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Geology

Riding the Waves of Change: A Revised Beach Cycle for Mixed Sand and Gravel Beaches

Article:  Roberts, T.M., Wang, P., Puleo, J.A., 2013.  Storm-driven cyclic beach morphodynamics of a mixed sand and gravel beach along the Mid-Atlantic Coast, USA.  Marine Geology 346, 403-421.  http://dx.doi.org/10.1016/j.margeo.2013.08.001

 

The movement and abundance of sand on sandy beaches has been described since the 1950’s (see Shepard [1950], Bascom [1953]).  The generalized description includes a strong seasonal pattern of sand that consists of 2 very different phases:

1. Winter Beach – Absence of berm, presence of offshore sandbar

2. Summer Beach – Presence of berm, absence of offshore sandbar

Figure 1. Typical seasonal Summer / Winter profiles beach profiles of a sandy beach. Note that the berm is the part of the beach where one might set up his/her beach chair and umbrella.

These distinctly different beach profiles are attributed to the frequency and duration of energetic storms.  During the winter, long-lasting storms with large breaking waves tend to sweep sand from the berm and deposit the sand offshore in sandbars.  During the summer, quieter weather and smaller waves slowly move the sandbar towards the beach, replenishing the sand lost from the berm.

In the highlighted study by Roberts et al., scientists investigate the dynamics of a different type of beach: Mixed Sand and Gravel (MSG) on the Delaware coastline.  Over a 4 year period, 2009 – 2011, 18 nearly monthly beach profile surveys, 60 sediment cores and 550 surface sediment cores were collected.  The beach profiles are essentially elevations taken in a line perpendicular to the shoreline, starting at the dunes, extending into the nearshore waters.  See Figure 2. to see how a beach profile can be taken.

Figure 2. A typical beach survey, using survey rods to record elevations from the dune to the ocean.  Other more sophisticated methods exist.  Photo from University of Rhode Island Geosciences.

Figure 2. A typical beach survey, using survey rods to record elevations from the dune to the ocean. Other more sophisticated methods exist. Photo from University of Rhode Island Geosciences.

An MSG beach, not surprisingly, is composed of mostly coarse sand and gravel, meaning the average size of sediment grains is larger than the classically discussed fine-sand beaches.  For this reason, the seasonal shape and cycle of an MSG beach differs from that of a sandy beach.

The study focused primarily on the last half of 2009, where 3 powerful storms impacted the Delaware coastline:

1. Distant Hurricane – Hurricane Bill – August 2009

  • Max Wave Height – 3.56 meters
  • Max Swell Period – 19 seconds
  • Dominant Swell Period – 5 to 15 seconds
  • No storm surge

2. Energetic Winter Storm – October 18th Storm – October 2009

  • Max Wave Height – 5 meters
  • Max Swell Period – 14 seconds
  • Dominant Swell Period – 7 to 12 seconds
  • 0.5 meter storm surge
  • 23 hours of 4 meter + wave height

3. Winter Storm + Hurricane – Nor’Ida – November 2009

  • Max Wave Height – 8.11 meters
  • Max Swell Period – 14 seconds
  • Dominant Swell Period – 11 to 14 seconds
  • 1 meter storm surge
  • 52 hours of 4 meter + wave height
Figure 3. Wave terminology.  A wave’s height is the vertical distance between the trough and crest of a wave.  A wave’s period is the amount of time between the arrivals of wave crests.  Higher swell periods are typical of more energetic waves, from a more distant source.

Figure 3. Wave terminology. A wave’s height is the vertical distance between the trough and crest of a wave. A wave’s period is the amount of time between the arrivals of wave crests. Higher swell periods are typical of more energetic waves, from a more distant source.

 

Hurricane Bill

During the distant passing of Hurricane Bill in August 2009, the built up beach berm typical of the summer profile scenario was already in place.  The more energetic long period swells and greater wave height caused sand to be pushed and dropped further up the beach.  This resulted in a 0.7 m thick layer of sediment added to the already inflated summer profile.  The source of the additional sediment appears to be from the nearshore region.

October 18th Storm

The long duration winter storm, referred to as the October 18th, 2009 storm had very different effects on the beach profile.  The summer profile that was enhanced by Hurricane Bill was greatly deflated by this storm.  Severe erosion of the beach berm redistributed a 1 meter thick layer of sediment to the nearshore region (0 – 4 meters water depth in this case).

Nor’Ida Storm

The combined winter storm and Hurricane Ida was a long lasting storm from November 11 – 14, 2009.  Severe erosion of the beach berm and the dunes occurred during this event.  Similar to the October 18th event, a 1 meter thick layer of sediment was added to the nearshore region.  The effects of the Nor’Ida storms are expected to be underestimated since beach profiles were not taken until 2 weeks after the event, at which significant recovery may have already taken place.  Figure 4 below shows monthly profiles displaying the beach response to these storm events.

Figure 4. Monthly beach profiles July – November 2009.  The “summer” profiles show a large beach berm.  The “winter” profiles, after the October 18th and Nor’Ida storms, show an absence of a berm and built up nearshore profile.  MHHW and MLLW are acronyms representing high (MHHW) and low (MLLW) tide water heights.

Figure 4. Monthly beach profiles July – November 2009. The “summer” profiles show a large beach berm. The “winter” profiles, after the October 18th and Nor’Ida storms, show an absence of a berm and built up nearshore profile. MHHW and MLLW are acronyms representing high (MHHW) and low (MLLW) tide water heights.

Post-storm beach recovery

The short-term (days to weeks) recovery of the beach after the October 18th and Nor’Ida storms is evident as soon as the storm begins to subside.  During the peak of storm intensity, the beach has been transformed into a flat, eroded profile with exposed gravel.  Within days, a discontinuous ridge of sediment shows the rapid progress in recovery.  By one month post-storm, a continuous ridge is welded to the beach, well on its way to recovery.  Figure 5 details the short-term recovery of a MSG beach.

Figure 5. Short-term post-Nor’Ida recovery.  Within days, the formation of a discontinuous sediment ridge (red arrow) is evident.  By one month a continuous ridge has been welded to the beach (yellow arrow).

Figure 5. Short-term post-Nor’Ida recovery. Within days, the formation of a discontinuous sediment ridge (red arrow) is evident. By one month a continuous ridge has been welded to the beach (yellow arrow).

 

Medium-term post-storm recovery (months +) typically involves the recovery of the eroded berm, shown by increases in profile elevation along the dry portions of the beach.  By July 2010 recovery of the beach to the typical “summer” profile had occurred.

 

Revised beach cycle for a Mixed Sand and Gravel beach

One of the largest differences between the classic sandy beach and MSG beach post-storm recovery is the absence of an offshore sandbar in the MSG beach.  The persistent absence of this sandbar feature suggests that wave energy is not responsible for this difference.  It is hypothesized that the concentration of gravelly deposits in the “swash” (breaking wave) zone tends to build a steep ramp near the water’s edge.  Although the exact mechanisms are still unclear, these gravelly deposits and steep ramps seem to inhibit the formation of an offshore sandbar under all wave conditions.  A comparison was drawn to a MSG beach in Florida. This particular beach receives additional sand through beach replenishment efforts, which leads to the formation of a temporary offshore sandbar.  Without this replenishment, an offshore sandbar will not form.  This suggests that the sediment characteristics of an MSG beach rather than the wave characteristics are responsible for the absence of an offshore sandbar.

A new and revised beach cycle has been proposed for an MSG beach.  The summer profile of an MSG beach is identical to that of the classic sandy beach, with the presence of a large berm.  The differences are in the winter profiles, where an MSG beach does not have an offshore sandbar.  Instead, there is a flat, planar built up nearshore profile in place of the classic offshore sandbar (Figure 6).  Also included is a transitional short-term post-storm profile showing beach recovery indicated by a ridge welded to the beach.

Figure 6. The classic beach cycle summer / winter profile for a sandy beach (A) and the revised beach cycle recovery / transitional / storm profile of an MSG beach (B).

Figure 6. The classic beach cycle summer / winter profile for a sandy beach (A) and the revised beach cycle recovery / transitional / storm profile of an MSG beach (B).

 

Conclusions

The classic summer / winter profile for a sandy beach has been a mainstay in coastal geology since the 1950s.  This cycle is not necessarily appropriate for mixed sand and gravel beaches such as those along the Delaware coastline.  The absence of an offshore sandbar in all wave conditions necessitated a revised beach cycle.  In the recovery phase, a berm is slowly rebuilt, and the nearshore profile becomes thinner.  This can be enhanced by long period swell events such as Hurricane Bill.  Large, energetic winter storms such as the October 18th and Nor’Ida events cause the storm beach profile which is characterized by the absence of a berm and the inflation of the nearshore storm deposit.  A transitional profile is achieved quickly after the storm event, in which a sediment ridge is built and welded to the beach.

It is evident that strong, damaging coastal storms are a major stress to coastal populations.  Large economic damage as well as loss of life are the harsh realities of living near the coastline.  For these reasons, it is important to have a good understanding of how the beach responds to these storm events.  Scientific studies, such as the one highlighted in this article are the first line of defense against damaging coastal storms, as they eventually are responsible for guiding better coastal engineering or better policy that will hopefully reduce the great societal damage in the future.

 

References:

Bascom, W.H., 1953.  Characteristics of natural beaches.  Proceedings of the 4th Coastal Engineering Conference.  American Society of Civil Engineers, pp. 163-180.

Shepard, F.P., 1950.  Beach cycles in Southern California.  Beach Erosion Board Technical Memo No. 20.  U.S. Army Corps of Engineers.

 

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