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Sand-ridge migration in St. Andrew Bay, Florida
Marine Geology -
ElsevierPublishingCompany, Amsterdam - Printed in The Netherlands
SAND-RIDGE MIGRATION IN ST. ANDREW BAY, FLORIDA G. G. SALSMAN~ W. H. TOLBERT AND R. G. VILLARS
U.S. Navy, Mine Defence Laboratory, Panama City, Fla. (U.S.A.) (Received September 29, 1965)
SUMMARY
This report presents the results of an investigation into the nature of a series of remarkably uniform sediment ridges found on the bottom of St. Andrew Bay, Florida. Scientific divers from the U. S. Navy Mine Defense Laboratory measured the ridges, obtained bottom samples, monitored their movement for a period of over 2 years, and installed a current meter on the bottom in order to record the flow responsible for their movement. It was found that the ridges were oriented normal to the current flow and were composed of a fine-grained quartz sand. They exhibited heights of 30-60 cm, wavelengths of 13-20 m, and were migrating into an area of the bay previously characterized as mud. During the 849-day period of the investigation, one of the ridges advanced a total of 11.44 m. The migration rate was found to vary periodically in response to tides, being greater than average during periods when the moon and sun were near maximum declination, and smaller than usual when they were near the celestial equator. Also detected was a gradual increase in ridge-migration speed apparently related to a decrease in ridge height and a long-term increase in current speed.
INTRODUCTION
In the fall of 1962, while conducting a fathometer survey in St. Andrew Bay, Florida, along the track indicated in Fig.l, a U. S. Navy Mine Defense Laboratory vessel passed over a series of remarkably uniform bottom undulations in approximately 11 m of water. These undulations, some of which are depicted in Fig.2, were asymmetric in shape with their steep slopes facing toward the northeast, and appeared to be transverse sand ridges migrating along the bottom in that direction. Nautical charts of the area (USCGS 489 and 869) indicated that the bottom was 12.2 m deep and composed of "mud". A series of dives was made to investigate the area and it was found that the undulations were, in fact, transverse sand ridges which, due to a man-made change in the bay's current regime, were migrating into an area previously dominated by mud. Some of the details of this investigation are presented in this paper. Marine Geol.,
4 (1966) 11-19
12
G. G. SALSMAN, W . H. TOLBERT A N D R. G . VILLARS 44'
42'
o,d
40'
BAR
GULF OF MEX/CO
85038 `
ENTRANCE
"~ 0
30* 0 2 '
2
NAUTICAL MILES T
Fig.1. Location map.
D E S C R I P T I O N OF T H E BAY A N D S T U D Y SITE
The St. Andrew Bay System is situated on the northern coast of the Gulf of Mexico approximately 450 km east of the Mississippi River delta. It is a typical tidal embayment which was apparently formed during the last major rise in sea level (the Holocene transgression), which, according to CURRAY (1960), took place 3,000-20,000 years ago. As sea level rose and flooded the valley of a local river system, ocean waves and longshore currents proceeded to build up a barrier bar across the mouth of the resulting coastal indentation, thus creating an embayment with a small natural channel near its southeasternmost corner (see Fig.l). The barrier bar, composed of a finegrained quartz sand, provided protection from wave action, and this, coupled with characteristically weak tides (range less than 1 m), provided an ideal environment for the deposition of the fine clays brought down to the bay by small surface streams, In 1934, a second entrance was cut through the bar by the Corps of Engineers. This entrance, 8 km northwest of the natural channel, is now used by all large vessels entering or leaving the bay. The hydrography of the bay system has been described by ICrnYE and JON~S (1961). They found that tidal currents within the bay, like the vertical rise and fall of the tide, display a diurnal period with ebb currents usually predominating over Marine Geol., 4 (1966) 11-19
SAND-RIDGE MIGRATION IN ST. ANDREW BAY
13
flood currents at the surface because of the effect of drainage. Current speeds seldom exceed 50 cm/sec, except near the bay entrances where speeds up to 120 cm/sec occur during tropic tide periods. Bottom sediments within the bay have been described by WALLER (1961), who divided the bay into two distinct sediment regimes--a sand regime near the entrances and in all areas shallower than 6 m, and a silty clay regime in deeper water. This classification is in good agreement with sediment data obtained by the U. S. Coast and Geodetic Survey during a 1934 survey and published by them as bottom notations on U.S.C.G.S. nautical charts No.489 and 869. It thus came as quite a surprise to the authors when a section of the bay bottom denoted as " m u d " on these charts was revealed to have the undulatory nature shown in Fig.2. The authors decided to investigate the area. Using self-contained underwater breathing equipment, they made a series of dives along the track shown in Fig. 1 and found that the undulations were sand ridges the crests of which were essentially perpendicular to the channel axis. Near mid-channel they displayed heights of 30-60 cm and wavelengths of 13-20 m. They were asymmetric in profile, and their steep faces were directed toward the northeast with slopes of approximately 30 °. Samples revealed the bottom to consist of a fine-grained quartz sand with a median diameter of 0.135 mm. Of the sediment particles 92 ~ had diameters between 0.062 and 0.500 mm, practically all of these being grains of white quartz sand. The portion of particles
Fig.2. Fathometer profile along channel axis. Marine Geol., 4 (1966) 11-19
14
G. G. SALSMAN, W. H. TOLBERT AND R. G. VILLARS
having diameters larger than 0.500 mm (2~o) were almost exclusively shell fragments, while the portion having diameters less than 0.062 mm (6%) were extremely fine quartz grains, silts, and clays. Observations have revealed that the sand grains comprising these ridges begin to move at current speeds as low as 10 cm/sec. Records from a Geodyne current meter (see SMITHand FREDK1N, 1963), mounted 55 cm above the mean sediment level at the study site, show that near bottom currents are primarily tidal in nature, flooding and ebbing in the manner typified by the lower curve in Fig.3. Flood currents generally set toward the northeast, ebb currents toward the southwest. These reversing currents exhibit a diurnal period and fortnightly strength variation analogous to the diurnal period and fortnightly range variation of the surface tide (upper curve of Fig.3). Peak flows are generally proportional to
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I
I
E 9O
~. ~o o~ ~3o go 60
MOONNEARMAXIMj~ UM
NORTHERLY DECLINATION
'
MOONAT ~ ' ~ CELESTIAL EQUATOR --
DAYS OF THE M O N T H ~
Fig.3. Typical tide and tidal current curves at study site.
the surface tide range, the maximum flood velocity (in cm/sec) being equal to seventenths (0.7) of the tide range (in era), and the maximum ebb velocity(in cm/see) being equal to one-tenth (0.1) of the tide range (in em). During periods of tropic tides (maximum lunar declination), the current floods for approximately 20 hours reaching a peak speed in the neighborhood of 40 cm/sec, while the ebb ttow lasts only 5 hours and reaches a peak speed of only 10 cm/sec. The tIood cycle characteristically displays three distinct peaks, the latter usually being the highest. During periods of equatorial tides (minimum lunar declination), the diurnal pattern practically disappears, and the current floods throughout the day at speeds usually less than 15 era/see. In addition to the fortnightly variation in current strength, there are possibly smaller amplitude variations with periods of 6 months and 18.6 years. These will be discussed in greater detail later in the paper. Marine Geol., 4 (1966) 11-19
SAND-RIDGEMIGRATIONIN ST. ANDREWBAY
15
From the above, it can be seen that flood currents at the study site not only flow faster and for longer periods of time than do ebb currents, but it is only during the flood cycle that currents exceed the critical erosional velocity of the sand (dashed lines in Fig.3). This occurs on the average of 25 ~ of the time during a fortnightly period, varying from 0 ~ during equatorial tides to 5 0 ~ during tropic tides. More sediment should thus be transported by the current during tropic tide periods than equatorial periods, the direction of this transport always being toward the northeast. Further, since the maximum current speed at the site is only four times larger than the critical erosional velocity, it can be expected that the primary mode of sediment transport is via bed-load.
SAND-RIDGE MIGRATION Observations made by divers at various times in the tide cycle have revealed, as expected, that the sand at the study site is not being eroded or transported during ebb periods; but during that portion of each flood cycle when the velocity exceeds 10 cm/sec, sand grains are eroded from the surface layer of each ridge, transported downstream over the crest, and deposited on the leeward slope. This accumulation of sand grains on the leeward slope causes the crest line to be displaced downstream. As each sand ridge passes a given point, however, many of the grains situated in the trough are left behind because the current transport capacity in the troughs is not great enough to remove all the grains deposited there by the previous ridge. The mean bed level is thus elevated a small amount as each ridge passes. Preliminary dives at the study site led to the above general observations, and also convinced the authors that the quantity of sand being moved by the current was not sufficiently great to be accurately measured by divers on a single descent. Therefore, on 14 November 1962, reference stakes were placed near the crest of one of the ridges (ridge 11, in Fig.2), and for a period of more than 2 years thereafter, periodic measurements of its dimensions and movement were made. These measurements are presented in Table I, where it can be seen that the ridge wavelength remained approximately 18 m during the entire period of study, while the ridge height decayed from 58 to 49 crn. Meanwhile, the ridge crest migrated slowly across the bottom toward the northeast, advancing 15.5 cm during the first 14 days, and a total of 1,144 cm during the 849-day study period, thus averaging 1.35 cm per day. A plot of the mean migration speed for the interval between each observation listed in Table I reveals that this speed was not constant, but varied considerably during the study period. Notice in Fig.4 that there is a tendency toward a cyclic variation of semi-annual period. This variation displays maxima near the time of the solstices and minima near the time of the equinoxes, in much the same manner as do surface tide ranges and tidal current strengths along the Gulf Coast. But whereas the amplitude of the latter two parameters relative to the mean was less than 11 ~o, the amplitude of the periodic ridge migrations relative to the mean was over 52 ~o. This
Marine Geol., 4 (1966) 11-19
16
G. G. SALSMAN, W. }t. TOLBERT AND R. G. VILLARS
TABLE I SAND-RIDGE MIGRATION DATA
Obs. date
14 N o v 28 N o v 3 Jan 13 Feb 22 M a r 3 May 21 M a y 14Jun 2 Jul 9 Aug 23 A u g 12 Sep 29 Oct 13 N o v 6 Dec 30 J a n 11 M a r 15 A p r 12 J u n I0 Jul 14 Oct 3 Dec 12 M a r
Ridge dimensions
62 62 63 63 63 63 63 63 63 63 63 63 63 63 63 64 64 64 64 64 64 64 65
Height (cmJ
Length (mj
58 --58 . . . . . . 55 --55 ---52 ---52 ----. . . . 49
18.2 --18.1 18.4 ---18.2 ---18.4 ---18.3 ----17.8
Days since start
Migration since start (cm )
Directiono/ movement (toward)
0 14 50 91 128 170 188 212 230 268 282 302 349 364 387 442 483 518 576 604 700 750 849
0.0 15.5 61.5 87.0 122.5 165.5 178.0 196.0 226.5 269.5 287.5 292.5 318.0 341.0 376.5 450.0 501.0 536,5 646.0 709.5 882.0 989.0 1,144.0
-NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE
-o
E o _c 1.0
o~
--time
~
rn0nths~---*
Fig.4. Ridge-migration rate as a function of time.
s u g g e s t s t h a t r i d g e s p e e d is n o t a l i n e a r f u n c t i o n t w e e n t h e s e is m o r e
of the flow velocity. The relation be-
e a s i l y s e e n i n F i g . 5 , w h e r e r i d g e s p e e d is p l o t t e d
against mean
flood velocity. These data points fall closely along a line expressed by the equation: C----- 3 . 1 2 . where
10 - 7 V 5
C is t h e r i d g e s p e e d i n c e n t i m e t e r s
per day and
V is t h e m e a n
flow velocity
Marine Geol., 4 (1966) I 1-19
SAND-RIDGEMIGRATIONIN ST. ANDREWBAY
17
in cm/sec. Similar empirical relationships have been developed by CHANG (1939) and LIU (1953), who found that small ridges in laboratory flumes migrate downstream at a rate approximately proportional to the 5th power of the mean flow velocity. Thus, the migration speed of a ridge is extremely sensitive to changes in water speed. Fig.4 also shows that the ridge-migration speed practically doubled in 2 years. There is a number of possible explanations for this increase, one related to a decrease in ridge height, and others connected with long term increases in the strength of the current. A portion of this 100~ increase in ridge-migration speed was obviously caused by the 15~o reduction in ridge height observed during the study. Since ridge speed is inversely proportional to ridge height, this should make a ridge move 17 ~ faster. Another portion of the increase was probably caused by a long term periodic variation in current speed due to a change in the inclination of the moon's orbit to the equator. This variation, which has a period of 18.6 years, was responsible for a i0 ~ increase in current strengths from 1962 to 1964; and although this increase is relatively small, it could produce more than a 50~o increase in ridge migration speed because of the high exponential dependence cited previously. Still another portion of the increase in ridge-migration speed could have been caused by a gradual increase in current strength due to a reduction in the size of the bay's natural entrance. There is evidence which indicates that sand migrating southeastward along the Gulf side of the barrier bar (see Fig. 1) is gradually pinching off the bay's old natural channel. This should cause the tidal flow to increase in the man-made channel and thereby increase the ridge-migration speed at the study site. Finally, there is the possibility that a 1962 dredging operation in the man-made entrance may have
I0
~_~
A
C= 3.12 x 10"7 V 5
II , [ ~ J , 1,1,1,] I0 115 20 25 30 40 50 60 70 80 meon flood velocity {V) in c m / sec
Fig.5. Ridge-migration speed as a function of mean flood velocity. Marine Geol., 4 (1966) 11-19
18
G . G . SALSMAN, W. H. TOLBERT AND R. G. VILLARS
altered hydraulic conditions at the study site. This effect, if present, must be small, as the dredging was done at least 3 km away and the quantity of sand removed was negligible compared to the size of the entrance. There are no means of accurately assessing the contribution of the latter two effects at this stage in the study. Accompanying the horizontal migration of the sand ridges at the study site has been a gradual rise in the mean bed level. It is difficult to evaluate the rate of rise at this time, for the ridges have not advanced one complete wavelength since the study began. But the following crude approximation can be made. About 120 cm beneath the trough level of ridge 11 the sediment type changes abruptly from sand to clay. This clay is apparently the " m u d " referred to as the surface sediment in pre-1934 nautical charts. Thus, some 120 cm of sand have been deposited at the study site in the 30 years since the Corps of Engineers excavated the man-made channel. During this 30 years some ten ridges have passed the study site. Six of these can be seen in Fig.2; four others, situated northeast of ridge 16, have diminished in height to the point that divers can see them, but the fathometer cannot. Thus, assuming that bed-load transport is the primary mode of sediment movement at the site, each ridge has apparently left behind a layer of sand averaging 12 cm thick. These ridges have moved into the study zone as the result of a man-made change in the bay's current regime.
CONCLUSIONS
During the past 2 years a series of remarkably uniform sediment ridges found on the bottom of St. Andrew Bay, Florida, have been investigated by scientific divers of the U. S. Navy Mine Defense Laboratory. These divers have found that the ridges are composed of a fine sand, are asymmetric in shape with steep slopes facing down current, have 30-60 cm heights, and 13-20 m wavelengths. A predominant flood current is causing them to migrate toward the northeast at an average rate of 1.35 cm per day, the rate being greater than average during periods when the moon and sun are near maximum declination, and smaller than average when they are near the celestial equator. These periodic variations indicate that the migration rate is extremely sensitive to changes in current speed, thus lending field support to the laboratory conclusions of other investigators. Near the leading edge of the ridge zone, where sand transport is primarily in the bed-load mode, each ridge passing a given point leaves behind a layer of sand averaging 12 cm thick. Approximately ten ridges have passed the study site since the Corps of Engineers excavated the bay's new man-made channel. This man-made change in the bay's current regime has caused some 120 cm of sand to be deposited in a location where the surface sediment was previously described as mud.
Marine Geol., 4 (1966) 11-19
SAND=RIDGEMIGRATIONIN ST. ANDREW BAY
19
REFERENCES
CHANG,,Y. L., 1939. Laboratory investigations of flume traction and transportation. Trans. Am. Soc. CivilEngrs., 104 : 1246-1313. CURRAY,J. R., 1960. Sediments and history of the Holocene transgression, continentalshell northern Gulf of Mexico. In: F. P. SHEPAP.D,F. B. PHLEGERand TJ. H. VAN ANDEL(Editors), Recent Sediments, Northwest Gulf of Mexico. Am. Assoc. Petrol. Geologists, Tulsa, Okla., pp.221-266. ICHIYE, T. and JONES, M. L., 2961. On the hydrography of the St. Andrew Bay system, Florida. Limnol. Oceanog., 6 (3) : 302-311. LIu, H. K., 1953. Ripple Formation and its Relation to Bed-Load Movement. Thesis, Univ. Minnesota, Minneapolis, Minn., Publ., 6389 : 160 pp. SMITH, P. F. and FREDKIN,E., 1963. Computers and transducers. In: R. D. GAUL(Editor), Marine Sciences Instrumentation. Plenum Press, New York, N.Y., 2 : 81-86. WALLER,R. A., 1961. Ostracods of the St. Andrew BaySyswm. M.S. Thesis, Dept. Biol. Sci., Florida State Univ., TaUahassee, Fla., 46 pp.
Marine Geol., 4 (1966) 11-19
ElsevierPublishingCompany, Amsterdam - Printed in The Netherlands
SAND-RIDGE MIGRATION IN ST. ANDREW BAY, FLORIDA G. G. SALSMAN~ W. H. TOLBERT AND R. G. VILLARS
U.S. Navy, Mine Defence Laboratory, Panama City, Fla. (U.S.A.) (Received September 29, 1965)
SUMMARY
This report presents the results of an investigation into the nature of a series of remarkably uniform sediment ridges found on the bottom of St. Andrew Bay, Florida. Scientific divers from the U. S. Navy Mine Defense Laboratory measured the ridges, obtained bottom samples, monitored their movement for a period of over 2 years, and installed a current meter on the bottom in order to record the flow responsible for their movement. It was found that the ridges were oriented normal to the current flow and were composed of a fine-grained quartz sand. They exhibited heights of 30-60 cm, wavelengths of 13-20 m, and were migrating into an area of the bay previously characterized as mud. During the 849-day period of the investigation, one of the ridges advanced a total of 11.44 m. The migration rate was found to vary periodically in response to tides, being greater than average during periods when the moon and sun were near maximum declination, and smaller than usual when they were near the celestial equator. Also detected was a gradual increase in ridge-migration speed apparently related to a decrease in ridge height and a long-term increase in current speed.
INTRODUCTION
In the fall of 1962, while conducting a fathometer survey in St. Andrew Bay, Florida, along the track indicated in Fig.l, a U. S. Navy Mine Defense Laboratory vessel passed over a series of remarkably uniform bottom undulations in approximately 11 m of water. These undulations, some of which are depicted in Fig.2, were asymmetric in shape with their steep slopes facing toward the northeast, and appeared to be transverse sand ridges migrating along the bottom in that direction. Nautical charts of the area (USCGS 489 and 869) indicated that the bottom was 12.2 m deep and composed of "mud". A series of dives was made to investigate the area and it was found that the undulations were, in fact, transverse sand ridges which, due to a man-made change in the bay's current regime, were migrating into an area previously dominated by mud. Some of the details of this investigation are presented in this paper. Marine Geol.,
4 (1966) 11-19
12
G. G. SALSMAN, W . H. TOLBERT A N D R. G . VILLARS 44'
42'
o,d
40'
BAR
GULF OF MEX/CO
85038 `
ENTRANCE
"~ 0
30* 0 2 '
2
NAUTICAL MILES T
Fig.1. Location map.
D E S C R I P T I O N OF T H E BAY A N D S T U D Y SITE
The St. Andrew Bay System is situated on the northern coast of the Gulf of Mexico approximately 450 km east of the Mississippi River delta. It is a typical tidal embayment which was apparently formed during the last major rise in sea level (the Holocene transgression), which, according to CURRAY (1960), took place 3,000-20,000 years ago. As sea level rose and flooded the valley of a local river system, ocean waves and longshore currents proceeded to build up a barrier bar across the mouth of the resulting coastal indentation, thus creating an embayment with a small natural channel near its southeasternmost corner (see Fig.l). The barrier bar, composed of a finegrained quartz sand, provided protection from wave action, and this, coupled with characteristically weak tides (range less than 1 m), provided an ideal environment for the deposition of the fine clays brought down to the bay by small surface streams, In 1934, a second entrance was cut through the bar by the Corps of Engineers. This entrance, 8 km northwest of the natural channel, is now used by all large vessels entering or leaving the bay. The hydrography of the bay system has been described by ICrnYE and JON~S (1961). They found that tidal currents within the bay, like the vertical rise and fall of the tide, display a diurnal period with ebb currents usually predominating over Marine Geol., 4 (1966) 11-19
SAND-RIDGE MIGRATION IN ST. ANDREW BAY
13
flood currents at the surface because of the effect of drainage. Current speeds seldom exceed 50 cm/sec, except near the bay entrances where speeds up to 120 cm/sec occur during tropic tide periods. Bottom sediments within the bay have been described by WALLER (1961), who divided the bay into two distinct sediment regimes--a sand regime near the entrances and in all areas shallower than 6 m, and a silty clay regime in deeper water. This classification is in good agreement with sediment data obtained by the U. S. Coast and Geodetic Survey during a 1934 survey and published by them as bottom notations on U.S.C.G.S. nautical charts No.489 and 869. It thus came as quite a surprise to the authors when a section of the bay bottom denoted as " m u d " on these charts was revealed to have the undulatory nature shown in Fig.2. The authors decided to investigate the area. Using self-contained underwater breathing equipment, they made a series of dives along the track shown in Fig. 1 and found that the undulations were sand ridges the crests of which were essentially perpendicular to the channel axis. Near mid-channel they displayed heights of 30-60 cm and wavelengths of 13-20 m. They were asymmetric in profile, and their steep faces were directed toward the northeast with slopes of approximately 30 °. Samples revealed the bottom to consist of a fine-grained quartz sand with a median diameter of 0.135 mm. Of the sediment particles 92 ~ had diameters between 0.062 and 0.500 mm, practically all of these being grains of white quartz sand. The portion of particles
Fig.2. Fathometer profile along channel axis. Marine Geol., 4 (1966) 11-19
14
G. G. SALSMAN, W. H. TOLBERT AND R. G. VILLARS
having diameters larger than 0.500 mm (2~o) were almost exclusively shell fragments, while the portion having diameters less than 0.062 mm (6%) were extremely fine quartz grains, silts, and clays. Observations have revealed that the sand grains comprising these ridges begin to move at current speeds as low as 10 cm/sec. Records from a Geodyne current meter (see SMITHand FREDK1N, 1963), mounted 55 cm above the mean sediment level at the study site, show that near bottom currents are primarily tidal in nature, flooding and ebbing in the manner typified by the lower curve in Fig.3. Flood currents generally set toward the northeast, ebb currents toward the southwest. These reversing currents exhibit a diurnal period and fortnightly strength variation analogous to the diurnal period and fortnightly range variation of the surface tide (upper curve of Fig.3). Peak flows are generally proportional to
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E 9O
~. ~o o~ ~3o go 60
MOONNEARMAXIMj~ UM
NORTHERLY DECLINATION
'
MOONAT ~ ' ~ CELESTIAL EQUATOR --
DAYS OF THE M O N T H ~
Fig.3. Typical tide and tidal current curves at study site.
the surface tide range, the maximum flood velocity (in cm/sec) being equal to seventenths (0.7) of the tide range (in era), and the maximum ebb velocity(in cm/see) being equal to one-tenth (0.1) of the tide range (in em). During periods of tropic tides (maximum lunar declination), the current floods for approximately 20 hours reaching a peak speed in the neighborhood of 40 cm/sec, while the ebb ttow lasts only 5 hours and reaches a peak speed of only 10 cm/sec. The tIood cycle characteristically displays three distinct peaks, the latter usually being the highest. During periods of equatorial tides (minimum lunar declination), the diurnal pattern practically disappears, and the current floods throughout the day at speeds usually less than 15 era/see. In addition to the fortnightly variation in current strength, there are possibly smaller amplitude variations with periods of 6 months and 18.6 years. These will be discussed in greater detail later in the paper. Marine Geol., 4 (1966) 11-19
SAND-RIDGEMIGRATIONIN ST. ANDREWBAY
15
From the above, it can be seen that flood currents at the study site not only flow faster and for longer periods of time than do ebb currents, but it is only during the flood cycle that currents exceed the critical erosional velocity of the sand (dashed lines in Fig.3). This occurs on the average of 25 ~ of the time during a fortnightly period, varying from 0 ~ during equatorial tides to 5 0 ~ during tropic tides. More sediment should thus be transported by the current during tropic tide periods than equatorial periods, the direction of this transport always being toward the northeast. Further, since the maximum current speed at the site is only four times larger than the critical erosional velocity, it can be expected that the primary mode of sediment transport is via bed-load.
SAND-RIDGE MIGRATION Observations made by divers at various times in the tide cycle have revealed, as expected, that the sand at the study site is not being eroded or transported during ebb periods; but during that portion of each flood cycle when the velocity exceeds 10 cm/sec, sand grains are eroded from the surface layer of each ridge, transported downstream over the crest, and deposited on the leeward slope. This accumulation of sand grains on the leeward slope causes the crest line to be displaced downstream. As each sand ridge passes a given point, however, many of the grains situated in the trough are left behind because the current transport capacity in the troughs is not great enough to remove all the grains deposited there by the previous ridge. The mean bed level is thus elevated a small amount as each ridge passes. Preliminary dives at the study site led to the above general observations, and also convinced the authors that the quantity of sand being moved by the current was not sufficiently great to be accurately measured by divers on a single descent. Therefore, on 14 November 1962, reference stakes were placed near the crest of one of the ridges (ridge 11, in Fig.2), and for a period of more than 2 years thereafter, periodic measurements of its dimensions and movement were made. These measurements are presented in Table I, where it can be seen that the ridge wavelength remained approximately 18 m during the entire period of study, while the ridge height decayed from 58 to 49 crn. Meanwhile, the ridge crest migrated slowly across the bottom toward the northeast, advancing 15.5 cm during the first 14 days, and a total of 1,144 cm during the 849-day study period, thus averaging 1.35 cm per day. A plot of the mean migration speed for the interval between each observation listed in Table I reveals that this speed was not constant, but varied considerably during the study period. Notice in Fig.4 that there is a tendency toward a cyclic variation of semi-annual period. This variation displays maxima near the time of the solstices and minima near the time of the equinoxes, in much the same manner as do surface tide ranges and tidal current strengths along the Gulf Coast. But whereas the amplitude of the latter two parameters relative to the mean was less than 11 ~o, the amplitude of the periodic ridge migrations relative to the mean was over 52 ~o. This
Marine Geol., 4 (1966) 11-19
16
G. G. SALSMAN, W. }t. TOLBERT AND R. G. VILLARS
TABLE I SAND-RIDGE MIGRATION DATA
Obs. date
14 N o v 28 N o v 3 Jan 13 Feb 22 M a r 3 May 21 M a y 14Jun 2 Jul 9 Aug 23 A u g 12 Sep 29 Oct 13 N o v 6 Dec 30 J a n 11 M a r 15 A p r 12 J u n I0 Jul 14 Oct 3 Dec 12 M a r
Ridge dimensions
62 62 63 63 63 63 63 63 63 63 63 63 63 63 63 64 64 64 64 64 64 64 65
Height (cmJ
Length (mj
58 --58 . . . . . . 55 --55 ---52 ---52 ----. . . . 49
18.2 --18.1 18.4 ---18.2 ---18.4 ---18.3 ----17.8
Days since start
Migration since start (cm )
Directiono/ movement (toward)
0 14 50 91 128 170 188 212 230 268 282 302 349 364 387 442 483 518 576 604 700 750 849
0.0 15.5 61.5 87.0 122.5 165.5 178.0 196.0 226.5 269.5 287.5 292.5 318.0 341.0 376.5 450.0 501.0 536,5 646.0 709.5 882.0 989.0 1,144.0
-NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE
-o
E o _c 1.0
o~
--time
~
rn0nths~---*
Fig.4. Ridge-migration rate as a function of time.
s u g g e s t s t h a t r i d g e s p e e d is n o t a l i n e a r f u n c t i o n t w e e n t h e s e is m o r e
of the flow velocity. The relation be-
e a s i l y s e e n i n F i g . 5 , w h e r e r i d g e s p e e d is p l o t t e d
against mean
flood velocity. These data points fall closely along a line expressed by the equation: C----- 3 . 1 2 . where
10 - 7 V 5
C is t h e r i d g e s p e e d i n c e n t i m e t e r s
per day and
V is t h e m e a n
flow velocity
Marine Geol., 4 (1966) I 1-19
SAND-RIDGEMIGRATIONIN ST. ANDREWBAY
17
in cm/sec. Similar empirical relationships have been developed by CHANG (1939) and LIU (1953), who found that small ridges in laboratory flumes migrate downstream at a rate approximately proportional to the 5th power of the mean flow velocity. Thus, the migration speed of a ridge is extremely sensitive to changes in water speed. Fig.4 also shows that the ridge-migration speed practically doubled in 2 years. There is a number of possible explanations for this increase, one related to a decrease in ridge height, and others connected with long term increases in the strength of the current. A portion of this 100~ increase in ridge-migration speed was obviously caused by the 15~o reduction in ridge height observed during the study. Since ridge speed is inversely proportional to ridge height, this should make a ridge move 17 ~ faster. Another portion of the increase was probably caused by a long term periodic variation in current speed due to a change in the inclination of the moon's orbit to the equator. This variation, which has a period of 18.6 years, was responsible for a i0 ~ increase in current strengths from 1962 to 1964; and although this increase is relatively small, it could produce more than a 50~o increase in ridge migration speed because of the high exponential dependence cited previously. Still another portion of the increase in ridge-migration speed could have been caused by a gradual increase in current strength due to a reduction in the size of the bay's natural entrance. There is evidence which indicates that sand migrating southeastward along the Gulf side of the barrier bar (see Fig. 1) is gradually pinching off the bay's old natural channel. This should cause the tidal flow to increase in the man-made channel and thereby increase the ridge-migration speed at the study site. Finally, there is the possibility that a 1962 dredging operation in the man-made entrance may have
I0
~_~
A
C= 3.12 x 10"7 V 5
II , [ ~ J , 1,1,1,] I0 115 20 25 30 40 50 60 70 80 meon flood velocity {V) in c m / sec
Fig.5. Ridge-migration speed as a function of mean flood velocity. Marine Geol., 4 (1966) 11-19
18
G . G . SALSMAN, W. H. TOLBERT AND R. G. VILLARS
altered hydraulic conditions at the study site. This effect, if present, must be small, as the dredging was done at least 3 km away and the quantity of sand removed was negligible compared to the size of the entrance. There are no means of accurately assessing the contribution of the latter two effects at this stage in the study. Accompanying the horizontal migration of the sand ridges at the study site has been a gradual rise in the mean bed level. It is difficult to evaluate the rate of rise at this time, for the ridges have not advanced one complete wavelength since the study began. But the following crude approximation can be made. About 120 cm beneath the trough level of ridge 11 the sediment type changes abruptly from sand to clay. This clay is apparently the " m u d " referred to as the surface sediment in pre-1934 nautical charts. Thus, some 120 cm of sand have been deposited at the study site in the 30 years since the Corps of Engineers excavated the man-made channel. During this 30 years some ten ridges have passed the study site. Six of these can be seen in Fig.2; four others, situated northeast of ridge 16, have diminished in height to the point that divers can see them, but the fathometer cannot. Thus, assuming that bed-load transport is the primary mode of sediment movement at the site, each ridge has apparently left behind a layer of sand averaging 12 cm thick. These ridges have moved into the study zone as the result of a man-made change in the bay's current regime.
CONCLUSIONS
During the past 2 years a series of remarkably uniform sediment ridges found on the bottom of St. Andrew Bay, Florida, have been investigated by scientific divers of the U. S. Navy Mine Defense Laboratory. These divers have found that the ridges are composed of a fine sand, are asymmetric in shape with steep slopes facing down current, have 30-60 cm heights, and 13-20 m wavelengths. A predominant flood current is causing them to migrate toward the northeast at an average rate of 1.35 cm per day, the rate being greater than average during periods when the moon and sun are near maximum declination, and smaller than average when they are near the celestial equator. These periodic variations indicate that the migration rate is extremely sensitive to changes in current speed, thus lending field support to the laboratory conclusions of other investigators. Near the leading edge of the ridge zone, where sand transport is primarily in the bed-load mode, each ridge passing a given point leaves behind a layer of sand averaging 12 cm thick. Approximately ten ridges have passed the study site since the Corps of Engineers excavated the bay's new man-made channel. This man-made change in the bay's current regime has caused some 120 cm of sand to be deposited in a location where the surface sediment was previously described as mud.
Marine Geol., 4 (1966) 11-19
SAND=RIDGEMIGRATIONIN ST. ANDREW BAY
19
REFERENCES
CHANG,,Y. L., 1939. Laboratory investigations of flume traction and transportation. Trans. Am. Soc. CivilEngrs., 104 : 1246-1313. CURRAY,J. R., 1960. Sediments and history of the Holocene transgression, continentalshell northern Gulf of Mexico. In: F. P. SHEPAP.D,F. B. PHLEGERand TJ. H. VAN ANDEL(Editors), Recent Sediments, Northwest Gulf of Mexico. Am. Assoc. Petrol. Geologists, Tulsa, Okla., pp.221-266. ICHIYE, T. and JONES, M. L., 2961. On the hydrography of the St. Andrew Bay system, Florida. Limnol. Oceanog., 6 (3) : 302-311. LIu, H. K., 1953. Ripple Formation and its Relation to Bed-Load Movement. Thesis, Univ. Minnesota, Minneapolis, Minn., Publ., 6389 : 160 pp. SMITH, P. F. and FREDKIN,E., 1963. Computers and transducers. In: R. D. GAUL(Editor), Marine Sciences Instrumentation. Plenum Press, New York, N.Y., 2 : 81-86. WALLER,R. A., 1961. Ostracods of the St. Andrew BaySyswm. M.S. Thesis, Dept. Biol. Sci., Florida State Univ., TaUahassee, Fla., 46 pp.
Marine Geol., 4 (1966) 11-19