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Coastal Mixing and Optics (CMO)

Ocean Physics Laboratory

Sediment Resuspension Events Forced by Hurricanes Edouard and Hortense

Presented by: Grace Chang
Ocean Physics Laboratory
University of California, Santa Barbara

In collaboration with:
T. Dickey, D. Manov, D. Sigurdson (UCSB)
A. Williams, J. Trowbridge (WHOI)
P. Hill (Dalhousie University)
Y. Agrawal (Sequoia Scientific, Inc.)

| Abstract | Oral presentation |


Physical and optical effects of Hurricanes Edouard and Hortense were observed during the fall deployment of the Coastal Mixing and Optics (CMO) experiment. The eye of Edouard passed within 110 km of the CMO mooring site on September 2, 1996. Two weeks later, the eye of Hortense passed within 350 km on September 14, 1996. Several newly developed oceanographic instruments were deployed at various depths on a mooring and a tripod in the Mud Patch located about 110 km south of Martha's Vineyard, MA on the Mid-Atlantic continental shelf in 70 m of water.

Winds peaked at 30 m/s, wave heights reached 3 m, and mixed layer depth increased from 10 m to greater than 50 m at the CMO site with the passage of Hurricane Edouard. Mean bottom shear stresses exceeded the estimated critical shear stress values for five days following the passage of the storm. A 10-fold increase in bottom shear stress, dissipation rate, and velocity standard deviation forced massive resuspension of bottom sediments to more than 40 m above the ocean bottom with beam c values increasing by a factor of 30. Sediment resuspension during the passage of Edouard was likely locally driven by near bottom wave and current processes. Optical data reveal that the resuspended sediment mainly consisted of detrital matter of low-density large particles.

Hurricane Hortense resulted in local winds of 12 m/s and wave heights of 2.5 m. The passage of this storm is more evident in the sediment resuspension event than the physical record. Mean shear stress did not exceed cirital values during the onset of the sediment resuspension, and dissipation rates remained at near mean values. Velocity standard deviation increased from 2.5 to greater than 15 cm/s. The result was a 20-fold increase in bottom beam c values. Wave-induced forcing likely caused sediment resuspension during the passage of Hortense.

Oral Presentation


This study was a part of the Coastal Mixing and Optics experiment (CMO), which is an Accelerated Research Initiative (ARI) funded by the Office of Naval Research (ONR). The objective of the CMO experiment was to study the mixing of ocean water on the continental shelf and the effect of mixing on the transmission of light through the water.

Specific Objectives

Some of the specific objectives that were met by the Ocean Physics Laboratory (OPL) with the passage of the two hurricanes were to:

1) Make distinctions among particle types, and
2) Relate optical and particle variability near the ocean bottom to physical processes affecting sediment resuspension.


The significance of this study is:

1) It provided us with a unique set of physical and optical observations during the passage of two hurricanes within a two week period,
2) The observations provide a data set for future modeling of near bottom physical, geological, chemical, and biological processes, and
3) The modeling enables the study of the fate of bottom materials, including sediment, organisms, and pollutants.

Site description


The CMO site is located in the Middle Atlantic Bight at 40.5 N latitude, and 70.5 W longitude. It is about 100 km south of Cape Cod, Massachusetts in approximately 70 m water depth. The site is in the Mud Patch, which is a 2-14 m thick lens of fine-grained material consisting of up to 95% silt plus clay. The first mooring deployment was from July 8 through September 25, 1996. The tripod was first deployed from August 9 through September 25, 1996. This presentation will cover the period from August 27 through September 21.

Mooring and tripod diagram

Slide 5

Several physical and optical instruments were deployed on a mooring and two tripods. The tripods were ~400 m from the mooring. Instruments were placed in Bio-Optical Systems (BIOPS) at 13.5, 37, and 52 m on the mooring, and at about 2 m above the bottom on the optical tripod. BIOPS instruments used in this study are the transmissometer, temperature sensor, WET Labs ac-9, which measures absorption and attenuation concurrently at nine wavelengths, and the WETStar fluorometer. BASS current meters, deployed at 7 depths above the bottom by Sandy Williams at WHOI, were also used in this study. We are concerned with the one at 0.38 meters above the bottom (mab). The sampling rate for the temperature sensors, fluorometers, and transmissometers was eight times per hour. The ac-9 sampling rate was once per hour, and the BASS current meters sampled at 2 Hz for 28 minutes and 49 seconds. Additional instruments used in this study include those on meteorological National Data Buoy Center (NDBC) buoy 44008, which was located about 90 km east of the CMO site.

Hurricane information

Slide 6

Hurricane Edouard reached category 4 status on the Saffir-Simpson hurricane scale with 140 mph winds on August 29, 1996 when it was about 700 km south of the CMO site. It maintained category 4 status for seven days, then decreased in intensity, and moved towards the north, where the eye reached within 110 km of the CMO site on September 2, 1996.

Hurricane Hortense also reached category 4 status with 120 mph winds on September 13, 1996 when it was approximately 800 km south of the CMO site. It dropped in intensity to category 3 the same day. The eye was nearest the CMO site on September 14, 1996, when it was within 350 km away. The distance of Hortense to the CMO site was more than three times greater than that of Hurricane Edouard.

NDBC Buoy 44008 data

slide 7

This plot shows meteorological data taken by instruments on NDBC buoy 44008. Barometric pressure dropped by about 45 mb with the passage of Hurricane Edouard, and about 25 mb with Hortense. Wind speed evaluated at 5 m above the sea surface increased from near zero to greater than 25 m/s with Edouard, and from about 4 m/s to 15 m/s with the passage of Hortense. The winds were more sporadic with Hortense due to the greater distance from the CMO site. Wave height increased dramatically with Edouard, increasing from less than 1 m to greater than 9 m. Hortense resulted in wave height increasing to about 4 m. The significant wave period was much longer for Hurricane Hortense due to the greater distance from the CMO site.

BIOPS data

slide 8

These plots show physical and optical time series data taken by BIOPS instruments. The top panel shows temperature at four depths, with the legend shown on the right. Summer stratification, which is common in this area, can be seen during the first several days of the time series. Stratification breaks down with the passage of Edouard, with mixed layer depths (MLD) increasing from about 12 m to greater than 65 m. This was calculated using a 1 degree temperature criterium. The water column remained mixed for several days, and then began to restratify on September 9. A few days following restratification, Hurricane Hortense passed over the mooring, remixing the water column with MLD increasing from ~20 m to ~50 m.

The bottom three panels show beam attenuation (or beam c) without the effects of water measured by the ac-9. (Notice the differences in scales of the y-axes) We obtained good spectral quality throughout the time series; the wavelengths are shown in the legend box on the right. Beam c is a measure of turbidity, which is an indication of sediment resuspension. Looking at the botom panel for Hurricane Edouard, sediment resuspension was greatest at the 68 m depth, with ac-9 beam c increasing from mean levels of 1 m^-1 to greater than 30 m^-1. The resuspension event can be seen in the 52 and 37 m depths as well, with a half day time lag in resuspension between each depth. The resuspension event was significant with Hortense, despite the great distance from the mooring. Beam c values increased to greater than 20 m^-1, with a time lag of three quarters of a day between each depth, and little resuspension at the 37 m depth.

What materials were resuspended?

slide 9

These plots are chlorophyll concentration measured by the WETStar fluorometer vs. beam c measured by the transmissometer. Notice the differences in scales of both the x and y-axes. These give us a rough indication of what materials may have been in the water column by the following method. High concentrations of chlorophyll and relatively low values of beam c are indicative of phytoplankton in the water. Conversely, high values of beam c with relatively low chlorophyll concentrations are indicative of detrital matter.

If we look at the top panel, which was for calm conditions before the passage of Hurricane Edouard, we see that there is mostly phytoplankton at the 37 m depth, with little detrital matter. At the 52 m depth, there was about half phytoplankton and half detritus. The material at the 68 m depth was mostly detrital matter. During the passage of Hurricane Edouard (middle panel), beam c values increase by greater than 10-fold. There is no longer any phytoplankton at any depth due to intense mixing of the water column. We suspect that the high chlorophyll concentrations at the 68 m depth were due to resuspended fluorescing matter off of the seafloor. After the passage of Edouard (bottom panel), the values of beam c and chlorophyll concentration are almost back to calm condition values. There is no phytoplankton at the 52 or 68 m depths, and some phytoplankton returned to the 37 m depth.

What were the mechanisms of resuspension?

slide 10

These plots are time series data measured or derived from the BASS current meters. The top panel is the magnitude of mean current velocity measured by the BASS current meter at 0.38 mab. The second panel is wave orbital velocity estimated by integrating the energy under the wave peak in the velocity spectra. The third panel shows mean current bottom shear stress computed using low-pass filtered current velocity and the law of the wall formulation. Combined wave and current bottom shear stress, computed using wave-orbital velocity, mean current velocity, and the model presented in Christoffersen and Jonsson (1985), is shown in the fourth panel. The bottom panel is dissipation rate, computed using shear velocities estimated from the Christoffersen and Jonsson model. The dissipation rate was near zero at calm conditions, increasing greatly during the passages of the two hurricanes.

Sediment resuspends when bottom stresses exceed a critical value. We found that the resuspension event forced by Edouard was induced by currents and waves, while the resuspension during Hortense was induced by waves, with little or no influence by the currents.

How do we know this?

If we look at the sediment resuspension events shown in the 68 m beam c record (green) and overlay it with the mean current bottom shear stress (red),

slide 11a

we see that there is a high correlation between mean current shear stress and bottom resuspension with Edouard; the R^2 is 0.68. But, there is no correlation between mean current bottom shear stress and the resuspension event with Hurricane Hortense (R^2=0.007).

If we do the same thing for the 68 m beam c record (green) and combined wave and current bottom shear stress (purple),

slide 11b

we see a high correlation between combined wave and current bottom shear stress and the resuspension events for both hurricanes. The R^2 for Edouard is 0.80, and for Hortense, it is 0.75. This is strong evidence that the resuspension event associated with Hurricane Edouard was forced by currents and waves, and that of Hortense was forced by waves, with little or no influence by the currents.

This is not unexpected because of the greater distance from Hortense to the CMO site, and boundary shear stresses associated with waves can be of order magnitude greater than those with a mean currents. This is due to the formation of a wave boundary layer within the current boundary layer which causes waves to resuspend material when the currents are too weak. The currents are then present to transport the material along or across-shelf.


In conclusion, we captured two consecutive hurricanes within a two week period during the CMO experiment. We found that beam c values increased from 1 m^-1 to greater than 30 m^-1 with Edouard, and to greater than 20 m^-1 with the passage of Hortense at the 68 m depth. The sediment resuspension event can be seen in the 37 m optical data as well. The resuspended material consisted of mostly detritus as seen in the chlorophyll concentration vs. beam c plots and supported by partitioned ac-9 absorption data, which will be presented later. Hurricane Edouard resuspension was locally driven by currents and waves, and Hurricane Hortense resuspension was remotely driven by waves, with little or no influence by the currents.