«SECONDARY CIRCULATION IN A SINUOUS COASTAL PLAIN ESTUARY A Dissertation Presented to The Academic Faculty By Susan Anne Elston In Partial Fulfillment ...»
Further, a better understanding estuarine dynamics could significantly aid in the placement and maintenance of freshwater drinking facilities (Dunne and Leopold 1978;
Prandle 1985) as well as the potential to reduce crop loss in marginally productive areas, e.g., rice paddies and crawfish farms (Clarke and Gibson 1987; Uehara 1984; Walter Stevens, personal communication). This is especially important for the United States, a nation whose major urban centers (for example: New York, Washington, D.C., Miami, Seattle, San Francisco, and San Diego) are expanding coastward, and whose major source of drinking water is from rivers and groundwater in the estuarine environment (World Resources Institute 1994).
The distribution of salt and momentum in an estuary vary in both time and space due to river discharge, changes in tidal amplitude and phase, wind stress, lateral mixing processes, and frictional processes at both the bottom and lateral boundaries (Savenije 1993). Classical estuarine studies (Pritchard 1952; Pritchard 1956; Bowden 1963;
Hansen and Rattray 1965) have often focused on the along-channel aspects of the salt and momentum balance. This is largely because the dominant scale of motion in an estuary is directed longitudinally, and older technology limited measurements to this direction. By assuming that estuarine transport occurs principally in the longitudinal direction, complex mathematical formulations become more tractable, yielding reasonable analytical solutions that explain a variety of observed phenomena. Although this classical picture explains much about the estuarine circulation, it obscures the additional contributions to the local and estuarine-wide salt and momentum balances made by lateral transport processes, of which secondary circulation is one of the primary mechanisms.
To improve our understanding of the salt and momentum balances in an estuary, detailed three-dimensional observations and enhanced conceptual models that include lateral variability are needed. The evolution of instrumentation necessary to obtain detailed field measurements along with the recent advances in numerical modeling techniques, access to high-speed computing facilities, and the development of new acoustic profiling instruments have brought renewed interest in understanding the effect of secondary circulation on estuarine dynamics. This dissertation will address how recently acquired three-dimensional observations, containing detailed measurements of lateral variability in both time and space, provide a unique opportunity to improve our understanding of how secondary circulation operates and its relationship to changes in the lateral momentum balance.
1.2 Lateral Mixing Lateral mixing and exchange are little understood and poorly defined processes in estuarine dynamics (Geyer and Signell 1992; Garrett 2002). Lateral mixing commonly refers to poorly resolved, rarely quantified non-linear interactions between river discharge, wind stress, and secondary processes such as transverse or secondary circulation. The redistribution of salt and momentum by lateral mixing is often mentioned in the literature as a significant contributing factor in altering the local salt flux and changes in larvae, momentum, and sediment distributions in an estuary. Scaling arguments are commonly used (Gill 1982; Dutton 1987; Pond and Pickard 1989; Holton
1992) as a way to neglect these smaller scale lateral processes. Consequently, advances on this subject, particularly in partially mixed estuaries, have been limited (Trowbridge et al. 1999).
While the ability to collect and analyze detailed observational data has greatly improved our knowledge of the three-dimensional flux of estuarine properties, few investigations appear to have been done which are able to quantify the contributions of secondary flows to changes in the lateral momentum balance. It is often difficult and expensive to collect the detailed comprehensive field data required for a proper examination of this problem. This is particularly true in the case of lateral studies, which need to collect simultaneously both longitudinal and lateral data (Kjerfve and Proehl 1979). Because of cost and time constraints, most recent field studies (Nunes and Simpson 1985; Huzzey and Brubaker 1988; Garvine et al. 1992; Wong 1994; Swift et al.
1996; Turrell et al. 1996; Valle-Levinson et al. 2000; Valle-Levinson, Wong, and Lwiza
2000) have focused on lateral mixing processes on very short time scales (on the order of a few tidal cycles) assuming nearly well-mixed conditions. To reduce complexity and better identify key processes, these studies have been generally conducted in straight reaches and under constant discharge conditions. While research by the previously mentioned authors has increased our understanding of lateral mixing processes, several questions still remain. For example: How does the strength and signature of secondary circulation differ between a channel bend and a straight reach? How does the signal of secondary circulation change between well-mixed and stratified conditions? What is the impact of changes in seasonal discharge on the signature of secondary circulation? What is the impact of drought and severe storms on secondary circulation? Does secondary circulation lead to enhanced lateral mixing? And if so, does this lead to an increase or decrease in the local salt flux?
The goal of this research is to understand the role of secondary circulation in estuarine mixing. Specifically, this research will examine the signal of secondary circulation on the tidal and fortnightly time scales (12.42 hours and 14 days, respectively) and quantify the contribution of secondary circulation to the lateral momentum balance in both time and space under different environmental conditions. The changes in the lateral momentum balance between spring and neap in both curved and straight channel reaches under different discharge and stratification conditions will receive special attention.
1.3 Dissertation Research Questions While the aforementioned studies have increased our understanding of lateral mixing processes and specifically secondary circulation, several questions linger on its role and impact in estuarine dynamics. What exactly is secondary circulation? How is it identified in acoustic data? Does it have a characteristic signal? Are there particular length and/or time scales critical to understanding secondary circulation? How is it modified under different environmental conditions? What are the broader influences and implications of secondary circulation? The structure of this dissertation will be guided by three main questions as they relate to secondary circulation and focus on the tidal and fortnightly time scales. Questions regarding secondary circulation on time scales longer than these variations are limited by the length and type of available data sets.
• What are the characteristics of secondary circulation in estuaries?
• What are the principal mechanisms that generate secondary circulation?
• How does the balance of mechanisms that generate secondary circulation change with time (over the tidal and fortnightly cycles) and with space (laterally in a
These questions will be addressed by focusing on the development of the following
specific objectives and tools:
• Identify and describe the signature of secondary flow in acoustic current data.
• Identify and parameterize mechanisms that drive secondary circulation.
• Quantify and compare a steady-state lateral momentum balance at four seasonal moorings during spring and neap tide.
• Develop methodology to grid, interpolate, and register three-dimensional irregularly spaced data to a prescribed mean lower low water datum (MLLW).
• Quantify and compare a steady-state lateral momentum balance in four lateral cross-sections at spring and neap during maximum ebb and maximum flood tides.
The remainder of this dissertation is divided into eight sections as described herein. Chapter 2 provides a detailed description of the study site, the Satilla River in southeast Georgia. Chapter 3 details background information on secondary circulation and important flow scaling parameters, including a discussion of the lateral momentum balance as a framework for analyzing the detailed three-dimensional Satilla River data sets. Chapter 4 gives a detailed description of the materials and methods used in this study, including sections on instrumentation, methods of deployment, and general data processing techniques.
The main chapters of this dissertation are Chapters 5 through 7. Each of these chapters is written in manuscript format, featuring a brief introduction, a site description and methods section, results, discussion, and conclusions. Written independently, Chapters 5, 6, and 7 will be submitted at the first available opportunity for publication.
Due to the format nature of these chapters, redundancy in some of the material is unavoidable. The focus of Chapter 5 is the fortnightly signal of secondary circulation.
Chapter 5 is a look at the characteristics of secondary circulation at one location in space during one seasonal mooring deployment. The temporal characteristics of secondary flow in four consecutive channel reaches and the effect of seasonal changes in freshwater input are examined in Chapter 6. Chapter 6 investigates the changes in secondary circulation at many locations during two seasonal mooring deployments. Chapter 7 examines the details of the spatial character of secondary flow in opposing channel bends by developing methodology to adjust, rotate, and grid irregularly spaced data to the mean lower low water (MLLW) datum. Chapter 7 also focuses on changes in the strength and character of secondary circulation in several lateral cross-sections during a series of seasonal surveys. Chapter 8 summarizes and provides additional discussion on the conclusions of Chapters 5 through 7. The final chapter of this dissertation, Chapter 9, provides recommendations and ideas for future research.
The Satilla River, in south Georgia, offers a unique site for better understanding of secondary circulation. Little studied, nor heavily impacted by human activities, this pristine river has one main sinuous channel and is joined near its freshwater limit by a large tidal creek. Easily accessible and subject to a wide variety of environmental conditions, it has been selected as the study site for this dissertation research.
The Satilla is a small mesotidal ‘blackwater’ river whose headwaters originate in the sandy coastal plain of Georgia (Figure 2.1). On average, it has 2.5 meter range semidiurnal tides with pronounced fortnightly variability and is little affected by clay materials from the Piedmont of Georgia. The predominant dark tea color of the water is due to tannic acid derived from humic materials found in the extensive flood plain swamps bordering the river. Anthropogenic influences such as development and pollution from industrial processes, residential sources, and agricultural runoff are low along the Satilla. This little-studied river, confined to within a 120 mile segment of the coast, offers a unique look at understanding the link between differing landscape characteristics, geologic setting, and flow rate. Because of its beneficial geographic location, the Satilla shares similar temperature extremes, rainfall patterns, and tidal regimes with other shallow [more in-depth studied] estuaries along Georgia’s coast (Howard and Frey 1975; Davis 1985; Sexton and Hayes 1996).
Figure 2.1: An aerial photograph of the Satilla River in southeast Georgia.
The Satilla has a drainage basin of 9143 km2 and a length of about 362 km from its headwaters to the Atlantic Ocean near Brunswick, Georgia (Dame et al. 2000) and has an average flow of 78 m3 s-1 (United States Geological Survey National Water Information System (USGS 2004)). The main channel is marked by nine channel bends and two short straight reaches between its mouth at St. Andrews Sound between Jekyll and Cumberland Islands and Woodbine, Georgia, at river kilometer 38. (River kilometer markings are shown in Figure 2.6). The majority of these bends are broadly concave (curve to the north, radius of curvature ~ 1200 m) each followed by more sharp convex bends (bend to the south, radius of curvature ~ 800 m). The bends are roughly three to four kilometers apart and start at six kilometers from the ocean (0 km) up to river kilometer 33. The river’s average channel width varies between 200 – 500 m and has an average depth in the thalweg (deep channel) of about 10 m. A triple junction near river kilometer 26 divides the flow of the Satilla River between its main branch to the south
communication from the upper watershed into the Satilla marsh system is minimal.
Dike-constructed local roadways and semi-permanent sediment fences due to continuous construction on nearby Interstate 95 likely contribute to the reduced upland exchange.
Figure 2.2: Extensive salt marshes at Sara’s Creek in the upper Satilla River.
The stand of trees in the background is located at Crows Harbor Reach (Courtesy S. Elston, 1999).
The river bottom is mostly made of fine to coarse sand, except in swampy areas where there is an additional overlay of organic muck (Howard and Frey 1975). Spartina alternaflora and Juncus roemerianus salt marshes border the Satilla up to just past the triple junction between the Satilla and WOC (Figure 2.2). Beyond the triple junction, flood plain swamps of black gum and cypress border the nearly fresh river (Sexton and Hayes 1996; Moran et al. 1999; Dame et al. 2000).
Morphologically, the Satilla, like many Georgia and South Carolina estuaries, is classified as an ebb-tidal system (Sexton and Hayes 1996). The Satilla exhibits several features of an ebb-tidal delta including a sinuous deep ebb channel, a weaker marginal flood channel, several swash bars (near river kilometer 2 around Horseshoe Shoal), and a terminal lobe, which defines the extent of the delta, that extends 2 kilometers out past the mouth of the river into St. Andrews Sound.