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Field measurements of longshore sediment transport during storms
Coastal Engineering 36 Ž1999. 301–321 www.elsevier.comrlocatercoastaleng
Field measurements of longshore sediment transport during storms Herman C. Miller
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US Army Engineer Waterways Experiment Station, Field Research Facility, Duck, NC, USA Received 2 April 1998; received in revised form 14 January 1999; accepted 3 February 1999
Abstract This paper presents an analysis of longshore sediment transport ŽLST. rates based on an accumulation of data obtained during five storms. Direct measurements of velocities and suspended sediment concentration were conducted at a minimum of nine positions across a barred profile in waves up to Hm0 s 3.5 m to provide a measure of the cross-shore distribution and total suspended-load sediment transport rates. The study was conducted at the US Army Engineer Waterways Experiment Station’s Field Research Facility, located in Duck, NC. Measurements were made using the Sensor Insertion System ŽSIS. which provides an economical means to collect the required information. The largest LST rate computed from the measurements was 1780 m3 hy1. Although the cross-shore distribution of the LST varied, it most often had two peaks associated with wave shoaling and breaking at the bar and near the beach. Comparisons of measurement results with predictions using the ‘CERC’ LST formula show the predicted rates were sometimes higher and other times lower; suggesting that additional terms may be required for short term predictions during storms. Comparisons to a ‘Bagnold’ type formulation, which included a velocity term that could account for wind and other effects on LST, show better agreement for at least one of the storms. These results are intended to help fill a void of information documenting the cross-shore distribution and LST rates, particularly during storms. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Longshore sediment transport; Field studies; Storm; Sediment transport formulae
1. Introduction Until recently, field data relating longshore sediment transport ŽLST. rates to wave and current processes were collected primarily by one of the following methods: Ž1. )
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sand accumulation at jetties and breakwaters ŽWatts, 1953; Caldwell, 1956; Bruno et al., 1981; Dean et al., 1987; Walton and Bruno, 1989; Kamphuis, 1991.; Ž2. sand tracers ŽKomar and Inman, 1970; Inman et al., 1980; Kraus et al., 1982; White, 1987.; or Ž3. sediment traps ŽHom’ma et al., 1965; Lee, 1975; Kana and Ward, 1980; Kraus et al., 1988.. These studies have provided valuable estimates of long-term transport rates at many sites around the world; however, most do not yield much information on details of the transport rates on a synoptic scale. With the advent of the optical backscatter sensor ŽOBS, Downing et al., 1981., the capability now exists to measure sediment concentration within the water column in great detail at time scales of fractions of a second. Simultaneous advances in acoustic sensors have permitted extensive documentation of ocean bottom characteristics. Together with electromagnetic and acoustic current meters, the capability now exists to study fluid–sediment interactions at a scale commensurate with boundary-layerrturbulence fluctuations thought to be important to these processes. The surf zone, however, can be extremely hazardous to sensitive equipment during large wave conditions and, since nearshore bottom-elevations can vary significantly over short intervals, studies of nearshore sediment transport have been limited to relatively short intervals of time during low or moderate wave conditions at only a few locations across the inner surf zone. Detailed measurements of LST rate and cross-shore distributions are very limited, particularly during many storms. To help fill this void, the US Army Engineer Waterways Experiment Station, Coastal and Hydraulics Laboratory’s Field Research Facility ŽFRF. has developed the Sensor Insertion System ŽSIS.. The SIS, Fig. 1, is a
Fig. 1. The Sensor Insertion System shown deployed on north side of pier.
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diverless instrument deployment and retrieval system that can operate in up to 5.6 m individual wave heights. The pier-based SIS utilizes an instrument array, which includes wave, sediment concentration, and velocity gauges, to make sediment transport processes measurements at any cross-shore position. A major advantage of the system is that it allows the positions of the measurements to evolve with the profile during a storm. This helps avoid common problems with fixed instrument installations and allows instrumentation to be kept at the desired positions above the bed even though erosion or accretion is occurring. Additionally, the cross-shore positioning is variable, and, as the profile features move, so can the instrument array. The primary purpose of the SIS is to have a system available throughout the year to make direct measurements from which LST rates can be determined during many storms. This will provide a wide range of processes and initial conditions from which variations in transport rates can be determined. It also provides the ability to obtain multiple measurements under similar conditions to reduce random measurement errors often associated with field data collection. The purpose of this paper is to present the results of an analysis of LST rates based on data obtained by the SIS during several storms. Direct measurements at a minimum of nine positions along the pier provide highly resolved cross-shore distributions and total suspended-load LST rates. The analysis shows a range of transport rates generated by a variety of cross-shore distributions of LST. The LST rates determined from these measurements are compared with popular prediction formulations to assess their applicability. The predictions are both higher and lower than the measurement based LST rates. This suggests that there are other terms that should be included in an accurate prediction method. This data set was taken at the FRF along the research pier, a 561-m long concrete and steel structure supported by widely spaced, Ž12-m apart., 1-m-diameter steel pilings. Although there has been considerable discussion about the potential contamination of measurements of this type due to a pier ŽMiller et al., 1983., considerable effort has been directed to assess the pier effect and operate the SIS in a manner that minimizes these effects. Although this assessment is not complete, preliminary estimates of the divergence in LST rates determined from erosionrdeposition patterns suggest that LST rates on the upstream side of the pier may only be minimally affected by the presence of the pier during storms.
2. Experimental design 2.1. Study area The field study was conducted at the FRF, which is located on the Atlantic Ocean near the village of Duck, North Carolina ŽNC., Fig. 2. The wave climate is among the most energetic on the US East Coast ŽLeffler et al., 1996.. Well known for violent ‘nor’easters’ ŽDolan and Davis, 1992., and the close passage of tropical hurricanes, it is an ideal location for studying sediment transport during storms. Tides at the FRF are semi-diurnal, with a spring-tide range of 1.2 m.
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Fig. 2. Study Site, Field Research Facility, Duck, NC, USA.
The beach is oriented north–northwest to south–southeast and is composed of fine to coarse sand ŽStauble, 1992.. The bimodal beach material has a main component around 0.25 mm and a secondary component near 1.0 mm. Sediments in the barrtrough region are more unimodal with a mean of 0.165 mm. Seaward of the storm bar the sediments are well sorted with a mean of 0.117 mm. 2.2. Measurement systems 2.2.1. WaÕes and winds Wave measurements are available from a 240-m long pressure gauge array located in a depth of 8 m slightly offshore and north of the research pier ŽLong and Oltman-Shay, 1991.. This array provides a high-resolution directional spectrum at the seaward end of all SIS transects. Wave directions are presented as directions of wave approach relative to the research pier, which is oriented 69.78 east–northeast of true north and is considered shore normal to the beach. Wind speeds and directions were obtained from the reference anemometer located at the seaward end of the FRF pier 19 m above MSL. Wind directions presented are the directions that the winds are coming from relative to shore normal unless otherwise specified. All wave heights referenced here are ‘zero-moment’ wave heights defined in terms of the total variance in the wave spectrum, i.e.,
(
Hm 0 s 4 E0
Ž 1.
where E0 is the total variance in the wave spectrum. Wave periods are given here in terms of the spectral peak period, Tp , the period associated with the maximum energy
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density in the frequency spectrum. Similarly, wave directions, a , are the direction of the wave component associated with the spectral peak. 2.2.2. Sensor Insertion System (SIS) The SIS is essentially a track-mounted crane Žwithout a lifting hook. with instrumentation mounted on the lower boom. The SIS can place instrumentation on the bottom in 9-m depth, 15 m away from the pier pilings. Table 1 gives a description of standard SIS instrumentation and Fig. 3 shows how this equipment was mounted on the SIS. 2.3. Data collection An accurate determination of the LST rate required sampling many cross-shore positions while the hydrodynamic processes remained stationary. It was anticipated that the tide would be the primary forcing mechanism causing significant variations in these processes during the time it took the SIS to complete a transect along the pier; therefore, to minimize water level variations, measurements were made within 1.5 h of high or low tide, whichever occurred during daylight hours. A precedent for the 3-h record lengths had been established at the FRF for resolving the directional spectra ŽLong, 1996.. With a 512-s data record, nine or more cross-shore SIS measurement locations could be sampled during this time. A high instrument sample frequency, 16 Hz, was used to document the intermittent resuspension events, see example Fig. 4. Each series of SIS measurements obtained while traversing the length of the pier is referred to as a ‘transect.’ The data set consists of single transects collected during five storms. Each transect includes measurements from between nine and eleven positions across the surf zone.
Table 1 Description of the standard SIS instrumentation Sensor type
Symbol
Function
Description
Optical backscatter sensor Electromagnetic current meter
OBS
Sediment concentrations
EMCM
Longshore and cross-shore velocities
Pressure gauge
PRE
Waves and water levels
Sonar
SONAR
Distance to bottom
Themistor Accelerometers
TEMP ACC
Water temperature Movement of SIS booms
Clinometers Underwater camera
CLIN VIDEO
Angle of SIS booms Documentation
Streamer traps
TRAPS
Suspended sediment
D&A Inst. Model OBS-3; dynamic range 0–80 g ly1 Hybrid: Marsh McBirney Model 512f 4-cm-diameter probes with custom electronics by Scripps Inst. of Oceanography Paroscientific Digiquartz Model 8DP020-3; accuracy 13 N my2 1 MHz custon built by William Boyd, EE, San Diego, CA YSI Model 44008 IC Sensors Model 3026-002-P with 2 g range Lucas with 0.018 resolution Outland Technology with close proximity, pan, tilt, and focus 100-mm mesh opening filter cloth
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Fig. 3. Instrumentation mounted on the SIS.
The 14 November 1995, 11 March, 12 March, and 2 April 1996 storms were associated with extra-tropical low-pressure systems that moved north along the coast following the passage of a cold front. The 27 March 1996 storm, also associated with a cold frontal passage, was caused by an Arctic high-pressure system. These storms are typical of the autumnrwinter variety of storms that effect the FRF, as opposed to during the springrsummer when more tropical storms can be expected. 2.4. Data analysis Computation of total suspended-load LST rate required determining the sediment flux at each measurement position and integrating through the water column and across the surf zone. The OBS sensors were post-experiment calibrated against a composite of suspended-sediment samples collected during the storms using 100-mm mesh streamer traps mounted on the SIS. Prior to computing sediment flux, the ‘turbidity’ offset was
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Fig. 4. Sample data time series for 256 s. Measurements made on 11 March 1996, 270 m offshore in 3.2 m depth. Note: concentration scale decreases towards top of figure reflecting less sediment in suspension with sensor distance above bottom.
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removed from the OBS concentration signal. Turbidity, which results from the suspension of microscopic organisms and very fine organic and inorganic matter, causes a mean offset in the concentration signal ŽLudwig and Hanes, 1990; Schoelhamer, 1993.. The offset was chosen as the mode in the lowest 5% of the concentration values, or, if absent, choosing the 5% value for each time-series distribution, as suggested by Beach and Sternberg Ž1988.. The longshore sediment flux is computed for each OBS and EMCM pairing, shown schematically in Fig. 5, by multiplying the instantaneous concentration, cŽ x, z,t i ., times the instantaneous longshore velocity, Õ Ž x, z,t i ., and time-averaging the products as suggested by Jaffe et al. Ž1984., Sternberg et al. Ž1984. and Hanes Ž1988.. FŽ x, z. s
1
N
Ý c Ž x , z ,t i . Õ Ž x , z ,t i . N
Ž 2.
is1
where F Ž x, z . is the longshore sediment flux, which is the rate per unit area in the x Žcross-shore. and z Žvertical. directions. An example of the flux at different distances above the bed for the storm on 2 April 1996 is given in Table 2. Each flux value is assumed to represent the flux through a portion of the water column from the mid-point between the sensors positioned below and above it, Fig. 5. The bottom concentration sensor was nominally 3 cm above the bed, but ranged from 2 to 5 cm. It was assumed that the flux was constant from the sensor to the bed.
Fig. 5. Schematic of the SIS instrumentation. Sediment concentration ŽOBS. and current meters ŽEMCM. are paired to compute sediment flux. Dashed lines separate regions of vertical integration. C s concentration, Vs velocity, Z s vertical distance.
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Table 2 Flux values for 2 April 1996 storm. Sensors buried in the bottom were not included Gauge pairings Del z Flux Žm3 my2 hy1 . Žm. OBS EMCM Position Žm. 488 430 393 357 335 277 238 201 174 Depth Žm. y6.4 y4.9 y4.1 y3.9 y4.0 y5.2 y4.0 y2.3 y1.8 1 2 3 4 5 6 7 8
1 2 2 3 3 4 4 4
0.08 0.12 0.08 0.08 0.07 0.03 0.03 0.04
0.51 0.98 1.68 1.86 0.70 1.38 0.84 1.64 2.30 2.18 2.83 2.21 1.74 1.03 28.31 23.40
5.17 19.21 7.94 12.36 9.00 10.29 10.11 7.86
2.06 3.86 3.81 8.07 9.02 7.11 na na
1.33 2.56 2.50 4.23 7.78 na na na
3.11 1.93 9.54 6.55 8.02 4.19 14.88 8.91 9.43 4.17 14.15 9.36 11.65 6.07 17.12 10.48 13.96 13.46 21.56 17.41 na 9.58 na 11.03 na 2.90 na 5.40 na 2.71 na 5.20
Swash approximately located at position 165 m. na s not available Žinstrument problem..
Summing through the water column yields M
GŽ x . s Ý F Ž x , zj . d zj
Ž 3.
js1
where GŽ x . is the vertically integrated flux per unit cross-shore length. Assuming the flux at each cross-shore location represents the mean flux from the midpoints between the next position inshore to the next position off shore, the total or ‘bulk’ suspended-load LST rate can be estimated by L
I s Ý GŽ xk . d xk
Ž 4.
ks1
where I s bulk longshore sediment transport rate. These rates were converted to volume transport rates, Q Žm3 hy1 ., considering the density of quartz sand as 2650 kg my3 , the density of seawater as 1025 kg my3 , and the solid fraction as 0.6. The flux was assumed to be constant from the innermost measurement location to the mean shoreline position.
3. Results 3.1. LST rates Measurement based LST rates, along with the wind and wave conditions for the five storm transects are shown in Table 3. Positive transport values in this table represent transports from north to south, while negative values denote transports from south to north. Because of high winds and local seas from the north during the 12 March storm, the SIS was operated on the north side of the pier. Subsequent calculations indicated that the LST rate for this time was actually dominated by swell out of the south. Thus, these measurements were inadvertently taken in the lee of the pier, at least in terms of the
310
Storm date
14-Nov-95 11-Mar-96 12-Mar-96 27-Mar-96 2-Apr-96
Winds
Waves
LST Rates
Speed Žm sy1 .
Direction Ždeg.
Height Žm.
Period Žs.
Direction Ždeg.
Measurements North side Žm3 hy1 .
8 16 16 15 10
146 60 80 30 70
1.9 2.8 3.5 1.8 1.6
7.6 7 12 6.7 7
14 10 y14 25a 26
530 1780 y450 560 670
South side Žm3 hy1 .
610
Computed CERC ŽUS Army Corps of Engineers, 1984. Žm3 hy1 .
Komar Ž1998. Žm3 hy1 .
590 1120 y2980 800 590
660 1210 y780 400 240
Measurement based LST rates are paired with predictions. CERC ŽUS Army Corps of Engineers, 1984. formulation assumes that LST is associated with the single peak in the wave spectrum. The formulation of Komar Ž1998. includes velocity terms that could account for processes other than the peak wave forcing. Note: directions relative to shore normal, qnorth–south. a Visually estimated; directional array data not available.
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Table 3 Wind, wave, and LST rates
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direction of LST. A measure of the effect of operating in the lee of the pier was investigated during the 14 November 1995 storm, when measurements were made on both the north and south sides at each cross-shore sampling location. As can be seen in Table 3, the transport rates differed by less than 15%. Conditions during transects taken on 14 November 1995, 27 March, and 2 April 1996 were quite similar in terms of wave heights and periods, 1.8 m and 7 s, respectively. Wave approach angles varied from 148 to 268 north of shore normal. Measurement based LST rates for all three transects were in the neighborhood of 600 m3 hy1 . Wave conditions during the 11 March 1996 transect were somewhat more energetic with a wave height of 2.8 m; however, the period for these waves was still only 7 s. The bulk transport rate for this transect was almost 1800 m3 hy1 , increasing almost linearly with wave height to the second power. As the storm continued to develop over the March 11–12 period, wave heights grew to 3.5 m by the time of the 12 March 1996 transect. Bulk transport rates on the 12th, however, actually diminished from the transport rates on the 11th and changed directions. Some apparent reasons for this decrease will be given in Section 4. 3.2. Cross-shore distribution of LST Bathymetric profiles during all transects had characteristic longshore bars as shown in Fig. 6. These profiles are presented as depth versus normalized offshore location, where
Fig. 6. Profiles measured with SIS. Non-dimensional cross-shore position normalized by bar crest position such that the swash Žapproximate shoreline position. is at 0 and the bar is at 1.
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Fig. 7. Cross-shore distribution of longshore flux. Positive values indicate transport from north to south and negative values indicate transport in opposite direction.
the approximate local mean waterline Žmean swash position. is at 0 and the bar is at 1. The bar position for the four transects between 11 March and 2 April 1996 varied by only q5 m. While 190 m separated the bar from the swash for all five storms, the bar
Fig. 8. Mean and cumulative cross-shore distributions of longshore flux with a mean profile for the four storms without flow reversals.
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position on 14 November 1995 was 40 m landward of the other four transects. Video and visual observations indicate the surf zone during these times typically began near position 1.2 in Fig. 6. Cross-shore distributions of longshore flux for all five transects are shown in Fig. 7. Although the distribution of transport varies, this Figure illustrates that it is often bimodal during the storm periods studied. The 12 March 1996 transect also has a bimodal distribution, however, there are direction reversals near cross-shore position 0.9 and seaward of position 1.4. These reversals occurred where the wave influence on longshore sediment transport apparently was minimized. Fig. 8 shows the mean distribution of LST for the four cases with no flow reversals, along with the cumulative amount of transport calculated starting at the shore and extending to locations seaward of the bar. From this Figure we see that as much as 20% of the total LST in these transects occurred seaward of the bar. It was observed that in this area the waves were shoaling, with only occasional wave breaking.
4. Discussion Presently, no reliable electronic instruments exist which can measure bed load and so called ‘sheet flow’ just above the bed within the surf zone; thus, it is difficult to quantify the contribution of bed load to the total transport rate during storms. Komar Ž1990., who summarizes the conflicting opinions of the importance of bed load found by Komar Ž1978., Inman et al. Ž1980., Kana and Ward Ž1980., Coakley and Skafel Ž1982., Downing Ž1984., Sternberg et al. Ž1984., and Kraus and Dean Ž1987., suggests that the relative roles of suspension versus bed-load transport on beaches must remain open and in need of further research. For the purpose of this discussion, we will make the tacit assumption that transport is reasonably well estimated by the instrumentation package on the SIS. Assuming a constant sediment flux between the bottom sensor and the bed suggests a constant sediment concentration and velocity. Although this probably underestimates the concentration and over estimates the velocity, it allows us to rely only on measured values. Future efforts will be required to provide a more definitive answer to the quantitative role of bed load in LST rates during storms. Another potentially important but difficult to document region of transport is the swash zone. Recent reviews of sediment transport investigations ŽSternberg et al., 1989; Komar, 1990; Kraus and Horikawa, 1990. indicate that between 5 and 60% of the total transport could be accounted for in the swash zone, with this percentage tending to be inversely related to wave height. Since we are dealing with storm conditions in the cases studied here, it is likely that the sampled area includes most of the transport; however, subsequent efforts are planned to investigate this hypothesis. 4.1. Longshore flux distributions and transport rates For almost five decades now, since the work of Watts Ž1953., engineers have attempted to relate LST rates to wave and current processes. This has produced hundreds of results that have been summarized by Bodge Ž1989., Sternberg et al. Ž1989., Komar Ž1990., Kraus and Horikawa Ž1990., Zyserman Ž1992., Schoonees and Theron Ž1993.,
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and others. However, since this investigation is focusing on transport rates over very short periods of time, it may not be appropriate to compare these results directly to many of the previous investigations, since they were primarily based on long-term averaging. The few specific studies that attempt to provide short-term estimates of LST rates based on multiple cross-shore measurements are discussed below. Kana and Ward Ž1980. made measurements along the FRF pier using the trap devices of Kana Ž1976. both during storm and post-storm conditions. They found the total load of sediment in suspension was approximately 10 times higher during the storm, and the longshore flux was as much as 60 times more than during the post-storm period. During their storm with 2.3 m waves, the longshore transport rate was over 900 m3 hy1 which falls within the 600 to 1700 m3 hy1 for storms with waves of 1.8 to 2.8 m, respectively, during this investigation. Also similar to this investigation was their finding that suspension dominated and sediment concentration peaked near the breaker location. Sternberg et al. Ž1984. used five arrays of OBS sensors with simultaneous current measurements at Leadbetter Beach, Santa Barbara, CA and found a single peak in the longshore sediment flux distribution near the mid-surf location between the bar and the beach. Similar peaks near the mid-surf position were found for this study on 14 November 1995 and 11 March 1996. They found the flux to always decrease shoreward and seaward which differs from the findings of this study which found peaks nearer the swash for the storms in 12 March, 27 March, and April, 1996. Their study with wave heights generally less than 1.8 m, resulted in a maximum transport rate of 217 m3 hy1 , which is much less than the rates for the present study. However, considering that their five measurement locations generally covered less than 60% of the surf zone and did not include the region where secondary peaks were observed during this study, the lower transport rates are understandable. An observation also seen in this study was that the lowest OBS sensor, located 3.5 cm above the bed, showed a wide range of concentrations, occasionally reaching a value of 180 g ly1 during individual suspension events. Kraus and Dean Ž1987. made measurements at the FRF in approximately 1 m waves. They found the cross-shore distribution took a number of different shapes, but frequently was characterized by peaks in the outer surf zone and at the swash similar to the peaks found in this investigation. Their maximum LST rate was 228 m3 hy1 during moderate 1.2-m waves. The present study was intended to extend their work into larger waves during storm conditions. For many years, visual observations of wave breaking has suggested that regions of wave breaking tend to be associated with high concentrations of sediment within the water column. Recently, OBS measurements from Yu et al. Ž1993. have confirmed that maximum values in the sediment concentration occur in the vicinity of wave breaking. Data from all transects studied here show this same trend, with highest concentrations occurring in regions of wave breaking and bore formation. On the other hand, during storm conditions on barred beaches, the region of maximum longshore currents is typically found within the trough area shoreward of the outer bar ŽSmith et al., 1993.. The results shown here indicate that the distribution of LST tends to be dominated more by the concentration factor in Eq. Ž2. than by the longshore current factor. The present study and the prior studies mentioned above suggest that one can not assume the sediment flux is always distributed across the nearshore in the same way as
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the longshore currents. It is also evident that the cross-shore distribution of longshore sediment flux varies, and that variation is apparently a function of the wave height and thus water level. Future investigations should include sequences of measurements during both high and low tides during the growth, peak, and waning stages of the storms. 4.2. Comparison to predictions Various arguments have been made for different relationships between wave conditions and LST rate ŽSayao and Kamphuis, 1983; Komar, 1990; Watanabe, 1992; Schoonees and Theron, 1994; Drake and White, 1995.. It is beyond the scope of this paper to examine all of these proposed relationships relative to the results of this field study; however, in order to provide some perspective to these results it is instructive to compare with some theoretical form. Two methods are presented; the first is the energy flux method, sometimes termed the ‘CERC’ formula. This formulation, arguably the most widely used in the world, requires only wave height, period, and direction as input. The other method is that of Inman and Bagnold Ž1963. which considers that the sand is mobilized by the mean wave stress and moved by the longshore velocity. Inclusion of the velocity is appealing because it helps account for transport mechanisms other than simply breaking waves, such as local winds and tides. 4.2.1. Comparison to the energy flux method The energy flux method is an empirical formulation is described in the Shore Protection Manual ŽUS Army Corps of Engineers, 1984. as Qs
KPlb
Ž rs y r w . gaX
Ž 5.
where Q is the volume transport rate and K s empirical coefficient, K s 0.39 when P lb is based on significant wave heights; Plb s longshore energy flux factor at breaking; rs s density of quartz sand; r w s density of water; g s acceleration of gravity; aX s solid fraction of beach sediment Ž aX s 1 y porosity.. The longshore energy flux factor is given in terms of the wave conditions at breaking. Plb s Ž ECg . bsin a cos a
Ž 6.
where E is the wave energy, Cg is the wave group velocity, and a is the wave crest angle relative to the beach. The wave height, period, and direction parameters were taken as the values associated with the peak of the directional spectrum determined at the FRF directional array. The values were shoaled to breaking, conserving wave energy flux according to Ed .a . Cgd .a . cos a d .a .s E b Cgb cos a b
Ž 7.
and the waves were refracted using Snell’s Law: sin a d .a . Ld .a .
s
sin a b Lb
Ž 8.
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where L is the wavelength, ‘d.a.’ is at the FRF directional array, and ‘b’ denotes breaking. The results given in Table 3 shows that the magnitude of the total transport seems to be consistent with the measurement based values, with the notable exception of the 12 March 1996 transect. This is not surprising, since at the time of this transect, the local seas and winds were acting in opposition to the swell waves and the CERC formulation assumes all of the energy is associated with a single peak in the wave spectrum. However, sometimes the predictions are both higher and lower than the measurement based values. This suggests that there are other terms, possibly accounting for such influences as the directional distribution of energy within the wave spectrum, local winds, grain size, beach slope, swash processes, or bed-load transport, that should be included in an accurate prediction methodology for LST. For the 12 March 1996 storm, where winds and local seas opposed the swell, the measurement based LST rate was an order of magnitude lower than predicted. Since this situation is not atypical of incident waves in coastal areas, it seems that simple LST formulae based on parametric representations of wave conditions alone may not be able to provide accurate estimates of total LST rates for some individual storms conditions. 4.2.2. Comparison to the Inman and Bagnold method A second formulation was compared to the measurement based LST rates. This method was chosen because it includes longshore and orbital velocity terms that should
Fig. 9. Predicted LST rates using CERC ŽUS Army Corps of Engineers, 1984. and the formulations of Komar Ž1998. compared to measurement based LST rates.
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provide more reasonable predictions when the LST rates were the result of more complicated processes and the assumptions of the CERC formulation were not valid. The analysis of Inman and Bagnold Ž1963. resulted in the following formulation I l s K X Ž ECg . b VrUm
Ž 9.
where K X is a dimensionless coefficient, V is an average longshore current velocity near the mid-surf position, and Um is the maximum horizontal orbital velocity of the waves evaluated at the breaker zone. Komar Ž1998., using the value of K X s 0.25 determined by Kraus et al. Ž1982. and linear-wave theory, reduces Eq. Ž9. to the following LST rate formulation when using significant wave heights, I l s 0.044 r gH 2 V .
Ž 10 .
Volumetric LST rates were determined with this formulation using currents from the approximate mid-surf position and the breaking wave statistics as described in the previous section. Comparisons of these results and those for the CERC formulation to the LST rates determined from the measurements are shown in Fig. 9. The diagonal line in the figure represents the best agreement. The formulation of Komar Ž1998. agrees much better with the measurement based LST rates than the CERC formulation during the 12 March 1996 transect. Recall that this was when swell waves opposed local seas and winds. Otherwise, both formulations are comparable. While the Komar formulation might be viewed as more fundamental because it considers the processes of sand transport, in so far as the waves mobilize the sand and many forces can contribute to the longshore velocity that move the sand, it does require velocity measurements that often are not available at a given project site.
5. Summary Many of the recent reviews of the longshore sediment transport literature have arrived at essentially the same conclusion, namely that comprehensive and well-instrumented synoptic field measurements of fluid processes, sediment transport, and nearshore morphologic change are essential for verifying theoretical and laboratory results. Almost all of the measurement based transport rates reported in the literature are for mild wave conditions with wave heights less than 1.8 m and bulk transport rates under 230 m3 hy1 . However, when considering an annual sediment budget, a few storms with much higher transport rates usually contribute a significant portion of the total. The consequence is that the conditions that are most important to predict are outside the range of our ability to calibrate our models. These are primary reasons for developing the SIS, which has now been successfully operated during five storms. Highly resolved processes measurements provided the basis for computing accurate LST rates during storm conditions with significant wave heights up to 3.5 m. Direct measurements at a minimum of nine positions across a barred profile in waves up to Hm0 s 3.5 m provided documentation of the cross-shore distribution and total suspended-load LST rates. The storms on 14 November 1995, 27 March, and 2 April 1996 had similar wave conditions, near 1.8 m, 7 s, and approaching 208 north of shore
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normal, and resulted in transport rates on the order of 600 m3 hy1 . An approximate 50% increase in wave height during the 11 March 1996 storm resulted in an approximate factor of three increase in the longshore transport rate. On 12 March, the wind and seas were from the north, while the spectral peak was associated with a southerly swell. The complexity of these opposing processes resulted in northward transport considerably lower than might be expected if all of the energy was associated with only the peak of the wave spectrum. The cross-shore distribution of the LST showed a variety of shapes, however two peaks were often observed. One peak was observed between the mid-surf position and the beach. On four occasions a second peak occurred on the seaward side of the bar associated with wave shoaling and breaking. The measurements showed approximately 20% of the sediment was transported alongshore seaward of the surf zone. It was observed that in this area the waves were shoaling, with only occasional wave breaking. Transport rates were computed by the energy flux method using the CERC formulation based on spectral peak wave parameters at breaking determined from the FRF directional array and were compared to the measurement based transport rates. The predictions compared reasonably well except on one occasion when the transport was complicated by opposing processes. Considering the CERC formulation assumes all of the wave energy was associated with a single direction and only the one oblique wave train is responsible for the longshore current, the poor performance is not unexpected when opposing processes are present. Komar’s formulation agreed with the LST rates computed from the measurements as well as the CERC formulation did for four of the storms. Komar’s formulation, which would be expected to account for additional processes that contribute to the generation of the longshore currents, agreed much better with measurement based LST rate for the 12 March 1996 storm when the processes were complicated by opposing forces. Where current measurements are available, Komar’s formulation would appear to have a wider application than the CERC formulation. This is an ongoing study designed to fill the void of information directly relating storm processes to LST rates. While additional effort needs to be placed in conducting bed load and swash transport measurements, the SIS provides the capability to document a wide range of processes and initial conditions from which variations in LST rates can be determined. Obtaining multiple measurements under similar conditions also should lead to reduced random measurement errors often associated with field data collection.
6. Conclusions Many field studies have provided LST rates for coastal locations. Yet few have attempted direct measurements of the fluid processes and sediment concentrations during storms when wave heights exceed 1.8 m. This investigation extends those previous works into higher wave conditions. The SIS can operate during severe storm conditions with significant wave heights in excess of 3 m, wind speeds of 20 m sy1 , and currents of 2 m sy1 ; however, the effect of the research pier has not fully been quantified and additional experiments are needed to
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address this question. Additionally, the magnitude of the LST in the swash zone should be quantified in the future. For three similar storms with wave heights near 2 m, the transport rates were on the order of 600 m3 hy1 . A 50% increase in wave height during a fourth storm resulted in a factor of three increase in the longshore transport rate. The cross-shore distribution of the LST often showed a bimodal distribution with a peak associated with wave shoaling and breaking on the seaward side of the bar and another peak between the mid-surf and beach locations. Approximately 20% of the sediment transported alongshore occurred seaward of the surf zone. Predicted LST rates using the ‘CERC’ formulation were consistent with the values determined from measurements, except on one occasion. On that occasion the winds and seas opposed swell waves and the formulation presented in ŽKomar, 1998. agreed much better with the measurement based LST rates. For the other storms, Komar’s formulation and the CERC formulation were sometimes higher or lower than the measurement based LST rates. This suggests additional terms are required for an accurate prediction of LST rates under all storm conditions.
Acknowledgements This work was conducted under the Sediment Transport Processes Work Unit, Coastal Sedimentation and Dredging Area, of the Coastal Research Program, of the U.S. Army Corps of Engineers. Permission was granted by the Chief of Engineers to publish this information. The author thanks Jay Lockhart, John Housley, Bob Hamilton, Doug Nyman, Don Resio, and, especially, Doug Charlton for helping make the SIS and this research program possible.
References Beach, R.A., Sternberg, R.W., 1988. Suspended sediment transport in the surf zone: response to cross-shore infragravity motion. Mar. Geol. 80, 61–79. Bodge, K.R., 1989. A literature review of the distribution of longshore sediment transport across the surf zone. J. Coastal Res. 5 Ž2., 307–328. Bruno, R.O., Dean, R.G., Gable, C.G., Walton, T.L., 1981. Longshore sand transport study at Channel Islands Harbor, CA. TR CERC-81-2, WES Vicksburg, MS. Caldwell, J.M., 1956. Wave action and sand movement near Anaheim Bay, CA. Tech. Memo. No. 68. US Army Beach Erosion Board, 21 pp. Coakley, J.P., Skafel, M.G., 1982. Suspended sediment discharge on a non-tidal coast. In: Proc. 18th Coastal Eng. Conf., Cape Town. Am. Soc. Civ. Eng., pp. 1283–1304. Dean, R.G., Berek, E.P., Bodge, K.R., Gable, C.B., 1987. NSTS measurements of total longshore transport. In: Proc. Coastal Sediments ’87, New Orleans, LA, Vol. 1, pp. 652–667. Dolan, R., Davis, R.E., 1992. An intensity scale for Atlantic Coast northeast storms. J. Coastal Res. 8 Ž4., 840–853. Downing, J.P., 1984. Suspended sand transport on a dissipative beach. In: Proc. 19th Coastal Eng. Conf., Houston. Am. Soc. Civ. Eng., pp. 1765–1781. Downing, J.P., Sternberg, R.W., Lister, C.R.B., 1981. New instrument for the investigation of sediment suspension processes in the shallow marine environment. Mar. Geol. 42, 19–34.
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Drake, T.G., White, T.E., 1995. Compilation of suspended-load point-transport theories. TR CERC-95-18, WES Vicksburg, MS. Hanes, D.M., 1988. Intermittent sediment suspension and its implications to sand tracer dispersal in wave-dominated environments. Mar. Geol. 88, 175–183. Hom’ma, M., Horikawa, K., Kajima, R., 1965. A study of suspended sediment due to wave action. Coastal Eng. Jpn. 3, 101–122. Inman, D.L., Bagnold, R.A., 1963. Littoral processes. In: Hill, M.N. ŽEd.., The Sea, Vol. 6. Wiley-Interscience, New York, pp. 529–553. Inman, D.L., Zamplo, J.A., White, T.E., Hanes, D.M., Waldorf, B.W., Kastens, K.A., 1980. Field measurements of sand motion in the surf zone. In: Proc. 17th Coastal Eng. Conf., Sydney. Am. Soc. Civ. Eng., pp. 1215–1234. Jaffe, B.E., Sternberg, R.W., Sallenger, A.H., 1984. The role of suspended sediment in shore–normal beach profile changes. In: Proc. 23rd Coastal Eng. Conf., Venice. Am. Soc. Civ. Eng., pp. 2680–2693. Kamphuis, J.W., 1991. Alongshore sediment transport rate. J. Waterway, Port, Coastal, Ocean Eng. 117 Ž6., 624–640, Am. Soc. of Civ. Eng. Kana, T.W., 1976. A new apparatus for collecting simultaneous water samples in the surf zone. J. Sed. Pet. 46, 1031–1034. Kana, T.W., Ward, L.G., 1980. Nearshore suspended sediment load during storm and post-storm conditions. In: Proc. 17th Coastal Eng. Conf., Sydney. Am. Soc. Civ. Eng., pp. 1158–1174. Komar, P.D., 1978. Relative quantities of suspension versus bedload transport on beaches. J. Sed. Pet. 48 Ž3., 921–932. Komar, P.D., 1990. Littoral sediment transport. In: Herbich, J.B. ŽEd.., Handbook of Coastal and Ocean Engineering, Chap. 25, Vol. II. Gulf Pub., Houston, TX. Komar, P.D., 1998. Beach Processes and Sedimentation, 2nd edn. Prentice-Hall, Upper Saddle River, NJ. Komar, P.D., Inman, D.L., 1970. Longshore sand transport on beaches. J. Geophys. Res. 75 Ž30., 5914–5927. Kraus, N.C., Dean, J.L., 1987. Longshore sediment transport rate distributions measured by trap. In: Proc. Advances in Understanding of Coastal Sed. Proc., New Orleans. Am. Soc. Civ. Eng., pp. 881–896. Kraus, N.C., Horikawa, K., 1990. Nearshore sediment transport. In: Le Mehaute, B., Hanes, D.M. ŽEds.., The Sea, Chap. 22, Vol. 9, Part B. Wiley, New York. Kraus, N.C., Isobe, M., Igarashi, H., Sasaki, T.O., Horikawa, K., 1982. Field experiments on longshore sand transport in the surf zone. In: Proc. 18th Coastal Eng. Conf., Cape Town. Am. Soc. Civ. Eng., pp. 969–988. Kraus, N.C., Gingerich, K.J., Rosati, J.D., 1988. Towards an improved empirical formula for longshore sand transport. In: Proc. 21st Coastal Eng. Conf., Malaga. Am. Soc. Civ. Eng., pp. 1182–1196. Lee, K.K., 1975. Longshore currents and sediment transport in west shore of Lake Michigan. Water Resour. Res. 2 Ž6., 1029–1032. Leffler, M., Baron, C., Scarborough, B., Hathaway, K., Hodges, P., Townsend, C., 1996. Annual Data Summary for 1994 CERC Field Research Facility. TR CERC-96-6, WES Vicksburg, MS. Long, C.E., 1996. Index and bulk parameters for frequency–direction spectra measured at CERC Field Research Facility, June 1994 to August 1995. MP CERC-96-5, WES Vicksburg, MS. Long, C.E., Oltman-Shay, J., 1991. Directional Characteristics of waves in shallow water. TR CERC-91-1, WES Vicksburg, MS. Ludwig, K.A., Hanes, D.M., 1990. A laboratory evaluation of optical backscatterance suspended solids sensors exposed to sand–mud mixtures. Mar. Geol. 94, 173–179. Miller, H.C., Birkemeier, W.A., DeWall, A.E., 1983. Effects of CERC research pier on nearshore processes. In: Proc. Coastal Structures ’83, Arlington. Am. Soc. Civ. Eng., pp. 769–784. Sayao, O., Kamphuis, J.W., 1983. Littoral sand transport: review of the state of the art. Report No. 78. Department of Civil Engineering, Queen’s University, Kingston, Canada. Schoelhamer, D.H., 1993. Biological interference of optical backscatterance sensors in Tampa Bay, Florida. Mar. Geol. 110, 303–313. Schoonees, J.S., Theron, A.K., 1993. Review of the field-data base for longshore sediment transport. Coastal Eng. 19, 1–25. Schoonees, J.S., Theron, A.K., 1994. Accuracy and applicability of the SPM longshore transport formula. In: Proc. 24th Coastal Eng. Conf., Kobe. Am. Soc. Civ. Eng., pp. 2595–2609.
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Smith, J.M., Larson, M., Kraus, N.C., 1993. Longshore current on a barred beach: field measurements and calculations. J. Geophys. Res. 98 ŽC12., 22717–22731. Stauble, D.K., 1992. Long-term profile and sediment morphodynamics: field research facility case history. TR CERC-92-7, WES Vicksburg, MS. Sternberg, R.W., Shi, N.C., Downing, J.P., 1984. Field investigation of suspended sediment transport in the nearshore zone. In: Proc. 19th Coastal Eng. Conf., Houston. Am. Soc. Civ. Eng., pp. 1782–1798. Sternberg, R.W., Shi, N.C., Downing, J.P., 1989. Suspended sediment measurements. In: Seymour, R.J. ŽEd.., Nearshore Sediment Transport, Chap. 11. Plenum, New York. US Army Corps of Engineers, 1984. Shore Protection Manual, Vols. I and II. Coastal Engineering Research Center, WES Vicksburg, MS. Walton, T.L., Bruno, R.O., 1989. Longshore transport at a detached breakwater, phase II. J. of Coastal Res. 5 Ž4., 679–691. Watanabe, A., 1992. Total rate and distribution of longshore sand transport. In: Proc. 23rd Coastal Eng. Conf., Venice. Am. Soc. Civ. Eng., pp. 2528–2541. Watts, G.M., 1953. A study of sand movement at South Lake Worth inlet, Florida. Tech. Memo No. 42. US Army Beach Erosion Board, 24 pp. White, T.E., 1987. Nearshore sand transport. PhD thesis, Univ. of California, San Diego, 210 pp. Yu, Y., Sternberg, R.W., Beach, R.A., 1993. Kinematics of breaking waves and associated suspended sediment in the nearshore zone. Continental Shelf Res. 13 Ž11., 1219–1242. Zyserman, J.A., 1992. A critical review of available data for calibration andror verification of sediment transport models. In: Proc. 23rd Coastal Eng. Conf., Venice. Am. Soc. Civ. Eng., pp. 2567–2580.
Field measurements of longshore sediment transport during storms Herman C. Miller
)
US Army Engineer Waterways Experiment Station, Field Research Facility, Duck, NC, USA Received 2 April 1998; received in revised form 14 January 1999; accepted 3 February 1999
Abstract This paper presents an analysis of longshore sediment transport ŽLST. rates based on an accumulation of data obtained during five storms. Direct measurements of velocities and suspended sediment concentration were conducted at a minimum of nine positions across a barred profile in waves up to Hm0 s 3.5 m to provide a measure of the cross-shore distribution and total suspended-load sediment transport rates. The study was conducted at the US Army Engineer Waterways Experiment Station’s Field Research Facility, located in Duck, NC. Measurements were made using the Sensor Insertion System ŽSIS. which provides an economical means to collect the required information. The largest LST rate computed from the measurements was 1780 m3 hy1. Although the cross-shore distribution of the LST varied, it most often had two peaks associated with wave shoaling and breaking at the bar and near the beach. Comparisons of measurement results with predictions using the ‘CERC’ LST formula show the predicted rates were sometimes higher and other times lower; suggesting that additional terms may be required for short term predictions during storms. Comparisons to a ‘Bagnold’ type formulation, which included a velocity term that could account for wind and other effects on LST, show better agreement for at least one of the storms. These results are intended to help fill a void of information documenting the cross-shore distribution and LST rates, particularly during storms. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Longshore sediment transport; Field studies; Storm; Sediment transport formulae
1. Introduction Until recently, field data relating longshore sediment transport ŽLST. rates to wave and current processes were collected primarily by one of the following methods: Ž1. )
Tel.: q1-252-261-6840, ext. 240; Fax: q1-252-261-4432; E-mail: [email protected]
0378-3839r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 3 9 Ž 9 9 . 0 0 0 1 0 - 1
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sand accumulation at jetties and breakwaters ŽWatts, 1953; Caldwell, 1956; Bruno et al., 1981; Dean et al., 1987; Walton and Bruno, 1989; Kamphuis, 1991.; Ž2. sand tracers ŽKomar and Inman, 1970; Inman et al., 1980; Kraus et al., 1982; White, 1987.; or Ž3. sediment traps ŽHom’ma et al., 1965; Lee, 1975; Kana and Ward, 1980; Kraus et al., 1988.. These studies have provided valuable estimates of long-term transport rates at many sites around the world; however, most do not yield much information on details of the transport rates on a synoptic scale. With the advent of the optical backscatter sensor ŽOBS, Downing et al., 1981., the capability now exists to measure sediment concentration within the water column in great detail at time scales of fractions of a second. Simultaneous advances in acoustic sensors have permitted extensive documentation of ocean bottom characteristics. Together with electromagnetic and acoustic current meters, the capability now exists to study fluid–sediment interactions at a scale commensurate with boundary-layerrturbulence fluctuations thought to be important to these processes. The surf zone, however, can be extremely hazardous to sensitive equipment during large wave conditions and, since nearshore bottom-elevations can vary significantly over short intervals, studies of nearshore sediment transport have been limited to relatively short intervals of time during low or moderate wave conditions at only a few locations across the inner surf zone. Detailed measurements of LST rate and cross-shore distributions are very limited, particularly during many storms. To help fill this void, the US Army Engineer Waterways Experiment Station, Coastal and Hydraulics Laboratory’s Field Research Facility ŽFRF. has developed the Sensor Insertion System ŽSIS.. The SIS, Fig. 1, is a
Fig. 1. The Sensor Insertion System shown deployed on north side of pier.
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diverless instrument deployment and retrieval system that can operate in up to 5.6 m individual wave heights. The pier-based SIS utilizes an instrument array, which includes wave, sediment concentration, and velocity gauges, to make sediment transport processes measurements at any cross-shore position. A major advantage of the system is that it allows the positions of the measurements to evolve with the profile during a storm. This helps avoid common problems with fixed instrument installations and allows instrumentation to be kept at the desired positions above the bed even though erosion or accretion is occurring. Additionally, the cross-shore positioning is variable, and, as the profile features move, so can the instrument array. The primary purpose of the SIS is to have a system available throughout the year to make direct measurements from which LST rates can be determined during many storms. This will provide a wide range of processes and initial conditions from which variations in transport rates can be determined. It also provides the ability to obtain multiple measurements under similar conditions to reduce random measurement errors often associated with field data collection. The purpose of this paper is to present the results of an analysis of LST rates based on data obtained by the SIS during several storms. Direct measurements at a minimum of nine positions along the pier provide highly resolved cross-shore distributions and total suspended-load LST rates. The analysis shows a range of transport rates generated by a variety of cross-shore distributions of LST. The LST rates determined from these measurements are compared with popular prediction formulations to assess their applicability. The predictions are both higher and lower than the measurement based LST rates. This suggests that there are other terms that should be included in an accurate prediction method. This data set was taken at the FRF along the research pier, a 561-m long concrete and steel structure supported by widely spaced, Ž12-m apart., 1-m-diameter steel pilings. Although there has been considerable discussion about the potential contamination of measurements of this type due to a pier ŽMiller et al., 1983., considerable effort has been directed to assess the pier effect and operate the SIS in a manner that minimizes these effects. Although this assessment is not complete, preliminary estimates of the divergence in LST rates determined from erosionrdeposition patterns suggest that LST rates on the upstream side of the pier may only be minimally affected by the presence of the pier during storms.
2. Experimental design 2.1. Study area The field study was conducted at the FRF, which is located on the Atlantic Ocean near the village of Duck, North Carolina ŽNC., Fig. 2. The wave climate is among the most energetic on the US East Coast ŽLeffler et al., 1996.. Well known for violent ‘nor’easters’ ŽDolan and Davis, 1992., and the close passage of tropical hurricanes, it is an ideal location for studying sediment transport during storms. Tides at the FRF are semi-diurnal, with a spring-tide range of 1.2 m.
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Fig. 2. Study Site, Field Research Facility, Duck, NC, USA.
The beach is oriented north–northwest to south–southeast and is composed of fine to coarse sand ŽStauble, 1992.. The bimodal beach material has a main component around 0.25 mm and a secondary component near 1.0 mm. Sediments in the barrtrough region are more unimodal with a mean of 0.165 mm. Seaward of the storm bar the sediments are well sorted with a mean of 0.117 mm. 2.2. Measurement systems 2.2.1. WaÕes and winds Wave measurements are available from a 240-m long pressure gauge array located in a depth of 8 m slightly offshore and north of the research pier ŽLong and Oltman-Shay, 1991.. This array provides a high-resolution directional spectrum at the seaward end of all SIS transects. Wave directions are presented as directions of wave approach relative to the research pier, which is oriented 69.78 east–northeast of true north and is considered shore normal to the beach. Wind speeds and directions were obtained from the reference anemometer located at the seaward end of the FRF pier 19 m above MSL. Wind directions presented are the directions that the winds are coming from relative to shore normal unless otherwise specified. All wave heights referenced here are ‘zero-moment’ wave heights defined in terms of the total variance in the wave spectrum, i.e.,
(
Hm 0 s 4 E0
Ž 1.
where E0 is the total variance in the wave spectrum. Wave periods are given here in terms of the spectral peak period, Tp , the period associated with the maximum energy
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density in the frequency spectrum. Similarly, wave directions, a , are the direction of the wave component associated with the spectral peak. 2.2.2. Sensor Insertion System (SIS) The SIS is essentially a track-mounted crane Žwithout a lifting hook. with instrumentation mounted on the lower boom. The SIS can place instrumentation on the bottom in 9-m depth, 15 m away from the pier pilings. Table 1 gives a description of standard SIS instrumentation and Fig. 3 shows how this equipment was mounted on the SIS. 2.3. Data collection An accurate determination of the LST rate required sampling many cross-shore positions while the hydrodynamic processes remained stationary. It was anticipated that the tide would be the primary forcing mechanism causing significant variations in these processes during the time it took the SIS to complete a transect along the pier; therefore, to minimize water level variations, measurements were made within 1.5 h of high or low tide, whichever occurred during daylight hours. A precedent for the 3-h record lengths had been established at the FRF for resolving the directional spectra ŽLong, 1996.. With a 512-s data record, nine or more cross-shore SIS measurement locations could be sampled during this time. A high instrument sample frequency, 16 Hz, was used to document the intermittent resuspension events, see example Fig. 4. Each series of SIS measurements obtained while traversing the length of the pier is referred to as a ‘transect.’ The data set consists of single transects collected during five storms. Each transect includes measurements from between nine and eleven positions across the surf zone.
Table 1 Description of the standard SIS instrumentation Sensor type
Symbol
Function
Description
Optical backscatter sensor Electromagnetic current meter
OBS
Sediment concentrations
EMCM
Longshore and cross-shore velocities
Pressure gauge
PRE
Waves and water levels
Sonar
SONAR
Distance to bottom
Themistor Accelerometers
TEMP ACC
Water temperature Movement of SIS booms
Clinometers Underwater camera
CLIN VIDEO
Angle of SIS booms Documentation
Streamer traps
TRAPS
Suspended sediment
D&A Inst. Model OBS-3; dynamic range 0–80 g ly1 Hybrid: Marsh McBirney Model 512f 4-cm-diameter probes with custom electronics by Scripps Inst. of Oceanography Paroscientific Digiquartz Model 8DP020-3; accuracy 13 N my2 1 MHz custon built by William Boyd, EE, San Diego, CA YSI Model 44008 IC Sensors Model 3026-002-P with 2 g range Lucas with 0.018 resolution Outland Technology with close proximity, pan, tilt, and focus 100-mm mesh opening filter cloth
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Fig. 3. Instrumentation mounted on the SIS.
The 14 November 1995, 11 March, 12 March, and 2 April 1996 storms were associated with extra-tropical low-pressure systems that moved north along the coast following the passage of a cold front. The 27 March 1996 storm, also associated with a cold frontal passage, was caused by an Arctic high-pressure system. These storms are typical of the autumnrwinter variety of storms that effect the FRF, as opposed to during the springrsummer when more tropical storms can be expected. 2.4. Data analysis Computation of total suspended-load LST rate required determining the sediment flux at each measurement position and integrating through the water column and across the surf zone. The OBS sensors were post-experiment calibrated against a composite of suspended-sediment samples collected during the storms using 100-mm mesh streamer traps mounted on the SIS. Prior to computing sediment flux, the ‘turbidity’ offset was
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Fig. 4. Sample data time series for 256 s. Measurements made on 11 March 1996, 270 m offshore in 3.2 m depth. Note: concentration scale decreases towards top of figure reflecting less sediment in suspension with sensor distance above bottom.
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removed from the OBS concentration signal. Turbidity, which results from the suspension of microscopic organisms and very fine organic and inorganic matter, causes a mean offset in the concentration signal ŽLudwig and Hanes, 1990; Schoelhamer, 1993.. The offset was chosen as the mode in the lowest 5% of the concentration values, or, if absent, choosing the 5% value for each time-series distribution, as suggested by Beach and Sternberg Ž1988.. The longshore sediment flux is computed for each OBS and EMCM pairing, shown schematically in Fig. 5, by multiplying the instantaneous concentration, cŽ x, z,t i ., times the instantaneous longshore velocity, Õ Ž x, z,t i ., and time-averaging the products as suggested by Jaffe et al. Ž1984., Sternberg et al. Ž1984. and Hanes Ž1988.. FŽ x, z. s
1
N
Ý c Ž x , z ,t i . Õ Ž x , z ,t i . N
Ž 2.
is1
where F Ž x, z . is the longshore sediment flux, which is the rate per unit area in the x Žcross-shore. and z Žvertical. directions. An example of the flux at different distances above the bed for the storm on 2 April 1996 is given in Table 2. Each flux value is assumed to represent the flux through a portion of the water column from the mid-point between the sensors positioned below and above it, Fig. 5. The bottom concentration sensor was nominally 3 cm above the bed, but ranged from 2 to 5 cm. It was assumed that the flux was constant from the sensor to the bed.
Fig. 5. Schematic of the SIS instrumentation. Sediment concentration ŽOBS. and current meters ŽEMCM. are paired to compute sediment flux. Dashed lines separate regions of vertical integration. C s concentration, Vs velocity, Z s vertical distance.
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Table 2 Flux values for 2 April 1996 storm. Sensors buried in the bottom were not included Gauge pairings Del z Flux Žm3 my2 hy1 . Žm. OBS EMCM Position Žm. 488 430 393 357 335 277 238 201 174 Depth Žm. y6.4 y4.9 y4.1 y3.9 y4.0 y5.2 y4.0 y2.3 y1.8 1 2 3 4 5 6 7 8
1 2 2 3 3 4 4 4
0.08 0.12 0.08 0.08 0.07 0.03 0.03 0.04
0.51 0.98 1.68 1.86 0.70 1.38 0.84 1.64 2.30 2.18 2.83 2.21 1.74 1.03 28.31 23.40
5.17 19.21 7.94 12.36 9.00 10.29 10.11 7.86
2.06 3.86 3.81 8.07 9.02 7.11 na na
1.33 2.56 2.50 4.23 7.78 na na na
3.11 1.93 9.54 6.55 8.02 4.19 14.88 8.91 9.43 4.17 14.15 9.36 11.65 6.07 17.12 10.48 13.96 13.46 21.56 17.41 na 9.58 na 11.03 na 2.90 na 5.40 na 2.71 na 5.20
Swash approximately located at position 165 m. na s not available Žinstrument problem..
Summing through the water column yields M
GŽ x . s Ý F Ž x , zj . d zj
Ž 3.
js1
where GŽ x . is the vertically integrated flux per unit cross-shore length. Assuming the flux at each cross-shore location represents the mean flux from the midpoints between the next position inshore to the next position off shore, the total or ‘bulk’ suspended-load LST rate can be estimated by L
I s Ý GŽ xk . d xk
Ž 4.
ks1
where I s bulk longshore sediment transport rate. These rates were converted to volume transport rates, Q Žm3 hy1 ., considering the density of quartz sand as 2650 kg my3 , the density of seawater as 1025 kg my3 , and the solid fraction as 0.6. The flux was assumed to be constant from the innermost measurement location to the mean shoreline position.
3. Results 3.1. LST rates Measurement based LST rates, along with the wind and wave conditions for the five storm transects are shown in Table 3. Positive transport values in this table represent transports from north to south, while negative values denote transports from south to north. Because of high winds and local seas from the north during the 12 March storm, the SIS was operated on the north side of the pier. Subsequent calculations indicated that the LST rate for this time was actually dominated by swell out of the south. Thus, these measurements were inadvertently taken in the lee of the pier, at least in terms of the
310
Storm date
14-Nov-95 11-Mar-96 12-Mar-96 27-Mar-96 2-Apr-96
Winds
Waves
LST Rates
Speed Žm sy1 .
Direction Ždeg.
Height Žm.
Period Žs.
Direction Ždeg.
Measurements North side Žm3 hy1 .
8 16 16 15 10
146 60 80 30 70
1.9 2.8 3.5 1.8 1.6
7.6 7 12 6.7 7
14 10 y14 25a 26
530 1780 y450 560 670
South side Žm3 hy1 .
610
Computed CERC ŽUS Army Corps of Engineers, 1984. Žm3 hy1 .
Komar Ž1998. Žm3 hy1 .
590 1120 y2980 800 590
660 1210 y780 400 240
Measurement based LST rates are paired with predictions. CERC ŽUS Army Corps of Engineers, 1984. formulation assumes that LST is associated with the single peak in the wave spectrum. The formulation of Komar Ž1998. includes velocity terms that could account for processes other than the peak wave forcing. Note: directions relative to shore normal, qnorth–south. a Visually estimated; directional array data not available.
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Table 3 Wind, wave, and LST rates
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direction of LST. A measure of the effect of operating in the lee of the pier was investigated during the 14 November 1995 storm, when measurements were made on both the north and south sides at each cross-shore sampling location. As can be seen in Table 3, the transport rates differed by less than 15%. Conditions during transects taken on 14 November 1995, 27 March, and 2 April 1996 were quite similar in terms of wave heights and periods, 1.8 m and 7 s, respectively. Wave approach angles varied from 148 to 268 north of shore normal. Measurement based LST rates for all three transects were in the neighborhood of 600 m3 hy1 . Wave conditions during the 11 March 1996 transect were somewhat more energetic with a wave height of 2.8 m; however, the period for these waves was still only 7 s. The bulk transport rate for this transect was almost 1800 m3 hy1 , increasing almost linearly with wave height to the second power. As the storm continued to develop over the March 11–12 period, wave heights grew to 3.5 m by the time of the 12 March 1996 transect. Bulk transport rates on the 12th, however, actually diminished from the transport rates on the 11th and changed directions. Some apparent reasons for this decrease will be given in Section 4. 3.2. Cross-shore distribution of LST Bathymetric profiles during all transects had characteristic longshore bars as shown in Fig. 6. These profiles are presented as depth versus normalized offshore location, where
Fig. 6. Profiles measured with SIS. Non-dimensional cross-shore position normalized by bar crest position such that the swash Žapproximate shoreline position. is at 0 and the bar is at 1.
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Fig. 7. Cross-shore distribution of longshore flux. Positive values indicate transport from north to south and negative values indicate transport in opposite direction.
the approximate local mean waterline Žmean swash position. is at 0 and the bar is at 1. The bar position for the four transects between 11 March and 2 April 1996 varied by only q5 m. While 190 m separated the bar from the swash for all five storms, the bar
Fig. 8. Mean and cumulative cross-shore distributions of longshore flux with a mean profile for the four storms without flow reversals.
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position on 14 November 1995 was 40 m landward of the other four transects. Video and visual observations indicate the surf zone during these times typically began near position 1.2 in Fig. 6. Cross-shore distributions of longshore flux for all five transects are shown in Fig. 7. Although the distribution of transport varies, this Figure illustrates that it is often bimodal during the storm periods studied. The 12 March 1996 transect also has a bimodal distribution, however, there are direction reversals near cross-shore position 0.9 and seaward of position 1.4. These reversals occurred where the wave influence on longshore sediment transport apparently was minimized. Fig. 8 shows the mean distribution of LST for the four cases with no flow reversals, along with the cumulative amount of transport calculated starting at the shore and extending to locations seaward of the bar. From this Figure we see that as much as 20% of the total LST in these transects occurred seaward of the bar. It was observed that in this area the waves were shoaling, with only occasional wave breaking.
4. Discussion Presently, no reliable electronic instruments exist which can measure bed load and so called ‘sheet flow’ just above the bed within the surf zone; thus, it is difficult to quantify the contribution of bed load to the total transport rate during storms. Komar Ž1990., who summarizes the conflicting opinions of the importance of bed load found by Komar Ž1978., Inman et al. Ž1980., Kana and Ward Ž1980., Coakley and Skafel Ž1982., Downing Ž1984., Sternberg et al. Ž1984., and Kraus and Dean Ž1987., suggests that the relative roles of suspension versus bed-load transport on beaches must remain open and in need of further research. For the purpose of this discussion, we will make the tacit assumption that transport is reasonably well estimated by the instrumentation package on the SIS. Assuming a constant sediment flux between the bottom sensor and the bed suggests a constant sediment concentration and velocity. Although this probably underestimates the concentration and over estimates the velocity, it allows us to rely only on measured values. Future efforts will be required to provide a more definitive answer to the quantitative role of bed load in LST rates during storms. Another potentially important but difficult to document region of transport is the swash zone. Recent reviews of sediment transport investigations ŽSternberg et al., 1989; Komar, 1990; Kraus and Horikawa, 1990. indicate that between 5 and 60% of the total transport could be accounted for in the swash zone, with this percentage tending to be inversely related to wave height. Since we are dealing with storm conditions in the cases studied here, it is likely that the sampled area includes most of the transport; however, subsequent efforts are planned to investigate this hypothesis. 4.1. Longshore flux distributions and transport rates For almost five decades now, since the work of Watts Ž1953., engineers have attempted to relate LST rates to wave and current processes. This has produced hundreds of results that have been summarized by Bodge Ž1989., Sternberg et al. Ž1989., Komar Ž1990., Kraus and Horikawa Ž1990., Zyserman Ž1992., Schoonees and Theron Ž1993.,
314
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and others. However, since this investigation is focusing on transport rates over very short periods of time, it may not be appropriate to compare these results directly to many of the previous investigations, since they were primarily based on long-term averaging. The few specific studies that attempt to provide short-term estimates of LST rates based on multiple cross-shore measurements are discussed below. Kana and Ward Ž1980. made measurements along the FRF pier using the trap devices of Kana Ž1976. both during storm and post-storm conditions. They found the total load of sediment in suspension was approximately 10 times higher during the storm, and the longshore flux was as much as 60 times more than during the post-storm period. During their storm with 2.3 m waves, the longshore transport rate was over 900 m3 hy1 which falls within the 600 to 1700 m3 hy1 for storms with waves of 1.8 to 2.8 m, respectively, during this investigation. Also similar to this investigation was their finding that suspension dominated and sediment concentration peaked near the breaker location. Sternberg et al. Ž1984. used five arrays of OBS sensors with simultaneous current measurements at Leadbetter Beach, Santa Barbara, CA and found a single peak in the longshore sediment flux distribution near the mid-surf location between the bar and the beach. Similar peaks near the mid-surf position were found for this study on 14 November 1995 and 11 March 1996. They found the flux to always decrease shoreward and seaward which differs from the findings of this study which found peaks nearer the swash for the storms in 12 March, 27 March, and April, 1996. Their study with wave heights generally less than 1.8 m, resulted in a maximum transport rate of 217 m3 hy1 , which is much less than the rates for the present study. However, considering that their five measurement locations generally covered less than 60% of the surf zone and did not include the region where secondary peaks were observed during this study, the lower transport rates are understandable. An observation also seen in this study was that the lowest OBS sensor, located 3.5 cm above the bed, showed a wide range of concentrations, occasionally reaching a value of 180 g ly1 during individual suspension events. Kraus and Dean Ž1987. made measurements at the FRF in approximately 1 m waves. They found the cross-shore distribution took a number of different shapes, but frequently was characterized by peaks in the outer surf zone and at the swash similar to the peaks found in this investigation. Their maximum LST rate was 228 m3 hy1 during moderate 1.2-m waves. The present study was intended to extend their work into larger waves during storm conditions. For many years, visual observations of wave breaking has suggested that regions of wave breaking tend to be associated with high concentrations of sediment within the water column. Recently, OBS measurements from Yu et al. Ž1993. have confirmed that maximum values in the sediment concentration occur in the vicinity of wave breaking. Data from all transects studied here show this same trend, with highest concentrations occurring in regions of wave breaking and bore formation. On the other hand, during storm conditions on barred beaches, the region of maximum longshore currents is typically found within the trough area shoreward of the outer bar ŽSmith et al., 1993.. The results shown here indicate that the distribution of LST tends to be dominated more by the concentration factor in Eq. Ž2. than by the longshore current factor. The present study and the prior studies mentioned above suggest that one can not assume the sediment flux is always distributed across the nearshore in the same way as
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the longshore currents. It is also evident that the cross-shore distribution of longshore sediment flux varies, and that variation is apparently a function of the wave height and thus water level. Future investigations should include sequences of measurements during both high and low tides during the growth, peak, and waning stages of the storms. 4.2. Comparison to predictions Various arguments have been made for different relationships between wave conditions and LST rate ŽSayao and Kamphuis, 1983; Komar, 1990; Watanabe, 1992; Schoonees and Theron, 1994; Drake and White, 1995.. It is beyond the scope of this paper to examine all of these proposed relationships relative to the results of this field study; however, in order to provide some perspective to these results it is instructive to compare with some theoretical form. Two methods are presented; the first is the energy flux method, sometimes termed the ‘CERC’ formula. This formulation, arguably the most widely used in the world, requires only wave height, period, and direction as input. The other method is that of Inman and Bagnold Ž1963. which considers that the sand is mobilized by the mean wave stress and moved by the longshore velocity. Inclusion of the velocity is appealing because it helps account for transport mechanisms other than simply breaking waves, such as local winds and tides. 4.2.1. Comparison to the energy flux method The energy flux method is an empirical formulation is described in the Shore Protection Manual ŽUS Army Corps of Engineers, 1984. as Qs
KPlb
Ž rs y r w . gaX
Ž 5.
where Q is the volume transport rate and K s empirical coefficient, K s 0.39 when P lb is based on significant wave heights; Plb s longshore energy flux factor at breaking; rs s density of quartz sand; r w s density of water; g s acceleration of gravity; aX s solid fraction of beach sediment Ž aX s 1 y porosity.. The longshore energy flux factor is given in terms of the wave conditions at breaking. Plb s Ž ECg . bsin a cos a
Ž 6.
where E is the wave energy, Cg is the wave group velocity, and a is the wave crest angle relative to the beach. The wave height, period, and direction parameters were taken as the values associated with the peak of the directional spectrum determined at the FRF directional array. The values were shoaled to breaking, conserving wave energy flux according to Ed .a . Cgd .a . cos a d .a .s E b Cgb cos a b
Ž 7.
and the waves were refracted using Snell’s Law: sin a d .a . Ld .a .
s
sin a b Lb
Ž 8.
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where L is the wavelength, ‘d.a.’ is at the FRF directional array, and ‘b’ denotes breaking. The results given in Table 3 shows that the magnitude of the total transport seems to be consistent with the measurement based values, with the notable exception of the 12 March 1996 transect. This is not surprising, since at the time of this transect, the local seas and winds were acting in opposition to the swell waves and the CERC formulation assumes all of the energy is associated with a single peak in the wave spectrum. However, sometimes the predictions are both higher and lower than the measurement based values. This suggests that there are other terms, possibly accounting for such influences as the directional distribution of energy within the wave spectrum, local winds, grain size, beach slope, swash processes, or bed-load transport, that should be included in an accurate prediction methodology for LST. For the 12 March 1996 storm, where winds and local seas opposed the swell, the measurement based LST rate was an order of magnitude lower than predicted. Since this situation is not atypical of incident waves in coastal areas, it seems that simple LST formulae based on parametric representations of wave conditions alone may not be able to provide accurate estimates of total LST rates for some individual storms conditions. 4.2.2. Comparison to the Inman and Bagnold method A second formulation was compared to the measurement based LST rates. This method was chosen because it includes longshore and orbital velocity terms that should
Fig. 9. Predicted LST rates using CERC ŽUS Army Corps of Engineers, 1984. and the formulations of Komar Ž1998. compared to measurement based LST rates.
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provide more reasonable predictions when the LST rates were the result of more complicated processes and the assumptions of the CERC formulation were not valid. The analysis of Inman and Bagnold Ž1963. resulted in the following formulation I l s K X Ž ECg . b VrUm
Ž 9.
where K X is a dimensionless coefficient, V is an average longshore current velocity near the mid-surf position, and Um is the maximum horizontal orbital velocity of the waves evaluated at the breaker zone. Komar Ž1998., using the value of K X s 0.25 determined by Kraus et al. Ž1982. and linear-wave theory, reduces Eq. Ž9. to the following LST rate formulation when using significant wave heights, I l s 0.044 r gH 2 V .
Ž 10 .
Volumetric LST rates were determined with this formulation using currents from the approximate mid-surf position and the breaking wave statistics as described in the previous section. Comparisons of these results and those for the CERC formulation to the LST rates determined from the measurements are shown in Fig. 9. The diagonal line in the figure represents the best agreement. The formulation of Komar Ž1998. agrees much better with the measurement based LST rates than the CERC formulation during the 12 March 1996 transect. Recall that this was when swell waves opposed local seas and winds. Otherwise, both formulations are comparable. While the Komar formulation might be viewed as more fundamental because it considers the processes of sand transport, in so far as the waves mobilize the sand and many forces can contribute to the longshore velocity that move the sand, it does require velocity measurements that often are not available at a given project site.
5. Summary Many of the recent reviews of the longshore sediment transport literature have arrived at essentially the same conclusion, namely that comprehensive and well-instrumented synoptic field measurements of fluid processes, sediment transport, and nearshore morphologic change are essential for verifying theoretical and laboratory results. Almost all of the measurement based transport rates reported in the literature are for mild wave conditions with wave heights less than 1.8 m and bulk transport rates under 230 m3 hy1 . However, when considering an annual sediment budget, a few storms with much higher transport rates usually contribute a significant portion of the total. The consequence is that the conditions that are most important to predict are outside the range of our ability to calibrate our models. These are primary reasons for developing the SIS, which has now been successfully operated during five storms. Highly resolved processes measurements provided the basis for computing accurate LST rates during storm conditions with significant wave heights up to 3.5 m. Direct measurements at a minimum of nine positions across a barred profile in waves up to Hm0 s 3.5 m provided documentation of the cross-shore distribution and total suspended-load LST rates. The storms on 14 November 1995, 27 March, and 2 April 1996 had similar wave conditions, near 1.8 m, 7 s, and approaching 208 north of shore
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normal, and resulted in transport rates on the order of 600 m3 hy1 . An approximate 50% increase in wave height during the 11 March 1996 storm resulted in an approximate factor of three increase in the longshore transport rate. On 12 March, the wind and seas were from the north, while the spectral peak was associated with a southerly swell. The complexity of these opposing processes resulted in northward transport considerably lower than might be expected if all of the energy was associated with only the peak of the wave spectrum. The cross-shore distribution of the LST showed a variety of shapes, however two peaks were often observed. One peak was observed between the mid-surf position and the beach. On four occasions a second peak occurred on the seaward side of the bar associated with wave shoaling and breaking. The measurements showed approximately 20% of the sediment was transported alongshore seaward of the surf zone. It was observed that in this area the waves were shoaling, with only occasional wave breaking. Transport rates were computed by the energy flux method using the CERC formulation based on spectral peak wave parameters at breaking determined from the FRF directional array and were compared to the measurement based transport rates. The predictions compared reasonably well except on one occasion when the transport was complicated by opposing processes. Considering the CERC formulation assumes all of the wave energy was associated with a single direction and only the one oblique wave train is responsible for the longshore current, the poor performance is not unexpected when opposing processes are present. Komar’s formulation agreed with the LST rates computed from the measurements as well as the CERC formulation did for four of the storms. Komar’s formulation, which would be expected to account for additional processes that contribute to the generation of the longshore currents, agreed much better with measurement based LST rate for the 12 March 1996 storm when the processes were complicated by opposing forces. Where current measurements are available, Komar’s formulation would appear to have a wider application than the CERC formulation. This is an ongoing study designed to fill the void of information directly relating storm processes to LST rates. While additional effort needs to be placed in conducting bed load and swash transport measurements, the SIS provides the capability to document a wide range of processes and initial conditions from which variations in LST rates can be determined. Obtaining multiple measurements under similar conditions also should lead to reduced random measurement errors often associated with field data collection.
6. Conclusions Many field studies have provided LST rates for coastal locations. Yet few have attempted direct measurements of the fluid processes and sediment concentrations during storms when wave heights exceed 1.8 m. This investigation extends those previous works into higher wave conditions. The SIS can operate during severe storm conditions with significant wave heights in excess of 3 m, wind speeds of 20 m sy1 , and currents of 2 m sy1 ; however, the effect of the research pier has not fully been quantified and additional experiments are needed to
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address this question. Additionally, the magnitude of the LST in the swash zone should be quantified in the future. For three similar storms with wave heights near 2 m, the transport rates were on the order of 600 m3 hy1 . A 50% increase in wave height during a fourth storm resulted in a factor of three increase in the longshore transport rate. The cross-shore distribution of the LST often showed a bimodal distribution with a peak associated with wave shoaling and breaking on the seaward side of the bar and another peak between the mid-surf and beach locations. Approximately 20% of the sediment transported alongshore occurred seaward of the surf zone. Predicted LST rates using the ‘CERC’ formulation were consistent with the values determined from measurements, except on one occasion. On that occasion the winds and seas opposed swell waves and the formulation presented in ŽKomar, 1998. agreed much better with the measurement based LST rates. For the other storms, Komar’s formulation and the CERC formulation were sometimes higher or lower than the measurement based LST rates. This suggests additional terms are required for an accurate prediction of LST rates under all storm conditions.
Acknowledgements This work was conducted under the Sediment Transport Processes Work Unit, Coastal Sedimentation and Dredging Area, of the Coastal Research Program, of the U.S. Army Corps of Engineers. Permission was granted by the Chief of Engineers to publish this information. The author thanks Jay Lockhart, John Housley, Bob Hamilton, Doug Nyman, Don Resio, and, especially, Doug Charlton for helping make the SIS and this research program possible.
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