Deltas on the move; Making deltas cope with the effects of climate c

KvR report 001/2006Delta, estuary, river mouth….There are many definitions of “deltas”. A common definition of a delta (Wikipedia)is: “the mouth of a river where it flows into an ocean, sea, desert or lake, buildingoutwards (as a deltaic deposit) from sediment carried by the river and deposited asthe water current is dissipated”. This definition includes the processes(transportation of sediment and the movement of water). The Van Dale Dutchdictionary provides a less process-oriented description: a delta is “land enclosed bythe arms into which a river divides itself at its mouth.” Geologists use a definitionthat is based primarily on the form: “a deposit of sediments made by a stream atthe place of its entrance into an open body of water, resulting in progradation of theshoreline” (Visser WA, 1980). A common factor in all these definitions is that adelta consists of a plain/area of land where a river flows into a sea, ocean or lake.Commonly a delta is seen as a different type of river mouth than an estuary.However, the underlying processes shaping deltas and estuaries are the same.The type of shape depends on which of the geomorphological and hydrodynamicprocesses dominate at a given place at a certain time. Also the scale at which ariver mouth is studied plays a role. Typically a river mouth may divide itself indifferent channels before entering the sea. The mouth of these different channelsoften take the shape of an estuary, whereas the larger picture is that of a delta.This is true for e.g. the Ganges delta (see photo) but also for a country like theNetherlands. In summary: the difference between a delta and an estuary is not asclear at might appear initially.<strong>Deltas</strong> on the move2.2 A game of sand and waterThe four most important natural factors that control the processes in a delta are: riverprocesses, coastal processes, climate factors and tectonics.Where a river enters an ocean or sea, sediments transported by the river aredeposited. As the flowing river water bumps into the calmer waters of the ocean, itloses its velocity and transport capacity and the sediment load of the river isdropped. The amounts can be enormous; e.g. the Huang He (Yellow River, seephoto) brings in over 1 billion ton of sediment a year in the Pacific Ocean (Hart &Coleman, 2004). As layer upon layer is deposited a platform of sediment, the delta,is built up and rises above sea level. At the seaside of the delta erosive forces likecurrents, tides and waves play a role. As long as the net rate of sediment supplyexceeds the rate of removal a delta will build seawards.Sediment plume of the Huang He inthe Bohai Gulf (Hart & Coleman,2004).However, it is of little consequence for the people living in the coastal areaconcerned whether their home is on a real delta or not. All these flat coastal areaswill be heavily influenced by climate change. As this human aspect forms the focusof this study, the concept of deltas will be interpreted broadly in this report.Ganges delta (Hart & Coleman,2004).A delta consists mainly of three sub environments (Figure 2.1): the delta plain (whereriver processes dominate), the delta front (where river and marine processes areboth important) and the prodelta (where marine processes dominate).The delta plain is the actual platform of sediment where the river processesdominate. It usually contains multiple river channels (distributaries) and a widevariety of non-marine to brackish environments like swamps, marshes, tidal flats andinterdistributary bays. The delta front is the site where the most active deposition in adelta takes place, particularly at the mouths of the streams where the coarsestsediment is deposited. The prodelta is the marine area where the fine sedimentssettle out to form the gently sloping front of a delta.1213