Wave and Bottom Sediment Interactions over a Submerged Sand Bank during the Winter Cold-Front Season, Western Louisiana

Transcription

1 Wave ad Bottom Sedimet Iteractios over a Submerged Sad Bak durig the Witer Cold-Frot Seaso, Wester Louisiaa Daijiro Kobashi 1, Felix Jose 2, ad Gregory Stoe 1 1 Coastal Studies Istitute ad Departmet of Oceaography ad Coastal Scieces, Louisiaa State Uiversity, Bato Rouge, LA Coastal Studies Istitute, Louisiaa State Uiversity, Bato Rouge, LA ABSTRACT The Louisiaa coast is experiecig severe coastal lad losses due to geological processes ad huma itervetio. Repleishig the eroded beaches ad the barrier islads with sad from offshore borrow sites is a plausible way to restore them. To assess the potetial effects of sad miig from these offshore baks, i terms of physical processes, a field survey was coducted o March 2004 at Sabie Bak, 15 km offshore i wester Louisiaa. Some of the data from the survey are preseted here. The results show that waves strogly affect the bottom sedimet re-suspesio rather tha currets, durig cold frot ad post-frotal phases that occurred durig the deploymet. Shear velocity ad turbidity have a positive correlatio, although the liear coefficiet of determiatio (R 2 ) was Maximum ormalized cross-correlatio coefficiet betwee the two parameters was The above results suggest that the waves greatly affect the bottom sedimet ad both parameters have some degree of positive correlatio, which is ot ecessarily liear. The o-liear correlatio may be associated with bottom roughess ad sedimet type as well as wave parameters. A simplificatio of the relatioship usig a stability parameter is also examied. INTRODUCTION Louisiaa s coast has uique characteristics i terms of a low eergy wave eviromet, high lad subsidece, ad frequet cold frots i witer ad hurricaes i summer (Wright et al., 1997; Friedrichs et al., 2000; Stoe, 2000; Stoe et al., 2004). Historical research shows that alog the Louisiaa coast, waves are too weak to re-susped sedimet durig fair weather; durig cold frots sedimet re-suspesio ca occur (Wright et al., 1997; Friedrichs et al., 2000). Over the decades, the Louisiaa coast has bee udergoig severe coastal lad loss due to sedimet compactio, sedimet deficit from the Mississippi River ad its distributaries, wave/wid erosio, ad hurricae overwash (Pelad et al., 1988; Roberts, 1997). Oe of the possible solutios to prevet coastal lad loss is to supply sad from potetial offshore sites (Drucker et al., 2004; Stoe et al., 2004). Sabie Bak, which is a remat of palaeo-beach ridges, submerged due to rapid sea-level rise durig the late Holocee, is thought to be oe of the potetial offshore sad borrow sites (Morto ad Gibeaut, 1995; Drucker et al., 2004; Rodriguez et al., 2004). However, submerged sad baks are thought to reduce the eergy of approachig waves ad storm surges, ad also serve as a ursery for bethic habitats (Stoe et al., 2004; U.S. Geological Survey, 2003). Therefore, to as- Kobashi, D., F. Jose, ad G. Stoe, 2006, Wave ad bottom sedimet iteractios over a submerged sad bak durig the witer cold-frot seaso, wester Louisiaa: Gulf Coast Associatio of Geological Societies Trasactios, v. 56, p

2 Kobashi et al. Figure 1. Map of Sabie Bak ad our observatio statios. sess the potetial effects of sad miig, i terms of physical processes, a field survey of waves, currets ad sedimet trasport was udertake for a period of 32 days i March, 2004 at Sabie Bak, to determie wavebottom sedimet iteractios, which had ot previously bee udertake i this area (Fig. 1). The objective of this paper is to preset the characteristics of waves ad bottom sedimet iteractios over a shallow sad bak, Sabie Bak, based o the field survey. THREE DISTINCT METEOROLOGICAL PATTERNS Meteorological data obtaied from a earby Natioal Data Buoy Ceter (NDBC) buoy statio, SRST2, show that there appear to be three distict meteorological patters: fair weather, cold frots ad post-frotal phases (Kobashi et al., 2005). Betwee October ad May, frequet cold-frot passages (i.e., times per seaso) occur alog the orther Gulf of Mexico (Roberts et al., 1989). Typically wids blow from souther quadrats durig fair weather, but oce cold frots pass the area, wid directio dramatically chages from south to orth, wid speed icreases, ad air temperature drops (Stoe, 2000). Durig our survey period, 11 cold frots passed the survey area. SIGNIFICANCE OF WAVES ON BOTTOM SEDIMENTS DURING COLD FRONTS AND POST-FRONTAL PHASES Time series of wids, waves, currets, optical backscatter sesor (OBS) data represetig turbidity, shear velocity ad shear stress due to waves are give i Figure 2. I geeral, waves are strogly associated with the local wid field. Variace of sigificat wave height (H S ) was high durig high-wid evets such as cold frots 396

3 Wave ad Bottom Sedimet Iteractios over a Submerged Sad Bak durig the Witer Cold-Frot Seaso, Louisiaa Figure 2. Time series plots of (a) wid, (b) variace of HS, (c) variace of curret speed, (d) shear velocity (U*), (e) shear stress (τw), ad (f) turbidity (OBS). Secod ad third figures show the variaces at the middle statio (dash-dotted lie) ad at the oshore statio (sigle lie). Red shaded triagles show the passages of cold frots. The lie parallel to x-axis at the fifth figure shows the threshold of sedimet re-suspesio based o Shields parameters. 397

4 Kobashi et al. (red shaded triagles i Figure 2) ad post-frotal phases (double arrow i Figure 2) at both oshore ad middle statios (Kobashi et al., 2005). H S was also correlated with turbidity (OBS) data ad cosequetly shear-stress values that exceed the threshold of sedimet re-suspesio were also associated with high turbidity. Both bottom currets at the oshore statio ad currets at 1 m above the bottom (ab) at the middle statio were associated with wids ad waves; however, the relatioship betwee wids ad currets at the middle statio was ot as clear as that at the oshore statio. Shear stress due to waves is associated with the three meteorological coditios. Shear stress values exceeded the threshold of sedimet re-suspesio durig cold frots ad post-frotal phases ad was lower tha the threshold durig fair weather. However, it is oted that ot all of the cold frots cause higher shear stress tha the threshold, but that depeds o the itesity ad the duratio of the storms. WAVES AND BOTTOM SEDIMENT INTERACTIONS I Figure 3, a time series of shear velocity ad OBS data obtaied durig cold frots ad post-frotal phases are preseted. These two parameters are strogly correlated; high turbidity occurs durig the period of high shear velocity. However, there exist time lags betwee these two variatios (see Sheremet et al., 2005). The correlatio betwee turbidity ad shear velocity is show i Figure 4 (left). It is apparet that there is a positive correlatio betwee the two, although ot liear. A coefficiet of determiatio of 0.43 was calculated. The maximum ormalized cross-correlatio coefficiet is Sedimet cocetratio is thought to be related to sedimet type, bottom bed forms as well as wave-iduced shear velocity. Vicet ad Gree (1990) proposed that sedimet re-suspesio is associated with wave-iduced thresholds ad vortex sheddig from a rippled bed. Gle ad Grat (1987) show a detailed theoretical approach of sedimet cocetratio ad waves. The relatioship ca be characterized usig a stability parameter (z/l), ad the parameter ca be described as the sum of the sedimet cocetratio ad sedimet parameters for each sedimet class at each depth level as (Gle ad Grat, 1987; Vicet ad Gree, 1990). z L = κ U 3 * g( S 1) * w f * C ( z) (1) where z/l is stability parameter, L is Moi-Obukhov legth, κ is vo-karma costat (= 0.4), g is gravitatioal acceleratio (9.8 m/sec 2 ), U * is shear velocity, S is relative desity of sedimet i class, w f is settlig velocity of the sedimet i class, ad C(z) is sedimet cocetratio (SSC) at a certai depth (z). Here, κ, g, S, ad w f are time-idepedet ad kow values for each class of sedimet. Turbidity ad SSC have a positive liear correlatio (Sheremet et al., 2005). I additio, i the atmospheric surface layer, the Moi-Obukhov legth is ofte assumed to be costat (Gle ad Grat, 1987). Therefore, if these coditios ca be applied ad values for each sedimet class at each depth level are kow, Equatio 1 ca be trasformed as follows. OBS = a * U 3 * 1 a = κ g( S 1 ) w (2) f z L (3) where a is a kow value for each sedimet class at each depth level. Equatio 2 shows that the OBS value ca be represeted as a fuctio of a ad the shear velocity. Figure 4 (right) shows the relatioship betwee the OBS ad the shear velocity icludig parabolas with differet arbitrary a values. From Equatio 3, higher a values suggest lower sedimet desity (S), lower settlig velocity (w f ), ad high stability parameter (z/l). Also, higher a values show higher turbidity for the same shear velocity (Fig.4, right). Gle ad Grat (1987) oted that higher settlig velocity has lower sedimet cocetratio ad vice versa. From Equatios 2 ad 3, higher a values suggest low settlig velocity ad have high turbidity which is proportioal to SSC. Therefore, our hypothesis follows the iterpretatio of Gle ad Grat (1987). It should be reiterated, however, that these data ad aalyses are prelimiary ad serve as a precursor for more detailed future work. 398

5 Wave ad Bottom Sedimet Iteractios over a Submerged Sad Bak durig the Witer Cold-Frot Seaso, Louisiaa Figure 3. Time series plots of (a) shear velocity (U * ) ad (b) turbidity (OBS). Figure 4. The relatioship betwee shear velocity (U * ) ad turbidity (OBS) with liear regressio lie (left) ad hypothetical parabolas with arbitrary a values (right). 399

6 Kobashi et al. CONCLUSIONS The followig coclusios ca be draw from the above described aalysis: Sigificat wave height is correlated with turbidity (OBS) depedig o the itesity ad the duratio of the storms. Shear stress due to waves was higher tha the threshold for sedimet re-suspesio durig cold frots ad post-frotal phases. Shear velocity ad turbidity have a positive correlatio although the liear regressio coefficiet of determiatio (R 2 ) was 0.42; the maximum ormalized cross-correlatio coefficiet was The above results suggest that waves durig severe weather coditios are more effective i sedimet resuspesio tha currets. Also, sedimet re-suspesio is likely to occur oly durig cold frots ad post-frotal phases durig our survey period. The correlatios betwee shear velocity ad turbidity are ot liear ad SSC which is proportioal to turbidity is iferred to be proportioal to a parabola oce other parameters which ca be kow for each sedimet class ad each depth level are determied. ACKNOWLEDGMENTS This research is supported by the U.S. Mierals Maagemet Service, U.S. Departmet of Iterior, uder the project, Wave-Bottom Iteractio ad Bottom Boudary Layer Dyamics i Evaluatig Sad Miig at Sabie Bak for Coastal Restoratio, Southwest Louisiaa. The authors would like to thak the Coastal Studies Istitute Field support group for ivaluable help durig the field survey. REFERENCES CITED Drucker, B. S., W. Waskes, ad M. R. Byres, 2004, The U.S. Mierals Maagemet Service Outer Cotietal Shelf Sad ad Gravel Program: Evirometal studies to assess the potetial effects of offshore dredgig operatios i federal waters: Joural of Coastal Research, v. 20, o. 1, p Friedrichs, C. T., L. D. Wright, D. A. Hopworth, ad S. C. Kim, 2000, Bottom-boudary-layer processes associated with fie sedimet accumulatio i coastal sea ad bays: Cotietal Shelf Research, v. 20, p Kobashi, D., F. Jose, ad G. W. Stoe, 2005, Hydrodyamics ad sedimetary resposes withi bottom boudary layer: Sabie Bak, wester Louisiaa: Gulf Coast Associatio of Geological Societies Trasactios, v. 55, p Gle, S. M., ad W. D. Grat, 1987, A suspeded sedimet stratificatio correctio for combied wave ad curret flows: Joural of Geophysical Research, v. 92, o. C8, p. 8,244-8,264. Morto, R. A., ad J. C. Gibeaut, 1995, Physical ad evirometal assessmet of sad resources: Sabie ad Heald baks, Secod phase : Texas Bureau of Ecoomic Geology, Austi, 244 p. Pelad, S., R. Boyd, ad J. R. Suter, 1988, Trasgressive depositioal systems of the Mississippi delta plai: A model for barrier shorelie ad shelf sad developmet: Joural of Sedimetary Petrology, v. 58, o. 6, p Roberts, H. H., 1997, Dyamic chages of the Holocee Mississippi River delta plai: The delta cycle: Joural of Coastal Research, v. 13, o. 3, p Roberts, H. H., O. K. Huh, S. A. Hsu, R. J. Rouse, ad D. A. Ruckma, 1989, Witer storm impacts o the cheier plai coast of southwester Louisiaa: Gulf Coast Associatio of Geological Societies Trasactios, v.39, p Rodriguez, A. B., J. B. Aderso, F. P. Siriga, ad M. Taviai, 2004, Holocee evolutio of the east Texas coast ad ier cotietal shelf: Alog-strike valiability i coastal retreat rates: Joural of Sedimetary Research, v. 74, o. 3, p

7 Wave ad Bottom Sedimet Iteractios over a Submerged Sad Bak durig the Witer Cold-Frot Seaso, Louisiaa Sheremet, A., A. J. Mehta, B. Liu, G. W. Stoe, 2005, Wave-sedimet iteractio o a muddy ier shelf durig Hurricae Claudette: Estuarie, Coastal ad Shelf Sciece, v. 63, p Stoe, G. W., 2000, Wave climate ad bottom boudary layer dyamics with implicatios for offshore sad miig ad barrier islad repleishmet i south-cetral Louisiaa: OCS Study MMS , U.S. Departmet of the Iterior, Mierals Maagemet Service, Gulf of Mexico OCS regio, New Orleas, Louisiaa, 90 p. Stoe, G. W., D. A. Pepper, J. Xu, ad X. Zhag, 2004, Ship shoal as a prospective borrow site for barrier islad restoratio, coastal south-cetral Louisiaa, USA: Numerical wave modelig ad field measuremets of hydrodyamics ad sedimet trasport: Joural of Coastal Research, v. 20, o. 1, p U.S. Geological Survey, 2003, Assessmet of fish commuities associated with offshore sad baks ad shoals i the orthwester Gulf of Mexico: U.S. Geological Survey Cruise Report, 17 p. Vicet, C. E., ad M. O. Gree, 1990, Field measuremet of the suspeded sad cocetratio profiles ad fluxes ad of the resuspesio coefficiet γ0 over a rippled bed: Joural of Geophysical Research, v. 95, o. C7, p. 11,591-11,601. Wright, L. D., C. R. Sherwood, ad R.W. Sterberg, 1997, Field measuremets of fair-weather bottom boudary layer processes ad sedimet suspesio o the Louisiaa ier cotietal shelf: Marie Geology, v. 140, p

8 402 NOTES