Delta dynamics: lessons from the Chao Phraya

Deltas cover about 0.6% of the Earth's surface area, but contain 4% of the world's population (about 300,000,000 people), a proportion that is outpacing world population growth (1.6 vs 1.1% per annum; Edmonds et al, 2017, EGU, Geo. Res. Abs., 19). Yet these regions are peculiarly vulnerable to sea level rise (SLR), land subsidence (often through water extraction), and the effects of climate and other extremes (typically cyclones, but also tsunamis), which can both impact on human populations directly (flooding), or indirectly (e.g. erosion of sediments and therefore deltaic retreat) (Renaud et al, 2013, Curr. Op. Env. Sust., 5(6)). Conversely, deltas are, by definition, areas of natural sediment accumulation, from river estuaries, which are stabilised by the establishment of inter-tidal vegetation. How vulnerable individual deltas are depends on the balance of these processes, including any upstream impoundment of sediment in dams. A case in point is the Chao Phraya delta, located in the upper Gulf of Thailand. Recent work, using long-term historical data, identifies retreat of this delta as being due primarily to subsidence, with loss of mangroves, sediment input depletion, and SLR as being exacerbating factors (Bidorn, 2016, PhD Thesis, Florida State University). The objective of this project is (1) to build a mathematical model of delta retreat / formation, in which SLR and land subsidence are prescribed background forcings and validate it via the Chao Phraya data-set; (2) conduct a sensitivity study using different scenarios of SLR and subsidence to explore how much upstream sediment input (natural flooding or dredging from dams) and mangrove replanting can mitigate these effects; (3) using the dependencies and behaviours observed in this study, infer behaviours that may be seen as generic to (tropical) delta morphodynamics, and which will therefore be valuable to assess long-term sustainability of these regions.

In year 1 the student will conduct a literature search on sediment transport processes (3 months), followed by (about 4 months) learning and running the existing code (Dodd et al, 2008, Coast Eng, 55), including verification tests. The existing model will thereafter be augmented by the inclusion of vegetation (initially by altering the bed friction factor, although other approaches will be considered), by including fluvial sediment input (building on the work of Dodd et al, 2008), and by extending the sediment description to cohesive sediments (by altering thresholds of mobilisation and settling (Pritchard and Hogg, 2003, J. Geophys. Res., 108). The student will then-15 months into the project--build a discretized representation of the Chao Phraya delta for use in the model. This will be aided by an 8-week visit to Chulalongkorn University in Bangkok so as to become familiar with the site (Dr Sriariyawat will accompany the student on site visits to the delta, to conduct update surveys, and to take sediment cores), as well as to become familiar with available validation data sets (2 months). Thereafter validation tests will be run (3 months). Likely scenarios of SLR / subsidence will then be identified, with reference to the aforementioned local data, as well as global forecasts. Underpinned by these background conditions, the sensitivities to sediment input and mangrove extent will be explored via multiple model runs, and the variable space populated so as to note dependencies and, in particular, infer nonlinear feedbacks. During this time, and with the knowledge of deltas already acquired, the student will also assess other tropical deltas (e.g. Yellow, Yangtze, Nile, Irawaddy, Mekong), so as to build a picture of the degree to which the Chao Phraya test case is generic. This will lead into general conclusions, the student write-up, including journal publications