fluvial

**CE 598** ** Cherie De Vore (cdevore@unm.edu) ** Fluvial geomorphology is essential for understanding flow and sediment movement in a river. Changes in river channels over time and space are related to the dynamic relationship between local geologic conditions, hydrology, sediment regime and ultimately human induced alterations to the system. The forms produced by these relationships are also key to understanding the larger fluvial catchment down to the scale of the channel. The discipline of geomorphology is necessary, when integrated with other disciplines, for assessing engineering designs for restoration or other ecosystem services. Many components of a fluvial system may be investigated at different scales and time periods, but no factor can be isolated because of the variable interaction between geology, hydrology and hydraulics. Understanding geomorphic responses to anthropogenic alterations to a river system as well as natural disturbances is essential to effective water resources management and mitigation. Fundamental principles of geomorphology are employed in a case study of an active eroding bank in Bernalillo, New Mexico at the Bernillo Priority Project site. For centuries fluvial systems have been exploited to respond to the demands of society to harness the resources of rivers for the benefit of mankind. The understanding of fluvial geomorphology is an essential component of creating effective fluvial projects that address these alterations. Ultimately, the independent and dependent controls of the fluvial system must work in concert with economic and engineering management strategies.
 * Fundamentals of Fluvial Geomorphology and Applications to River Restoration **
 * Abstract **

Fluvial geomorphology is the science of river form and structure and the processes that create them. Applications of fluvial geomorphology are significant as growing concerns over multi-scale environmental change, water resource constraints and river restoration become the focus of human impacts to fluvial systems. The geomorphology and dynamics of a river system are defined by influences of climate and geology as well as vegetation and soils in the surrounding environment. In addition, the sediment and flow regimes of a system are determined by these parameters within a well defined spatial boundary (Figure 1). The river system can be characterized as an open energy and matter system, allowing exchanges with the outside environment. Although there is great variability in space or location, considerable variability of a fluvial system also happens through time. Described by Schumm as a process-response system, changes in the fluvial system through time and space are the result of erosional and depositional processes (1988).
 * I. ** ** Introduction **

The drainage basin is a fundamental spatial unit encompassing a fluvial network and is typically a well-defined topographic and hydrologic entity (Knighton 1998). The properties of the channel network are examined in concert with its modes of change or evolution. The analysis of a network begins with its foundation represented by two related concepts, stream order and drainage density (Knighton 1998). Stream order is one component of the fluvial network composition. Horton (1945) laid the foundation for network analysis through stream ordering, and it was later redefined by Strahler (1952). The method of classifying segments of channel by stream order is the basis of the combined Horton-Strahler approach.
 * II. ** ** The Fluvial Network **

The headwater of each perennial stream is designed an order of 1. Two 1st order streams joining at a confluence make a 2nd order reach downstream. Two 2nd order streams downstream of their confluence make a 3rd order stream and this can continue down into the system as illustrated by Figure 2. In the case of the junction of two streams with different stream orders meeting at a confluence, the segment downstream takes on the order equal to that of the higher order stream (Knighton 1998). A second type of stream order classification method uses link magnitude as its basis. Each junction in the network is a link and the link magnitude is the sum of the links, which is more in accord with the physical reality of stream networks.



__ Stream Classification __ The classification of a fluvial system is one significant component to the management of a stream and development of potential restoration or design. The morphology of the channel is one mechanism used to classify stream types. One of the earliest morphology based classifications was the Leopold and Wolman approach, which identified rivers according to their pattern. Patterns included meandering, braided or straight. More comprehensive approaches by Montgomery and Buffington, as well as Rosgen are useful for understanding stream condition in different settings and influences. In particular, the Rosgen classification (1994) provides one frame of reference for communicating stream morphology and conditions. Both quantitative and qualitative studies are integrated into this system that is a systematic way to name river characteristics. The combination of appearance, sediment relationships, reach attributes and equilibrium state provide some context for evaluating stream type with this classification system. Eight major variables influence stream pattern morphology and they include channel width, depth, velocity, discharge, channel slope, roughness of channel materials, sediment load and sediment size (Rosgen 1994).

The broad geomorphic characterization of streams using this procedure is described under Level 1 of the system that integrates landform, valley morphology, channel relief and pattern, shape and dimension. The influences of climate, depositional history and life zones on channel morphology help to delineate broad descriptions of stream types (Figure 3). Vertical or lateral modes of adjustment and energy distribution can be inferred in these broad stream types (Rosen 1994). The morphological descriptions described by slope changes and dominant channel-material particle sizes make up Level II of the Rosgen method. 42 initial major stream types are the result of this level (Figure 4). A data set of 450 rivers throughout the United States, New Zealand and Canada were use to refine ranges of delineative criteria for channel type. This level can incorporate field measurements due to changes in short distances along the river channel under the influence of geology changes and tributaries. The Rosgen classification and others can assist in organizing the observation of river data into a reproducible system used by professionals across many disciplines.





__ Stream Equilibrium __ The status of a system between the standard range of deposition and erosion is characterized by the product of sediment load and sediment size with the product of stream discharge and stream slope. All streams try to maintain a state of equilibrium by balancing the capacity of the stream to transport/discharge the sediment with the amount of sediment delivered to the channel from the watershed over the long term. One equilibrium concept is described as dynamic equilibrium because a fluvial system is never static. The stream’s dynamic equilibrium can be expressed with the “stream power proportionality” equation developed by Lane (Zaimes & Emanuel 1997).

The parameters of the equation include Qs (bed material), D50 (median grain size of the bed material), Q (dominant discharge) and S (stream slope). The resisting forces of the system are characterized by both Qs and D50. The driving forces are represented by Q and S. The resisting and driving forces are proportional to each other in Lane’s Equation. All four parameters must be balanced for the system to be in equilibrium. Disturbance of the system leads to aggradation and degradation (Figure 5).



The interaction of a number of variables in geomorphic processes is numerous. Variables that perturb the system must be met by an opposing force or state, which can mechanically be described as (a) uniformly distributed rate of energy expenditure and (b) minimum total work expended in the system (Langbein and Leopold, 1964). The concept of quasi-equilibrium is appropriate for fluvial systems because it is a condition with mean or intermediate position between two opposing tendencies concerning energy utilization, which are dynamic in any given system. Understanding these perturbations, which operate at different temporal and spatial scales, helps to explain the alteration of depth, width and plan form of a channel. These alterations are important for helping to identify problems in a river system and planning for subsequent restoration plans.

__ Sediment Transport __ Flow through an open channel is dependent on the principle forces of gravity and friction. Gravity acts on the downslope direction to move water through the channel and friction opposes downslope motion. The interaction between the two forces controls the capacity of the flow to transport and erode sediment in the channel. The load carried by streams during sediment transport can be described as dissolved load, wash load and bed material load (Knighton 1998). Sediment deposition happens when the shear velocity of the flow falls below the settling velocity of the sediment particle.

__ Stream Channel Formation __ The channel form is influenced by both the character of the flow regime and the magnitude of discharge in the system. Adjustments to the geometry of the fluvial system involve a number of variables. Four degrees of freedom represent different planes of adjustment (Figure 6). The channel bed slope, channel pattern, bed configuration and cross sectional form are adjustable variables of channel form. In addition to more adjustable constraints, the longitudinal profile is one component that is the least changeable. It is the diagrammatic representation of the change of elevation with distance.




 * III. ** ** Channel Characteristics **

__ Flow Cross Sections __ Flow in self formed channels is expected to show average form and geometry, which dynamically adjust to carry sediment and water discharges. The nature of that geometry can be described by flow cross sections. Channel pattern and bed topography are systematically related to local variations in cross sectional form. Flow geometry is a function of continuity, flow resistance and sediment transport. Hydraulic geometry assumes discharge is the dominant independent variable. Figure 7 illustrates the velocity profile in a meandering channel, the thalweg and cutbank view of the channel.

__ Bankfull Discharge __ The lateral characteristics of a stream are important for migration, floodplain inundation and a key feature known as bankfull discharge. This parameter is defined as the amount of flow that fills the channel to the top of the banks prior to overtopping the banks and flooding adjacent floodplain. Because changes in geomorphology occur at the bank, such as downcutting or dredging, flow required to fill the stream to bankfull is much more than the stream’s natural bankfull discharge. The dominant discharge Q is the single value most responsible for channel geometry, or the channel forming flow (Knighton 1998). Even within the same basin, bankfull discharge can have highly variable recurrence intervals.



** IV. ** ** Human Disturbance of the River basin ** Evidence for river channel change includes direct observations, historical records, sedimentary evidence and dating techniques. Various types of change during the evolution of a drainage basin can be natural evolution or human induced. Anthropogenic alteration of fluvial systems including dams, levees, channelization and other alterations, disrupt geomorpholoigcal processes, habitat, and the natural flow regime. There is considerable interest in restoring fluvial systems and their floodplains because of their valuable functions to both ecosystems and society, including water quality enhancement, flood control, erosion control and even endangered species habitat. Environmental disruptions of channel stability, water quality and habitats have gathered a growing movement in evolving river restoration and management.

This section of the paper discusses restoration measures at a site that integrates components of fundamental geomorphology and river engineering. A general overview and understanding of geomorphological changes are discussed in one area of the Albuquerque Reach of the Rio Grande before and after restoration treatment actions were implemented for bank stabilization. Before the Bureau of Reclamation project began in 2006, the Rio Grande was eroding the east river bank in Bernalillo, NM just south of Highway 550 at the Bernalillo Priority Site project area (Figure 9). The channel was meandering and making its way close to flood control levees and irrigation systems critical to the surrounding community. The river made an abrupt bend towards the east levee system at the project area site, which brought the bank less than 100 feet from the levee. Though it is useful to examine small scale changes to the bank line and open channel hydraulics, this section of the paper focuses on the planform or pattern scale of the system. Prior to this project, other attempts at preventing erosion included riprap placement but even at high flows, erosion potential was still eminent. Breaching the levee system and flooding adjacent parcels of Pueblo, private and Project facilities were the impetus for this BOR project, as well as considerations for endangered species habitat.
 * V. ** ** River Engineering and Management: NM Case Study at Bernalillo Bridge Priority Site **

Resulting from floods in the early 1950s and late 1940s, the Rio Grande was channelized to control sedimentation and flooding. Jetty jacks were placed along the floodway to control the location of the channel but nevertheless, bank erosion began to occur under the jetty jacks at the Bernalillo Priority Site. In addition, exotic vegetation is associated with highly altered hydrologic regimes so consideration of riparian vegetation removal is important for planning erosion control measures. Salt cedar and Russian Olive are present at this site.

Dietrich Kinzli (2009) used a design flow of 6,000 cfs (cubic feet per second) in a habitat restoration study, and it was chosen because it represents the general bankfull discharge of the Rio Grande in this particular reach. Google Earth images from June 21, 2005 illustrate a high runoff event that persisted from mid-April well into July as seen by the hydrograph at Alameda Bridge (Figure 10). According to the definition provided by Dietrich Kinzli, bankfull was achieved and the channel inundated during this period of time. Figure 11 shows how close the inundation is to the levee during such high flow conditions.

The channelization period for the Albuquerque Reach occurred between the 1950s and the 1970s from Cochiti to the south near Elephant Butte, New Mexico. The design for channel widths achieved by the channelization had widths for clear channel area between jetties that varied between 900 and 500 feet. The channelization process included installing jetty jacks, clearing floodways, and the use of low flow conveyance channels (Bureau of Reclamation 2005).

Background hydrologic conditions are important in considering design methods for erosion control. Different characteristics of flow will have impacts on geomorphic characteristics of the Rio Grande. Though the Middle Rio Grande receives discharge from a few tributaries, the water supplied from higher portions of the basin defines the hydrology of the Albuquerque Reach. Along with a huge shift in peak flow regime in the 1950s, the construction of Cochiti dam in the 1970s dramatically altered the flow and sediment characteristics of the system (Bureau of Reclamation 2009).

Understanding the historical geomorphology of the reach helps in planning for channel restoration. The Middle Rio Grande was a braided system with wider channels than what is currently observed today (Figure 12). The Albuquerque Reach tended to aggrade, which drove the movement of the channel laterally across a wide floodplain. With a historically sandy bed, much of the Albuquerque Reach is in a transition state trending towards a coarsening gravel bed and the formation of vegetated islands from bars that were active in the late 1980s (Scurlock, 1998 & Crawford et al., 1993).



The Rio Grande is deepening, narrowing, and transitioning to gravel in the Bernalillo reach and meanders resulting from changes in flow regime and sediment supply. Flood control facilities on the Rio Grande and its tributaries contribute to the decrease in sediment supply. In addition to the unbalanced sediment regime, increases in sediment transport capacity can also cause major changes in channel characteristics. The response of the river is to entrain more sediment through bank erosion. Meandering and lateral progression is the river’s adjustment to increases in transport capacity. Changes in land use or other large scale modifications to the hydrology are common factors that drive changes in sediment transport capacity.
 * i. ** ** Case Study: Current Conditions and Proposed Actions **

Proposed actions for the Bernalillo Priority Site included a number of treatments that included removal of jetty jacks, excavation of a secondary channel, installation of bendway wiers and installation of rootwads and debris piles. The jetty jack removal essentially provides better access for construction and project features. The secondary channel was excavated to split the river flow into two channels. Located at the upstream end of the sharp bend, the same gradient as the main channel is characteristic in the second channel but is expected to develop a much deeper thalweg. Up to 1/3 of the flow during high flow conditions is expected to flow through the secondary channel and up to 1/6 at low flow conditions (US Fish & Wildlife 2005).

In addition, bendway weirs were installed on the outer bank to reduce deepening and reduce adjacent river bank erosion. Though they promote bank stability during high flows, some scour is expected to occur near the toe of each weir which would promote the development of a new thalweg into the channel. There were a total of 13 weirs installed in the channel constructed primarily of rip rap. Finally, the main channel was realigned in an attempt to lengthen the bend located near the levees. Final measures of bank stabilization included the installation of rootwads at the weirs to mitigate flows directed at the bankline from the eastern channel. The rootwads also buffer eddies that cause erosion behind the last weirs. Debris piles at the tip of the island ensure split flow and help to prevent erosion of the island between the two channels (U.S. Fish & Wildlife 2005).

Bendway weirs are useful for deflecting the flow and energy away from the outer bank at the Bernalillo Priority Project site. Figure 13 illustrates the initial close proximity of the bankline to the eastern levees in comparison to a much more aligned main channel with weirs in place. There are also implications for endangered species habitat as the weirs slow down the velocity of the stream and create slackwaters for the silvery minnow. This project investigation highlights the reach’s low annual peak flows, channel narrowing, structural changes and incision in the system. These criteria are dynamic in the Rio Grande and the combined influence of human activities make restoration and erosion treatment difficult for the Albuquerque Reach. Project maintenance, especially for the weirs, though not usually highlighted in many proposed action plans, should be a consideration for Bernalillo Priority Site, as well as others within the system.
 * ii. ** ** Case Study: Remarks **



Rivers are a fundamental part of the human and natural environment. A summary of geomorphological fundamentals illustrate that as agents of erosion and transportation, rivers have the ability to move water and sediment supply from land surfaces to the ocean over various spatial and temporal scales. In their mechanical and hydrological work, rivers develop a wide range of channel forms and networks. Because rivers are open systems exchanging energy and mass with its surroundings, any modification to a river resulting from human or engineered changes has implications for surrounding ecology and basin morphology. Flood and sediment control as well as other traditional river engineering practices have created instability and environmental problems, with a good example illustrated by the Bernalillo Priority Project site in this paper. However, river engineering has the potential to enhance and preserve the conservation value of the river. A combination of active and passive restoration approaches are possible in any number of projects and applications of fluvial morphology are critical in this endeavor. Independent and dependent controls on the fluvial system, character and behavior should be considered in what Knighton proposes as the interrelationship of the fluvial system (Figure 14).
 * VI. ** ** Conclusion **



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