1. Introduction
Deltas with estuaries are among the most fertile and densely populated areas of theworld . The combined effects of variable river discharge and tidal forcing, generating non-linear interactions, render channel networks in deltas with highly dynamic and complex environments.Today, many deltas face threats from changing boundary conditions. Among those, sea level rise and land subsidence have received ample attention both in academic research and in the media. Flood vulnerability relates to peak surface levels rather than to the mean surface level relative to the delta. Here we show that peak surface levels, in turn,may not be linearly related to mean surface levels, and are strongly dependent on the tidal motion. Variation of the tidal elevation amplitudes in a delta can be caused by changes in conditions at the landward and the seaward boundaries of the delta, but more often, human activity including dredging, sand extraction, dam construction and confinement or widening of channels play a dominant role. The effect of engineering works on water levels ranges from basin-wide effects from large-scale interventions and dredging, to local changes from smaller-scale measures. Since most engineering works are constructed over the timespan of several years, the effects on water levels and tides can be seen as an abrupt change in the system, whereas the effects of sea level rise or land subsidence are gradual. Partially due to a lack of data, not much is known about the short-term and local influences of engineering works on water levels in tidal river networks. So far, studies have mainly focused on long-term trends and changes caused by human-induced climate change, or long-term dredging. The objective of this paper is to quantify and understand the spatiotemporal variability in tidal and subtidal water levels in a delta channel network in response to engineeringworks, relative to sea level rise. The Rhine–Meuse tidal river network in the Netherlands is chosen as a subject of study, where up to 70 years of historical surface water levels is available at 13 measurement stations. It represents an area subject to both sea level rise, tidal intrusion and a large number of engineering works, including channel deepening, damconstruction and the creation of shortcut canals. Existing hydraulic studies in this regionmainly focus on the morphological consequences of large-scale engineering works. Recent estimates of future flood risks consider sealevel rise to be an important factor increasing high water levels, without demonstrating the direct influence of sea level rise on high and lowwater levels. Here,we showthat the effect of sea level rise over the past 70 years on high and low water levels is overwhelmed by human-induced changes in tidal amplitudes.
1. Introduction
Deltas with estuaries are among the most fertile and densely populated areas of theworld . The combined effects of variable river discharge and tidal forcing, generating non-linear interactions, render channel networks in deltas with highly dynamic and complex environments.Today, many deltas face threats from changing boundary conditions. Among those, sea level rise and land subsidence have received ample attention both in academic research and in the media. Flood vulnerability relates to peak surface levels rather than to the mean surface level relative to the delta. Here we show that peak surface levels, in turn,may not be linearly related to mean surface levels, and are strongly dependent on the tidal motion. Variation of the tidal elevation amplitudes in a delta can be caused by changes in conditions at the landward and the seaward boundaries of the delta, but more often, human activity including dredging, sand extraction, dam construction and confinement or widening of channels play a dominant role. The effect of engineering works on water levels ranges from basin-wide effects from large-scale interventions and dredging, to local changes from smaller-scale measures. Since most engineering works are constructed over the timespan of several years, the effects on water levels and tides can be seen as an abrupt change in the system, whereas the effects of sea level rise or land subsidence are gradual. Partially due to a lack of data, not much is known about the short-term and local influences of engineering works on water levels in tidal river networks. So far, studies have mainly focused on long-term trends and changes caused by human-induced climate change, or long-term dredging. The objective of this paper is to quantify and understand the spatiotemporal variability in tidal and subtidal water levels in a delta channel network in response to engineeringworks, relative to sea level rise. The Rhine–Meuse tidal river network in the Netherlands is chosen as a subject of study, where up to 70 years of historical surface water levels is available at 13 measurement stations. It represents an area subject to both sea level rise, tidal intrusion and a large number of engineering works, including channel deepening, damconstruction and the creation of shortcut canals. Existing hydraulic studies in this regionmainly focus on the morphological consequences of large-scale engineering works. Recent estimates of future flood risks consider sealevel rise to be an important factor increasing high water levels, without demonstrating the direct influence of sea level rise on high and lowwater levels. Here,we showthat the effect of sea level rise over the past 70 years on high and low water levels is overwhelmed by human-induced changes in tidal amplitudes.
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