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GEOG 361 Sedimentary & Ecological Flows: Process, Form and Management Lecture 3: Alluvial Fans.

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Presentation on theme: "GEOG 361 Sedimentary & Ecological Flows: Process, Form and Management Lecture 3: Alluvial Fans."— Presentation transcript:

1 GEOG 361 Sedimentary & Ecological Flows: Process, Form and Management Lecture 3: Alluvial Fans

2 Definition Fan-shaped deposits of sediment Due to change in –gradient –lateral constraint –resistant forces Natural or human

3 Importance Scientific importance Socio-economic importance

4 Importance

5 Structure fan-shaped concave ~symmetric Merge bajadas Fine distally Channel incision Hetergeneous Scale invariant

6 Factors Affecting Structure External factors Affect fan: Size, Slope, Hydraulics, Sediment Transport efficiency determines fan slope angle: –drainage basin area –average annual rainfall –sediment supply Incised channels steeper than fan slope Young fans steeper than old fans

7 Questions Why are alluvial fan depositional areas, which are potentially so hazardous, also so attractive for human habitation? What are the key factors that determine alluvial fan morphology and stratigraphy and how do they each control the fan characteristics?

8 Flow Processes Formation by: –stream flow –debris flow Flooding not predictable –location unpredictable –may change rapidlly (avulsion) Flow spatial structure: –sheetflow at apex –then channelized flow –channels only active over small part of fan

9 Deposition Processes Debris flow deposit creates long, thin topographic high Similar process for channelised streamflows: constant deposition elevates bed

10 Deposition Processes Laboratory experiments: –braided channels grow headward –distal depositional lobes –deposition migrates up-fan, back-filling channels –cycle at range of scales –frequent avulsion Unstable at geomorphic & engineering time scales

11 Channel Avulsion Filling of channel until overtopped Maybe –Nodal or random –Local or regional –Full or partial Occurs at engineering timescales c.f. New Orleans

12 Growth

13 Long Timescale Processes Over long timescales: –Aggrade and prograde at decreasing rate –Accumulate in topograhic lows –Zones of subsidence At largest scales: –Driven by tectonics –Uplift causes formation –Fans ultimately subducted

14 Questions Explain how channel avulsion produces alluvial fans which are much larger than the flow features which create them, and why fans formed from streamflow and debris flow are so similar in structure Are alluvial fans stable or unstable features over engineering and geomorphic timescales, and with what external factors are they tending towards equilibrium?

15 Hydrology Triggering requires rainfall intensity-duration thresholds to be exceeded

16 Hydrology Fans often act as aquifers

17 Hydrology Groundwater drainage modelling

18 Dating Rock varnish microlamination (VML) Dating Rock varnish: –coating on exposed rock –thickness ~100 µm –Well-preserved in arid regions Microlaminations: –two types of layers: form micro-stratigraphy Carries climate record –yellow layers = dry periods –black layers = wet periods

19 VML Dating Fan in Death Valley Units identified by: –fan morphology –VML Age estimates: – yr BP Deposition during wet periods

20 VML Dating

21 Modelling Alluvial fan evolution difficult to study in the field Modelling crucial Random-walk models Diffusion models Laboratory models

22 Questions What is the key hydrological role of alluvial fans? How does VML dating allow the stratigraphy of alluvial fans to be interpreted? What is the most effective way of modelling alluvial fans?

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