Geolog Geology y 103 Lecture #11
Channe Channell configu configurat ration ion,, Braided Braided strea streams ms th Reading: Boggs, 5 edition, pp !15"!!0
- Last time: We talked about alluvial fans - Today: We will look at bar forms and one type of fluvial deposits: braided streams - Next time: we will compare braided and meandering streams, discuss anastomosing streams $ Channel characteristics of braided vs. meandering streams:
treams are classified !named" based on c#annel form, sinuosity and braiding $" %#annel form: - %#annels are straig#t, braided or meandering: meandering: %ee &igure 3'! from Reinec( and %ingh, p !5)
- #ows map-view examples of eac# c#annel type &" inuosity: - 's defined as ratio of c#annel lengt# to down-valley distance *ra+ picture on oard
- inuosity is low !().*": produces straig#t or braided streams streams #ig# !+).*": produces meandering meandering or anastomosing anastomosing streams
%" &raiding parameter - efers to number of bar forms meander wavelengt# %ee &igure 3'1 from Reinec( and %ingh, 1)-.
- enerally: - &raided streams #ave + ) bar form per meander - /eandering streams #ave ( ) bar form per meander - We will only deal wit# meandering and braided streams in detail - #ould be aware t#at combinations of bedforms and c#annel forms exist !alt#oug# t#ey are less common" %ee /ale !! from Reinec( and %ingh, p !5) $ Braided streams:
$" eneral c#aracteristics:
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- inuosity ( ).* - %#annel divides and re0oins fre1uently - + ) bar per meander - 2low may be less consistent !more episodic" - $lso called a 3bedload4 stream: #as a #ig# percentage of material transported as bedload rat#er t#an suspended load !coarse grain si5es: gravel to sand, minor clay" %ee &igure 1 from iall 1)'-$ &lu2ial sedimentology, p .00 %ee &igure -10a from Boggs, th edition, p !55 http:44+++uoregonedu4millerm4raidedhtml
- 6ig# gradient %ee &igure 3'0 from Reinec( and %ingh 1)-.$ *epositional %edimentary 6n2ironments, p !5-
- Longitudinal bars !see below" %ee &igure http:44+++physicalgeographynet4fundamentals4107html
- andy, non-co#esive bank material - Little floodplain material !c#annel deposits dominate" %ee &igure '"11 from *a2is 1))!$ *epositional systems, p !1-
- 2luvial arc#itecture: channel deposits are preserved in t#e rock or sediment record %ee &igure -11 from Boggs, th edition, p !5.
&" %ommon &ar forms: - 7 common bar forms: Longitudinal, linguoidtransverse, lateral %ee &ig 10' from Boggs, 3 rd edition
- T#is is a bit more complex t#an t#e general c#aracteristics listed above - Longitudinal bar forms: - $re oriented parallel to flow - 2ine upward t#roug# t#e bar and fine downstream !8 directions" - 'nternal structure: massive or crudely bedded http:44facultyggu+yoedu4heller4%ed8!0%trat8!0Class4%edstrat4slidesho+99'htm
- Transverse bars: - 9riented transverse !at an angle" to t#e current
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- sually build out from t#e side - Lateral bars: - 2orm in lower energy conditions - &uild out from t#e bank !attac#ed to bank" - ;aleocurrents often diverge !flow around bars" %ee &igure 10) from Boggs, p 31! 3 rd edition
<" ummary: - eneral pattern: - Leave large, s#eet-like coarse deposits - are, discontinous clay drapes -
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