W W W W Wood in Ri ood in Ri ood in Ri ood in Ri ood in Riv v v v ver Rehabilitation and Management...

American Fisheries Society Symposium xx:xxx–xxx, 2003© Copyright by the American Fisheries Society 2003

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WWWWWood in Riood in Riood in Riood in Riood in Rivvvvver Rehabilitation and Managementer Rehabilitation and Managementer Rehabilitation and Managementer Rehabilitation and Managementer Rehabilitation and Management

TIMOTHY B. ABBE

Herrera Environmental Consultants, Inc.2200 Sixth Avenue, Suite 1100, Seattle, Washington 98121, USA

ANDREW P. BROOKS

Centre for Catchment and in-Stream ResearchGriffith University, Nathan, Queensland, Australia 4111

DAVID R. MONTGOMERY

Department of Earth and Space SciencesUniversity of Washington, Seattle, Washington 98195, USA

Abstract.—Wood induces hydraulic, morphologic, and textural complexity into fluvial sys-tems in forested regions around the world. Snags and logjams can create complex networksof channels and wetlands across entire river valleys and historically posed a significant ob-stacle to navigation. The clearing of wood from rivers and riparian forest land along streamsand rivers reduced or eliminated the supply of wood into rivers in many regions of theworld. Ecological restoration of fluvial environments increasingly includes the placement ofwood. But few guidelines exist on appropriate methods for emulating natural wood accu-mulations, where and how to place wood, its longevity, the hydraulic and geomorphic con-sequences of wood, and how to manage systems where wood is reintroduced. Importantissues to understand when placing wood in rivers include the watershed and reach-scalecontext of a project, the hydraulic and geomorphic effects of wood placements, possiblechanges in wood structures over time, and how it may impact human infrastructure andsafety. Engineered logjams constructed in Washington, USA and New South Wales, Austra-lia offer examples of how wood reintroduction can be engineered without the use of artifi-cial anchoring to form stable in-stream structures as part of efforts to rehabilitate fluvialecosystems and provide ecologically sensitive means to treat traditional problems such asbank stabilization and grade control.

IntroductionIntroductionIntroductionIntroductionIntroduction

The geomorphic effects of wood on fluvial sys-tems range in scale from controlling bed forms andinfluencing channel patterns to floodplain devel-opment (e.g., Wolff 1916; Davis 1901; Guardia1933; Keller and Swanson 1979; Lienkaemper andSwanson 1987; Harwood and Brown 1993; Abbeand Montgomery 1996, 2003; Buffington andMontgomery 1999; Brooks 1999; Brooks andBrierley 2002). Snags and logjams can be the prin-cipal mechanism creating habitat complexity notonly within an active channel, but also by induc-ing localized flooding and creating and sustain-ing secondary channels and wetlands (Triska 1984;

Sedell and Froggatt 1984; Abbe 2000; Collins andMontgomery 2002). Examples of these complexfluvial systems with numerous side channels thatextend across much of a river valley are increas-ingly rare (Figure 1). Habitat complexity directlyor indirectly related to wood clearly benefits manyaquatic ecosystems (e.g., Pearsons et al. 1992;Quinn and Peterson 1996; Lehtinen et al. 1997;Inoue and Nakano 1998), and reduced fish popu-lations can reflect the extensive loss of physicalcomplexity in fluvial systems resulting from woodremoval, channelization, and floodplain develop-ment (e.g., Shields and Smith 1992; Beechie et al.2001; Collins et al. 2003; Pess et al. 2003). Reha-bilitation of fluvial ecosystems depends on re-es-

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tablishing natural processes and conditions thatcreate and sustain physical complexity, such asrestoring instream structure and cover, mature ri-parian forests, floodplain connectivity, and chan-nel migration zones (CMZs). Reintroduction ofwood is an important part of river rehabilitationbut must be balanced with the potential for physi-cal changes that are ecologically beneficial to havenegative consequences to existing human devel-opment. The use of wood to restore streams andrivers should be based on sound science, engineer-ing, and an understanding of the magnitude towhich the system may change.

Although the role of wood in many parts ofthe world remains to be investigated, it has beenfound to play a significant role in the ecology andmorphology of streams and rivers in a wide rangeof climates and physiographic regions, includingAsia (e.g., Inoue and Nakano 1998; Rikhari andSingh 1998), Australia and New Zealand (e.g.,

Mosley 1981; Nanson et al. 1995; Gippel et al.1996a; Weigelhofer and Waringer 1999; Brooksand Brierley 2002; Webb and Erskine 2003), Eu-rope (e.g., Gregory et al. 1993; Piegay 1993;Maridet et al. 1996; Piegay and Gurnell 1997;Gurnell and Sweet 1998; Hering et al. 2000a, 2000b; Diez et al. 2001; Kail 2003), northeastern NorthAmerica (e.g., Zimmerman et al. 1967; Thompson1995; Beebe 1997), southeastern North America(e.g., Veatch 1906; Guardia 1933; Diehl 1997;Wallerstein et al. 1997), southwestern NorthAmerica (e.g., Haden et al. 1999), and northwest-ern North America (e.g., Keller and Swanson 1979;Harmon et al. 1986; Lienkaemper and Swanson1987; Andrus et al. 1988; Robison and Beschta1990; Nakamura and Swanson 1993; Maser andSedell 1994). Historical management of wood inrivers has focused on the removal of snags andlogjams, which were considered threats to navi-gation, flooding, and even fish passage (e.g.,

FIGURE 1. Snags and logjams contribute to the development of a complex mosaic of anastomosing channels,wetlands, and floodplain forest in river valleys such as Taiya River in southeast Alaska.

WOOD IN RIVER REHABILITATION AND MANAGEMENT 3

Ruffner 1886; McCall 1984; Maser and Sedell 1994;Collins et al. 2002). Additional river managementpractices that have reduced the supply of woodinclude dams, bank stabilization, and removal ofriparian forests. In many regions, retention ofwood was further diminished by the loss of largetrees capable of forming stable snags, as well aseffects associated with channel straightening,levees, and navigation improvements. But it isworth noting that, through much of history, woodhas been considered a viable material for instreamstructures and extensively utilized for infrastruc-ture in rivers, such as bulkheads, spur dikes,bridge abutments and piers, weirs, and dams.

Large scale efforts to reintroduce wood tostreams and rivers have been limited to the Pa-cific Northwest of North America. An increase inthe passive reintroduction of wood associatedwith the reforestation of riparian areas, such asoccurring in parts of the United States and Eu-rope, will have physical consequences, such asincreased channel width and complexity (e.g.,Davies 1997; Trimble 1997; Collins and Montgom-ery 2002), increasing flood inundation, and redi-recting channels (Figure 2). Such physical effectscan improve ecological conditions, but they maynot be compatible with existing or future devel-opment and can pose a significant challenge toefforts to restore streams and rivers. Channelclearing (removal of snags and logjams) contin-ues to be a common practice in river managementaround the world despite international recogni-tion of the ecological benefits of wood and stud-ies demonstrating that significant quantities ofwood can be left in some channels without ad-verse impacts on flow conveyance (e.g., Shieldsand Gippel 1995; Gippel et al. 1996b). Becausewood is not simply a “cosmetic” feature in streamsand rivers, greater accountability for potential ef-fects may be demanded for projects incorporat-ing direct or passive wood reintroduction. Suc-cessful rehabilitation of fluvial ecosystems willdepend on understanding the dynamics and ef-fects of wood and identifying and resolving po-tential problems (e.g., Maridet et al. 1996; Abbe etal. 2003). While the scientific understanding of thephysical dynamics and effects of wood is still de-veloping, substantial progress has been made re-garding the patterns in which wood accumulates,its hydraulic and geomorphic effects, mechanicsand longevity, and the performance of woodplacements (e.g., House and Boehne 1985, 1986;Murphy and Koski 1989; Gregory et al. 1993;Shields and Gippel 1995; Abbe and Montgomery

1996, 2003; Gippel et al. 1996a, 1996b; Abbe et al.1997, 2003; Wallerstein et al. 1997, 2001, 2002;Gurnell and Sweet 1998; Syndi et al. 1998;Braudrick and Grant 2000).

Wood influences fluvial geomorphology byaltering both the hydraulics and distribution offlow within a fluvial corridor and, thereby, thedeposition and transport of sediment. Instreamwood can provide stable flow obstructions thatnot only store sediment, but can also sustain andeven enlarge pools in aggrading channels thathave been subjected to increases in sediment sup-ply (Lisle 1995; Lisle and Napolitano 1998). Natu-ral wood accumulations can dam headwaterchannels (e.g., Keller and Swanson 1979; Marston1982) and redirect the course of large rivers (Wolff1916; Guardia 1933; Sedell and Froggatt 1984;Triska 1984; Abbe and Montgomery 1996; Collinsand Montgomery 2002). Numerous studies havedescribed significant geomorphic effects of woodon relatively small channels (e.g., Zimmermanet al. 1967; Keller and Swanson 1979). For ex-ample, instream wood can account for 8% tomore than 80% of the elevation loss of montaneforest channels (Tally 1980; Marston 1982; Th-ompson 1995; Abbe 2000). Montgomery andBuffington (1997) show that reach-scale channelmorphologies associated with lower gradientscan be imposed upon much steeper channels bythe presence of wood. In the coast ranges of west-ern Washington and Oregon, USA, stable logsteps transform stream segments from bedrockto alluvial channels (Montgomery et al. 1996,2003). When wood is removed from thesestreams, the sediment transport capacity exceedssupply and the channel segments revert to bed-rock. Stable instream wood accumulations canalso increase pool frequency (Andrus et al. 1988;Robison and Beschta 1990; Montgomery et al.1995; Abbe and Montgomery 1996; Beechie andSibley 1997), and individual snags or log jamscan have both local effects on channel bed tex-ture (Buffington and Montgomery 1999) andbroader effects on stream morphology (e.g.,Keller and Tally 1979; Nakamura and Swanson1993; Montgomery and Buffington 1997). In somesand-bed channels, virtually all pools can be at-tributed to either the direct or indirect control ofwood (Brooks and Brierley 2002; Webb andErskine 2003).

Wood can also have reach-scale effects onchannel form in large rivers (e.g., Davis 1901;Russell 1909; Wolff 1916; Triska 1984; Abbe andMontgomery 1996; Collins and Montgomery

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FIGURE 2. (a) The Deschutes River approximately 20 km south of Olympia, Washington on 4 February 2002.Flow is to the north (location 1). The large logjam (2) began forming in the 1990s and grew substantially in thewinter of 2001–2002. As a result, river stage upstream of the logjam increased about 1.3 m during low flowconditions (~7 m3/s versus a mean daily discharge of 14 m3/s). Higher water elevations inundated adjacentfloodplain areas (locations 3–5) and threatened low-lying homes (location 6). (b) Newly developed channelthrough relatively young floodplain on left side of logjam (location 3 on photo a). (c) Newly developed channelthrough mature cedar floodplain on right side of logjam (location 4 on photo a). All photographs were takenduring low flow conditions, and arrows depict direction of flow. Despite the significant increase in the extentand complexity of aquatic habitat resulting from the logjam, it was cleared by private landowners in the fall of

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2002). The anastomosing channel pattern (i.e.,multiple channels separated by vegetated islands)of some large alluvial rivers has been attributedto an abundance of snags and logjams (Sedell andFroggatt 1984; Harwood and Brown 1993; Abbe2000; Collins and Montgomery 2002; Abbe andMontgomery 2003). Effects of wood accumula-tions can extend beyond the confines of thebankfull channel to the development of flood-plains and even terraces (Abbe 2000; Abbe andMontgomery 2003). In some circumstances, it isclear that wood accumulations were instrumen-tal in vastly expanding the area of land subjectedto inundation and fluvial processes (Ruffner 1886;Veatch 1906; Guardia 1933; Triska 1984; Collinsand Montgomery 2001; Brooks et al. 2003).

Observations of geomorphic change that oc-curred after removal or addition of wood illus-trate the magnitude to which wood can controlriver gradient and store immense quantities ofsediment. A particularly striking example is givenby clearing of wood from the Red River in Louisi-ana, USA, which caused portions of the river toincise more than 4 m (Veatch 1906; Guardia 1933;Harvey et al. 1988a, 1988b), increased the rivergradient by an order of magnitude, and resultedin a sixfold increase in sediment transport capac-ity (Harvey et al. 1988a). Conversely, large woodaccumulations can result in substantial aggrada-tion of channels and floodplains. After a singleyear of unexceptional flows, the channel bed up-stream of a logjam on Alta Creek, a tributary ofthe Queets River in Washington State, aggradedover 4 m (Abbe and Montgomery 2003). Channelaggradation upstream of wood accumulations re-sults in distinctive landforms that resemble ter-races but have surface gradients less than thoseof the valley in which they occur.

Brooks (1999) presents a dramatic example ofthe effect of instream wood debris and floodplainforests on the geomorphology of rivers in south-east Australia by comparing two adjacent sand-bedrivers: the relatively undisturbed Thurra River andthe extensively disturbed Cann River. By early inthe 20th century, the Cann River floodplain foresthad been extensively cleared. The channel was thenprogressively cleared of wood through the 20thcentury. Prior to channel incision, the Cann Riverhad not experienced any other significant types ofdisturbance (e.g., dams, grade control), and, out-side the valley bottom, the catchment remains un-der forest cover. Hence, channel and floodplainclearing represent the only feasible explanationsfor the river’s present condition.

Relict channels and sedimentology of theCann River’s floodplain indicate that the riveroriginally resembled the nearby Thurra River,which has remained undisturbed. The sand-bedThurra River has a narrow sinuous channel withvery high wood loading (0.032–0.044 m3/m2

within the wetted perimeter) that flows througha heavily forested floodplain (Brooks and Brierley2002). Over the last 100 years, however, and par-ticularly the last 30 years, the channel capacity ofthe Cann River increased 700%, its morphologi-cal bankfull discharge has increased 45-fold, meanannual sediment transport has increased 850-fold,and peak instantaneous sediment transport capac-ity has increased up to 40,000-fold. Bank stabilityparameters have also fundamentally changed, ashave roughness characteristics of the channel andwood (Brooks 1999; Brooks et al. 2003). The ThurraRiver has exhibited no such changes, demonstrat-ing not only the profound effect wood and ripar-ian vegetation have on channel morphology andprocesses, but also on the evolution of the flood-plain. A key finding from the undisturbed ThurraRiver was that, in addition to the wood containedwithin the channel wetted perimeter, wood com-prised around 7% by volume of the channel bedto a depth of 1.8 m (Brooks and Brierley 2002).The hydraulic roughness and stability impartedby this wood, coupled with macrophytes grow-ing on the bed, prevented incision into what mightotherwise be a highly unstable channel, therebyallowing the long-term aggradation of the chan-nel and the entire floodplain (Brooks and Brierley2002).

In gravel-bed rivers, natural logjams can formstable “hard points” within the channel migra-tion zone, and that can limit bank erosion (Abbeand Montgomery 1996; O’Connor et al. 2003).Channel migration at these hard points is re-tarded, and the radius of curvature of river bendsis reduced, forming tighter meanders over time(Abbe and Montgomery 2003). For example,throughout the Queets River valley in northwestWashington, USA, the radius of curvature of chan-nel meanders associated with logjams is consid-erably less than for alluvial meanders with no log-jams (Figure 3). This effect on channel geometrycan have important consequences to hydraulicprocesses within the river, which, in turn, can re-sult in additional geomorphic effects. Using asimple model for super-elevation of a water sur-face around a bend (Chow 1959), a reduction inthe channel’s radius of curvature, Rc, will increasewater elevations at the outside of the bend:

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∆ = ×2

c

W Uh

R g ,

where ∆h is the increase in water elevation aroundthe bend (super-elevation), W is channel width, gis the acceleration of gravity, and U is the meanflow velocity around the bend. Manning’s equa-tion provides simple means of estimating veloc-ity:

=1 2

2 3s hU

n ,

where S is the hydraulic gradient, h is the meanwater depth, and n is Manning’s roughness coef-ficient. Differences in super-elevation betweenmeanders with and without logjams, estimated

using a simple model in which all channel vari-ables except for radius of curvature, are depen-dent on drainage area. The results indicate thatsuper-elevation around bends associated with log-jams is as much as 0.5 m more than meanderswithout logjams (Figure 4). These increased wa-ter elevations will increase the frequency ofoverbank flows and be more prone to initial de-velopment of avulsions and side channels.

Introducing a flow obstruction to a streamcauses several indirect consequences related to thedrag imposed on the flow. An increase in flowdepth upstream of an obstruction can result frombackwater effects. Because a flow obstruction re-sults in a stagnation point where the velocity isessentially zero (Abbe and Montgomery 1996), theflow depth at that point is equivalent to the spe-cific energy of the flow or U2/(2g). For a channelwith a mean velocity of 3 m/s, the velocity head

FIGURE 3. Radius of curvature of alluvial channel meanders in the Queets River, Washington. Meanders withlogjams (solid triangles) have a significantly lower radius of curvature than meanders with no logjams (opencircles) for the same reaches of the river system. Stable logjams form “hard points,” which limit channelmigration and result in the radius of curvature diminishing over time until formation of an avulsion or cutoffmoves channel away from the logjam (modified from Abbe and Montgomery 2003).


upstream of a snag or logjam would be about 46cm, or 82 cm for a flow of 4 m/s. If the snag orlogjam was located along the bank, these localincreases in water elevation can deliver flow tosecondary channels and the floodplain at lowerdischarges than it would otherwise take if the ob-struction were not there. Localized increases inwater elevations along riverbanks due to super-elevation and velocity head provide a mechanismto explain how logjams can contribute to the de-velopment of channel avulsions.

Gippel et al. (1996b) found that channel back-water effects are insignificant when wood occu-pies less than 5% of the bank-full cross-sectionalarea (a blockage coefficient of 0.05) but rapidlybecome more pronounced when more than 10%of the cross-sectional area is blocked. Increases inwater elevation upstream of wood can be pre-dicted given the water depth and Froude numberof flow downstream of the obstructed area andthe blockage and drag coefficients of the wood(Gippel et al. 1996b).

The stability of an individual piece of wooddepends on whether the sum of resisting forces isgreater than the sum of driving forces. The prin-cipal driving forces are buoyancy (function of dis-placed volume of water) and drag (function offlow velocity and cross-sectional area of woodwithin flow). The principal resisting forces arenormal forces (function of the weight of wood andany surcharge due to overlying wood or sedi-ment), bed friction (function of the wood’s bedfootprint and the bed material), skin friction (func-tion of the burial depth of wood into substrate),and passive earth pressures (function of the depthof sediment leeward of wood and bed material).The strength of wood can also influence its stabil-ity in small, steep channels where the wood issubjected to significant bending movements thatcould break a piece into smaller, more mobilepieces. Wood stability in large channels is prima-rily dependent on how the wood interacts withthe channel bed, which is a function of the shapeand size of the wood and the type of substrate

FIGURE 4. Significant reductions in a channel’s radius of curvature can result in elevated water elevationsaround the outer bank of the meander due to super-elevation. Results of a simple model for estimating super-elevation along the Queets River, assuming only a change in radius curvature between meanders with andwithout logjams are plotted.

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(Abbe 2000). Resisting forces increase exponen-tially with burial of the wood due to skin friction,passive earth pressures, and surcharge.

Wood tends to be most efficiently transportedthrough the deepest portion of a channel (thal-weg), where it is subjected to the greatest buoy-ant and drag forces and least resistance. Just likea boat, floating wood has draft and will experi-ence the least resistance where the water is deep-est. Because only the largest snags are stable inthe thalweg, stable wood accumulations paradoxi-cally can initiate in the thalweg where large snagsbecome deeply embedded in the river (Figure 5).Once stable, a snag begins to redirect flow andtrap other wood moving through the system(Abbe and Mongtomery 1996). As the channelcontinues to migrate, snags that formed in thethalweg become incorporated into bars and flood-plains (Abbe and Montgomery 1996, 2003). It isimportant to differentiate the wood that is stablein the thalweg from the material that deposits onbars or other depositional features. There will al-ways be smaller pieces of wood that deposit inareas of diminished depth and velocity, such asbars. But it is those pieces of sufficient size andshape to create stable flow obstructions that arecrucial for altering channel morphology and re-taining mobile wood or drift within the system

(Keller and Swanson 1979; Nakamura andSwanson 1993; Abbe 2000; Abbe and Montgom-ery 1996; Abbe et al. 2003).

The draft of a tree bole increases substantiallyif it retains some of its root mat. The centroid of asymmetrical tree bole will tend to be along thecenter line running through the middle of the bole.The centroid of a simple cylindrical log with noroot mat is mid-way along the log’s length. Situ-ated on a smooth bed, a simple cylindrical log willdistribute its mass along its entire length, result-ing in relatively low normal stress and low resis-tance (Abbe 2000). Logs with specific gravity lessthan unity will be buoyant at some water depthless than the log diameter. In contrast, the cen-troid of a bole with an attached root mat is lo-cated much closer to the basal end of the log, andmass tends to be concentrated on a small area ofthe bed resulting in relatively high normal stress.The root mat also raises the centroid elevation,and the buoyant depth becomes more a functionof the root mat diameter than of the bole diam-eter. This means that significantly greater flowdepths are typically required to mobilize a logwith a root mat than without. If a snag’s resistingforces are still greater than the driving forces atthe time bedload transport is initiated, bed defor-mation occurs around the snag. Once partially em-

FIGURE 5. Snags embedded in thalweg of Mendenhall River, Alaska. Large trees eroded from outer bank ofmeander fall into river and become deeply imbedded in channel. Note crown of buried snag exposed in toe ofpoint bar to left as river migrates to the right.


bedded, a snag can prove extremely difficult tomove.

Factors that tend to increase wood stability—such as larger size, denser wood, and depositionin deep portions of the river where the wood ismore likely to remain saturated—tend to also in-crease the wood’s resistance to decay and its lon-gevity in a system. Larger or key members alsohave high lignin contents and lower surface areato volume that tend to further reduce decay rates(Bisson et al. 1987; Melillo et al. 1983). Carbondating of buried wood demonstrates that largelogs can last hundreds to thousands of years inboth high gradient gravel-bed and low-gradientsand-bed rivers (Murphy and Koski 1989; Nansonet al. 1995; Brooks 1999; Abbe 2000; Hyatt andNaiman 2001). Wood can remain in near pristineconditions indefinitely if it is kept either com-pletely dry or saturated in anaerobic conditions.For example, timber piles used in the foundationof St. Mark’s in Venice were found to be so wellpreserved they were left to support the recon-structed tower after already serving for more than1,000 years (Jacoby and Davis 1941). Even therapid decay rates associated with species such asblack cottonwood Populus trichocarpa can be sig-nificantly reduced in near anaerobic conditions(Van Der Kamp and Gokhale 1979). Successfulreintroduction of wood to rivers should focus onkey pieces, the snags that have the most substan-tial geomorphic influence and longest longevity.Likewise, wood structures that incorporate keypieces will more likely to persist in the system.

IntegrIntegrIntegrIntegrIntegration of ation of ation of ation of ation of WWWWWood intoood intoood intoood intoood intoRiRiRiRiRivvvvver Restorer Restorer Restorer Restorer Restorationationationationation

Large-scale efforts to reintroduce wood to streamsin the Pacific Northwest of North America, par-ticularly in rural forest land, were well underwayin 1980s (e.g., House and Boehne 1985, 1986).While most wood reintroduction projects are wellintended, the majority have been based simply onsubjective decisions and have lacked rigorous sci-entific and engineering basis. Unfortunately,guidelines for stream restoration have little or nodiscussion of the mechanics and geomorphic ef-fects of wood, nor do they provide natural ana-logs for the wood placements described (Rosgenand Fittante 1986; Rosgen 1996; Fischenich andMorrow 1999). The poor performance of manyrestoration projects is in part due to an insuffi-cient understanding of the fluvial processes a

project will be subjected to, how the project willinfluence these processes, and the consequencesto habitat (Frissell and Nawa 1992). Preconceivedperceptions that wood is inherently unstable andinadequate physical explanations of why woodwas naturally stable in many streams and rivershave led to the widespread use of steel cables andartificial anchors or ballast for wood placements(e.g., D’Aoust and Millar 2000; Fischenich andMorrow 1999; Shields et al. 2000; Nichols andSprague 2003). However, the stability of naturalwood obviously never depended on such meth-ods. For situations in which channel degradationhas created conditions that are more inhospitablefor wood stability than had naturally existed, suchas incised channels or where large trees are nolonger available, artificial means of stabilizationmay be necessary. A quantitative assessment ofsite conditions and a force balance analysis canprovide the means to evaluate the stability of aproposed wood placement and help determinewhere artificial ballast is appropriate (Abbe et al.1997; D’Aoust and Millar 1999, 2000; Castro andSampson 2001; Shields et al. 2000). When cable isused in wood structures, it should only be usedto secure logs tightly to one another or directly torock ballast so that all the components act as oneunified structure (D’Aoust and Millar 2000). Cableanchoring (e.g., dead-man or duck-billed anchors)is commonly used in wood placements (Fische-nich and Morrow 1999) but poses significant risksthat should be considered. A flexible medium,such as a cable, will not prevent wood from mov-ing up and down or side to side with fluctuatingstage or turbulence. Movement of the wood willmove the cable, and an oscillating or vibratingcable will tend to cut away the material withinwhich it is set. The cable can become exposed tocreate an entanglement hazard or simply fail andliberate the log that it was intended to secure. Onthe contrary, stable wood structures can be de-signed without the use of any cable (Abbe et al.1997, 2003; Brooks et al. 2001).

The observation that logjams act as a naturaltype of bank protection over long periods of timeled to the idea that similar structures could be “en-gineered” to provide bank protection that betterreflects the natural character of rivers than tradi-tional engineering measures, such as rock revet-ments, bulkheads, and spur dikes. The term engi-neered logjams (ELJs) was proposed to refer to ageneral group of structures based on the premiseof emulating natural fluvial systems using scien-tific observations and engineering design prin-

10 ABBE ET AL.

ciples (Abbe et al. 1997). “Restoring” the naturalquantities of wood loading in rivers associatedwith natural logjams may be unrealistic becausethe geomorphic consequences might be incompat-ible with social needs and require time scales be-yond the realm of planning. But there are manysituations where instream wood can be reintro-duced with no adverse impacts and even enhancehabitat while solving traditional engineeringproblems. Unlike conventional river engineeringsolutions for flood risk reduction (e.g., bank andbridge protection, grade control, snag manage-ment), ELJ technology is founded on engineeringdesigns that emulate boundary conditions andprocesses of the natural fluvial system. This ap-proach can offer the distinct advantage of contrib-uting to the restoration or rehabilitation of the riv-erine ecosystem, while still complying withconstraints imposed by human development.

Design Considerations for EngineeredLogjams

ELJ design begins with a clear statement of theproject objectives. The type of ELJ selected de-pends on site conditions and project objectives.The type of structure appropriate for a site is de-pendent on local climate, hydrology, channel andbank characteristics, bed material, objectives, andconstraints. Analysis of flow conditions at the site,assessment of the conditions required for bedmobility, and a force balance of individual logsand the complete ELJ are fundamental parts ofthe design process. Both theoretical models andempirical relationships are utilized to evaluate fac-tors such as buoyancy, drag, resistance, decay, andbed scour. Other design considerations includeELJ size and spacing, wood budget, sedimenttransport, channel migration and avulsions, po-tential flood effects, and safety issues. Such analy-ses contrast with more traditional river restora-tion efforts that use generalized conceptualguidelines (House and Boehne 1985, 1986; Rosgenand Fittante 1986; Fischenich and Morrow 1999).

Careful consideration should be given to howand why wood is reintroduced into the fluvialenvironment, not only in regard to wood stabili-zation, but also how it is likely to respond to thesystem (e.g., existing flux of mobile wood, chan-nel migration rates, riparian forest conditions) andthe potential effects wood placements may have(e.g., increasing erosion, flood, avulsion risks, orhazards to human safety). Evaluating a project

within a watershed and reach-scale context andhow individual structures may alter the systemor be altered (e.g., changes in size) over time servesas a means of assessing project performance.

Any ELJ project begins with a geomorphicanalysis of past, present, and probable future con-ditions of the fluvial system. Has the riverchanged during historical times; could these typeof changes impact the project? If so, further analy-sis is needed to identify the factors responsiblefor those changes. ELJs may be used to reverseriver changes previously identified or used to in-voke changes to mitigate impacts elsewhere. Anassessment of the potential direct and indirect con-sequences of predictable changes, such as signifi-cant increases in ELJ size or channel changes thatcould affect the ELJs, can weigh the relative im-portance of project maintenance, which may tem-per the advisability of an ELJ project. An exampleof change commonly associated with relativelylow-gradient alluvial rivers is lateral channel mi-gration and bank erosion. The rate of channel mi-gration and planform development of a river re-sults from a complex interaction of numerousvariables, but significant changes in certain fac-tors, such as sediment supply, hydraulic geom-etry of the channel, channel planform, or bankvegetation, can accelerate or decelerate the migra-tion rate. Assessment of the local geomorphic con-text, the characteristics and availability of largewood, and evidence for the type of natural woodjams is necessary to determine the appropriatetype of ELJs for a particular site. The flow regime(hydraulics) of the channel and the availability ofmaterial for construction and subsequent recruit-ment will also influence ELJ design. Finally, in-frastructure that affects hydraulic and geomorphicboundary conditions, such as bridges and the like,may also influence ELJ design.

Selecting an appropriate design for a projectdepends on the project objectives and constraints,the type of channel and its characteristics (gradi-ent, size, bed material), and floodplain conditions.Several types of problems common in river engi-neering can be effectively addressed with differ-ent types of ELJs, such as grade control (i.e., halt-ing or retarding channel incision), bank pro-tection, habitat rehabilitation, and creation of sidechannels. Geomorphic conditions influencing ELJdesigns include bed and bank stability, site loca-tion in the channel network, disturbance history,and changes in channel conditions or processes.

The first step in ELJ design is to determinethe appropriate type of ELJ for the project pur-


pose and location. In some instances, such as con-trolling channel alignment upstream of a bridgeand reducing debris accumulation on bridge piers,the location is set and the ELJ is selected to fit thelocation and purpose. Another situation mightprioritize establishment of pool habitat for fish-eries enhancement. In this instance, the purposeand type of ELJ may be set, and the challenge isto determine the most suitable location within thestream reach of interest. The objectives of theproject and the specific conditions and context ofthe project reach will determine how many jamsmay be necessary for an ELJ project.

Confidence in ELJ stability and longevity in-creases if the structures are evaluated for the rangeof discharges and stages expected to occur overthe design life of the structure. The expected stageassociated with higher frequency flood events isparticularly important in the design of ELJs. AnELJ structure is designed to resist floatation uti-lizing its own mass and frictional resistance ratherthan employing artificial means, such as cabling.However, several additional factors reduce buoy-ancy and add resistance to an ELJ, so that, whensubmerged at some high stage, the ELJ does notbecome unstable. Both designed surcharge andnatural sedimentation, as well as the accumula-tion of additional wood over time, all serve to fur-ther stabilize an ELJ. In effect, postconstructionsedimentation and wood recruitment increasesafety factors in the design of ELJ structures.

Because floatation is a key design issue, cer-tain physical sites are well suited for ELJs. Siteswith unconstrained alluvial channels and flood-plains are ideally suited for wood structures thatcan be securely buried into the substrate. Thelarger shear stresses necessary to mobilize gravelmake it more difficult for snags to become em-bedded than if they were situated in a sand-bedchannel. But scour depths around a snag or log-jam in a sand-bed channel are significantly greater,which can either undermine and destabilize thewood or bury it even deeper, firmly securing it. Ifburied so deep that only a small portion of thewood interacts with the flow, it may not have thedesired effects. Confined channels, particularlythose with bedrock beds, are more difficult sitesfor building stable instream wood structures.Wood structures have been applied with somesuccess to incised channels, though typically withthe addition of steel cable anchoring (House andBoehne 1985, 1986; Shields et al. 2000). In loca-tions such as Australia, where timber with highspecific gravity is available, stable wood structures

without cable or artificial ballast have been builtin incised, deep channels (Brooks and Brierley2002).

Wood Stability and Longevity

Based on historical accounts, describing the diffi-culties typically encountered in removing natu-ral snags and logjams from rivers (e.g., Ruffner1886; McCall 1984), and studies of the age of woodin a variety of river systems (e.g., Nanson et al.1995; Abbe and Montgomery 1996; Gippel 1996b;Brooks 1999; Abbe 2000), it is clear that naturalwood can be remarkably stable over long periodsof time. Field studies of natural log jams haveshown that the stability of wood accumulationsis linked to the presence of one or more immobilesnags or key members (Keller and Swanson 1979;Murphy and Koski 1989; Abbe and Montgomery2003; Fox et al. 2003). Key member stability insteep channels is typically provided by interac-tion with pre-existing boundary conditions, suchas bedrock outcroppings, boulders, or channelbanks, and the strength properties of the tree,which determine its ability to withstand rupture.If the tree breaks, the smaller pieces become moresusceptible to transport downstream. Log diam-eter, length, and strength are the principal factorsinfluencing wood stability in this environment(Abbe 2000).

Once the root mat becomes partially embed-ded in the channel bed, it becomes more difficultto move. A sediment buttress downstream of a rootmat can form in situ by the accumulation of sedi-ment in the leeward eddy or if the root mat sinksinto the bed under its own weight. If an embed-ded rootwad begins to slide downstream, it willpush up a mound of sediment, similar to a bull-dozer, making further downstream movementincreasingly difficult. Rootwads of natural snagsare commonly observed to be at least partiallyembedded in the channel bed. Assuming a cohe-sionless substrate, the magnitude of resistance willdepend on the depth to which the snag extendsinto the substrate, the surface area of the embed-ded portion of the root mat, the submerged weightof the alluvium, the friction angle of the alluvium,and the skin friction between the wood surfaceand alluvium. The importance of partial burial inthe stability of wood is clearly illustrated by tim-ber piles. A properly placed pile remains stable,even at its maximum buoyancy (fully submerged),despite having no ballast or anchoring. The sta-bility of piles is entirely due to skin friction be-

12 ABBE ET AL.

tween the pile surface and the substrate. Frictionalresistance of timber piles in various types of allu-vium range from less than 1,000 N/m2 to morethan 10,000 N/m2 (Jacoby and Davis 1941). It cantake remarkably little burial to provide enoughskin friction to compensate for buoyancy; for ex-ample, a cylindrical pile 10 m long, 0.6 m in di-ameter, and a specific gravity of 0.5 would requirea burial depth of 2.4 m for a substrate of loosesand (Figure 6). Skin friction tends to increase withincreasing grain size and compaction of the sub-strate. Compact sand provides more than threetimes the skin friction of loose sand (Jacoby andDavis 1941). Since skin friction increases as a prod-uct of 2πR, where R is pile radius and wood vol-ume increases as a function of π2R, the relativeimportance of skin friction resistance varies in-versely with diameter.

Additional resistance to longitudinal forcesis provided by the passive earth pressure of sedi-ment downstream of the snag. This is the depthof sediment buttressing the downstream or lee-ward side of a buried log or root mat. The passiveearth pressure per unit width of buried wood, PP,in N/m, is a function of the depth of burial andphysical characteristics of the substrate (CanadianGeotechnical Society 1985):

λ=

2

2s

P P

zP K ,

where γs, = submerged bulk density of substratein N/m3, z is the depth of the buried wood seg-ment, KP = (1 + sinφ)/(1 – sinφ) = coefficient oflateral passive earth pressure, and φ = internal fric-tion angle of the substrate in degrees. For a bur-

FIGURE 6. Skin friction for a timber pile as a function of burial depth in loose sand with a frictional resis-tance of 9.8 kN/m2 (Jacoby and Davis 1941). Pile is 9.1 m (30 ft) in length, 0.6 m (2 ft) in diameter, and has aneffective density of 500 kg/m3. The pile is assumed to be completely submerged and thus have a constantupward buoyant force of 13.0 kN. Net resistance is equal to the skin friction—buoyant force (a negative valueequals an upward force or unstable configuration). Factor of safety is net resistance divided by the buoyantforce.


ied segment of a circle, such as a partially buriedroot mat (Figure 7):

=dP dxP PP ,

where dPP, = incremental passive earth pressureof buried segment of width dx = (xi + 1 – xi) and theaverage depth of increment, z, between xi andxi + 1. Summing each increment of the buried seg-ment provides an estimate of the total resistingforce imposed by passive earth pressure, FP:

= ΣdP dxP PF .

The following example offers an illustrationof how bed deformation (scour and lowering ofwood into bed and sediment deposition down-stream of wood) can contribute sufficient resis-tance to stabilize a snag. Assuming an alluvial

substrate composed of moderately rounded coarsegravel with a median grain size of 36 mm, φ = 35o,then KP = 3.7. The following assumptions also ap-ply:

• substrate has a submerged bulk densityof 1,200 N/m3,

• snag has a homogeneous specific grav-ity = 0.5.

• snag is symmetrical about the axis of itsbole

• snag bole axis is parallel to bed surfacewith root mat facing upstream

• snag radius = 2 m• water depth = 2 m• incident flow is uniformly distributed

with a constant velocity = 3 m/s• root mat is small relative to channel (no

blockage effect on drag)

FIGURE 7. Two-dimensional representation of snag root mat with radius Rrw oriented normal to flow (in thedownstream-facing view shown, the bole extends into the page). Water depth upstream of the root mat hu isassumed to extend to lowest point of root mat (ground level upstream of root mat gu), giving a conservative(high) measure of drag. Ground level downstream of root mat gd represents the depth of sediment to the lee ofthe root mat for measuring passive earth pressures for each root mat increment of width dx and depth z. AngleΘ defines the arc of the buried segment of the root mat.

14 ABBE ET AL.

• drag coefficient = 1.5 (Gippel et al. 1996b)• submerged area of root mat subjected to

flow remains constant with respect toburial

Based on these assumptions, the snag is buoy-ant and therefore doesn’t exert any normal forceon the bed. The driving force exerted on the snagby flow is a function of the snag’s submerged areanormal to flow, the flow velocity, and the dragcoefficient associated with the shape of the snag:

ρ=

2o

2D

D

C A UF ,

where FD is the drag imposed on the snag, CD isthe drag coefficient, U is velocity of flow, Ao is thecross-sectional area of the snag normal to flow,and r is the fluid density. Without any burial ofthe root mat, the snag has no resisting force andis unstable (Figure 8). Based on the assumptionsof a constant uniform flow and that the submergedarea, Ao, of the root mat remains constant, the drag

force imposed on the snag remains constant (Fig-ure 8). The resisting force rapidly increases withthe depth of sediment downstream of the root mat,and a factor of safety of 1 is reached at a depth ofabout 1.0 m when the resisting and driving forcesare equivalent at 42 kN (Figure 8). With an addi-tional burial depth of 0.5 m (total depth of 1.5 m),the factor of safety = 2.8.

Sediment deposition on a tree bole can addsubstantial surcharge and thus further increase thestability of a snag or logjam. Field observationsshow that sediment commonly accumulatesdownstream of the eddy created by the root matand buries part or all of the tree bole. In some cases,the entire root mat becomes buried deep withinthe bed. Fluvial sediments composed of coarsesand and gravel have dry bulk densities of 1,400–2,200 kg/m3, about 3–4 times the densities of mostwoods, so overburden depths less than the logradius can be sufficient to negate any positivebuoyancy of the wood.

If channel bed material is likely to be mobi-

FIGURE 8. Simple example of driving and resisting forces on a snag with its root mat facing upstream and boleparallel to flow. Rootwad radius and flow depth are assumed to be 2 m, and therefore, snag is buoyant (noresisting force with no sediment leeward of root mat). Bed substrate is gravel and root mat subjected to auniform constant flow velocity of 3 m/s.


lized under flow conditions in which a tree boleremains stable, then the stability of the bole shouldincrease due to accumulation of sediment imme-diately downstream. As flow moves past an ob-struction (such as stable wood debris), a separa-tion envelope in which flow moves upstream willform directly behind the obstruction, separatedfrom the downstream flow of water by a shearzone. Sediment moving in the main flow is ag-gressively entrained by vortices in the shear zoneand transferred into the recirculation zone whererapid deceleration leads to deposition. Such sedi-mentation influences debris stability in severalways: forming a buttress, which the rootwad mustmove up and over or plow through; adding sur-charge or weight to buried portions of the treebole; and increasing frictional resistance withburial, similar to the “embeddedness” concept bywhich most ship anchors work. Partial burial of atree can increase the tree’s resistance several foldand can help to explain why many snags and log-jams remain stable even when completely sub-merged and subjected to extreme flows.

If an ELJ remains stable after bedload trans-port has been initiated, then accumulation of sedi-ment in the downstream separation envelopeshould further stabilize the structure. In mostgravel-bed channels, bedload transport begins atflows near or below bank-full stage. Hence, keymembers designed to be stable in flows exceedingbank-full conditions should become more stableafter experiencing several bed-mobilizing events.

Another factor that can influence a force bal-ance analysis of wood stability is the effective den-sity of wood based on its moisture content. Mostdry woods have densities less than water (1.0 kg/m3) and cannot exceed a density of 1.5 kg/m3, thedensity of the cellulose and ligin within wood(Harmon et al. 1986). As wood gets wet, its effec-tive density can increase substantially, sometimesto values sufficient to cause it to sink, as illustratedin samples taken from wood placed in the NorthFork Stillaguamish River, Washington, USA (Fig-ure 9). Stability of wood through time can also beinfluenced by root cohesion and surcharge pro-vided by vegetation growing above a snag or log-jam.

Log and channel dimensions in riversthroughout western Washington provide an em-pirical model of wood stability using dimension-less variables of log size: the ratio of log diameterto bank-full depth, D/hbf, and the ratio of loglength to channel width, L/wbf (Abbe and Mont-gomery 2003). Stable logs, or key members, form

the critical components holding wood accumula-tions in jams. In surveys of large alluvial chan-nels, stable logs had minimum values of L/wbf

equal to 0.1 and values of D/hbf rarely below 0.8.In channels exceeding 40 m in bank-full width,all key members consisted of a bole and a rootmat with a diameter greater than the bank-fulldepth.

The buttressing or taper of a tree’s bole con-centrates a substantial portion of a tree’s mass nearits base, which significantly affects the log’s massand buoyant depth. A root mat both raises the cen-ter of mass or centroid, up to more than five timesthe tree’s diameter breast height, and the root matitself acts like a plow, further increasing resistance.The presence of an attached root mat substantiallyincreases the stability of a snag by elevating thecentroid and increasing the frictional resistance(Abbe and Montgomery 1996; Abbe 2000; Braud-rick and Grant 2000; Castro and Sampson 2001).

The availability of material to form key,stacked, and racked members is important whenconsidering the use of ELJs. In some areas, theavailability of large trees for use as key membersmay be a critical obstacle to the use of ELJ tech-nology. The size of key members increases withboth expected discharge and flow depth and canbe related to the size of the channel, as discussedin the design section of this document. Costs forinstallation can vary greatly depending on theproximity of key members to the site location. Asthe size requirements for key members increase,transporting them becomes increasingly more dif-ficult and expensive. Smaller racked members canaccount for 70–90% of the logs used in some typesof ELJs that project into the flow, whereas sometypes of ELJ structures used for grade control orto harden a bank may have no racked membersat all (Abbe et al. 2003).

Appropriate sizes, locations, and spacing forwood structures depends upon the project goals,the channel and its migration zone, human con-straints, and acceptable risk. Bank protectionprojects, for example, could consist of continuousnatural log revetments (e.g., bank-full bench jams,flow deflection jams, chatoic cribs) and inter-spersed structures (e.g., meander jams, flow de-flection jams, crib groynes). The specific spacingof ELJ structures can be based on experimentalstudies of flow deflection by rock groynes and piledikes (Klingeman et al. 1984). Our experience todate suggests that a spacing of about four timesthe protrusion into the main flow provides a con-servative value for ELJ spacing; however, a spac-

16 ABBE ET AL.

ing of up to seven times may prove acceptable inmany situations, depending on the bend radiusof curvature. A tighter bend will require closerspacing of the structures than a straight reach.Long-term project success will be better ensuredby evaluating project designs under different sce-narios representing the range of channel condi-tions, such as changes in the direction of incidentflow on a structure (or set of structures), channellocation, radius of curvature, bed elevations, orwater surface profiles.

Wood Structure Dynamics Over Time

A key concern among many designers in regardto using wood structures is the longevity of wood

as a material. How long wood lasts depends onfactors such as the type of wood, the environmentin which it is located, its size, and the age of treeat death. If wood remains saturated in freshwa-ter, it can last almost indefinitely; it is not uncom-mon to recover premium grade timber that hasbeen submerged in lakes for over a century. Bolesof large deciduous trees found in river gravels ofthe ancient Rhine River in Germany have beendated at more than 18,000 years old and yet werestill in excellent condition (Becker and Schirmer1977; Becker 1980). Eucalyptus and pine logsfound in floodplain sediments and across streamsin southeast Australia and northwest Tasmaniahave yielded 14C ages as old as 13,000–17,000 yearsb.p. (Gippel et al. 1996a; Nanson et al. 1995; Brooks

FIGURE 9. Densities of 109 core samples from trees used in engineered logjams constructed in North ForkStillaguamish River, Snohomish County, Washington, 1998. Green (open circles) and saturated (solid squares)of same data set are plotted as a function of their oven dry densities. Tree species in data set include Tsugaheterophylla, Thuja plicata, Picea sitchensis, Pseudotsuga menziesii, and Populus trichocarpa.


and Brierley 2002). Several studies have shownthat large boles comprising wood jams in rela-tively small streams found in coniferous forestscan last for hundreds of years based on the age oftrees growing on top (Keller and Tally 1979; Tally1980; Hogan 1987; Murphy and Koski 1989). Woodfrom 30 buried logjams exposed in alluvial banksof the Queets River, Washington dated from mod-ern to 1,324 ± 20 years b.p., averaging about 400years b.p. (Abbe 2000). In contrast, log jams com-posed of relatively small pieces debris, such asslash commonly left behind after timber harvest,last only several years (Lisle 1986b).

Few data exist for absolute decay rates of dif-ferent types of wood in fluvial environments, butthere is an extensive literature on decay rates ofwood on forest floors (see Mackensen and Bauhus1999 for a recent review). Such studies of the decayrates of timber under moist aerobic conditions ona forest floor can provide a conservative estimatefor timber decay used in the construction of woodstructures in rivers. Accelerated laboratory tests todetermine decay rates of some typical Australianeucalyptus species came up with turnover times(i.e., the time taken for a block of wood 5 cm × 1.25cm × 1.25 cm to decay to 95% of its initial mass)ranging from around 30–375 years. The authors ofthese experiments acknowledge that these rates areconservative for the species being tested, given thatthe experimental blocks have much higher ratiosof surface area to volume than large logs. Speciesthat might typically be used in river rehabilitationprojects in Australia, such as river red gum Euca-lyptus camaldulensis, forest red gum E. tereticornis,or iron bark E. paniculata, have values at the upperend of this range.

Models for wood longevity can be applied toestimate the design life of individual logs basedon tree species, log diameter, and environmentalsetting (Abbe 2000). Decay rates can vary substan-tially for the same species and settings, but, ingeneral, the longer and more continuously thewood remains submerged, the longer it will last.Assuming a constant decay rate, k, and knowingthe initial mass of a log, M(o), the log’s mass atsome time, M(t), can be predicted using the simpledecay model:

∆= ( )( ) ( ) k tM t M o e ,

where Dt is the number of years from when firstplaced in the stream, assuming its mass was M(o)when first placed. The effective diameter of a logafter some time can also be predicted using thesame model and assuming that decay proceeds

uniformly into a cylindrical log from its perim-eter (Figure 10). This type of procedure can be usedto specify the size of logs that will most likely meetminimum specifications over the design life of astructure. Wood decay is a complex and highlyvariable process dependent on the species andphysical condition of the wood, microclimate andorganisms breaking down the wood, so informa-tion regarding local decay processes and rates willimprove design predictions.

A substantial increase in ELJ size can occurover time if it collects debris moving through thechannel. Changes in the ELJ size can subsequentlyobstruct a larger portion of the channel cross-sec-tional area and result in significantly higher wa-ter surface elevations than anticipated in the origi-nal project design. The risk will depend onchanges in channel position relative to the ELJsand the flux of wood moving through the systemat particular flow levels. An examination of thewatershed and stream system above the site isimportant to get some understanding of the fluxof drifting wood through the project reach andwhether its accumulation at the ELJs is likely tocreate management problems.

Wood reintroduction projects will benefitfrom adaptative management plans that provideprotocols for determining when additional woodaccumulation may become a problem and provid-ing guidelines for what, where, and when to re-move selected pieces. These plans can also includeinspection criteria for assessing the structural in-tegrity of ELJs and what to do if maintenance be-comes necessary. Failure to establish a reasonableinspection and maintenance program is likely toelevate risk of project failure or development ofadverse effects due to evolution of an ELJ to con-ditions outside the scope of project design.

Examples of Engineered LogjamProjects

Although ELJs remain a relatively new technol-ogy, they have been successfully used in a grow-ing number of contexts. Demonstration projectsinclude three ELJs constructed in the upperCowlitz River in 1995 (Abbe et al. 1997), five ELJsin the North Fork Stillaguamish River in 1998, andseven ELJs in the Cispus River in 1999 (Abbe etal. 2003). Each of these projects have experiencedmultiple overtopping flows, and all remain intactin 2003.

Nineteen ELJ structures built on the WilliamsRiver, New South Wales, Australia in September–

18 ABBE ET AL.

October 2000 (Brooks et al. 2001) show that ELJsmay be applied beyond the Pacific Northwest. InSeptember 2000, 19 ELJs incorporating 430 logswere constructed in a 1.1-km-long reach of the Wil-liams River as part of an experimental reintroduc-tion of wood into Australian Rivers. The ELJ struc-tures included both flow deflection structures alongthe river’s banks and channel-spanning grade con-trol structures intended to prevent channel incisionand trap additional sediment within the projectreach. All of the structures were built without anyartificial anchoring (i.e., no cabling or importedballast). The study site experienced six flows that

overtopped most of the structures in the first 6months after construction (Figure 11). Three ofthese peak flows exceeded the mean annual flood,inundating 18 of the ELJs by 2–3 m and 1 by 0.5 m.All the structures remain intact and, thus far, theyare performing as intended, with a net increase of40–60 m3 of sediment storage per 1,000 m2 of bedsurface within the test reach and a substantial in-crease in channel complexity. A control reach 5 kmupstream experienced a net export of 15 m3/1,000m over the same period.

As of 20 March 2002, the five ELJs in the NorthFork Stillaguamish have been completely inun-

FIGURE 10. Simple decay model to estimate mass and effective diameter of a log after some time. Dimension-less mass and diameter are plotted as a function of time. M(t) is its mass after time t, and M(o) is the log’s initialmass when placed. Diameter is derived assuming uniform decay from the perimeter of a cylindrical log to itscentral axis. Two different species, Pseudotsuga menziesii (Douglas fir) and Populus trichocarpa (cottonwood),are presented using decay rates for logs lying on the ground in areas west of the Cascade Range of Oregon andWashington (Harmon et al. 1986).


FIGURE 11. View from upstream of 2 of 15 engineered logjams constructed in the Williams River, northwestNew South Wales, Australia. (a) As-built structures in September 2000, (b) March 2001 flood during whichpeak flow overtopped structures by approximately 2 m with velocities exceeding 3 m/s, and (c) same structuresin 2002 after six overtopping flows.

a

b

c

20 ABBE ET AL.

dated by at least 30 flows and have successfullymet objectives for increasing pool frequency andcover, bifurcating the river into two perennialchannels, protecting 200 m of eroding bank, andimproving a chronic problem of drift accumula-tion on a bridge directly downstream of the project(Abbe et al. 2003). The ELJs were designed to col-lect mobile wood (i.e., drift or flotsam) upstreamof the bridge while increasing the conveyance ca-pacity of water and flotsam beneath the bridgeby improving channel alignment. Flotsam has yetto rack up on the bridge’s center piling, eventhough hundreds of logs have been deposited onfour of five ELJs. Of the logs that moved and weresubsequently found in 1999, those that were ini-tially located upstream of at least one ELJ trav-eled from 200 to 600 m. Logs that moved down-stream of the bridge, where there are significantflow obstructions in the river, traveled much fur-ther, from 2 to 11 km. This data suggests that theposition and frequency of logjams reduces thetravel distance of wood and increases wood resi-dence time within the fluvial system.

The successful performance of ELJ projectsto date is largely attributed to the comprehensivegeomorphic, engineering, and ecological analy-sis leading to design and implementation. Manyquestions remain regarding the science, engineer-ing, risk assessment, and long-term performanceof ELJs. For this reason, implementation of addi-tional projects still should be viewed as appliedresearch that contributes to advancing the exist-ing body of knowledge. In situations where natu-ral recovery of river corridors is unlikely or un-duly constrained by human development, how-ever, ELJs offer exciting possibilities for reintro-ducing wood to rehabilitate fluvial environments.

ConclusionsConclusionsConclusionsConclusionsConclusions

The use of wood in river restoration should befounded on emulating natural processes througha comprehensive approach to understanding cur-rent and historical conditions within a river reachand its watershed, identification of opportunitiesand constraints within the project reach, and ap-plication of sound engineering practice. Projectsconstructed to date in the United States and Aus-tralia offer examples of engineered logjams thathave experienced overtopping flows and havesuccessfully met objectives to rehabilitate habitat,protect banks, reverse channel incision, and re-duce risks to bridges. Specifically, several projectshave demonstrated that ELJs (1) can remain stable

in 20+ year recurrence interval floods; (2) can beeffective at redirecting even large channels to con-trol local bank erosion; (3) can dramatically en-hance physical habitat, such as pool frequency anddepth, and cover; and (4) need not exacerbate lo-cal flooding or erosion. The application of ELJtechnology in these projects demonstrates the po-tential to address both river engineering and habi-tat concerns. As with any relatively new technol-ogy, further experimental applications arewarranted before formalization of design stan-dards. However, site-specific geomorphic and sta-bility analyses based on general principles of ELJdesign appear to provide a reasonable frameworkfor the assessment, design, and implementationof successful projects.

The last several decades have seen an unprec-edented growth of grassroots community effortsto rehabilitate urban and rural streams in both theUnited States and Australia. Widespread recog-nition by the scientific community and the publicregarding the environmental and economic costsassociated with human alteration of fluvial sys-tems has probably never been greater. In manyregions, dramatic changes are occurring in gov-ernment policies regarding fluvial systemsthrough increased regulation of land use withinfloodplains and channel migration zones. But thelong-term success of efforts to rehabilitate or re-store the physical and biological attributes of flu-vial systems will depend on development of man-agement and engineering strategies that betteremulate natural processes. Experience to datedemonstrates that, in the overall context of thesechanging approaches to river management, woodcan be an integral part of strategies aimed at re-habilitating and maintaining natural conditionswhile still attaining reliable engineered solutionsto local problems.

AcAcAcAcAcknoknoknoknoknowledgmentswledgmentswledgmentswledgmentswledgments

Initial development of engineered logjam technol-ogy was made possible through a grant from theWatershed Science Institute of the Natural Re-sources Conservation Service, U.S. Department ofAgriculture, with special thanks to CarolynAdams. Australian research is funded by Landand Water Australia grants MQU9 and GRU29,in collaboration with the Department of Land andWater Conservation, Hunter Region, and theHunter Catchment Management Trust. SeleneFisher was a tremendous help in editing our initialdraft, and three anonymous reviewers helped to


significantly improve the final manuscript. MaryLou White and Washington Trout coordinatedmonitoring efforts in the North Fork StillaguamishRiver. The Cispus project was made possible byDonna Ortiz de Anaya and Brenda White. Andthanks to Jill Silver and Jim Turner for their stead-fast commitment to protecting rivers.

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