Residence time of bedform-driven hyporheic exchange

Salmon that spawn in streams deliver marine-derived nutrients (MDN) that catalyze trophic productivity and support rearing juvenile salmon. Salmon spawning also affects hyporheic exchange and movement of dissolved MDN through the stream bed by creating redd topography that induces pumping exchange and by winnowing fine sediment and loosening the bed, which alters hydraulic conductivity and bed porosity. The spatial extent of spawning within the channel likely governs the volume and rate of dissolved MDN exchanged with the stream bed through this process. To explore this issue, we used a two-dimensional groundwater model to predict changes in hyporheic volume, flux, and mean hydraulic residence time of dissolved MDN as a function of the proportion of the bed surface occupied by redds (P). Predictions indicate that hyporheic volume and flux systematically increase with P, while the mean hydraulic residence time of dissolved MDN in the hyporheic zone decreases sharply with P, from 5.79 h on an unspawned bed (P = 0) to 0.03 h for a mass-spawned bed (P = 1.0). Shorter residence time results from hyporheic flux increasing faster than hyporheic volume with higher P. Implications for uptake of dissolved MDN are explored with Damköhler numbers, defined as the ratio of the mean hydraulic residence time to a biogeochemical rate of interest. Given the considerable influence of spawning on hyporheic exchange, additional research is needed to determine conditions under which bioassimilation of dissolved MDN is limited by nutrient supply, extent of the hyporheic zone, or processing rate of MDN in stream beds.

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Quantifying Vertical Hyporheic Exchange and hyporheic residence time in thalweg paths of meandering streams characterized by multiple riffle-pool sequences morphology

10.5194/hess-2019-446 ◽

2019 ◽

Author(s):

Aminreza Meghdadi ◽

Morteza Eyvazi ◽

Zohre Najatijahromi ◽

Bahram Saghafian

Keyword(s):

Residence Time ◽

Hyporheic Exchange ◽

Subsurface Water ◽

Upward Movement ◽

Meandering Streams ◽

Mean Values ◽

Sediment Bed ◽

Regional Flow ◽

Vertical Hydraulic Conductivity ◽

Bed Materials

Abstract. Riffle-pool sequences in the thalweg paths of meandering streams are of pivotal importance to the hyporheic exchange pattern in a fluvial network, but the complex hydrodynamic, morphological, and sedimentary features of riverbed sediments increase the difficulties associated with vertical hyporheic exchange (VHE) quantification. This study applied depth-dependent radon (222Rn) and diel temperature variations to quantify VHE and residence time (tr). The study was conducted in four different hyporheic areas with riffle-pool sequences in the third-order Ghezel-Ozan River, located in north-west Iran. The mean values of temperature-derived VHE (VHET) and radon-derived VHE (VHERn) were 0.67±0.32 m/day and 0.63±0.36 m/day, respectively. Due to effects of sediment bed heterogeneity on temperature variation and 222Rn activity at downwelling and upwelling points, there were discrepancies between radon-derived (trRn) and temperature-derived residence time (trT), with mean values of 2.11±1.17 days and 1.87±1.26 days, respectively. The value of trT was well within uncertainty boundaries at a 95 percent confidence interval (p<0.05) and was lower than trRn at the downwelling points. The analysis of vertical diel temperature, radon and electrical conductivity variations revealed subsurface water exchange to be greatly affected by larger scale regional flow. The comparison between VHET and VHERn with VHE obtained from PHAST model simulation (VHEPHAST) revealed a higher correlation between VHET and VHEPHAST (R2=0.96) than with VHERn (R2=0.76). Furthermore, vertical hydraulic conductivity (Kv) of the sediment-bed materials, calculated in situ by the permeameter test, indicated not only that Kv was up to 21 % higher in areas dominated by upward movement than at downwelling points, but also principle component analysis (PCA) demonstrated the dependence of Kv on porosity, VHE, and %sand of the stream-bed materials. This study provides evidence that vertical flux in the hyporheic zone is mainly affected by stream sinuosity and regional subsurface flow, and that the temperature method is more suitable than radon activity to quantify hyporheic exchange patterns.

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Analytical representations of the Residence Time Distribution (RTD) associated with Hyporheic Exchange beneath Dune-like bedforms at different sediment bed depths

10.22541/au.162443405.53862463/v1 ◽

2021 ◽

Author(s):

Ahmed Monofy ◽

Fulvio Boano ◽

Stanley Grant

Keyword(s):

Residence Time ◽

Time Distribution ◽

Residence Time Distribution ◽

Hyporheic Zone ◽

Hyporheic Exchange ◽

Mixing Processes ◽

Sediment Bed ◽

Distribution Parameters ◽

Log Normal ◽

High Degree

The hyporheic exchange below dune-shaped bedforms has a great impact on the stream environment. One of the most important properties of the hyporheic zone is the residence time distribution (RTD) of flow paths in the sediment domain. Here we evaluate the influence of an impervious layer, at a dimensionless sediment depth of d_b^*=(2πd_b)⁄λ where λ is the dune wavelength, on the form of the hyporheic exchange RTD. Empirical RTDs were generated, over a range of d_b^(* ) values, from numerical particle tracking experiments in which 10000 particles sinusoidally distributed over a flatbed domain were released. These empirical RTDs are best represented by the Gamma, Log-Normal and Fréchet distributions over normalized bed depth of 〖0 <=d〗_b^(* )≤1.2, 〖1.23.1, respectively. The depth dependence of the analytical distribution parameters is also presented, together with a set of regression formulae to predict these parameters based on d_b^(* )with a high degree of accuracy (R^2>99.8%). These results contribute to our understanding of the physical and mixing processes underpinning hyporheic exchange in streams and allow for a quick evaluation of its likely impact on nutrient and contaminant processing (e.g., based on the magnitude of the Damköhler number).

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Analytical representations of the Residence Time Distribution (RTD) associated with Hyporheic Exchange beneath Dune-like bedforms at different sediment bed depths

10.22541/au.162443405.53862463/v2 ◽

2021 ◽

Author(s):

Ahmed Monofy ◽

Fulvio Boano ◽

Stanley Grant

Keyword(s):

Residence Time ◽

Time Distribution ◽

Residence Time Distribution ◽

Hyporheic Zone ◽

Hyporheic Exchange ◽

Residence Times ◽

Damkohler Number ◽

Sediment Depth ◽

Depth Effect ◽

Damköhler Number

The hyporheic exchange below dune-shaped bedforms has a great impact on the stream environment. One of the most important properties of the hyporheic zone is the residence time distribution (RTD) of flow paths in the sediment domain. Here we evaluate the influence of an impervious layer, at a dimensionless sediment depth of db*=2πdbλ where λ is the dune wavelength, on the form of the hyporheic exchange RTD. Empirical RTDs were generated, over a range of db*values, from numerical particle tracking experiments in which 10000 particles sinusoidally distributed over a flatbed domain were released. These empirical RTDs are best represented by the Gamma, Log-Normal and Fréchet distributions over normalized bed depth of 0<=db*≤1.2,1.2<db*≤3.1, and db*>3.1, respectively. The depth dependence of the analytical distribution parameters is also presented, together with a set of regression formulae to predict these parameters based on db*with a high degree of accuracy (R2>99.8%). These results contribute to our understanding of the physical and mixing processes underpinning hyporheic exchange in streams and allow for a quick evaluation of its likely impact on nutrient and contaminant processing (e.g., based on the magnitude of the Damköhler number). Keywords: Dunes, bedforms, residence times distribution, sediment depth effect, Hyporheic residence times, analytical representation, two parametric distributions, Damköhler Number.

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