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

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KvR report 001/2006PrecipitationChanges in precipitation primarily influence river discharge. If extremely high waterlevels occur more often, erosive flooding will increase and more sediment will betransported by the river. In drier climate areas, extra precipitation may also result inthe development of more overgrowth in the hinterland. This overgrowth “traps”precipitation, resulting in less extreme high water situations. The plants also protectthe soil against erosion. In cooler climates, the formation of peat moors has the samedelaying effect on the drainage of water. It is difficult to predict the exact influence ofchanges in the precipitation on deltas. During peak discharges fresh water plumesmay extend over many km’s in coastal waters. If the sea level rises, or the riverdischarge decreases, salt water can gradually displace freshwater, reaching farinland, as was the case in the Casamance estuary (Figure 2.2). Salt water can thusinfluence areas a long way inland.StormWhen storms become more severe, the waves combined with currents will washaway more sand from the coast. This sand will end up on the foreshore and will laterbe partially deposited back on the coast. However, some of this sand can also endup deep in the sea or be transported elsewhere, making it unavailable for the delta.<strong>Deltas</strong> on the move2.6 SummaryThe four most important natural factors that control the processes in a delta are: riverprocesses, coastal processes, climate factors and tectonics. Sediments that areprincipally transported by the rivers forms the most important “building blocks” for adelta. The water ensures that the sediment ends up in the right place in the delta.The influence of the river, tides, waves and currents varies from delta to delta. Somedeltas are dominated by the river, whereas other deltas may be influenced more bywaves or tidal currents.As long as the natural processes in a delta are allowed to run their course andsufficient sediment is transported, a delta can grow at the same rate as a rising sealevel. It is therefore essential for the continued existence of the delta that sufficientsediment is available and that the dynamic processes are not hindered. If thepotential of system-based measures is to be mapped out, it is important to first gain agood overview of the dominant processes at work in a delta. Appropriate systembasedmeasures can then be implemented on the basis of this.Figure 2.2:Longitudinal salinity distribution in Casamance estuary. During theSahelian drought of the early 1980sthe Casamance estuary turned hyper saline with dramatic ecologicalchanges as a result (Savenije et al., 1992).salinity (g/kg)1501209060feb-79mei-78mrt-84mei-843000 40 80 120 160 200 240 280 320distance from mouth (km)1819
KvR report 001/2006<strong>Deltas</strong> on the move3. <strong>Deltas</strong> and menUnder natural circumstances deltas are well adapted to changes in sea levels,sediment loads, river discharges etc. Climate change in itself therefore is not a threatto deltas. However, over the past centuries human interventions have in many placesparalyzed the system’s inherent natural resilience to climate change. These may beinterventions in the delta itself (e.g. dikes preventing the influx of sediments buildingup the area), but also interventions elsewhere in the watershed (e.g. afforestationdecreasing sediment loads) or in the sea (e.g. off-shore harbor developmentaffecting long-shore drifts). This chapter deals with human interventions in deltas andwhether or not they fit within a system-based approach.3.1 Why do people live in deltas?Coastal zones and deltas have always been attractive settlement sites. In 2001 overhalf the world's population lived within 200 km of a coastline. Eight of the top tenlargest cities in the world are located by the coast.One of the main reasons why people settle in deltas is the availability of food. Thesoil is generally fertile and easy to cultivate. The freshwater of the river can be usedfor irrigation. In addition, there are often rich fishing grounds where the sea and rivermeet. This has set a precedence for populations to naturally migrate towards coastalareas.Satellite photograph of theNile and its delta. The colorsshow the night sky brightness(Cinzano et al., 2002).An example of a densely populated delta is the Ganges. Between 115 and 143million people live here, despite from risks from floods caused by monsoons, heavyrunoff from the melting snows of the Himalayas, and tropical cyclones. It is believedthat upwards of 300 million people are supported by the Ganges Delta, andapproximately 400 million people live in the Ganges River Basin, making it the mostpopulous river basin in the world. Most of the Ganges Delta has a population densityof more than 520 people per square mile, making it one of the most denselypopulated regions on earth (Wikipedia). Other examples of densely populated deltas2021
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