Fluvial GeomorphologyCE 598 River Restoration

Lyle C. Begay

February 20, 2012

1. Introduction

Fluvial Geomorphology (FGM) is the study of streams and their interaction with the geologic terrain it has formed therein. It is a multidisciplinary synthesis of engineering, biology, geology, earth science, hydrology and other science disciplines. It is a developing practice in the area of natural stream channel design stemming from movements to restore river systems. Fluvial Geomorphology FGM is a holistic science-based view of restoring stream channels by empirical studies such as sediment transport, river discharge, channel size, mapping and others.

2. History of the Fluvial Geomorphology Field

Fluvial geomorphologyFGM has its roots in the fields of engineering and geoscience that have progressed over the past two centuries. The term fluvial is defined as of, relating to, or inhabiting a river or stream and geomorphology is the scientific study of landforms and the process that shapes them (en.wikipedia.org). In essence fluvial geomorphologyFGM is the study of landscapes formed from a flowing body of water. Geomorphology is a sub discipline of geoscience and holds a closer relationship to contemporary fluvial geomorphologyFGM. In its essence, geoscience adheres to the inductive scientific method of observation, hypothesis and experimentation. Experimental geoscience is based on a Baconian methodology by conducting research and producing data for future studies.

In contrast, engineering applies mathematical Newtonian mechanics in order to design structures and systems to promote quality of human life. Since the 18 th century engineering design was applied to river systems in order to alleviate flood risk, develop nautical transportation, energy generation and recreational use. The engineering practices provided the framework for early river management, developing laws and standards for water operations. Detention dams, locks and stream channelization were all measures of river control and stabilization but recently have shown costly effects. Such examples include maintenance cost, bed armoring, sediment loading, non-point source pollution and riparian degradation.

Over the past several decades the environmental disruptions of channel stability, water quality and habitats have gathered a growing movement in evolving river restoration and management. A cooperative approach to stream restoration requires understanding the biophysical nature of

the river based on its geographic structure and the region’s fluvial process. This incorporated with hydrologic and hydraulic engineering data allows for a more rounded approach to river rehabilitation and non-degrading structures.

Society has become disenchanted with full exploitation of riverine systems and governments such as the US and UK has required environmental consideration in their developments. At the same time fluvial methods have expanded and literature has been appropriated especially during the past three decades. Understanding of a river systems involves understanding its historical evolution and definable future involving the implementation of river design. This understanding established the demand for geomorphologists to develop a body of knowledge into rather unknown topic areas such as gravel bed-rivers in extreme environments and continues so today. Specialists in fluvial geomorphologyFGM arose in the 1980s, such as Schumm who describe morphologists as investigators that evaluate models of the landscape by deductive reasoning and measurements of erosion based on extrapolation of empirical relations. During this time Schumm described the components of the fluvial system in three parts and the sediment process which will be discussed further. Leopold and Baker further described the fluvial process in geomorphic terms by publishing field and laboratory research which illustrate water impact on varying landforms, drainage basin patterns and methods for management.

In management discourse policy makers and legislation aligned engineers and geomorphologists during the turn of the century. The two schools of thought formed a relationship around water with differing views; the engineer with intervening institutionalized solutions and the geomorphologist with long-term independent observations. Together the field of FGM has evolved with the bond between the two camps over numerous case projects.

Table 1: Comparison of skills in FGM IntegrationEngineering GeomorphologyDesign Experience Field ExperienceHydraulics Sediment Supply/TransportProject Timescales Longer TimescalesSpecialist Function Generalist BreadthProtection Techniques Erosion/Deposition ProcessesSimple Channel Forms Complex/Sinuous Channel DynamicsReach Scale Basin ScaleAccredited Standards Personal InsightsHard Hat Chest Waders

3. Applications

The following are examples of case studies implemented with environmentally conscious river design as taken from the text Applied Fluvial Geomorphology for River Engineering and Management (Thorne et al., 1997). In simplifying the amount of input from other disciplines (biology, chemistry, law, etc), four classes of geomorphological consultations are represented in the studies.

Direct Intervention in Channel Management

Channel Designs for Flood Protection at Environmentally Sensitive Sites

The first case study was an evaluation of flood relief systems in England and Wales conducted for the UK Ministry of Agriculture. The objectives of the project were to: 1) evaluate performances of implemented systems in terms of discharge capacity, channel stability, construction & maintenance costs and conservation value and 2) develop guidelines for the design of stable flood alleviation/land drainage schemes on mobile-bed channels on the basis of objective 1.

Table 2: Classification of Flood Alleviation SchemesUrban Rural

Upland Brecon WDB Byton RF

DuffieldWSCRB Conistone FP

Matlock DBRKendal WDBRExeter WDRMTiverton WD

Lowland Banbury WDS RedhillDCTW

Sidmouth DBR Abridge ABath DBR Colyford RMFTrowbridge DB Newton Flotman AMBury St. Edmunds WDCB Saffron Walden WDTSaffron Walden WDTColyton RMF

W Widening R WeirsD Deepening F Flood BanksS Straightening M Diversion ChannelT Trapezoidal Section P Partial DredgingC Concrete-Lined Channel A 2 Stage ChannelB Bank Revetment

The sites were chosen based on their representation of traditional engineering techniques and evaluated on their riverine environment impacts. The study sites were divided into upper and lower river environments to better compare different systems as can be seen in table 2 (Torne et al., 1997). The sediment transport equation displayed the best forecasting of erosion/deposition in the reaches coupled with the Hey resistance function to evaluate discharge conditions produced comparable measures of environmental impact of each structure. The only stable schemes were those incorporating distant flood banks due to the lack of sediment transport regime alteration (erosion/deposition). Two-stage channels are viable due to minimal siltation if applicable, otherwise bank-adjacent levees would be the preferred over diversion channels and resectioning.

Indirect/Contingent Intervention: Protecting and Maintaining Channel Capacity

Erosion Protection: Banks

Headward and bank erosion at meander bends along a reach of the River Roding in Loughton, Essex, UK was intensified by channel straightening. Geomorphologists were commissioned to evaluate the use of submerged vanes to alleviate the rate of erosion. The number, size and location of the vanes were calculated to oppose torque forces in the stream and subsequent trials and theory provided for higher degrees of erosion dissipation. Each vane was 3 m long and 1 m high and installed so as to be submerged even during low flow conditions. The developed vane was installed in January 1989 with a fixed position with a main and minor anchoring pile. A survey was carried out after installation and indicated erosion rates were significantly reduced along with bank undercut, over-steepened and mass failures.

Geomorphology of Channels and Water Resource Management

Regulating Reservoirs and Channel Stability

Severn-Trent and Welsh Water Authorities acquired consultation on the maximum levels of release that could be made to the Wye and Severn rivers which would allow their natural stability to be maintained. The basic necessity was to establish release thresholds in regard for bed material entrainment downstream from the outfalls, making the choice of sampling location important. Sites were determined from sinuosity changes implying net erosion and natural instability.

4. Fluvial Process

In order to understand the geomorphic classifications in stream restoration the fluvial processes must be introduced. The hydrologic cycle pictured below (figure ) illustrates the water scheme in its most general form. The location in which the water cycle occurs, the highest elevations of the land form the boundaries of a watershed, the crown shaped area of land draining water to the

lower elevations where streams form. Moved by the force of gravity the water infiltrates the soil along its route, transpires into the atmosphere and/or collects into streams and forms a flowing concentrated body of water. This reaction between earth and water is the focus of Fluvial GeomorphologyFGM and is dependent on the driving and resisting forces of a river system in response to erosion and transportation of debris sediment ().

The basic mechanics of the fluvial process involve the balance of the river system’s potential energy. The energy of the flowing water produced in the system (driving force) is balanced by the system’s ability to consume that flow’s energy (resisting force). This balance, or dynamic equilibrium, is a function of the river’s slope, earth materials (sediments), roughness, flow velocity, climate, and channel width and depth. As a fluvial geomorphologist, these factors are accounted for from the site location itself with a good understanding of the river system’s history.

Time factorization itself is a distinction of the river’s life course and can indicate what changes have occurred to the system. It is a fluvial geomorphologist’s task to appropriate when and what type of event occurred to form the current system and its future possibilities. It is worthwhile to note that river system parameters are in a constant flux, continually adapting in response to others; therefore it is more apt to use the term quasi-equilibrium.

A widely accepted representation of the equilibrium concept is Lane’s Balance relationship which illustrates the changes in a system dependent on four major factors in the formula

(Resisting) QsD50 α QwS (Driving)

where Qs is the bed material, D50 is the median grain size of the bed material, Qw is the dominant discharge, and S is the stream slope. Lane’s balance of resisting vs. driving forces shows that a change in one factor causes a change in another resulting in degradation, bed

Figure 1. : The hydrologic cycle. Figure 2.

scouring/polishing, or aggradation, the deposition of sediment. By this relationship, the system is in equilibrium if the sediment load is transported in and out of the reach.

As mentioned in section 2, Schumm (1977) categorized the fluvial process with regards to the sediment load as a system in three parts (Figure )(1977). The three sediment zones are designated: (1) the upper sediment production zone, (2) the middle sediment transfer zone and (3) the lower sediment deposition zone. The upper sediment production functions as the initial runoff of water from higher elevations with steeper slopes allowing for deeper incisions and cuts into the earth. This zone sees the most erosion activity and develops steeper watercourses such as cascading streams and waterfalls. The middle transfer zone operates as a conveyor belt with softer slopes and is the dominant focus of river restoration. This river reach is the longest of the three zones and human populations more commonly reside in these areas and see meandering or braided streams. The lower sediment deposition zone may be a lake, delta, or reservoir where the sediment load is dropped off. In river rehabilitation, all three zones must be considered so as not to significantly disrupt the transfer of sediment from zone 1 to zone 3.

Figure 3. : Lane’s river energy balance scheme.

Since restoration efforts concentrate on the transfer zone, two fluvial landforms that can be discussed within this region are the floodplains and terraces. Floodplains are flat areas of land that reside along a river and are formed from the deposition of sediments during flood events. Terraces themselves are ancient, abandoned floodplains of a river and are step like in cross section emanating from the river. The river is usually the lowest point as it is the main driver downcutting into the earth and over time erodes into a new floodplain. Terraces can reveal the events the land has undergone such as changes in climate, vegetation, tectonic shifts, discharge, and sediment load. More recently human influence such as the installation of dams and high runoff from urbanization has altered the flow regime and produced more terraces.

Figure 4.:The fluvial system in three zones sediment source, transfer, and accumulation.

Figure 5. : Terrace and floodplain cross-section.

5. Channel Patterns

The fluvial process in the transport zone of concentration form several different plan-view patterns. Each pattern is a result of the river system’s hydraulic adjustment of slope, water discharge and sediment load. The primary types of channels are the straight, meandering and braided forms (Figure Figure #). In order to understand the geologic formation of the channel patters, the river cross section features must be defined. In figure Figure #, a straight and meandering reach are shown to accumulate bed material called bars on opposing sides of the stream sequentially. The dashed line represents the thalweg and connects the deepest parts of the channel and in most cases the line of fastest flow. The riffles are shallow areas and are successive between bars while pools are deep areas just opposite of bars.

The interaction of riffles and pools are the physical result of the balance relation and are important in illustrating how sediment, flow, and bedforms maintain equilibrium (figure Figure 7). Riffles, being shallower, have higher flows compared to deeper pools where bank erosion takes place. The excavating pool may allow sediment to be removed from the bank surface, a process called corrasion. Two others processes are overhanging bank ledges failing and falling over time and pre-existing vertical cracks in the bank meeting intersecting waters and collapsing to the surface.

Figure 7. : Features of (A) straight (B) meandering rivers. T=thalweg, B=bar,

R=riffle, P=poolChannel patterns Schumm (1985)

6. Fluvial Geomorphology in Action

The practice of fluvial geomorphology takes into account a gradient of physical variables associated with the stream from the entire watershed and its climate to the soil sediment type. Figure 5 8 illustrates the spatial scale of a river system (figure Figure 9) reviewed in a river restoration project of channel in the transfer zone.

Characterization of the river reach conditions usually begins at the macro-level with a calculation of the watershed area. The drainage area features can be determined by means of a Geographic Information System (GIS) with a topographic map layer such as the U.S. Geological Service (USGS) digital line graphs. The area size depends on the length of the restoration reach and project magnitude but delivers a representative computer model. Next is the determination of land use utilizing the National Resource Conservation Service (NRCS) Curve Number which demonstrates the percentage of impervious surfaces in the watershed resulting in stormwater runoff. The Curve Number accounts for the regional climate and soil types infiltration rate. This land use survey information will be utilized in determining bankfull elevation and characteristics.

Next, the river channel pattern can be evaluated by measuring its sinuosity and radius of curvature at bends. The sinuosity (K) can be determined by dividing the stream length by its valley length (figure Figure 10). Two relevant measurements are the stream’s belt length, the straight line distance between the two outermost bends, and meander wavelength. The calculations and restoration recommendations can be found in the Stream Restoration A Natural Channel Design Handbook.

Figure 8. : Spatial images of fluvial geomorphic investigation

Figure 9. : Diagram illustrating sinuosity calculation.

Next, the channel cross-section dimensions can be obtained by a field site visit and surveying measuring tools. Several sites should be chosen based on the geomorphologist’s judgement with the suggestion that one permanent cross-section be placed over a riffle and another over a pool. Next, the bankfull elevation is determined which is the highest point the stream can reach before it overtops the levee onto the floodplain. Concurrently, the bankfull discharge, velocity, depth, and entrenchment ratio can be calculated as well. The determination of bankfull discharge can be subjective and because of that regional curves have been developed by the APWA American Public Works Association (APWA) (American Public Works Association) to correlate drainage area to bankfull discharge.

An analysis of the bed and bank material can be done after the reach’s longitudinal profile is calculated into percentages of riffle and pool. Pebble samples can be obtained by their respective profile percentage. For example six riffles and four pools counted in a reach allows for ten pebbles to be gathered at each riffle accounting for 60% of the reach profile. The pebble samples are then measured along their axes as shown in Figure 11. The movement of sample retrieval proceeds from the left bank to the right looking upstream. The data is collected until all 100 pebbles are accounted for with their size and type and recorded in a spreadsheet.

Rosgen Classification System

One major stream classification system utilized is the Rosgen Method. The field measurements performed are used to describe the stream type and conditions. The system is broken into four levels 1) geomorphic characterization, 2) morphological description, 3) stream condition assessment, and 4) validation and monitoring. The Rosgen System utilizes various diagrams comparing qualitative and quantitive values which will not be explored here.

The first level is a description of the stream by its slope, cross-section and plan view characteristics. The second level involves field measurements described in Fluvial Geomorphology FGMin Action and assigns a numbered stream type. The third level is an evaluation of stream conditions and predicts its stability, erosion pattern, flow regime, riparian changes, and others. The fourth level is a validation of level three by measurements of sediment transport, stream flow, and stability.

Figure 10. : Pebble measurement along axes

Montgomery Buffington Classification System

This classification goes beyond Rosgen’s and accounts for all three of Schumm’s zone types. This classification system accounts for the source zone’s colluvial system and the deposition zone’s bedrock system. The Montgomery Buffington matrices classify the river’s response to sediment input.

Figure 11. : Montgomery Buffington sediment activity along river course.

The different reaches are classified by their geomorphic response to sediment transport and the Montgomery Buffington classification system takes the whole fluvial system into account. In the figure above from the fgmorph.com website the source zone on the left is an area where sediment transport is limited in the streams meaning that sediment loads exceed the stream’s ability to carry it therefore more deposition occurs. In the third deposition zone where bedrock is the hydraulic ability of the stream is greater than the sediment load and is termed supply limited allowing for more scouring to occur. That is why in the first source zone there is minimal geomorphic change in the channel while changes in the flatter third deposit zone sees physical land adjustment with high sediment load.

7. Biological Consideration

In stream restoration the vegetation and aquatic life benefit from a river restored to a more balanced system where the driving and resisting forces are in quasi-equilibrium. In a system that is in quasi-equilibrium, the channel does not go through extreme geomorphic changes. This allows for water temperature and chemistry to remain stable. With more sustained river discharge, extreme erosion events are less likely to occur allowing for bankside vegetation to remain and continue the nitrogen cycle. Trees provide shade for the water, organic detritus, food, and hiding places for wildlife.

The riparian vegetation along the stream bed stabilizes the bank and filter pollutants and sediment allowing for better water quality for aquatic species, reduce flow velocities and trap sediment (Simon, Bennett, and Neary, 2004). Natural stream processes allow the channel bed material to be mixed without aggradation or degradation. The sorting of bed material results in habitat diversity through space openings for resting, feeding, and reproduction. The pools in a rehabilitated river system provide a retreat for fish out of high velocity currents.

Conclusion