Introduction

Overwash is the flow of water and sediment over the crest of the beach (and adjacent dunes) that does not directly return to the water body where it originated (ocean, sea or bay). One commonly distinguishes run-up overwash due to wave overtopping and inundation overwash due to total flooding. Washovers are the morphologial and sedimentological product of overwash. In this project we explore the possibilities of a more natural and dynamic type of shoreline management for the West Frisian barrier islands, the Netherlands. This new approach includes the option to reactivate washovers systems in order i) to create a more dynamic, flexible and sustainable type of coastal behaviour; ii) to increase the sediment budgets of barrier islands to cope with sea level rise and iii) to re-establish more vital and rejuvenated beach-dune-salt marsh ecosystems that demonstrate a high biodiversity.

Objectives

The main objectives of the present studies are:

• to reconstruct the morphological development of washovers in a typical mesotidal barrier island environment, characterized by drumstick-shaped barriers, numerous tidal inlets, large tidal basins and well-developed dune systems;
• to measure, model and explain the main hydrodynamic factors and conditions that affect overwash in these conditions;
• to propose and design the development of re-activated washovers based on field and modelling results.

Figure 1: Inundation overwash on barrier island Schiermonnikoog during storm surge in November 2007 (falling tide).

Methods

The morphological part of the study is based on an extensive analysis of aerial photographs (1927-2007), coastal profile data, satellite images, local maps and a number of dedicated field surveys (2006 and 2007). Hydrodynamic conditions during overwash are determined by using monitoring data, field measurements and observations (2007 and 2008) and initial modelling with Delft3D (done by Delft Hydraulics). Storm and post-storm conditions were also observed using video-based field observations and aerial reconnaissance.

Figure 2: Typical funnel-shaped washover on the barrier island Schiermonnikoog. The washover has a narrow throat and a length which largely exceeds the width.

Results

The present studies focused on the barrier island of Schiermonnikoog. Washover formation starts with run-up overwash and inundation overwash on a bare sand flat. Shortly after, dune-building processes result in newly developing dune complexes alternating with both broad and narrow washovers. Simultaneously, inundation overwash creates subaqueous deposition at the basin side, creating washover deltas. As soon as the dune building processes continu to proceed in a seaward direction, sheltered areas become occupied by marsh vegetation. The location of the washovers becomes more or less fixed, tidal creeks at the basin side demonstrate a headward erosion and the washover drainage pattern gradually interacts with the tidal creeks. The former overwash deltas are no longer maintained and gradually disappear. Comparing past and present developments, we observe that for decades overwash is observed at the same places, located at gaps in the foredunes. As a result, the washover systems have developed a typical funnel-shaped morphology with a narrow throat. In landward direction sandy washover systems gradually transform into muddy marshes with tidal creeks.

During the pilot study inundation overwash was observed during storm surges in 2006 and 2007, respectively. Inundation overwash at the seaward side was simultaneously accompanied by flooding from the basin side and inundation overwash was initially not able to reach the back-barrier basin. Video-based field observations clearly show that dune erosion is mainly a product of the impact of low-frequency waves. Surprisingly enough though, this erosion is hardly reflected in the amount of washover deposition and most of the eroded sediment is simply deposited in front of the duneface.

Figure 3: Digital elevation measurements of the east coast of the barrier island Schiermonnikoog showing the presence of a series of washovers. The washovers are connected with tidal marsh creeks in the back-barrier system.

Future work

This study will be incorporated in a larger coastal zone research programme for the Dutch barrier islands, integrating hydrodynamical, morphological, ecological and hydrological studies to develop a new coastal zone management strategy for the North Sea facing coast of these islands. In this framework we will further focus on the hydrodynamic processes and conditions during overwash and the potential of re-activated washovers to trap sediment and to restore ecological values on the islands.

Please contact Piet Hoekstra for more info on this project.