NEARSHORE WAVE AND SEDIMENT PROCESSES: AN EVALUATION OF STORM EVENTS AT DUCK, NC

Transcription

1 NEARSHORE WAVE AND SEDIMENT PROCESSES: AN EVALUATION OF STORM EVENTS AT DUCK, NC By JODI L. ESHLEMAN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2004

2 Copyright 2004 by Jodi L. Eshleman

3 This thesis is dedicated to my parents, who provided unconditional support throughout my entire education.

4 ACKNOWLEDGMENTS I would like to thank the Army Corps of Engineers Field Research Facility (FRF) in Duck, NC for providing the data used in this investigation. I extend my greatest appreciation to the staff at the FRF, who gave me the opportunity to gain some valuable field experience and were always willing to offer advice and encouragement. I acknowledge specifically Kent Hathaway, Chuck Long, and Bill Birkemeier, whose input was vital to this research. I thank all of the FRF for spending countless hours helping me with everything from analyzing data through interpretation. I would also like to thank Rebecca Beavers for taking the time to provide additional sediment data. I thank my supervisory committee chair (Dr. Robert G. Dean) for his continual support and encouragement throughout this process, and for always finding time for my questions and concerns. I thank Dr. Robert Thieke for providing the teaching assistantship that allowed me to continue this research. I also thank Dr. Robert Thieke and Dr. Andrew Kennedy for serving on my supervisory committee. I also thank Jamie MacMahan: his patience and insight were invaluable assets to this investigation. iv

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS... iv LIST OF TABLES... vii LIST OF FIGURES... viii ABSTRACT... xi CHAPTER 1 INTRODUCTION...1 Study Location, Characteristics, and Instrumentation...2 Geographic Location...2 Wave and Weather Conditions...3 Bipod Instrumentation...4 Chapter Contents GENERAL NEARSHORE CHARACTERISTICS...8 Introduction...8 Quality Control...10 Data Screening...10 Representative Data...11 Analysis...11 Current Influence...12 Bipod Depth (m)...13 Bottom Change (cm)...13 Current (cm/s)...13 Wave Influence...17 Combined Waves and Currents...19 Current Direction...19 Wind...20 Variance of Total Current Acceleration...21 v

6 3 SEDIMENTS...24 Introduction...24 Analysis...26 Sediment Characteristics...26 Previous sediment data...26 Sonar evaluation...26 Velocity Profile Calculations...29 Critical shear velocity...30 Error estimates...33 Combined wave-current influence...34 Apparent hydraulic roughness WAVE TRANSFORMATION IN THE NEARSHORE...42 Introduction...42 Analysis...44 Development of Analytical Spectrum...44 Dataset...45 Development of Directional Spectrum from the Data...48 Determination of m Values...51 Comparison of Fourier coefficients...52 Two-sided nonlinear fit...53 Comparison of Data to Linear Wave Theory Calculations...57 Wave direction...60 Refracted m values...62 Wave height comparisons...65 Energy flux comparisons...66 Friction factor...68 Reynolds Stresses...72 Discussion CONCLUSIONS...77 LIST OF REFERENCES...81 BIOGRAPHICAL SKETCH...85 vi

7 LIST OF TABLES Table page 2-1 Erosion events of 3 cm or greater Event-Based comparison of erosion events of 3 cm or greater Theoretical Fourier coefficients for different m values Measured mean wave directions at peak frequency Average % energy loss values between bipods Friction factor estimates from bottom current meter Reynolds stresses for October vii

8 LIST OF FIGURES Figure page 1-1 Field Research Facility location Bipod instrumentation Bipod locations at initial deployment in November 1997 filtered mean current comparison with sonar May 1998 filtered mean current comparison with sonar October 1997 mean orbital velocity estimates vs. sonar measurements October 1997 cross-shore orbital velocity estimates vs. sonar measurements Wind vectors measured at the Field Research Facility October 1997 current-wind comparison November 1997 current-wind comparison Median grain size variation with water depth X-ray images of boxcores Sonar histogram at 13 m bipod, August 30, Sonar histogram at 5.5 m bipod, August 12, Sonar histogram at 8 m bipod, August 31, Sonar histogram at 13 m bipod, August 29, Shield s curve Shear velocity vs. sonar at the 13 m bipod, October 18, Shear velocity vs. sonar at the 8 m bipod, August 19, Shear velocity vs. sonar at the 5.5 m bipod, October 18, viii

9 3-11. Surface roughness variation with mean currents at 5.5 m bipod for October 15-21, Surface roughness variation with mean currents at 8 m bipod for October 15-21, Surface roughness variation with mean currents at 13 m bipod for October 15-21, Coordinate system Ratios of Fourier coefficients Significant wave heights measured in October 1997, November 1997, May 1998, and August Bathymetry in vicinity of bipod instrumentation Measured spectra for November 7, 1997 time= Error versus m value comparison for October 20, 1997 time= Comparison of measured and best-fit spectra from matching coefficients for October 20, 1997 time= Error between spectra fitted from coefficient ratios for October 20, 1997 time= Comparison of measured and best-fit spectra from curve fitting for October 20, 1997 time= Error between spectra from curve fitting for October 20, 1997 time= Variation of m values with frequency range at 8 m Directional spectrum variation with frequency at 8 m bipod for October 19, 1997 time= Energy density spectral values for October 19, 1997 time= Mean wave direction comparison for October 19, 1997 time= Measured and calculated wave direction differences for October 19, 1997 time=700 at 5.5 m bipod Average measured and calculated wave direction differences for October Measured and refracted directional spectra for November 13, 1997 time= ix

10 4-18 Comparison of refracted and measured m values over frequency range for November 13, 1997 time= Significant wave height ratios versus cross-shore position Average measured and predicted energy flux values Surveyed bathymetry in vicinity of bipod instrumentation Histogram of calculated friction factors at all current meters Friction factor variation with wave height at the bottom current meter...74 x

11 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science NEARSHORE WAVE AND SEDIMENT PROCESSES: AN EVALUATION OF STORM EVENTS AT DUCK, NC By Jodi L. Eshleman May 2004 Chair: Robert G. Dean Major Department: Civil and Coastal Engineering Pressure, sonar, and current measurements were recorded at 5.5 m, 8 m, and 13 m water depths in the outer surf zone and inner continental shelf region off the coast of Duck, NC. This unique data set was analyzed to investigate erosion thresholds and wave evolution. A mean current threshold of 20 cm/s and combined wave and current threshold of 60 cm/s were identified for bed elevation decrease. Shear velocity was determined to be a good indicator of bottom elevation change at the 8 m and 13 m bipods, with erosion beginning 0 to 3 hours after it crossed a movement threshold of 1.17 cm/s. Surface roughness estimates at these same two water depths decreased with increasing mean currents. The combination of measured near bottom pressure and horizontal velocity components provides the basis for determining a directional spectrum. A simplified analytical directional spectrum based on a single cosine curve of varying power (m) was used to approximate these measured directional spectra. A nonlinear least squares curve xi

12 fit to each side of the measured directional spectrum proved the most accurate method of determining the best representation of m values. Refracted mean wave directions were slightly overestimated by the theory and the decrease in width of the directional spectra with decreasing water depth was overestimated. Also, energy flux calculations combining shoaling and refraction theory showed smaller measured than predicted energy flux values with inshore distance (sometimes by more than one third) emphasizing the importance of considering energy loss in calculations for engineering design and planning. A representative friction factor for each record was determined by accounting for frictional energy loss in the energy flux calculation, using velocity time series measured at the bottom current meter. Calculated friction factors varied throughout storm events, but most fell within a range of 0 to 0.2. A representative value of 0.17 was identified for this location through the use of average energy flux and energy loss values over all storm events. Reynolds stresses were calculated and were found to be consistently different at the current meter at 0.55 m elevation, a result that remains unexplained. xii

13 CHAPTER 1 INTRODUCTION The inner continental shelf off the open mid-atlantic coast is a wave-driven environment, where sediment transport and nearshore circulation are primarily forced by wind-generated ocean surface waves (Wright 1995). This is a friction-dominated region, where boundary layers may occupy the entire length of the water column, transmitting effects of wind blowing on the water surface to the seabed (Wright 1995). Wave propagation is largely characterized by transformation through refraction, diffraction, energy dissipation, and shoaling. Mean currents are another important component in the nearshore zone, and can be driven by waves, wind, tides; and gradients in pressure, temperature, and density, among other things. Within this dynamic environment, sand movement is not uniform in all directions and at all locations. Harris and Wiberg (2002) suggest that gradients in bed shear stress may create gradients in suspended sediment flux. These cross-shelf gradients in sediment flux will in turn create cross-shelf gradients in sediment size as the higher orbital velocities on the inner shelf move finer sediment offshore (Harris and Wiberg 2002). Alongshore sediment flux is also an important component of the sediment transport discussion. Beach and Sternberg (1996) found that alongshore sediment flux is dependent on breaker type, and this information should be incorporated into sediment transport models. Their measurements showed that plunging waves were responsible for the greatest portion of suspended load and sediment flux, but other breaker types and 1

14 2 nonbreaking waves combined still contributed almost half of the total suspended load and sediment flux (Beach and Sternberg 1996). There is variation in sand transport throughout the water column as well. Many investigators have found an inverse relationship between distance above the bed and suspended sediment concentration (Beach and Sternberg 1996; Conley and Beach 2003). A study conducted in the surf zone during the SandyDuck experiment showed that the increasing importance of wave-driven transport near the bed might lead to a reversal in the net cross-shore transport direction in the water column. The directions of transport at the bed may dominate even if much of the water column has an opposing transport direction since more than half of the depth-integrated net transport occurs within 5 cm of the bed (Conley and Beach 2003). Study Location, Characteristics, and Instrumentation Geographic Location Field data were obtained on the inner continental shelf off the coast of the Army Corps Field Research Facility (FRF) in Duck, North Carolina. The FRF facility is located on the Outer Banks of North Carolina, on the central portion of the Currituck Spit, which extends southeast continuously for over 100 km from Cape Henry, Virginia to Oregon Inlet, North Carolina (Figure 1-1). It is located in the southern portion of the Middle Atlantic Bight ( N; W) and bordered by Currituck Sound, a low-salinity estuarine environment, on the west; and the Atlantic Ocean on the east. Ocean tides are semi-diurnal, with a mean range of approximately 1 m (Birkemeier et al. 1981).

15 3 Figure 1-1. Field Research Facility location (from Wave and Weather Conditions Wave heights vary seasonally along the Outer Banks, with peak waves occurring in October and February, and mild conditions prevailing in late spring and early summer months (Birkemeier et al. 1981). A compilation of wave statistics for the time period of 1985 through 1995 resulted in an average annual wave height of m, and a mean annual wave period of s (Leffler et al. 1998). There have been many observations of water masses that interact with currents in the area, including low salinity slugs from the Chesapeake Bay and warm, clear Gulf Stream currents (Birkemeier et al. 1981).

16 4 The majority of storm events that affect the Atlantic coast originate in the middlelatitude westerly wind belt and are often termed extra-tropical (Dolan et al. 1988). Tropical storms, including hurricanes, also affect the region, but less frequently. Dolan et al. (1992) discuss the importance of extra-tropical storms for erosion and note that they often generate wave heights that are comparable to or greater than those from hurricanes. A study that examined 1,349 northeast storms on the Atlantic coast found a distinct seasonality of frequency and duration, with maximum values in the winter and minimum in the summer (Dolan et al. 1988). When specifically examining extra-tropical storms, the most significant contribution to erosion is from northeasters. Xu and Wright (1998) determined that even though comparable winds from the southerly directions sometimes caused high wave heights, the alongshore current magnitudes recorded during these storms were only one-fifth of those achieved during northeasters. Cross-shore current magnitudes were also smaller, but differences were not as large as for alongshore currents (Xu and Wright 1998). Bipod Instrumentation The initial bipod instrumentation was deployed in October 1994 as part of a multi-year monitoring program to study shore-face dynamics on the inner continental shelf of the Field Research Facility in Duck, NC (Howd et al. 1994). The instrumentation consisted of three current meters at varying elevations, a pressure sensor, and a sonar altimeter, which were all attached to a bipod frame, secured by two 6.4 m pipes jetted vertically into the seabed (Beavers 1999). The original bipods collected data until October 1997 using three Marsh-McBirney electromagnetic current meters, which often experienced significant noise. The current meters were replaced with Sontek Acoustic Doppler Velocimeters for the SandyDuck experiment in October 1997, and the bipods

17 5 were redeployed at depths of 5.5 m, 8 m, and 13 m relative to NGVD. They remained operational at these three simultaneous locations through December The data used in this analysis were collected during this second deployment. Figure 1-2 shows the bipod setup where A, B, and C are electronic housings; P is the pressure sensor; and S the sonar altimeter. General bipod locations at the time of initial deployment in 1994 are pictured with respect to local bathymetry in Figure 1-3. Figure 1-2. Bipod instrumentation A,B,C = electronic housings, P is pressure sensor, and S is sonar altimeter (from Beavers 1999) The bipod packages each contained three SonTek Acoustic Doppler Velocimeters (ADV), which sampled at 2 Hz and were located at elevations of 0.2 m 0.55 m and 1.5 m above the sea floor. The end of the frame containing the current meters was oriented toward the southeast to minimize interference of current meters and vertical supports with orbital velocity measurements, since storm events of interest would have primarily northeast waves (Beavers 1999). Digital Paroscientific gauges were used for pressure measurements, operating at 38 k Hz and a sampling rate of 2 Hz (Beavers 1999). A Datasonics PSA-900 sonar altimeter was used to record bottom elevation. The range was

18 6 modified from 30 m to 3 m to increase the resolution, sampling at 1 Hz with a beam frequency of 210 khz (Beavers 1999). Tests by Green and Boon (1988) of response characteristics found this model of altimeter to be accurate to one centimeter. Current meter and pressure data are output in 34-minute segments, with a 10-minute break in data every 3 h. Average values for each record represent a mean value for a 34-minute burst. Sonar measurements are determined through a histogram filtering technique, taking the highest bin value for the burst. Figure 1-3. Bipod locations at initial deployment in 1994 (from Beavers 1999) The lowest ADV records bottom measurements for a duration of 9 min every three hours to provide a second measurement of bottom elevation (Beavers 1999). This beam has a frequency of 4 MHz and a beam width of one degree, creating a 2 cm diameter footprint; whereas, the sonar altimeter operates at a frequency of 210 khz and has a 10-degree beam width, creating a 20 cm diameter footprint (Beavers 1999). The differences in beam frequency and footprint size provide different optimum operating

19 7 conditions. Beavers (1999) suggests that the ADV is more reliable under non-storm conditions and the sonar altimeter provides a better estimate of bottom elevation under storm conditions, when suspended sediment concentration within the water column is high. Chapter Contents The purpose of this investigation is to examine sediment movement and wave evolution within the inner continental shelf and outer surf zone region through the analysis of field measurements. Chapter 2 focuses on determining general relationships between waves, currents, and sonar measurements. Chapter 3 describes sediment characteristics, and discusses some aspects of bottom roughness by analyzing velocity profiles. Chapter 4 is the heart of the investigation and utilizes the data collected at all three bipod locations to provide a comparison with analytical predictions of evolution of wave characteristics.

20 CHAPTER 2 GENERAL NEARSHORE CHARACTERISTICS Introduction Many researchers have attempted to establish a relationship between statistical properties of velocity measurements and sediment movement. Although there have been some velocity moments that have seemed more relevant than others, there does not seem to be any one parameter that shows a consistent significant correlation to sediment transport. A study by Guza and Thornton (1985) examined velocity moments from measurements at Torrey Pines Beach in San Diego, CA and found that oscillatory asymmetries and combined current-wave variance terms are significant to cross-shore transport. Several studies have shown that the oscillating velocity terms move sediment onshore and the mean velocities move sediment offshore (Guza and Thornton 1985; Osborne and Greenwood 1992; Conley and Beach 2003). Measurements of sandbar migration at Duck, NC showed maximum values of velocity asymmetry and acceleration skewness near the bar crest (Elgar et al. 2001). Hoefel and Elgar (2003) found that extending an energetics model to include fluid accelerations resulted in better predictions of onshore bar migration between storms. Velocity measurements taken in the surf zone during SandyDuck showed no significant correlation between velocity moments and wave driven transport, although acceleration skewness showed the strongest relationship (Conley and Beach 2003). These suggest that velocity asymmetry and acceleration skewness seem to have the strongest ties to sediment transport in past experimental results. 8

21 9 There has been previous work with similar instrumentation done at this location. Several studies included instrumented tripods deployed during storm and fair-weather conditions, which also included suspended sediment measurements (Wright et al. 1986; Wright et al. 1991; Wright et al. 1994). The first tripod deployment was at a single depth and did not show a relationship between bed level changes and increased mean or orbital velocities. There was a gradual change in the bottom elevation throughout the middle and final stages of the storm, followed by significant accretion that was hypothesized to be the result of a migrating bedform. Suspended sediment measurements did not show a response to the onset of the storm or peak with mean currents, but peaked with oscillatory flow, suggesting that waves are the dominant source of sediment resuspension (Wright et al. 1986). The second study consisted of three separate deployments at the Field Research Facility and the results suggested that it is the near-bottom mean flows, not oscillatory components that play the dominant role in transporting suspended sediment (Wright et al., 1991). Mean flows may also play a role in the direction of sediment movement. A study conducted at Duck showed the tendency of a mean cross-shore velocity threshold around 30 or 40 cm/s directed offshore to be the divider between landward and seaward bar migration (Miller et al. 1999). Both mean and oscillatory flows are essential to the analysis and they are not independent. The wave boundary layer creates resistance for the current above and slows down that flow. Waves are often thought to be more efficient at initiating motion, whereas currents are more efficient at net transport, but the two interact nonlinearly (Grant and Madsen 1976). There are two critical differences between previous studies and this dataset. These include the length of time and number of instrumentation packages deployed.

22 10 Many other studies have included one or two instrument packages deployed simultaneously for individual storm events or short periods of fair-weather conditions, but not three instrument packages with continuous measurements for this length of time. Some previous investigations have been carried out with this specific dataset that focused on the sonar data. Sonar Altimeter measurements were compared to surveyed profiles to discuss discrepancies in depth of closure concepts. The predicted depth of closure was around 8 m, yet for some storm events, the 13 m bipod showed the greatest change in bottom elevation. Net and range of seabed elevation changes were examined during storm events that were defined by wave thresholds. Finally, a comparison of sonar records to diver collected boxcores served to validate the sonar record and showed the sonar was capable of monitoring long-term bottom stratigraphy (Beavers 1999). The current and sonar measurements were also used as forcing and validation for a bottom boundary layer and sedimentation model (Keen et al. 2003). These analyses have shown some interesting relationships, but have neglected a major component of the dataset: the current measurements. The first level of this analysis focuses on the currents and the manner in which they affect the bottom during all weather conditions, not just storm events. Quality Control Data Screening Different levels of screening were applied in an attempt to eliminate noise and assure that the measurements presented here are representative of actual conditions in the nearshore environment. Spikes were removed using polynomial interpolation. Beam correlation and intensity values output by the ADV were used to identify low quality data. The second level of data screening was accomplished through determining several

23 11 quality control parameters for each record. The quality control parameters included signal-to-noise ratios for current and pressure measurements, and a z test value based on a ratio of the wave heights calculated from pressure and current measurements. Data with signal-to-noise ratios less than 1.5 or z test values outside the range of 0.5 to1.5 were not used. Representative Data The following analysis is based on four months during which the described data standards were satisfied. These months: October 1997, November 1997, May 1998 and August 1998 were chosen for several reasons. They include a nearly complete data set that has been successfully edited. They have z-test values near 1, signal to noise ratios of 2 or higher, and wave directions that are consistent for all three current meters, suggesting that biofouling and problems with current meter rotation were minimal. They have bottom measurements from the lowest current meter recorded every three hours, so that trends in the sonar measurements can be validated. In addition to data quality, they encompass significantly different seasonal variations. Measured significant wave height values range from less than 1 m to almost 4 m, spanning storm and mild weather conditions. It is important to note that the data from these months includes some problems, but knowledge of data quality can be combined with analysis techniques to obtain results that account for the limitations of the data. Analysis Our knowledge of the dynamics of the nearshore system leads to the recognition that no single statistical property can explain sediment movement. Sand transport is governed by a complex combination of many factors. The following discussion of

24 12 erosion refers to bed elevation decrease, rather than transport initiation, which cannot be measured by the available instrumentation. The continuity equation gives the following relationship between bed elevation, z, and gradients in cross-shore, alongshore and vertical sediment transport components at the bed, q x, q y, and q z respectively. dz q q q x y z ( 1 p) = + + dt x y z where p represents the porosity. It is important to recognize that it is possible to have sediment transport without bed elevation change; however, if the bottom sediment is suspended or the gradient in cross-shore or alongshore sediment transport is positive, the bottom elevation will decrease. This initial analysis is an attempt to discern which properties appear to play a more significant role when considered individually. Current Influence There appears to be a mean current threshold of approximately 20 cm/s for erosion in most of the data. There is usually some erosion when the total mean current reaches 20 cm/s, yet there may be erosion for smaller currents. The currents were filtered with a cutoff of one day to remove tidal influences and to facilitate a comparison with sonar altimeter data. Table 2-1 shows events of bed elevation decrease of 3 cm or greater and the associated currents. There are many times when the mean current is very high and the bottom change is small and vice versa, indicating the possibility that other forces may be involved. One important thing to note is that often times erosion occurs when significant wave heights are fairly low. 54% of the erosion events identified in Table 2-1 occurred when the significant wave height was less than 2 m, which is often considered as the threshold between storm and calm conditions. This reinforces the need to examine

25 13 Table 2-1. Erosion events of 3 cm or greater Bipod Depth Bottom Change Current H mo Time since last event (m) (cm) (cm/s) (m) T (s) (days) Oct Nov May Aug

26 14 the changes occurring during all types of conditions, since it is not necessarily during storm events that the sediment is moving. Table 2-2. Event-Based comparison of erosion events of 3 cm or greater Date Current (cm/s) Bottom Change (cm) Oct Oct Oct Oct Oct Nov Nov Nov Nov Nov Nov May May May May Aug Aug Aug Aug Aug Aug Aug Aug Aug Major erosion events appear to be fairly consistent between bipods, although the magnitudes of bottom elevation changes are usually different. Table 2-2 presents the data from Table 2-1 by date, to facilitate a comparison between bipods (- represents < 3 cm of bottom change). An interesting situation occurs in November 1997 and August of 1998, where the current reaches one of the maximum values for the month (above the 20 cm/s

27 15 threshold), but the erosion is not consistent at all three bipods. Both instances show significant erosion at the 5.5 m and 8 m bipods (ranging from 8-13 cm), and less than 3 cm of erosion at the 13 m bipod. Figure 2-1 shows one case occurring around November 24, Currents are positive onshore and south. Figure 2-1. November 1997 filtered mean current comparison with sonar a) 5.5 m b) 8 m c) 13 m A situation occurred in May of 1998 that seems to be the reverse of this last observation and occurs around May 13 (see Figure 2-2). The currents were at their highest values for the month, in excess of 40 cm/s, causing minimal erosion at the 5.5 m and 8 m bipods and a more significant change at the 13 m bipod. For this situation, the ADV bottom measurement showed less erosion, indicating that there may have been fluffy material at the 13 m bipod that the sonar had trouble penetrating and the higher

28 16 frequency ADV picked up. Here we see the 5.5 m and 8m bipods showing significant erosion at the onset of currents over 20 cm/s, but then the erosion leveled off as the currents continued to increase. The 13 m bipod did not show as much erosion initially, but as currents continued to increase there was a spike where the sonar based bottom elevation dropped 15 cm. If this sediment was of the finer type that is sometimes seen at 13 m, a higher current may have been required for movement due to cohesive properties, and once the current reached that threshold, the entire layer moved. The cause of this cannot be explained with certainty without more detailed sediment information. Figure 2-2. May 1998 filtered mean current comparison with sonar a) 5.5 m b) 8 m c) 13 m

29 17 Wave Influence The root mean square current speeds were examined in different frequency ranges. These values were multiplied by the square root of two to obtain the amplitudes as substitutes for orbital speeds and represent the significant values for orbital speed. This should provide some insight into which frequency components contribute the most. A high frequency range from 0.04 to 0.35 Hz was identified to examine sea swell and a low frequency range of to 0.04 Hz to investigate any infragravity contributions. Figure 2-3. October 1997 mean orbital velocity estimates vs. sonar measurements a) 5.5 m b) 8 m c) 13 m (solid black line represents sonar) The infragravity orbital speed was always smaller, but reached 20 cm/s at the 5.5 m bipod during storm events. Results for October 1997 are presented in Figure 2-3, which shows

30 18 a peak at the maximum erosion event for the month, and slight increase for smaller erosion and accretion events. Figure 2-4. October 1997 cross-shore orbital velocity estimates vs. sonar measurements a) 5.5 m b) 8 m c) 13 m (solid black line represents sonar) This suggests that the infragravity component may be significant for this dataset. This is an interesting observation because other studies have had conflicting results for this location in the past. Wright et al. (1994) took similar measurements at this location at a depth of 13 m during the Halloween storm of 1991 and showed a significant infragravity component, reaching 20 cm/s near the peak of the storm, which is similar to these findings. They suggest that roughly half of the infragravity energy emanates from the surf zone (Wright et al. 1994). An earlier attempt to quantify wave reflection found no significant quantity of long wave energy, either incident or reflected from

31 19 measurements taken at a depth of 6.5 m (Walton 1992). One possible explanation for this difference may be a difference in significant wave heights, since values recorded during this study were never greater than 3.5 m and those recorded during the Halloween storm reached 6.5 m. The plot of the cross-shore components (Figure 2-4) shows that the cross-shore is the major component of the R.M.S seaswell velocity, which enforces the need to consider cross-shore velocity for sediment transport even when mean values are small. The current amplitude increased at most erosion events for the month, but not all. Combined Waves and Currents Another approach to considering erosion causes examined the combined wave and current maximum velocities. The sum of the amplitude (used to estimate orbital velocity) and the component of the mean current in the wave direction was calculated. These are all positive values since they were taken in the wave direction and negative or positive wave orbital velocities could cause erosion. This was an attempt to consider not just the mean current or wave orbital velocity, but their combined effect; however, this did not take into account any nonlinear interactions between waves and currents, but provided a rough estimate of combined velocity. This analysis did not show any consistent threshold between maximum velocity and sediment movement, but there was a relationship between the two. Very high combined velocities in the range of 60 to100 cm/s always seemed to be associated with erosion, but below this range the effect varied. Current Direction The maximum erosion events for the month always occurred with a southerly longshore current and usually a downwelling (seaward) flow in the cross-shore component. One example is around the 28 of August 1998 in which a northerly

32 20 longshore current reversed direction and caused the most significant erosion for the month. Wind Measurements of wind magnitude and direction are obtained from FRF wind gages 932 and 933. Figure 2-5 shows a vector plot of these values. Most peaks in mean longshore current velocity that are above the 20 cm/s threshold appear to coincide with peaks in longshore wind velocity. The notable exception to this is the month of November 1997, where scatter plots show a poor correlation between the longshore wind velocity and longshore mean current. Figure 2-5. Wind vectors measured at the Field Research Facility

33 21 The cross-shore currents showed no significant correlation to cross-shore wind. Currentwind comparisons for the month of October 1997 are provided in Figure 2-6. Current and wind measurements are positive onshore and south. Another study by Xu and Wright (1998) of wind-current correlation at this same general location has shown that it is dependent on wind direction. The correlations were broken into quadrants, and it was found that the current and wind speeds are most correlated with winds from the Northeast or Northwest direction, showing much higher R squared values than winds blowing from the Southeast or Southwest (Xu and Wright 1998). Variance of Total Current Acceleration There seems to be some relationship between the variance of the acceleration of the total current and bottom change on a month-to-month basis. This follows some previous observations discussed in the introduction, although the acceleration skewness did not show any significant trends. There is an increase in this variance at times of maximum erosion for the month; however, this same trend was seen when considering current variance. Erosion due to the acceleration variance cannot be distinguished from erosion due to the current variance, since they differ only by the square of the radial frequency.

34 22 Figure 2-6. October 1997 current-wind comparison

35 23 Figure 2-7. November 1997 current-wind comparison

36 CHAPTER 3 SEDIMENTS Introduction Past observations of bottom change from sonar altimeter measurements have often been supported by visual observation or other instrumentation since sonar measures only the elevation at a single point. One limitation of the analysis lies in the inability to distinguish whether sediment is moving as suspended load or bed load from a single sonar altimeter. It is possible to have sediment transport without elevation change, yet the sonar can only capture variation in the bed elevation. Topographic features moving, including bed forms or ripples, can also cause problems since they may be measured as the representative bed elevation, but their existence is localized. There have been observations of non-uniform topography in the vicinity of the bipod instrumentation. An array of seven sonar altimeters deployed in the surf zone during the SandyDuck experiment captured mega ripples which ranged from 15 cm to 30 cm high and moved through the sonar range in a period of about ten hours (Gallagher et al. 1998). The first tripod deployment by Wright et al. (1986) was supplemented by diver observations, pictures and side-scan sonar measurements. Side scan sonar images showed sediment lobes of fine material overlying coarser material which were up to one meter high and thought to be the cause of the rapid accretion at the 8 m tripod at the end of the storm event (Wright et al. 1986). Bottom features such as these are difficult, if not impossible to discern from sonar altimeter measurements alone. 24

37 25 An important parameter when discussing sediment transport in relation to bottom topography is the bed shear stress, or equivalently, the shear velocity, u = * b τ / ρ. The von Karman-Prandtl equation u c u ( z) * c 1 = ln κ z z 0 can be used to estimate shear velocity and hydraulic roughness length values from measurements of the mean current at different elevations. Madsen et al. (1993) calculated shear velocity and apparent hydraulic roughness from the log-profile method for the data collected at the 13 m tripod during the Halloween storm of These estimates showed shear velocity values on the order of 2 to 3 cm/s and apparent hydraulic roughness values generally between 0.1 cm and 1 cm. These values were found using data obtained with similar instrumentation and at a very similar location, therefore the values found in the subsequent analysis are expected to be the same order of magnitude. They found a value of 1.5 mm for the Nikuradse sand grain roughness for a movable flat bed (Madsen et al. 1993). Past analysis of the dataset examined in this thesis used diver-collected boxcores to supplement sonar measurements. Beavers (1999) discussed the geologic features of these cores and the correlation between core layers and sonar records in detail. The positive correlation between these two records indicates that scour around the pipes was not appreciable, which will be assumed in the following analysis. Shear stress calculations showed that when the shear stress is a maximum, seabed elevation decreases and when shear stress decreases, the seabed elevation increases (Beavers 1999).

38 26 Analysis Sediment Characteristics Previous sediment data Sediment data are available for the period from locations adjacent to the bipod instrumentation. Figure 3-1 compares median grain size versus elevation for this time period and implies little variation in sediment size at the 5.5 m and 8 m bipods. Most measurements remain within a ½ φ unit range. There is significantly more variation at the 13 m bipod with a 3/2φ unit range. The left panel of Figure 3-2 shows an x-ray of a boxcore taken at a water depth of 13.2 m on August 18, The white region shows a section where the material is too fine for the x-ray to register and grain size analysis determined a median φ value of 4.02 for this section, as opposed to 3.10 for the rest of the sediment column (Beavers 1999). The right panel of Figure 3-2 shows a boxcore taken in 1992 at a water depth of 14 m, which also shows a layer of fine, silty material (Nearhoof 1992). It is important to be aware of the sediment range at the 13 m bipod when considering sonar measurements, since there are instances when the sonar may have trouble recognizing fine, silty material. Sonar evaluation Bed form migration can register on sonar measurements and changes in elevation may reflect localized change from large-scale bed forms moving through the sonar footprint. Tests of ripple fields under a similar sonar altimeter showed that the ripple could not be resolved if its height above the bed is more than eight times its wavelength (Green and Boone 1988). The possibility of non-uniform topography is difficult to resolve and one method of addressing this was to examine sonar histograms. One notable

39 27 observation is that the raw sonar measurements were very noisy, although the outline of the bottom could generally be distinguished. A representative sonar value for each 34-minute record was taken as the max bin value of the histogram for that time series. The hypothesis was that at times when there was silty material at the 13 m bipod, the sonar might show two peaks, at the top and bottom of this layer. Another possibility was that during storms, the histograms may show more spread if the sonar was unable to consistently penetrate the suspended sediment in the water column Boxcore Samples ( ) -6 Profile Line 62 ( ) Elevation (m,ngvd) Duck 94 Samples-Oct Duck 94 Samples-Aug SandyDuck 97 Samples Median Grain Size (phi) Figure 3-1. Median grain size variation with water depth (data from FRF) Sonar histograms are included for each bipod location during the month of August In most instances, there is a very well defined peak at a specific value, and smaller peaks or spreading at depths less than this value (Figure 3-3). Secondary peaks are within a few centimeters of the major peak. There are times when this histogram

40 28 deteriorates and the peak is less well defined with a greater spread (Figure 3-4). These times do not always correlate with storm events as anticipated. A. B. Figure 3-2. X-ray images of boxcores A) h=13.2 m on August 18, 1997 (Beavers 1999) B) h=14 m in 1992 (Nearhoof 1992) At the 5.5 m and 13 m bipods there is rarely spreading at depths greater than this peak, which lends confidence to sonar estimates of the bottom where spreading is most likely an indicator of noise within the water column. One very interesting observation is that the 8 m bipod shows almost all of the spreading and secondary peaks in the histograms to be at depths greater than the histogram peak, as evident in Figure 3-5. This is consistently different from the other bipods and remains unexplained. Another interesting situation occurred on August 29, 1998 and lasted for approximately a day, showing the variation in sonar histogram values with time at the 13 m bipod. Here two distinct peaks occurred that are over 20 cm apart and are of similar magnitude. One peak is at m and another at m (Figure 3-6). This occurs immediately after the major storm event for the month. One possible explanation is that there is a layer of very fine sediment here and the sonar sometimes pings off the top and

41 29 sometimes penetrates the layer. An observation about this August storm event is that there are strong northerly alongshore currents reversing direction and reaching 100 cm/s in the southerly direction, and this is the only time that this alongshore-current reversal occurred within the four months analyzed. This dual sonar peak phenomenon was not repeated and with the limited sediment information available it cannot be explained with any degree of certainty. Figure 3-3. Sonar histogram at 13 m bipod, August 30, 1998 Velocity Profile Calculations The velocity profile method discussed earlier is used to estimate shear velocity and hydraulic roughness length values from the measured mean current values from the three different current meters. Bed elevation values from ADV measurements are

42 30 incorporated to account for changing current meter elevations with time and bottom change. Figure 3-4. Sonar histogram at 5.5 m bipod, August 12, 1998 Critical shear velocity One attempt to examine further the shear velocity relationship with bottom change was to calculate a critical shear velocity value from the range of sediment sizes shown in Figure 3-1 for each bipod. This method utilized the form of Shield s curve shown in Figure 3-7 to determine a value of shear stress ( τ * ) for each mean sediment diameter based on an abscissa value of ζ * D υ = 2 3 ( ρ ρ) s ρ g The ordinate value of

43 31 τ * τ c = ( ρ ρ)gd s is used to determine a critical shear velocity through the relationship τ = 2 c u *c ρ The next step was to identify the times during the four months analyzed when the bottom was just beginning to erode. A daily plot was generated for each of these times of erosion initiation to facilitate a visual comparison of shear velocity with measurements of sonar. Figure 3-5. Sonar histogram at 8 m bipod, August 31, 1998 The critical range of shear velocity values were overlain on the shear velocity plots to determine if the bottom elevation began to change around the same time that the shear velocity crossed the threshold for movement.

44 32 Figure 3-6. Sonar histogram at 13 m bipod, August 29, 1998 Most situations at the 8 m and 13 m bipod locations show the bottom beginning to erode zero to three hours after the shear velocity crossed the critical threshold. Figure 3-8 shows a particular time when the two occur almost simultaneously. The blue line represents shear velocity with the red lines identifying the range of critical values for different sediment sizes, and the circles marking where the bottom begins to erode and the shear velocity crosses the threshold. Figure 3-9 shows a particular situation at the 8 m bipod where we see a phase lag between the two, and erosion does not occur until approximately 3 h later. At the 5.5 m bipod, the shear velocity value is always below the threshold for movement when sonar measurements show the bottom beginning to erode.

45 33 One example is included as Figure Peaks in shear velocity show some relationship to peaks in tidal currents. Figure 3-7. Shield s curve Error estimates It is important to address the error in estimates of shear velocity, since values are calculated from a velocity profile method that fits a curve to three current measurements, leaving only two degrees of freedom. Error estimates for shear velocity are calculated for a 90% confidence interval using the student t distribution and they are very large, around an order of magnitude higher than most calculated values of shear velocity. This is a limitation of the method and data available, and most field measurements would demonstrate similar error estimates when using vertical arrays of individual current meters for measurements. Another factor is that the chosen critical value of shear velocity is dependent on sediment size, but the range of values does not vary widely when considering the range of sediment sizes measured at bipod locations. After taking

46 34 into account the limitations of the shear velocity estimates the time lags between the shear velocity reaching a critical value and initiation of sonar change do not appear to be unreasonably long. The critical shear velocity seems to be a good indicator of when erosion will begin at the 8 m and 13 m bipods. Figure 3-8. Shear velocity vs. sonar at the 13 m bipod, October 18, 1997 (Note blue line is shear velocity, black line is sonar, red lines are critical shear velocity) Combined wave-current influence Another concern regarding shear stress and shear velocity estimates is that they are calculated based only on the current values and they do not take into account wave orbital velocities. The shear stress associated with combined waves and currents is different than with either alone, because of the turbulence generated by the wave-current interaction (Grant and Madsen 1979).

47 35 Figure 3-9. Shear velocity vs. sonar at the 8 m bipod, August 19, 1998 (Note blue line is shear velocity, black line is sonar, red lines are critical shear velocity) This shear stress would be larger than that given by the mean current. Grant and Madsen s model was applied to current measurements taken during the Halloween storm and the wave boundary layer was estimated to be a maximum of 11.6 cm thick, much lower than their bottom current meter at a 29 cm elevation (Madsen et al. 1993). This lends confidence to the assumption that our bottom current meter at an elevation of 20 cm above the bed is also outside that wave boundary layer. Analytical models have been tested which calculate a shear velocity based on both wave and current influence. These not only account for the individual wave and current influence, but also any nonlinear interactions between the two. Wiberg and Smith (1983)

48 36 Figure Shear velocity vs. sonar at the 5.5 m bipod, October 18, 1997 (Note blue line is shear velocity, black line is sonar, red lines are critical shear velocity) compared shear velocity estimates calculated from currents alone to those calculated from two different models, those of Grant and Madsen, and Smith. The field data used for the analysis was collected at a similar depth, 18 m, and in an area with a similar sediment size. They found that the shear velocities calculated using the wave-current models are similar to the values obtained from the measured average velocity profiles, although the estimates of surface roughness are very different (Wiberg and Smith 1983). This suggests that recalculating shear velocities with an added wave influence would not alter the estimates significantly. It raises the concern that estimates of the surface roughness may not be characteristic of actual values, and often may be higher by up to an order of magnitude. This paper also suggested that scour under the instrument frame

49 37 caused original estimates of surface roughness from the data collected by Drake and Cacchione to be unrealistically high (Wiberg and Smith 1983). This may allow the surface roughness calculated through the velocity profile method to be used as a quality control parameter for the data, indicating situations where settling or scour might be a concern. Apparent hydraulic roughness An investigation into estimates of bottom surface roughness showed that some values are at the extreme limits for the sediment size present. A general idea of the magnitude of surface roughness values that are expected is determined from D z 0 = 30 Choosing a representative sediment size of 3φ (0.125 mm) yields a value of surface roughness on the order of 10-6 m. Calculated values may range from 10-1 m to m with extreme outliers exhibiting a broader range, reaching m at the 5.5 m bipod. Many of these values do not appear to significant physical meaning, but a relative comparison yields some interesting observations. One observation that occurs following a storm event is a decrease in surface roughness values estimated from velocity profiles. One possible explanation for this is that after the storm event, there is more suspended sediment, which may inhibit turbulence and subsequently cause greater velocities. Increased velocities would lower the surface roughness estimate by shifting the velocity profile. The 5.5 m bipod shows more scatter than the 8 m and 13 m bipods with a significant number of measurements reaching m or m. The surface roughness values calculated here do not always show a decrease with increasing currents. This may

50 38 be realistic considering that this bipod may be inside the surf zone during peak waves and currents and may be influenced by breaking waves. Figure Surface roughness variation with mean currents at 5.5 m bipod for October 15-21, 1997 Four different storm events were analyzed, one in each month, and all showed this decrease in surface roughness at the 8 m and 13 m bipods as mean currents increased. During periods of consistently large currents and waves there is a gradual decrease in surface roughness throughout the entire period. Plots of mean currents versus surface roughness trends for each bipod are included in Figures 3-11,12,13 for a single storm event in October of The different colors represent current measurements at three different elevations. At the 8 m bipod, we see a decrease in surface roughness values with the initial increase in currents to 40 cm/s, but then the currents increase rapidly to

51 39 80 cm/s and the values of surface roughness are relatively stable. Perhaps the initial fine sediment has been removed with the first current increase. Figure Surface roughness variation with mean currents at 8 m bipod for October 15-21, 1997 Comparison of surface roughness estimates at the 8 m and 13 m bipods for the week of October show emerging trends. They both commence decreasing about the same time and have similar ending values, but there appears to be a phase lag between the two. The 13 m surface roughness values seem to increase first and decrease sooner than those at 8 m. Surface roughness values vs. mean currents at the 5.5 m bipod show much greater variation and do not follow the same relationships at the other two bipods. Surface roughnesses for the entire month of October were found to be largest during times of low currents and waves. These are on the order of one centimeter and

52 40 would indicate unrealistically large bedforms, although this observation is consistent at all three bipods. These are on the same order of magnitude as the surface roughness estimates that were calculated by Smith and Wiberg (1983) when they used velocity profiles to estimate roughnesses and did not account for wave-current interaction. Figure Surface roughness variation with mean currents at 13 m bipod for October 15-21, 1997 The possibility of error in shear velocity and surface estimates has already been addressed, but another potential problem is that the presence of bed forms would alter the shape of the velocity profile. Extensive research has examined bed forms in rivers and their effect on velocity profile estimates. Smith and McLean (1977) conducted a study in the Hood River in Oregon, which found that the velocity profile over a bedform has a