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Dimensions of Fluvial-Tidal Meanders: Are They Disproportionally Large? Jasper R.F.W

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Dimensions of Fluvial-Tidal Meanders: Are They Disproportionally Large? Jasper R.F.W https://doi.org/10.1130/G45144.1 Manuscript received 9 May 2018 Revised manuscript received 26 July 2018 Manuscript accepted 16 August 2018 © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 11 September 2018 Dimensions of fluvial-tidal meanders: Are they disproportionally large? Jasper R.F.W. Leuven*, Barend van Maanen, Bente R. Lexmond, Bram V. van der Hoek, Matthijs J. Spruijt, and Maarten G. Kleinhans Faculty of Geosciences, Utrecht University, Princetonlaan 8A, 3584 CB Utrecht, The Netherlands ABSTRACT meandering channel deposits in Palaeozoic Many of the world’s major river systems seemingly have one or a few disproportionally outcrops are orders of magnitude smaller than large meanders, with tight bends, in the fluvial-tidal transition (e.g., the Thames in the UK, in modern systems (Davies and Gibling, 2011, and the Salmon River in Canada). However, quantitative studies on meanders have so far 2013) and are possibly biased toward maximum primarily focused on rivers without tidal influence or on small tidal meanders without river meander dimensions and sinuosity, because inflow, providing relations between channel geometry and meander characteristics (length, meanders at the point of cut-off have the highest amplitude, and sinuosity). Physics-based predictions of meander size and shape for the fluvial- preservation potential (e.g., Durkin et al., 2017). tidal transition zone remain untested for a lack of data. Therefore, it remains unclear whether A possible exception is found in seismic data of the dimensions of meanders in the fluvial-tidal transition zone are indeed disproportionally the Cretaceous McMurray Formation (Canada), large, and whether meander characteristics can be used as an indicator for tidal influence. which has kilometer-scale meander bends (Hub- Here, data from 823 meanders in 68 fluvial-tidal transition zones worldwide are presented bard et al., 2011; Durkin et al., 2017). Both the that reveal broad-brush relations between channel geometry and meander dimensions. Our morphology and the facies in core data are simi- results show that fluvial-tidal meanders indeed become larger in the seaward direction, but lar to that of a fluvial environment (Mossop and the dimensions are proportional to local channel width, as in rivers. Sinuosity maxima are Flach, 1983; Hubbard et al., 2011), although an exception, rather than the rule, in the fluvial-tidal transition zone. Surprisingly, the width trace fossils indicate brackish water, which is of the upstream river correlates with estuarine channel width and tidal meander size even interpreted as the fluvial-tidal transition zone though river discharge constitutes only a fraction of the tidal prism. The new scaling relations (La Croix and Dashtgard, 2015; Gingras et al., can be used to constrain dimensions of rivers and estuaries and their meanders. 2016). Therefore, it would be useful to have a complementary indicator that can discriminate INTRODUCTION are much smaller (typically 2–200 m wide) than between fluvial, fluvial-tidal, and tidal meanders The shapes and dimensions of meanders fluvial and estuarine meanders. Moreover, they on the basis of geometry for the full range of are a major characteristic of meandering rivers predominantly form under ebb-dominant flow scales on which they occur on Earth. (Leopold and Wolman, 1960). Typical scaling and may thus be comparable with small river Lack of data and empirical generalizations relations exist between meander dimensions, meanders (Kleinhans et al., 2009). leave hypotheses untested, such as the hypoth- river discharge, and local channel width (e.g., The majority of the world’s largest cities are esis that fluvial-tidal meanders are very tight, Inglis, 1949; Leopold and Wolman, 1960; Hey located near estuaries and tidal rivers (Ashworth with sinuosities peaking above 2.5 at the bed- and Thorne, 1986) (Table DR1 in the GSA Data et al., 2015) in which meandering channels are load convergence zone (Dalrymple et al., 2012). Repository1). However, these empirical relations ecologically valuable but also threaten bank The limited number of analyzed cases so far have never been tested in the fluvial-tidal transi- stability, affect shipping lanes, and influence does not demonstrate whether sinuosity peaks tion zone and in estuaries (Fig. 1), where one of flood safety. Therefore, it is of interest whether significantly beyond the natural temporal and the signatures of the environment is that local the meander-forming mechanisms are similar to along-channel variability within meandering tidal prism (the volume of water leaving an estu- those of their fluvial counterparts. Physics-based systems, which occurs due to bend cutoffs, and ary at ebb tide) increases in the seaward direction theory can predict dominant meander lengths the scatter in combined observations from mul- (Leuven et al., 2018) with exponentially increas- based on forming mechanisms (e.g., Solari et tiple systems (Howard and Hemberger, 1991; ing channel width. Observations of tidal mean- al., 2002; Schramkowski et al., 2002; Seminara, Camporeale et al., 2007; Hubbard et al., 2011) ders show that their planform dimensions scale 2006; for review, see Leuven et al., 2016), but (Fig. 1B). Moreover, a sinuosity peak implies similarly as in rivers (Marani et al., 2002; Solari the resulting predictions have never been tested a disproportional scaling relation between et al., 2002; D’Alpaos et al., 2017; Finotello et al., due to a lack of data from natural systems. meander dimensions and channel dimensions 2018). However, these observations mainly cover Geological outcrops could provide an alter- predicted by theory, because it requires a peak meanders on tidal flats and salt marshes, which native source of data. However, preserved in meander amplitude or drop in meander length. 1 GSA Data Repository item 2018343, supplementary figures, tables, and a .kml file with the recorded polygons of fluvial-tidal meanders, is available online at http://www.geosociety.org/datarepository/2018/ or on request from [email protected]. *E-mail: [email protected] CITATION: Leuven, J.R.F.W., van Maanen, B., Lexmond, B.R., van der Hoek, B.V., Spruijt, M.J., and Kleinhans, M.G., 2018, Dimensions of fluvial-tidal mean- ders: Are they disproportionally large?: Geology, v. 46, p. 923–926, https://doi.org/10.1130/G45144.1. Geological Society of America | GEOLOGY | Volume 46 | Number 10 | www.gsapubs.org 923 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/46/10/923/4338811/923.pdf by guest on 26 September 2021 Observations from aerial photography and pre- A fluvial-tidal transition zone (3) fluvial meanders dictions from physics-based theory (Solari et al., large 2002) suggest that these meanders are longer meander than those of their fluvial counterparts. (1) meanders in multi-threaded zone small meanders Therefore, we test whether fluvial-tidal mean- approximate ders are indeed disproportionally large when 1 km tidal limit (2) fluvial-tidal meanders 52°32’ N 4°01’ W compared with their local channel dimensions B and upstream and downstream meanders. We derive scaling relations that can be used to obtain former meanders broad-brush dimensions of meanders along the fluvial-tidal continuum, for application in geo- morphology and geological reconstruction of 3 km meander belts. To this end, a new data set was 12°28’ S 132°24’ E collected with characteristics of 823 meanders C in 68 fluvial-tidal transition zones from around the world. Data were compared with typical rela- tions previously found for rivers and for small tidal meanders. 1 km 45°42’ N 63°18’ W DATA COLLECTION AND ANALYSIS We recorded channel planform polygons Figure 1. Aerial photographs of fluvial-tidal meanders (Google Earth™). River flow is from right to left. A: Dovey River, Wales, UK. B: South Alligator River, Northern Territory, Australia. for 68 rivers that transition into seas (Table C: Salmon River, Cobequid Bay, Canada. DR2). Because hydrodynamic data are gen- erally unavailable, the tidal limit was defined as the landward location where channel width approaches a constant value within a factor two of the most upstream river width. Meanders were digitized beyond that point, such that sub- A 66 B sequently, the tidal limit could be derived from 4 the along-channel width profile (Figs. DR1 and 10 DR2). To allow for comparison between sys- 57 ) tems, along-channel distance was normalized with the tidal limit. In cases where the single threaded river transitions into an estuary with 103 braided channels and bars, we digitized the main 20 continuous channel. This approach resulted in a y R2 ±2σ 2 13x1.2 0.6 3.3 y R ±2σ classification of (1) main meanders in the multi- 1.1 0.88 0.75 2.8 Meander length (m 23x 0.42 4.3 25x 1.2 threaded zone, (2) single-threaded fluvial-tidal 52x 0.54 4.1 Data Leopold (1964) 102 meanders, or (3) fluvial meanders (Fig. 1A). Fluvial meanders Data Marani (2002) The channel centerlines and along-channel Single-threaded fluvial-tidal meanders Marani et al. (2002) width profiles were automatically determined Main meanders in multi-threaded zone using GIS software (following the approaches C D of Davies and Woodroffe [2010] and Leuven et al. [2018]). All centerlines were smoothed with a Polynomial Approximation with Exponential 101 ) Kernel (PAEK) algorithm using a tolerance of 0.6× the local channel width, to prevent the cen- terlines from being too sensitive to local varia- tions in channel width. 3 10 Meander length (λ) and amplitude (a) were calculated from the centerlines as follows (Fig. y R2 ±2σ DR3). Inflection points were determined from Meander amplitude (m 0.73x 1.5 0.21 8.9 channel curvature (Howard and Hemberger, 2 2x1.4 0.02 9.6 10 1.4 2 ±2 1991; Marani et al., 2002; Schwenk et al., 2015).
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