Morphodynamics of Tidal Channels In the Open Coast Macrotidal Flat, Southern Ganghwa Island In Gyeonggi Bay, West Coast of Korea

Abstract

Morphodynamics of intertidal channels were monitored in order to understand their implication on the architecture of inclined heterolithic stratification (IHS) in the open-coast Yeochari macro tidal flat on southern Ganghwa Island in Gyeonggi Bay, west coast of Korea. The tidal flat is divisible into narrow salt marshes in the upper intertidal zone, a concave-up upper to middle intertidal zone with small tidal creeks, and a channelized lower intertidal zone. Channels in the lower intertidal zone are 200-600 m wide and 1-2.5 m deep at bankful stage. They are sinuous in planform and have well-defined point bars and cutbanks. IHS is exposed along the cutbanks. High-precision leveling of the tidal channel bank unveiled that a point bar in the lower intertidal zone migrated about 400 m in 39 months. Channel migration is pronounced during the summer rainy season, when the point bar migrated as fast as 40 m per month, which led to rapid sediment accumulation of as much as 40 cm per month. In contrast, channel migration rate during winter is notably reduced, down to less than 1 m per month. Point-bar geometry alternates between a concave-up profile in summertime and a convex-up profile during the remainder of the year. Enhanced ebb currents during the rainy season, due to increased runoff discharge caused by heavy precipitation, accelerated point-bar migration. Remarkable rill erosion induced by heavy precipitation especially during low tide led to the rapid accumulation of sediment at the lower part of the point bar and the channel base, creating concave-up point-bar geometry. During the remainder of the year, the point bar retains a convex-up profile by continued sedimentation with insignificant rill erosion. Wave activity, particularly during winter, seems to facilitate deposition in the upper part of the point bar by providing more sediment into the channel. The present study illuminates the fact that the stratigraphic architecture of IHS of intertidal origin is controlled largely by monsoonal weather and to a lesser degree by tidal process even in a macrotidal environment. Careful examination of IHS geometry may hint at the depositional setting in terms of tidal frame (intertidal versus subtidal) and climate regime (monsoonal versus non-monsoonal).

Incipient Tidal Bar and Sill Formation

“…Point bars are a shared in‐channel element of tidal channels and rivers (Barwis, ; Bridges & Leeder, ; Brivio et al, ; Choi et al, ; Choi & Jo, ; de Mowbray, ; Ghinassi et al, ; Pearson & Gingras, Solari et al, ; Tank, ), but only in cases where the channel is relatively narrow and deep such that the active BI is unitary and where channel curvature is strong enough to force the bar to be nonmigratory. However, in the absence of bank strength the experiments showed behaviour similar to river braiding.…”

Morphodynamic evolution and stratal architecture of translating tidal point bars: Inferences from the northern Venice Lagoon (Italy)

“…4A), suggesting that the lack of any rhythmicity in the distribution of mud drapes would be ascribed to intense bioturbation. Millimetric sandy laminae interbedded within bar-top mud were formed during storm events, when wave winnowing suspended the muddy fraction and concentrated sandy grains (Green & Coco, 2007Carniello et al, 2011Carniello et al, , 2012Carniello et al, , 2014Fruergaard et al, 2011;Choi & Jo, 2015). These grains were occasionally spread on the bar top by strong tidal currents, which also caused local concentration of shells and shell fragments in bar sand.…”

“…In fluvial settings, this flow pattern develops a clear downstream-fining grain-size trend (Jackson, 1976) that is welldeveloped in downstream-migrating point bars (Smith et al, 2011;Durkin et al, 2015;Ghinassi & Ielpi, 2015). This flow configuration is also experienced by tidal meanders, although the differential paths of the high-velocity streamlines during a tidal cycle (Mutti et al, 1985;Choi et al, 2004;Fagherazzi et al, 2004;Dalrymple & Choi, 2007;Li et al, 2008;Hughes, 2012;Choi & Jo, 2015) allow the same side of the bar (for example, the seaward side) to cyclically experience both the outward-directed flow (for example, flood flow) and the secondary helical current (for example, ebb flow). These opposing and offset currents, highlighted by Fenies & Faug eres (1998) showing orientation of bedforms along a point bar of the Arcachon Lagoon (south-west France), contribute to explain the sedimentary features of the study bar.…”

Tidal currents and wind waves controlling sediment distribution in a subtidal point bar of the Venice Lagoon (Italy)

“…These bars show an overall stratal architecture which is not far from that defined by classical models for their fluvial counterparts (Allen, ; McGowen & Garner, ; Brice, ; Jackson, ; Nanson, ; Thomas et al ., ), since both are characterized by laterally‐accreted deposits, as well as accretional and erosional processes along the inner and outer bank, respectively (e.g. Bridges & Leeder, ; Barwis, ; Allen, ; De Mowbray, ; Marani et al ., ; Seminara et al ., ; Solari et al ., ; Pearson & Gingras, ; Choi et al ., ; Choi & Jo, ; Brivio et al ., ; Gugliotta et al ., ; D'Alpaos et al ., ; Ghinassi et al ., ). Beyond such affinities, however, several features differentiate between tidal and fluvial point bars: (i) tidal channels are affected by periodic flow reversal; (ii) tidal discharge fluctuates within a defined range, whereas fluvial discharge is commonly characterized by a more marked variability; (iii) active tidal channels experience conditions of slack water; and (iv) tidal channels typically widen seaward in response to the increase in the flowing tidal prism (Lanzoni & D'Alpaos, ), whereas fluvial channels exhibit constant width over longer distances.…”