Bank stability - Paper
Genna Slape
CE 598
Mark Stone

Introduction: Will write last. will state paper objective

Objective: Erosion, sediment transport, and deposition are the fundamental processes that govern fluvial geomorphology. Being that these fundamental processes occur naturally and have literally formed the world we live in, why are they a concern for river managers, ecologists, engineers, etc.? This question leads to the grander, more difficult question: Are the geomorphic alterations, specifically stream bank alterations, occurring within a fluvial system consequential of anthropogenic applications and efforts, or rather resultant from natural hydrologic functions such as flood events?

The purpose of this paper is to address these questions, not necessarily answer them.

Fluvial Geomorphology: A Natural Process!

- Fluvial Geomorphology definition: the examination of the processes that occur in a river system and the land forms which they create.
- Emphasize that a river is one feature within a dynamic and complex system (overall watershed), in which governing thresholds exist.

Problems caused by Erosion:

- Strong focus on ecological and hydrological consequences
- Economic consequences as well

Diversity of Form of Channels: Influences on Erosion

- channel substrate: alluvial vs. non-alluvial. Focus of paper is on alluvial channels, as this is more applicable to the gila and rio grande systems.
- Channel flow variations, both between distinct systems, as well as the flow variations that occur temporally and spatially along the same channel.

Processes of Erosion, Transport, and Deposition

Erosion
- How river flows shape their channels.
- Stream power impacts (open channel hydraulics)
- Bank erosion in alluvial channels
- Bank materials and weakening processes
- Modeling procedures
- Bank failure mechanisms

Identifying Erosion as Problematic: Channel Stability Classification

Three classifications professionals (scientists, river managers, engineers, etc) have identified:
- Absolutely stable: absolute stability is achieved through channelization.
- Dynamically stable: state alluvial channels seek
- Unstable: problematic indicator. Since we often think of nature as having the ability to self heal, is an unstable state indication of human interference?

Physical responses that indicate unstable channel conditions:

- Incised channel

Causes of System Instability: System Responses

- Upstream causes: dams and diverges which cause changes in discharge and sediment supply
- Downstream causes: backwater or basewater level fluctuating due to cutoffs or channelization
- Basin wide causes: land use practices or changes effects runoff rates

System instability vs. Local Instability
Local instability: bank erosion that occurs as a result of site-specific processes or factors, rather than as a result of system dis-equilibrium.

Bank Stabilizing Techniques

- Direct reinforcement - lateral protection (local solutions)
- Energy management - bendway weirs, spur-dikes, etc. (stream-scale scale solutions)
- Bioengineering and hybrid approaches
- lessons learned

Conclusion

  • Implications for River management efforts

Still evolving, will write last


What I’ve scribbled down so far:

River management efforts tend to place a lot of emphasis is put on the geomorphic stability of a fluvial system, when attempting to determine the quality of a system, and thus the river management applications. This places a great importance and need to understand the erosion, transportation and deposition mechanics and functions of a system, as these embody the fundamental process of geomorphic alterations.

Historically, river and land-use manager focused on the negative impacts that erosion posed on structures such as bridges, roads, buildings, etc. However, more recently, as our society is becoming more environmentally concerned, the hydrological and ecological consequences of erosion and increased sediment loads are becoming a focul concern.
Although river and stream channels are only one feature within an overall hydrologic functioning system, and erosion does occur on the larger watershed scale this paper will focus on the erosion dynamics that occur within the fluvial channel.

Erosion and sediment deposition are simply processes that occur in fluvial geomorphic processes, which occur as a result of the hydraulic forces originating from the in-stream flow as well as other physical considerations.

Fluvial systems are extremely diverse and dynamic. Every system has erosion, sediment transport and deposition occurring synonymously.

It is important to understand the basic fluvial geomorphic processes to understand the erosion and sediment issues we are facing. Fluvial geomorphology is the study of the interactions of river channel forms and processes at a range of space and time (Ro Charlton).

Rivers and streams naturally and continuously reshape their channel geometry and spatial context through erosion of the channel boundary, referring to the channel bed and banks. This a natural process that depends on a fragile balance between the erosive abilities of the hydraulic forces occurring the channel as well as other physical occurences, and the resistivity or strength of the channel formation. The resistivity of the channel boundary depends on several properties including: Slope and length of channel, channel material, channel coverage. These properties basically determine the channels ability to dissipate energy within the channel flow. Energy is generated within a channel B: provide specific reasons for variability i.e. bank material, vegetation type, climate. What are types of soils that may have more resistance to erosion?

Channel substrate is defined as the material in which the channel is formed, and is significant in contributing to the channel dynamic of the system. Generally channels can be categorized into two opposite classifications based on the substrate of the channel: alluvial or non-alluvial. Alluvial channels are formed in substrate comprised of unconsolidated material ranging in size, such as gravel, sand, and fine sediment and clay material, that has been deposited within the channel. The Middle Rio Grande is an example of an alluvial channel comprised of fine silts and clay. At the opposite spectrum, the channel boundaries in non-alluvial channels can are comprised of less transportable material such as boulders and even bedrock. Other systems can be identified as a combination of alluvial channel.

Not surprisingly alluvial channels are subject to extensive geomorphic alterations, the extent and temporal rate depending on the system’s distinct. The rate of change may be gradual or rapid, natural or man-induced. Alluvial channels are dynamic systems and the types of change that the system will experience over time include: width, depth, alignment, and stability. The types of process that result in alluvial system alterations include:

Non-alluvial channels may not by subject to erosion, except in the occurrence of large flow events, that exert enough energy on the channel boundaries to cause erosion.
(Has there been any research on the effects of clear vs. turbid water and the impacts on alluvial vs. non-alluvial channels? As I understand it, clear water has more energy and causes bank instability, yet it seems to flow in non-alluvial systems, while alluvial systems seem to have less stability with their fine substrate. I probably have this concept all wrong, but maybe there's an avenue in there to expand your paper on.)
The channelization epidemic that has occurred in response to human’s desire to achieve technological and engineering advances, has resulted in the evolution of many natural alluvial channelization to concreted or non-alluvial systems.

The extent and nature of erosion that occurs in a channel, determine whether a channel is identified as being in one of the three classifications:
-Absolutely Stable,
- Dynamically stable,
- Unstable
Classifying a channel as one of these categories is not as simple as entering data into a simple plug and chug equation and getting an explicit answer. While some systems are obviously more resistant to erosion, stability classification depends largely on perspective, particularly on temporal scale perspective. One time scale that has been a focus for many river management projects, is the engineering time scale which is defined to range over a 50-year span. Absolutely stable channels are identified as systems that experience change over the engineering time scale. Dynamically stable systems undergo feature location alterations., while maintaining dynamic relations over the engineering time-scale. Bank location, meander locations, and sand and gravel bars are examples of features that may be subject to spatial alterations in a system that is considered to be dynamically stable. Unstable systems experience major changes in depth, width, sediment transport, flow, sinuosity, and planform over a period of days, months, or years.

Headcuts, knick points, and incised channel features are consequential of erosion and observable indicators of channel instability. (I assume you are going to go more into depth on these terms.)

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Figure – Headcut Erosion

In exploring the issues surrounding erosion and sedimentation, the question often becomes how has man influenced these problems. Because fluvial systems are no where near simple, it is often difficult to quantatively address this question. Because the erosion and sedimentation are fundamental in the occurrence of natural geomorphic processes, it may be safe to say that man’s influences inflict the same effect, it is the magnitude of the system variables and effects that are effected.

the ability of flowing water to carve a channel and transport debris depends on two principal external forces acting on the water flowing in an open channel; gravity and friction (shear stress)

Things that cause erosion and sediment volume problems:
- mining
- bridges
- Channelization which increases slope and flow velocity; removes meandering

why problem
- Causes environmental problems; sediment transport affects the water quality
- Sediment alone degrades water quality for municipal supply, recreation, industrial use, hydroelectric facilities and aquatic life. Sediment also serves as catalyst and carrier for other pollutants
Meghan's comments: Looks like you have a lot to expand on. Sounds like it's going to be a great paper. I'm sure you're aware that it needs a little more work to flow better, but that's the purpose of a rough draft right? I wonder if there is anything out there about a before and after type study, say with the removal of a dam? It sounds like out of everything we have discussed thus far in class, this topic could be the least time consuming to study. Also, I am curious if you were going to discuss the methods of correcting sedimentation issues. I think it was Jeff Samson's presentation that talked about how to restabalize banks-what type of equipment to use, where to put excess material... Overall looks like a really interesting topic, excited to see more.

Bekah's Comments:

Good stuff here, but I think your paper would be stronger if you focused primarily on bank stability rather than thinking about erosion as a whole (although of course you need to consider it in the context of bank stability). Use bank stability as the jumping off point to discuss erosion issues. I would also recommend keeping your outline headings as part of your final draft to keep your reader focused on what you are talking about. My other recommendation is to use specific examples of what you are discussing throughout. give examples of bank stabilization projects that have failed or been successful. Provide specific data about what constitutes a channelized reach. Demonstrate channel forms that have higher erosion rates...

A few references to help:

Couper, P., 2003. Effects of silt-clay content on the susceptibility of river banks to subaerial erosion. Geomorphology, 56, 95-108.
Florsheim, J.L., Mount, J.F., Chin, A., 2008. Bank erosion as a desirable attribute of rivers. Bioscience, 58(6), 519-529.
Montgomery, D.R., 1997. What's best on the banks? Nature, 388, 328-329.
Micheli, E.R., Kirchner, J.W., 2002. Effects of wet meadow riparian vegetation on streambank erosion. 2. Measurements of vegetated bank strength and consequences for failure mechanics. Earth Surf. Process. Landf., 27, 687-697.

Main Sources of Data:

Charlton, Ro. Fundamentals of Fluvial Geomorphology. New York: Routledge 2008.

Julien, Pierre Y. Erosion and Sedimentation. New York: Cambridge University Press 1995.

Pollen-Bankhead, Natasha. Hydrologic and hydraulic effects of riparian root networks on streambank stability: Is mechanical root-reinforcement the whole story? Geomorphology 116 (2010): 353-362.

Simon, Andrew. Bank and near-bank processes in an incised channel. Geomorphology 35 (2000): 193-217.

Williams, David T. “Fluvial Geomorphology and Its Use in River Stabilization.” DNRC Water Resources Division. 3-4, December 2009.