Massive submarine sand dunes in the eastern Juan de Fuca Strait, British Columbia
Mosher, D.C.; Thomson, R.E. (2000). Massive submarine sand dunes in the eastern Juan de Fuca Strait, British Columbia, in: Trentesaux, A. et al. (Ed.) Marine Sandwave Dynamics, International Workshop, March 23-24 2000, University of Lille 1, France. Proceedings. In: Trentesaux, A.; Garlan, T. (Ed.) (2000). Marine Sandwave Dynamics, International Workshop, March 23-24 2000, University of Lille 1, France. Proceedings. Université de Lille 1: Lille. ISBN 2-11-088263-8. 240 pp., more |  |
| Authors | | Top | - Mosher, D.C.
- Thomson, R.E.
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| Abstract | A field of very large submarine sand dunes in the eastern Strait of Juan de Fuca, British Columbia, has been surveyed with seismic reflection, sidescan sonar and multibeam bathymetric systems. The field is comprised of approximately 2.6x107 m3 of sand and fine gravel (mean size > 0.5 mm). The largest of these dunes measures 25 m in height, 300 m in wavelength and 1200 m in width, comprising 9x106 m3 of material. Its wavelength to water depth ratio (L/h) is 3.3 (typical for intertidal dunes) and height to water depth ratio (H/h) is 0.28 (larger than typical in any environment). Its vertical form index (L/H) is 12 indicating a steep dune, a dimension that is atypical for large dunes. Its modified symmetry index (MSI = [(stoss-side length/lee-side length)-1]) is 0.6, indicating only slight asymmetry. The lee slope is about 20° and stoss slope is 12°. The larger dunes in this field are classified as linear, slightly asymmetrical, stoss-erosional, lee-deposition, two-dimensional, very large dunes. They show net zero climb. A subsidiary set of sand waves exists on the larger dunes at an oblique angle. Whether the dunes are modern or relict is unconfirmed, but conditions do exist today that could construct these features. Currents in excess of 100 cm/s can be expected at depth in the Strait. These currents have the potential of eroding and transporting material up to granule size. It is likely that deep flood tides and estuarine circulation during summer months construct the dunes. Most of the time the flow transports material only near the crest of the bedforms, giving them a concave-up appearance. The estuarine circulation may reverse during winter months, though less strong, resulting in reworking of the dunes and causing the near-symmetrical profile more typical of tidal bedforms. Clinoform bedding suggests net eastward migration. This combination of estuarine circulation, tidal forcing, bathymetric control accelerating currents, source material and size of the source material has resulted in the construction of some of the highest and steepest submarine bedforms documented in the literature. The question of dune age and stability remains unanswered but is of critical engineering importance with the presence of pipelines, sewage outfall and fibre optic cables in the area. |
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