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Modification of Meander Migration by Bank Failures

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Modification of Meander Migration by Bank Failures JournalofGeophysicalResearch: EarthSurface RESEARCH ARTICLE Modification of meander migration by bank failures 10.1002/2013JF002952 D. Motta1, E. J. Langendoen2,J.D.Abad3, and M. H. García1 Key Points: 1Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA, • Cantilever failure impacts migration 2National Sedimentation Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Oxford, Mississippi, through horizontal/vertical floodplain 3 material heterogeneity USA, Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, USA • Planar failure in low-cohesion floodplain materials can affect meander evolution Abstract Meander migration and planform evolution depend on the resistance to erosion of the • Stratigraphy of the floodplain floodplain materials. To date, research to quantify meandering river adjustment has largely focused on materials can significantly affect meander evolution resistance to erosion properties that vary horizontally. This paper evaluates the combined effect of horizontal and vertical floodplain material heterogeneity on meander migration by simulating fluvial Correspondence to: erosion and cantilever and planar bank mass failure processes responsible for bank retreat. The impact of D. Motta, stream bank failures on meander migration is conceptualized in our RVR Meander model through a bank [email protected] armoring factor associated with the dynamics of slump blocks produced by cantilever and planar failures. Simulation periods smaller than the time to cutoff are considered, such that all planform complexity is Citation: caused by bank erosion processes and floodplain heterogeneity and not by cutoff dynamics. Cantilever Motta, D., E. J. Langendoen, J. D. Abad, failure continuously affects meander migration, because it is primarily controlled by the fluvial erosion at and M. H. García (2014), Modification the bank toe. Hence, it impacts migration rates and meander shapes through the horizontal and vertical of meander migration by bank failures, J. Geophys. Res. Earth Surf., 119, distribution of erodibility of floodplain materials. Planar failures are more episodic. However, in floodplain 1026–1042, doi:10.1002/2013JF002952. areas characterized by less cohesive materials, they can affect meander evolution in a sustained way and produce preferential migration patterns. Model results show that besides the hydrodynamics, bed Received 16 AUG 2013 morphology and horizontal floodplain heterogeneity, floodplain stratigraphy can significantly affect Accepted 31 MAR 2014 meander evolution, both in terms of migration rates and planform shapes. Specifically, downstream Accepted article online 1 APR 2014 meander migration can either increase or decrease with respect to the case of a homogeneous floodplain; Published online 9 MAY 2014 lateral migration generally decreases as result of bank protection due to slump blocks; and the effect on bend skewness depends on the location and volumes of failed bank material caused by cantilever and planar failures along the bends, with possible achievement of downstream bend skewness under certain conditions. 1. Introduction Slump blocks produced by mass failure of river banks may modify bank erosion rates by buffering or but- tressing of the bank face and toe [Wood, 2001] and by reducing the shear stress acting on the bank because of a shifting of the locus of high streamwise velocity away from the bank [Kean and Smith, 2006a, 2006b]. Thorne [1982] investigated the role of slump blocks on the bank retreat cycle and introduced the concept of “basal endpoint control” (BEC) comprising three removal scenarios: (i) impeded removal, where bank mass failures supply material to the base of the bank at a higher rate than it is removed; (ii) excess basal capacity, where the rate of fluvial erosion at the bank toe exceeds the supply rate of failed material; and (iii) unim- peded removal, where the supply and removal processes are in balance. The BEC concept reflects the role of slump blocks in shear stress partitioning at river banks and bank buffering, with consequent reduction of lat- eral erosion [Wood, 2001]. The impact of this process on meander migration rates and shapes has not been investigated so far. Feedback between mass failure processes and meander migration is complex, because (i) slump blocks are not uniformly distributed along the river but concentrate where the hydrodynamic con- ditions and the topographic and geotechnical properties of the banks induce mass failure processes; (ii) bank-material stratification affects slump block volumes, which may impact bank protection and therefore migration rates and patterns; and (iii) differing rates of slump block supply through mass failure and their consequent weathering, erosion, and removal make the impact of bank protection by slump blocks time dependent. Different methods have been used to incorporate bank protection through slump blocks in models of bank evolution, channel evolution, and meander migration. Langendoen and Simon [2008] and Motta et al. [2012a, 2012b] increased critical shear stresses for hydraulic erosion to indirectly account for bank protection and MOTTA ET AL. ©2014. American Geophysical Union. All Rights Reserved. 1026 Journal of Geophysical Research: Earth Surface 10.1002/2013JF002952 match observed migration. Darby et al. [2007] and Rinaldi et al. [2008] may also have indirectly accounted for slump block protection by calibrating the erodibility parameter to match calculated and measured eroded volumes. Xu et al. [2011] and Parker et al. [2011] described the phenomenon more explicitly by introducing an armor factor as a function of the bank height [Xu et al., 2011] or the cantilever failure volume caused by fluvial erosion of the lower cohesionless layer [Parker et al., 2011]. The research presented here uses the physically and process-based method for bank erosion developed by Langendoen and Simon [2008] and used in meander migration calculations by Motta et al. [2012a, 2012b]. This method explicitly considers the impact of both horizontal and vertical heterogeneity of floodplain materials on cantilever and planar failure mechanisms. Using this methodology in combination with a slump block dynamics model, we analyze how the horizontal and vertical heterogeneity of floodplain materials affects meander migration and enhances planform complexity. This paper therefore represents the next step in the progression of our research on the factors affecting meander migration patterns: we show that, besides the functional form used for bank erosion [Motta et al., 2012a] and the horizontal structure of the floodplain [Motta et al., 2012b], combined horizontal and vertical structure of the floodplain affects meander migration rates and shapes through explicit inclusion of the mechanism of slump block protection. Slump block impact on meander migration has been much less investigated than riparian vegetation, which can affect bank erosion through several mechanisms and depending on the type of vegetation [Micheli and Kirchner, 2002]. Herbaceous vegetation increases bank strength due to the reinforcement of bank soils by roots [Micheli and Kirchner, 2002]; riparian trees increase the roughness of the channel boundary to flow, reducing turbulence intensity and Reynolds stresses [e.g., Lopez and Garcia, 2001], and produce large woody debris affecting primary and secondary flow [e.g., Daniels and Rhoads, 2004]. Research has shown that meander migration coefficients are reduced by riparian vegetation [Johannesson and Parker, 1985; Odgaard, 1987; Pizzuto and Meckelnburg, 1989; Micheli and Kirchner, 2002; Micheli et al., 2004] and meander shapes are influenced by the spatial distribution of biomass density [Perucca et al., 2007]. In this paper we investigate the effects of slump blocks in both reducing erosion rates and modifying patterns of meander migration. We also discuss how riparian trees can interact with slump block dynamics and how that can be incorporated in our modeling framework. As in Motta et al. [2012b], the studied migration scenarios do not consider meander cutoff and its associated planform complexity. Note that current computational algorithms for meander cutoff are quite simplified and only model neck cutoff [e.g., Sun et al., 1996; Xu et al., 2011]. Any inclusion of such algorithms may therefore distort our results. 2. RVR Meander Model of Meander Migration 2.1. Hydrodynamics and Bed Topography The model for hydrodynamics and bed topography implemented in RVR Meander is analytical and obtained from linearization of the two-dimensional depth-averaged Saint Venant equations of motion. It follows the approach first developed by Ikeda et al. [1981] and adopts the secondary flow correction derived by Johannesson and Parker [1989a], who introduced an “effective centerline curvature”—the secondary current cell strength—which lags behind the local channel curvature and determines the bed transverse slope through a coefficient of proportionality named scour factor. Johannesson and Parker [1989a] and Camporeale et al. [2007] provide details of the analytical solution. Important model assumptions are spatially and temporally constant channel width, bed topography is only a function of channel planform (no free response of sediment), and spatially constant friction coefficient. The assumption of constant channel width during meander migration, while generally being supported by empirical observations [Ikeda et al., 1981] and adopted by many authors
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