**Course Title: Floodplain Management**

**Module 2:** Stream Systems on Dynamic Earth

**Session 6**: Stream Mechanics Framework -- Evolution and Flow Analysis

**Author**: Dr. Donald R. Reichmuth, P.E. President, Geomax, P.C.
Spokane, WA

(E-mail -- <Geomax@comcast.net>)

**Time 75 minutes**

# 

# Objectives: (PP1-2, Title and Objectives)

At the end of this session, the students will be able to:

6.1 Summarize how floodplains first evolved after glacial retreat 15,000
years ago.

6.2 Explain the basic concepts of sediment transport.

6.3 Show how stream patterns develop in bedrock areas and in alluvial
valleys.

6.4 Summarize how streams are shaped by peak flows, stream gradient and
sediment supply.

6.5 Illustrate how gravity systems transport water and sediment.

6.6 Describe secondary currents and stream features that are formed by
their action.

6.7 Describe how in-channel obstructions cause random energy
dissipation.

6.8 Discuss stream mechanics within the context case study floodplains

**Scope:**

Floodplain management requires knowledge of where we have been, before
reasonable and sustainable decisions can be made on how we should
proceed in correcting past errors and minimizing future conflicts that
impact our irreplaceable riparian areas. This session traces drainage
evolution since the last ice age and provides the physical basis for
determining how stream characteristics are modified when flow, energy,
and sediment inputs change. Stream flow will be analyzed from three
levels (flow downhill, secondary current flow, and random eddy flow) to
explain the physical channel patterns that exist.

**Readings:**

***Student and Instructor Reading:***

Knighton, David. 1998. *Fluvial Forms & Processes, A New Perspective*.
Oxford Univ. Press Inc., Chapter 6 \-- Channel Changes Through Time.
Pages 260-335.

Lee, Douglas. August 1983. "The Land of the River," *National Geographic
Magazine*, pages 226-252.This article provides insight into changes that
are occurring on the Mississippi River Delta.

***Instructor Reading:***

Knighton, David. 1998. *Fluvial Forms & Processes, A New Perspective*.
Oxford Univ. Press Inc.\
This book provides an excellent list of references concerning stream
behavior.

**General Requirements:**

The students should start to apply the concepts learned in this session
to their individual Case Studies. The instructor should spend a short
time in class checking student progress with their Case Studies and
answering questions that they may have.

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**Objective 6.1** **Summarize how floodplains first evolved after
glacial retreat 15,000 years ago. (PP3, Post Glacial Retreat)**

**Requirements:**

The information in this section is presented as a lecture using Power
Point slides:

**Remarks:**

Major stream system responses occurred when the last glacial period
ended.

I.  **Glacially impacted streams reorganized their drainage networks.**

    A.  Often preglacial valleys were filled with glacially derived
        sediment.

    B.  Many streams were forced to join sections of preglacial valleys
        together.

        1.  Occasionally this required new channel sections to be cut
            across drainage divides.

            a.  Erosion was rapid in these sections.

            b.  Often these new valley sections were over-steepened and
                relatively unstable.

        2.  Groundwater is still able to flow through the preexisting
            valleys.

        3.  The Ohio River is an example of such a reorganized system.
            **(PP4, Ohio River)**

    C.  Sometimes when channels were being excavated in the glacial
        fill, they would become trapped on the bedrock areas outside of
        the original valleys.

        1.  When this occurred, it was common for a waterfall to
            develop.

        2.  The stream energy is concentrated at these locations as the
            streams cut their way through the bedrock.

            a.  The stream is attempting to reach its preglacial grade.

            b.  The waterfalls are short-lived (geologically speaking)
                surface features.

            c.  Once the bedrock is excavated, the stream will become
                stable at its preglacial grade.

        3.  Niagara Falls is an example of this phenomenon.

II. **Restoration of the preglacial stream gradients is the long-term
    trend.** **(PP5, Long Term Regional Trends)**

    A.  Higher sections of many catchments are now experiencing
        degradation.

        1.  This often happens at elevations above 400 feet msl (present
            mean sea level).

        2.  The excess glacial fill is being removed. **(PP6, Channel
            Evolution)**

        3.  When smaller side valleys are present, excavation is usually
            delayed until the master stream has reached grade.

            a.  These valleys often contain lakes.

            b.  The Finger Lakes in northwestern New York are an
                example. **(PP7, Blocked Valleys Example)**

            c.  The lakes near Coeur d'Alene, ID are another example.

    B.  In lower elevation sections (approximately 20 to 400 feet msl)
        valley filling dominates.

        1.  Sea level raised about 400 feet in the last 15,000 years to
            reach the present mean sea level (msl).

        2.  This filling has caused the absolute base level to rise.

        3.  Below 20 feet elevation, tectonic activity controls steam
            processes.

    C.  Climate and land use changes since the last glaciation have
        accelerated stream responses.

        1.  Proboscidians (mammoths and mastodons) were keystone species
            that significantly altered the postglacial landscape.
            **(PP8, Post Glacial Conditions)**

            a.  Their large size and strength allowed them to modify the
                vegetative cover **(PP9, Paleo-Floodplain Development)**

            b.  Modern elephants in Africa currently serve the same
                function.

        2.  Paleo-Indian hunters eliminated many species 11,000 years
            ago. **(PP10, Paleo-Indian Hunters)**

        3.  Beaver stabilized many drainages after the ice retreated.

            a.  Giant Beaver (Castoroides ohioensis) were the initial
                beaver species to actively modify postglacial streams.
                **(PP11, Giant Beaver)**

                1.  They weighed over 200 kg. (450 lbs.) and were 2.5
                    meters (8 feet) long.

                2.  They ranged widely across the eastern U.S. **(PP12
                    Giant Beaver Fossil Sites)**

                3.  They became extinct about 10,000 years ago.

                4.  Climate change and hunting pressure probably ended
                    their ability to survive.

            b.  Modern beaver continued stabilizing river basins after
                the Giant Beaver went extinct. **(PP13, Pre-Columbus
                Floodplain Development)**

                1.  Their historic range covered most of North America.
                    **(PP14, Historic Beaver Range)**

                2.  They numbered in the millions before Europeans
                    arrived.

                3.  Fur trapping after European arrival decimated their
                    populations.

        4.  The elimination of archaic Megafauna; modern beaver and
            human activities have had serious impacts on stream
            stability.

        5.  The postglacial, beaver-stabilized drainage contains
            numerous ponds and wetlands. **(PP15, Post Glacial Model)
            (PP16, Beaver Stabilized Drainage)**

            a.  The beaver dams allowed very little sediment transport
                to occur.

            b.  When beaver are removed from drainages, vertical and
                horizontal erosion begins. **(PP17, Beaver Lost)**

            c.  Stream gradients and water tables lower. **(PP18, Lost
                Base Flow Storage)**

            d.  Streams develop high flood peaks and lower base flow.

        6.  Beaver introduction (or reintroduction) into a drainage
            basin usually causes significant change. **(PP19, Effects Of
            Beaver Introduction)**

            a.  For example, in 1946, 25 pairs of Canadian beaver were
                introduced into the Tierra del Fuego region at the
                southern tip of South America.

                1.  There are no predators to control population growth.

                2.  There are now over 100,000 beaver in the area.

                3.  Wetlands are being created and native trees
                    destroyed.

                4.  Without some type of population control the system
                    in not sustainable.

**Supplemental Considerations:**

The instructor should discuss local attitudes and impacts caused by
beaver and provide some examples of how nearby streams are affected.

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**Objective 6.2 Explain the basic concepts of sediment transport.**

**Requirements:**

The information in this section is presented as a lecture using Power
Point slides:

**Remarks:**

I.  The erosion products (sediment) transported through by a river
    system have many forms. (PP20, Basic Concepts)

    A.  Rock material that is completely dissolved during erosion is
        called the dissolved load.

        1.  This material is carried in solution.

        2.  Calcium and magnesium become ionized and are transported in
            this fashion. These ions produce "hard water".

    B.  Fine material that is carried in suspension is called suspended
        load.

        1.  Clay minerals are so fine that they rarely precipitate in
            fresh water.

            a.  Upon reaching salt water, the mineral grains flocculate
                into larger aggregations and settle to the ocean floor.

            b.  These materials form shale deposits.

        2.  Silt and sand sized particles are mostly concentrated near
            the streambed.

            a.  During peak flow conditions this material can travel
                significant distances without touching the streambed.

            b.  When flow volumes decrease, these materials often
                deposit on the bed in quieter locations.

            c.  At low flow, silt and sand often can be seen moving
                along the streambed as dunes.

        3.  Coarser fractions (gravel, cobbles and boulders) move along
            the streambed.

            a.  Movement normally occurs only during peak flow periods.

            b.  Individual particles often move only short distances
                each time they are mobilized. (Movement is
                "steady-by-jerks")

            c.  It is common for the largest available particles to coat
                the streambed.

                1.  This coating layer is call the "Bed Armor"

                2.  Often the bed armor is only one rock thick.

                3.  The bed armor slows bed scour.

                4.  The bed armor normally becomes sealed which limits
                    water loss to the underlying valley fill.

II. Portions of stream cross-section carry different sediment loads.

    A.  Upper portions of the cross-section carry cleaner water, called
        top water.

    B.  The lower parts of the cross-section carry much of the suspended
        load and all of the bed load, called bottom water.

    C.  When top water and bottom water are split by some in-stream
        structure, the resulting two downstream flows will have
        significantly different characteristics.

        1.  The cleaner top water will become more erosive.

        2.  The dirtier bottom water will tend to cause deposition to
            occur within the channel because there is not sufficient
            flow to transport the mobilized sediment load.

**Supplemental Considerations:**

Presenting a local example of where Topwater and Bottomwater are
separated by an in-stream structure should spark student interest.

**Objective 6.3 Show how stream patterns develop in bedrock areas and in
alluvial valleys**. **(PP21, Stream Patterns)**

**Requirements:**

The information in this section is presented as a lecture using Power
Point slides:

**Remarks:**

The detailed study of stream patterns to determine underlying geologic
conditions is a common tool used in Geomorphology, but is beyond the
scope of this course. The discussion will be limited to explaining the
difference that occurs between bedrock and alluvial valley reaches.

I.  **The steam patterns that develop on bedrock areas are primarily
    controlled by the underlying geologic structure.**

    A.  When tectonic uplift in an area occurs, the available potential
        energy to erode the surface increases.

    B.  Streams, in drainages that are being uplifted, slowly etch their
        way into the bedrock as weathering occurs so that a stable
        gradient is preserved.

    C.  Over time, this etching process often causes streams to shift
        locations to weaker bedrock areas.

        1.  Weaker areas include: faults, joints, and softer rock
            formations.

        2.  Folding sedimentary beds causes another class of weakness.

            a.  Anticlines (upward projecting folds) cause tension and
                cracking to occur along the anticlinal axis that creates
                a weak path.

            b.  Synclines (downward projecting folds) cause compression
                to develop along the synclinal axis. This compression
                causes the rock to be more erosion resistant.

            c.  After long periods of weathering the surface topography
                will develop highs on the synclinal axis and valleys
                along the anticlinal axis.

        3.  The underlying weaknesses can have serious ramifications
            when structures such as dams are placed in river valleys.

    D.  The drainage pattern that develops generally exploits the
        underlying weaknesses which helps define the basic rock
        structure.

    E.  Drainage pattern names such as shown on slide **(PP21, Stream
        Patterns)** help define the underlying structure.

II. **Different processes control streams that flow on alluvial fill.**

    A.  Braiding occurs when the stream is depositional.

    B.  When a stream has a stable grade and is able to transport the
        supplied sediment load, it will tend to meander.

    C.  Delta patterns always occur when base level is a body of water.

    D.  Often a transitional pattern of double channels occurs between
        braided and meandering reaches.

III. **The lower Mississippi River Valley is an example of how steam
     patterns can change over time.**

     A.  The lower valley consists of: **(PP22, Lower Mississippi
         River)**

         1.  The Mississippi River floodplain.

         2.  The Mississippi River Delta.

         3.  The Atchafalaya Basin.

     B.  At the end of the last ice age, 16.000 years ago, the
         Mississippi River was located in a bedrock trench (canyon).
         Base level was the Gulf of Mexico, which at that time was 400
         feet lower than current sea level. **(PP23, Mississippi Valley
         Cross-Section (16K))**

     C.  By 9,000 years ago, base level had raised to minus 100 feet
         below current sea level. **(PP24 Mississippi Valley
         Cross-Section (9K))**

         1.  The Mississippi River had a braided pattern as it deposited
             sand and gravel to fill its trench.

         2.  The River moved back and forth across its floodplain as the
             trench was filled.

     D.  By 4,000 years ago, the Gulf of Mexico had reached it current
         elevation and the base level of the Mississippi River
         stabilized. **(PP25, Mississippi Valley Cross-Section (4K))**

         1.  The River became mostly meandering because:

             a.  The sediment load had significantly diminished.

             b.  Base level was stable.

             c.  The River was able to deposit its fine-grained silt
                 load in backswamp areas during frequent over bank
                 floods.

         2.  The Lower Mississippi River Valley continued to **aggrade**
             slowly from the silt deposits that occurred.

     E.  By 3,000 years ago, the **Delta** had become fully developed.
         **(PP26 Mississippi River Delta Evolution)**

         1.  As deltaic lobes pushed out into the Gulf of Mexico, the
             gradient of the Mississippi River became lower.

         2.  At intervals (3000, 1700, 1000, 900, and 500 years before
             present, BP), the Mississippi jumped to a new location to
             shorten the distance to the Gulf. These jumps increased
             river grade so more energy was available to transport the
             silt load.

     F.  Over the last 150 years, the lower valley has been massively
         altered by human activity in attempts at stopping **overbank**
         flooding and **backswamp** silt deposition.

         1.  **Levees** have been constructed.

         2.  Channels have been dredged throughout the Delta

         3.  Small drainages in the Atchafalaya Basin have been damned.

         4.  Control structures have been built on the Mississippi
             River.

     G.  The Mississippi River is currently attempting to reach the Gulf
         by utilizing the Atchafalaya River channel.

         1.  This path is considerably shorter than the current path.

         2.  Control structures have been built near Angola, Mississippi
             in an attempt to stop this change.

         3.  The flood of 1992 almost resulted in this change taking
             place.

         4.  Currently cleaner top water is diverted to the Atchafalaya
             River during high water while the bottom water is kept in
             the Mississippi River.

             a.  Top water tends to scour

             b.  Bottom water tends to deposit its suspended load.

             c.  These actions increase the likelihood that a channel
                 change will occur.

     H.  It is unrealistic to believe that a process that has been
         active for 16,000 years can be stopped by human intervention
         for more than a short period.

         1.  Floodplain managers must realize that a channel change will
             occur.

         2.  Plans should now be made to minimize the resulting
             multibillion-dollar losses. (It will probably be the
             biggest loss suffered in the U.S.)

**Supplemental Considerations:**

The instructor should relate the potential loss that will occur when the
Mississippi River changes course with a local disaster that the students
understand.

**Objective 6.4 Summarize how streams are shaped by peak flows, stream
gradient and sediment supply.**

**Requirements:**

The information in this section is presented as a lecture using Power
Point slides:

**Remarks:**

I.  Stream patterns are diagnostic of stream behavior. (PP27 Stream
    Characteristics)

    A.  There are four primary stream patterns as defined by vertical
        behavior. (Note: These patterns form a continuum of behavior
        rather than four separate and distinct patterns).

        1.  Erosional (Degrading)

            a.  Structurally controlled usually by bedrock.

            b.  Confined to narrow "V" shaped valleys.

            c.  Show only minor shifting.

            d.  May have "Stair-Step" profiles.

        2.  Stable (No vertical change evident).

            a.  Meandering pattern

            b.  Flat-bottomed valleys

            c.  Slowly shifting meander loops

            d.  Frequent overbank flow during peak flows.

        3.  Transitional (Slight filling)

            a.  Double Channels with one dominant.

            b.  Found in short reaches between meandering and braided
                sections.

            c.  Stable, vegetated islands that remain for long periods.

            d.  Frequent overbank flow during peak flows.

        4.  Depositional (Extensive filling)

            a.  Braided Pattern

            b.  Flat bottomed valleys

            c.  Shallow, unstable channels.

            d.  Islands change often during high water.

    B.  Stream patterns change when controlling parameters change.

        1.  Peak Flow

            a.  Stream patterns normally only change during peak flow
                events.

            b.  When peak flows are increased, the stream will become
                more erosive and the stream pattern will shift toward
                the more erosive pattern.

        2.  Gradient

            a.  The stream gradient determines the amount of Potential
                Energy that can be converted to Kinetic Energy in a unit
                length of channel.

            b.  An increase in gradient allows the stream to do more
                work including mobilizing and transporting sediment.

            c.  When gradients are increased, the stream will become
                more erosive and the stream pattern will shift toward
                the more erosive pattern.

        3.  Sediment Supply

            a.  Most sediment produced by weathering is moved by streams
                (Note: Glaciers, Landslides, Wind and similar processes
                can also move sediment).

            b.  When the amount of available sediment is decreased, the
                stream will become more erosive and the stream pattern
                will shift toward the more erosive pattern.

        4.  Note: When peak flow decreases, gradient decreases and/or
            sediment load increases, the changes will be toward the
            Depositional pattern.

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I.  Detailed behavior of Erosional Streams includes:

    A.  The streams normally are downcutting through bedrock. (PP28,
        Erosional Streams)

        1.  Valleys are narrow and steep sided.

        2.  Valleys usually are "V" shaped.

    B.  As downcutting progresses, underlying weaknesses in the bedrock
        are often exploited. (PP29, Erosional Streams \-- Structure)

        1.  Joints, faults, and variations in rock strength can control
            stream alignment.

        2.  When the channel follows linear weaknesses, an angular
            stream pattern develops.

    C.  In reaches of steep valley gradient, a Stair-Step Profile
        normally develops. (PP30, Stair-Step Profile)

        1.  The steps can be formed by large rock clusters, uprooted
            trees, hard bedrock layers, or any other debris that can
            form an in-stream dam.

        2.  Excess energy is dissipated when water plunges over these
            structures.

        3.  The stream gradient that develops between the steps (drops)
            will be flatter than the overall valley gradient.

            a.  The flatter gradient will be stable for the given site
                conditions.

            b.  Often small sediment (gravel) deposits form above each
                step (barrier).

            c.  These gravel deposits contribute significantly toward
                improving the reaches' fish habitat.

II. Detailed behavior of meandering streams include: (PP31, Meandering
    Streams)

    A.  Truly meandering streams are always found on alluvial valley
        fill.

        1.  Overbank flow occurs frequently (typically every two or
            three years).

            a.  Natural levees form along the channel banks when the
                coarser fraction of the suspended sediment load can no
                longer be transported and deposits.

            b.  Slack water deposits of fine-grained soil form away from
                the river during these events.

            c.  These slack water deposits form highly productive
                agricultural areas.

            d.  Most early agriculturally based civilizations began in
                these type areas.

        2.  Occasionally, if an area is being tectonically uplifted, a
            meandering stream can cut through its alluvial fill and get
            trapped on bedrock with its meandering pattern still intact.

            a.  These channels should be considered erosive.

            b.  Overbank flow is considerably less frequent.

    B.  The channels show long term (100 years plus) vertical stability.

    C.  Normally the meander loops systematically shift downstream.

        1.  They tend to erode the outside of their bends

        2.  They deposit bars on the inside of their bends.

        3.  This action forms a Meander Belt that often includes the
            entire valley floor.

        4.  The rate of movement is moderated by bank vegetation and bed
            armor.

    D.  Channel size is determined by the interaction between bank
        vegetation and peak flows.

        1.  During summer low flow periods, bank vegetation tends to
            expand into the active stream channel.

        2.  During peak flow, some of this encroaching vegetation is
            washed away so that a functional channel remains.

        3.  The location on the bank where a balance is struck between
            these competing forces is called the Ordinary High Water
            Mark.

        4.  Normally a meandering stream will become unstable if the
            stabilizing vegetation is removed from the bank area.

            a.  The stream will eat away the unprotected bank.

            b.  The channel will widen and often become shallower.

            c.  The mobilized bank material also can cause secondary
                depositional problems downstream.

    E.  The channel bed is normally protected by bed armor.

        1.  The bed armor consists of the largest available particles
            that are mobilized by the stream during peak flow events.

        2.  Often these large particles form a continuous, interlocking
            layer only one rock thick over the bed.

        3.  Because these large particles are the most difficult
            (largest & heaviest) to mobilize, the underlying finer,
            mixed fraction is to some degree protected.

        4.  The bed armor also becomes a seal on the stream bed that
            limits water loss to the underlying less consolidated
            alluvium. This improves surface flow particularly during
            drought period.

        5.  In valleys that have been glaciated, the stream may be
            armored with glacially derived particles.

            a.  Usually the glacially derived material is much larger
                than what can be moved solely by flowing water.

            b.  These lag deposits become immovable in the channel.

    F.  The sediment that moves into the alluvial valley from side
        drainages or valley slopes is carried downstream by the
        meandering stream at a rate that causes the valley to neither
        neither fill nor degrade. A dynamic equilibrium is maintained.

III. Detailed behavior of transitional streams includes: (PP32,
     Transitional Streams)

     A.  Transitional streams normally occur in short sections between
         meandering and depositional streams.

     B.  Usually they show slight depositional tendencies.

     C.  They are characterized by relatively stable double channels.

         1.  At any given time, one channel will be dominant.

         2.  Stable, vegetated islands occupy the areas between the
             channels.

         3.  Modest gravel plugs block the head if each section of the
             non-dominant flow path.

     D.  During peak flow, the water is split between the two available
         flow paths.

         1.  Most of the dirtier bottom water is confined to the
             dominant channel.

         2.  The cleaner top water passes over the gravel plugs and
             flows through the non-dominate channel.

         3.  The dominant channel tends to become depositional while the
             non-dominant channel becomes more erosive because it is
             carrying cleaner water.

         4.  Over time, the dominant channel becomes filled and the
             non-dominant channel is cleaned and enlarged. Frequently,
             head cuts can be seen excavating the non-dominant channel.

     E.  Channel dominance will abruptly switch at some point during a
         peak flow event.

         1.  The switch is normally triggered when the gravel plugs are
             destroyed by the headcuts moving up through the
             non-dominant channel.

         2.  When the switch occurs, gravel plugs will immediately form
             in the old dominant channel.

         3.  At this point, the cycle begins all over again.

     F.  Dominance switching can have a cycle that typically is from 20
         to 40 years.

IV. Detailed behavior of Depositional Streams include: (PP33, Common
    Causes For Deposition)

    A.  Depositional systems are characterized by braided channels.

    B.  Depositional Streams always occur in alluvial valleys.

    C.  Channels normally are shallow and wide.

    D.  There are three common causes for deposition to occur.

        1.  When peak flows in a stream are reduced.

            a.  Removal of top water from a stream can have the most
                pronounced effect.

        2.  When the stream gradient is reduced.

            a.  This typically occurs when base level is raised.

            b.  Construction of storage reservoirs is a common cause.

        3.  When the sediment supply is increased.

            a.  Glaciation almost always increased sediment supply.

            b.  Landslides can increase the local sediment supply.

            c.  Changes in land cover often mobilize more sediment.

    E.  There are two other depositional patterns that can create
        special problems.

        1.  Alluvial fans have unique behavior. (PP34, Other
            Depositional Patterns \-- Fans)

            a.  Channels across fans are not well defined.

            b.  Land use patterns on alluvial fans often create
                "waste-lands".

                1.  In the arid West, water is often diverted at the fan
                    apex for irrigation. (PP 35, Typical Fan
                    Utilization)

                2.  The stream channels often become dewatered.

                3.  The original channels then become "Flood Channels"
                    and only carry water during peak flow conditions.

                4.  Because the "Flood Channel" are unstable, they are
                    often treated as waste land and have been used for
                    such things as dumps or landfill sites.

            c.  Flows normally are quite shallow but can spread
                laterally.

            d.  During peak flow events, it is possible for water to
                reach any part of the fan.

            e.  There are really no flood proof areas on an alluvial
                fan.

            f.  Alluvial fans on side drainages tend to force main stem
                channels to the far side of the main valley.

                1.  The main stream can be pinched between the fan and
                    the far valley wall.

                2.  Flood levels on the main stem upstream of the fan
                    can be increased as the pinching occurs.

                3.  A steeper gradient develops in the pinched section
                    and rapids can form.

                4.  The pinching can become particularly severe when
                    dams on the main stem reduce peak flows.

            g.  The following alluvial fan case study on Teton Creek in
                eastern Idaho illustrates some of the problems that can
                be encountered.

                1.  Case Study shows the critical geologic controls that
                    occur. (PP 36, Alluvial Fan)

                2.  Teton Creek Schematic Diagram illustrates the
                    various depositional environments that occur on the
                    fan. (PP 37, Teton Creek Schematic Diagram) Also the
                    physical of the Topographic Apex and the
                    Hydrographic Apex are shown and defined.

                3.  The currently active portion of the alluvial fan
                    starts at the Hydrographic Apex and terminates at
                    the Spring Line.

                4.  The Standard FEMA Model has been used to define a
                    100-Year Floodplain and Floodway but this model is
                    not appropriate for the currently active portion of
                    the fan. ( PP28, Standard FEMA Model).

                5.  The proper model for the currently active portion of
                    the fan should be the Alluvial Fan Floodplain Model.
                    (PP39, Alluvial Fan Floodplain Model).

                6.  The creek channel in the reach between the
                    Topographic Apex and the Hydrographic Apex has
                    incised through glacial deposits and flows in one
                    dominate channel. (PP 40, Fan Apex Area Details) The
                    Standard FEMA Model should be used in this section.
                    Once the channel leaves the incised section near the
                    ID-WY border, the active alluvial fan begins. This
                    location is the Hydrographic Apex. Below this point
                    the Alluvial Fan Floodplain Model should be used.

                7.  Typical Flood Channel shows one of many flood
                    channels that are located on the active alluvial
                    fan. (PP 41, Typical Flood Channel) The channels are
                    shallow and usually vegetated but any upstream
                    blockage of the current dominant channels could
                    cause some of the temporarily abandoned channels to
                    become active again. Clearly the Standard FEMA Model
                    is not satisfactory in the type area.

            h.  Alluvial fans pose unique flood hazards because the flow
                paths are unpredictable and the water depths during
                major flooding events are usually not great. (PP42,
                Unique Alluvial Fan Flood Hazards).

            i.  Model Comparison illustrates the relative Flood Hazard
                for two points: one near the currently active channel
                and the other remote from the active channel. PP 43,
                Model Comparison). The Riverine Flood Hazard (where the
                Standard FEMA Model applies) is much greater for the
                point near the currently active channel. The Alluvial
                Fan Flood Hazard (where the Alluvial Fan Floodplain
                Model applies) is the same for both points.

        2.  Lateral Replacement occurs when a stream migrates laterally.
            (PP45, Other Depositional Patterns -- Lateral Replacement)

            a.  It is common for coarser material to be deposited on the
                floodplain as finer material is eroded from the
                embankment that is being attacked.

            b.  The embankment that is being attacked can produce
                significantly more material than is being deposited when
                sediment from the eroding bank is considerably finer
                than the deposited material. This occurs because it is
                much easier to transport finer material.

            c.  Often, this process is driven by:

                1.  Gently dipping sedimentary bedrock that contains
                    shale formations.

                2.  Regional tectonic tilting especially when there is
                    an uplift component.

                3.  A mobilized, upstream supply of coarse bed load.

            d.  Infrastructure located between the stream and the
                eroding formation is especially vulnerable because the
                stream has a strong tendency to shift laterally.

**Supplemental Considerations:**

The instructor should identify some local site-specific examples where a
stream has changed its pattern and explain to cause for the change.

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**Objective 6.5 Illustrate how gravity systems transport water and
sediment.**

**Requirements:**

The information in this section is presented as a lecture using Power
Point slides:

**Remarks:**

Stream flow patterns are more easily analyzed when the flow components
are broken into one of three levels of behavior. These levels are:

> 1\. **Flow Downhill**
>
> 2\. **Secondary Currents**
>
> 3\. **Random** **Eddies**

This section will concentrate only on the first level -- **Flow
Downhill**. **(PP46 Stream Flow Analysis)**

I.  **Gravity causes objects (including water) to fall toward the center
    of the earth's mass. (PP47 Gravity Systems)**

    A.  The height of the object above some reference point (datum)
        determines that object's **potential energy**.

    B.  If the object is allowed to free fall (fall without retarding
        forces acting to restrain the object, i.e. falling in a vacuum
        for example) then the **potential energy** is totally converted
        to **kinetic energy.** (Free fall is often referred to as
        **frictionless** motion).

        1.  **Kinetic energy** is determined by the object's velocity.

        2.  If the object falls a specific distance (h) and starts from
            rest (zero velocity) then the object's final velocity (V)
            will be proportional to the square root of the fall distance
            (h). V = (2gh)^1/2^ where g = acceleration of gravity
            constant

        3.  Energy cannot be created nor destroyed; therefore, the
            object's velocity will continue to increase as the fall
            height increases.

    C.  Normally when objects (including water) fall, retarding forces
        are encountered that cause work to be done. This work uses up
        potential energy so the object will not fall as fast as
        predicted in the free fall case described above. This is termed
        **friction loss.**

    D.  **Friction losses** normally increase the faster the object
        falls and at some fall velocity all additional potential energy
        contributions are converted into Friction Loss and the object
        continues to fall at a constant velocity. This limited fall
        velocity is called the object's **terminal velocity** for that
        specific set of conditions.

    E.  For example, a man jumping out of an airplane without a
        parachute will have a considerably higher **terminal velocity**
        than a man jumping with a parachute. This occurs because the
        parachute causes large **friction losses** to develop that uses
        up the **potential energy**. Clearly, this is an advantage when
        the men reach the ground!!

    F.  Exactly the same principal applies to stream flow.

        1.  As the stream falls (flows to a lower elevation), it must
            either convert the **potential energy** released to
            increased **stream velocity** or the **potential energy**
            must be used up in **friction losses.**

        2.  If there is insufficient **friction losses**, a threshold
            **stream velocity** will be reached that is capable of
            causing serious stream bed and bank erosion to develop.

II. **Friction losses can take many forms.** **(PP48, Designer's
    Toolkit).**

    A.  Turbulent flow generates heat and uses up **potential energy**.

    B.  Details of channel size and configuration create **friction
        losses**.

        1.  Channels are significantly lengthened when the stream's
            **sinuosity** (meander size) is increased. Because the water
            must pass over greater amounts of bed friction is added.

        2.  Changing the channel cross-section shape and size creates
            additional turbulence that uses up **potential energy.**

            a.  Most energy is expended when the channel size increases
                (i.e. flowing from a **riffle** into a **pool** for
                instance).

            b.  Flow from pools into riffles creates minimum turbulence;
                therefore little energy is expended it this process.

        3.  Forcing water to flow through vegetation creates
            considerable turbulence and associated **friction loss**.
            This is particularly true when flow goes out-of-bank and
            flows across the floodplain.

        4.  In-stream obstructions (downed trees and boulders for
            example) create turbulence and use energy.

        5.  In-stream drops use considerable energy as the flow changes
            velocity and spills over the structure.

            a.  In steep valleys **stair-step profiles** develop
                naturally that use this method to dissipate energy.

            b.  **Beaver dams** create drops that expend energy

            c.  **Grade control structures** made out of rock or other
                materials can create stable in-stream drops that expend
                considerable energy.

III. **Sediment transport is extremely sensitive to stream velocity.
     (PP49, Flow Velocity Relationships)**

     A.  For example, if the average stream velocity is doubled from 4.5
         feet per sec. to 9 feet per sec., the stream's sediment
         capacity increases from moving a 2-inch diameter rock weighing
         about 1 lb. to moving a 10-inch diameter rock weighing about 90
         lbs.

     B.  Average stream velocities at flood stage commonly exceed 9 feet
         per sec.

     C.  If the average velocity reaches 16 feet per sec, 3.5 feet
         diameter rocks weighing 6000 lbs. can be moved.

     D.  Major floods like the one that occurred in Big Thompson Canyon,
         in Colorado had velocities exceeding 30 feet per sec. and
         boulders weighing 500,000 lbs. moved.

     E.  It is important to differentiate between **Average Stream
         Velocity** and **Instantaneous Stream Velocity.**

         1.  **Instantaneous Stream Velocity** is the velocity that a
             particular rock encounters within the stream's boundaries.

         2.  **Instantaneous Stream Velocity** is increased considerably
             when structures are placed within the streams that increase
             turbulence.

**Supplemental Considerations:**

The instructor could demonstrate energy conversion and friction losses
in a gravity system by a simple experiment involving of dropping a
feather, a piece of paper, and a piece of chalk. Try wetting all three
items and repeating the test. Does it make a difference? Is the added
weight of the water a factor? Finally, wad up the paper into a small
ball and repeat the drops.

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**Objective 6.6 Describe secondary currents and stream features that are
formed by their action.**

**Requirements:**

The information in this section is presented as a lecture using Power
Point slides:

**Remarks:**

This section will explore the second level (**Secondary Currents**) of
stream flow analysis. Much of the systematic and repeatable features
seen in stream channels and along stream banks are formed by the actions
of **Secondary Currents**.

I.  **Channel features determined by secondary currents include: (PP50,
    Channel Features)**

    A.  In vertically stable (**meandering**) streams:

        1.  Meander loop shape.

        2.  The in-stream location of the **thread (thalweg).**

        3.  The erosion and deposition patterns that occur along bends
            and on bars.

        4.  The shape of sand/gravel bars that form within the channels.

        5.  The location of the mid-channel undertow that develops.

    B.  In depositional (**braided**) streams.

        1.  Mid-channel island formation.

II. **Secondary bed currents in a typical meander stream are controlled
    primarily by bed friction. (PP51, Bed Friction Controls)**

    A.  During **bank full flow** there is a set of dominant, **helical
        cells** that are continuously moving down the channel.

        1.  On the bed, these cells diverge from each other.

        2.  This sweeping action tends to move **bed load** away from
            the center of the channel. This creates a sediment deficient
            path that becomes the **thread**.

        3.  On the outside of bends one of these cells sweeps up along
            the bank causing increased bend erosion.

        4.  The matching cell carries sediment up onto the bar located
            on the inside on the bend.

        5.  Many experiments have been conducted that demonstrate that
            material that erodes from one side of the stream ultimately
            deposits on a downstream bar on the [same side of the
            channel.]{.underline} Very little material crosses the
            stream's **thread**.

            a.  Most material deposits on the bar immediately downstream
                from the bend erosion site.

            b.  When the eroded material fails to deposit on the first
                bar, it continues to travel downstream past the next bar
                on the opposite side of the stream and deposit on the
                next bar on the same side of the stream that the erosion
                originally occurred on.

        6.  At the surface, the two primary cells converge and flow
            downward. This creates an **undertow** in the middle of the
            stream.

            a.  Many swimmers have drowned in these **undertows**.

            b.  In the fall and winter floating debris and ice follow
                this **undertow** path.

            c.  In the 1850s paddle wheel boat pilots would follow this
                trash line in the fall when flows were low. This allowed
                them to navigate over the **thread**, which is the
                deepest path on the river.

        7.  Cross sections across the channel during **bank full flow**
            demonstrate how sediment is directed in the channel.
            **(PP52, Channel Features Active)**

            a.  In areas where the cells flow downward, cleaner water is
                directed against the bed. This causes more scour to
                occur at these locations.

            b.  In areas where the cells flow upward, sediment
                (especially **bed load**) is left behind. This caused
                deposits to form.

            c.  When the system is heavily loaded with sediment, these
                mid-channel deposits can become large enough to be
                exposed at the surface when flow recedes. This action
                creates the depositional braided channels.

    B.  The map view of the secondary currents shows that a second set
        of **helical flow** cells are present during bank full flow
        conditions.

        1.  These cell pairs are not continuous.

        2.  They are confined to areas of the bars located on the
            insides of the bends.

        3.  They function to distribute sediment eroded from the bends
            onto the bars.

        4.  A common feature of gravel bars is the small embayment that
            forms at the lower end of bars usually near the bank. These
            **embayments** mark the location where the down flowing in
            the discontinuous pair of cells has created a mini-thread
            and has become sediment deficient.

III. **When flow recedes to normal conditions only the primary,
     continuous pair of cells remains. (PP53, Channel Features Stable)**

     A.  The bars in the inside bends become exposed and the location of
         the mini-thread can be seen.

     B.  If the stream was overloaded with sediment, mid-channel islands
         will be exposed. This is the cause of braiding.

**Supplemental Considerations:**

The instructor should consider discussing local examples of features
that have been formed by secondary cell action.

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**Objective 6.7 Describe how in-channel obstructions cause random energy
dissipation.**

**Requirements:**

The information in this section is presented as a lecture using Power
Point slides:

**Remarks:**

This section will explore the third level (random eddies) of stream flow
analysis. Much of the individual character that is seen in streams is
caused by random features that cause local variations in to flow
patterns to develop.

I.  Random eddy production is caused by many natural and manmade
    features that are within the stream's boundaries. (PP54, Random Eddy
    Production)

    A.  Downed trees are one of the most common random features. (PP55,
        Random Energy Dissipation)

    B.  Rock outcrops and boulders that are too big for the stream to
        move affect currents.

        1.  In glaciated valleys, large boulders are often left in the
            channels.

        2.  Landslides can also introduce large boulders into the
            channels.

    C.  Many manmade structures now intrude into stream channels.

        1.  Bridge piers are very common obstructions.

        2.  Bank protection structures such as jetties and riprap cause
            unnatural behavior.

II. Obstructions cause flow patterns to vary.

    A.  Turbulence can be concentrated by the obstructions.

    B.  Velocity variations caused by obstructions can cause
        unpredictable scour.

    C.  Changes in cross section size and shape develop when
        obstructions change flow patterns.

**Supplemental Considerations:**

The instructor should consider discussing local examples of manmade
features that alter natural flow patterns.

**Objective 6.8 Discuss stream mechanic within the context case study
floodplains.**

**Remarks:**

A.  The instructor should spend some class time answering student
    questions concerning their Case Studies. To insure that the Case
    Studies are progressing, the instructor should ask the students
    questions based on the Session 2 Handouts.

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**References:**

[Classic Reference]{.underline}

Leopold, Wolman & Miller. 1964. *Fluvial Processes in Geomorphology*. W.
H. Freeman & Co.\
The book was republished in 1995 by Dover in an unabridged and unaltered
paper-back form.

[Drainage Patterns]{.underline}

Howard, A.D. 1967. "Drainage Analysis in Geologic Interpretation: A
Summation." *Amer. Assoc. of Petroleum Geologists Bulletin*, v. 51, No.
11, Nov. 1967, pgs. 2246-2259

[Alluvial Fan References:]{.underline}

Committee on Alluvial Fan Flooding, National Research Council, 1996,
Alluvial Fan Flooding, National Academy Press. Found at: 
<http://www.nap.edu/readingroom/books/all/>

FEMA, 2000, Guidelines for Determining Flood Hazards on Alluvial Fans.
Found at:  <http://www.fema.gov/mit/tsd/ft_alfan.htm>