Retention time and dispersion associated with submerged aquatic canopies

[1] The shear layer at the top of a submerged canopy generates coherent vortices that control exchange between the canopy and the overflowing water. Unlike free shear layers, the vortices in a canopy shear layer do not grow continuously downstream but reach and maintain a finite scale determined by a balance between shear production and canopy dissipation. This balance defines the length scale of vortex penetration into the canopy, δe, and the region of rapid exchange between the canopy and overflow. Deeper within the canopy, transport is constrained by smaller turbulence scales. A two-box canopy model is proposed on the basis of the length scale δe. Using diffusivity and exchange rates defined in previous studies, the model predicts the timescale required to flush the canopy through vertical exchange over a range of canopy density and height. The predicted canopy retention times, which range from minutes to an hour, are consistent with canopy retention inferred from tracer observations in the field and comparable to retention times for some hyporheic regions. The timescale for vertical exchange, along with the in-canopy velocity, determines the minimum canopy length for which vertical exchange dominates water renewal. Shorter canopies renew interior water through longitudinal advection. Finally, canopy water retention influences longitudinal dispersion through a transient storage process. When vertical exchange controls canopy retention, the transient storage dispersion increases with canopy height. When longitudinal advection controls water renewal, dispersion increases with canopy patch length.

[1]  A. Roshko,et al.  On density effects and large structure in turbulent mixing layers , 1974, Journal of Fluid Mechanics.

[2]  T. Tsujimoto Fluvial processes in streams with vegetation , 1999 .

[3]  G. Katul,et al.  A Note On The Contribution Of Dispersive Fluxes To Momentum Transfer Within Canopies , 2004 .

[4]  E. F. Bradley,et al.  Turbulent flow in a model plant canopy , 1976 .

[5]  H. Nepf,et al.  Mixing layers and coherent structures in vegetated aquatic flows , 2002 .

[6]  H. Nepf Drag, turbulence, and diffusion in flow through emergent vegetation , 1999 .

[7]  James E. Saiers,et al.  Solute transport and storage mechanisms in wetlands of the Everglades, south Florida , 2005 .

[8]  J. Finnigan,et al.  A wind tunnel study of air flow in waving wheat: Two-point velocity statistics , 1994 .

[9]  N. Kouwen,et al.  Modern approach to design of grassed channels , 1992 .

[10]  B. Amiro,et al.  Drag coefficients and turbulence spectra within three boreal forest canopies , 1990 .

[11]  Fu‐Chun Wu,et al.  Variation of Roughness Coefficients for Unsubmerged and Submerged Vegetation , 1999 .

[12]  Enrique R. Vivoni,et al.  Flow structure in depth-limited, vegetated flow , 2000 .

[13]  Marco Ghisalberti,et al.  The limited growth of vegetated shear layers , 2004 .

[14]  G. W. Thurtell,et al.  Some observations of turbulence and turbulent transport within and above plant canopies , 1974 .

[15]  D. Joseph,et al.  Boundary conditions at a naturally permeable wall , 1967, Journal of Fluid Mechanics.

[16]  F. Browand,et al.  Vortex pairing : the mechanism of turbulent mixing-layer growth at moderate Reynolds number , 1974, Journal of Fluid Mechanics.

[17]  W. Dade,et al.  Grain‐Size, Sediment‐Transport Regime, and Channel Slope in Alluvial Rivers , 1998, The Journal of Geology.

[18]  J. Fisher,et al.  The role of current velocity in structuring eelgrass (Zostera marina L.) meadows , 1983 .

[19]  P. Champion,et al.  The influence of aquatic macrophytes on the hydraulic and physico-chemical properties of a New Zealand lowland stream , 1999, Hydrobiologia.

[20]  Ronald Smith A delay-diffusion description for contaminant dispersion , 1981, Journal of Fluid Mechanics.

[21]  J. Harvey,et al.  Predicting changes in hydrologic retention in an evolving semi-arid alluvial stream , 2003 .

[22]  M. Raupach,et al.  Averaging procedures for flow within vegetation canopies , 1982 .

[23]  M. J. Dwyer,et al.  Turbulent kinetic energy budgets from a large-eddy simulation of airflow above and within a forest canopy , 1997 .

[24]  Ian R. Wood,et al.  Longitudinal Dispersion with Dead Zones , 1977 .

[25]  K. Sand‐Jensen,et al.  Influence of submerged macrophytes on sediment composition and near-bed flow in lowland streams , 1998 .

[26]  J. Finnigan,et al.  A wind tunnel study of air flow in waving wheat: Single-point velocity statistics , 1994 .

[27]  John D. Wilson,et al.  A second-order closure model for flow through vegetation , 1988 .

[28]  J. Taylor An Introduction to Error Analysis , 1982 .

[29]  Marco Ghisalberti,et al.  Mass Transport in Vegetated Shear Flows , 2005 .

[30]  H. Nepf,et al.  Prediction of velocity profiles and longitudinal dispersion in emergent salt marsh vegetation , 2005 .

[31]  Brian J. Wagner,et al.  1 – Quantifying Hydrologic Interactions between Streams and Their Subsurface Hyporheic Zones , 2000 .

[32]  Jeremy B. Jones,et al.  Retention and Transport of Nutrients in a Third-Order Stream in Northwestern California : Hyporheic Processes , 2007 .

[33]  Marcelo H. García,et al.  Mean Flow and Turbulence in a Laboratory Channel with Simulated Vegatation (HES 51) , 1996 .

[34]  J. Finnigan,et al.  Coherent eddies and turbulence in vegetation canopies: The mixing-layer analogy , 1996 .

[35]  K. Niklas THE SCALING OF PLANT AND ANIMAL BODY MASS, LENGTH, AND DIAMETER , 1994, Evolution; international journal of organic evolution.

[36]  Alexander N. Sukhodolov,et al.  Evolution of mixing layers in turbulent flow over submersed vegetation: Field experiments and measurement study , 2006 .

[37]  G. Katul,et al.  Momentum Transfer and Turbulent Kinetic Energy Budgets within a Dense Model Canopy , 2004 .

[38]  George M. Hornberger,et al.  A mixing layer theory for flow resistance in shallow streams , 2002 .

[39]  S. Chikwendu,et al.  Slow-zone model for longitudinal dispersion in two-dimensional shear flows , 1985, Journal of Fluid Mechanics.

[40]  Nicholas Kouwen,et al.  FLEXIBLE ROUGHNESS IN OPEN CHANNELS , 1973 .

[41]  M. Schulz,et al.  The influence of macrophytes on sedimentation and nutrient retention in the lower River Spree (Germany). , 2003, Water research.

[42]  M. Raupach,et al.  Experiments on scalar dispersion within a model plant canopy part I: The turbulence structure , 1986 .

[43]  The role of the submergent macrophyte Triglochin huegelii in domestic greywater treatment , 1999 .

[44]  M. Raupach Drag and drag partition on rough surfaces , 1992 .

[45]  G. Katul,et al.  Principal Length Scales in Second-Order Closure Models for Canopy Turbulence , 1999 .

[46]  F. Triska,et al.  Retention and Transport of Nutrients in a Third‐Order Stream: Channel Processes , 1989 .

[47]  Anne F. Lightbody,et al.  Prediction of velocity profiles and longitudinal dispersion in salt marsh vegetation , 2006 .

[48]  E. Murphy Longitudinal dispersion in vegetated flow , 2006 .

[49]  E. Prepas,et al.  Nutrient dynamics in riverbeds: The impact of sewage effluent and aquatic macrophytes , 1994 .

[50]  T. Day,et al.  Longitudinal dispersion in natural channels , 1975 .

[51]  Luca Ridolfi,et al.  The Effect of Vegetation Density on Canopy Sub-Layer Turbulence , 2004 .