LECTURE 1: INTRODUCTION, HISTORY, AND DEFINITIONS

Transcription

1 LECTURE 1: INTRODUCTION, HISTORY, AND DEFINITIONS Basic Definitions Brief History of Fluvial Geomorphology Views of River Channels and Drainage Basins channels collect material produced from the landscape all of the products exported by erosion leave the landscape through the channel network (except by wind) Human Organization hydraulic cultures and the rise of agriculture transportation watershed management and planning Time Scales of Interest Drainage Basin Components Hillslopes & Hollows Channels - definable banks Valleys - convergent topography - valley walls & floors Floodplains Channel Types alluvial vs. non-alluvial Other Basic Definitions Floodplains and terraces Channel patterns meandering, straight, and braided The bankfull channel The bankfull flood The hydraulic geometry of channels ESS Spring 2006

2 A FEW BASIC DEFINITIONS Fluvial: Of, found in, or produced by a river; from latin fluvius Geomorphology: The science dealing with the nature and origin of the earth's topographic features; from greek: Geo - earth morphos - form -ology - science Drainage Basin: The drainage area which contributes water to a particular channel or set of channels. Synonymous with watershed (America) and catchment (everywhere else). Channel: A zone of concentrated flow and sediment transport within definable banks. ESS Spring 2006

3 A BRIEF HISTORY OF FLUVIAL GEOMORPHOLOGY Date Event 3000 BC King Menes dammed the Nile 3000 BC Nilometers in use to gauge Nile 2200 BC Emperor Yu mapped river networks 300 BC Aristotle - subterranean condensation feeds springs 0 Vitruvius - springs arise from percolation of rain and snow through rock strata to the foot of mountains AD 100 Romans built impressive aqueducts but had little understanding of hydrology or hydraulics Middle Ages Based on Ecclesiastes 1:7 it became heresy to doubt the subterranean sea-water theory; "All the rivers run into the sea, yet the sea is not full; unto the place from whence the rivers come thither they return again". ESS Spring 2006

4 17 th Perrault measured rainfall and compared estimates of the century total for Seinne basin with runoff and concluded that rainfall was adequate to feed the river - beginning of modern quantitative hydrology. 18 th Development of basic hydraulics - Chezy in particular century showed that flow velocity varies with water slope. 19 th Expansion of empirical hydrology and qualitative century geomorphology 20 th Development of river-basin based hydrologic and land century use planning, and quantitative geomorphology. ESS Spring 2006

5 A BRIEF REVIEW OF THE MAIN ACTORS OF 18 TH, 19 TH, AND EARLY 20 TH -CENTURY GEOMORPHOLOGY James Hutton ( ) An early non-catastrophist: the processes we see operating today are sufficient to explain the evolution of the earth s surface. if the succession of worlds is established in the system of nature, it is in vain to look for anything higher in the origin of the earth. The result, therefore, of our present enquiring, is that we find no vestige of a beginning,--no prospect of an end. (1788) William Buckland ( ) Restatement of geologic creationism, particularly diluvialism: evidence included erratics, drift, striations, river terraces, and underfit streams. Obviously these were the product of a flood, the Flood. Furthermore, if we only have 6000 years of Earth history, change must be catastrophic. Sir Charles Lyell ( ) A return to uniformitarianism in all spheres, biological as well as geological. Called for exclusively uniform processes: Never was there a dogma more calculated to foster indolence, and to blunt the keen edge of curiosity, than this assumption of the discordance between a former and the existing causes of change. (1833) In later years, Lyell became enamored of the work of marine currents and waves, believing that the most valleys without obvious structural control were eroded by the ocean as the land gradually rose (the theory of marine dissection ). In the second half of the 19 th century, the glacial theory of landscape evolution arose, providing an alternative explanation for many of the features previously identified as diluvial or of marine dissection. There arose also a renewed appreciation of the power of subaerial erosion by fluvial action, in part a consequence of expeditions to the tropics (where a year s worth of English rainfall could fall in 24 hours). ESS Spring 2006

6 G. K. Gilbert ( ) Gilbert considered only subaerial erosion and stressed the dynamic equilibrium between landscapes and erosion: more resistant rocks erode slower, steeper slopes eroded faster, and the transport of eroded material depends on the slope angle and the amount of water available. A landscape in equilibrium will experience uniform lowering and not change its form, as all slopes are adjusted to their respective rock resistance. Emphasis here is on the mechanisms of geomorphic work, not just deciding if the agent of change is ice or water a precursor to process geomorphology. William Morris Davis ( ) Davis published The Geographical Cycle in 1899, which has influenced all broad-scale geomorphic thinking ever since. He believed that three variables affect the landscape: (1) structure, (2) process, and (3) time but in the end, only time matters. He defines three stages of landscape evolution, defined in terms of time only: youth, where a landscape has been uplifted and remnants of the pre-uplift topography still exist (now called erosion surfaces ); maturity, with all the original topography consumed and with slope and relief at a maximum; and old age, where flattened slopes and wide floodplains have removed all relief but for the most resistant monadnocks. Compare to Gilbert, where the landscape retains its form over time. The crux of Davis s model was best articulated by him in 1905: the scheme of the cycle is not meant to include any actual examples at all, because it is by intention a scheme of the imagination and not a matter for observation : The problem, of course, is that if we never observe the cycle in nature. Many geomorphologists now wonder whether it worth carrying around its conceptual baggage. ESS Spring 2006

7 VIEWS OF RIVER CHANNELS AND DRAINAGE BASINS A channel has two basic functions within a drainage basin. It must convey all of the 1. WATER, and 2. SEDIMENT that the drainage basin delivers by the various runoff and hillslope processes. In order to accomplish these tasks, any channel must take on a particular form, by which we mean its width, depth, sinuosity, and distribution of such small-scale features as pools and bars. In addition to accomplishing its fundamental "tasks" of moving water and sediment from uplands to outlet, the form of the channel will also be affected locally, by bank vegetation, fallen trees, bank sediments, tributary inputs, and bank modifications; systemically, by the progressive inclusion of increasing tributary areas with their own particular influxes of water and sediment; and temporally, by the sporadic disturbances to a watershed occasioned by large storms, fires, or human activity. Our study of channel geomorphology is the understanding of how these factors affect channel form, and how to interpret or to predict that form even with less-than-perfect information. ESS Spring 2006

8 HUMAN ORGANIZATION: As the land manager sees the river: Activities and processes, linked through economics and human actions irrigation, navigation, etc ESS Spring 2006

9 As the engineer sees the river: Adjacent sets of isolated, independent processes and problems bank erosion, flooding, etc ESS Spring 2006

10 As the geomorphologist sees the river: (1) Landscape is a system that produces and transports runoff and sediment. (2) Channel network is like the veins of the landscape. (3) Channels collect sediment produced on hillslopes and transport it to basin outlets. (4) Channels influenced by sediment production, transport, routing, and storage processes. ESS Spring 2006

11 TIME AND SPATIAL SCALES OF INTEREST: ESS Spring 2006

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13 CHANNEL TYPES. Although we will say more about the classification of river and stream channels later, we must make an initial discrimination between two distinct types of channels: Alluvial channels: channels formed in and by sediment transported by the river (= "alluvium") under its current hydrologic and climatologic regime (and so which could be transported again) Non-alluvial channels: channels not formed in alluvium, such as those: bounded by bedrock or concrete; deeply incised into hillslope deposits; choked by relatively immovably objects such as large boulders; rimmed with thick and deeply rooted bank vegetation; Alluvial, "self-formed" channels are free to adjust their shape in response to changes in flow, because they a competent to move the material that forms their boundaries. The detailed hydrodynamics of how these channels establish their preferred dimensions and shape are complex and still not fully understood. However, we can recognize similarities in the behavior of these channels worldwide, expressing in readily measured ways the net result of processes only imperfectly understood. We will use these empirical characteristics extensively to predict channel behavior; but remember that they only work satisfactorily on alluvial channels! ESS Spring 2006

14 SOME OTHER USEFUL, BASIC DEFINITIONS FLOODPLAINS AND TERRACES To a geomorphologist, a floodplain is the surface that has been built by a river channel under the current hydrologic and sedimentological regime. It is composed of alluvium, the sediment carried by the river. An alluvial channel is bounded by a floodplain; conversely, a channel formed within a true floodplain is by definition alluvial. In contrast, a terrace is also a constructed surface and also underlain by alluvium, but it has not formed under the current regime of the river. Instead it represents floodplain formation at an earlier time when, for whatever reasons, deposition was occurring at a higher elevation. (Note that if the earlier deposition occurred at a lower elevation than at present, the remnant terrace would be buried by the modern floodplain and so we could not see it.) This definition of a floodplain differs from that normally offered by an engineer or a planner. To those disciplines, the floodplain is a particular strip of ground that is inundated by a flood of a particular recurrence interval. Thus some may speak of the 10-year floodplain, or the 100-year floodplain. There is no requirement that this area of inundation correspond to ESS Spring 2006

15 any discernible feature on the landscape, although there are some useful correlations that we will explore later for alluvial channels. The floodplain may be absent or may not correlate with the valley bottom where: channels have "inherited" a valley geometry from some other geologic process (such as glaciation) in the recent past; or channels have undergone a significant change in hydrologic regime (as a result of climate change, diversion, or watershed disturbance) that leads to incision, or entrenchment, of the channel. ESS Spring 2006

16 CHANNEL PATTERNS, by which we mean the appearance of the channel in map view, are another way in which channels are categorized. Each of the following patterns (meandering, straight, braided, and anabranching) could plausibly be either alluvial or non-alluvial, but the variety of common alluvial channel patterns is much greater. Meandering rivers are the most common type of channel, where the main thread of the flow (the thalweg) oscillates from one side of the channel to the other. Pools and riffles form in predictable locations along meandering rivers, which become more precisely fixed in place as the magnitude of the meanders increases. In most natural channels the ratio of channel length to straight-line down-valley distance lies between 1.5 and 2. Where this ratio, called the sinuosity, is less than 1.3 the channel is not termed "meandering" but instead is "sinuous" or "straight." Straight rivers are naturally uncommon because they are inherently unstable: any minor perturbation of the flow, such as caused by a hard projection or a small hollow in the bank, will tend to establish the oscillation of the thalweg that leads to concentrated scour of pools, pointbar formation, and a meandering pattern. ESS Spring 2006

17 Braided rivers are identified wherever the flow divides into more than one thread. Braided channels are not as common as meandering ones, but they are of special interest because their rates of lateral shifting and of bank erosion are generally very much greater. Irregular but very active transport and deposition of sediment characterize the braided environment. The outlets of mountain glaciers are classic environments for braided channels, but this pattern is also common at mountain rangefronts, where steep alpine drainages reach the flat lowlands and must abruptly deposit a large fraction of their sediment load in response to the decline in overall valley gradient. This last setting gives rise to an important landform commonly associated with braided channels, namely alluvial fans. Anabranching rivers are similar to braided rivers in that flow is divided into multiple channels but they differ in that the area between the channels is stable and may even develop mature forest. Most common in vegetated environments where high bank strength (due to roots) and stable logjams can, respectively, retard lateral channel migration and split the flow into multiple channels. ESS Spring 2006

18 THE BANKFULL CHANNEL The "size of a channel" only has meaning for alluvial channels; we typically call the feature so measured the bankfull channel. The surface at the top of the bankfull channel is the floodplain, which is inundated whenever the river or stream experiences a bankfull flow. The most reliable ways of identifying the bankfull channel is to determine the elevation of the currently active floodplain. Recognition of active floodplains, discriminating floodplains from terraces, and identifying the associated bankfull channel, are key tasks of river planners. Williams (1978) reviewed various methods of identifying these features, which include: 1. The height of the "valley flat," or prominent surface on the valley floor; 2. The elevation of the active floodplain, which is the surface of frequent inundation by floods and is typically the lowest level of perennial vegetation; 3. Various relationships between channel width and depth at a particular cross section, particularly the elevation at which the width-to-depth ratio of the channel reaches a minimum value (see below), or the elevation at which a plot of cross sectional area vs. top width of the flow changes most abruptly. In general, multiple determinations at multiple sites is the most reliable approach! ESS Spring 2006

19 One common method of determining the bankfull depth involves plotting the ratio of the flow width to the depth versus the height above the channel bed (see example above). Near the bed the flow is wide and shallow, so the width-to-depth ratio is high. At flood stages the flow spreads out across the floodplain and the width-to-depth ratio is also high. The bankfull flow depth can be approximated by the flow depth that corresponds with the minimum width-to-depth ratio. ESS Spring 2006

20 THE BANKFULL FLOOD What is the recurrence interval of the bankfull flood? on non-alluvial channels there is no bankfull channel...this is a meaningless question (although it doesn t stop people from asking it!) on alluvial channels, most channels fill somewhere between Q 1.5 and Q 2, i.e. the discharge with a recurrence interval (which = 1/[probability of annual exceedence]) of 1.5 to 2 years. But: this relationship is not universal, or a result of theoretical analyses, or the fourth Law of Thermodynamics. It just seems to work out that way, in most (but by no means all) channels! ESS Spring 2006

21 THE HYDRAULIC GEOMETRY OF CHANNELS In its most common definition, the hydraulic geometry refers to the way in which a channel's width, depth, and velocity change with changes in discharge. Although we might acknowledge that other parameters of a channel's form can also vary (such as slope, roughness, or degree of meandering), these three parameters have the singular property that Q = w d u and so a change in Q must be fully reflected by changes in the width, depth, and velocity. Discharge in a stream system can change in two ways (see sketch next page): (1) The general increase in discharge as we move downstream and so collect runoff from a progressively greater drainage area. This is measured by the downstream hydraulic geometry. (2) The changing dimensions of the flow at a single gauging location as discharge changes during the passage of a flood. This type of change is measured by the at-a-station hydraulic geometry. ESS Spring 2006

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23 By convention, the hydraulic geometry relationships are written with the following symbols: w = aq b d = cq f u = kq m Multiplying these three equations together, w d u = a c k Q (b+f+m), And because Q = w d u, Q = a c k Q (b+f+m), and so a c k = 1, and b + f + m = 1. ESS Spring 2006