Saved or starved? The importance of sediment management in determining the future response of estuaries and deltas.

Abstract

The primary mechanism which forms and maintains estuary and delta morphology and elevation is sedimentation. Sediment deposited annually may offset elevation loss due to relative sea-level rise (SLR). So, to quantify the amount of sediment that is delivered is of crucial importance to predict if estuaries and deltas will be saved by or starved of sediment and therefore sustain their elevation or sink. Here, we assess and quantify sediment budgets and sediment management; and evaluate morphological responses of estuaries and deltas to both anthropogenic and climate changes over various timescales. I focus on urbanized deltas which are undergoing a high degree of sediment management like the Rhine-Meuse delta (RMD), the Netherlands. The Rhine--Meuse delta has an annual loss of sediment. Predictions for future sediment loss are the largest in scale in the 3500 year history of the delta. It is caused by extensive dredging and an uneven distribution of sediment over the channel network. Most sediment delivered by upstream rivers and from the coastal boundary becomes trapped in the very deep mouth area. Meanwhile, the tidal cross-cut channels and southern part of the system are sediment starved. This uneven distribution has negative implications for ecosystems including the loss of intertidal areas, causing danger to infrastructure (dikes, groynes, underground cables and tunnels), increasing flood risk and leading to higher dredging volumes (and costs) annually. The thesis finds that sediment management techniques i.e. sand mining and dredging are causing negative sediment budgets in many urban deltas. This is the primary threat in the short term (next 100 years) for most European estuaries and deltas. SLR will affect dredged and undredged estuaries differently. The floodplains and intertidal areas of dredged estuaries will actually take longer to drown than undredged estuaries. In undredged estuaries, the uneven elevation decreases the rate of drowning. However, the excess energy due to SLR will manifest itself as either meander expansion in unconstrained estuaries or as bed and bank erosion in constrained/embanked estuaries. This thesis suggests that many of the world's estuaries and deltas are at risk of land loss due to a lack of sediment. To that end, solutions which can enhance sedimentation and raise land in estuaries and deltas are urgently required. In our review of existing and planned sedimentation enhancing strategies (SES), it was found that 79% of strategies can offset even the most extreme of sea-level rise rates but available space, particularly in urban deltas is a vital challenge in their implementation. For centuries, channel networks have been shaped by port development and global growth in ship size. In such urbanized multi-channel systems, dredging and sediment management are causing an imbalance in hydrodynamics and sediment transport leading to erosion, loss of intertidal areas and increased flood risk. To tackle these issues, two things are required: 1) a positive sediment budget - enough sediment to build and sustain the desired morphology and to protect areas at risk and 2) sediment reconnectivity - protecting sinking areas by reconnecting them to fluxes of water and sediment.