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