Drainage Basin Geomorphology

Transcription

1 Drainage Basin Geomorphology Fundamentals & Basin Morphometry Dr Steve Darby Source:

2 The Fluvial System: Components Sources, transfers, sinks (Schumm, 1977) Independent variables Climate, geology, land cover/use, tectonics, etc Dependent variables See this lecture Source: Schumm, S.A The Fluvial System, Wiley, New York

3 The Fluvial System: Functions Terrestrial and aquatic ecosystems Transports matter from sources to sinks Water Solids (sediment, organic matter, carbon) Solutes (mineral and carbon solutes) Nutrients (N, P, etc) Mont Mine Glacier, Switzerland Steve Darby LANDSAT S (~8S, 143E) LANDSAT Image from: NASA John Stennis Space Center

4 Global Sediment Yield 70% of the total ( Mt a -1 ) load comes from 10% of the land 3 rivers (Ganges, Brahmaputra & Huang He) carry 20% of the load Source: Skinner, B. J., and Porter, S. C. (1995). The Dynamic Earth, An Introduction to Physical Geology (3rd Edition), New York, John Wiley and Sons.

5 Global Denudation Rates Calculated by estimating total sediment/solute load and dividing by basin area to give average lowering rate in mm ka -1 Global mean is ~ 8 mm ka -1 Masks huge variations amongst the world s large drainage basins Chari Basin, interior Africa (3 mm ka -1 ) Brahmaputra Basin, eastern Himalayas (677 mm ka -1 ) Scale dependent Haast River, Southern Alps, NZ (4,717 mm ka -1 ) Huangfuchan, China (19,814 mm ka -1 )

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7 The Scientific Method Observation INDUCE Formulate Theory (Hypothesis/Model) DEDUCE Make Prediction REVISE THEORY Observation Test Theory (Hypothesis/Model) PASS New Observations? Robust Theory/Model/Hypothesis

8 Drainage Basins Area of land that contributes water to a stream/river Also known as catchments (UK) or watersheds (US)

9 Drainage Basin Components Boundaries Watersheds or divides Landscape elements Hillslopes Channels Channel heads Valleys Interfluves Materials Bedrock Regolith Colluvium vs. alluvium HyMap airborne imaging spectrometer data (19 June 2000) draped over DEM based on digitised OS 1: map contours for portion of Highland Water Research Catchment, New Forest, UK

10 Drainage Basin Components Drainage divide/watershed Topographical Hydrological Military Interfluve A region of higher land separating two rivers in the same drainage basin Source: Figure 14.4 from Christopherson, R.W Geosystems: An Introduction to Physical Geography (5 th Edition), Prentice Hall, New Jersey.

11 Drainage Basin Components Hillslopes area of land between drainage divide and a channel or valley Channel a passage for water to flow through Valley elongated lowland between mountains, hills, or other uplands, usually with a well developed drainage network Rocky Mountain NP, Colorado Photo courtesy of USGS photo library

12 Where do channels begin? Valley side gullies, Rif Mountains (N Morocco) Frans Kwaad 2002

13 Channel Heads Abrupt channel head South of Lake Balaton, Hungary Diffuse channel head Rambla Mofar, near Turre, Almeria Professor Mike Kirkby Professor Mike Kirkby

14 Hillslope Morphology Hillslope morphology is complex and multi-dimensional, so many parameters can be used as descriptors steepness, length, shape (i.e. rounded), degree of linearity, concavity or convexity (in plan and/or in profile), etc. Hillslopes are usually mantled in sediment (colluvium) Sediment synonyms: Soil, regolith The significance of all these metrics is that water and gravity mediated sediment transfer is strongly controlled by slope and the way in which slope varies across the landscape

15 Slope Profiles Basic hillslope profile shapes linear, convex, concave all have average slope of 0.1 Elevation (m) Linear Hillslope Profile Distance Across (m) Convex Hillslope Profile Concave Hillslope Profile Elevation (m) Elevation (m) Horizontal Distance (m) Distance Across (m)

16 3D Slope Morphology Hillslopes are 3-dimensional Slope curvature controls the flow of surface and subsurface water across a landscape areas with concave plan curvatures have focused flows, whereas convex plan curvatures have dispersed flows areas with convex profile curvature have accelerated flows, concave profile curvature have decelerating flows L = linear, V = convex, C = concave Source:

17 NOTE CONVEX PLAN CURVATURE NOTE RUNOFF CONCENTRATION IN CONCAVE CURVATURE Hillslope Morphology: Examples HIGH RELIEF, STEEP GRADIENT, LINEAR IN PROFILE Badlands NM, South Dakota LOW RELIEF, LOW GRADIENT, CONCAVE PLAN CURVATURE River Severn Catchment, UK Deborah Tappan Philip Owens

18 Drainage Basin Morphometry Morphometry The quantitative measurement of form Provides a systematic basis for description and comparison (e.g. between basins, model predictions vs. observations) Topology versus topography Topological variables represent network structure Implies that two basins may look utterly different but be functionally similar if their topology is similar Dimensional versus non-dimensional variables All things can be measured with 3 scales: M, L, T Dimensional: area (L 2 ), velocity (L/T), density (M/L 3 ) Non-dimensional: slope (L/L) Non-dimensional variables can be compared across basins of different scale

19 Morphometric Variables Basic descriptions of basin shape & network structure Drainage patterns and network characteristics Basin size and shape (e.g. elongation ratio) Drainage density Topographic characteristics Analysis of slopes and curvature Basic mapping and DEM cartography Basin elevation Hypsometry Relief (local and total) Ruggedness

20 Drainage Patterns Redrawn from Summerfield (1991, Fig , p406); itself modified from Morisawa (1985, Fig. 10.3)

21 Drainage Patterns Used to infer underlying geological/other controls TYPE STRUCTURAL CONTROL Dendritic Lack of structural control; rock/sediment of uniform resistance Parallel Closely spaced faults; steep topography; non-cohesive sediments Radial Volcanic cones, domes Trellis Tilted or folded alternately resistant/weak sedimentary units Rectangular Joints or faults Annular Eroded dome in alternate resistant/weak sediments Centripetal Calderas, craters, tectonic basins Deranged Glaciated terrain Source: M. Summerfield (1991, Table 16.2, p406)

22 Image from: NASA John Stennis Space Center Drainage Patterns: Examples Allegheny Plateau, E West Virginia DENDRITIC DRAINAGE LANDSAT ~ 38N, 80W Valley & Ridge province, NW Virginia TRELLIS DRAINAGE N (~ 15N, 120E) Mount Pinatubo, Phillipines RADIAL DRAINAGE Image from:

23 Image from: NASA John Stennis Space Center Drainage Patterns: Examples LANDSAT Val D Herens, Switzerland PARALLEL DRAINAGE N (~ 44N, 74W) N New York State, USA DERANGED DRAINAGE

24 Network Structure: Stream Order Scale-independent system for comparing streams A stream with no tributaries is 1 st order Strahler (1952): A stream of order N forms at the confluence of two streams of order (N -1) Shreve: N is the sum of the stream order of the tributaries Source: Summerfield (1991, Figure 9.1, p208)

25 Some Data The largest river is the 12 th order Amazon Assuming 1 st order streams are identified correctly! Over 80% of the total length of Earth s rivers are 1 st or 2 nd order Basin area increases non-linearly with increasing stream order Bifurcation Ratio (R b ) R b = N x /N x+1 Strahler says R is typically ~ 3 Hence N 1 > for the Amazon (N 1 = 3 x-1 ) Source areas make up in numbers what they lack in size Sulphur Creek, CA (2 nd Order) Wheeling Jesuit University Steve Darby Wheeling Creek, Ohio (2 nd Vs. 3 rd Order Basins)

26 Drainage Basin Shape Basin shape influences the fluxes and delivery of runoff and sediment from headwater reaches Can be represented using the Elongation Ratio (E) E = (2/L) (A/π) 0.5 E is dimensionless Source: Redrawn from Benda, L. et al Water Resources Research, 40, W05402, doi: /2003wr

27 Significance Runoff delivery to streams Elongated basins have flatter hydrographs Influences confluence geomorphology (Benda et al., 2004) Tributaries of elongated basins have less impact on main stem geomorphology Source: Redrawn from Benda et al

28 Drainage Density Extent to which a landscape is channelled reflects the balance between erosive and resisting forces So, likely influenced by climatic and geologic factors Ratio of total stream length to drainage area (ΣSL/A) DD (has dimensions L -1 ) varies across a wide range e.g. 5 km -1 on permeable rocks, to 500 km -1 on badlands Can be problematic to define where channels are indistinct or ephemeral Locating channel heads is critical to accurate estimation

29 Examples N (~ 40N, 100W) LANDSAT Middle Loup River, Nebraska SEMI-ARID - Low Drainage Density LANDSAT N (~ 5N, 70W) Upper Orinoco Basin, Venezuela HUMID - High Drainage Density Images from: NASA John Stennis Space Center

30 Physical Significance (?) Drainage density values can sometimes surprise Perhaps best interpreted as a long-term measure of the relative difference between rainfall and infiltration Nazca, Peru (~15S, 75W) Steve Darby

31 Drainage Basin Topography Elevation is the vertical height above a datum Relief (R) refers to differences in elevation at local (e.g. for a hill) or total basin (difference in elevation between the drainage divide and the basin outlet) scales LOW RELIEF HIGH RELIEF ~ 50m ~ 1000m Martin Geertsema HyMap airborne imaging spectrometer data (19 June 2000) draped over DEM based on digitised OS 1: map contours for portion of Highland Water Research Catchment, New Forest, UK

32 Drainage Basin Ruggedness Measured with the ruggedness number RN = R DD Measures the extent to which basin topography is dissected DEM of the headwaters of the Smith River in Oregon an example of a rugged basin with high relief

33 Smith River, Oregon Drainage Basin Topography Elevation is spatially distributed so representing the topography of a drainage basin with a single parameter is hard

34 DEM visualisation and analysis DEMs can be used for visualisation and to extract variables As accurately as the base data allow Image Source: USGS Image Source:

35 Terrain Analysis with GIS ELEVATION SLOPE ASPECT PROFILE CURVATURE TANGENTIAL (PLAN) CURVATURE FLOWLINES Source: Mitasova, H. and Mitas, L Terrain Analysis and Erosion Modelling. Accessed online at on 12/5/05

36 Drainage Basin Hypsometry Provides a means of summarising the distribution of elevations in a landscape Source: Redrawn from Summerfield (1991, Fig. 9.5, p211), which is itself based on Strahler (1952) Bull. Geol. Soc. Amer. 63, Figs 1 & 2.

37 Examples & Significance Provides a basis for comparing basin form Distinct hypsometries are associated with different processes Fluvial (concave) Tectonic (linear/convex) Glaciated (shoulder) Stage of evolution? HI implies age (Strahler, 1952) Source: Montgomery et al Geology, 29(7),

38 Why does Basin Form Matter? Relief influences steepness, which controls the energy available for driving forces (runoff, gravity) Basin form influences the connectivity between landscape units and hence the potential to store sediment Implies form-process interaction Rapaälven River, Sweden Hans Andren Sebaskachu River, Labrador Norm Catto

39 Conclusion Morphometric analysis is needed to provide a systematic basis for describing & comparing drainage basin geomorphology & to test hypotheses Morphometry can highlight links between basin forms and environmental variables, but it provides no direct insight into formative processes Care is required: Underlying data are derived from maps, DEMs, etc. Relationships depend on the basic quality of acquired data Relationships can be scale dependent

40 Urumchi, NW China Matthias Jakob