Jeroen Stark

Location
The Columbia River Littoral Cell (CRLC) comprises the coastal area of southwest Washington and northwest Oregon along the U.S. Pacific Northwest coast (Figure 1). The headlands of Point Greenville and Tillamook Head form the natural boundaries for the coastal cell with a total length of 165 km. The Columbia River Estuary, Willapa Bay and Grays Harbor divide the CRLC into four sub-cells, named after the beaches between the headlands and the estuaries; North beach, Grayland Plains, Long Beach and Clatsop Plains. This study focuses mainly on the Mouth of the Columbia River (MCR), its ebb-tidal delta and adjacent coast.


Figure 1 - Overview of the Columbia River Littoral Cell 


Historical change in the Columbia River Mouth 
During the last century, the morphology of the MCR changed significantly. This morphological change was mainly caused by the construction of two large entrance jetties starting in the late 19^th century. Before jetty construction, the MCR was a highly dynamic area, with continuously adjustment in channel configuration and shoal positions. At its mouth, the Columbia River was historically flanked by two large shoals, Peacock Spit and Clatsop Spit.As authorities decided that navigational safety should be improved, plans for jetty construction were made. Construction of the North Jetty and the South Jetty took place in phases between 1885 and 1917. In a response to jetty construction large amounts of sand eroded from the entrance channel and ebb-delta shoals, while a new outer delta formed in deeper water more to the northwest. Waves dispersed the former flanks of the ebb-delta onshore and finally alongshore away from the river mouth, causing adjacent coasts to advance several meters per year on average in an initial response. In the entrance itself, the channel deepened and moved to the north, scouring the bed near the North Jetty.

Besides jetty construction, other human influences on the morphological change in the MCR are a reduction of sediment supply from the river into the estuary due to the construction of numerous dams in the Columbia River, dredging activities within the estuary and entrance channel and dredged material disposal in the MCR. Early dredging activities started in the same period as jetty construction. These activities were intensified from the 1950s and on, with the purpose to maintain the authorized channel depth. To maintain the authorized dimensions, the entrance channel requires annual dredging of 3 to 5 Mm³ of fine-to-medium sand at the MCR. Disposal of dredged material takes place at designated placement sites in and around the MCR and the Columbia River estuary.

During the last decades, accretion rates on Long Beach slowed down. In the most southern part on Benson Beach, close to the North Jetty, the beach even began to erode rapidly (several meters per year on average). Apparently, the sediment source from the ebb-tidal delta depleted as the delta flanks migrated further offshore. Along with that, the delta might have lost its sheltering function for the coast, causing the southern part of Long Beach and the jetties themselves to get more exposed to higher waves during storm conditions. The receding behavior of the tidal shoals also increases the depth near the jetties, which eventually might destabilize them. Recently, dredging management programs have been initiated in an effort to restore the old ebb-tidal delta flank of Peacock Spit. Especially a shallow water placement site just west of the North Jetty is currently being used to restore the protective and sediment feeding functions of the ebb-tidal shoal. By keeping dredged sediment in the littoral system and shoring up the shoals, it is intended that coastal retreat on Benson Beach reduces or stops and future jetty damage is prevented. Early observations indicate that material disposed on a placement site west of the North Jetty might indeed be feeding the coastal system.


Problem description
Severe erosion on some beaches in southwest Washington starting in the 1990s resulted in the call for a better understanding of processes driving coastal change in the CRLC. Physical processes that might well play a role in the transport of sediments between the ebb-tidal shoals and the adjacent coast are wave-induced transports, transports through currents caused by tide and river outflow, density gradients and displacement of material due to dredging and disposal activities in the MCR. Long-term morphological modeling is required to make adequate predictions for the future development of the coastal system. A process-based numerical model such as Delft3D is a good tool in achieving this, taking into account the complexity of the area. Besides that, it could also be very helpfull for investigating dredging and disposal strategies and their long-term effects.

This study is about modelling the long-term morphological development of the Columbia River Mouth and the interaction of the ebb-tidal delta with the adjacent coast of Long Beach. By doing so the long-term sediment supply from ebb-tidal delta towards the coast can be analyzed. A process-based numerical model for the MCR has already been constructed in a collaboration between the United States Geological Survey (USGS) and Deltares using the Delft3D modelling system (Figure 2). The model consists of 3 domains; sea, estuary and river. Basically this study will continue on the work in (Moerman, 2011), in which this existing Delft3D model of the Columbia River Mouth was used for long-term modelling for the first time. Important aspects in this study are the dredging and disposal activities, since these activities play a major role in the morphological development of the area nowadays and they are not yet incorporated in the model.


Figure 2 - Overview of the MCR flow domain consisting of the grids: sea (black), estuary (red) and river (blue)


Research Objectives
Main objective form this study is to investigate the effect of historical dredge and disposal activities on the long-term morphological development of the Columbia River Mouth. Besides, the long-term effect of dredging and disposal strategies for the Columbia River Mouth will be investigated

For investigating the effect of historical dredging and disposal activities or strategic placement of dredged material, it is necessary that the Delft3D model represents the relevant processes for sediment transport on the ebb-tidal delta and along the coast adequately. The ability of the model for doing so can be tested by comparing model results with historical bathymetric surveys. Historical dredging and disposal activities in and around the mouth of the Columbia River should be implemented in the model. After this, the development of the sediment supply from ebb-tidal shoals towards the coastal cell can be studied. Eventually, when the capabilities of the model are sufficient, the long-term effect of future dredge and disposal strategies can be investigated. In short the objectives are:

• Construction / Improvement of the existing Delft3D model so it gives a better representation of the hydrodynamic and morphodynamic processes involved.  
• Calibration of the model settings based on the 1926-1958 interval;
• Implementation of schematized historical dredge and disposal activities in and around the MCR in the Delft3D model.  
• Modelling (hind cast) the long-term morphological development of the MCR over the period 1958 – 1999 and comparing them with the observed bathymetric data, to analyze the effect of dredging activities on the long-term development of the delta.
• Analysis of the development of sediment supply from ebb-tidal delta towards the Long Beach coastal cell.
• Performing predictive model simulations to investigate the impact of management programs for dredge and disposal activities on the morphological development of the ebb-tidal delta and the coastal system.