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THE CHANNEL EVOLUTION MODEL (CEM), CHANNEL INCISION, FLOODPLAIN BENCHES, & CAN STREAMS REPAIR THEMSELVES?.
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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 modelIn an idealized stream Types I-V will occur in sequence (series)
Channel Evolution Model Overview 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 Type 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
< Type I is upstream of active incision
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
< Type II reaches are actively incising, although mass wasting of bank has not been initiated (h<hc)
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!!!
Headcut moving upstream on Johnson Cr, MS. CEM Type II
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 headcuts
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
< In the Type III reach, mass wasting of the banks with rapid channel widening is the dominant process
CEM Type III Bellefontaine 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
Headword migration of knickpoints stopped by twin road culverts, north MS. A vehicle CEM Type III
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!! Huge problems with perchlorate interception from the groundwater table I am standing on the roots of dead wetland plants, over 2,200 acres of wetlands lost
Then 2,400 Acres of Wetlands
Now 2000 acres of wetlands lost ! Photo by Gerry Hester
Channel Evolution Model Type 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
< 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
Channel Evolution Model Type 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
< Type V reaches represent a state of dynamic equilibrium with a balance between sediment supply and sediment transport capacity.
Old floodplain bench (hundreds of feet wide) is now a disconnected terrace CEM Type V, Middle Fork Worsham Cr. Duck Hill, MS New floodplain bench 6 ft wide
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
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). ALISO CREEK - PIX BY DERRICK 5-10-2009
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.
