Vignettes > Geomorphic history controls the locations of fresh-water wetlands on barrier islands, Virginia's Atlantic shore

Geomorphic history controls the locations of fresh-water wetlands on barrier islands, Virginia's Atlantic shore

Rich Whittecar
Old Dominion University, Ocean Earth and Atmospheric Sciences
Author Profile

Shortcut URL: http://serc.carleton.edu/75360

Location

Continent: North America
Country: United States
State/Province:Virginia
City/Town: Virginia Beach
UTM coordinates: UTM 18S 407375 E, 4085870 N

Setting

Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy

Figure 1. Fresh-water pond in cypress forest at First Landing State Park, Cape Henry, Virginia. Note that the water is stained with tannin formed by the decay of needles and leaves. (UTM 18S 407375 E, 4085870 N) Details


Figure 2. Cross-section through a hypothetical barrier island underlain by extremely thick sand beds. The lower dotted line shows the relative position of the freshwater-saltwater interface. The upper dotted line shows the position of the water table as calculated by the equation from Fetter (2001) where H is hydraulic head (elevation of top of saturated zone) K is hydraulic permeability of the sediments W is the rate of recharge L is half the width of the island X is the distance from the side of the island, and G equals 40, based on the ratio of the density of sea water and fresh water. Details


Figure 3. Aerial photo of the Cape Henry, Virginia area, with a topographic map inset. Note the many long, narrow beach ridges thought to have prograded oceanward during periods of abundant sediment supply, available during portions of the Holocene sea level rise. Cypress swamps between the ridges develop where the water table rises higher than the swales. Large dunes along the coast stabilized under pine forests that prospered after European settlers suppressed forest fires. North is to the top. (photo taken March 24, 1994, obtained at Google Earth; UTM 18S 409023 E, 4085633 N). Details


Figure 4. Aerial photo of Parramore Island, Virginia. The central portion of this island appears to be a transgressive barrier; less than 200 m wide with few dunes, this area experiences overwash frequently. This image was taken 1 day after slow-moving tropical storm Ida set up several days of extraordinarily high tides. In addition to the fresh washover fans of sand lapping onto the mucky peats of the lagoon, note the two sets of large offshore bars (lines of breaking waves), developed during the storm by sediment moved offshore from the beach. North is to the upper right corner. (photo taken November 15, 2009, obtained at Google Earth, UTM 18S 440300 E, 4157347 N). Details


Figure 5. Image of north end of Hog Island, Virginia, an area of accretion during historic times. Swales between the dune ridge contain marshes dominated by salt-tolerant Spartina sp., which indicate frequent inundation by the ocean during storms. Dune sands contain small amounts of fresh groundwater. North is to the right. (photo taken May 23, 2009, obtained at Google Earth, 18S 441165 E, 4145454 N). Details


Figure 6. Fresh-water marsh formed between dune ridges on a barrier island with a history of human settlement and dune enhancement until the early 20th century, but is now nearly deserted. Managers of this area at False Cape State Park in Virginia, and adjacent Back Bay Wildlife Preserve, attempt to maintain natural habitats particularly attractive to migrating waterfowl. (UTM 18S 420233 E, 4053750 N). Details


Figure 7. Cypress swamp and pond at First Landing State Park, Virginia. Covering a significant portion of the Cape Henry beach ridge complex in Virginia Beach, the park attracts more than a million visitors annually. It is surrounded by a military base and housing subdivisions. The major issues that arise as park managers work to maintain several coastal zone habitats commonly stem from excessive foot and pleasure boat traffic and groundwater withdrawal from nearby wells. (UTM 18S 407375 E, 4086869 N). Details


Description

Fresh-water ponds on low sand islands
Native Americans, pirates and the early European colonists used them. Ship-wreaked sailors owe their survival to them. Fresh-water ponds somehow seem out of place, though, scattered along the narrow barrier islands lining the wind-swept ocean shore of the U.S. from Virginia to Florida. First-time visitors to these swamps and marshes often presume their dark tea-colored waters are brackish or salty (Fig.1). But they are even better than "fresh," British colonists soon discovered, for if they used the tannin-rich water from ponds in these cypress forests for ocean voyages back to Europe, the pond water stayed drinkable longer than fresh water from other sources. But on some barrier islands interior wetlands are brackish or saline, unfit to drink. For any given pond, the difference between having salty or fresh water stems from the history of the geomorphic processes that formed the barrier island.

Barrier island hydrology

The source of the fresh water, of course, is precipitation, nearly all as rain in Virginia. Rain that drips through the tree canopy, soaks into the sand dunes and overwash flats, and percolates down past thirsty roots will saturate the island sediments. Think of the barrier island as a long, low sand hill surrounded and underlain by dense salt water; fresh water mounds under the middle of the hill (island) and leaks out along the edges. As rain water accumulates underground it seeps down and laterally, pushing aside the denser salty water before being forced to pass upwards through the floor of the ocean or the lagoon behind the island. These processes develop a lens of fresh water in the saturated zone, usually highest and thickest in the middle of the island (Fig. 2).

Fetter (2001) provides a useful equation for the shape of the top of this fresh-water lens - the "water table" – that forms beneath an oceanic island made of uniform sand. His equation shows that the height of the water table (H) is a function of the rate of water added to the island (W=recharge), the density of the underlying salt water, the permeability of the sediments beneath the island (K), and the width of the island (2L)(Fig.2). Wetlands and ponds form where the water table rises above the land surface. In general, the greater the recharge and wider the island, the higher the water table can rise in the middle. The more undulating the island surface, the greater the number of separate wetlands and ponds that will develop.

Barrier island formation and landforms

Both the width and the complexity of an barrier island's surface reflect the dynamic history of that portion of the island. Island segments with the greatest overall height and width usually formed during a period of shoreline progradation. During that time, longshore transport moved a surplus of sediment onto this part of the island, widening the beaches and generating a succession of back-beach dune ridges as the island shore grew oceanward. These "prograded" barrier island segments ("regressive" islands of Leatherman, 1988) tend to be relatively wide and have parallel lines of sandy hills and swales. An excellent example of this setting, Cape Henry, lies on the southern side of the mouth of the Chesapeake Bay (Fig. 3). Over 3 km wide, this barrier beach complex consists of a mosaic of sandy ridges separated by ponds and cypress swamps (Fig. 1). The water table can rise to more 2.5 meters elevation in the middle of such a wide island during wet years. In 1607 colonists who made their first landing here, and eventually settled at Jamestown, replenished their water supplies from ponds in the island's interior.

Narrow island segments, far more common than wide sets of beach ridges, often experience flooding from both the ocean and lagoon (Fig. 4). Large tidal surges driven by storm winds overtop many islands every few years, particularly those capped only with the low, discontinuous dunes found on barrier islands unaltered by humans. These "trangressive" islands experience shoreface retreat during large storms, but sediment eroded from the beach zone and spread across the island as washover fans can build out the backside of the island. As these islands migrate landward, washover fans form thin carpets of sand that bury peat and mud deposited in the lagoon; this geomorphic history creates an island with thin surficial aquifers that hold less groundwater than the thick sand masses which lie beneath prograded island segments. In addition, on many islands sound-side flooding occurs often. Strong steady winds blowing across wide lagoons ("sounds") can raise water levels and flood island interiors through swales connected to the lagoons. Thus with their flatter surfaces, lower overall permeability and island widths, and frequent saline water incursions, narrow barrier islands usually have fewer fresh-water ponds and wetlands.

These characterizations of prograding and transgressive islands must be modified for islands with complicated histories. For example, barrier islands with old beach ridges formed by progradation when sea level was lower may now experience frequent inundation as sea levels rise (Fig. 5). Overwash and sound-side flooding that soaks the swales may create saline marshes, rather than fresh-water ponds, far into the island interior. Conversely, many transgressive islands along the Atlantic shore have tall, continuous dune lines built to prevent storms from flooding roads and resort communities. Most of these tall dunes date to grass-planting efforts and other ecosystem alterations made by the Depression-era Civilian Conservation Corps of the 1930s. Some of these dune fields have grown large enough to support fresh-water wetlands in interdune swales (Fig. 6), even on islands where overwash was a common event in the 1800s.

Who cares?
Fresh-water coastal wetlands need protection. Many state parks and habitats maintained by non-governmental organizations focus on preserving both salt-water and fresh-water ponds, marshes, and swamps in coastal regions (Fig.7). As sea-level rise accelerates and hydrologic conditions fluctuate during the coming decades, the management and survival of these preserves will require awareness of the dynamic geomorphic processes, past and future, endemic to these special regions.

Associated References


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