Fluvial Geomorphology and Its use in River

Stabilization – Part 1

Instructor:

David T. Williams, Ph.D., P.E.David T. Williams and Associates, Engineers

david@dtwassoc.com

What is Fluvial Geomorphology?

Fluvial – Fluvius = River

Geo – of or relating to earth, ground or soil

Morphology – Study of form and structure

Geomorphology - the study of landforms, the processes that created them, and the history of their development.

Fluvial geomorphology - the examination of the processes that operate in river systems and the landforms which they create or have created.

Fluvial Geomorphology

Numerous Time Scales

Geologic Paleo Historic Engineering (50 yrs) Event Time to Retirement

Things to Remember

A River is part of a System:

System is:

• Dynamic• Complex• Thresholds Do Exist• Geomorphology Provides Historical Perspective• Size / Power of Stream is Important

A River is Part of System

The entire watershed forms a system Rainfall => Runoff (Land Use) Runoff Transports Sediment (Land

Use/Cover) Slope / Meanders Impact Transport Sediment Size Impacts Transport and

Erosion/Deposition Manmade Features Impact Everything Nature has its own plans and goals!!!

• It’s not always what we expect!

River Sizing/Shape Relationships

Lane Leopold & Wolmann Schumm Kennedy Regime Theory

Channel Forming Discharge Rosgen/Thorne Lots of Uncertainty and Discussion

Fluvial Geomorphology

Streams are part of watershed Changes that impact streams:

Changes in land use Changes in hydrology

• Reservoirs, M&I Outfalls, Irrigation

Diversions into/out of watershed Timing of delivery – M&I outfalls

• Hydropower / Detention Basins

Flood Control Features

Lanes Equation

Sediment Load x Bed Sediment Size (D50)is proportional to

Stream Discharge x Stream Slope

Qs D50 α Q S

Lanes Balance

Watershed Location

Upper Watershed – Erosion Produces sediment

Middle Watershed – Transportation Transports most of sediment produced

Lower Watershed – Deposition Delivers sediment to sink

Ocean, lake, delta, broad plain

Watershed Location

Sediment Transport

IT’S NOT THAT SIMPLE

Every river/stream reach has erosion, transport and deposition

occurring at the same time!

Transport Reach

Point or mid-channel bars are always changing

Outer banks erode

Inner banks deposit

Sediment moves from outer bank to inner banks and bars

Meanders move down valley

What is Stable?

Absolute Stability – Doesn’t Change in Engineering Time Scale

Think Concrete / Bed Rock / Etc.

Dynamic Stability – Changes but Relationships are Constant in Engineering Time Scale

Unstable – Major Changes in Width, Depth, Flow, Sediment Transport, Sinuosity, Planform or all of These are Occurring over a Period of Days, Months or Years

Absolutely Stable

Absolutely Stable

Little Cottonwood Creek, Salt Lake City, UT

Absolutely Stable

Who says concrete channels aren’t green?

Dynamically Stable

Unstable

WES Stream Invest.

Unstable?

Stability

Rivers want to be Dynamically Stable Not Necessarily Constant / The Same

• Bank Location• Meander Locations• Sand / Gravel Bars• Anything having to do with location of features

Remember Locations Move under Dynamic Stability!

StabilitySome Streams are more Stable

Clay Bed & Banks

Rock Outcrops or Banks• Mountain Streams / Torrents• Steep Sediment Starved Systems

Concrete / Designed Channels

Stability

Less Stable Systems

Silt Bed & Banks Sand or Gravel Banks Any Non-cohesive / Uncemented Banks Braided Systems

• High Sediment Loads Fill Flow Areas• Channels Constantly Moving

Channels without Bedrock Controls

Stability Depends on Perspective

How fast does an unstable river move?

How long does change take to be classed stable?

If a river moves at 10’ per year it’s probably stable until it gets to 50’ of your house!

Stream Stabilization

Rivers are Mobile

Hard Points• Protect Permanently (Engineering Time)• Concrete• Riprap

Soft Points• Protect for a While then Fail (Melt Away)• Bio-Engineering

System Instability Features Headcuts

channel bottom erosion progressing upstream indicating a readjustment of slope, discharge, and sediment.

Knick Point location in the thalweg where there is an abrupt

change in elevation.

Incised Channel Channel that is not hydraulically connected with

its floodplain due to erosion.

Causes of System Instability

Upstream Causes Changes in discharge and sediment supply due

to dams or diversions. Downstream Causes

Base level lowering due to cutoffs or channelization.

Basin wide Causes Land use change such as urbanization that

alters discharge and sediment. Complex/ Multiple Causes

System Instability Causes(Profile Adjustment)

AGGRADATION Upstream increase in

sediment (construction) Downstream rise in the

base level (sea level rise, deposition in dams)

Basin-wide increase in sediment yield (soil erosion)

DEGRADATION Upstream reduction in

sediment load (dams) Downstream increase

in stream power (base level change)

Basin-wide reduction in sediment yield (soil conservation)

Local Instability

Definition: Localized instability such as bank erosion that is not caused due to systemic dis-equilibrium in the watershed, but results from site-specific factors or processes. (example, eroding outer bank in a meandering channel)

Note: Stream bank erosion may also be a symptom of system instability.

Causes of Local InstabilityStream Bank Erosion

Parallel Flow Impinging Flow Piping Freeze/Thaw (tension cracks) Sheet Erosion Rilling/Gullying Wind Waves Others

Types of Local InstabilityStream Bank Failure

Rotational Slip (Slumps) Shallow Slides Piping Failure Pop-out Failure Block Failure (Slab Failure) Soil/Rock Fall Wet Earth Flow Others

Hard Points in Mobile Rivers

What is your Purpose Restoring River (allows adjustment) Fixing One Bank in Place Fixing River Reach in Place

• Does it Matter if Concrete or Bio-engineering? Fixed in Place = Fixed in Place!

Be Careful Who You Criticize!• Biologists & Single Purpose Refuges• Engineers & Single Purpose Projects• Stream Restoration & Fixed Designs

Channel Evolution Model

What Happens to Channels Over Time?

(It’s a “Natural” Process!)

Simon’s Modification of Schumm’s Model (courtesy, G.

Athanasakes, Stantec)

Stage I

Stage II

Stage III

Stage IV

WES Stream Invest.

Stage V

WES Stream Invest.

Stage V

Coalville, Utah

River Mechanics Branch of fluvial geomorphology that

quantifies the relationship between process and form in rivers.

Channel Characteristics and Definitions Hydrologic Classification Channel Pattern/Planform Channel Geometry (cross-section) Channel Profile (slope)

Channel Process-Form Relationships Channel Classification

Hydrologic Classification

Arid Zone Flow Occupancy: 0-50 percent Ephemeral Streams Intermittent Streams

Humid Zone Flow Occupancy: 50-100 percent Intermittent Streams Perennial Streams

Channel Geometry Pools and Riffles (Crossings)

Riffle-Pool channels (sand and gravel) Step-Pool channels (boulders and cobbles,

steep slopes > 3%-5%)

Cross-Section Shape

Channel Bars Point bars Alternate bars Mid-channel bars

Terrace

Bankfull Depth

2nd Terrace 1st Terrace

Geomorphic Floodplain

Bankfull Width

Natural Channel Geometry (courtesy, G. Athanasakes, Stantec)

Channel Profile (Slope)

Product of discharge (Q) and slope (S) is defined as STREAM POWER.

Stream power is the ability of the channel to do work.

Channel slope is defined as the water surface slope or the stream bed slope.

Channel Process-Form Relationships

W = Ca x QaD = Cb x QbV = Cc x Qc

Qs = Cd x Qd

where:

W = width, Q = water discharge, D = mean depth,

V = mean velocity, Qs = suspended sediment load

Ca, Cb, Cc, Cd, a, b, c, d are numerical constants

Copeland Method for Sand BedsLess than 50% cover on banks

Copeland Method for Sand BedsMore than 50% cover on banks

Copeland Method

Copeland Range of Solutions