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GEOMORPHOLOGY EXAM #2
Streamflow
- Running water is the most important
surface process in shaping the Earth.
- Other processes may be more
important locally.
Runoff
Overland flow, Horton Overland Flow, Sheetflow
o Laminar flow over surface
- thin layer
- very little erosive power
- gentle slope (<5%)
‘RILLS’
o Gullies (generally 1-2 ft. deep)
-Not prominent features
(Rills)
‘STREAMS’
o Semi-Permanent features
o Discharge (Q)
-Q = VA
-Q = wdv
(Stream)
Factors of Runoff
o Basin Characteristics
-Vegetation
-Soil Characteristics
-Infiltration rate
o Channel Charteristics
-Gradient
-% or angle
-length of slope
-Width
-Depth
o Precipitation
-Intensity
-Duration
-Antecedent precipitation (prior wetting condition)
Channel Geometry
- Cross sectional area
- Wetted perimeter
- Relationship of (W)Width, (D)Depth,
(V)Velocity to (Q)Discharge
Streamflow
o Discharge (Q) – the volume of water
moving past a given point in a certain
amount of time.
Ft.3 / S or M
3 / S
o ‘Thalweg’ – where stream flows
fastest (top, middle of stream)
(Thalweg)
Measurement of Stream Discharge (Q)
WIDTH
Varies: Hourly, daily, seasonally, and along the stream.
All measurements of Q should be made together
Measure perpendicular to flow & banks
Measure with fiberglass or steel-tape in small streams,
For large streams use electronic or surveying instruments, or map.
The tape will slag/stretch when measured long distances.
DEPTH
Varies with Discharge (Q), and from bank to bank across the stream channel.
Even considerably in a short distance.
Deeper on the outside bends of stream channel.
Critical measurement of Q
Avoid obstructions, abutments as they distort flow
Small Streams = Use a graduated pole
VELOCITY
Varies from top-to-bottom, and side to side
Also along the course of the channel, varies as f(Q, and A)
Float method (simple but accurate)
Bobber, Stick, Rubber Ducky (0.85 * “surface” V)
Current Meter = very accurate
Price Meter (measure 0.6 of depth, to get average for that segment)
Tracers
Dyes or salts
THE MANNING EQUATION
V = velocity (ft/s , m/s)
n = Manning roughness coefficient (unit-less)
k = 1.49 in English units, 1.0 in SI units
R = hydraulic radius (ft , m)
s = slope of channel (ft/ft , m/m)
Laminar & Turbulent Flow
Laminar Flow – streamline flow, slow moving water.
Turbulent Flow – faster and velocity changes.
Competence
The ‘Maximum Size’ of sediment that the stream is moving, and whether these clasts are likely
to be entrained at the velocity corresponding to bankfull discharge.
Steps;
Measure the intermediate diameter of the 10 largest gravel clasts that you can find at the channel
cross-section. The critical velocity required to initiate motion of these clasts was established by
‘Costa’ to be:
Dmax = mean intermediate axis diameter in m/m of the five largest clasts.
Boundary Layer – edge of every flow, where flow speed decreases due to friction.
Bed Shear Stress – corresponding to the forces that tend to roll particles along the bed
and the pressure difference above and below the grain in which tend to lift them off the
bed called Hydraulic Lift.
Hydraulic Shear
- Hydrodynamically Smooth Flow
Velocities may be very low and may
Be laminar, even if turbulent above.
- Transitional Flow
Protuberances project through laminar flow.
- Fully Rough Flow
No laminar layer exists
Reynolds Number
Dimensionless ratio of driving to resisting forces. p = Density
d = Depth
Rn = pdv / μ v = Velocity
μ = Molecular Viscosity
Froude Number
Resistance of an object moving through water.
Streaming F < 1
Critical F = 1
Shooting(Rapid) F > 1
6th
Power Law (Rubey)
Shows that a small increase in velocity produces a very large increase in size of a particle that
can be moved.
r = Radius of the particle
k = Constant, Gravity, Density
v = Flow Velocity
Load – material that a stream is moving. Dependent upon topographic relief, Lithologies
in the basin, Climate, and vegetation processes acting in the basin.
Dissolved Load – Typically 50 -100 parts per million, Chemical Weathering, Carries
greatest quantity at ½ total load, ‘not’ related to Velocity.
Suspended Load – to remain in suspension a particle’s settling velocity must be less than
the turbulent velocity of the stream.
‘STOKES LAW’ – particle size is most important.
Bed / Tractive Load – granular material supported by the bed.
Traction – rolling & sliding
Saltation – bouncing
Hjulstroms Diagram Recurrence Interval
Flood - temporary overflow of a river Hydrograph
onto adjacent lands not normally covered
by water.
1993 Flood (Upper Mississippi, Lower Missouri, and Illinois River)
o 144 Days of Flooding!
Four Principle Reasons for the Flood
o Higher than normal precipitation for the first half of 1993
o Precipitation could not be accommodated by local streams,
storms dumped large volumes of rainfall.
o Ground was saturated, because of cooler conditions in 1992
o River systems have been altered by humans, draining
wetlands that slow water (80% since 1940).
Big Thompson River Flood (Loveland, Colorado)
o 10 – 12 inches of rain fell in 1 day!
o 8 inches fell in 2 hours!
o Killed 139 people
o $35.5 million in damage
Reasons for the Flood
o Steep walls of canyon allows easy runoff
o Weak winds (20 mph @ 10,000 feet) allowed
storm to not move along
o Unstable air continued to rise, condensing
water vapor, making a huge storm cloud
Paleofloods – the study of sediment size
- ‘Slackwater Sediments’ – sediments deposited
during a flood.
o Silt & fine Sand
o Deposited in areas sheltered from main flow velocities.
-Tributary Mouths
-Downstream from sudden channel widening
-Overbank deposits on high terraces
-Caves in bedrock along channel
Channel Patterns Straight (uncommon)
Meandering
Corkscrew motion
Shifting rivers along channel bottoms
Point Bars (deposited)
Cut Banks (eroded)
Deep & narrow
Causes = turbulent flow reflects water to the side of channel, which
then reflects to the other side resulting in ‘Helical Flow’.
Wavelength
(W = KQx ) or (L = 106 Qm
0.46) or (L = 1890 Qm
0.34/ M
0.94)
Sinuosity Index – main stream length / Valley length (larger the # = more meandering)
Braided
Steep & Shallow down the river’s profile
Total width = greater, but Individual Channel’s width = lower
Channel position changes
Total # of channels are greater than others and vary
‘ERODABLE BANKS’ – non-cohesive sand
Covered in vegetation
Wide & Shallow, higher hydraulic shear stress (greater erosion)
Abundant load & rapid variations in Q (discharge)
Helps sustain pattern
High Sed Load
Divisions of main trunk into several branches
‘Braid Bars’ – islands in a braided stream
Chezy Equation
Shows gradients in ‘BRAIDED STREAMS’ are higher
Shape of channel (R) affects flow velocity (R = hydraulic radius)
To transport load (Q), it must maintain a constant (V) velocity
If (R) changes, (S) must change to maintain (C)
Degradation & Aggradation
DEGRADATION
Hydraulic action on bank (drag & lift)
Pothole drilling (abrasion & eddies)
Battering of wetted perimeter (coarse material, cobbles, boulders)
AGGRADATION
‘Long Term’ deposition of sediments in a channel
Increase in load
Decrease in (V) velocity
The Drainage Basin
The total area of land which contributes water
to a drainage network of stream channels.
Has a common outlet at some point
Also called ‘Watershed’ = U.S.
‘Catchment’ = European
Boundary is called a ‘Divide’
‘Drainage Divide’
‘Watershed’ = European
Form
Can be quantified
Reflects hydrologic behavior & characteristics
Area, Slope, Shape, Stream Network
Topo maps & aerial photos used to acquire information
Largest stream is at the mouth
Systems for Channel Hierarchy
STRAHLER METHOD
Basin order & Channel order
Doesn’t take into consideration lower order streams along its coarse that influence Q (discharge)
SHREVE METHOD (Modified Strahler)
Divided network into separate links
Allows magnitude of links to reflect # of first order streams feeding it
Simpler & more widely used
Linear Morphometic Relationships
Bifurcation Ratio Rb = No / No + 1
Length Ration RL = Lo / Lo + 1
Drainage Density D = ∑ L / A
Relief Ratio Rh = H / Lo
Low Density – coarse texture class < 8
Medium – medium texture 8 – 24
High – fine (texture) 24 – 200
Badlands – ultrafine > 200
Drainage Patterns Dendritic – most common, branching pattern, uniform conditions.
Parallel – streams parallel to another, steep gradient, similar to dendritic.
Trellis – folded/tilted sedimentary rock formations, rock units in the way.
Rectangular – controlled by joint patterns, rock fractures at 90o angles.
Radial – deriving from a common point, usually a mountain, hill, volcano.
Annular – circular 90o angles, domes & basins.
Multibasinal – swampy areas, lakes, very deranged (no order), from glaciation.
Contorted – highly metamorphic areas, drainages that follows ridges & valleys.
Alluvial Fans
Profile is generally concaves up
Can be a series of straight segments
Head / Apex = front of Alluvial Fan
‘Dis-Tributaries’- streams of the Alluvial Fan
‘Bajada’ – coalesced fans (Joining Alluvial Fans)
Fan Size
f( drainage basin area, geology, climate, avail space)
Bajada
Delta
Top-Set Beds – (Mud, freshest layer)
Foreset Beds – (Sands)
Bottom-Set Beds – (Fine Clay & Muds)
3 TYPES of DELTAS
Floodplains
Topographically flat surface occupying much of a valley bottom & typically underlain by
unconsolidated sediments.
Produces beds / ‘Lamina’ (very thin sediment layers)
Reoccurrence Interval = 1.5 years on most perennial streams
Coarse grains @ the bottom, Fine grains @ the top
Development;
Lateral migration
Cut bank
Point bar deposition
Overbank flooding
Result of vertical accretion
Fine grained material
Sediments;
‘Channel Lag’ – coarse gravel, small clasts winnowed out.
‘Channel Fill’ – poorly sorted mix of sand, silt, clay, and gravel.
‘Crevasse Splay’– any material spread out on a flood plain though breaks in Natural Levees.
Meander Scrolls –begins as a point bar during bankfull discharge. Vegetation establishes
on new ridges, stream undercuts banks, lateral migration ensues.
Stream Capture
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