EPS 50: Lecture 24 Rivers and Streams
Stream flow and transport
Stream morphology Streams, rivers and human intervention
Water on Planet Earth
Rivers & lakes only 0.009 % of water Streams are most important agent of landscape modification and erosion in most environments Worldwide, streams carry ~16 billion tons of sediment and ~3 billion tons of dissolved matter each year Pre-human transport might have been only ~50% of this
Studying Rivers and Streams
Fresh water supply Transportation Agriculture Renewable, clean energy resource Production of fertile floodplain soil Flood hazard to communities
The Waters of California
CA stream flows:
(1) Sierra Nevada (plate tectonics) (2) W-E winds (the hydrologic cycle)
CAs most valuable and contested resource 150 years of damming, diverting and polluting Most water goes to Central Valley agriculture
EPS 50: Lecture 24 Rivers and Streams
Stream flow and transport
Stream morphology Streams, rivers and human intervention
How Stream Waters Flow
Depends on flow velocity, geometry (depth), viscosity (fluid dynamics) The viscosity (low) and velocity (high) of stream water usually results in turbulent flow
Smooth sheet-like flow at a low velocity, streamlines are parallel Usually confined to edges and top of stream Irregular swirling flow Occurs at most rates of stream flow Keeps particles in suspension
Laminar flow
Turbulent flow
Stream Discharge (Q)
Five important conditions:
Channel width Channel depth Velocity Gradient Bed roughness
Q = v x A (depth x width)
30 m3/s 180 m3 /s
water cross section x
velocity
Discharge (m3/s) = width (m) depth (m) velocity (m/s)
Stream Flow Continuity
Given constant discharge
Reduction in area ==> faster flow Changes in slope and roughness influence velocity and must be compensated by change in area
A balance of driving and resisting forces determines the nature of river flow :
Q=vA
Driving force = water weight x sin(bed slope) Resisting force = river-bed area x river-bed shear stress
Stream Sediment Transport
Increased Flow Velocity Increased suspended sediment Increased bed load transport Increased saltation, rolling and sliding
suspended load
saltation
bed load
Particle Size vs. Current Velocity
Cohesion of clay and silt particles
Bedforms Associated with Velocity
structure produced by a migrating nearshore bar, Pliocene terrace deposits, Monterey Bay, California
www.usgs.gov
ripples
ripples on dunes
Stream Erosion
Physical weathering, abrasion, plucking & pot holes Potholes form by pebbles and gravel grinding inside eddies Waterfall undercutting and headward erosion Chemical and biological factors also contribute to erosion
Missoula Flood(s) - 15,000 years ago at close of ice age
~100 individual floods perhaps 1 every 50 to 150 years evidence: scour, huge-scale sediment deposits, wave-cut platforms
Work and graphics by David Finlayson, Univ. of Washington
When the ice dam broke, it sent ~800 meter wall of water racing at 100 kmph. Ripple marks are 15-20 meters in scale.
Floating ice dams and subsequent flooding has now been observed in Greenland, Alaska and Himalayas (albeit at smaller scale).
Catastrophism vs. Uniformitarianism Harlan Bretz first proposed features were due to catastrophic flooding, initially met with skepticism.
EPS 50: Lecture 24 Rivers and Streams
Stream flow and transport
Stream morphology Streams, rivers and human intervention
The shape of a river
Rivers form channels, valleys and floodplains The shape of river flow varies from straight to sinuous and meandering to braided The longitudinal profile of a stream represents an attempt to achieve grade or equilibrium (reduce and distribute work in natural system) Adjustments to profile, channel cross section and channel patterns result from changes in sediment supply, discharge and slope
Stream Longitudinal Profile
All streams, large and small show the same concave-up profile Result of balance of erosion (incision) and deposition Base level controls the elevation of the longitudinal profile
Erosion-dominant
Deposition-dominant
Base Level
Longitudinal Profile and Capacity
A B C
Knickpoints
Sudden breaks in slope due to faulting, lithology, tributaries, dams, etc. Rapids develop at knickpoints High gradient and high stream power downstream => Erosion Low gradient and low competence upstream => Deposition Headward migration results
Mount, 1995
10
Dams and Stream Longitudinal Profiles
Dams result in deposition upstream & erosion downstream
The stream deposits sediment in the upper part of the reservoir
The sedimentdepleted stream begins to erode downstream of the dam
Stream Meanders and Floodplains
Erosion on the cutbank Deposition on point bar Meander Neck Meander Cutoff Oxbow Lake
11
Why Meander?
Lateral migration of streams creates river flood plain Sinuosity reflects balance of energy efficiency and distribution as a function of load, gradient and discharge Local disturbances in flow resistance encourage meandering
Point Bar
Meandering channel
Braided Streams
Some streams have multiple channels with numerous sand bars and repeatedly diverging and joining channels forming an interlacing network
12
Why Braid?
Attempt to dissipate excess energy Related to steep gradients, highly variable water discharge, abundant coarse load, and easily eroded bank material
Channel Form
Transition between straight, meander and braided streams is complex threshold function of discharge, slope and sediment load Large rivers and low slopes tend to form floodplain morphology in broad valleys
Mount, 1995
13
Streams and Tectonics
Antecedent streams Longitudinal profile adjusts to tectonics Terraces are uplifted and deformed by folding
Streams and Pre-existing Structure
Downcutting can cause a stream to be superimposed on a pre-existing structure Left-behind terraces follow longitudinal profile
Delaware water gap
14
Alluvial Fans
Form at mountain fronts Widening from narrow stream valley to broad valley Loss of competence and capacity Often related to tectonic uplift
Drainage Networks
15
Major American River Systems
Drainage divides bound drainage basins
EPS 50: Lecture 24 Rivers and Streams
Stream flow and transport
Stream morphology Streams, rivers and human intervention
16
Floods!
Historically, many major cities are built in floodplains US flood cost is ~$1.5 billion/yr Our attempts to harness rivers are only partly successful
Building a Floodplain, One Flood at a Time
Low Natural Levee Overbank flow results in the flooding of the floodplain
Decreased flow velocity results in deposition of suspended sediment
1996 Liuzhou, China
17
Discharge Changes and Floods
Stream flow fluctuations can be immense Hydrologists often characterize discharge as X-year floods The Eel River had a discharge of ~752,000 cfs on Dec. 23, 1964 (> 1993 Mississippi flood)
Skytomish River, Washington
1993 Mississippi Flood
1992 1993
18
1993 Mississippi Flood
500 - year flood (90-day volume) Dense levee system confined stream flow and led to sharp increases in peak flow and catastrophic flooding ~500 levee breaks allowed for discharge on flood plains which reduced impact on major cities downstream Total estimated damage 16 billion 48 people killed
The Problem With Levees
Levees protect from flooding on regular basis BUT flow constriction during floods causes water to flow faster and deeper increasing stream power Flooding occurs upstream (backup) and downstream during biggest floods
19
The Problem With Levees
Levees protect from flooding on regular basis BUT flow constriction during floods causes faster and deeper flow, increasing stream power Flooding occurs upstream (backup) and downstream during biggest floods
Flood enhancement through ood control: Criss & Shock, Geology, 2001 Flood stages for constant discharge
have increased 2-4 m over the past century, mostly attributable to channelization
Build more levees ?
Mitigation Strategy
FEMA Land buyouts & levee strengthening and building European model make more room for the river Now counterbalanced by massive construction in flood plain: Since 1993 28,000 homes, 26% increase in population, 26.8 sq km (6630 acres) of new commercial and industrial development totaling 2.2 billion dollars.
20
