area are scoured, as described by Cooper (1993), aggradation on the lower floodplain channel may reach a critical elevation causing local avulsion. Thus, the lower floodplain is a region of frequent avulsion as the channel jostles the effects of transgression and loss of sediment transport capacity.…”

Depositional history of estuarine infill during the last postglacial transgression (Gulf of Cadiz, Southern Spain)

“…This may be an effect of the small accommodation space owing to the high topographic elevation of the thalweg relative to the highstand, but space constraints may lead to considerably complex facies architectures. The importance of fluvial is also on the geometry of the bedrock valley (Cooper, 1993) which in part is controlled by the geological framework (Dabrio et al, et al, It is important to remark that the Holocene filling of estuaries does not with fluvial conglomerates as by et al (1997), because these are of Late Pleistocene age ( Fig. 5 and T able 1), and were incised by rivers the erosional surface of the sequence u'--''-,.u ...... ''u The faunal content of the lower of the muddy central basin facies indicates restricted living conditions that m ust be related to the first of drowning of the ca.…”