Persistent sand bars explained by geodynamic effects



[1] The persistent nature of intertidal sand bars has been the subject of much speculation concerning the hydrodynamic mechanisms involved, but its origin remains enigmatic. Here, we introduce salient geophysics in contrast to the physics of fluids above the sediments. The geophysical evidence combined with theoretical modeling and analysis demonstrates that the geodynamic processes ensuing during exposure periods have a profound impact, yielding the persistent nature of the intertidal bars under severe hydrodynamic forcing which would otherwise lead to unstable bar behavior. The feedback between the effects of the dynamics of suction, i.e. negative pore water pressure relative to atmospheric air pressure, and sediment transport and morphology is found to play a crucial role in the intertidal bar morphodynamics. Our finding may fundamentally alter the current perspective, leading to a new level of understanding, of sediment transport and bar behavior at waterfronts that are ubiquitous in rivers, estuaries, and coastal seas.

1. Introduction

[2] Sand bars are common morphological features in rivers, estuaries, and coastal seas. In the marine environment, they are situated in subtidal and/or intertidal zones. Sand bars play an important role in beach stability since they reduce the energy of waves by breaking them, thereby preventing severe erosion. The hydrodynamics and associated sediment transport processes involved have thus been extensively investigated to understand the sand bar morphodynamics [e.g., Hoefel and Elgar, 2003; Masselink et al., 2006; Ruessink et al., 2007]. Sand bars typically move offshore during storms and move back onshore to form a berm under calm wave conditions. However, there are persistent intertidal sand bars that are subdued and static even in the presence of sufficiently strong waves, but their origin remains an “enigma” [Masselink et al., 2006].

[3] In the present study, we aim to unravel the origin of such persistent sand bars. To this end, we introduce our recent findings on the salient physics involved in intertidal sediments, which contrasts sharply with the physics of fluids above the sediments. We have previously demonstrated that the dynamics of suction, that is, negative pore water pressure relative to atmospheric air pressure, play a substantial role in the temporal and spatial evolution of voids and surface shear strength in cyclically exposed and submerged sediment [Sassa and Watabe, 2007]. In this paper, we explore the role of such geodynamic processes in the intertidal bar morphodynamics.

[4] The organization of this paper is as follows. We first review the intertidal bar morphology in relation to the prevailing hydrodynamic and sediment transport characteristics. We then describe the physical evidence concerning the effects of the suction dynamics, followed by a description of their modelling and analysis in the context of sediment transport and bar morphology.

2. Persistence of Intertidal Sand Bars in the Presence of Waves and Currents

[5] Intertidal sand bars can generally be categorized into three main types depending on their amplitudes and slopes: slip-face bars, low-amplitude ridges, and sand waves. Slip-face bars present the most pronounced and dynamic morphology, with bar heights generally exceeding 1 m. They migrate offshore during storms and remigrate onshore under prolonged calm wave conditions [Roelvink and Stive, 1989], a characteristic common to subtidal bars [e.g., Hoefel and Elgar, 2003; Ruessink et al., 2007]. Here, subtidal bars refer to those submerged all the time. By contrast, although the low-amplitude ridges, and especially the sand waves, are fairly static, they experience a series of wave processes, including shoaling, wave breaking, swash and return flow, in the course of water level changes during tides [Masselink et al., 2006]. Indeed, various authors have explored the hydrodynamic mechanisms involved, suggesting that they are the results of multiple wave breaking and undertow development [e.g., Dolan and Dean, 1985], of surf zone processes due to wave breaking and return flow [e.g., Carter, 1988], of swash processes through profile steeping [e.g., King, 1972], of shoaling waves [e.g., Boczar-Karakiewicz and Davidson-Arnott, 1987], and of standing infragravity waves [e.g., Short, 1975]. Despite the development of these theories to explain the formation of multiple sand bars, the origin of their persistent nature remains unclear [Komar, 1998; Masselink et al., 2006].

[6] Here, we consider two representative examples showing the morphodynamic stability of intertidal multiple sand bars. Figure 1a shows the results of field surveys performed during a 7-year period from 1994 to 2000 on the Banzu intertidal flat located on the east coast of Tokyo Bay, Japan [Furukawa et al., 2000]. The soils were fine-grained sands with D50 in the range of 0.17 to 0.23 mm. Multiple sand bars were present on the lower intertidal zone, with heights of 0.1 to 0.2 m and lengths of 40 m on a gentle slope of 1/1000. There were temporal variations in the average ground heights due primarily to the net deposition with an average rate of 0.04 m/year [Uchiyama, 2007]. However, except for the one at the offshore front, the bar locations remained stationary.

Figure 1.

Results of field surveys showing the morphodynamic stability of intertidal sand bars at (a) Banzu sandy flat [Furukawa et al., 2000] and (b) Okoshiki beach [Yamada and Kobayashi, 2007]. MHWS, MLWS, and MLWN refer to the mean high water spring, mean low water spring, and mean low water neap, respectively.

[7] Figure 1b shows the results of 26 field surveys performed during a 3-year period from 2003 to 2005 on Okoshiki beach located in Ariake Bay, Japan [Yamada and Kobayashi, 2007]. The soils were fine-grained sands with D50 in the range of 0.12 to 0.33 mm. Multiple sand bars with heights of 0.15 to 0.5 m and lengths of 30 to 50 m were present on a mild slope of 1/300. Except for the offshore fronts, the bar locations remained essentially the same.

[8] During the periods of both surveys, the two different sites experienced occasional seasonal events such as storms and typhoons [Furukawa et al., 2000; Yamada and Kobayashi, 2007]. This fact, together with the above field results, indicates the persistent nature of the intertidal sand bars in the presence of waves and currents.

3. Suction Dynamics and Its Effects on Sediment Transport and Bar Morphology

3.1. Geophysical Evidence

[9] Sediments in intertidal zones are cyclically exposed and submerged. Accordingly, there are temporal changes in the groundwater level, causing dynamic changes in the suction state of the sediments [Sassa and Watabe, 2007]. Suction represents the tension of moisture in the sediment and is defined by

equation image

where ua is the atmospheric air pressure and uw is the pore water pressure in the sediment. By definition, suction is equal to zero at the groundwater level.

[10] Through a combination of field, experimental, and theoretical investigations, we have revealed the following [Sassa and Watabe, 2007]. The dynamics of suction in association with tide-induced groundwater level fluctuations bring about a significant cyclic elastoplastic contraction in repeatedly exposed, yet saturated sediments. Such suction-induced void state changes give rise to distinct variations in the surface shear strengths of the sediments, the magnitudes of which depend strongly on the intensity of the suction dynamics ensuing there.

[11] At the Banzu intertidal flat, the bars experienced larger groundwater level variations, thereby undergoing stronger suction dynamics than the troughs. As a result, the bars became denser and developed significantly higher surface shear strengths, beyond three-fold magnitudes, than the troughs [Sassa and Watabe, 2007].

[12] The marked contrast in the bar-trough stratigraphy due to the suction-induced cyclic elastoplastic contraction has also been confirmed by the application of a high resolution surface-wave seismograph and by insitu sediment sampling and physical soil tests [Watabe and Sassa, 2008].

[13] Overall, these effects of the suction dynamics yield a close relationship between the distributions of the surface shear strengths and the variations of the ground heights in the bar-trough intertidal sediments.

3.2. Modeling and Analysis

[14] Sediment becomes mobile when the surface shear stress exerted due to waves and currents exceeds a threshold shear stress of the sediment, i.e., the shear strength of a single particle resting on the sediment surface. Under severer conditions, the thickness of the mobile layer increases with increasing sediment transport rate, which is constrained by a unique relationship at the bottom of the mobile layer, namely, that the shear stress must be equal to the shear strength there [Nielsen, 1992]. Indeed, Figure 2 clearly shows that the sediment transport rate is a function of both the shear stress and shear strength of the sediment. Although significant advances have been made in understanding the evolutions of the shear stresses, current approaches to sediment transport modelling explicitly assume the shear strengths to be fixed in the sediments [Hoefel and Elgar, 2003; Hsu and Hanes, 2004; Masselink et al., 2006; Ruessink et al., 2007]. In view of Figure 2, this means that for given sediment grain size, the internal friction angle of sediment, ϕ is constant: for example, Ruessink et al. [2007] adopted constant values of tan ϕ in the range between 0.1 and 0.5 in their numerical analyses. The geophysical evidence, however, has clearly shown that the intertidal sediments exhibit distinct variations of the surface shear strengths due to the effects of the suction dynamics.

Figure 2.

Sketch showing sediment transport rate Q as functions of both shear stress τ and shear strength τ* of sediment. Here m denotes the lumped parameter: submerged unit weight of sediment multiplied by tan ϕ where ϕ is internal friction angle of sediment. The dotted line and arrow represent respectively the enhanced shear strength and the resulting reduced mobile layer thickness, which is indicative of the reduced sediment transport rate due to the effects of suction dynamics.

[15] Below, we will describe a simple physics-based model for the effects of the suction dynamics on sediment transport and bar morphology.

[16] The equation of continuity for sediment mass in a cross-shore direction x can be expressed by [e.g., Hoefel and Elgar, 2003]

equation image

where z < 0 is the ground height which is taken downwards from just above a given intertidal bar morphology on the shore side, n is the porosity of the sediment, and Q is the cross-shore sediment transport rate per unit length in the alongshore direction. The cross-shore sediment transport direction is cyclic in space and time due to the intertidal hydrodynamic characteristics [Masselink et al., 2006]. Q may take its simplest form

equation image

where κ = 2π /L and ω = 2π/T are the wave number and angular frequency of Q, respectively, and A represents the maximum sediment transport rate, which depends on both the given shear stress and the shear strength in the sediment. Considering the above-described close relationship between the shear strength and the bar-trough ground height distributions due to the effects of suction dynamics yields

equation image

where a is a parameter constrained by the given shear stress on the sediment, and ∣z∣ is a dimensionless ground height normalized by the extent of the intertidal zone, set at 1 m here. Equation (4) shows that when a is constant, lower sediment transport rates ensue at bars than at adjacent troughs due to their higher shear strengths, in view of Figure 2.

[17] Analysis of the intertidal bar morphodynamics was performed on the basis of equations (2) to (4). With a given initial geometry and wave and sediment conditions, equation (2), incorporating equations (3) and (4), was solved using a finite difference method. The ground height distributions obtained were used to update equations (2) to (4). Calculations continued for a target number of time steps. In view of the intertidal sand bars described in the former section of this paper, the initial bar geometry was set as: length 40 m, height 0.25 m, slope 1/500. The parameters used were: T = 1 year, L = 40 m, n = 0.45, a = 0.0075 m2/day. Here, the value of a was determined by ensuring that the prescribed sediment transport rate was large enough to cause dynamic bar movement without the effects of the suction dynamics. This was achieved by setting A = a in equation (4) for the purpose of comparison.

3.3. Results and Discussion

[18] The sand bar behaviors with the effects of suction dynamics are plotted in Figure 3. In the absence of the geodynamic effects, in other words, solely under the influence of hydrodynamic agents, dynamic morphological changes ensue due to the repeated erosion and deposition. Namely, while the bar heights remain essentially constant, the sand bar undergoes periodic offshore and onshore movements. By contrast, the geodynamic effects alter the bar behavior sharply, as shown in Figure 3a. The morphological changes become markedly suppressed in such a manner that the bar heights vary but their locations remain the same, indicating the persistent nature of the sand bars.

Figure 3.

Results of analysis with the effects of suction dynamics, showing (a) the persistent nature of the bar and (b) the transformation of the bar behavior in the offshore direction. Figure 3a depicts the upper onshore part shown in Figure 3b.

[19] Let us mention here the basis for close-up of this particular bar behavior for presentation. In view of the initial bar geometry shown in Figure 3a, the ground height ∣z∣ of the bar crest is approximately one-third of that of the trough. Following the descriptions in the preceding subsection, this corresponds to the assumption of about three-fold increase in shear strength at the bar crest than the adjacent trough in the analysis, which is consistent with the field evidence described above.

[20] The results demonstrate that a simple yet realistic consideration of the effects of suction dynamics can account for the persistence of the intertidal sand bars subjected to a severe hydrodynamic forcing which would otherwise lead to unstable bar behavior. It is important, however, to remark that the way in which the geodynamic effects manifest themselves can vary depending on a number of factors, including bar morphology, slope, location in the cross-shore direction, and sediment grain size. One such example is illustrated in Figure 3b, showing that the bar behavior becomes gradually dynamic in the offshore direction. This behavior stems from the decreasing effects of the suction dynamics, as the groundwater level relative to ground surface level becomes shallower with descending ground height in the offshore direction. Indeed, one can observe the sequence of processes of bar generation, migration, and development and decay at the offshore fronts. These results conform to the general features of the observed bar behavior in the field, as described in section 2 of this paper.

[21] Also, under conditions where the sediment becomes unsaturated, i.e. suction exceeds air-entry suction of the sediment, due to the coarse grain size with low air-entry suction and/or prolonged exposed bar height with excessive suction [Sassa and Watabe, 2007], for example, in the case of slip face bars as referred to in section 2 of this paper, the effects of suction dynamics may become less pronounced, allowing dynamic bar movement under the given hydrodynamic forcing.

[22] The above results and discussion emphasize the importance of properly considering the interplay between the prevailing hydrodynamics and the geodynamic effects in the intertidal bar morphodynamics.

4. Conclusions

[23] Recent findings about the salient physics involved in intertidal sediments have led to a substantial new insight into the intertidal bar morphodynamics. Namely, the morphodynamic stability of the intertidal sand bars, which has thus far remained elusive, has been shown to manifest itself due to the interplay between the effects of the suction dynamics and sediment transport and morphology. The present finding is relevant to sediment bars which experience periodic exposure events, such as in rivers, estuaries, and coastal seas. Hence, it may effectively contribute to the engineering design and maintenance of such morphological features, which are often crucial for disaster reduction, as well as for conservation and restoration of habitats with diverse ecological activity.


[24] We gratefully acknowledge the helpful comments of anonymous reviewers. This study was supported by the grant-in-aid for scientific research (Nos. 18360232 and 20360216) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.