# Presentation: 550534

## A Day Trip to Middle Ground with REMUS and Swath BathymetryRich Signell, USGSAl Plueddemann, WHOI

**A** **Day Trip to Middle Ground with ****REMUS and Swath Bathymetry**Rich Signell, USGSAl Plueddemann, WHOI

- Middle Ground is a sand ridge located between Martha’s Vineyard and Falmouth. The crest of the ridge is found at 5 meters depth. Tidal circulation is the primary physical force responsible for maintaining the ridge.  This Ikonos satellite image from June 5, 2001 shows evidence of strong tidal currents crossing the ridge at an oblique angle.

- J. Dungan Smith (1969) first discussed the dynamic setting and origin of Middle Ground sand ridge. Smith discussed the movement of the sand waves covering the ridge and net movements of the sand waves are consistent with counterclockwise of net circulation of water around the ridge. Theory and field measurements indicate that the present morphology of the ridge is in dynamic equilibrium with the tidal current system, and that large-scale morphological changes occur within a period of only a few years.

**Submetrix SEA SWATHplus 234kHz**

- GPS antenna

- SWATHplus Transducer Head

- Transducer on each side with 4 staves:
- (1 transmit, 3 receive)

- Interferometric sonar was used for this project. This type of sonar is used in shallow water surveying (<30m) and has a wide swath (6-10 times water depth). Horizontal and vertical accuracy, when coupled with RTK-GPS and a motion sensor is 5-10 cm.

- L~100 m, h~3 m, H=20 m

- L~50-100 m, h~2-4 m, H=17 m

**(a)**

**(b)**

**(c)**

**(d)**

- Several transects highlight variability in the shape of the sand wave fields.  A northern pass (a) has longer wavelength, ebb-dominated, westward migrating waves, while a southern pass (b) has shorter wavelength, flood-dominated eastward migrating waves

- The lee sides of the sand waves approach angle of repose (c), and sub-bottom profiling with a 4-24 kHz chirp system reveals a flat gravel floor beneath the sand ridge (d).   This system was not sufficient to penetrate further than 4-5 m of the 800 micron sand, so the question of underlying geologic control remains.

- REMUS

- TIDE

- REMUS made several passes (g) across and along the bank. The instrument recorded currents, water depth, density, temperature, and salinity.  The cross-bank transects showed large scale current shear induced by the ridge, as well as small-scale current structure presumably induced by the sand wave field.

**(e)**

**(f)**

- Future ideas:
-  Repeat bathymetry mapping of this region to observe temporal changes in bank and relate to hydrodynamics
- Try again on cross-sandwave ADCP surveys with REMUS
- Map eastern end of Middle Ground to see if there is evidence of transfer of sand from beach at West Chop
- Repeat the sub-bottom profiling with a more powerful chirp system to see if there is evidence of geologic control under the ridge crest
- With ROMS model, explore spring-neap variability, more realistic sediment distribution, and detailed modeling of survey region including sand waves

**(g)**

- A tidal model of Vineyard Sound was produced using ROMS, inspired by a Redfield1 (1978) finding that the region acts as a channel with interfering tidal waves.  The model shows tidal flow crossing Middle Ground obliquely (h) and counterclockwise residual circulation with strong convergence of sand on the bank crest (i). The rate of sand convergence would build the bank by ~2 m/yr.
- 1Redfield, A.C., 1978. “The Tide in Coastal Waters”, *Journal of Marine Research, *36, 255-294.

**(h)**

**(i)**