Puget Sound Beaches

General Characteristics 

The beaches of the Puget Lowland exhibit three characteristics simultaneously that are typically treated as separate beach types in the scientific literature: (1) mixed sand and gravel composition, (2) meso- to macro-tidal, and (3) low-energy (estuarine) wave environment. Although modern research into beach forms and processes dates back to the 1940s, most of this research has focused on beach environments that are well-sorted sand, high-energy and micro-tidal. As a result, many conventional beach models make assumptions about the wave environment, sediment properties and transport mechanics that are not appropriate for Puget Sound.

The following paragraphs are an excerpt from the introductory chapter of my Ph.D.
dissertation. The text and images are copyrighted by myself. However, you are welcome to use this material provided you make a proper note of the source of the material.

Mixed Sediment

 Figure 1: An example mixed sediment beach in Puget Sound (Fay Bainbridge State Park)

Mixed sediment beaches (Figure 1) are found where the primary source of sediment to the littoral system contains a mixture of sand and gravel. This beach substrate is common in previously glaciated regions but is comparatively rare on a world-wide basis. As a result, beaches composed of mixed sand and gravel material are less well represented in the scientific literature (Mason and Coates, 2001). The term mixed beach is applied where the substrate is composed of a homogeneous mixture of sand and gravel and also to beaches where the foreshore is composed of gravel with a sandy low-tide terrace (Pontee et al., 2004). It
might also be appropriate to consider many gravel beaches in this class (also called shingle or coarse-clastic) since gravel is rarely found without a sizable fraction of sand in the substrate.

Most mixed beaches in the literature have a composite profile with a distinct break in slope between the upper foreshore and the low-tide terrace (Mason and Coates, 2001; Kulkarni et al., 2004). The break in slope is typically accompanied by a break in sediment grain size with a pebble-sand mixture found above the break and mostly coarse sand found below. Sediment size is responsible for beach hydraulic conductivity, which in turn, controls the beach profile and groundwater flow.

Figure 2: Comparison of high tide (above) and low tide (below) wind waves. Note that the high tide photograph shows the waves breaking very near the shoreline at a large angle to the beach while in the low-tide photograph the waves have broken offshore and are moving inshore as less energetic, shore-parallel bores. (Cama Beach, Camano Island) 

Composite profiles can have a dramatic affect on wave hydrodynamics from low to high tide under the same incident wave conditions (Figure 2) (Miles and Russell, 2004). At low-tide the waves propagate across the dissipative low-tide terrace. These waves are characterized by spilling breakers, weak undertow, strong longshore currents, and a dominance of infragravity frequencies. At high-tide, the waves are not affected by the terrace, instead they break directly on the steep reflective beach. Here they are characterized by plunging breakers, large gravity-band cross-shore variance and a strong undertow.

Large Tides

Beaches exposed to large tidal ranges are affected morphodynamically in two distinct ways: (1) through tidal modulation of wave processes, and (2) through pumping of the beach groundwater system (Anthony and Orford, 2002; Horn, 2002b; Turner, 1993). Large vertical and horizontal tidal excursions modulate wave transport processes by distributing wave energy over a relatively larger area than would have been possible on a micro-tidal beach. By widening the sediment transport zone and reducing the per-unit-area wave energy available for sediment transport, large tides act to reduce or retard morphological change
on the beach (Anthony and Orford, 2002). Tidal currents also impose a unidirectional, shore-parallel current that can last for several hours per day. Sediment mobilized by oscillating wave currents will adjust to the presence of the tidal current and drift slightly down the beach in response.

Large diurnal tidal ranges lead to large changes in the elevation of the beach ground water table. The beach water table is an equilibrium surface at which the pore water pressure is equal to atmospheric pressure. Below the water table, pore water pressure is greater than atmospheric pressure; above the water table it is lower. The shape and elevation of the beach water table is a function of beach morphology (mainly permeability), tidal state, wave conditions, rainfall and terrestrial water sources but generally, the surface dips landward during a rising tide and seaward during a falling tide. Observations of the water table have revealed that the water table rises rapidly but drains slowly (Horn, 2002b).

Figure 3: Beach seepage face, Cama Beach, Camano Island (top) and seep-induced rilling of the low-tide terrace, Kitsap Memorial State Park, Hood Canal (lower). Upper photo by H. Shipman.

The beach ground water table plays several important roles in shaping beach morphology. First, during flood tides, the swash lens washes over unsaturated sand and water percolates into the beach attenuating the wave run up spectrum, which leads to deposition of coarse material high on the swash lens. During the ebb tide, the swash lens is acting over saturated sediments. The mean water surface often drops faster than the beach can drain leading to a decoupling of the mean water surface and the beach ground water table (Figure 3). The positive pore water pressure of the saturated sediments causes exfiltration of ground water from the water table down to the mean water line. This moving zone is called the seepage face and can lead to sediment mobilization in two different ways: First the exfiltration of groundwater acts to fluidizes the surface layer of sediments decreasing the critical threshold for entrainment (Masselink and Hughes, 1998). Second, seepage on the beach face transports fine-grained sediments off of the foreshore and onto the terrace even in the complete absence of waves. Sometimes the drainage is sufficient to form small riverlets which carve shallow rills in the upper terrace (Figure 3). The role of seepage may be especially important in very sheltered environments where wave energy is rarely a strong geomorphic agent.

Low-Energy

Low-energy beaches are the least well studied of the major beach types (Nordstrom and Jackson, 1992; Hegge et al., 1996; Jackson et al., 2002). A “low-energy” beach is defined by Jackson et al. (2002) as a beach where:

• non-storm significant wave heights are minimal (e.g., < 0.25 m)
• significant wave heights during strong on-shore winds are low (e.g., < 0.50 m)
  
• beachface widths are narrow (e.g., < 20 m in microtidal environments)
  
• morphologic features include those inherited from higher energy events.
  

This later point presents problems for researchers attempting to establish a link between beach form and offshore wave climate. In low-energy environments, normal wave conditions may not achieve the critical energy thresholds necessary to mobilize beach sediments leaving the beach out of equilibrium with the incident wave climate (Hegge et al., 1996; Anthony, 1998; Jackson et al., 2002).

In the scientific literature the term “low-energy” applies to two broad classes of beaches: those that are sheltered from nearby high energy environments where wave energy is a dissipated fraction of waves on  he outer water body (such as lee shores and coastal lagoons); and those that are fetch-limited where topographic enclosure limits wave generation to the local basin only (Jackson et al., 2002). The key (dynamic) difference is the presence or absence of steady background wave energy. In sheltered water bodies, swell from the external basin propagates into the low-energy basin more or less continuously, while in fetch-limited water bodies, waves do not occur unless there is a local wind disturbance. In western
Washington, Willipa Bay, Grays Harbor and the Strait of Juan de Fuca are sheltered from the Pacific Ocean and so receive significant energy from ocean swell. Alternatively, the main basin of Puget Sound, Hood Canal, Saratoga Passage and Port Susan are all more-or-less isolated from the Pacific and each other such that most wave energy is generated by local winds (fetch-limited). 

Like mixed sediment beaches, low-energy beaches often exhibit a composite profile composed of a steep foreshore above a low-gradient terrace (Nordstrom, 1992; Jackson et al., 2002). The composite profile combined with the small incident waves leads low-energy beaches to exhibit different wave shoaling and breaking behavior than higher energy coasts. When the tides are high, low-energy waves are not affected by wave shoaling until they are within a few meters of the shoreline. This fact truncates the surf zone to a meter or two at most before the wave plunges directly onto the beach face (Figure 2). In addition, short-period waves are less affected by refraction so they have a tendency to approach the shoreline at relatively large angles, increasing the potential for longshore currents for a given wave height (Jackson et al., 2002). Under these conditions, the swash zone is relatively more important as an energy dissipation mechanism than the surf zone. Likewise, sediment transport in the swash zone is a more important component of the gross sediment transport than sediment transport in the surf zone (Horn, 2002a). 

Figure 4: Eelgrass (Zostera marina) on the low-tide terrace, Cama Beach (top). Oyster (Crassostrea gigas) bed on Kitsap Memorial State Park, Hood Canal (bottom). Upper photo by H. Shipman.

During low tides on composite profiles the opposite conditions occur. In this case, the large, low-gradient terrace acts to dissipate much of the wave energy as surf before the waves reach the upper foreshore. Refraction may be significant turning the waves parallel to the shoreline before they break. Suspended sediment transport mechanisms across the surf zone are a relatively more important component of the bulk sediment transport than the weakened swash zone. In addition, in Puget Sound, the lower foreshore and terrace are often the home of dense communities of flora and fauna (Figure 4) that can introduce significant roughness to the bed (shellfish beds for example) increasing fluid turbulence or in the case of sea weeds and grasses, act to buffer waves as they cross the low-tide terrace (Fonseca and Fisher, 1986; Fonseca and Cahalan, 1992). However, the extent to which sea grasses and kelp in Puget Sound are acting to influence wave shoaling and breaking dynamics is not known.

For More Information...

Fore more information about the beaches of Puget Sound please see my Ph.D. dissertation (9.5 Mb PDF).

References Cited

Anthony E.J., 1998. Sediment-wave parametric characterization of beaches. Journal of Coastal Research, 14(1), 347–352.

Anthony E.J. and Orford J.D., 2002. Between wave- and tide-dominated coasts: the middle ground revisited. Journal of Coastal Research, SI 36, 8–15. (ICS 2002 Proceedings).

Fonseca M.S. and Cahalan J.A., 1992. A preliminary evaluation of wave attenuation by four species of seagrass. Estuarine, Coastal and Shelf Science, 35, 565–576.

Fonseca M.S. and Fisher J.S., 1986. A comparison of canopy friction and sediment movement between four species of seagrass with reference to their ecology and restoration. Marine Ecology - Progress Series, 29, 15–22.

Hegge B., Eliot I. and Hsu J., 1996. Sheltered sandy beaches of southwestern Australia. Journal of Coastal Research, 12(8), 748–760.

Horn D.P., 2002a. Beach groundwater dynamics. In 29th Binghamton Geomorphology Symposium; Coastal Geomorphology, Elsevier, Amsterdam, Netherlands, volume 48. pp. 121–146.

Horn D.P., 2002b. Mesoscale beach processes. Progress in Physical Geology, 26(7), 271–289.

Jackson N.L.; Nordstrom K.F.; Eliot I. and Masselink G., 2002. “low energy” sandy beaches in marine and estuarine environments: A review. Geomorphology, 48, 147–162.

Kulkarni D.C.; Levoy F.; Monfort O. and Miles J.R., 2004. Morphological variations of a mixed sediment beachface (Teignmouth, UK). Continental Shelf Research, 24, 1203–1218.

Mason T. and Coates T.T., 2001. Sediment transport processes on mixed beaches: A review for shoreline management. Journal of Coastal Research, 17(3), 645–657.

Masselink G. and Hughes M., 1998. Field investigation of sediment transport in the swash zone. Continental Shelf Research, 18, 1179–1199.

Miles J.R. and Russell P.E., 2004. Dynamics of a reflective beach with a low tide terrace. Continental Shelf Research, 24, 1219–1247. 

Nordstrom K.F., 1992. Estuarine Beaches: An Introduction to the Physical and Human Factors Affecting Use and Management of Beaches in Estuaries, Lagoons, Bays and Fjords. Elsvier Applied Science, London.

Nordstrom K.F. and Jackson N.L., 1992. Two dimensional change on sandy beaches in meso-tidal estuaries. Zeitschrift f¨ur Geomorphologie, 36(4), 465–478.

Pontee N.I.; Pye K. and Blott S.J., 2004. Morphodynamic behaviour and sedimentary variation of mixed sand and gravel beaches, Suffolk, UK. Journal of Coastal Research, 20(1), 256–276.

Turner I., 1993. Water table outcropping on macro-tidal beaches: A simulation model. Marine Geology, 115, 227–238.