Storm-induced sand transport and bedform genesis at beach and shoreface environments of the Maine Coast
- Maine Geological Survey, Augusta, ME (United States)
- Univ. of Maine, Orono, ME (United States). Dept. of Geological Sciences
Hurricane Bob passed over a taught-wire mooring array of current meters in Saco Bay, Maine on 19 August 1991. Current speeds and directions from Bob are compared to more common extratropical northeaster storms that dominate the extreme wind and wave conditions in the Gulf of Maine. Currents, sampled and averaged to produce 30 minute vectors and burst-mode 1 second vectors, yield combined wave, tide, and wind-driven flows up to 40 cm/s, sufficient to induce sand transport. Comparison of the author's data with wind and wave measurements at the nearby Portland Large Navigation Buoy suggest sand transport events occur many times a year. During storms, 10 cm/s tidal currents may enhance or impede wind-driven offshore-directed bottom flow (coastal downwelling). Preliminary results suggest that a rapidly moving northeaster with a peak wind velocity of 7.7 m/s (15 knots) during a rising tide will lead to net seaward transport of sand during the ebb portion of the tidal cycle. Hurricane Bob data show 35 cm/s downwelling during the approach of the hurricane and 30 cm/s onshore-directed coastal upwelling following landfall. Upwelling lasted twice as long (24 hours) as downwelling and is believed to have been the cause of observed beach accretion. Repeated sidescan sonar surveys of Saco Bay and Cape Small's Kennebec River paleodelta indicate the persistence of large shore-normal ribbons of ripple bedforms (rippled scour depressions) across the nearshore during a period when storms reworked the seabed and could have reshaped the bedform field. Calculations of the threshold of motion and wave orbitals under storm conditions explain the origin of large ripples in coarse sand and gravel as well as plane beds in medium sand. The spatial pattern of each bed type on the shoreface in the 10--40 meter depth range may be due to downwelling, although further study is needed.
- OSTI ID:
- 5722474
- Report Number(s):
- CONF-9303211-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:2; Conference: 28. annual Geological Society of America (GSA) Northeastern Section meeting, Burlington, VT (United States), 22-24 Mar 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GEOLOGIC DEPOSITS
DEPOSITION
EROSION
MAINE
ATLANTIC OCEAN
BAYS
COASTAL REGIONS
CONTINENTAL SHELF
ENVIRONMENTAL TRANSPORT
FLOW RATE
FLOWMETERS
HURRICANES
OFFSHORE SITES
SAND
SEDIMENTS
TIDE
WATER CURRENTS
COASTAL WATERS
CONTINENTAL MARGIN
CURRENTS
DEVELOPED COUNTRIES
DISASTERS
MASS TRANSFER
MEASURING INSTRUMENTS
METERS
NORTH AMERICA
SEAS
STORMS
SURFACE WATERS
USA
580000* - Geosciences