Ad J.F. van der Spek

Extracting storm-surge data from coastal dunes for improved assessment of flood risk

Future changes in climate and sea level are likely to increase the threat from storm surges in many coastal regions. Mitigation of this threat requires an understanding of storm surge magnitude and frequency, and the relationship of these variables to climate parameters. This understanding is currently limited by the brevity of instrumental records, which rarely predate the twentieth century. However, evidence of former storm surges can be recorded in coastal dunes, because the dune topography may trap high-magnitude deposits at elevated locations. Here we combine a range of techniques to extract storm-surge data from coastal dune sediment. The sediment is tracked in the subsurface with ground-penetrating radar to assess its height and extent, and its age is determined with good precision through optically stimulated luminescence dating. The probable age of the sediment (A.D. 1775 or 1776) is within a period of increased storminess in northwest Europe, and the local magnitude of the event is likely to be greater than any on instrumental record. By utilizing coastal dunes for storm surge analysis, our approach provides a valuable new source of information for understanding storm surge risk, which is vital for the protection of coastal regions.

Recent Coastal Dune Development: Effects of Sand Nourishments

Abstract BAKKER,M.A.J.; VANHETEREN, S.; VONHÖGEN, L.M.; VAN DER SPEK, A.J.F., and VAN DER VALK, L., 2012. Recent coastal dune development: effects of sand nourishments. Much of the Dutch coast has been subject to structural erosion. From 1990 onward, sand nourishments have been used under a government policy of dynamic preservation. Annual monitoring and field inspections show that the structural erosion has decreased or even turned into coastal progradation after 1990. The monitoring data concern only morphodynamics and thus supply limited information on system-related geological processes driving the observed changes. Recently acquired ground-penetrating radar (GPR) data help establish the origin of sedimentary elements within the beach-foredune area, determine their decadal-scale preservation potential under the present nourishment policy, and demonstrate temporal and spatial accretion/erosion variability along nourished coasts. GPR images from a nonnourished retrograding barrier section show historical storm surge deposits within the eroding foredune and accumulations of natural eolian sediment farther landward. GPR images from a heavily nourished, prograding site show that the accreted foredune and beach consist of nourishment embankments (20%), wind-blown units derived from nourished sand (70%), and progradational beach deposits (10%). The net volume of accretion at this site is approximately 200 m3/m. Remarkably, almost all sand nourished before 2000 has been washed away, except for embankments constructed in 1990. Analysis of meteorological data suggests that 1999 storm surges are responsible for this erosion. The relative longevity of post-2000 nourishments can be attributed to a combination of shoreface nourishment and favorable meteorological conditions. During a storm surge in 2007, water-lain embankments proved to be more resistant against wave erosion than nourished sand redistributed by wind, indicating the importance of grain size, roundness and packing in the durability of nourishments.

EVIDENCE AND IMPLICATIONS OF MIDDLE- TO LATE- HOLOCENE SHOREFACE STEEPENING OFFSHORE THE WESTERN NETHERLANDS

The concept of equilibrium shoreface profiles with depths and slopes determined by wave regime and seabed-sediment characteristics is still frequently used by coastal engineers to model landward migration of coastal-barrier systems as a result of sea-level rise. Although useful in some circumstances, it has come under increased scrutiny when used to predict long-term changes. The geological record of the western Netherlands contains evidence that on a timescale of millennia, the width and slope of the shoreface have experienced dramatic changes. During the middle-Holocene transgression maximum, when relative sea level was about 5 meters lower than today, the coastline was located some 10 kilometers landward of its present position. Remnants of even older tidal-channel fills are present at or near the seabed at 14-16 meters below mean sea level in a zone that extends up to 12 kilometers seaward of the present coastline. This latter observation shows that the seabed has not been much lower than its current position since at least the middle Holocene. Thus, it is clear that during the maximum transgression, the shoreface reached depths of more than 10 meters only beyond about 20 kilometers from the contemporary coastline. Currently, similar depths are reached within 2 kilometers from shore. The upper 3-6 meters of the tidal-channel fills and the adjacent Pleistocene river deposits have been truncated by later shoreface erosion, providing an indication of sediment volume made available to the modern coastal tract. It is likely that the eroded sediment was used for coastal progradation following the middle-Holocene transgression maximum, around 6000 years ago. Modern shoreface-nourishment techniques, including the planned ‘sand engine’ mega-nourishments, mimic this natural process. Sand is extracted on the inner shelf, beyond the defined depth of closure, and dumped on the middle and upper shoreface for further redistribution by tidal and wave processes.

From Transgression to Regression: Coastal Evolution near The Hague, The Netherlands, around 5000 Years BP

The coast of the western part of The Netherlands changed from a transgressive, tide-dominated system, characterised by tidal inlets and tidal basins, into a prograding barrier coast, between 5500 BP and 3200 BP. Ypenburg, SW of The Hague, is situated on the seaward boundary of a transgressive tidal basin which evolved from a predominantly subtidal into a intertidal flat area. During this evolution the landward expansion of the basin stopped. The seaward side of the tidal flat area was reworked and built-up by waves; it formed a transgressive barrier. The limited thickness of the wave-dominated deposits and the lack of shells of open-marine molluscs indicate that this coast line was sheltered from open North Sea conditions. The infilling of the tidal basin and the associated decreasing tidal channel are the onset of coastal regression. The formation of a beach barrier further seaward was the start of coastal progradation.

Challenges in developing 'building-with-nature' solutions near tidal inlets

In the Building-with-Nature(BwN) philosophy, the focus of erosion control no longer is predominantly on counteracting destructive forces, but just as much on stimulating constructive forces. It implies a shift towards a pro-active approach, implying that BwN interventions require a long-term perspective and understanding of the coastal system at a large scale. Furthermore, BwN solutions also require understanding of the social system of stakeholders involved in the coastal problem. It is the interaction between these two that creates a series of challenges for BwN, including the necessity to consider multiple scales in both coastal system and social system and the linkages between them. We argue that a way to meet those challenges may be found in a collaborative design approach (co- design), in which systems understanding is shared between all stakeholders involved in the BwN solution. This approach will be practiced and analyzed for its effectiveness in the CoCoChannel project.