Abstract
Abstract. Surface transient storage (STS) and hyporheic transient storage (HTS) have functional significance in stream ecology and hydrology. Currently, tracer techniques couple STS and HTS effects on stream nutrient cycling; however, STS resides in localized areas of the surface stream and HTS resides in the hyporheic zone. These contrasting environments result in different storage and exchange mechanisms with the surface stream, which can yield contrasting results when comparing transient storage effects among morphologically diverse streams. We propose a fluid mechanics approach to quantitatively separate STS from HTS that involves classifying and studying different types of STS. As a starting point, a classification scheme is needed. This paper introduces a classification scheme that categorizes different STS in riverine systems based on their flow structure. Eight STS types are identified and some are subcategorized based on characteristic mean flow structure: (1) lateral cavities (emergent and submerged); (2) protruding in-channel flow obstructions (backward- and forward-facing step); (3) isolated in-channel flow obstructions (emergent and submerged); (4) cascades and riffles; (5) aquatic vegetation (emergent and submerged); (6) pools (vertically submerged cavity, closed cavity, and recirculating reservoir); (7) meander bends; and (8) confluence of streams. The long-term goal is to use the classification scheme presented to develop predictive mean residence times for different STS using field-measurable hydromorphic parameters and obtain an effective STS mean residence time. The effective STS mean residence time can then be deconvolved from the transient storage residence time distribution (measured from a tracer test) to obtain an estimate of HTS mean residence time.
Highlights
Transient storage is the short-term storage of fluid due to the exchange of solutes and suspended particulates in the main flow with (1) recirculating in-stream flow structures, referred to as surface transient storage (STS); and/or (2) the hyporheic zone, referred to as hyporheic transient storage (HTS) (Bencala and Walters, 1983; Boulton et al, 1998; Briggs et al, 2009)
The purpose of this paper is to introduce a classification scheme that categorizes different types of Surface transient storage (STS) in riverine systems based on their flow structure
To implement the STS classification scheme and quantitatively separate STS from HTS in a tracer test, we propose a method that deconvolves the STS residence time distribution (RTD) from the total transient storage RTD to obtain the HTS RTD
Summary
Transient storage is the short-term storage of fluid due to the exchange of solutes and suspended particulates in the main flow with (1) recirculating in-stream flow structures, referred to as surface transient storage (STS); and/or (2) the hyporheic zone, referred to as hyporheic transient storage (HTS) (Bencala and Walters, 1983; Boulton et al, 1998; Briggs et al, 2009). Reach-averaging a stream’s total transient storage parameters couples the effects of STS and HTS, and does not provide information on the relative influence of surface and hyporheic exchange on solute entrainment and retention (Choi et al, 2000; Briggs et al, 2009). Briggs et al (2009) and Harvey et al (2005) utilized the two-zone transient storage model – developed by Choi et al (2000) – to differentiate STS from HTS by measuring tracer breakthrough in the STS and utilizing a transient storage model, partially parameterized with STS to determine HTS by inverse modeling Drawbacks to this approach include (1) additional data collection (e.g., velocity and concentration time series), and (2) the parameterization of two additional parameters, transient storage area and the mass exchange coefficient. The classification scheme described is a compilation of previous studies and is meant as a basis for future work and research directions to accurately quantify the effects of STS on stream solute transport
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