water Article The Straightening of a River Meander Leads to Extensive Losses in Flow Complexity and Ecosystem Services Tian Zhou 1 and Theodore Endreny 2,* 1 Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA; [email protected] 2 Department of Environmental Resources Engineering, College of Environmental Science and Forestry, State University of New York, Syracuse, NY 13210, USA * Correspondence: [email protected] Received: 13 April 2020; Accepted: 8 June 2020; Published: 11 June 2020 Abstract: To assist river restoration efforts we need to slow the rate of river degradation. This study provides a detailed explanation of the hydraulic complexity loss when a meandering river is straightened in order to motivate the protection of river channel curvature. We used computational fluid dynamics (CFD) modeling to document the difference in flow dynamics in nine simulations with channel curvature (C) degrading from a well-established tight meander bend (C = 0.77) to a straight channel without curvature (C = 0). To control for covariates and slow the rate of loss to hydraulic complexity, each of the nine-channel realizations had equivalent bedform topography. The analyzed hydraulic variables included the flow surface elevation, streamwise and transverse unit discharge, flow velocity at streamwise, transverse, and vertical directions, bed shear stress, stream function, and the vertical hyporheic flux rates at the channel bed. The loss of hydraulic complexity occurred gradually when initially straightening the channel from C = 0.77 to C = 0.33 (i.e., the radius of the channel is three-times the channel width), and additional straightening incurred rapid losses to hydraulic complexity. Other studies have shown hydraulic complexity provides important riverine habitat and is positively correlated with biodiversity. This study demonstrates how hydraulic complexity can be gradually and then rapidly lost when unwinding a river, and hopefully will serve as a cautionary tale. Keywords: river engineering; meander bend; CFD simulation; hydraulic complexity 1. Introduction River meanders are a common landform in alluvial systems, characterized by sinuous patterns of planform curvature of channel banks and bedform topography within those banks. Classic bedform features include point bars on the inner bank and pools along the outer bank centered at the meander bend apex, and riffles upstream and downstream of the bend [1–3]. The meander planform curvature generates a centrifugal force on river water and creates non-uniform depths and velocities, which shapes the bedform topography in alluvial rivers. As one travels streamwise downriver, the rising and falling of bedform topography generates drag and steering on river flows, creating a complex feedbacks between topography and flow [4]. The hydraulic features of meanders include streamwise velocity separating from the inner bank about the apex and attaching to the outer bank, a superelevated water surface forming along the outer bank, eddies forming downstream of the point bar, and helical corkscrew patterns of cross-channel flow. This hydraulic complexity is known to control the pressure distributions along the riverbed and therefore induce patterns of surface water Water 2020, 12, 1680; doi:10.3390/w12061680 www.mdpi.com/journal/water Water 2020, 12, 1680 2 of 18 downwelling into the streambed balanced by groundwater upwelling into the channel, a mixing process important for species and water quality called hyporheic exchange [5]. In a natural alluvial system, the meander bend planform will evolve through a repeating cycle of straight to curved channels with coordinated bedform adjustments. Initially, straight channels the viscosity of water and roughness of channel banks leads to depositional bars opposite erosional pools, which then experience meander elongation and a decreasing radius of curvature. Ultimately, the evolution culminates in a meander neck cutoff, at which point an oxbow lake may occupy the abandoned meander bend and the upstream and downstream sections of channel are joined by a straight channel, with a radius of curvature reset to infinity [6,7]. This repeating cycle of meander evolution is at different stages, initiation to completion, along the streamwise path of a river. Flows within meander bends generate hydraulic complexity [8–10] . The hydraulic complexity at a meander bend further influences river ecosystems. Gualtieri et al. [11,12] demonstrated how hydraulic complexity metrics succinctly represent flow patterns and structures utilized by riverine biota, and greater complexity is correlated with more habitat types and greater biologic diversity. Common hydraulic complexity metrics include river velocity and depth gradients, which are known to support species life cycles, such as spawning, feeding, and resting [11]. The importance of hydraulic complexity to riverine species richness [13] is in keeping with other ecological systems, where a positive correlation exists between biodiversity and habitat heterogeneity [14]. Riverine meander evolution has too often been cut short by river straightening, and as communities realized the associated loss of ecosystem services they are eager to restore these meanders as well as reduce their future loss [15]. River meanders have been removed to repurpose the floodplain for development or provide local flood water relief (while exacerbating flood levels downstream). However, human modification of rivers too often leads to unintended consequences on river flows, sediment transport, riverbank stability, and aquatic suitability, making river meander loss a significant public and private concern. A successful river management target is a self-sustaining system with dynamic equilibrium for hydrological, geomorphological, and ecological processes [16]. In an analysis of how to meander bend curvature affects hydraulics and hence ecosystems, it is best to keep constant variables of flow stage, i.e., depth, and bedform topography within the channel banks. Bankfull stage, when water is at the threshold of leaving the channel, is a standard depth for such analysis. The goal of this research is to quantify the extent that hydraulic complexity degrades when a highly evolved meander bend at the bankfull stage is straightened, while maintaining equivalent bedform topography associated with a highly sinuous meander. The research is approached from the perspective of valuing river flow hydrodynamics and surface water–groundwater exchange patterns associated with high sinuosity meanders, and views the straightening of the channel as a departure from, and the restoration of the sinuosity as a return to, a target value. In order to examine the independent adjustment of planform curvature and bedform topography, we used computational fluid dynamics (CFD) models to simulate flow dynamics in the channel water column and at or in the channel bed, including hyporheic fluxes across the channel bed [17–21]. The desired outcome of this study is the knowledge to help river managers and engineers better understand the hydraulic complexity associated with their river meanders and to guide their channel designs which increasingly rely upon river steering structures to create curvature or bedform. 2. Methods To systematically change planform curvature while keeping equivalent bedform topography, we used a commercial CFD model (Flow3D R ) as a virtual river system that coupled the surface water flow and groundwater movement in one domain. We describe the range of the channel curvature with variable C, defined as the ratio of the channel width, B to the centerline radius of channel curvature, R. A straight channel has a C = 0, while a high sinuosity meander bend might have a C > 0.5. Nine experiments with gradually changed curvature from a tight bend to a straight channel were simulated by maintaining equivalent cross-section profiles. The output of these simulations were then Water 2020, 12, 1680 3 of 18 used to investigate the hydraulic complexity response to the channel curvature changes. Note that the purpose of this study is to investigate the flow responses to meander curvature changes solely, not to bedform topography. There was no sediment transport and associated self-adjusted river bedform evolution included in the experiment setup. CFD Model The CFD model used finite-element methods to solve the velocity and pressure variables using the fully 3D transient conservation of mass and momentum equations. Flow depth was controlled by the pressure boundary conditions at both upstream and downstream boundaries. Flow surface was treated as a free surface boundary by applying the volume-of-fluid (VOF) technique [22] which computed fluid volume fraction as one variable in each grid cell. The mass continuity was represented as V ¶p ¶uA ¶vAy ¶wA uA F + x + R + z + x x = 0. (1) rc2 ¶t ¶x ¶y ¶z x The momentum conservation was represented as 2 ¶u 1 ¶u ¶u ¶u Ayv −1 ¶p + (uAx + vAyR + wAz ) − x = + fx (2) ¶t VF ¶x ¶y ¶z xVF r ¶x ¶v 1 ¶v ¶v ¶v Ayuv −1 ¶p + (uAx + vAyR + wAz ) + x = (R ) + fy (3) ¶t VF ¶x ¶y ¶z xVF r ¶x ¶w 1 ¶w ¶w ¶w −1 ¶p + (uAx + vAyR + wAz ) = + fz. (4) ¶t VF ¶x ¶y ¶z r ¶x In these equations, VF is the fraction volume open to flow, r is the fluid density, c is a constant (approximately the sound speed), p is pressure, t is time, Ai is the area fractions for flow in the i(x, y, z) direction, and ui(u, v, w) denote the velocities in i directions, fi is the viscous acceleration terms. R and x are transformation coefficients depend on the choice of the coordinate system is in use. When Cartesian coordinates (x, y, z) are used, x = 0 and R = 1. When cylindrical coordinates (r, q, z) are used, y derivatives in the equation will be converted to azimuthal derivatives: ¶ 1 ¶ ! , (5) ¶y r ¶q which is accomplished by using the equivalent form: 1 ¶ ¶ = R , (6) r ¶q ¶y where R = rm/r, y = rmq, and rm is a fixed channel radius. In the meantime, x = 1. Readers are referred to the FLOW-3D users manual (https://www.flow3d.com) for more information.