Deltas on the move - CcSP COM11

Where a river enters an ocean or sea, sediments transported by the river are deposited. 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 like currents, tides and waves play a role. As long as the net rate of sediment supply exceeds the rate of removal a delta will build seawards.

Their fertile soil, presence of fresh water and the proximity of transport routes over river and sea make them an ideal place to live and for economic activities. However there is a downside: lying in the low coastal zone makes deltas vulnerable to all kinds of disasters like hurricanes and tsunamis. In the future this vulnerability will only further increase, because of sea level rise as a result of climate change. The global temperature is set to increase by a few degrees, resulting in the weather becoming much more extreme in many places, with more storms and changed river discharges. The sea level will also rise faster than in the preceding centuries. These changes will have a severe impact on the natural processes in deltas and in the lives of those living in these areas.

As long as the natural processes in a delta are allowed to run their course and sufficient sediment is transported, a delta can grow at the same rate as a rising sea level. But inhabitants of deltas have been adapting their environment for centuries to suit their own needs. The main effects of human interventions is a shortage of sediment and the disruption of the natural processes. Consequently, the relative influence of the sea increases, resulting in erosion at the delta front and subsidence of the delta. As a result affected deltas are no longer able to respond to modified circumstances such as climate change. For inhabitants of the delta area, this means an increased risk of flooding.

Until recently, society relied on technical engineering measures. A re-appraisal of natural, geomorphological processes, however, is currently gaining in importance. A fuller consideration of natural, dynamic forces in a delta, in an integrated, systembased approach seems to be a more flexible and promising way to cope with climate changes. We explored the potential for soft system-based measures in deltas all over the world and distinguished four strategies:

1. physical measures aimed at the management of sediment
2. physical measures aimed at the management of water
3. adaptation
4. no action

Strategies 1 and 2 consist of permanent, physical measures that influence the water and/or sediment management of the delta. These measures can include full or partial system recovery or the steering of natural processes in the delta system. In strategy 3, no measures are carried out with regard to the actual delta processes but human use of the delta is modified. The distribution of people and activities in a delta can be regulated with spatial planning. In addition, solutions that limit the damage as much as possible, compensation by means of an emergency fund or insurance and the temporary evacuation of residents during floods are part of this strategy.

The DELTA website gives an accessible overview of compiled information on deltas across the World. The study is limited to two data sources for which larger sets of data were available in freely accessible form: the World Delta Database and the DIVA-tool. Information retrieved from these databases has been used to carry out preliminary, but quantitative evaluations for three categories of indicators along the world’s deltas: vulnerability, stocks at risk and the potential for system-based engineering solutions. The interactive DELTA tool makes it possible to rank deltas for all kinds of indicators so that it can be used for innovative system-based delta management.

From the available data three simple, generic indicators were extracted that provide a first, quantitative glance of vulnerability, stocks at risk and the potential for systembased measures. These are: 1/100 yr surge height, people potentially flooded in 2000 and area of the coastal plain between 0 and 2 m above mean sea level. In addition to these generic indicators, a range of specific indicators is provided that can offer detailed informed on specific aspects of vulnerability, risk and the potential of system-based measures.

The world’s deltas are quite diverse, and so will be their responses to climate change and the potential for soft measures to accommodate or counteract this. Different deltas are suitable for different ‘soft’ system-based approaches. Looking at a single indicator simplifies the complex reality. The real diversity can be visualized and studied using the DELTAS system for innovative delta management. Taking the 4 highest-ranking deltas for all individual indicators mentioned in section 6.4 gives a list of the following list of 20 deltas that might be interesting for the next phase:

The deltas that score three or more times with the best 4 are:

• Lena 4
• Ganges-Brahmaputra 3
• MacKenzie 3

For further interpretation, a subset can be created from these 20 deltas by applying several criteria. That is, however, also a more context-dependent (subjective) approach.

The deltaic complex of Rhine, Meuse and Scheldt has a particular position. It was not included in the present study, since it did not feature in the data bases used. Since system-based measures are equally applicable on delta-similar coastlines, this complex of river mouths will be included in the subsequent phase of this project. Because system-based measures have been applied here, we presume that the area is potentially promising.

Download the project final report