Formation of deep scour holes at the junction of tidal creeks: An hypothesis

Marine Geology, 33 (1979)M9--M14

M9

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

Letter Section FORMATION OF DEEP SCOUR HOLES AT THE JUNCTION OF TIDAL CREEKS: AN HYPOTHESIS

BJORN KJERFVE 1, CHUN-CHIEN SHAO 1 and FRANK W. STAPOR Jr. 2

1Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina, Columbia, S.C. 29208 (U.S.A.) 2Marine Resources Research Institute, S. C. Wildlife & Marine Resources, Charleston, S.C. 29412 (U.S.A.) (Received August 15, 1978; revised and accepted May 23, 1979)

ABSTRACT

Kjerfve, B., Shao, C.-C. and Stapor Jr., F.W., 1979. Formation of deep scour holes at the junction of tidal creeks: an hypothesis. Mar. Geol., 33: M9--M14. Holes, 15--30 m deep and 2--5 times deeper than adjacent tidal channels, occur commonly at the junction of marsh drainage creeks in coastal South Carolina. They extend through marsh and channel sediments and are often cut 5--15 m into the underlying consolidated Tertiary Cooper marl limestone bedrock. We hypothesize that these holes primarily scour from excessive macro-scale turbulence during the flood portion of the tidal cycle, when the flow in the main channel branches into the tributaries and is highly variable in speed and direction. The holes are then swept clean of unconsolidated sediments by the swift ebb tidal current.

INTRODUCTION

Bathymetric charts of the central portion of coastal South Carolina reveal the existence of deep subaqueous holes at the junctions of tidal creeks. The reason for these holes has n o t previously been explained. Our hypothesis is that t h e y m a y form in tidal environments where the flow direction alternates over the tidal cycle. They are believed to be scoured b y turbulence. This primarily takes place during the flood tide when the flow divides into branch channels and excessive macro-scale turbulence exists, which tends to surpress normal b o u n d a r y layer development. Although c o m m o n features in tidal creeks, we are not aware of the existence of similar holes at the junction of river tributaries with main channels. Fig. 1 shows the location of 25 major scour holes where we have made observations and measurements which indicate that the holes exhibit many similar features. Here, we will concentrate our attention to one deep hole, the one located at the first branching of Dewees Creek landward of the Intracoastal Waterway (Fig. 1A). Dewees Creek flows across a Spartina alterniflora

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salt marsh, located between a Pleistocene mainland and a series of Holocene barrier islands. Quaternary deposits of unconsolidated silty clays and sands, 10--15 m thick, overlie the Tertiary Cooper marl bedrock (Force, 1978) which is very fine-grained limestone that is consolidated but not indurated (Malde, 1959). The deep hole at Dewees Creek has a m a x i m u m mean low water (MLW) depth of 24.7 m and is cut more than 9 m into the Cooper marl bedrock. The three channels leading toward the hole have a 7.6-m average m a x i m u m MLW depth. The hole is cone-shaped, forming an approximate ellipse at the depth corresponding to the adjacent channel bottoms. The major axis measures 260 m and the minor axis 80 m. Although not the deepest scour hole in South Carolina, the Dewees Creek hole ranks among the deepest and largest. Diver observations and grab samples indicate that the base of the hole is essentially sand-free with the Cooper marl bedrock exposed. In contrast, adjacent intertidal creek banks are composed of silty clay marsh sediments.

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Quartz sand-rich sediments (fine-grained, 1/4 to 1/8 mm diameter) make up the creek beds. Current studies in Dewees Creek and tributaries indicate that both ebb and flood tidal currents are capable of entraining this bed material. As the flow is ebb-dominant, net transport is directed oceanward (Shao, 1977). These ebb-dominant tidal currents not only introduce sand from upstream but also sweep the Cooper marl exposure clean and keep it clean enough for burrowing organisms, mainly pelecypods of the family Pholadidae, to thrive Although several modes of formation are possible, all indications suggest that the deep holes are due to active current scour. The holes are much larger than potholes in rivers (Strahler, 1971, p. 627) and differ from these in that no grinder besides sand, is present. There is no evidence that they are related to dredging or man-made structures. Nor are they likely to be sinkholes. Although the Cooper marl is leached locally beneath its contact with overlying Quaternary sands and clays (Malde, 1959), no other example of chemical weathering-produced relief is known. Further, the Cooper marl erosion surface slopes gently and uniformly seaward (Fig. 1) with topographic relief present only along existing river channels and pre-Holocene channels, now filled with Pleistocene coastal deposits (Force, 1978; Colquhoun et al., 1972). Thus it is highly unlikely that the holes are recently exhumed relict features. We suggest that the scour primarily takes place during flood tide when the flow in Dewees Creek is forced to divide into the two branch channels. The flood current exhibits a significantly greater short-term (~ 15 min) velocity variability than the ebb current, i.e., macro-scale turbulence occurs to a greater extent, normal boundary layer development is suppressed, and thus the potential for scour is significantly enhanced. This argument is qualitatively supported by laboratory studies (Taylor, 1944; Law and Reynolds, 1966), which indicate the existence of excessive turbulence during dividing flow, i.e. flood tide. Also, attempts at momentum and energy balance in junctions point to highly complex flow kinematics and dynamics as the flow divides (Ellis, 1969; Law and Reynolds, 1966; Taylor, 1944). Analytical treatment of dividing channel flow is intractable, whereas combining channel flow, i.e. ebb tide, has analytical solutions (Shao, 1977; Ellis, 1969; Law and Reynolds, 1966; Taylor, 1944). These indicate that only a slight loss of energy occurs at the junction on the ebb tide. We therefore argue that due to a greater velocity variability or more macro-scale turbulence during flood than ebb flow, the tidal holes are most likely to scour during the flood portion of the tidal cycle. Theoretical projections of complex flood flow kinematics are supported by current meter measurements made 1 m off the bottom (at a MLW depth of 23.7 m) in the deepest part of the Dewees Creek hole (Fig. 1A, B). A 10-day time series of current speed (Fig. 2) and direction (Fig. 3) was made with a General Oceanics 2010 current meter. Each flood tide exhibits highly variable flow because of macro-scale turbulence. The ebb current speed is significantly greater and the direction fluctuates to a lesser extent. The average peak ebb current in the hole (60 cm/s) is sufficient to entrain and transport sand-sized sediment (Komar, 1976).

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The hypothesis of maximum scour taking place during flood tide is further supported by the asymmetric shape o.~ the hole (Fig. 1B). Fathometer traces along Dewees Creek, extending into the branch channels, reveal steeply sloping sides (steep side; Fig. 1B) on the landward edge of the hole. Toward the ocean the sides slope more gently (gentle side; Fig. 1B) suggesting the possibility of ebb-tide sediment deposition away from maximum scour in the hole. The formation of deep tidal scour holes is hypothesized to be due to hydranlic action primarily during the flood ~ide because of excessive macroscale turbulence. Scour is known to occur with increased turbulence (Russel, 1967), which should be most intense at the steep side of the hole. Scour could

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Fig. 4. Scatter diafram showing a relationship between the relative depth of the tidal s c o u r hole, Dr, and the junction angle, a (see Fig. 1A). The scour hole at Dewees Creek is indicated with an open circle. The solid line is the best least squares fit with a coefficient of determination of 0.3.

also be due to the presence of a secondary (helicoidal) flow in the hole (Blatt et al., 1972, pp. 138--139). However, the existence of a secondary current in the hole has neither been d o c u m e n t e d nor would it necessarily only occur during the flood tide. The depth of tidal scour holes is at least partially related to the junction angle (Fig. 1A) (Shao, 1977). The relative depth, Dr, is the ratio of maximum hole depth to average maximum depth of the channels leading into the hole. With a great junction angle, ~, the relative water depth is large (Fig. 4). Leastsquares analysis, assuming error in b o t h Dr and ~ (Burington and May, 1970, pp. 150--159), yields: Dr = 0.04~ - 1.4 for 51 ° ~< ~ ~< 148 ° with a coefficient of determination r 2 = 0.30. Although 30% of the relative depth variation is attributable to the junction angle, other u n k n o w n factors also play an important role in controlling the depth of tidal scour holes. These m a y include channel erodability, magnitude of maximum currents, and supply of sediment. ACKNOWLEDGEMENTS

Support for current meter installations was provided b y the S.C. Coastal Council to the Marine Resources Division of the S.C. Wildlife and Marine Resources Department. Lucy Kjerfve did the drafting. This is Contribution No. 281 from the Belle W. Baruch Institute for Marine Biology and Coastal Research.

M14 REFERENCES Blatt, H., Middleton, G. and Murray, R., 1972. Origin of Sedimentary Rocks. Prentice-Hall, Englewood Cliffs, N.J., 634 pp. Burington, R.S. and May Jr., D.C., 1970. Handbook of Probability and Statistics with Tables, McGraw-Hill, New York, N.Y., 2nd ed., 462 pp. Colquhoun, D.J., Bond, T.A. and Chappel, D., 1972. Santee submergence, example of cyclic submerged and emerged sequences. Geol. Soc. Am. Mere., 133: 475--496. Ellis, J., 1969. A study of unsteady flow in branched channels. Proc. Inst. Cir. Eng. (London), 44: 341--348. Force, L., 1978. Structure contour map of the Pleistocene near Charleston. S.C.U.S. Geol. Surv., Open File Report. Komar, P.D., 1976. Transport of cohesionless sediments on continental shelves. In: D.J. Stanley and D.J.P. Swift (Editors), Marine Sediment Transport and Environmental Management. Wiley, New York, N.Y., pp. 107--126. Law, S.W. and Reynolds, A.J., 1966. Dividing flow in axi open channel. J. Hydraul. Div., Proc. Am. Soc. Cir. Eng., 92(HY2): 207--231. Malde, H.E., 1959. Geology of the Charleston phosphate area, South Carolina. U.S. Geol. Surv. Bull., 1 0 7 9 : 1 0 5 pp. Russel, R.J., 1967. River Delta Morphology. Louisiana State Univ. Press, Baton Rouge, La., 55 pp. Shao, C.-C., 1977. On the Existence of Deep Holes at Tidal Creek Junctions Thesis in Marine Science Program, Univ. of South Carolina, Columbia, S.C., 31 pp. Strahler, A.N., 1971. The Earth Sciences. Harper & Row, New York, N.Y., 2nd ed., 824 pp. Taylor, E.H., 1944. Flow characteristics at rectangular open channel junctions. Trans. Am. Soc. Cir. Eng., 109: 893--902.