1. Fluvial Geomorphology Environmental Hydrology Lecture 20

2. Fluvial Geomorphology Study of the role of running water in shaping the landscape

3. Key concepts in fluvial geomorphology Characteristics of rivers as function of landscape position (geography) How rivers accomplish “work” How rivers respond to change

4. Geomorphic Work Running water exerts force   Landscape offers resistance

5. Geomorphic Work Lane’s Balance, Ward & Trimble, Fig 9.20 Q S  Q s d Where: Q = dischargeQs = sediment discharge S = sloped = median sediment size Q*S = stream power

6. When does geomorphic work occur? Applied stress Rate of movement Frequency of occurrence frequency x rate ~ “work” after Wolman, M. G. and J. P. Miller, 1960. Magnitude and Frequency of Forces in Geomorphic Processes. Journal of Geology, 68(1): 54-74.

7. When does geomorphic work occur? see Ward & Trimble, Fig 6.14

8. “Effective” width, depth

9. Geomorphic work expressed in channel characteristics Winooski Falls, Photo by Jim Westphalen Channel dimensions (cross-section) Channel profile (longitudinal view) Channel pattern (plan view)

10. Channel dimensions Geometry or cross-sectional shape of the channel Width, depth Bankfull width, depth (i.e. “effective”) Floodplain Terrace (abandoned floodplain) see also Ward & Trimble, Fig 6.13

11. Helical flow creates variations in shear stress across the cross section

12. Channel dimensions see also Ward & Trimble, Fig 6.13

13. What happens if force-resistance balance is disturbed?

14. Channel evolution Pre-DevelopmentDowncutting Widening Restored Floodplain Graphic from A. Ward after Schumm

15. Channel Profile Elevation Distance downstream  Longitudinal relationship between elevation and distance downstream (slope) Uniform rate of work Minimum rate of work

16. Channel Profile Ward & Trimble, Fig 6.8

17. Channel Profile Ward & Trimble, Fig 6.27 (from Montgomery & Buffington, 1997. GSA Bulletin, 109, 596-611.) “Colluvial Channels” “Alluvial Channels”

18. Channel Pattern Meandering StreamBraided Stream Plan (aerial) view of channel geometry Ward & Trimble, Fig 6.10

19. Channel Pattern Sinuosity (S) = L c / L v where L c = channel length L v = valley length

20. Helical flow induces meander formation Cut banks

21. Meander Geometry Ward & Trimble, Fig 6.11

22. How do these concepts scale? Graphic from A. Ward

23. How do these concepts scale? Ward & Trimble, Fig 6.15 “Hydraulic Geometry” of stream channels