Why Chesapeake Bay’s Beaches Are moving and Eroding
The currents that move sand are shaped by a variety of natural processes that operate over a variety of different scales in time and space [FIGURE 1]. On one end of the spectrum, the gradual rise of sea level has caused shorelines, coasts, beaches, bays, marshes, wetlands, maritime forests, and barrier islands to migrate tens to hundreds of miles toward land over time periods that span thousands to millions of years. These coastal systems can move dramatically towards land by continuously “rolling over” the landscape without losing sediment (gravel, sand, silt, clay), vegetation, and organisms.
The everyday forces shaping our beaches.
At the other end of the spectrum, over shorter time scales and more local space scales, daily and seasonal physical forces take over the job of moving sand and beaches around. These forces are mostly waves, tides, storms, and yes, humans. These are what we see and experience in our lifetimes.Everyday, non-storm waves mostly work to move sand along (or parallel to) the shore. Tides, storm waves, and storm surge tend to move sand perpendicular to the shore - onshore and/or offshore. In places like the Chesapeake Bay that are affected by extratropical storms (aka nor’easters), beach changes can happen seasonally – alternating between wider, flatter beaches in the winter and steeper, narrower beaches in the summer.
Sand In, Sand Out: The Beach Budget Explained
If more sand leaves a coastal area or beach than comes in, that beach will have a sand deficit and erode. You get the budget analogy…. Beach erosion is defined as a net loss of sand, not necessarily the movement of sand (beaches) across the landscape. Under a regime of rising sea levels and stronger storms, beaches move toward land while they erode. [FIGURES 2 and 3]
And, yes, humans can have an effect of sand availability just like waves, tides, and storms. Shoreline structures can have the unintended consequence of removing sand that could be available to resupply beaches by not allowing the land to erode or trapping sand that might have gone to another beach.
The fate of a beach depends on many factors, including its “rollover” potential, the supply of sand to keep it “nourished,” and the frequency and intensity of storms. Because the processes at this scale can vary widely from place to place, it’s important to understand how these processes operate at a local scale.
A key factor in determining if and how fast a beach will erode is the amount of sediment (sand) available to resupply the amount of sand that’s left a beach. You can think of this as a “financial budget.” If more sand (or money) comes into a coastal area or beach (or bank account) than leaves it, that beach (or bank account) will have a sand (or money) surplus, build up, and accrete (or get rich!).
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Signs of a changing shore
A closer look at the physical changes shaping the Chesapeake Bay’s coastal landscape.
Figure 1
The coastal and ocean processes that affect a beach’s short- and long-term migration across a landscape range from non-storm waves that break on a particular beach many times a day to global sea-level changes (eustatic) and regional sea-level changes (relative). In the Chesapeake Bay, the sinking of the land from groundwater extraction and the longer-term effects of the last glacial period has produced some of the highest “relative” sea-level rise rates on the U.S. east coast.
Figure 2
The Bay’s shoreline encountering living trees is a sure-tell sign of rapid beach erosion. Trees are usually setback some distance away from the shoreline because saltwater makes it harder for them to take up water and nutrients they need to survive.
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Figure 3
Marsh grasses do not survive well when exposed to waves and that is the reason why they are found behind (landward of) beaches and barrier islands. Seeing marsh cropping out on the beach or where waves break is a sign that significant, rapid beach erosion has occurred. (See figures 4 and 5 for more information)
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Figure 4
Tree stumps in the "nearshore" wave zone and vegetation growing seaward of the beach not only make boat traffic hazardous but are also more signs of significant beach erosion taking place.
Figure 5
Many beaches in the Chesapeake Bay are thin, slender ribbons of sand that can move rapidly over the landscape behind them if the land is low-lying as most are. And, most of them are eroding and narrowing because there isn't enough sand in the system to ensure the beach rolls over itself without losing sand. That's why shoreline stabilization structures or other interventions are needed when waterfront developments get constructed. Development and infrastructure draw a "hard line in the sand" to encroaching beaches and shorelines.
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