THE CHANNEL EVOLUTION MODEL (CEM),

CHANNEL INCISION, FLOODPLAIN BENCHES, &

CAN STREAMS REPAIR THEMSELVES?

Channel Evolution Model (Schumm, et al. 1984)

Originally developed to describe erosion evolution of Oaklimiter Creek, Calhoun

City/Derma, MS.

A location-time substitution conceptualization is used to generate a five-reach type incised channel

evolution model

In an idealized stream Types I-V will occur in sequence (series)

Channel Evolution ModelOverview

The primary value of the model is to determine the evolutionary state of the channel from a field reconnaissance.

The evolution sequence provides an understanding that reaches of a stream may differ in appearance, but channel form in one reach is associated with other reaches by an evolving process.

Form, process, and time relate dissimilar reaches of the stream.

The CEM was originally designed to help understand a stream’s response to a downstream disturbance (typically bed lowering), although upstream disturbances are handled well too. From C. Watson

Channel Evolution Model

Channel Evolution ModelType I Reach Characteristics

Type I reaches are generally characterized by a U-shaped cross section with little or no sediment stored in the channel bed.

Type I reaches are located upstream of the actively degrading reach and have not yet experienced significant bed or bank instabilities.

From C. Watson

Ih < hc

IIh < hc

IIIh > hc

IVh hc

Vh < hc

PRIMARY KNICKPOINT

PRECURSORKNICKPOINT

SECONDARYKNICKPOINT

Type I is upstream of active incision

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A dappled Dr. Watson in a CEM Type I stream

CEM Type I

Stream connected with its floodplain, bed stable

Channel Evolution Model Type II Reach Characteristics

Immediately downstream of Type I reaches, Type II reaches are encountered. Bed degradation is the dominant process in the Type II reach.

Type II channels are steepened reaches where the sediment transport capacity exceeds the sediment supply.

Although the channel is actively degrading in a Type II reach, the bank heights (h) have not exceeded the critical bank height (hc). Therefore, banks are not geotechnically unstable.

From C. Watson

Ih < hc

IIh < hc

IIIh > hc

IVh hc

Vh < hc

PRIMARY KNICKPOINT

PRECURSORKNICKPOINT

SECONDARYKNICKPOINT

Type II reaches are actively incising, although mass wasting of bank has not been initiated (h<hc)

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A large knickpoint, Niagara Falls (American Falls) (This headcut moves on

average 2.5 ft per year)

Hard Dolomite overlaying weaker

Rochester Shale, could result in a large riffle over

time

Note large amount of stone at toe of falls

Derrick 6-5-2009

So much rock fell that the Corps dewatered the American Falls in 1969 to see what was going on!!

Derrick 6-5-2009

A headcut has to move upstream over time

(toward the headwaters of the stream), if the

waterfall does not move, it is not a headcut!!!

CEM Type IIHeadcut moving

upstream on Johnson Cr,

MS.

Typically knickpoints will not occur in non-cohesive

materials (sands, etc.). Sand will not stand vertically with water

flowing over it.

CEM Type II A series of small headcut

s

Dr. Watson with large headcut in a

CEM Type II stream, Johnson

Creek, MS. Banks bad upstream

Looking US at a North Miss. stream, CEM Type II upstream (downcutting) & Type III (almost immediately

twice as wide) in foreground.

Channel Evolution Model Type III Reach Characteristics

As bed degradation continues, the bank heights and angles will continue to increase.

When the bank heights have exceeded the critical bank height for stability, mass failures (geotechnical instability) begin to occur in the Type III reaches.

The dominant process in the Type III reach is channel widening.

From C. Watson

Ih < hc

IIh < hc

IIIh > hc

IVh hc

Vh < hc

PRIMARY KNICKPOINT

PRECURSORKNICKPOINT

SECONDARYKNICKPOINT

In the Type III reach, mass wasting of the banks withrapid channel widening is the dominant process

<

CEM Type III, rapid over widening of stream

CEM Type III, banks fail, trees fall in

CEM Type III-bridges too

short

CEM Type IIIBellefontaine Creek,

{sand & clay bed, rural, slope <1%} April 2005,

rapid widening

Bellefontaine Creek about 700 ft US of the previous picture. CEM Type II, but the headcut is coming, followed by channel widening

HEADCUTS GONE BAD!!

CEM Type III

Headword migration of knickpoints stopped by twin

road culverts, north MS.

A vehicle

Las Vegas Wash, NV. has degraded from a 1 ft deep by 100 ft wide channel in 1975, to a 40 ft deep by 1,000 ft wide channel in 1995!!

I am standing on the roots of dead wetland plants, over

2,200 acres of wetlands lost

Huge problems with perchlorate

interception from the groundwater table

Then

2,400 Acres of Wetlands

Now

2000 acres of wetlands lost !

Photo by Gerry Hester

Channel Evolution ModelType IV Reach Characteristics

The Type IV reaches are downstream of the Type III reaches and represent the first manifestation of the incised channel returning to a new state of dynamic equilibrium.

In the Type IV reach, geotechnical bank instabilities and channel widening may continue, but at a much reduced rate.

From C. Watson

Ih < hc

IIh < hc

IIIh > hc

IVh hc

Vh < hc

PRIMARY KNICKPOINT

PRECURSORKNICKPOINT

SECONDARYKNICKPOINT

Channel widening continues at a much reduced rate in the Type IV reach. The first manifestation of a new

equilibrium emerges.

<

CEM TYPE IV, North Miss. stream, overwidened, cannot process sediment

CEM Type IV, MS., should be a single-thread channel

CEM Type IV, Illinois

Channel Evolution ModelType V Reach Characteristics

Type V reaches represent a state of dynamic equilibrium with a balance between sediment transport capacity and sediment supply.

Bank heights in the Type V channel are generally less than the critical bank height, and therefore, geotechnical bank instabilities do not exist.

From C. Watson

Ih < hc

IIh < hc

IIIh > hc

IVh hc

Vh < hc

PRIMARY KNICKPOINT

PRECURSORKNICKPOINT

SECONDARYKNICKPOINT

Type V reaches represent a state of dynamicequilibrium with a balance between sediment supply

and sediment transport capacity.

<

CEM Type III-IV-V

CEM Type V, Middle Fork Worsham Cr. Duck Hill, MS

Old floodplain bench (hundreds of feet wide) is now a disconnected terrace

Long Creek, Batesville, MS, - CEM Type V stream, old floodplain (abandoned by stream), new floodplain bench

CEM Type V

SO WHAT DOES THAT HAVE TO DO WITH ALISO CR. @ THE BEACH?? (Southern CA). IN MINUTES WE WILL SEE THE CEM TYPE 2 & 3 WORKING. Looking at Pacific

Ocean wave action building a beach berm

ALISO CREEK - PIX BY DERRICK 5-10-2009

Aliso Cr. had pooled up on the beach behind a low berm ocean waves had built. Stream has just broken through the berm & is

flowing toward the Pacific Ocean

ALISO CREEK - PIX BY DERRICK 5-10-2009

ALISO CREEK - PIX BY DERRICK 5-10-2009

Two minutes later flow has increased.

ALISO CREEK - PIX BY DERRICK 5-10-2009

Looking toward the ocean, in 3 minutes the channel went from CEM Type 2 (lowering) to Type 3 (widening).

The ponded water surface elevation dropped 39 inches in 10 minutes, kind of like a dam break.

ALISO CREEK - PIX BY DERRICK 5-10-2009

A CEM QUICK QUIZ,

LET’S THINK!!

Stream degradation?? Or local scour?? Air observed under the RDB

highway bridge pier, Interstate 20, Clear Cr. Bovina, MS

Odd protection for LDB Interstate 20 bridge pier, left bank, Clear Creek, Bovina, MS, Air under protection.

Definitions Aggradation - The geologic process by which a stream bed is raised in

elevation by the deposition of material that was eroded and transported from other areas. Typically a stream that is undergoing aggradation over a long section of its length has an excess supply of sediment. Aggradation is the opposite of degradation.

Degradation ‑ A progressive lowering of the channel bed due to scour, usually caused by a shortage of sediment, and/or an increase in discharge. Degradation can occur along the entire stream length, a certain reach of a stream, (i.e. downstream of a dam, reservoir, etc.), or system-wide (every stream in the watershed is undergoing degradation). Degradation is the opposite of aggradation.

Incised stream or channel - A stream that has degraded to the extent

that it is now hydraulically disconnected from its original floodplain. Incised is usually understood to mean that the 1.5-yr flood event is contained within the top banks of the stream. Incision of the main channel usually results in the eventual base lowering of all tributaries, resulting in destabilization of the entire watershed.

Original bridge

Widening streams make for some double ugly bridges !! Akron OH

Conceptually, for an incised system you can:

raise the stream, lower the floodplain, do a little of both,

or possibly roughen the channel

Lower either one or both banks of the floodplain?

Construct bankfull flood benches on one or both banks?

Some designs are very complex, here a three-

stage channel

Mini floodplain

bench

Active channel

Wet meadow Channel forming discharge

(floodplain) bench

Mini floodplain

bench

MINI FLOODPLAINBENCH ON THE

OTTAWA RIVER, OTTAWA HILLS, OH

Looking DS on the Ottawa River, OH, (sand-gravel, less than 1% slope. urban, pool-riffle-pool) note slower water within the veg on bench along left bank, Ottawa River, Ottawa Hills, OH

Looking DS, close-up shot Ottawa River, note drag from plants vectoring thalweg away from

veg.

Looking DS, close-up shot Ottawa River @ low flow 7/26/07, note how narrow the floodplain bench is

BankFloodplain

bench

CAN STREAMS REPAIR

THEMSELVES???Sometimes, let’s

think

FURNACE CR. BETHLEHEM, PA.

Stream is only 4 years old. Dam was breached & stream

is adjusting down through

reservoir sediment.

How do streams dissipate excess energy?

Transverse bars direct stream

energy toward outer bank…

Furnace Cr., PA

Which increases stream length & reduces stream slope….

Furnace Cr., PA

Dam

Stream arranged bedload to form pools & riffles, increasing the amplitude of the vertical sine wave of the stream

Furnace Cr., PA

Straightened (with grade control), vs. the old natural rough & meandering channel

From Brian Winkley

From Brian Winkley

From Brian Winkley

From Brian Winkley

This PowerPoint presentation was developed & built by Dave Derrick.

Any questions or comments, call my personal cell @ 601-218-7717, or email @ d_derrick@r2d-eng.com

Enjoy the information!!