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New Innovative Habitat-Creating Bank Protection Method

New Innovative Habitat-Creating Bank Protection Method
New Innovative Habitat-Creating Bank Protection Method

ASCE EWRI Conference

Emerging and Innovative Technologies- Understanding, Measuring, and Implementing Water Resources Sustainability through Innovative Strategies

Hydraulics and Waterways-Bank Stabilization

New Innovative, Habitat-Creating Bank Protection Method

Tim Abbe1, Jack Bjork2, Al Zehni3 and James Park4

1Cardno ENTRIX, 200 First Ave West, Suite 500, Seattle, WA 98119. PH (206) 790-1079; FAX (206) 269-0098; tim.abbe@https://www.doczj.com/doc/8c5110330.html,

2Cardno ENTRIX, 200 First Ave West, Suite 500, Seattle, WA 98119. PH (206) 269-0104; FAX (206) 269-0098; jack.bjork@https://www.doczj.com/doc/8c5110330.html,

3Pierce County Public Works, 2702 42nd St, Tacoma, WA. 98409. PH (253) 798-4677; FAX (253) 798-2725

4Washington Department of Transportation, MS 47331, 310 Maple Park Ave, Olympia, WA 98504. PH (360)705-7415; FAX (360) 705-6833; parkj@https://www.doczj.com/doc/8c5110330.html,

ABSTRACT

The traditional method to control river bank erosion and channel migration has been rock revetment. Although this approach can be effective, it often has negative impacts on aquatic habitat. In the Pacific Northwest billions of dollars have been spent on restoration, restrictions and spillage of water at hydropower dams to stop the decline of threatened and endangered salmon. We developed a new innovative method for bank protection which combines the habitat benefits of large woody debris and the stability and erosion resistance of concrete dolos. The first application was along the Lower Puyallup River where a complex revetment of dolos and logs was constructed in 2009 to protect a segment of the river’s North Levee in the highly developed city of Fife, Washington. Regulators considered the work to be self-mitigating. The second application, now being designed, will provide 1,300 LF of protection along State Route 20 in northern Washington where the Skagit River has caused repeatedly obliteration of the highway. The project reach is a Wild and Scenic River near the North Cascade National Recreation Area, has numerous spawning redds of endangered steelhead and Chinook salmon and resource agencies would not allow further placement of a rock revetment. The dolos will be made of colored concrete and will have a roughened, bark-like surface. Connected to logs, the units can be placed with a crane or excavator without dewatering or river diversions, a substantial cost savings. The resultant is bank protection which has both a large surface area of logs and a huge volume of interstitial spaces which enhances physical complexity critical to habitat. This new approach is in the process of being patented and offers tremendous potential for more sustainable bank protection and maintenance.

INTRODUCTION

River bank erosion is one of the most widespread threats to infrastructure and property

in the world. When bank erosion threatens major facilities like levees, roads and developed areas, it can result in major environmental, social and economic damages. Traditional bank protection has focused on limiting economic and social damages, but come at a high cost to aquatic and riparian habitat. Over the last 30 years the increasing awareness of the cumulative environment impacts of bank protection, particularly under the U.S. Endangered Species Act, has led to a wide range of bank protection alternatives. Many of these alternatives fall under the umbrella of “bio-engineering”. These techniques focus on the role of riparian vegetation in limiting erosion. But in large channels the toe scour that drives most bank erosion occurs well below the root depth of vegetation and requires more structural elements. The most common approach for toe protection is placement of large rock. Thus many bioengineered bank protection projects only differ from a traditional blanket revetment by the presence of vegetation along the upper bank. Other bank protection methods that deflect flow away from the bank have increased in popularity because they create more diverse habitat conditions for fish, structures such as bend way weirs, spur dykes, rock vanes, and engineered logjams. They offer different ways to create more physical complexity within the river channel – a key objective for protecting aquatic habitat. Engineering solutions that add physical complexity add uncertainty to predicting river response, requiring more extensive analysis. Compared to traditional rock revetments, more complex structures will induce more complex response and may be subject to more severe conditions such as greater drag, shear and scour. They are also more difficult and, therefore, more expensive to build. Despite the challenges, the negative environmental impacts of rock revetments continues to drive changes in bank protection. This is very much the case along the Pacific Coast where many rivers have ESA-listed salmonids. If the project is found to impact aquatic and riparian habitat then mitigation is likely to be required that is costly and increases permitting times. Additionally, construction is usually limited to very short periods when impacts to threatened and endangered species will be minimized. The best solutions are those that demonstrate positive enhancements and thus deemed “self-mitigating”.

An example of innovation coming out the Pacific Northwest (PNW) has been development of engineered logjam technology (ELJs) which takes a naturally occurring feature of river systems and integrates into into river management and restoration (Abbe et al. 1997, 2005). In the last 15 years ELJs have become a widely applied technique in the restoration of Pacific Northwest rivers (Abbe et al. 2003a, b; Abbe and Brooks 2011). While there are many different applications for ELJs within fluvial environments and many more different variations in ELJ design, the underlying philosophy is to emulate natural systems (Abbe et al. 1997). When properly designed and constructed, ELJs have been very successful in enhancing aquatic habitat and achieving bank protection goals. One of the major differences between ELJs and bioengineering is that ELJs are in-stream structures which induce local scour and therefore must be designed to accommodate scour. Just as in the case of rock structures and bridge piers, scour poses the most serious threat to an ELJ. To design for scour we have three basic options for any structure: 1) it must go deeper than depth of scour, 2) not going as deep and thus limiting scour depth proximal to the structure, or 3) the structure settles with scour without compromising its integrity or performance. The first

option involves river bed excavation that brings up many challenges for permitting and construction. The second option still involves either excavation or pile driving. Both of the first two options almost always require measures to isolate the construction area. Because of the buoyancy of wood structures it is critical they are either deeply embedded (e.g., piles) or ballasted. The last option simply places material into the river bed that will settle as scour occurs but is designed so that as the structure deforms it still achieves the desired bank protection goals. A good example is a dynamic revetment in which rock launches into scour holes. Obviously using wood in this type of approach can only be done if the wood also settles when subjected to scour and doesn’t simply get washed away. Thus individual wood must have attached ballast or the wood structures such as cribs be designed to retain their ballast and still perform if structural elements undergo changes in their position and orientation as they settle into the riverbed.

This paper focuses on the development of a solution to meet the following objectives:

1.Provide long-term protection to critical infrastructure (levees and roads)

2.Create complex edge habitat for salmonids

3.Integrate large quantities of large woody material (LWM)

4.Minimize construction impacts and time

5.Minimize cost

6.Limit the need for mitigation

Many bank protection protections on the Pacific Coast typically come with these objectives and thus created incentives to develop new ideas based on sound science and engineering. We will describe one project in which we were faced with these objectives and developed a unique solution never before done.

THE LOWER PUYALLUP RIVER

Bank erosion along the right bank of the Lower Puyallup River at River Mile (RM) 5.26 presents a potential threat to the integrity of the river’s North Levee protecting the city of Fife, WA (Figure 1). This portion of the Lower Puyallup used to have a tortuous meandering channel that went from went from one side of its valley to the other. Between 1914 and 1930 the river was straightened and levees constructed. The project site is located about 5 miles upstream of Puget Sound and 2500 feet downstream of the 66th Ave Bridge (Figure 1). The Puyallup River flows northwest between North Levee road along the river’s right bank and State Route (SR) 167 along the left bank. The river has a 2457 km2 basin that originates from 12 glaciers on the West and North sides of Mount Rainier (4392 m). The river carries high loads of bedload and suspended sediment. Estimated peak flows at the project site are listed in Table 1. The estimated 2-yr flood stage is approximately equivalent to the top of bank along the silt bench. Water elevations increase about 9.5 feet from the 2-yr to 10-yr flood stage, completely inundating the silt bench. Water elevations increase approximately 2 feet from the 10-yr to 100-yr floods.

Figure 1. Lower Puyallup River, Pierce County, WA. Project site is located along North Levee which protects the city of Fife.

Table 1.Flood recurrence intervals and magnitudes

Flood Recurrence Interval (yrs) Peak Flood Magnitude

(cfs) (cms)

1 9,000

255

2 22,000

623

5 33,000

935

10 41,000

1161

50 46,000

1303 100 48,000

1359 Throughout this reach of the river, the inboard side of the North Levee is abutted by an inset floodplain referred to as the “Silt Bench”(Figure 2). The silt bench is approximately 50-70 feet in width. The original levee design included a concrete slope slab mattress on the inboard slope of the levee (Figure 2) with a cedar brush mattress along its toe for erosion control. This cedar brush mattress is believed to have initiated sedimentation along the North Levee that ultimately formed the silt bench. The current ground elevation of the silt bench is estimated to be about 5-10 feet above the original cedar brush mattress.

Much of silt bench is covered by riparian vegetation dominated by native deciduous trees. The silt bench forms a buffer between the levee and the river which is considered integral to protecting the levee itself. Between the years of 2000 and 2004, a 250 feet section of the silt bench experienced approximately 20 feet of lateral erosion that eliminated riparian vegetation between the utility road and the North Levee (Figure 3). This erosion posed a serious threat of losing the silt bench and threat to the integrity of the levee.

Historic photos clearly show that the North Levee silt bench encroached out into the

river over time and gradually become more densely vegetated. No further encroachment of the bench into the river occurred after 1982, after which both banks of the river have been relatively stable. Based on historic airphotos, a loss of riparian trees

at the project site suggests bank erosion began shortly before 2002. Colonization of riparian vegetation would have played an important role in development and stabilization of the silt bench by addition soil cohesion and hydraulic roughness, the later slowing down flows and promoting sedimentation. Recent cross-section surveys show that most of the erosion at the project site occurred between 2002 and 2007. Given the lack of toe protection along the North Levee, Pierce County Public Works did not want to risk erosion getting into the upper levee prism.

Figure 2. Generic cross-section geometry of Lower Puyallup North Levee near the project site with silt bench intact.

The silt bench is composed almost exclusively of sand whereas the point bar along the south bank is composed of larger sand and gravel. A boring into the silt bench was done to determine grain size distributions of subsurface materials. At a depth of 5 ft the D50 = 4 mm and the D95 = 30 mm. At a depth of 17.5 ft the D50 = 0.08 mm and the

D95 = 2 mm. The scour analysis used an intermediate value of 2 mm. As a sensitivity analysis, the scour analysis was also performed using a grain size of 0.20 mm. Substrate mobility is typically expressed as a function of basal shear stress.

The shallower depths and coarser grain size of the point bar mean it is less likely to erode opposed to finer grained sand located in deeper water along the silt bench. But this is based on some important assumption that there are no other factors (such as wood debris) contributing to energy losses along the right bank of the river and the sediment has no cohesion. The primary factors contributing to energy loss along a river bed are roughness elements. The cumulative effect of different roughness elements (e.g., bed/bank roughness and wood debris) reduces the energy available for moving sediment. This is referred to as roughness or shear stress partitioning (Manga and Kirchner 2000, Hygelund and Manga 2003):

τE = τ0 – (τGS + τLWD)

where τ0 = basal shear stress (Pa)

τE = effective basal shear stress available for sediment transport

τGS = stress borne by bed roughness

τLWD = stress borne by wood debris

Relatively small quantities of stable wood debris can account for significant reductions in the effective shear stress available for sediment transport. In the case of the Puyallup River silt bench, snags and woody vegetation clearly play a substantial role in stress partitioning. At high flows woody vegetation further partitions the shear stress, and adds cohesion to otherwise cohesionless materials, increasing bank resistance to erosion. Other alternatives considered involved costly structural elements (i.e., timber or sheetpile wall, a rock revetment with riparian planting, or engineered logjams). The alternatives all encountered opposition from regulatory agencies and stakeholders including the Puyallup Tribe. The project was of special concern to the Puyallup Tribe not only because of the salmon fishery but because they actually own the riverbed below ordinary high water. Each of the alternatives also entailed significant construction challenges with regards to dewatering and grading. Placement of an adequate toe on the rock revetment would require a major effort to control water in the work area. The natural process by which the silt bench formed and has been sustained was not accounted for in the alternatives. None of the three alternatives would look or function like the existing stable vegetated shorelines observed along the river, particularly with regards to stress partitioning. The engineered logjam alternative addressed some of the regulatory concerns but was incompatible with the county’s zero-rise flood policy. Thus the county was not only looking at a set of expensive alternatives, but substantial mitigation costs required by the Puyallup Tribe.

The project team at Cardno ENTRIX conducted a geomorphic and hydraulic assessment of the project reach in which processes of erosion and the role of vegetation were evaluated (Abbe 2008). To best meet the project objectives of the County and stakeholders a new design approach was developed that used self-settling interlocking roughness elements intended to simulate the role of natural snags and LWM found along the river’s banks. A conceptual model of river bank processes was presented that incorporated the role of vegetation and LWM (Abbe 2008). Sections of the silt bench shoreline devoid of vegetation, such as the project site, are clearly at the highest risk of experiencing further erosion. Trees substantially increase frictional roughness along the bank at high flows and along the toe of the bank when they fall in (the snag visible in Figure 3 marks old shoreline and remained intact from 2004-2010). In many locations along the river where there is more of a riparian buffer snags accumulate more wood debris and together with other riparian vegetation stabilize the bank (Figure 4). In this way, the vegetation not only limits bank erosion but provides excellent complexity and cover for fish. The stress partitioning provided along these rough channel boundaries encourages sedimentation which is followed by more plant colonization, rebuilding banks that had been subjected to erosion. To simulate these conditions yet provide the structural confidence and longevity needed for protecting an important levee, a new idea was needed. So we sought out an artificial means of simulating a tree trunk that would eliminate uncertainty with regards to stability and longevity. The answer was a complex shaped self-ballasted element similar in size to a tree. Simple rock revetments have several common characteristics: rock must be big enough not to move and placed deep enough not to be undercut. To get around the massive size that would be required of individual rocks in some coastal environments, coastal engineers focused on using

geometry instead of size to create more effective clasts for constructing shoreline revetments. The result has been a variety of inter-locking pre-cast concrete elements (USACE CEM 2006) that offer the closest analog meeting the goals laid out above for the Puyallup project site. The most suitable option for simulating a snag are dolosse (individually referred to as “dolo”), large pre-cast unreinforced concrete “jacks”.

Figure 3. Erosion along Lower Puyallup River at project site in 2007. Photo is looking downstreamt. Maintenance road sits on silt bench. Original shoreline located about 20 ft out from existing bank.

Figure 4. Natural conditions in which fallen trees form embedded snags and capture wood material along the forested banks of Lower Puyallup River (looking downstream). These conditions are very important for juvenile salmonids, particularly in a channelized river that doesn’t offer much cover. Note sand deposits upstream and downstream of the wood.

We know of at least one site in Washington State where dolosse were used in a river for bank protection (Nooksack River in Whatcom County). What made the Lower Puyallup Project unique was the addition of wood. Abbe (2008) laid out a concept in which an individual dolo was combined with a log to form a more complex form that increased roughness, stability and environmental benefits. Each combined element of dolo and logs offered a self-settling, interlocking unit that could be lowered into the river. Cumulatively, these individual elements could be used to form a complex revetment that emulated the natural conditions found along the river (Figure 5). We know of no other project that has ever done anything similar.

Figure 5. Concept visualization of pre-cast concrete elements (e.g., dolosse and logs) used to simulate natural snags and ballast natural wood to substantially roughen shoreline of silt bank. View is looking upstream.

The concept provided a design that could accommodate scour (because units would simply settle into the riverbed, while remaining linked to the matrix of the entire revetment. The entire structure could be fit within the erosion prism of the project site thereby having no more impact on water surface elevations than the original shoreline prior to erosion. Good bathymetry is critical to estimating the number of units that will be needed and how the revetment may deform with scour – thus influencing the structure’s initial geometry and ultimate number of units needed.

The individual dolo units used in the project have a volume of 106 ft3 and a surface area of 180 ft2. Each dolo weighs 8 tons and was delivered by truck. The volume of wood attached to the dolo ranges from 50 to 100% of the dolo volume. Since approximately 3 logs were used for each dolo, the total surface area of wood was more than twice that of the dolosse. For the Puyallup River project chain was used to attach the timber to each dolo. Once in place, additional logs and small woody material was placed within the primary matrix. The Puyallup project used 60 individual dolo units four layers deep over a distance of 225 ft. The project used over 250 logs. Each dolo and log unit was lowered into place (Figure 6) with a single 400 series excavator with one person on the ground. No backfill was placed to reconstruct the original bank; it was assumed that natural sedimentation would rebuild it. The project was completed in less than two weeks. The total construction cost of the project was approximately $372,000, of which the pre-cast dolosse were about $207,000. This was more than a million dollars less than estimated costs of other alternatives which would have also required mitigation. Thus far the project has achieved all the objectives and performing as intended. The structure has formed a complex interstitial, low velocity area along the length of the project (Figure 7). Sedimentation within the matrix is occurring as expected. The only criticism of the project is the bright white color of the concrete used for the dolosse (Figure 9). Natural weathering and sedimentation is already changing the color. And riparian vegetation is expected to rapidly cover the entire revetment. Observations of the project (personal communication, Tom Nelson, Pierce County Public Works):

1.The structure has stopped all erosion along the project site.

2.Juvenile salmon, coho and either chum or pink salmon were observed rearing in

and around the dolo/log matrix of the project site.

3.The edge and interior of the structure has created low velocity areas. Measured

flow velocities of about 8 ft/s about 10 ft off the structure reduce to zero to within

5 ft of the revetment.

4.Sedimentation is occurring within the structure.

5.The recently planted willows and other plants at the silt bench have a 90%

survival rate.

Figure 6. Individual dolo and timber unit being lowered into place;2009 construction

Figure 7. Completed dolo and timber revetment structure, 2009 (looking downstream).

Figure 8. Project site on April 2010. Road is situated on top of the North Levee. Bare area between road and river is maintenance road. 3 of 4 layers of dolo and log revetment are visible. Flow is from right to left.

The Skagit River

The success of the Lower Puyallup River dolo and log revetment provided a general prototype for other projects. Cardno ENTRIX was contracted in 2010 by the Washington Department of Transportation (WSDOT) to assess erosion and develop designs to protect a 1500 ft section of the right bank of the Skagit River along State Route 20, the northern most highway crossing the Cascade Mountains. The Skagit River is the largest river draining into Puget Sound and has 2 yr and 100 yr peak flows of 36,000 and 92,500 cfs, respectively.

The blanket rock revetment built along SR 20 had been subjected to chronic failures and repairs. Additionally, resource agencies would not allow further construction of rock revetments. The impacts of the repairs were driving development of a long-term solution that could better emulate natural conditions and enhance habitat. The project site is adjacent to spawning beds of endangered steelhead and Chinook salmon, and is federally designated as a Wild Scenic River, bringing up major restrictions on any bank protection strategy. On top of the regulatory constraints, the project site presents major challenges including deep water and high velocities along the bank. The upper segment of the project has water depths of over 15 ft during construction conditions.

It was determined the site was a good candidate for applying the dolo and log approach. The challenge would be creating a structure that incorporated ELJ features and using dolo units that much more resembled trees. The solution created a textured and colored concrete that resemble natural tree bark (Figure 9). Each dolo would also have logs attached. The complete unit is called a “dolotimber?”(copyright 2011 Tim Abbe and CardnoENTRIX). These units could include any type of simulated concrete log attached to real timber and take on any form and configuration that creates a complex shape intended to partition shear stress. Just like the Puyallup project, each dolotimber?could be lowered directly into the river, a major advantage for the Skagit River site. The dolotimbers? could also be used for ELJ structures that included lots of logs and small debris inserted into the dolotimber? matrix. This approach was determined to be lower in cost than other alternatives.

Figure 10. Example of concrete texture and coloration included in a dolotimber?.

Figure 11. Physical model of a complex dynamic revetment constructed using dolotimbers? (copyright 2011 Tim Abbe and CardnoENTRIX). References

Abbe, T. 2008. Preliminary assessment and erosion protection concept for Lower Puyallup River North Levee. Fife, Pierce County, Washington. Report by

ENTRIX, Inc. for Pierce County Water Programs Division, Tacoma, WA. 45 p. Abbe, T. and Brooks, A. in press. Geomorphic, engineering and ecological considerations when using wood in river restoration. American Geophysical

Union.

Abbe, T., D.R. Montgomery, and C. Petroff. 1997. Design of Stable In-Channel Wood Debris Structures for Bank Protection and Habitat Restoration: An Example

from the Cowlitz River, WA. pp. 809-816 in: S.S.Y. Wang, E.J. Langendoen,

and F.D. Shields Jr. (eds.), Proceedings of the Conference on Management of

Landscapes Disturbed by Channel Incision. University of Mississippi. Abbe, T., A. Brooks, and D.R. Montgomery. 2003a. Wood in River Restoration and Management. pp. 367-389 in: S. Gregory et al. (ed.), Wood in World Rivers.

American Fisheries Society, Bethesda, Maryland.

Abbe, T., G. Pess, D.R. Montgomery, and K.L. Fetherston. 2003b. Integrating Engineered Logjam Technology in River Rehabilitation. pp. 443-490 in: D.R.

Montgomery, S. Bolton, D. B. Booth, and L. Wall (eds.), Restoration of Puget

Sound Rivers. University of Washington Press, Seattle, Washington.

Abbe, T., D.R. Montgomery, C.A. Adams, R.C. Riley, K.M. Robinson, and E.L .

Owens, 2005. Bank protection and habitat enhancement using engineered log

jams: an experimental approach developed in the Pacific Northwest. USDA

Natural Resource Conservation Service.

Hygelund, B. and Manga, M. 2003. Field measurements of drag coefficients for model large woody debris. Geomorphology 51, 175-185.

Manga, M., Kirchner, J.W., 2000. Stress partitioning in streams by large woody debris.

Water Resources Research 36, 2373– 2379.

USACE CEM. 2006. Coastal Engineering Manual. U.S. Army Corps of Engineers.

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