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].
 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 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.
 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.