Annotated Bibliography of River Avulsions Pat Dryer Geography 364 5/14/2007

Transcription

1 Annotated Bibliography of River Avulsions Pat Dryer Geography 364 5/14/2007

2 1 Table of Contents Introduction 2 Annotations I. River avulsions and their deposits 2 II. Channel avulsion on alluvial fans in southern Arizona 3 III. Modeling the effect of vegetation on channel pattern in bed 4 load rivers IV. Interrelations of channel processes, changes and sediments 6 in a proglacial braided river V. Channel morphology and bed load pulses in braided, 7 gravel-bed streams Conclusion 8

3 2 Introduction Channel avulsions are a very interesting geomorphic event. A river avulsion is a natural event in which stream flow gets diverted out of an established channel and forms a new permanent course on the adjacent floodplain. Avulsions are typically features of aggrading floodplains. Channel avulsions can happen frequently in some river systems and very rarely occur in others. The exact cause of river avulsions is not fully understood but there seems to be some contributing factors that influence avulsions. Avulsions can cause millions of dollars of damage and injure or kill numerous people. Annotations I. Slingerland, R., and Smith, N.D., 2004, River avulsions and their deposits: Annual Review of Earth Planet Science, v. 32, p This paper focuses on summaries the various styles of avulsions, review of present knowledge of avulsions, describing sedimentology and stratigraphy of alluvial floodplain deposits, and emphasis on the role avulsions play in creating them. To accurately understand river avulsions occur one must first understand several terms such as bed load, sedimentation, and channel patterns. In this paper the author s quantify avulsions by sedimentation, bifurcation, and bed load transportation rates.

4 3 There are six main types of river avulsions including; partial, nodal, local, full, random, and regional. In a full avulsion all flow is diverted out of the parent channel into a new avulsion channel. A partial avulsion involves only part of the flow to be diverted typically resulting in anastomosing channels. Nodal avulsions are recurring events that originate from a fixed area of a floodplain such as a fan apex. Random avulsions happen throughout active channel systems. Local avulsions form new channels that rejoin the parent channel downstream. Regional avulsions happen at a very large scale and effect flow everywhere downstream from the avulsion. Avulsion frequencies vary greatly among modern river systems. The lowest rate is 28 years for the Kosi River in India to up to 1400 years for the Mississippi River. Stratagraphic records suggest much longer intervals between avulsions. Avulsions seem to be a function of aggrading floodplains and sedimentation rates. Avulsions occur more frequently when large amounts of sediment are introduced into systems, such as glacial outwash streams with braided channels. On the other hand avulsions may never happen on river systems with aggradation rates near or below zero. This article was very well written and clearly describes many key factors that influence river avulsions. The text was written for a scientific audience but easily understood by those who have some knowledge of the subject area. This paper was relevant to my topic and helped me understand many factors that influence avulsions. II. Field, J., 2001, Channel avulsion on alluvial fans in southern Arizona: Geomorphology, v. 37, p

5 4 This paper looks as channel avulsions on alluvial fans in southern Arizona and how they happen. John Field uses aerial photography to study channel patterns in the past and has also developed a descriptive model for explaining avulsions. Channel avulsions are of great interest when studying alluvial fans. The fan shape is a result of avulsions diverting flow to different portions of a fan. Avulsions occur very frequently on alluvial fans when sediment laden water reaches bankfull flow on channels. Channel abandonment occurs when overland flow form the main channel directs headward erosion of smaller channels on the fan surface. This headward erosion can eventually reach the parent channel and cause an avulsion to occur via stream capture. There are many stages to the avulsion process that can help identify areas of a fan surface that are more prone to flooding. These stages include; on-fan channels, new channels, adjusted channels, aggrading channels, and abandoned channels. Overtime the new channels become stabilized and the whole process starts over. Channel avulsions on fans are not completely random because of their relative position of low banks along the main channel and smaller channels draining the fan surface. This paper was written for scientific audiences which are experts on channel morphology. Some of the concepts were hard to understand. The research was well designed and would be easily recreated in other areas with alluvial fans. Some of the information in the paper is relevant to my topic but is not the most relevant topic for my source of river avulsions. III. Murry, A.B., and Paola, C., 2003, Modeling the effect of vegetation on channel pattern in

6 5 bed load rivers: Earth Surface Processes and Landforms, v. 28, p This paper looks at how vegetation can play a role in controlling avulsions. The main focus of this paper is on the role of vegetation in the development of single-thread channel patters either straight or sinuous. To study this relationship between vegetation presence and avulsions can be modeled with cellular modeling or rule-based modeling in non-linear dynamics. In each cell lattice the elevation, water and sediment discharge are defined in arbitrary units. This paper modified a simple numerical stream-pattern model to look at the sediment stabilization by roots on channel patterns in bed load streams. Vegetation enhances bank resistance to erosion resulting in one single incised channel instead of multiple channels typical of braided systems. Streams with a high sediment load can cause frequent avulsions that destroy vegetation along banks that limit erosion. The authors developed a stability chart that depicts if vegetation can anchor the stream banks before avulsions take away the vegetation and further promote erosion. This article was well written and makes some very interesting conclusions. Vegetation is often overlooked when looking at avulsion events. Once an avulsion occurs and takes away vegetation it takes time for the vegetation to regenerate and restabilize the banks. This article was written for a scientific audience interested in biological factors in river avulsions. I would have liked more information on how single channel streams can change into braided steams through avulsions. This article would be useful for my topic and would help create a well rounded paper from both the physical science of avulsions to biological factors.

7 6 IV. Ashworth P.J., and Ferguson, R.I., 1986, Interrelations of channel processes, changes and sediments in a proglacial braided river: Physical Geography, v. 68, p For this paper research was conducted in July and August 1984 on the Lyngsdalselva River, a braided proglacial river north of the Arctic Circle in Norway. The study area was downstream of the glacier snout at the head of the outwash plain, where bed load materials were cobble sized. This study focused on two main branches of the stream before an avulsion happened and then incorporated the new channel into the research as time progressed. Researches monitored discharge, velocity, shear stress, and transportation rates to try and quantify the avulsion. The researches placed 188 painted pebbles into the stream during the high flow event in August. The next morning only 50 of the pebbles could be located. The remaining pebbles were most likely buried during the event. When discharge decreased 88 more painted pebbles were introduced into the stream. After some time pebbles were found at or near the source and as far as 17 meters downstream from their initial position. This indicates decreasing mobility of sediment as discharge dropped. Results from this research indicate that discharge was mostly constant, but sunny weather in August produced larger diurnal peaks and higher discharge. During this time of high flow several new channels were formed through the process of avulsions and new bars were also deposited along the channel. The study showed that avulsion had a greater probability of occurring during times of high sedimentation and high discharge. No

8 7 avulsions were recorded during the times of constant discharge. This paper was very well written and was easily comprehendible. The research was well planned and executed. The way the writers presented the information made the data easily comprehendible. This paper would be relevant to a paper about stream avulsions and would provide useful information. V. Ashmore, P., 1991, Channel morphology and bed load pulses in braided, gravel-bed streams: Physical Geography, v. 73, p The researches in this paper used small-scale hydraulic laboratory models based on Froude modeling to investigate: the influence of gradient and discharge on cross sectional areas of graded streams, braided stream morphology, and occurrence of bed load pulses. The researches repeated several transect measurements of braided channels and found several different combinations of slope and discharge, which is an influential factor in local avulsions. Goals of this paper include using hydraulic modeling of braded streams to use as an approach to understand braided stream geometry and interaction of bed-load transfer. From this of understanding of the goals, the paper has three objectives; demonstrate experimentally that gravel-bed braided streams have stable cross-section geometry and braiding intensities, variations in morphological characteristics of a stream that occur at constant slope and discharge, and the downstream transfer of bed load. Periodic pulses in bed load over time can be related to avulsion events in braided streams. These pulses are generated by local erosion and aggradation of the stream bed. This process creates migrating bars and is the primary means of sediment transport.

9 8 Increases in bed load can cause channels to shift (avulsions) and bars to migrate downstream (bed load transport). This paper was poorly written and very difficult to understand. It seems as though some of the research was left out of the discussion and it limits the ability to fully understand this study. The research seems sound and scientific but seems to be lacking in great detail of what actually was studied. This paper is not very relevant to my topic therefore would not be used in a paper written on river avulsions. Conclusion River avulsions occur in a variety of different environments and can have a variety of trigger events. The avulsions can alter the landscape and create channels that were not previously there. The frequency of avulsions greatly varies from environment to environment and depends on things such as bed load transportation, sedimentation, and discharge. Rivers are dynamic and avulsions will continue to occur wherever fluvial processes are occurring. Avulsions are typically features of aggrading floodplains and streams. The exact cause of river avulsions is not fully understood at this point in time.