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Chapter 5 the Channel Bed – Contaminant Transport and Storage

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Chapter 5 the Channel Bed – Contaminant Transport and Storage CHAPTER 5 THE CHANNEL BED – CONTAMINANT TRANSPORT AND STORAGE 5.1. INTRODUCTION In the previous chapter we were concerned with trace metals occurring either as part of the dissolved load, or attached to suspended particles within the water column. Rarely are those constituents flushed completely out of the system during a single flood; rather they are episodically moved downstream through the processes of erosion, transport, and redeposition. Even dissolved constituents are likely to be sorbed onto particles or exchanged for other substances within alluvial deposits. The result of this episodic transport is that downstream patterns in concentration observed for suspended sediments is generally similar to that defined for sediments within the channel bed (although differences in the magnitude of contamination may occur as a result of variations in particle size and other factors) (Owens et al. 2001). While this statement may seem of little importance, it implies that bed sediments can be used to investigate river health as well as contaminant sources and transport dynamics in river basins. The ability to do so greatly simplifies many aspects of our analyses as bed materials are easier to collect and can be obtained at most times throughout the year, except, perhaps during periods of extreme flood. Moreover, bed sediment typically exhibits less variations in concentration through time, eliminating the need to collect samples during multiple, infrequent runoff events. Sediment-borne trace metals are not, however, distributed uniformly over the channel floor, but are partitioned into discrete depositional zones by a host of erosional and depositional processes. Thus, to effectively interpret and use geochemical data from the channel bed materials, we must understand how and why trace metals occur where they do. In this chapter, we will examine the processes responsible for the concentration and dispersal of sediment-borne trace metals, and the factors that control their spatial distribution within the channel bed. We begin by examining the basic mechanics involved in sediment transport, including an analysis of open channel flow. We will then examine how these basic processes produce spatial variations in sediment- borne trace metal concentrations, both at the reach scale and at the scale of the entire watershed. 127 128 5 The Channel Bed – Contaminant Transport and Storage 5.2. SEDIMENT TRANSPORT 5.2.1. Modes of Transport Our understanding of sediment transport processes is hampered by the difficulties of obtaining measurements in natural channels. Rivers are characterized by ever changing discharge and velocity and often are bounded by erodible, inhomogeneous bed and bank materials. In addition, river water is usually opaque, thereby making visual examination of the channel bed impossible. In order to overcome these diffi- culties, investigators have supplemented field investigations with laboratory studies that allow the transport of sediment to be observed in flumes where slope and discharge can be changed, and other selected variables can be held constant. These laboratory studies have been instrumental in deciphering the mechanisms respon- sible for initiating particle motion, and for predicting the relationships between sediment transport rates and discharge. Nonetheless, our ability to predict the size and quantity of sediment (and thus, contaminants) that a river can carry under a given set of flow conditions remains less than perfect. The movement of sediment in a channel varies with both time and space. It is a function of the size of the sediment that is being transported as well as the energy that is available to perform mechanical work. In the previous chapter, we examined the downstream flux of fine-grained particles transported within the water column as part of the suspended load. These suspended particles have little, if any, contact with the channel bed. They are held in suspension by short, but intense, upward deviations in the flow as a result of turbulent eddies which are generated along the channel margins. The more frequent and intense the turbulence, the more material, and the larger the grains, that can be held within the water column (Mount 1995). We generally think of the suspended load as consisting of only fine grained sediment. However, during extreme floods in steep, gravel bed-rivers, particles a few tens of centimeters in diameter have been observed to move in true suspension. Suspended sediment typically moves at a rate that is slightly slower than that of the water, and may travel significant distances downstream before coming to rest. In contrast, bedload, which is transported close to the channel bottom by rolling, sliding, or bouncing (saltating), is unlikely to be moved great distances before it is deposited and stored, either temporarily or semi-permanently, within the channel. It is important to recognize that sediment which moves as suspended load during one flood event, may move as bedload during another, depending on the hydraulic conditions to which the particles are subjected. The ability of rivers to transport sediment is described in terms of competence and capacity. Competence refers to the size of the largest particle that a river can carry under a given set of hydraulic conditions. Capacity is not concerned with the size of the material, but is defined as the maximum amount of sediment that a river can potentially transport. Note that capacity represents a theoretical maximum. As such, it is almost always larger than the actual sediment load being carried by the river. Sediment Transport 129 It should come as no surprise that as discharge increases the amount of sediment being transported also will increase. The relationship, however, is much more complicated than one might think. For example, the correlation between the suspended load and discharge is generally quite poor because rivers can typically transport more fine-grained sediment than is supplied from the watershed; the load is controlled by supply rather than discharge. In contrast, the amount of material available for transport as bedload generally exceeds that which can be carried by the river. The correlation between discharge and bedload transport rates, then, is much better than for suspended load, but it is still less than perfect. In the case of bedload, the correlation is significantly influenced by complexities associated with particle entrainment (discussed below). Another important problem is that bedload transport is difficult to measure because the movement of particles along the channel bed is primarily associated with floods when flow depths and velocities are elevated. Working in these conditions can be difficult and dangerous. In addition, where bedload transport rates have been accurately measured along a heavily instrumented river, they are found to vary across the width of the channel bed and through time (Leopold and Emmett 1977; Hoey 1992; Carling et al. 1998). These variations are at least partly related to the movement of large-scale bedforms along the channel floor. The periodic movement of a bedform through a cross section hinders the use of hand held instruments to measure bedload transport rates because they can only sample a single location for a short period of time. The difficulties of directly measuring bedload has led to the use of mathematical models to estimate a river’s load for given a set of channel, sediment, and flow conditions (Meyer-Peter and Muller 1948; Einstein 1950; Bagnold 1980; Parker et al. 1982). The accuracy of the estimated transport rates can be difficult to assess because of a general lack of direct measurements with which to verify the model outputs. Nonetheless, it is generally accepted that these transport models only provide estimates of bedload transport (perhaps within 50 to 100% of the actual value). Gomez and Church (1989), for example, examined ten transport formulas and found that none of them were entirely adequate for predicting bedload transport in coarse-grained (gravel-bed) rivers. 5.2.2. Channelized Flow The ability of a river to transport sediment or sediment-borne trace metals is governed by the balance between driving and resisting forces. The principal driving force is gravity (g), which is oriented vertically to the Earth’s surface and equal to 981 m/s2. The component of this gravitational force that acts to move the available mass of water in the channel downslope is equal to g sin , where is the average slope of the channel (Leopold et al. 1964). The resisting forces are more difficult to quantify and originate from a variety of sources. Attempts in recent years to identify these sources, and their relative importance, has met with only limited success. Nonetheless, the total resistance to flow can be broadly categorized into three principal components referred to as free surface resistance, channel resistance, 130 5 The Channel Bed – Contaminant Transport and Storage and boundary resistance (Bathurst 1993). Free surface resistance is caused by disruptions in the water surface by waves or abrupt changes in gradient. Channel resistance represents the loss of energy as water moves over and around undulations in the channel bed and banks, or is distorted by changes in the channel’s cross- sectional or planimetric configuration. The final component, boundary resistance, is produced by the movement of water over individual clasts or microtopographic features (e.g., dunes and ripples). Resistance produced by dunes and ripples is referred to as form drag, whereas resistance due to individual particles is called grain roughness. Other factors such as vegetation, suspended sediments, and internal viscous forces within the water also affect the resistance to flow. Hydraulic engineers have been concerned with quantifying the relationships between the driving and resisting forces inherent in channelized flow for centuries. The net result of these studies has been the formulation of a number of resistance equations presented in Table 5.1. The Manning equation is perhaps the most widely used of these in the U.S., although the Darcy-Weisbach equation is growing in popularity within the academic community.
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