HISTORIC AND PRESENT-DAY HYDRODYNAMICS OF INTERTIDAL BEDFORMS IN A COASTAL LAGOON: PHOTOGRAMMETRIC ANALYSES AND FIELD SURVEYS

The hydrodynamics of the Pleasant Bay-Chatham Harbor lagoon on Cape Cod, Massachusetts were significantly altered after the formation of a new inlet as a result of an extratropical storm on 7 January, 1987. This abrupt change in hydrodynamics provides the opportunity to test a new method of quantifying historic hydrodynamics using photogrammetric analyses.

We determined the spacing of inter- and subtidal bedforms via photogrammetric analyses of rectified vertical aerial photographs. This information coupled with assumptions about height of the bedforms and sediment size yields quantitative data concerning velocities associated with the formation of bedforms at the time the photographs were taken. Historic aerial photographs traditionally were taken after storm events to qualitatively document system response; this method provides quantitative data for these areas. The evolution of ebb and flood channels, horizontal segregation of flow, a flood-tidal delta and associated sediment bodies were documented from 1938 to the 2004, with 24 individual years of photographs. To ground-truth the accuracy of this new method data from a spring tide bedform survey were used. This survey was completed 4 June, 2004, 7 days prior to a series of high-resolution vertical aerial photographs of the lagoon.

The flood-tidal delta was relatively stable from the early 1960s to 1982 (the last aerial photograph before inlet formation), with little migration of the ebb shield, (<30 m) and/or change in surface area (<30%). Since new inlet formation the flood tidal delta migrated rapidly from 1982 to 1987 (>80m) with significant changes in surface area (>40%). Bedforms present on the flood-tidal delta in the rectified 1982 photographs show predominantly 2D, sand waves (mean spacing of 15.5 m, n=31). Flow velocities needed to generate these bedforms would be approximately 0.7 - 0.8 m/s. Field surveys done in June 2004 show 2D to 3D, megaripples (mean spacing of 2.9 m (n=38). Flow velocities needed to generate these bedforms would be approximately 1.0 - 1.5 m/s. This method was successful in quantifying historic hydrodynamics in the lagoon and could be used to quantify response to storms, long term changes in coastal processes and anthropogenic alterations.