A06_Specs_LTPBR-Manual_Chptrs-4-6.pdf

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CHAPTER 4: MIMICKING & PROMOTING WOOD ACCUMULATION & BEAVER DAM ACTIVITY WITH PALS & BDAs

– MIMICKING & PROMOTING WOOD ACCUMULATION &

BEAVER DAM ACTIVITY WITH POST-ASSISTED LOG STRUCTURES &

BEAVER DAM ANALOGUES

Prepared by:

Scott M. Shahverdian, Joseph M. Wheaton, Stephen N. Bennett, Nicolaas Bouwes, Reid Camp, Christopher E. Jordan, Elijah Portugal, and Nicholas Weber

Recommended Citation:

Shahverdian, S.M., Wheaton, J.M., Bennett, S.N., Bouwes, N., Camp, R., Jordan, C.E., Portugal, E. and Weber, N., 2019. Chapter 4 – Mimicking and Promoting Wood Accumulation and Beaver Dam Activity with Post-Assisted Log Structures and Beaver Dam Analogues In: J.M. Wheaton, S.N. Bennett, N. Bouwes, J.D. Maestas and S.M. Shahverdian (Editors), Low-Tech Process-Based Restoration of Riverscapes: Design Manual. Utah State University Restoration Consortium, Logan, Utah. 66 pp.

This work is licensed under a Creative Commons Attribution 4.0 International License.

IMPLICATIONS FOR PRACTICE

• Post-assisted log structures (PALS) and beaver dam analogues (BDAs) are hand-built structures. PALS mimic and promote the processes of wood accumulation; whereas BDAs mimic and promote beaver dam activity.

• PALS and BDAs are permeable, temporary structures, built using natural materials.

• BDAs differ from PALS in and that BDAs create ponds using a variety of fill materials; PALS are built with only woody material, which tends to be larger diameter than the woody material used for BDAs.

• PALS and BDAs are both intended to address the broad impairment of structural starvation in wadeable streams, but can also be used to mitigate against a range of more specific impairments.

• PALS and BDAs can be built using a variety of natural materials, and built to a range of different shapes, sizes and orientations.

• PALS and BDAs are most likely to achieve restoration goals when built in high numbers.

• Some PALS and BDAs are likely to breach and/or lose some wood, but when many structures are installed, that material will accumulate on downstream structures or in natural accumulation areas leading to more complexity.

http://creativecommons.org/licenses/by/4.0/

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INTRODUCTION

The systematic and widespread removal of large woody debris (LWD) and beaver has resulted in simplified and degraded riverscapes (Wohl, 2005; Wohl, 2013). Historically, large woody debris and beaver dams were ubiquitous throughout North American riverscapes (Naiman et al., 1988). Beaver dams exert a major influence on streams by influencing hydrologic and geomorphic processes and have been shown to elevate water tables (Westbrook et al., 2006), maintain channel-floodplain connectivity (Burchsted et al., 2010), increase riparian areas (Cooke and Zack, 2008), attenuate peak flows and elevate baseflow (Nyssen et al., 2011), and increase sediment retention (Butler and Malanson, 1995). Large woody debris has been shown to influence hydrologic and geomorphic processes in similar ways to beaver dams by creating fish habitat and spawning areas and promoting sediment and nutrient retention (Gurnell et al., 2002; Roni et al., 2015; Wohl, 2014). Importantly, many of the processes beaver dams and large woody debris influence are often directly related to stream restoration goals (Beechie and Bolton, 1999). The introduction of habitat structures has been practiced for at least a century (Thompson and Stull, 2002), with restoration focused on the creation of discrete habitat features, often pools for fish, rather than emphasizing how structures could enable and promote processes.

Figure 1 – The vision that guides the use of post-assisted log structures (PALS) and beaver dam analogues (BDAs) is ‘Stage 0,’ where large wood accumulation and beaver dams force the floodplain connectivity, multiple channels and complex physical instream and riparian habitat.

To address the scope of degraded streams (Chapter 1: Shahverdian et al., 2019a), cost-effective and scalable restoration methods are critical. The approach to restoration described throughout this manual, and the design of low-tech process-based restoration projects described in this chapter is informed by the vision of physically complex valley bottoms and multi-thread channels described as ‘Stage 0’ (Cluer and Thorne, 2014, Figure 1).

We describe the design process for two types of low-tech structures, post-assisted log structures (PALS) and beaver dam analogues (BDAs). PALS are woody material of various sizes pinned together with untreated wooden posts driven into the substrate to simulate natural wood accumulations. BDAs are channel-spanning, permeable structures, with a uniform crest elevation, constructed using woody debris and fill material, to form a pond and mimic natural beaver dams. We introduce the term complex to describe a group of low-tech restoration structures designed to achieve

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specific objectives. A complex may be composed of a single type of structure, or a mix of structure types. In general, complexes range in size between 2 – 15 structures. Complex design is described in Chapter 5 (Shahverdian et al., 2019b).

First, we discuss some of the key low-tech restoration principles that inform the use and application of PALS and BDAs;

next we detail the form, function and design considerations for PALS and BDAs; then we describe how PALS and BDAs are likely to change through time, as well as trade-offs associated with each structure type. We conclude by outlining some of the common misconceptions and pitfalls that practitioners may encounter when employing the use of PALS and BDAs. This chapter does not address large-scale planning and assessment that is required in order to determine if low-tech restoration structures are an appropriate restoration technique (Chapter 3: Bennett et al., 2019b) or complex-level design (Chapter 5: Shahverdian et al., 2019b). A history of the recent development and use of PALS (Appendix B) and BDAs (Appendix C) can be found in the Appendix.

KEY PRINCIPLES FOR DESIGNING POST-ASSISTED LOG STRUCTURES AND BEAVER

DAM ANALOGUES

While the use of instream restoration structures, often referred to as habitat structures, is not new, we contend that an explicit linking of the how structural additions are conceptualized within a process-based framework is lacking, and has led to their misapplication (see Chapter 1: Shahverdian et al., 2019a). Here we briefly review the key low-tech process-based restoration principles (Chapter 2: Wheaton et al., 2019) that inform the design of PALS and BDAs.

Strength in Numbers – Focus on the Treatment, Not the Structure

Low-tech restoration structures are intended to be implemented in high numbers (Figure 2). The importance of any individual structure is limited when understood in the context of an entire project. As such, the emphasis is not on any particular structure, but rather the total number of structures and density at which they are built. Maintaining a focus on the larger context helps practitioners reduce the time and resources spent designing individual structures. The design of individual structures is a rapid (3-5 minutes) process that does not require high resolution hydraulic, topographic or hydrologic data.

It’s Okay to be Messy

The beaver dams and large woody debris that low-tech restoration structures emulate are diverse, characterized by a range of shapes and sizes. There is no ‘ideal’ restoration structure. At the scale of an entire restoration project, there should be a range of PALS and BDAs shapes and sizes. Different structures shapes, sizes and locations can be designed to promote specific outcomes at the structure scale. Building a diversity of structure types accommodates variability and uncertainty in stream flows and is more likely to encourage the recovery of degraded processes (e.g., erosion, deposition, overbank flow) that are crucial to meeting restoration goals. Different structures can be designed to affect different processes during different flow conditions (i.e., baseflow vs high flow). Low-tech restoration structures are designed in the field, most often built using locally available materials, and intended to have lifespans similar to the natural features they mimic, whether beaver dams or large woody debris.

DEFINITIONS

Post-Assisted Log Structures (PALS) – woody material of various sizes pinned together with untreated wooden posts driven into the substrate to mimic natural wood accumulations.

Beaver Dam Analogues (BDAs) – a permeable, channel-spanning structure with a constant crest elevation, constructed with a mixture of woody debris and fill material to form a pond and mimic a natural beaver dam.

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Figure 2 – An example of typical density of structure placement shown at the reach scale (upper and lower right) and at the riverscape scale in lower left. Not only are a high number of structures built, a diverse mix of structure types are used to achieve complex-level objectives (see Chapter 5 for design: Shahverdian et al., 2019b) Figure adapted from Camp (2015a).

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POST-ASSISTED LOG STRUCTURES (PALS) & BEAVER DAM ANALOGUES (BDAS)

Post-assisted log structures (PALS) are a low-tech restoration structure that mimic and promote accumulation of large woody debris (LWD) and are designed to influence hydraulic, hydrologic and geomorphic processes (Figure 3). PALS are designed to influence hydraulics across a range of flows, and depending on the design, may force the creation of an upstream pond. While PALS influence hydraulics at all flows, they are most likely to force geomorphic change during high flows and as such require posts to provide temporary stability. PALS can be built in a range a shapes and sizes, best described by their location within the channel and desired function, but in general consist of larger diameter and longer length material than used in the construction of BDAs. PALS can be used to achieve a range of restoration outcomes including: creating high flow refugia for aquatic species; increasing channel-floodplain connectivity at high flows; increasing physical complexity by altering patterns of erosion and deposition; and promoting channel incision recovery by forcing channel widening and aggradation.

Figure 3 – A post-assisted log structure (PALS) so buried in wood accumulation and sediment, it is hard to recognize.

Beaver dam analogues (BDAs) are man-made structures that mimic the form and function of natural beaver dams.

BDAs are temporary, permeable structures built with or without posts using a combination of locally available woody material sediment and fill material. The design and implementation of BDAs is a simple, non-destructive and cost-effective method to restore the processes that are responsible for physically complex channel and floodplain habitat.

They can be used to support existing populations of beaver by increasing the stability of existing dams; create immediate deep-water habitat for beaver translocation (Figure 4); or they can be used to promote many of the same processes affected by natural beaver dams (e.g., increased channel-floodplain connectivity).

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Figure 4 – An example of using beaver dam analogues to mimic the deep water cover provide by a natural beaver dam and provide a safe release site for a colony of four beaver (2 shown in A). The beaver are immediately comfortable and curious with the safety of water in their new suggested home. These beaver expanded this and another BDA complex within a few months of their release. From: Shahverdian and Wheaton (2017).

Below we first describe the form of the various low-tech structures to provide context and terminology necessary to discuss their function. Next, we describe the functions of PALS and BDAs.

Form: Structure Type, Dimensions, and Material

Types of PALS are differentiated by their position in or relative to the channel. We define PALS types as channel-spanning, bank-attached, mid-channel, and on the floodplain (Figure 5). Unsecured wood (“seeding”) can also be added within groups of PALS to increase wood density but defer to the system where the wood will accumulate (Chapter 5: Shahverdian et al., 2019b). The size and height of the structure can vary depending on specific objectives. PALS are built to a height that is necessary to achieve a certain objective (e.g., create a scour pool or reconnect a floodplain-see next section). The orientation of structures (relative to flow) can be as varied as natural wood accumulations but generally channel-spanning and mid-channel PALS are built roughly perpendicular and bank-attached PALS are built angled upstream, perpendicular, or downstream. PALS are generally built with woody material that can be moved and placed by one to four people (i.e., shrubs, branches, logs, and/or trees 1-1.5 ft (30-45 cm) diameter and 10-16 ft (3-5

m) long). Generally, a wide range of sizes are used; large pieces are positioned first and pinned in place with medium and small pieces used to fill in gaps and make the structure less porous. This simulates natural racking of small material on a natural log jam.

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Figure 5 - PALS can be built in a range of shapes, sizes and in different channel locations. (A) bank-attached, (B) mid-channel, (C) channel-spanning, (D) channel-spanning, (E) mid-channel, (F) channel-spanning, (G) bank-attached, and (H) channel-spanning.

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Figure 6 - Representative photos of the diversity of possible BDA shapes, sizes, locations, and building material. (A) post-assisted and willow weave (B) postless, sage and juniper (C) postless willow, using existing willow for stability (D) postless, juniper (E) post-assisted and juniper (F) postless willow and juniper (G) postless juniper (H) postless sage.

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Like natural beaver dams, BDAs can be built in a range of environments and in a variety of shapes and sizes using a range of natural materials. We define primary BDAs as a relatively taller structure meant to mimic a beaver primary dam that is used to create a pond that supports and underwater entrance to their lodge and food cache (woody winter food storage). Often their crest elevation is equal to, or greater than bankfull elevation. They may be completely within the bankfull channel or extend onto the adjacent floodplain. Secondary dams mimic beaver dams that extend deeper water to other foraging locations or back up water to the base of a primary dam to reduce the hydraulic head created by the primary dam. They generally have a lower crest elevation, near or below bankfull. BDAs have a uniform crest elevation such that water flows equally over the entire crest rather than concentrating flow in a particular location. The crest planform may be straight or convex. BDAs may be constructed with or without untreated wooden posts driven into the streambed (Figure 6). They can be built from a range of woody material including riparian species such as willow and cottonwood, as well as upland species such as juniper and sagebrush. In all cases, BDAs incorporate locally sourced sediment ranging from silt and sand to coarse cobble, placed on the upstream face of the structure to protect the base of the structure from scour. Although rarely approaching a true beaver dam, this sediment reduces dam permeability and forces upstream pond formation. While the height and length of BDAs may vary according to location and objective, all BDAs share a common cross-sectional form that resembles a pyramid. Rather than a vertical wall, BDAs should have a broad base which promotes stability by reducing the potential for scour as water moves through and over the structure.

Function: How PALS and BDAs Influence Hydraulic, Hydrologic and Geomorphic Processes

Here, we distinguish the processes that are influenced by low-tech structures into three categories: hydraulic, hydrologic and geomorphic. Hydraulic refers to the changes in the depth and velocity of water, which ultimately drive both hydrologic and geomorphic responses. Hydrologic refers to changes in the timing and magnitude of the movement of water through the streams and ultimately watershed. Geomorphic refers to the characteristic topographic forms created from the changes in patterns of erosion and deposition that result from altering hydraulics. The manner in which structures influence hydraulic, hydrologic and geomorphic processes depend on their specific form and location.

Here we describe how structures influence hydraulic, hydrologic and geomorphic processes in a general sense. For clarity, we address hydraulic, hydrologic, and geomorphic processes separately, however in practice the hydraulic response to low-tech structures forces both hydrologic and geomorphic responses (Figure 7 and Figure 8).

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Table 1 – Summary of typical hydraulic, hydrologic and geomorphic effects of post-assisted log structures (PALS) and beaver dam analogues (BDAs). *indicates that influence may be minor compared to other structure types.

Type Hydraulic Hydrologic Geomorphic PALS Channel-spanning create upstream backwater or pond, and plunge hydraulics downstream increase frequency and magnitude of overbank flow, increase hyporheic flows channel aggradation, channel avulsion, bank erosion, dam and plunge pool formation, bar formation

PALS Bank-attached force convergent flow (deeper and faster), create eddy behind structure force overbank flows* bank erosion, scour pool formation, bar formation, sediment sorting, channel avulsion

PALS Mid-channel force flow separation, create eddy in lee of structure force overbank flows* bank erosion, scour pool formation, bar formation, sediment sorting, channel avulsion

Primary BDA create deep slow water increase frequency and magnitude of overbank flow, increase hyporheic flows channel aggradation upstream, bar formation, bank erosion (if breached on ends), sediment sorting

Secondary BDA create deep slow water increase frequency and magnitude of overbank flow, increase hyporheic flows channel aggradation, channel avulsion, bank erosion, dam pool formation, bar formation

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Figure 7 - From Bouwes et al. (2016b): Expected changes following the installation of beaver dam analogues (BDAs). Beaver-made dams and BDAs slow and increase the surface height of water upstream of the dam. Beaver ponds above, and plunge pools below dams change the plane bed channel to a reach of complex geomorphic units providing resting and efficient foraging opportunities for juveniles. Deep pools allow for temperature stratification and greater hydraulic pressures forcing downwellings to displace cooler groundwater to upwell downstream, increasing thermal heterogeneity and refugia. Dams and associated overflow channels produce highly variable hydraulic conditions resulting in a greater diversity of sorted sediment deposits. Gravel bars form near the tail of the pond and just downstream from the scour below the dam, increasing spawning habitat for spawners and concealment substrates for juveniles. Complex depositional and erosional patterns cause an increase in channel aggradation, widening, and sinuosity and a decrease in overall gradient, also increasing habitat complexity. Frequent inundation of inset floodplains creates side channels, high-flow refugia and rearing habitat for young juveniles, and increase recruitment of riparian vegetation. Flows onto the floodplain during high discharge dissipates stream power, and reduces the likelihood of dam failure. The increase in pond complexes and riparian vegetation increases refugia for beaver and their food supply and caching locations, resulting in higher survival and more persistent beaver colonies. Beaver will maintain dams and the associated geomorphic and hydraulic processes that create complex fish habitat.

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Figure 8 - Conceptual model used in the Asotin Intensively Monitored Watershed (IMW) study of the expected geomorphic and steelhead responses of adding post-assisted log structures (PALS). The increase wood loading by adding PALS is expected to increase flow complexity, creating: deposit and erosion of different substrates sizes; areas of slow water above and behind structures provide resting areas; fast water where convergent jets can scour bottom substrate creating pools or undercut banks; and shear zones at the interface between fast and slow water that is energetically efficient for juvenile steelhead foraging. The deposition of gravels from scour or changes in water velocity provides areas where juveniles can hide and adults can build redds. Wood and undercut banks also provide steelhead cover from predators. The increase in geomorphic complexity including changes in the number and diversity of geomorphic units, channel sinuosity, overbank flows, variables widths is expected to move the stream from a degraded stable state that was locked in by dense young riparian vegetation, to a dynamic stable state (Stage 0) that is capable of recruiting more wood and maintain more complex fish habitat.

Hydraulic PALS and BDAs influence hydraulics in diverse ways and during multiple flow conditions. Changes in depth and velocity are the foundation for changes in hydrologic and geomorphic changes. The primary hydraulic impact of BDAs is to create slow-moving, deep water upstream of the structure. Although seemingly simple, the complex topography this creates (Bouwes et al., 2016b) (Figure 9), including the formation of gravel bars, is easily observed following the breaching of a BDA or beaver dam. In a plane bed channel previously dominated by large cobble, pond deposits behind the BDA are sorted from larger to smaller as water approaching the dam face slows diminishing the capacity to suspend larger sediment sizes. This deposition also leads to channel aggradation. Along homogenized and simplified streams, deep-water habitat (e.g., pools) is often limited. BDAs force dam pools that provide flow and temperature refugia for fish (Bouwes et al., 2016b). Furthermore, by immediately creating deep water, BDAs can create an important habitat feature for successful beaver translocation (McKinstry and Anderson, 2002).

PALS create more variable flow patterns and force areas of high and low velocity and shallow and deep water (Camp, 2015a). Channel-spanning PALS can force deeper, slower velocity water upstream of the structure and increase

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velocity as water flows over the top of the structure. Channel-spanning PALS can rack up material that reduces their permeability and can provide a similar function as a BDA. Mid-channel PALS force flow to split into two separate flow paths, and often create eddies in the lee of the structure (Figure 10). Water split around a mid-channel structure is often faster and shallower initially, but may force scour pools on either side of the structure or channel widening. Bank-attached PALS shunt flow to the opposite side of the stream from the bank it is attached to causing water to converge, increase in velocity and depth. As flow moves past a bank-attached structure, flow diverges and forms eddies, where low is slower and often shallower. The force of these hydraulic responses will be influenced by the size, shape, degree of channel constriction, and orientation of the PALS (i.e., form). Diverse hydraulics provide important habitat characteristics (e.g., energy refugia, predation refugia, prey delivery, oxygen delivery) for fish and other aquatic species that enable them to satisfy their specific life-stage needs.

In general, as flows become constricted, the energy dissipated on the stream bed or bank becomes higher per unit area (i.e., increase in unit stream power), increasing the ability of the water to scour. These constricted flows, such as what can be accomplished with a bank-attached PALS, can be further accentuated by forcing flows to a hard surface such as boulder, making the constriction smaller. Taller, less porous structures create a greater hydraulic head. This potential energy can be focused through a constriction or alternatively, this energy can be dissipated over a structure to prevent scouring, such as in a channel-spanning PALS or BDA. Structures also increase stream roughness, slowing water, and promoting bar development.

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Figure 9 - Digital elevation models (DEMs) and water depth distributions for a A) typical reach with beaver dam analogues (BDAs) (i.e., successfully mimicking and promoting beaver dam activity) and B) without BDAs (i.e., structurally-starved control) from Bouwes et al. (2016b).

Treatment area with BDAs has more channels and greater water depth variability than the control area without BDAs. Note: the red dashed line delineates the extent of a temperature experiment.

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Figure 10 - Hypothesized hydraulic and geomorphic responses associated with bank-attached, mid-channel, and debris jam post-assisted log structures (PALS) from Figure 3.5 from Camp (2015a). Note: what is labeled as ‘debris-jam’ is referred to in this chapter as ‘channel-spanning’.

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Figure 11 - Observed hydraulic and geomorphic responses associated with deflector and mid-channel PALS relative to the magnitude of flows from Camp (2015a). Deflectors (bank-attached) PALs shunt flow, and mid-channel PALS split flow (channel-spanning PALS not depicted). The thickness of the arrows (responses) signifies the magnitude of flow required to initiate observed responses (thin – low flow; medium – typical flood; thick – large flood) based off of empirical findings of their prevalence.

Hydrologic BDAs alter the timing and magnitude of water delivery by forcing temporary storage in ponds and groundwater. BDAs can increase channel-floodplain (i.e., lateral) connectivity by influencing the frequency, duration, and extent of overbank flows. BDAs may increase overbank flows both by channel aggradation and increased instream roughness raising surface flows (Figure 12). BDAs can also be strategically placed to activate side channels or high flow channels (i.e., diversifying residence time of water). Depending on local geomorphic setting and BDA design, BDAs can produce channel-floodplain connectivity and overbank flows during baseflow conditions or during high flow conditions. Increased overbank flow can recharge ground water and raise the water table, providing the water resources necessary to promote riparian expansion; attenuate peak flows and increase baseflow. Water recharge and an increase in the hydraulic head of surfaces waters, may also force water through hyporheic pathways that can produce cool zones of upwelling that provide temperature refugia (Weber et al., 2017).

PALS influence stream hydrology by increasing instream roughness, which promotes channel-floodplain connectivity.

Like BDAs, PALS can be used to divert flows into side-channels or high-flow channels. By increasing water depth or diverting flows into stream banks, PALS may also force increased hyporheic flow and exchange and produce areas of upwelling downstream by slowing water and increasing water depth (i.e., surface water and groundwater exchange).

The hydrologic impact of PALS are most likely more pronounced during high flow conditions (i.e., flow attenuation (see Riverscapes Principle 4 – “inefficient conveyance of water is often healthy” in Chapter 2: Wheaton et al., 2019));

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however, channel-spanning PALS that have sufficiently racked up material to decrease porosity similar to a BDA may be able to force overbank flows even at low discharges.

Figure 12 – Complex of four BDAs forcing overbank flows and inundation of the floodplain where the project goal was to restore the stream to perennial flows. The same magnitude flows (spring-runoff here) prior to installation of these BDAs had no overbank flow.

Geomorphic By altering local hydraulics, PALS alter patterns of erosion and deposition (Figure 10 and Figure 11). These patterns of erosion and deposition create a greater diversity of geomorphic units. Depending on the specific location and structure type, PALS can force: bank erosion, channel widening, lateral migration, channel avulsions, scour pools, plunge pools, bar creation, sediment sorting, and channel aggradation. Some processes, such as channel avulsions and bank erosion are essential processes for the ongoing recruitment of natural large woody debris necessary to sustain physical complexity.

BDAs can lead to increased sediment retention, channel aggradation, and sediment sorting. Increased sediment retention, especially of fine sediment, can increase water quality. Deposition of sediment behind the dams can cause channel aggradation leading to increased channel-floodplain connectivity and accelerated channel incision recovery.

BDAs that breach can also lead to geomorphic changes such as increase in channel width and sinuosity (Pollock et al 2014; Figure 13). Additionally, BDAs can not only quickly connect relic channels, but also create new channels. BDAs can force additional pathways onto a floodplain surface that can eventually result in the formation of another channel when return flows head-cut back to the structure. If BDAs are occupied by beavers, these geomorphic processes are likely accentuated, but, additionally, beavers mechanically create their own channels and tunnels that can lead to further side channel formation.

The geomorphic complexity that is added by the addition of structures is critically important in improving habitat quality for flora and fauna. Perhaps equally important is the increase quantity of aquatic and mesic habitat that structure creates by increasing surface and subsurface water area (Bouwes et al., 2016b).

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Figure 13 – Expected geomorphic responses following the Cluer and Thorne (2013) channel evolution model (from Stage 3 to 0) after the installation (a) of BDAs, their initial ‘failure’ by end-cutting (b), subsequent repair (c) and aggradation leading to floodplain reconnection in an incised system. Figure from Pollock et al. (2014). In practice, PALS can force the same processes of channel-widening and aggradation as BDAs.

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Structure Location

Unlike traditional restoration, which is often characterized by a limited number of instream structures, or stream miles treated, low-tech restoration structures can, and should, be implemented over the maximum possible spatial extent (Chapter 1: Shahverdian et al., 2019a). This means working across a range of geomorphic settings and flow regimes, including incised channels, channels with extensive floodplain, and channels at various stages in their channel evolution (Cluer and Thorne, 2014). The location of a structure constrains what processes it can promote and therefore the structure type that will be most effective (Figure 14). Below, we discuss how the structure setting can influence their performance as well as outline the variables practitioners need to consider when designing an individual structure.

Figure 14 - Structure design is informed by relative location, (i.e., structure configuration within a complex), structure objectives (the function) and form (structure type, size, shape).

The natural variability between riverscapes as well as within riverscapes suggests that there are innumerable forms that PALS and BDAs can take. In other words, no single structure is ‘right’, and the entire treatment (number of structures, or miles treated) is more important than individual structures. However, project managers should consider multiple factors when designing an individual structure. Recognizing and working with these attributes will increase the ability of structures to promote the “system to do the work.” Below we discuss some general attributes to consider when designing low-tech structures.

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Table 2 - General flow, geomorphic, and vegetation attributes to consider when designing PALS and BDAs.

Characteristic Importance Flow Existing flow patterns Enhancing natural flow convergences and divergences using existing geomorphic features such as bars and meander bends is more cost-effective than working against such patterns

Stream power Stream power (a product of discharge and slope) is a useful metric that represents how much power a stream has to do geomorphic work. Higher slopes and higher discharge will be able to do greater geomorphic work, but also put more physical pressure on the structure

Geomorphic Channel width Wider channels will require more material to build geomorphically effective structures, but are often natural areas of deposition

Bank susceptibility to erosion

Highly resistant banks, whether because of vegetation or lithology, are less likely to erode and provide a source of material or promote increased sinuosity. Structures that force convergent flow against highly resistant banks are more likely to force scour of the channel bed

Channel bed substrate Sand bedded streams and rivers have more highly mobile and erodible channel beds, making the PALS and BDAs more vulnerable to scour

Bank material Locate structures adjacent to banks with specific grain sizes (e.g., gravel to support spawning habitat) if composition of bars (downstream) is important to initiate different geomorphic processes (e.g., aggradation)

Bank height If sediment recruitment is a goal to promote channel aggradation, taller banks will provide more sediment per unit length eroded than shorter banks.

Vegetation Presence/absence and type Vegetation may increase resistance to bank erosion and channel widening, but it may also be an important target when recruiting large woody debris into the system. Directing flow at well-vegetated banks may help create undercut banks and provide good fish cover

Flow Regime The flow regime within the project area is important information generally obtained during the planning phase that helps inform the design of individual structures. The flow regime is useful when estimating the forces that will be exerted on any given structure to provide some guidance on how stable a structure needs to be. Additionally, estimating the bankfull height (1-2 year recurrence interval flood) will help determine how tall a structure needs to be to meet some structure objectives (e.g., floodplain access). A cursory survey of the project area can reveal the effects of previous floods – key in on those indicators and use PALS to replicate the results.

Local Sediment Sources PALS tend to induce more geomorphic change when there is a local sediment source upstream. Whether it is in the form of a bar, erodible bank, sediment slug, or caused by erosion from upstream structures, PALS cannot accumulate and sort sediment if it is not being delivered.

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Channel Geometry, Planform, Slope Because the width, depth, and slope of a channel will influence the forces exerted on the structure, consider the materials and time required to construct a structure, and also what objectives are realistic. In general, structures built in a high gradient narrow channel with high banks (e.g., incised channels) will experience greater force than those built in wider channels with low banks. The forces exerted on a structure also depends on the height and amount the structure constricts or spans the channel, regardless of the channel geometry.

These considerations are also important for considering spacing of structures. If BDAs or channel-spanning PALS are used to pond water, the height of the structure and the channel slope will determine where an upstream BDA becomes redundant. In higher gradient channels BDAs will need to have higher crest elevations to create larger ponds.

The sinuosity and number of channel threads are important considerations when planning locations for structures.

Straight, single-thread channels require less consideration for structure placement because the imposed forces are relatively homogenous. The forces (e.g., shear stress) in a sinuous area are more variable. For example, the amount of force will be higher on the outside of a meander bend than the inside. Use this distribution of forces to your advantage when placing a structure to increase their effectiveness and stability. For example, PALS can be placed at the head of side channel junctions to encourage flow path separation, or small PALS can be quickly built to improve side channel habitats.

Channel-Floodplain Connectivity The degree of channel-floodplain connectivity influences the force exerted on a structure at high flows. Where channel-floodplain connectivity is high (i.e., minimal elevation distance between the channel and floodplain) and flows reach or exceed the bankfull elevation frequently, high flows will disperse across the floodplain, increasing flow width and decreasing the force on any given structure. Where channel-floodplain connectivity is limited, and flows are incapable of dispersing, high flows will exert their full force on the structure, increasing the probability of a breach, blowout, or movement downstream. Because restoring channel-floodplain connectivity is a common restoration goal, locating opportunities (e.g., low bank, relic channels) where structures may increase connectivity to promote groundwater recharge, off-channel habitat creation, or riparian expansion is often a major consideration.

Some of the factors listed above are consistent at the scale of entire projects (e.g., flow regime) while others may vary over short length scales (e.g., channel geometry). Some of these factors can be evaluated remotely, while others require field visits.

Structure Design

The design of individual PALS and BDAs depends on the site-specific conditions outlined in the previous section.

Based on those considerations there are a number of structure attributes practitioners must decide upon, including:

structure type, height, width (both laterally and longitudinally), orientation to flow, percent constriction (PALS only), and whether to use posts for additional stability (Table 3). A specific consideration when building PALS is the hydraulic purchase of the structure (Figure 15). Hydraulic purchase refers to the different flow stages at which a PALS will be able to influence flow (BDAs influence flow at all stages). What geomorphic changes PALS are able affect depends on what flows they are capable of interacting with.

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Figure 15 – A key placement consideration is defining at what flow stage the structure will engage with hydraulics or obstruct flows (i.e., hydraulic purchase). At the design stage, there is a choice about whether to build for immediate (i.e., low-flow) hydraulic purchase (e.g., mid-channel PALS in background of A), or only to activate at typical floods or rarer floods (e.g., channel-spanning piece in A). Wood that is long enough that it spans past the entire width of the channel, will only be engaged in overbank flows. Here, there was no wood accumulation for three years through typical floods, but a larger rare flood eventually came through and impressive responses associated with wood accumulation resulted.

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Table 3- Design decisions for individual PALS and BDAs.

Design Decisions Description

Location/type The location of PALS and BDAs constrains the influence they are able to have

Percent constriction (PALS only)

The percent of the channel constricted by a PALS influences its ability to force convergent flow and do geomorphic work

Size The height, width, and thickness of any given structure determines how much of the flow it is able to influence, shunt or back up water, as well as its stability

Orientation How a structure is oriented with respect to the flow will influence hydraulic and geomorphic response

Posts The number of posts used is an important logistical consideration that influences the time and resources required to build a particular structure. It also influences the overall stability of the structure

PALS and BDA Complexes

All low-tech restoration structures should be designed as part of a larger-scale project. While individual structures (PALS and BDAs) may have local influence, they are unlikely to achieve restoration goals unless they are part of a more widespread effort (Chapter 1: Shahverdian et al., 2019a; and Chapter 2: Wheaton et al., 2019). A complex is a group of structures, often between 2 and 15 individual structures that are designed to work together. A complex may be composed of a single structure type (i.e., BDAs) or a mix of structure types. Like natural beaver dam complexes (Figure 16), complexes are more likely to be able to influence hydrologic and geomorphic processes when built in clusters. Individual PALS and BDAs that are part of a complex help to increase the stability of any given structure within the complex by altering flow timing, magnitude and pathway at the reach scale. Furthermore, individual structures can be located in such a way as to reduce the potential for scour and to maximize the ability to achieve restoration goals.

Complexes are discussed in detail in Chapter 5 (Shahverdian et al., 2019b).

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Figure 16 – The natural beaver dam complex should be the inspiration for designing a complex. Like natural beaver dams and accumulations of large woody debris, low-tech structures are more likely to achieve restoration goals when built to work together to influence hydraulic, hydrologic and geomorphic processes.

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PALS AND BDAS CHANGE OVER TIME

In this chapter, we described the form and function of intact PALS and BDAs. However, PALS and BDAs are not intended to be permanent structures, and will change over time in response to flow conditions, wood accumulation, and sediment delivery. Deciding how to allocate limited restoration funds and developing realistic expectations for both PALS and BDAs is critical for designing effective low-tech restoration projects. In this section we describe common trajectories for both PALS and BDAs.

All PALS have a less than one-year design life (i.e., designed to withstand a typical mean annual flood), but their actual life-spans may extend in decades. This indicates that structures are not built to be permanent structures and are not guaranteed to withstand high flow events. However, like natural accumulations of large wood and natural beaver dams, many individual structures are likely to persist beyond one year.

PALS

PALS can be specifically intended to affect geomorphic change during high flows and are therefore likely to both force geomorphic changes and experience structural changes. Because PALS mimic and promote accumulation of large woody debris, it is common for structures to increase in size as large woody debris is trapped by existing structures (Figure 17). PALS may trap wood naturally delivered to the channel or lost by upstream PALS. PALS may also trap enough bedload to bury the main channel or cause an avulsion that reroutes the main flow around a PALS or complex, leaving structures high and dry. Mid-channel and bank-attached PALS can become channel-spanning debris jams if they capture enough woody material from upstream. None of the scenarios should be considered failures, unless they cause harm to the system or infrastructure, because the PALS still provide structure to the channel and floodplain, leaving it more resilient than it was prior to treatment. PALS can be maintained by adding more large woody debris and/or posts as they decay or otherwise lose material over time. Whether a PALS changes from mid-channel to channel-spanning, or channel-spanning to bank-attached is not of special importance. Instead, evaluating how the complex has changed (Chapter 5: Shahverdian et al., 2019b) is more important in determining future management actions.

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Figure 17. Example of PALS evolution over the course of one year by promoting processes of wood accumulation. A and B show a mid-channel becoming a bank-attached, C and D show a bank-attached becoming a debris jam, and E and F show a bank-attached becoming a mid-channel. The geomorphic changes imposed by the presence of the PALS in each example shows clear alterations to the channel bed and hydraulics.

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BDAs

The specific evolution of any particular BDA depends on flow conditions, sediment regime, beaver activity and maintenance done by restoration practitioners. Common outcomes for BDA include: blowouts (defined as a complete loss of the BDA), breach (an end section or middle section fails), sedimentation, remaining structurally intact but no longer ponding water, and intact and ponding water. If high flows occur, blowouts or breaches can occur where all or part of a BDA is washed downstream. While not the design intent, breached BDAs can still provide significant instream restoration benefit. In short, following a breach a BDA begins to function like a bank-attached or mid-channel PALS. In systems with high bedload transport, BDAs may force channel aggradation that reaches the BDA crest elevation.

Depending on restoration objectives, this may represent a successful outcome, present new opportunities, or require new action. In cases where reconnection with the floodplain is the restoration goal it may be appropriate to build a new structure on top of the existing structure in order to continue the process of incision recovery. However, if the objective is the creation and maintenance of pool habitat (e.g., for fish), then filled-in BDAs will need to be rebuilt or replaced with another structure type to meet those objectives. In the absence of maintenance, whether by beaver or restoration practitioners, BDAs are unlikely to continue to force upstream ponding during typical flows, in which case they effectively evolve into channel-spanning PALS. Such a structure may or may not meet restoration objectives and require either rebuilding (to maintain pond habitat) or be sufficient (to promote channel aggradation and floodplain connectivity). For restoration practitioners, predicting and monitoring different structure responses can help improve restoration effectiveness and implementation efficiency.

USING PALS AND BDAS

In this chapter, we have presented a parallel discussion of PALS and BDAs. In practice, a low-tech restoration project can utilize any combination of PALS and BDAs to achieve restoration goals. In many cases local stream conditions, often at the sub-reach scale (101 -102 m) will lend themselves to a particular structure type. The decision to design a PALS or a BDA is based on both physical parameters of the site and restoration goals as well as pragmatic considerations on how to allocate limited project resources. Because PALS require fewer resources per structure than BDAs, more PALS than BDAs can be built for a given amount of funding. In accordance with low-tech restoration principles we suggest that the total number of structures and structure density is the single most important factor in any restoration project and as such often recommend strategies maximize the total number of structures. However, PALS and BDAs mimic and promote distinctive processes, regardless of logistic concerns. As will be elaborated in the design chapter, the structures that most appropriately invoke the process that matches the complex objective should be used.

In areas with easily accessible floodplain or relic channels, BDAs can immediately increase floodplain connectivity, or activate another channel by forcing immediate overbank flows, even during baseflow conditions. Where restoration may incorporate other strategies such as riparian plantings, immediate increase in water resources may be desirable to increase the success of plantings. Where beaver translocation or the expansion of existing beaver populations is a goal, creating immediate pond habitat may encourage the successful colonization of a particular reach and reduce the likelihood of predation. In incised streams, characterized by narrow width and high banks (Stage 2-4 Cluer and Thorne (2013) or Stage 2, Pollock et al. (2014)), PALS are a more cost-effective way to promote channel widening and aggradation. If channel widening is the goal of restoration in order to promote incision recovery (Pollock et al., 2014), channel widening would necessarily result in the effective breaching of BDAs. In such a case, bank-attached or channel-spanning PALS can achieve the same restoration objectives with less resources per structure, enabling restoration practitioners to build more structures and expand their restoration treatment.

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CONCLUSION

PALS and BDAs are low-tech restoration structures that mimic and promote the processes of wood accumulation and beaver dam activity. They are permeable, temporary structures that can be built by hand using natural materials. Both PALS and BDAs influence hydraulic, hydrologic and geomorphic processes in similar ways. The design process of PALS and BDAs requires considering flow conditions and local geomorphic context (e.g., gradient, planform, cross-section geometry). Both PALS and BDAs can be used to address common restoration objectives such as, increased instream complexity and increased channel floodplain connectivity. Therefore, the decision to use particular structure type is driven both by the restoration objective as well as logistic considerations, and the knowledge that greater numbers of individual structures are more likely to achieve restoration goals.

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– APPENDIX

APPENDIX A. FREQENTLY ASKED QUESTIONS ABOUT PALS AND BDAS

Since we began building beaver dam analogs (BDAs) and post-assisted log structures (PALS) in 2009, we have been asked many questions about their function, design, construction, effectiveness, and their potential negative impacts to the riverscape or aquatic species. Often the same questions come up over and over again.