Interaction between human engineering and natural processes of delta evolution: A Bayesian network modelling approach

Abstract

Deltaic environments are low-lying coastal areas which often concentrate population and socio-economic values. They constitute hotspots of climatic, environmental and anthropogenic change and are exposed to both rapid and slow onset hazards, such as storm surges and land subsidence. The frequency and severity of these hazards is expected to increase reflecting natural and anthropogenic forcings. There is a long history of engineered interventions being used to counteract these hazards in deltas. Large scale engineered structures, such as dyke rings and polders, have played an important role in shaping both environmental and socio-economic conditions in many populated deltas. This may lead to a feedback between natural and human processes, providing an example of coevolution. Coevolution starts when humans first implement engineered interventions, which reduces the flood hazard in the short-term, but also alters and often disrupts hydrological and natural deltaic processes. This in turn drives the need for additional engineered interventions. The aim of this thesis is to investigate how engineered adaptations have shaped populated deltas through time, and how these might influence their future development.
An historical analysis of five representative highly populated deltas identified generic evolutionary pathways for deltas and key cause-effect relationships. The triggers and impacts of engineered interventions within the deltas were also analysed. The type of engineering intervention used within deltas varies with hazard type, level of economic development and land use. While less economically developed deltas use less advanced methods of engineered interventions, all the deltas were experiencing coevolution, as defined above. In addition, the deltas in wealthy developed countries (e.g., the Rhine, Netherlands) have reached a state of lock-in. This is where they are entirely dependent on structural interventions and their enhancement to protect the delta area from increasingly hazardous events- i.e. they have no other adaptation choice. All the other deltas are heading in this same direction.
To explore these processes further, a conceptual model of a generic delta in a developing country was developed, including the key natural and human processes shaping the delta and causing coevolution. This conceptual model was operationalised using Bayesian Statistics and the literature, supplemented by expert opinion, as needed. Over time, exploratory model simulations exhibited delta coevolution. Human aspects of the model had a greater long term influence on the development trajectory of the delta compared to the natural aspects, which generated short term, localised implications. Pressured deltas, i.e. deltas exposed to sea level rise, upstream engineering and pressures to develop the economy, are likely to follow a similar development pathway to developed country deltas, becoming more urbanised and thus locked-in to structural protection interventions. An alternative innovative adaptation approach to dykes based on controlled sedimentation to raise land elevations reduced the likelihood of delta flooding and its adverse impacts. This counters lock-in and increases choices in long-term delta management. As deltas become increasingly vulnerable locations, due to relative sea-level rise and climate change, rising populations and increased development, this thesis emphasises the importance of recognising and addressing the adverse long-term effects of traditional engineered interventions. Innovative management approaches are required to develop interventions that satisfy not only hazard management in the short-term but promoting sustainable deltas and avoiding lock-in in the long-term. Building elevation via controlled sedimentation within deltas is a potential solution explored within this study. However, application of this method requires a fundamental change of preference from hard engineered interventions by delta stakeholders.

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Identifiers

ORCID for Amy Charlotte Welch: orcid.org/0000-0002-3363-1672

ORCID for Robert Nicholls: orcid.org/0000-0002-9715-1109

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