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Migration of swash zone, step and microtopographic features during tidal cycles on an estuarine beach, Delaware Bay, New Jersey, U.S.A.
Marine Geology, 92 (1990) 147-154
147
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Letter Section
Migration of Swash Zone, Step and Microtopographic Features During Tidal Cycles on an Estuarine Beach, Delaware Bay, New Jersey, USA KARL F. NORDSTROM and NANCY L. JACKSON Center[or Coastal and Environmental Studies, Rutgers University, New Brunswick, N J 08903 (U.S.A.) (received August 31, 1989; revision accepted January 12, 1990)
Abstract Nordstrom, K.F. and Jackson, N.L., 1990. Migration of swash zone, step and microtopographic features during tidal cycles on an estuarine beach, Delaware Bay, N e w Jersey, USA. Mar. Geol., 92: 147-154. A time-series analysis of changes in surface elevation on the foreshore of an estuarine beach was conducted to compare movement of topographic features in the swash and breaker zones with results derived from study of ocean beaches. Wave heights on the upper foreshore during the fieldstudy averaged 0.15 m, with an average period of 2.4 s. The average height of the beach step was 21 ram. Smaller elevation changes, up to 12 m m in height, occurred at 1015 rain periods. There were more small elevation changes, greater longshore variations in these changes, and greater variation in the form of the beach step during the rising tide than during the falling tide. The variation in the small elevation changes and the beach step are attributed to greater short-term fluctuations in beach saturation during rising tides, causing greater variability in the velocity of swash and backwash.
Introduction
Studies of morphologic changes on sandy beaches during tidal cycles record rhythmic changes in beach profile on foreshores, associated with migration of the zone of sediment reworking by waves and swash (Duncan, 1964; Strahler, 1966; Sallenger and Richmond, 1984; Howd and Holman, 1987). Strahler (1966) documented patterns of deposition and scour that migrate at the rate of movement of the swash and breaker zones. Short-term sediment-level changes that migrate independently of these features are documented in the swash zones of ocean beaches. These changes have 0025-3227/90/$03.50
been called oscillations, and their formation is attributed to low-frequency waves (Sallenger and Richmond, 1984; Howd and Holman, 1987). Depictions of the form of the beach profile that incorporate both the overall step form and sediment-level oscillations do not exist. Most studies of short-term microtopographic beach change have been conducted in ocean environments, where it is difficult to monitor sediment-level changes under the high-energy conditions occurring in the lower swash zone (Sallenger and Richmond, 1984) or in water seaward of this zone (Eliot and Clarke, 1988). These constraints make it difficult to obtain readings of the temporal and spatial variability
© 1990 Elsevier Science Publishers B.V.
148
of topographic changes in the lower swash zone and the breaker zone in both the shore-normal and longshore directions. It is assumed that sediment-level fluctuations vary at different stages in the tidal cycle and under different wave conditions. This study is designed to provide perspective on the effects of water-level changes at different stages in the tidal cycle by comparing surface-elevation changes at rising and falling tides on the same day on a low-energy estuarine beach, during periods of near-constant wave-energy inputs. The less energetic estuarine environment allows for a sampling plan that extends from the upper limit of swash to a location seaward of the breakers at all stages in the tidal cycle. This provides the opportunity to compare and relate topographic changes associated with the beach step to the changes in the swash zone and to show how these changes vary at different locations on the beach. The opportunity is also provided to examine the applicability of models of tidal-cycle and short-term changes developed on swell-dominated ocean beaches to fetch-restricted estuarine beaches.
Description of experiment Study-site characteristics The field site is a sand beach on a transgressive estuarine barrier in Delaware Bay, New Jersey (Fig. 1 ). Tides are semi-diurnal, with a mean range of 1.6 m and a spring range of 1.9 m (NOAA, 1989). M a x i m u m fetch is 48 km to the northwest. Dominant waves are locally generated wind waves. Visual wave data gathered at a location 4.9 km north of the site over a 1 yr period (Nordstrom and Jackson, 1989) indicate a mean significant breaker height in this region of 0.19 m, with a breaker period of 2.7 s. Mean longshore current velocity, measured just landward of the breaker zone, is 0.13 m s - 1. Net drift is to the north, but southerly transport is common. The beach has a 6.20 ° (11%) planar upper
K.F. NORDSTROM AND N.L. JACKSON
foreshore and a low-tide terrace of <0.50 ° ( < 1% ). The steep upper foreshore and low wave energies result in a narrow surf/swash zone (2-3 m wide on non-storm days) and a conspicuous step that migrates with the plunge point of the breaking waves. A wide, dissipative surf zone, with spilling breakers and no conspicuous step, occurs on the low-tide terrace during low water levels. The overall form of the beach at the field site is common for shorelines with a large tidal range relative to wave weight (Inman and Filloux, 1960). These beaches retain steep slopes on the upper foreshore under all but the largest storms (Nordstrom, 1980).
Field methods The experiment was conducted between September 27 and 30, 1988. Wave heights and nearbottom horizontal water velocities were monitored using three Marsh-McBirney Model 511 bidirectional electromagnetic current meters placed on the upper foreshore and low-tide terrace (Fig. 2). T w o pressure transducers were co-located with two of the current meters. Surface sediment samples were taken on the upper foreshore and on the low-tide terrace to determine sediment characteristics at different elevations on the beach. N e t changes in beach elevation were measured at 5 m intervals using a 15 m X 35 m grid. All locations are identified by horizontal distance seaward of an arbitrary, shore-parallel baseline on the upper beach. Each sampling point for beach elevation data was identified by a 6.4 mm diameter 0.91 m long rod driven 0.6 m into the sand. Locations on the grid are identified as north (N), middle (M) and south (S). Elevations were measured at all sites at low tide each day. Additional elevation measurements were taken at 5 rain intervals on the rods on the upper foreshore while the swash and breaker zones migrated through the 15, 10 and 5 m sampling lines. The elevation value is the mode of ten readings taken near slack water in the u p r u s h backwash cycle.
MIGRATION OF FEATURES DURING TIDAL CYCLES ON AN ESTUARINE BEACH
149
OCEAN are Bay
Delaware FIELD SITE
Bay
0
5kin 75 a
Fig. 1. S t u d y area.
o
,oS ,.o /
• 8AMPLE POINT FOR STEP MIGRATION CURRENT METER ( { ~ w r r H PRE88URE TRANSDUCER) --
SWASH LIMIT DURING EXPERIMENT
Fig. 2. Field s a m p l i n g location.
In addition to the ten readings at each rod, the 5 min sampling period allowed sufficient time for visual observations of the location of highest breakers and m a x i m u m runup excursions. The north, middle and south rods were sampled in the same sequence, so the intervals
within each times series are similar. The swash and step zones migrate through the three sampling stations on each shore-parallel line at slightly different times. This time offset occurs because of the differences in sampling times as well as slight differences in the elevations of the beach at the sampling stations. The range in elevations at the 10 m rods at the beginning of the study was 35 ram. To minimize disruption of flow, all readings were taken while standing on the downdrift side of the rods. The multiple readings at 5 rain intervals smooth the variability associated with high-frequency water motions and minimize the effects of measurement error. Comparison of averages of consecutive series of ten readings reveals differences of < 1.8 ram. The 5 min time interval is considerably shorter than the 30 rain interval used by Duncan (1964) and Strahler (1966), but was required because of the rapid excursion
150
of the narrow swash zone up the foreshore. The interval is longer than that used by Sallenger and Richmond (1984) and Howd and Holman (1987), but it is considered sufficiently short to characterize the form of the beach step through time and sufficiently long to allow for accurate measurements at the three rods on each shoreparallel line.
K.F.NORDSTROMANDN.L.JACKSON
RISING TIDE (0920-1030) 0.06 " 0.04 ,~
~
NIO
0.02 0.00 -0.02 -0.04 $10
"0.06
Results Mean grain size is similar at the 5 m and 10 m sampling locations (0.56 and 0.53 ram) but coarser at the 15 m sampling locations (1.55 mm). Sediments are finer on the low-tide terrace (0.27 ram). Wave height during the period when the breaker zone and swash were at the 10 m and 5 m lines averages 0.15 m with a period of 2.4 s.
Variability o[ surface-elevation changes
MIO
OSCILLATIONS
I
0
20
I
40
I
60
I
80
FALLING TIDE (1350-1510) 0.06 N10
0.04 0.02
MIO
0.00 -0.02
$10
-0.04 -0.06
Average step height at all nine sampling locations during the 3 day period is 21 ram. Step height varies from a minimum of 8 mm to a maximum of 36 mm at M10 (the middle shorenormal line, 10 m from the baseline) during falling tide on September 29 (portrayed in Fig. 3 ). These step heights correspond to the dimensions of steps measured on sheltered beaches under similar wave energies (Otvos, 1965; Hughes and Cowell, 1987). The time series presented in Figs. 3 and 4 identify elevation changes as the swash zone and step migrate past sampling points. The data for September 29 are presented because this is the day with highest wave energies, and the tidal range was sufficiently great that all three rows of sampling stations were within the surf zone during daylight when the 5 rain readings could be taken. The width of the swash zone varies during the tidal cycle. The average length of swash uprush on September 29 is greatest at high water (Table 1 ). The duration of the swash is greatest at the 5 m locations, which are in the swash at
I
100
I
0
20
40
FALLING
60
80
TIDE
100
(1530-1625)
0.06 0.04
,,~,.m~t~"d~'~
N15
0.02 M15
0.00 -0.02
S15
-0.04 -0.06
I
I
I
I
20
40
60
80
I
100
MINUTES Fig. 3. Time series of surface-elevation changes measured in the breaker and swash zones on September 29, 1988. High water was at 1156; low water was at 0523 and 1816.
the high water still-stand. The values of swash duration at the 10 and 15 m locations do not include the time that the sampling stations are under the influence of the backwash vortex bayward of the swash zone, so the duration of
151
M I G R A T I O N OF F E A T U R E S D U R I N G T I D A L C Y C L E S O N A N E S T U A R I N E B E A C H
TABLE1
TABLE2
Average length and excursion rate of swash, September 29, 1988
Highest cross correlation coefficients for surface-elevation changes at 10 m sampling stations, September 29, 1988
Location in 5 m grid
Uprush Swash zone Duration length excursion rate of swash (m) (mm min -1) (min)
Sites
Coefficient
Lag (5 rain intervals)
0.703 0.606 0.631
+ 1 0 +2
0.885 0.763 0.891
- 1 -2 - 1
Rising tide Rising tide 10 m High water 5 m Falling tide 10 m 15 m
2.1 3.2 1.7 0.8
60 18 55 55
35 175 30 15
N10 and M10 N10 and S10 M10 and S10
Falling tide
pronounced sediment levelchanges in Fig. 3 exceeds these time periods. The greatestchanges in surface elevation are associated with migration of the step (Fig. 3 ). Smaller oscillationsoccur in some of the time series. Step height and sediment-level oscillations are lowest along the 15 m line (Fig. 3) where breaker and swash energy are lessat lowwater levels when waves are partially dissipated on the low-tide terrace.The short period of time that the 15 m line is exposed to breaker and swash processes also tends to damp gross and net changes in sediment level. Inspection of data for the 10 m sampling locations (Fig. 3) indicates that there is greater longshore variation in surface-elevation oscillations during rising tide than during falling tide,and there are more oscillationswithin the swash zone during risingtide. The results of a cross correlation analysis with the trend and mean removed from the time series (Table 2) confirm greatervariationduring risingtide.The correlations are higher on the fallingtide, and the lag times show a regular progression from
Method used is identified in S G C (1986). The time lags occur partly because of small differences in beach elevations and sampling times and partly because of variation in the sediment-level oscillations.
north to middle to south. Correlations are lower on the rising tide, and the lag times show no clear relationshipbetween space and time. Sediment-level oscillationshave the shortest periodicitiesat location $10, and there are more oscillationsat this site during the 3 days than at the other sampling points 10 m from the baseline (Fig. 3). There are only three oscillations at this site (Fig. 3) because of the rapid migration of the narrow swash zone through the sampling station at mid-tide (Table 1). There are more oscillationsin the record for the sampling stations 5 m from the baseline (Fig. 4) where the step approached to within 0.3 m of the stations during the relativestill-standnear high tide.The amplitude of the largestof these oscillationsis 12 ram. The time seriesfrom the swash zone at N 5 (Fig. 4) shows greater long-
Record
0.06 0.04
N10 and N10 N10 and S10 MIO and SIO
RISING TIDE
I A
Record
I
J e
I
FALLING TIDE
0.02 ~J
0.00 -0.02
2'0
4'0
6'0
8'0
100
120
140
1;0
i
180 200 MINUTES
Fig. 4. Time series of surface-elevation changes measured in the swash zone near high water on September 29, 1988.
152
K.F. NORDSTROMAND N.L.JACKSON
TABLE 3 Sediment-elevation statistics for 5 m sampling stations Location
Net change (m)
Heights ( R M S in meters)
N5 Rising Falling
+ 0.014 - 0.008
0.011 0.008
M5 Rising Falling
+ 0.007 - 0.008
0.009 0.005
$5 Rising Falling
+ 0.003 - 0.011
0.013 0.010
Heights (root mean square) are calculated as twice the standard deviation of the time series.
period oscillations during rising water level than during falling water level. This finding is supported by comparison of the heights of oscillations at the 5 m stations with the mean and trend removed (Table 3). Records of water level and horizontal velocities taken near high water on the rising and falling tides (records A and B, Fig. 4) show that there is little difference in process conditions just bayward of the step. Mean water levels recorded on the pressure transducers during the rise and fall differ by only 0.027 m. There is a difference of only 0.01 m in wave heights at the 12.5 m instrument station (Fig. 2). Root mean square onshore velocities at the 7.5 m station, within 1.6 m of the breakers, are 0.30 and 0.29 m s-1 on the rise and fall, and offshore velocities are 0.26 m s -1 at both these times. The magnitudes of the sediment-level oscillations at these times are 12 mm on the rising tide and 6 mm on the falling tide.
Relationship between elevation changes and beach form A time series of elevation changes may be viewed as a portrayal of the form of the beach if the rate of tidal rise and rate of motion of beach features past a sampling point is con-
stant. This assumption appears valid for the three series presented in Fig. 3, which are located near mid-tide level. A slight difference in the exaggeration of relief occurs at falling tide relative to rising tide because the rate of movement of still water level is 4% greater, but this difference is too small to obscure the conspicuous differences. To convert the time series to a conceptualization of step form, the left-hand side of Fig. 3 should be viewed as the landward side on the rising tide and the bayward side on the falling tide. The surface-evaluation changes and the forms associated with these changes (Fig. 3) are similar to those seen in the composite diagram of phases of beach change presented in Strahler (1966). The scour phase preceding the step deposition phase is identifiable on several of the time series, and the step deposition phase is identifiable on all of the time series. The initial deposition phase identified by Strahler {1966) near the upper limit of swash is difficult to distinguish from measurement error, and this phase is not conspicuous in the time series.
Discussion and conclusions The frequencies of the oscillations in the swash zone at Delaware Bay fall within the 615 min oscillations identified by Sallenger and Richmond (1984), and are roughly comparable to the 8-10 min oscillations identified by Howd and Holman (1987). The heights of the oscillations are about 20-25% of the heights reported in those studies. The frequency of sediment-level measurement in the present study enabled a more detailed characterization of the form of the step than was possible in Strahler (1966), and greater relative differences are revealed during rising tide and falling tide. The greater variety in the form of the step and the magnitude of swash oscillations during rising water levels than during falling water levels suggests that the differences may be related to the degree of beach saturation, as discussed by Wadell
MIGRATION OF FEATURES DURING TIDAL CYCLES ON AN ESTUARINE BEACH
(1976). Fluctuations in the elevation of the beach water table occur at incident wave frequencies as well as at the frequencies of surf beat, shelf waves, and sub-tidal and tidal changes (Duncan, 1964; Wadell, 1976; Eliot and Clarke, 1988). Fluctuations are less frequent than incident wave periods, because the beach acts as a low-pass filter (Wadell, 1976). The low water table during rising tide permits greater swash infiltration, reducing backrush. The greater swash energy, relative to backrush, during rising tide, moves more sand onshore, so there is a greater likelihood of formation of sediment-level oscillations than during falling tide and a greater likelihood that the oscillations will be larger. The fluctuations also may be more common during the rising tide because there is a greater likelihood that parts of the beach face are alternately saturated and unsaturated at that time. The water table is high relative to the position of the swash on the falling tide (Duncan, 1964). There is less potential for percolation into the beach, and this decreases the potential for deposition. An alternative model for generation of sediment-level oscillations in the swash zone is that they are formed near the step at the seaward extreme of the swash in response to a temporary rise in water level due to a long period wave (Howd and Holman, 1987). Their model predicts erosion on the steep face of the step, and deposition of sediment landward of the step where the slope is flat. The migration of this perturbation on steep beaches is believed to be driven by the dominant swash oscillations at the incident gravity-wave frequency or its first subharmonic ( H o w d and Holman, 1987). The results of the present study do not confirm or refute H o w d and Holman's model, b u t they strengthen the evidence that beach water-table fluctuations play a role in the generation of sediment-level oscillations. If long-periods waves generate the oscillations without regard to water-table fluctuations, there should be little difference between the time series on rising and falling tides.
153
The results of this field study document the existence of sediment-level oscillations on an estuarine beach that are similar to those identified on ocean beaches, although the magnitude of these oscillations is lower. The phases of beach change associated with migration of the swash during tidal cycles identified by Duncan (1964) and Strahler (1966) are also documented. The sediment-level oscillations may be envisaged as smaller bedforms superimposed on the larger scale step form. The results of this study indicate that there is a difference in the variability of sediment-level oscillations and in the form of the beach step during the rising tide and falling tide. These differences are believed to be related to the degree of beach saturation.
Acknowledgements Financial assistance for this research was provided by the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Office of Coastal Zone Management under provisions of Section 305 of the Federal Coastal Zone Management Act (P.L. 92-583) in cooperation with the State of New Jersey Department of Environmental Protection, Division of Coastal Resources.
References Duncan, J.R., Jr., 1964. The effects of water table and tide cycle on swash-backwash sediment distribution and beach profile development. Mar. Geol., 2: 186-197. Eliot, I.G. and Clarke, D.J., 1988. Semi-diurnal variation in beachface aggradation and degradation. Mar. Geol., 79: 1-22. Howd, P.A. and Holman, R.A., 1987. A simple model of beach foreshore response to long period waves. Mar. Geol., 78: 11-22. Hughes, M.G. and Cowell,P.J., 1987.Adjustment of reflective beaches to waves.J. Coastal Res., 3: 153-167. Inman, D.L. and Filloux, J., 1960. Beach cycles related to the tide and local wind wave regime. J. Geol., 68: 225231. NOAA (National Oceanic and Atmospheric Administration), 1989. Tide Tables, 1989, east coast of North and South America. U.S. Dep. Commer., Natl. Ocean Serv., Washington, D.C. Nordstrom, K.F., 1980. Cyclic and seasonal beach re-
154 sponse: a comparison of oceanside and bayside beaches. Phys. Geog., 1: 177-196. Nordstrom, K.F. and Jackson, N.L., 1989. Processes and landform changes affecting management decisions on the Raritan Bay and Delaware Bay shorelines. Rutgers. Univ. Cent. Coastal Environ. Stud. Tech. Rep. 1043. Otvos, E.G., Jr., 1965. Sedimentation-erosion cycles of single tidal periods on Long Island Sound beaches. J. Sediment. Petrol., 35: 604-609. Sallenger, A.H., Jr. and Richmond, B.M., 1984. High fre-
K.F.NORDSTROMANDN.L.JACKSON quency sediment-level oscillations in the swash zone. Mar. Geol., 60: 155-164. SGC {Statistical Graphics Corporation), 1986. Statgraphics User's Guide. STSC, Rockville, Md. Strahler, A.N., 1966. Tidal cycle of changes in an equilibrium beach, Sandy Hook, New Jersey. J. Geol., 74: 247268. Wadell, E., 1976. Swash-groundwater-beach profile interactions. In: R.A. Davis Jr. and R.L. Ethington (Editors), Beach and Nearshore Sedimentation. Soc. Econ. Paleontol. Mineral. Spec. Publ., 24: 115-125.
147
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Letter Section
Migration of Swash Zone, Step and Microtopographic Features During Tidal Cycles on an Estuarine Beach, Delaware Bay, New Jersey, USA KARL F. NORDSTROM and NANCY L. JACKSON Center[or Coastal and Environmental Studies, Rutgers University, New Brunswick, N J 08903 (U.S.A.) (received August 31, 1989; revision accepted January 12, 1990)
Abstract Nordstrom, K.F. and Jackson, N.L., 1990. Migration of swash zone, step and microtopographic features during tidal cycles on an estuarine beach, Delaware Bay, N e w Jersey, USA. Mar. Geol., 92: 147-154. A time-series analysis of changes in surface elevation on the foreshore of an estuarine beach was conducted to compare movement of topographic features in the swash and breaker zones with results derived from study of ocean beaches. Wave heights on the upper foreshore during the fieldstudy averaged 0.15 m, with an average period of 2.4 s. The average height of the beach step was 21 ram. Smaller elevation changes, up to 12 m m in height, occurred at 1015 rain periods. There were more small elevation changes, greater longshore variations in these changes, and greater variation in the form of the beach step during the rising tide than during the falling tide. The variation in the small elevation changes and the beach step are attributed to greater short-term fluctuations in beach saturation during rising tides, causing greater variability in the velocity of swash and backwash.
Introduction
Studies of morphologic changes on sandy beaches during tidal cycles record rhythmic changes in beach profile on foreshores, associated with migration of the zone of sediment reworking by waves and swash (Duncan, 1964; Strahler, 1966; Sallenger and Richmond, 1984; Howd and Holman, 1987). Strahler (1966) documented patterns of deposition and scour that migrate at the rate of movement of the swash and breaker zones. Short-term sediment-level changes that migrate independently of these features are documented in the swash zones of ocean beaches. These changes have 0025-3227/90/$03.50
been called oscillations, and their formation is attributed to low-frequency waves (Sallenger and Richmond, 1984; Howd and Holman, 1987). Depictions of the form of the beach profile that incorporate both the overall step form and sediment-level oscillations do not exist. Most studies of short-term microtopographic beach change have been conducted in ocean environments, where it is difficult to monitor sediment-level changes under the high-energy conditions occurring in the lower swash zone (Sallenger and Richmond, 1984) or in water seaward of this zone (Eliot and Clarke, 1988). These constraints make it difficult to obtain readings of the temporal and spatial variability
© 1990 Elsevier Science Publishers B.V.
148
of topographic changes in the lower swash zone and the breaker zone in both the shore-normal and longshore directions. It is assumed that sediment-level fluctuations vary at different stages in the tidal cycle and under different wave conditions. This study is designed to provide perspective on the effects of water-level changes at different stages in the tidal cycle by comparing surface-elevation changes at rising and falling tides on the same day on a low-energy estuarine beach, during periods of near-constant wave-energy inputs. The less energetic estuarine environment allows for a sampling plan that extends from the upper limit of swash to a location seaward of the breakers at all stages in the tidal cycle. This provides the opportunity to compare and relate topographic changes associated with the beach step to the changes in the swash zone and to show how these changes vary at different locations on the beach. The opportunity is also provided to examine the applicability of models of tidal-cycle and short-term changes developed on swell-dominated ocean beaches to fetch-restricted estuarine beaches.
Description of experiment Study-site characteristics The field site is a sand beach on a transgressive estuarine barrier in Delaware Bay, New Jersey (Fig. 1 ). Tides are semi-diurnal, with a mean range of 1.6 m and a spring range of 1.9 m (NOAA, 1989). M a x i m u m fetch is 48 km to the northwest. Dominant waves are locally generated wind waves. Visual wave data gathered at a location 4.9 km north of the site over a 1 yr period (Nordstrom and Jackson, 1989) indicate a mean significant breaker height in this region of 0.19 m, with a breaker period of 2.7 s. Mean longshore current velocity, measured just landward of the breaker zone, is 0.13 m s - 1. Net drift is to the north, but southerly transport is common. The beach has a 6.20 ° (11%) planar upper
K.F. NORDSTROM AND N.L. JACKSON
foreshore and a low-tide terrace of <0.50 ° ( < 1% ). The steep upper foreshore and low wave energies result in a narrow surf/swash zone (2-3 m wide on non-storm days) and a conspicuous step that migrates with the plunge point of the breaking waves. A wide, dissipative surf zone, with spilling breakers and no conspicuous step, occurs on the low-tide terrace during low water levels. The overall form of the beach at the field site is common for shorelines with a large tidal range relative to wave weight (Inman and Filloux, 1960). These beaches retain steep slopes on the upper foreshore under all but the largest storms (Nordstrom, 1980).
Field methods The experiment was conducted between September 27 and 30, 1988. Wave heights and nearbottom horizontal water velocities were monitored using three Marsh-McBirney Model 511 bidirectional electromagnetic current meters placed on the upper foreshore and low-tide terrace (Fig. 2). T w o pressure transducers were co-located with two of the current meters. Surface sediment samples were taken on the upper foreshore and on the low-tide terrace to determine sediment characteristics at different elevations on the beach. N e t changes in beach elevation were measured at 5 m intervals using a 15 m X 35 m grid. All locations are identified by horizontal distance seaward of an arbitrary, shore-parallel baseline on the upper beach. Each sampling point for beach elevation data was identified by a 6.4 mm diameter 0.91 m long rod driven 0.6 m into the sand. Locations on the grid are identified as north (N), middle (M) and south (S). Elevations were measured at all sites at low tide each day. Additional elevation measurements were taken at 5 rain intervals on the rods on the upper foreshore while the swash and breaker zones migrated through the 15, 10 and 5 m sampling lines. The elevation value is the mode of ten readings taken near slack water in the u p r u s h backwash cycle.
MIGRATION OF FEATURES DURING TIDAL CYCLES ON AN ESTUARINE BEACH
149
OCEAN are Bay
Delaware FIELD SITE
Bay
0
5kin 75 a
Fig. 1. S t u d y area.
o
,oS ,.o /
• 8AMPLE POINT FOR STEP MIGRATION CURRENT METER ( { ~ w r r H PRE88URE TRANSDUCER) --
SWASH LIMIT DURING EXPERIMENT
Fig. 2. Field s a m p l i n g location.
In addition to the ten readings at each rod, the 5 min sampling period allowed sufficient time for visual observations of the location of highest breakers and m a x i m u m runup excursions. The north, middle and south rods were sampled in the same sequence, so the intervals
within each times series are similar. The swash and step zones migrate through the three sampling stations on each shore-parallel line at slightly different times. This time offset occurs because of the differences in sampling times as well as slight differences in the elevations of the beach at the sampling stations. The range in elevations at the 10 m rods at the beginning of the study was 35 ram. To minimize disruption of flow, all readings were taken while standing on the downdrift side of the rods. The multiple readings at 5 rain intervals smooth the variability associated with high-frequency water motions and minimize the effects of measurement error. Comparison of averages of consecutive series of ten readings reveals differences of < 1.8 ram. The 5 min time interval is considerably shorter than the 30 rain interval used by Duncan (1964) and Strahler (1966), but was required because of the rapid excursion
150
of the narrow swash zone up the foreshore. The interval is longer than that used by Sallenger and Richmond (1984) and Howd and Holman (1987), but it is considered sufficiently short to characterize the form of the beach step through time and sufficiently long to allow for accurate measurements at the three rods on each shoreparallel line.
K.F.NORDSTROMANDN.L.JACKSON
RISING TIDE (0920-1030) 0.06 " 0.04 ,~
~
NIO
0.02 0.00 -0.02 -0.04 $10
"0.06
Results Mean grain size is similar at the 5 m and 10 m sampling locations (0.56 and 0.53 ram) but coarser at the 15 m sampling locations (1.55 mm). Sediments are finer on the low-tide terrace (0.27 ram). Wave height during the period when the breaker zone and swash were at the 10 m and 5 m lines averages 0.15 m with a period of 2.4 s.
Variability o[ surface-elevation changes
MIO
OSCILLATIONS
I
0
20
I
40
I
60
I
80
FALLING TIDE (1350-1510) 0.06 N10
0.04 0.02
MIO
0.00 -0.02
$10
-0.04 -0.06
Average step height at all nine sampling locations during the 3 day period is 21 ram. Step height varies from a minimum of 8 mm to a maximum of 36 mm at M10 (the middle shorenormal line, 10 m from the baseline) during falling tide on September 29 (portrayed in Fig. 3 ). These step heights correspond to the dimensions of steps measured on sheltered beaches under similar wave energies (Otvos, 1965; Hughes and Cowell, 1987). The time series presented in Figs. 3 and 4 identify elevation changes as the swash zone and step migrate past sampling points. The data for September 29 are presented because this is the day with highest wave energies, and the tidal range was sufficiently great that all three rows of sampling stations were within the surf zone during daylight when the 5 rain readings could be taken. The width of the swash zone varies during the tidal cycle. The average length of swash uprush on September 29 is greatest at high water (Table 1 ). The duration of the swash is greatest at the 5 m locations, which are in the swash at
I
100
I
0
20
40
FALLING
60
80
TIDE
100
(1530-1625)
0.06 0.04
,,~,.m~t~"d~'~
N15
0.02 M15
0.00 -0.02
S15
-0.04 -0.06
I
I
I
I
20
40
60
80
I
100
MINUTES Fig. 3. Time series of surface-elevation changes measured in the breaker and swash zones on September 29, 1988. High water was at 1156; low water was at 0523 and 1816.
the high water still-stand. The values of swash duration at the 10 and 15 m locations do not include the time that the sampling stations are under the influence of the backwash vortex bayward of the swash zone, so the duration of
151
M I G R A T I O N OF F E A T U R E S D U R I N G T I D A L C Y C L E S O N A N E S T U A R I N E B E A C H
TABLE1
TABLE2
Average length and excursion rate of swash, September 29, 1988
Highest cross correlation coefficients for surface-elevation changes at 10 m sampling stations, September 29, 1988
Location in 5 m grid
Uprush Swash zone Duration length excursion rate of swash (m) (mm min -1) (min)
Sites
Coefficient
Lag (5 rain intervals)
0.703 0.606 0.631
+ 1 0 +2
0.885 0.763 0.891
- 1 -2 - 1
Rising tide Rising tide 10 m High water 5 m Falling tide 10 m 15 m
2.1 3.2 1.7 0.8
60 18 55 55
35 175 30 15
N10 and M10 N10 and S10 M10 and S10
Falling tide
pronounced sediment levelchanges in Fig. 3 exceeds these time periods. The greatestchanges in surface elevation are associated with migration of the step (Fig. 3 ). Smaller oscillationsoccur in some of the time series. Step height and sediment-level oscillations are lowest along the 15 m line (Fig. 3) where breaker and swash energy are lessat lowwater levels when waves are partially dissipated on the low-tide terrace.The short period of time that the 15 m line is exposed to breaker and swash processes also tends to damp gross and net changes in sediment level. Inspection of data for the 10 m sampling locations (Fig. 3) indicates that there is greater longshore variation in surface-elevation oscillations during rising tide than during falling tide,and there are more oscillationswithin the swash zone during risingtide. The results of a cross correlation analysis with the trend and mean removed from the time series (Table 2) confirm greatervariationduring risingtide.The correlations are higher on the fallingtide, and the lag times show a regular progression from
Method used is identified in S G C (1986). The time lags occur partly because of small differences in beach elevations and sampling times and partly because of variation in the sediment-level oscillations.
north to middle to south. Correlations are lower on the rising tide, and the lag times show no clear relationshipbetween space and time. Sediment-level oscillationshave the shortest periodicitiesat location $10, and there are more oscillationsat this site during the 3 days than at the other sampling points 10 m from the baseline (Fig. 3). There are only three oscillations at this site (Fig. 3) because of the rapid migration of the narrow swash zone through the sampling station at mid-tide (Table 1). There are more oscillationsin the record for the sampling stations 5 m from the baseline (Fig. 4) where the step approached to within 0.3 m of the stations during the relativestill-standnear high tide.The amplitude of the largestof these oscillationsis 12 ram. The time seriesfrom the swash zone at N 5 (Fig. 4) shows greater long-
Record
0.06 0.04
N10 and N10 N10 and S10 MIO and SIO
RISING TIDE
I A
Record
I
J e
I
FALLING TIDE
0.02 ~J
0.00 -0.02
2'0
4'0
6'0
8'0
100
120
140
1;0
i
180 200 MINUTES
Fig. 4. Time series of surface-elevation changes measured in the swash zone near high water on September 29, 1988.
152
K.F. NORDSTROMAND N.L.JACKSON
TABLE 3 Sediment-elevation statistics for 5 m sampling stations Location
Net change (m)
Heights ( R M S in meters)
N5 Rising Falling
+ 0.014 - 0.008
0.011 0.008
M5 Rising Falling
+ 0.007 - 0.008
0.009 0.005
$5 Rising Falling
+ 0.003 - 0.011
0.013 0.010
Heights (root mean square) are calculated as twice the standard deviation of the time series.
period oscillations during rising water level than during falling water level. This finding is supported by comparison of the heights of oscillations at the 5 m stations with the mean and trend removed (Table 3). Records of water level and horizontal velocities taken near high water on the rising and falling tides (records A and B, Fig. 4) show that there is little difference in process conditions just bayward of the step. Mean water levels recorded on the pressure transducers during the rise and fall differ by only 0.027 m. There is a difference of only 0.01 m in wave heights at the 12.5 m instrument station (Fig. 2). Root mean square onshore velocities at the 7.5 m station, within 1.6 m of the breakers, are 0.30 and 0.29 m s-1 on the rise and fall, and offshore velocities are 0.26 m s -1 at both these times. The magnitudes of the sediment-level oscillations at these times are 12 mm on the rising tide and 6 mm on the falling tide.
Relationship between elevation changes and beach form A time series of elevation changes may be viewed as a portrayal of the form of the beach if the rate of tidal rise and rate of motion of beach features past a sampling point is con-
stant. This assumption appears valid for the three series presented in Fig. 3, which are located near mid-tide level. A slight difference in the exaggeration of relief occurs at falling tide relative to rising tide because the rate of movement of still water level is 4% greater, but this difference is too small to obscure the conspicuous differences. To convert the time series to a conceptualization of step form, the left-hand side of Fig. 3 should be viewed as the landward side on the rising tide and the bayward side on the falling tide. The surface-evaluation changes and the forms associated with these changes (Fig. 3) are similar to those seen in the composite diagram of phases of beach change presented in Strahler (1966). The scour phase preceding the step deposition phase is identifiable on several of the time series, and the step deposition phase is identifiable on all of the time series. The initial deposition phase identified by Strahler {1966) near the upper limit of swash is difficult to distinguish from measurement error, and this phase is not conspicuous in the time series.
Discussion and conclusions The frequencies of the oscillations in the swash zone at Delaware Bay fall within the 615 min oscillations identified by Sallenger and Richmond (1984), and are roughly comparable to the 8-10 min oscillations identified by Howd and Holman (1987). The heights of the oscillations are about 20-25% of the heights reported in those studies. The frequency of sediment-level measurement in the present study enabled a more detailed characterization of the form of the step than was possible in Strahler (1966), and greater relative differences are revealed during rising tide and falling tide. The greater variety in the form of the step and the magnitude of swash oscillations during rising water levels than during falling water levels suggests that the differences may be related to the degree of beach saturation, as discussed by Wadell
MIGRATION OF FEATURES DURING TIDAL CYCLES ON AN ESTUARINE BEACH
(1976). Fluctuations in the elevation of the beach water table occur at incident wave frequencies as well as at the frequencies of surf beat, shelf waves, and sub-tidal and tidal changes (Duncan, 1964; Wadell, 1976; Eliot and Clarke, 1988). Fluctuations are less frequent than incident wave periods, because the beach acts as a low-pass filter (Wadell, 1976). The low water table during rising tide permits greater swash infiltration, reducing backrush. The greater swash energy, relative to backrush, during rising tide, moves more sand onshore, so there is a greater likelihood of formation of sediment-level oscillations than during falling tide and a greater likelihood that the oscillations will be larger. The fluctuations also may be more common during the rising tide because there is a greater likelihood that parts of the beach face are alternately saturated and unsaturated at that time. The water table is high relative to the position of the swash on the falling tide (Duncan, 1964). There is less potential for percolation into the beach, and this decreases the potential for deposition. An alternative model for generation of sediment-level oscillations in the swash zone is that they are formed near the step at the seaward extreme of the swash in response to a temporary rise in water level due to a long period wave (Howd and Holman, 1987). Their model predicts erosion on the steep face of the step, and deposition of sediment landward of the step where the slope is flat. The migration of this perturbation on steep beaches is believed to be driven by the dominant swash oscillations at the incident gravity-wave frequency or its first subharmonic ( H o w d and Holman, 1987). The results of the present study do not confirm or refute H o w d and Holman's model, b u t they strengthen the evidence that beach water-table fluctuations play a role in the generation of sediment-level oscillations. If long-periods waves generate the oscillations without regard to water-table fluctuations, there should be little difference between the time series on rising and falling tides.
153
The results of this field study document the existence of sediment-level oscillations on an estuarine beach that are similar to those identified on ocean beaches, although the magnitude of these oscillations is lower. The phases of beach change associated with migration of the swash during tidal cycles identified by Duncan (1964) and Strahler (1966) are also documented. The sediment-level oscillations may be envisaged as smaller bedforms superimposed on the larger scale step form. The results of this study indicate that there is a difference in the variability of sediment-level oscillations and in the form of the beach step during the rising tide and falling tide. These differences are believed to be related to the degree of beach saturation.
Acknowledgements Financial assistance for this research was provided by the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Office of Coastal Zone Management under provisions of Section 305 of the Federal Coastal Zone Management Act (P.L. 92-583) in cooperation with the State of New Jersey Department of Environmental Protection, Division of Coastal Resources.
References Duncan, J.R., Jr., 1964. The effects of water table and tide cycle on swash-backwash sediment distribution and beach profile development. Mar. Geol., 2: 186-197. Eliot, I.G. and Clarke, D.J., 1988. Semi-diurnal variation in beachface aggradation and degradation. Mar. Geol., 79: 1-22. Howd, P.A. and Holman, R.A., 1987. A simple model of beach foreshore response to long period waves. Mar. Geol., 78: 11-22. Hughes, M.G. and Cowell,P.J., 1987.Adjustment of reflective beaches to waves.J. Coastal Res., 3: 153-167. Inman, D.L. and Filloux, J., 1960. Beach cycles related to the tide and local wind wave regime. J. Geol., 68: 225231. NOAA (National Oceanic and Atmospheric Administration), 1989. Tide Tables, 1989, east coast of North and South America. U.S. Dep. Commer., Natl. Ocean Serv., Washington, D.C. Nordstrom, K.F., 1980. Cyclic and seasonal beach re-
154 sponse: a comparison of oceanside and bayside beaches. Phys. Geog., 1: 177-196. Nordstrom, K.F. and Jackson, N.L., 1989. Processes and landform changes affecting management decisions on the Raritan Bay and Delaware Bay shorelines. Rutgers. Univ. Cent. Coastal Environ. Stud. Tech. Rep. 1043. Otvos, E.G., Jr., 1965. Sedimentation-erosion cycles of single tidal periods on Long Island Sound beaches. J. Sediment. Petrol., 35: 604-609. Sallenger, A.H., Jr. and Richmond, B.M., 1984. High fre-
K.F.NORDSTROMANDN.L.JACKSON quency sediment-level oscillations in the swash zone. Mar. Geol., 60: 155-164. SGC {Statistical Graphics Corporation), 1986. Statgraphics User's Guide. STSC, Rockville, Md. Strahler, A.N., 1966. Tidal cycle of changes in an equilibrium beach, Sandy Hook, New Jersey. J. Geol., 74: 247268. Wadell, E., 1976. Swash-groundwater-beach profile interactions. In: R.A. Davis Jr. and R.L. Ethington (Editors), Beach and Nearshore Sedimentation. Soc. Econ. Paleontol. Mineral. Spec. Publ., 24: 115-125.