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Sediment Budgets As an Organizing Framework in Fluvial Geomorphology

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Sediment Budgets As an Organizing Framework in Fluvial Geomorphology CHAPTER 16 Sediment budgets as an organizing framework in fluvial geomorphology Leslie M. Reid1 and Thomas Dunne2 1USDA Forest Service Pacific Southwest Research Station, Arcata, CA, USA 2University of California, Santa Barbara, CA, USA 16.1 Introduction have been used and describes an approach for designing and constructing useful budgets. Fluvial geomorphology concerns the transport of weathered rock debris and its accumulation and organization into land­ The sediment budget defined formsthatevolvecontinuously. Whereastheoretical fluvial Asedimentbudgetdescribestheinput,transport,storage and geomorphology focuses on the mechanics of fluvial processes export of sediment in a geomorphic system. For example, and the principles governing the evolution of landforms, Fig. 16.1 encapsulates the operation of the sediment budget of a applied fluvial geomorphology is devoted to understanding small forested mountain catchment. This budget was based ini­ and designing strategies for coexisting with changing fluvial tially on qualitative field observations and mapping of sediment systems. Whenever a fluvial feature changes form, there is a sources and storage elements and the conceptual diagram was local imbalance in the movement of sediment to and from the then used to guide quantitative estimates of the various transfer site. An understanding of how a river system collects, transports ratesandstoragetimes of thesediment. and deposits sediment is therefore central to addressing both Sediment budgets can be designed to quantify the magnitude applied and theoretical questions regarding how changes in of a process or response rate, its location and its timing or to catchment conditions affect channels, how long the effects explore the influences contributing to a morphological change. will last and what the sequence of responses will be. Sediment They can be used to compare the likely outcomes of different budgets are tools for building that understanding. land-management options or climatic changes or to evaluate the Sediment budgets define the most fundamental aspect of significance and implications of climatic, tectonic or land-use landform evolution: mass conservation as it is achieved by mor­ changes that have already occurred. Sediment budgets provide phogenetic processes acting within the boundary conditions a framework for organizing both qualitative information about imposed by natural or anthropogenic controls. Information process interactions and quantitative information about process aboutwhetherthesedimentbudgetofaparticularfluvialland­ rates. Budgets can take many forms, describe many scales and form or a fluvial system is in a steady state — or the degree incorporate diverse levels of precision. The most commonly of imbalance if it is not — is as basic a descriptor as the mean used sediment budgets take the form of qualitative flowcharts annual flood, mean annual runoff or other commonly used that describe relationships between sediment sources and trans­ characterizations of fluvial systems. Quantifying sediment sup­ port processes. Long-term monitoring projects often are used plies and transport rates is typically more difficult to accomplish to provide more precise measurements of particular budget than measurement of hydroclimatic quantities, but technical components. advances during the past few decades have improved the capac­ Whether qualitative or quantitative, all sediment budgets are ity for defining the sediment budgets of fluvial systems. Even conceptually underlain by the continuity equation for sediment rudimentary sediment budgets can prevent oversights and guide transfer: the selection of the analytical tools needed for more detailed = analyses. Eventually, the degree of detail and sophistication of sediment input to a landscape element sediment output sediment budgeting is constrained by time and by the avail­ + change in sediment storage (16.1) ability of data sources and tools, many of which are described in detail in other chapters of this book. This chapter discusses where all terms are expressed as quantities per unit time. The the nature of sediment budgets, provides examples of how they basic equation can be refined in many ways. Changes in grain Tools in Fluvial Geomorphology, Second Edition. Edited by G. Mathias Kondolf and Hervé Piégay. © 2016 John Wiley & Sons, Ltd. Published 2016 by John Wiley & Sons, Ltd. 357 358 Chapter 16 Mixing, Weathering Bedrock Soil weathering Saprolite Ridge Creep, weathering Mixing, Side Creep, Soil Saprolite Weathering Bedrock weathering weathering slope Dissolved Dissolved Hollow load load transport discharge Debris flow Foot slope Suspended Suspended debris load load slide transport discharge Bed material attrition Sediment Debris torrent, Debris Sediment ‘Active’ in Debris Bank sediment Bedload Bedload high fan in main tributary fan erosion erosion in main transport discharge stream valley channel channels discharge Figure 16.1 Conceptual model of the sediment budget of a small mountainous watershed in the Oregon Coast Range. Rectangles represent storage elements, octagons indicate transfer processes and circles represent outputs. Solid lines indicate the transfer of sediment and dotted lines represent the migration of solutes. Dietrich and Dunne, 1978. Reproduced with permission of Schweizerbart. size can be accounted for by constructing the equation for following European colonization and subsequent amounts of different size classes, for example, and specific processes can be fluvial sediment transport in neighbouring lowlands. Both isolated. authors interpreted the disparities to indicate that large volumes Given the various forms that sediment budgets may take and of sediment must be stored on footslopes and valley floors the variety of problems to which they can be applied (Table 16.1), and would continue to contribute fluvial sediment long after it is clearly not useful to think of sediment budgeting as a single hillslopes restabilize. These studies emphasized the connec­ tool to be applied using a uniform protocol. Instead, sediment tions between sediment fluxes through landscape elements and budgeting represents a general approach to geomorphic problem highlighted the importance of changes in sediment storage. solving and the methods most useful for each budget depend on Although these sediment budgets were rudimentary by modern the intended application of that budget. standards, they revealed the significance of long-term, inter­ mittent transfer and storage of sediment throughout landscapes History and applications and stimulated decades of research. Geomorphologists have long used the concept that imbalances By the mid-1900s, methods had been developed to quantify between sediment supply and transport capacity cause aggra­ components of sediment regimes and long-term monitoring dation and degradation, and results of sediment production records and aerial photographs were becoming available. The and transport measurements were being used to understand idea of systematically quantifying the balance between sediment landscapes by the late 1800s. Hill (1896), for example, integrated inputs, transport rates and storage changes began to spread. At the results of landslide surveys in New Zealand to demonstrate first, sediment budgets simply involved systematic accounting that landslides could influence landscape evolution. Gilbert ofprocessmeasurements(forareview, seeReidand Dunne (1917) used a sediment budget to evaluate the impacts of 1996). More recently, field monitoring studies were coupled hydraulic mining in California on downstream navigation, with modelling to interpret, extend and generalize results. New and although later work in the same river system added detail methods of dating now allow long-term deposition rates to be (James 1997; Singer et al. 2013), Gilbert’s basic insight remains evaluated for large sediment sinks and analyses of cosmogenic useful. isotope concentrations in sediment are used to infer average Sediment budgeting as a concept for interpreting sparse data erosion rates over large catchments and regions. soon proved useful for analysing the landscape-scale effects of As methodological and conceptual difficulties were sur­ land use. Both Haggett (1961) in southeastern Brazil and Trim­ mounted, the organizing power of the sediment budget concept ble (1977) in the Southern Appalachian Mountains found large became more evident and the approach is now widely applied to disparities between landscape-averaged estimates of soil erosion quantify landform evolution under both natural and modified Sediment budgets as an organizing framework in fluvial geomorphology 359 Table 16.1 Examples of sediment budgets used to address issues in fluvial geomorphology. Problem and reference Why Regime Precision Time Method PDF DESY LNS AELS MFAESH Spatial focus: Catchment response Prioritize rehabilitation by erosion potential (Gellis et al. 2001) .D. .E.. ..S … S .FA … Proportion of sediment yield from landslides (Hovius et al. 1997) .D. .E.. .N. A … .FA … Sediment contribution to lake from cyclone (Page et al. 1994) .D. DESY .N. .E.. .FA.S. Effectiveness of soil conservation strategies (Phillips 1986) ..F DESY .N. ..L. .F. Downstream influence of upper-basin sediment (Phillips 1991) PD. DESY .N. ..L. .F. Relation of hillslope erosion to sediment yield (Reneau and Dietrich 1991) P.. .E.Y .N. A. F..SH Spatial focus: Channel system response Plan restoration using erosion distribution (Abernethy and Rutherfurd 1998) .DF DE.. L.S S .F.E.. Extent of channel
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