Borrow Site

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While some beach fill sites have the luxury of being located rather close to prospective sand sources, other coastlines may have no usable sediment within a reasonable distance.

Although sand compatibility is the primary concern when seeking borrow sites, the cost of that sand is determined by the distance of transport from the borrow site to the fill site. Typical borrow sites can be located offshore in the form of drowned barrier islands, oblique sand bodies and longshore sand bars, near to shore in the form of flood and ebb tidal deltas, or on land. Sand for filling a beach can also come from a navigational maintenance dredging activity. Instead of dumping sand removed from an inlet on the continental shelf, for example, it is deposited on the nearest beach. This is often referred to as "opportunistic dredge and fill."

Contents

Offshore sources

Dredge06.jpg

The following offshore borrow sites are listed according to their distance from the shoreline:

  1. Longshore sand bars Longshore sand bars are found along the Atlantic and Gulf coastlines, are parallel to shore and typically located in water ranging from 2 to 7 meters in depth. These bars are dynamic, formed by wave action and move seasonally and with storm patterns. While longshore sand bars are attractive sources of sand (because of sand compatibility and proximity to the fill site), they are also often integral parts of the active shoreface of a beach. Because of this relationship, removing sand from a longshore bar can alter the bathymetry (shape of the ocean bottom), change the local wave climate and increase erosion.
  2. Drowned barrier islands Located hundreds of meters to several kilometers from the beach at depths up to 30 meters, drowned barrier islands are accumulations of sand that likely formed during periods of low relative sea level and have become submerged during times of high relative sea level. Drowned barrier islands are composed of well-sorted sediment that is typically dispersed over a localized area of the seabed.
  3. Oblique sand bodies Called "oblique" because of their orientation at an angle to the shoreline, these sand bodies are located on the shallow continental shelf along the north and central Atlantic coastlines and along the Florida Gulf Coast. Oblique sand bodies can be up to a kilometer long, ten meters thick and hundreds of meters wide.[1] Though these bodies contain copious amounts of sand, they are usually located a kilometer or more from the shoreline at depths up to 30m, making acquisition difficult and costly.


Management for sand deposits within the Outer Continental Shelf (OCS) lies with the Marine Minerals Program of the US Bureau of Ocean Energy Management.

Nearshore sources

Nearshore sediment sources include flood tidal-deltas and ebb tidal-deltas located landward and seaward of tidal inlets, respectively. These bodies are generally formed by wave action and tidal influence on sand moving through inlets.[2] Sediment in these tidal-deltas is typically comprised of medium- to well-sorted quartz sand.[3]

  1. Flood tidal-deltas Flood tidal-deltas are found landward of tidal inlets. Though these sources can contain thousands of cubic meters of sand, they are not commonly used for nourishment projects because of high mud content. Because flood tidal areas are also productive biotic communities, sand extraction may result in substantial ecosystem damage.
  2. Ebb tidal-deltas Because these bodies are located seaward of tidal inlets, the shape and amount of sand contained in a given ebb tidal delta is determined by the dominant wave and current action. If an ebb tidal-delta is located in an area with high wave action, the delta is likely to be small. Ebb tidal-deltas not subject to extensive wave and current action, however, can be rather extensive. As is the case with longshore sand bars, these accumulations are part of the active shoreline and their use as sources of sand, along with the resulting change in bathymetry, can significantly alter wave interaction and cause significant shoreline impacts.


Upland sources

Upland sand sources are often in the form of relict sand dunes, dredge disposal areas and quarries. Upland sand sources are typically well sorted by wind and, as a consequence, lack shell content. Sediment size may also be too fine and undesirable for fill projects. Regardless, upland sources are often the most economically feasible option because transport requires only overland transport by truck.

Notes

  1. http://www3.csc.noaa.gov/beachnourishment/html/geo/borrow.htm
  2. Davis Jr., R.A. and Fitzgerald, D.M. Beaches and Coasts, Blackwell Publishing 2004.
  3. http://www.ozcoasts.org.au/conceptual_mods/sed_environs.jsp