FLUVIAL GEOMORPH ESS 426
Popular in Course
Popular in Earth And Space Sciences
This 23 page Class Notes was uploaded by Miss Jeanette Keebler on Wednesday September 9, 2015. The Class Notes belongs to ESS 426 at University of Washington taught by Staff in Fall. Since its upload, it has received 31 views. For similar materials see /class/192676/ess-426-university-of-washington in Earth And Space Sciences at University of Washington.
Reviews for FLUVIAL GEOMORPH
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 09/09/15
LECTURE 1 INTRODUCTION HISTORY AND DEFINITIONS Basic De nitions 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 amp Hollows Channels de nable banks Valleys convergent topography valley walls amp oors Floodplains Channel Types alluvial vs nonalluvial Other Basic De nitions Floodplains and terraces Channel patterns meandering straight and braided The bankfull channel The bankfull ood The hydraulic geometry of channels E55 426 11 Spring 2006 A FEW BASIC DEFINITIONS Fluvial Of found in or produced by a river from latin uvius Geomorphology The science dealing with the nature and origin of the earth39s topographic features from greek Geo earth morphos form eulogy science Drainage Basin The drainage area which contributes water to a particular channel or set of channels Synonyrnous with watershed America and catchment everywhere else Channel A zone of concentrated flow and sediment transport Within de nable banks E55 426 12 Spring 2006 A BRIEF HISTORY OF FLUVIAL GEOMORPHOLOGY 5 Date Event 39 Iquot I 3000 BC King Menes dammed the Nile I Ii 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 17 it became heresy to doubt the subterranean sea Water theory HAll the rivers run into the sea yet the sea is not full unto the place from whence the riVers come thither they return againquot ESS 425 13 Spring 2006 17th century 18th century 19th century 20th century E55 426 Perrault measured rainfall and compared estimates of the total for Seinne basin with runoff and concluded that rainfall was adequate to feed the river beginning of modern quantitative hydrology Development of basic hydraulics Chezy in particular showed that flow velocity varies with water slope Expansion of empirical hydrology and qualitative geomorphology Development of riverbasin based hydrologic and land use planning and quantitative geomorphology 14 Spring 2006 A BRIEF REVIEW OF THE MAIN ACTORS OF 18 19 AND EARLY 20TH CENTURY GEOMORPHOLOGY James Hutton 1726 1797 An early noncatastrophist the processes we see operating today are sufficient to explain the if the succession of worlds is established in the system of evolution of the earth s surface 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 beginningno prospect of an end 1788 William Buckland 1 784 185 6 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 1797 1875 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 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 uvial action in part a consequence of expeditions to the tropics where a year s worth of English rainfall could fall in 24 hours E55 426 15 Spring 2006 G K Gilbert 1843 1913 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 gemorphology William Morris Davis 1850 1934 Davis published The Geographical Cycle in 1899 which has influenced all broadscale 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 preuplift 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 E55 426 16 Spring 2006 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 Z 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 smallscale features as pools and bars In addition to accomplishing its fundamental quottasksquot of moving water and sediment from uplands to outlet the form of the channel will also be affected 0 locally by bank vegetation fallen trees bank sediments tributary inputs and bank modifications 0 systemically by the progressive inclusion of increasing tributary areas with their own particular influxes of water and sediment and 0 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 lessthanperfect information E55 426 17 Spring 2006 HUMAN ORGANIZATION As the land manager sees the river Activities and processes linked through economics and human actionsiinigation nawgation etc ESS 426 Land Er catchment use Water transfers AgricuIture Forestry Dramage EUII OPI IICEUO T I Pesticides y A 39 agigrgguahw x I Fisheries DEVELOPMENT Reservoirs w FISHERIES BorehOIeS Conservation Fisheries E H ham Ing I Amenity Hearing quot E39C Iw39w IMPOUNDMENT Ramcking Propagation Disease Fish ladders I treatment DrobIems I Passes Screens Counters Treatment works I l Ecological 1 considerations Conservation Boreholes Bieloglcal queiity BankSlde storage DISTRIBUTION nImaI Infestation TABLE ABSTRACTION problems RIVER USE Resoures E IuenL isposal s AUGMENTATION Water quality EcoIogy Fisheries INDUSTRIAL ABSTRACTION Industry Domestic Navigation FISher DIRECT DISCHARGE consumers TOXICIW EFFLUENT BIOLOGICAL TREATMENT SEWAGE TREATMENT TOXICITY TREATED EFFLUENT WESIewater DISCHARGE treatment Sewage DISCHARGE SLUDGE DISPOSAL treatment Biologicel efficiency ToxiCItv Odour NuIsanca DATA generation RIVER MANAGEMENT RESOURCE DEVELOPMENT AND PROTECTION Abstraction licences Discharges Consents Fisheries and Recreation HEATED EFFLUENTS Estuary Amenity Conservation Fish migration Water quality LAND DRAINAGE Conservation of habitat Aquatic weed control herbicides Spring 2006 As the engineer sees the river Adjacent sets ofisolated independent processes and problemsibank erosion ooding etc Hydraulic structures NavierStokzs equations SEDIMENT TR NSP Coastal problems ESS 426 19 SpIing 2006 l 0 1 2 3 A 5 m 5mm m thausonus or Feet As the gemnmphologist sees the ver 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 in uenced by sediment production transport routing and storage processes ESS 426 110 Spring 2006 TIME AND SPATIAL SCALES OF INTEREST 1O 1O 10 Time years 10 1O 10 ESS 426 Geologist Geomorphologist Engineer 10 3 10392 10391 100 1o1 102 103 104 Space km 111 Spring 2006 Drainage Basin Components 0 I I I o I l Hillslopes undissected elevated areas between valleys Hollows unchanneled valleys Hillslope Geomorphology Fluvial Geomorphology Channels concentrated transport of sediment and water within D hillslope hollow defined banks Floodplains relatively at channel oodplain land formed by a river in the 39 39 present climate and inundated every one or two years E88 426 112 Spring 2006 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 quotalluviumquot 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 39 bounded by bedrock or concrete 39 deeply incised into hillslope deposits 39 choked by relatively immovably objects such as large boulders 39 rimmed with thick and deeply rooted bank vegetation Alluvial quotselffor39medquot 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 E55 426 113 Spring 2006 SOME OTHER USEFUL BASIC DEFINITIONS FLOODPLAINS AND TERRACES To a geomorphologist a oodplain 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 oodplain conversely a channel formed within a true floodplain is by de nition 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 Abandoned oodplain HillSlOpe or terrace Floodplain Channel Valley at gt 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 lOyear floodplain or the lOOyear floodplain There is no requirement that this area of inundation correspond to ESS 426 114 Spring 2006 any discernible feature on the landscape although there are some useful correlations that we will explore later for alluvial channels The oodplain may be absent or may not correlate with the valley bottom Where 39 channels have quotinheritedquot a valley geometry from some other geologic process such as glaciation in the recent past or 39 channels have undergone a signi cant change in hydrologic regime as a result of climate change diversion or watershed disturbance that leads to incision or entrenchment of the channel E55 426 115 Spring 2006 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 downvalley distance lies between 15 and 2 Where this ratio called the sinuosity is less than 13 the channel is not termed quotmeanderingquot but instead is quotsinuousquot or quotstraightquot Alf l Meander wavelengi h A P Sinuosi l Sal curVaJhJ quot6 u k ltbank ull law 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 point bar formation and a meandering pattern E88 426 116 Spring 2006 Braided rivers are identi ed 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 logj ams can respectively retard lateral channel migration and split the flow into multiple channels E55 426 117 Spring 2006 THE BANKFULL CHANNEL The quotsize of a shaiiiiaiquot only has meaning for alluvial shaiiiias we typically call the feature so measured the bankfull channel The surface at the top of the bankfull shaiiiiai is the oodplain which is inundated whenever the iivai or stream experiences a bankfull ow The most reliable ways of identifying the bankfull shaiiiiai is to determine the elevation of the currently active oodplain Recognition of active oodplains discriminating oodplains 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 quotvalley atquot or prominent surface on the valley oor 2 The elevation of the active oodplain which is the surface of frequent inundation by oods 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 widthtodepth 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 ow changes most abruptly In general multiple determinations at multiple sites is the most reliable approach E55 426 118 Spring 2006 CHANNEL CROSS SECTION 1 30 g 20 quotquotquotquotquotquotquotquotquotquotquotquot quotquotquotquotquotquotquotquotquotquotquot quotI39I39I39I39I zI39Is39I I IcJI L39Iquot39siII IzaILLW quotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquot quot g 10 LEVEL 39 m m 00 I I 0 10 20 30 40 DISTANCE FROM LEFT BANK DATUM ft 30 DETERMINING BANKFULIL DEPTH 2 E m 25 m E4 E 20 39 MINIMUM c WD93 I AT DEPTH215ft o Equot I5 E S I 10 5 I I I I I I 4 I I l I I 00 05 10 15 20 25 30 HEIGHT ABOVE BED ft 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 E88 426 1 19 Spring 2006 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 Q15 and Q2 ie the discharge with a recurrence interval which l probability of annual exceedence of 15 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 PERCENT OF TOTAL STATIONS E88 426 40 30 20 From WILLIAMS BANKFULL DISCHARGE quotFIB dim u 20 RECURRENCE INTERVAL IN YEARS PARTIAL DURATION SERIES 025 05 1 2 5 10 50 200 r T r I T F I I ACTIVE FLOODPLAIN L 1 I 1 11 I d 101 105 125 2 5 10 50 200 RECURRENCE INTERVAL IN YEARS ANNUAL MAXIMUM SERIES Frequcncy distribution of recurrence intervals for bankfull ow 120 Spring 2006 THE HYDRAULIC GEOMETRY OF CHANNELS In its most common de nition the hydraulic geometry refers to the way in which a channel39s width depth and velocity change with changes in discharge Although we might acknowledge that other parameters of a channel39s form can also vary such as slope roughness or degree of meandering these three parameters have the singular property that Q rwd39M and so a change in Q must be fully re ected 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 ow at a single gauging location as discharge changes during the passage of a ood This type of change is measured by the at a station hydraulic geometry E55 426 121 Spring 2006 Rmsniom had nwhc ameh mmumwmb If we LIA3 0 9 4 Downsh em M 4 MI is seomch men urw b 6f siIeg 12t3 W E88 426 1 22 Spring 2006 By convention the hydraulic geometry relationships are written with the following symbols waQb dcQ 11ka Multiplying these three equations together fwd alclk2bfmy And because Q wd39u Q aflle m and so ack 1 and bfm1 E55 426 123 Spring 2006
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'