Shifting Sands
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
No topic brings more dread and discomfit than the discussion of our rapidly disappearing island. Erosion is the wearing away of the earth's surface by any natural process. The chief agent of erosion is running water; minor agents (world–wide) are glaciers, the wind, and waves breaking against the coast. Everything that we think of when we think of soil or sediment was created by a combination of erosion by wind and water and decomposition by fungi and bacteria. Our beaches would not exist if erosion had not broken down the parent material into fine little particles. Cape Cod and Nantucket are a product of the last continental glaciation, the Wisconsinian glacial stage during which the Laurentide Ice Sheet advanced and retreated. Erosion of glacial landforms like moraines, drumlins, outwash plains, and kames provides the primary source of sand and cobble for Massachusetts' 1,500 miles of beaches, dunes, and barrier beaches.
This week, two coastal geologists, Drs. Sue Halsey and Peter Rosen presented a public forum to discuss basics of island geology and coastal process and to explain how and why erosion happens. Peter Rosen has been coming to the Field Station with his Northeastern University geology students for many years. Each year he arranges a flyover of the island to observe changing conditions. Dr. Rosen’s thesis work was based on the formation and evolution of the cuspate spits protruding from Coatue. Over the years I have collaborated with a variety of geologists typically addressing sedimentation in nearshore areas and as an oceanographer, coastal geologic processes are part of the equation if you ever hope to understand how material is transported around the earth.
On and off for the past seven years, the UMass Boston Nantucket Field Station has been teaching visiting and local school kids how to do beach profiling while conducting regular measurements with citizens to document the loss of shoreline in some areas and the build-up in others. This project was started with Jim O’Connell, a coastal processes specialist working for the Massachusetts Coastal Zone Management (CZM) with the Woods Hole Oceanographic Institution Sea Grant Program and Cape Cod Cooperative Extension who is now based in Hawaii. The Commonwealth wanted to ground truth their aerials surveys and obtain more accurate information on the high tide line to compare to the snapshot they got when flying overhead and recording the high tide line. In some cases, the on-the-ground measurements indicated that they could be off as much as 150 feet which is a large error when sea level rise is included. With sea level that is slowly increasing from its decadal average of 1 foot every 100 years and the potential for more storms resulting from warmer sea surface temperatures, it is critical to determine what parts of the state are at risk.
First some beach basics, and no, I am not talking about what type of swimsuit to wear or whether SPF 30 or 60 is better (perhaps another column). The parts of a beach include in order of distance from the low tide line: Intertidal area, forebeach, midbeach/berm, and back beach. The following are some definitions of beach terms from a talk I gave at the Whaling Museum a couple of years ago:
The “foreshore/forebeach” is the sloping portion of the beach between high and low tide. The “swash zone” is the intertidal area of wet sand where the most recent high and low tide resided.
Wrack line: deposit of vegetation (algae, eelgrass, etc.) and other floating material on the shoreline marking extent of high water; wrack lines can usually be seen for the most recent high tide and the highest one of the week and sometimes are even evident for the highest tide of the year.
Grain size: diameter of sand particle;
A “berm” is nearly horizontal and is formed when the waves deposit sand. A storm berm can mark the highest limit of storm waves. Several berms can occur at spring and neap tide levels.
The “back beach” or “backshore” is rarely touched by wave action and ends at the edge of the first dune. The “active dune” or “primary dune” is the first dune. “Fixed dunes” or “secondary dunes” can follow, sometimes in great numbers.
A swale is the hollow between dunes, often close enough to the water table so that marsh plants or peatland plants can get established.
Longshore drift is the movement of sand grains along the beach by waves. Waves that approach the shore at an angle rush diagonally up the beach. The water then returns directly down the beach under the force of gravity. Sand grains carried by the rush and backwash of the waves are moved along the beach in a sawtooth fashion. Other grains are carried along just seaward of the beach by the longshore current, which is also generated by the oblique approach of the waves. Longshore currents and longshore drift are generally considered to be constructive processes. Unlike storm waves, they are not significant in coastal erosion. They are the continuing processes that nourish the beach and carry sand along the shore of a barrier spit to deposit it at the end of the spit so that the spit grows in length.
We’ve been documenting the loss of coastal bluff at Tom Nevers for almost 30 years now as buildings, roads, and tons of bluff material composed of poorly sorted sand, larger gravel and fine clays slough off the beach escarpment face and wash into the nearshore area to be carried along by longshore drift and deposited in sandbars off shore circling the island or on beaches down drift (typically toward Madaket).
Massachusetts includes 1500 miles of tidal shoreline, 78 coastal communities, 681 barrier beaches and 36,000 people live within 500 feet of a coastal shore. Approximately 72% of the Massachusetts shore is exhibiting a long-term erosional trend and this trend has accelerated since 1950.
Nantucket has the highest erosion rates in the state, with the southern side losing 12 feet per year on average. Storm generated erosion ranges over periods of hours (tropical cyclones) to several days (northeasters). Although the storm events are short-lived, the resulting erosion can be equivalent to decades of long-term erosion. The actual quantity of sediment eroded from the coast is a function of storm tide elevation relative to land elevation, the duration of the storm and the characteristics of the storm waves. During severe coastal storms, it is not uncommon for the entire berm (dry beach above the normal high water line) and part of the dune to be removed from the beach. The amount of erosion is also dependent on the pre-storm width and elevation of the beach. Repeated small storms can do a lot of damage because the beach is more vulnerable to sand loss. In fact, the cumulative effects of two closely spaced minor storms can often exceed the impact of one severe storm.
It is easiest to think of our shifting shoals and the sandbar we live in as a system. Most of the sand stays in the system, but is transported in storm events offshore in the water, to create much steeper and shorter wintertime beaches. During summer’s gentler wave climate; with more southerly breezes, the sand moves back into place and the beaches lengthen by many feet, become much flatter and typically a bit lower in elevation. Our profiles show this process occurring each year in areas like Codfish Park in Sconset, which can shorter by 50-100 feet in the winter and become much steeper. Many beach areas develop a “stair-step effect” as mini escarpments are created as storms carve out larger chunks of shore. This material migrates around the island in shoals offshore. Each of these shoals may protect a part of the island for dozens of years, but then it eventually migrates (in the case of the eastern side off Sankaty) south; or on the southern shore, to the west, exposing areas to erosion that previously were protected. Nantucket has experienced trend reversals, in which sand has built up for many years, in an accretion cycle, then reverses to an eroding cycle to show a deceptively minor average erosion of only a foot or so per year.
For instance on the southern shore near Surfside, Massachusetts Coastal Zone Management found “In many cases, short-term shoreline fluctuations can be orders of magnitude greater than the long-term rate of shoreline change. For example, Nantucket's southeast shore has a long-term average shoreline change rate of +0.10 feet per year ("net" accretion of 2.1 feet between 1846-1978), suggesting a relatively stable area. However, between 1846 and 1978 the shoreline accreted 238 feet, then eroded 236 feet. This same phenomenon occurred at Codfish Park on the eastern shore of Nantucket. Unfortunately, many homes were constructed during the accretion phase. Since the trend reversed to erosion beginning in the mid-1950s, many houses have been lost to erosion and storms. “ http://www.fathom.com/feature/122398/
According to the Army Corps of Engineers, the most important cause of human-induced erosion is interruption of sediment sources and longshore sediment transport. Examples include the armoring of sediment sources with seawalls, revetments, and bulkheads, and the interruption of longshore sediment transport by the construction of groins and jetties. Coastal erosion, sand transport, and deposition are the natural processes that are responsible for the Cape as we know it. The cliffs on E/NE side of island erode to form the beach below and to the south and north (nodal point pushes some sand north, some south, node itself moves too). Jetties and groins generally do not stop erosion, but interfere with longshore drift and longshore currents to stop the passage of sand along the beach.
Although hurricane erosion can be serious and dramatic, in the long run, it is the northeast storms that do the most damage (woodshole.er.usgs.gov/staffpages/boldale/capecod/quest.html “Coastal Erosion on Cape Cod: Some Questions and Answers by Robert N. Oldale”).
When we build groins or jetties, we are typically diverting some of the longshore drift and transport of sand. Hard structures stop sand from being supplied by a beach front to build the beach directly below or down drift (“down current”) from the area. This is called starving a beach and you can see in many areas where hard structures like revetments have been built, that the beach below is essentially removed unless it is supplemented by additions of sand. Cusps form when wind and waves come from along-shore, or at an angle to the beach. If wind comes onshore directly, cusps are less likely to form.
How does sand accumulate on a beach?? It seems we are always talking about losses. Sand is transported over the sea bed towards the beach when waves 'stumble' such that their crests become narrower than their troughs. This produces a swift forward flow followed by a slower backward flow. On the beach the top of the wave breaks and mingles with the foot of the wave, both dashing forward with a force driven by the energy from the height and speed of the collapsing wave. The resulting rush of water is fast and strong and moves sand effortlessly up the beach. The water then comes to a rest as the sand particles settle out. The water then begins flowing back down the beach, first slowly and then faster until it dislodges cohesive sand grains. But the forcing power is much less than that of the on-rushing waves. The particles that settled at the top of the forward rush stay because of the hysteresis (lagging behind) between erosion speed and settling speed. Thus sand settles on the beach only when the tide recedes.
It is always helpful to collect several hundred grams of beach sand and bring it back for measuring through a series of sieves which sort the material into various grain sizes. The average amount of energy and wave and wind action that a beach endures is measured in its tiny grains of sand. Large poorly sorted and angular (sharper edged) material indicates a beach that experiences high energy, large waves, and windy conditions. Fine sand that is well sorted indicates that the beach experiences relatively gentle waves that leave these finer particles behind.
