Elsevier

Ecological Engineering

Volume 23, Issue 3, 1 November 2004, Pages 189-203
Ecological Engineering

Effects of wetland depth and flow rate on residence time distribution characteristics

https://doi.org/10.1016/j.ecoleng.2004.09.003Get rights and content

Abstract

The residence time distribution (RTD) representing the hydraulics of a wetland is an important tool for modeling and designing treatment wetlands for optimal constituent removal. To correctly use RTD results, it is necessary to understand the conditions under which this distribution remains stable. Dye tracer experiments were conducted on a stormwater treatment wetland to investigate hydrologic factors affecting RTD characteristics. Dye was introduced into the inflow under normal flow conditions and during simulated storm flows, providing a range of flow rates and water levels. Dye distribution in the outlet was measured using an in situ fluorometer. Results indicate that flow rates did not have a significant effect on RTD characteristics. The RTDs normalized for volume and flow demonstrated a greater amount of short-circuiting and a larger mixing scale when water depth increased, demonstrating that water level can have a direct impact on the RTD of a wetland. This effect suggests that more than one RTD may be necessary for analyzing a wetland subject to changing water levels. For the wetland in this study, increasing the water depth elicited a decrease in hydraulic efficiency. Understanding such factors that affect hydraulic efficiency will aid in the design and management of wetlands.

Introduction

The residence time distribution (RTD) is a tool that has advanced the science and engineering of treatment wetlands. A pulse of a non-reactive tracer chemical dissolved into a wetland inlet is used to measure an RTD. In the ideal case, a wetland experiences plug flow, in which water and its dissolved constituents run uniformly and without dispersion from inlet to outlet. If a tracer pulse is added to an ideal wetland inlet, all of the tracer will exit at the same time, the residence time. In real wetlands, flow is nonideal. Nonideal flow comprises different flow path lengths, flow velocities, and mechanisms of diffusion and mixing, causing a distribution of residence times, an RTD (Levenspiel, 1972). When a tracer pulse is introduced into a nonideal wetland, the outflow tracer concentration is an RTD reflecting the dispersive nature of the system. For simplicity, real wetlands are traditionally, though arguably inaccurately (Kadlec, 2000), modeled as ideal, plug-flow systems. RTD analysis has been developed to more accurately model treatment wetland performance, improving on the ideal wetland model, and leading to better understanding and design of wetlands.

Many applications of RTD analysis have been explored in wetland science and engineering, but the consistency and stability of these have not been well studied. To strengthen the efficacy of RTD analysis, its range of utility must be understood more fully. The present research study explores a range of hydrologic conditions that affect RTD stability. By performing tracer studies during hydrologic manipulation of a small wetland, this study explores the sensitivity of the RTD to changing flow rates and water levels. Because RTD analysis is used to compare the dispersive characteristics of wetlands independent of volume and flow rates, understanding the influence of variations of volume and flow rates is needed to improve the application of RTD analysis.

In order to compare RTDs between different wetlands or dissimilar conditions, each RTD must be normalized by removing the units of flow rate, system volume, and tracer mass. A raw RTD can be measured by adding a pulse of tracer to an aquatic system and then measuring the resulting outflow tracer concentration as a function of time. Because the outlet concentration is a function of the tracer mass and wetland volume, these factors can be used to normalize the concentration axis into dimensionless units (Werner and Kadlec, 1996). Increasing the system volume or decreasing the flow rate lengthens the retention time, thus extending the raw RTD in time. Conversely, decreasing the volume or increasing the flow shortens the retention time, compressing the raw RTD. The horizontal orientation of the RTD curve is therefore a function of volume and flow. For this reason, it is common to use volume and flow to normalize the time axis into dimensionless units (Fig. 1) (Levenspiel, 1972, AWWARF, 1996). This normalization procedure, essential for comparing RTDs of systems under different hydrologic conditions, is complicated by pulsed flow, where there may not be one single system volume or flow rate. Werner and Kadlec (1996) developed a dynamic normalization procedure for RTD analysis of pulsed systems. While hydrologic changes stretch or compress the raw RTD, the normalization procedure removes these effects, isolating the dispersive and mixing characteristics of the system (Fig. 1). Dispersive and mixing characteristics are important to resolve in a treatment wetland because they affect the system's treatment performance.

Wetland engineers use RTD analysis to quantify wetland design characteristics that affect treatment efficiency. In order to characterize the performance of a wetland, it is useful to reduce the RTD to a single number, the hydraulic efficiency. The hydraulic efficiency represents the ability of a wetland to distribute its flow uniformly throughout its volume, maximizing contact time of pollutants in the system and optimizing the ability to break down these pollutants. Thackston et al. (1987) introduced the concept of hydraulic efficiency in wetlands by quantifying the relative position of the RTD's centroid. The RTD centroid is a measure of the average residence time of the RTD, representing the true retention time of the system. Ideally, the true retention time of the system is equivalent to the theoretical retention time of the system, determined by the system volume and flow rate. The centroid of an RTD falls below the theoretical retention time when short-circuiting causes the loss of effective retention volume (Fig. 2). The scale of mixing in a wetland also affects its hydraulic efficiency. Complex systems, such as wetlands, are often described in equivalent number of continually stirred tank reactors, or CSTRs (Levenspiel, 1972, Kadlec and Knight, 1996). A wetland can be compared to an equivalent volume of many small CSTRs or a few large CSTRs. Many small CSTRs in series represent a system with a small mixing scale, which has a small RTD spread and resembles plug flow (Fig. 2). As the number of equivalent CSTRs decreases, the mixing scale and the spread of the RTD increase (Fig. 2). A large RTD spread is considered inefficient for a chemical reaction system (Levenspiel, 1972). A metric of hydraulic efficiency may quantify the short-circuiting, the mixing scale, or a combination of both (Persson et al., 1999). The measurement of hydraulic efficiency is a simple and effective method of characterizing wetlands, identifying factors that affect treatment wetland performance.

Wetland shape affects the RTD and the hydraulic efficiency. Zones of diminished mixing (ZDM), where the area of a wetland is not being optimally utilized for pollutant treatment (Thackston et al., 1987, Kadlec, 1994), lower the hydraulic efficiency by reducing the effective volume (Fig. 2). Wetland shape and aspect ratio are important design considerations because they affect the location and abundance of ZDM (Walker, 1998, Persson et al., 1999). RTD analysis has been used to design wetland shape to reduce ZDM, optimizing treatment efficiency (Koskiaho, 2003).

Internal structure can affect flow dynamics and therefore hydraulic efficiency of a wetland. Aquatic macrophytes, for example, enhance lateral (Nepf et al., 1997, Nepf, 1999) and vertical (Nepf and Koch, 1999) diffusion and flow in a wetland, potentially enhancing the wetland's hydraulic efficiency. If patchy vegetation creates stagnant ZDM, however, the hydraulic efficiency may decrease (Thackston et al., 1987). The influence of vegetation on flow patterns indicates that seasons or ecological succession may influence the characteristics of wetland flow. Basin morphology also affects dispersion and flow paths through a wetland, strongly influencing the wetland's RTD (Koskiaho, 2003). The inlet and outlet locations and their structure also affect hydraulic efficiency. RTD analysis has been used to optimize inlet (Shilton and Prasad, 1996) and outlet structures (Konyha et al., 1995) or both (Ta and Brignal, 1998) for maximum treatment efficiency.

Although hydrologic conditions, such as flow rate and wetland depth, may affect hydraulic efficiency, few studies have quantified these effects. Natural flooding events in riparian wetlands have caused observable changes in RTD characteristics (Stern et al., 2001). The water level in a treatment wetland, either caused by flooding or controlled by wetland design and management, should similarly affect hydraulic efficiency. Basin morphology and vegetation, for example, would likely have different effects relative to different water depths. Flow rate should also affect RTD characteristics and hydraulic efficiency. Turbulent diffusion, enhanced by vegetation and other obstructions, is related to the Reynolds number and thus flow velocity (Nepf et al., 1997). Turbulent diffusion may therefore dominate the diffusive and mixing mechanisms at higher flow rates. Different mechanisms of flow dispersion would govern dynamics of low flow. Convective circulation (Oldham and Sturman, 2001) and diffusion driven by wind or molecular movement (Kadlec and Knight, 1996, Keller and Bays, 2000) would likely dominate during long retention times. Because flow rate and depth affect wetland hydraulics, studies are needed to quantify their effects on RTD characteristics.

Understanding depth and flow factors is important for determining conditions under which an RTD or its derived hydraulic efficiency metrics can be applied. Typically, a single RTD is used to characterize a wetland, a procedure that assumes that each wetland embodies a single, intrinsically stable RTD. This is a useful simplification for analyzing the efficiency of a treatment wetland; however, it remains to be demonstrated under which circumstances the intrinsic RTD assumption is valid. A similar uncertainty arises when a tracer study is performed under pulsed conditions. Producing a normalized RTD from pulsed conditions is a practical means of assessing a wetland subject to storm flow (Werner and Kadlec, 1996). Studies have not yet demonstrated to what extent such an RTD reflects the wetlands local and transient conditions versus how much it represents the intrinsic state of the wetland. In conventional waste treatment facilities, it is common to run tracer tests at various flow rates and water levels to assess the system performance under varying conditions (AWWARF, 1996). A similar protocol would be beneficial for a treatment wetland if studies demonstrate a flow or depth effect on wetland RTD characteristics.

The purpose of this study was to investigate the sensitivity of the normalized RTD to different applied depths and flow rates. To accomplish this goal, a series of dye tracer experiments was run on a small constructed wetland under controlled flow rates and water levels. Water level and flow rate were regulated with adjustable-depth control structures and flow-adjustable irrigation water.

The results of this study were intended to elucidate two areas of wetland science and engineering. First, the results will help determine under which hydrologic conditions an RTD remains valid for a wetland. Nonideal flow models and hydraulic characterizations will become more robust when the limitations to their underlying RTDs are understood. This will lead to better modeling and design of wetlands. Second, because depth and flow rate can be controlled in engineered wetlands (Kadlec and Knight, 1996, Mitsch and Gosselink, 2000), understanding how hydraulic efficiency is affected by these parameters will influence how treatment wetlands are designed and managed.

Section snippets

Site description

This study was performed in the spring and summer of 2003 on a 250-m2 wetland constructed on the Waterman Agricultural and Natural Resource Laboratory at the Ohio State University in Columbus, Ohio (Fig. 3). The wetland had three adjacent inlet structures that discharged water from storm flow, overland flow, and nursery pad irrigation runoff, and one controlled outlet structure that discharged to another wetland cell (discharge structures from Agri Drain Corp.). To simulate storm flow, water

Results

The controlled depth varied by factor of 2.4 between treatments and the average low and high flow rates varied by a similar factor of 2.7 (Table 1). Dye recovery for all experiments ranged from 75 to 95% with an average of 84%. There were no consistent differences between the RTD plots at different flow rates. None of the parameters for hydraulic efficiency differed significantly between high and low flow rates (Fig. 5): λt varied from 0.51 at low flow to 0.53 at high flow (P = 0.53), λp changed

RTD sensitivity to hydrologic influences

The results of this study indicate that the RTD of a wetland is sensitive to changes in water depth, but not significantly sensitive to changes in flow rate. If this result is characteristic of wetlands in general, then it indicates that a change in wetland depth by a factor of 2.4 is enough to elicit a significant change in RTD characteristics. If wetland management or hydrologic factors cause depth changes of a similar magnitude, more than one RTD will be needed for an appropriate hydraulic

Conclusion

This study demonstrates that changing water levels can significantly affect the flow characteristics through a wetland. The magnitude of depth change in this study, a factor of 2.4, sets a baseline for identifying hydrologic changes that are likely to affect the RTD. This study also indicates that minor fluctuations in flow rate, at least within the factor of 2.7 tested in this study, may not significantly affect the RTD. This supports analysis of normalized RTDs across various flow rates, such

Acknowledgements

We would like to thank Noel Cressie for his assistance and advice. We are also appreciative of the design and construction work done by Dan Gill, Tim Salzman, and Alex Daughtery. For the technical assistance of Chris Gecik, Kevin Duemmel, and Carl Cooper, we are greatly indebted. Many thanks also go to Mark Schmittgen for his assistance on the farm, to Chris Keller for his advice on using dye tracers, and to Mark Benjamin for supplying a draft of his publication in press. This study would not

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