720 Olive Way, Suite 1900 Seattle, Washington 98101 Phone 206.287.9130 Fax 206.287.9131 www.anchorqea.com D RAFT MEMORANDUM To: Rob Wenman, Pierce County Public Works and Utilities Date: From: Betsy Severtsen, Anchor QEA Project: 150767-01.01 Re: Peter Hummel, Anchor QEA Levee Planting Program Recommendations Pierce County (County) is developing a vegetation management plan that is part of the Pierce County System Wide Improvement Framework (SWIF) for river levees. The SWIF addresses U.S. Army Corps of Engineers (USACE) levee inspection and management requirements while incorporating habitat enhancement for aquatic species, in response to tribal and other stakeholder concerns. This memo documents the planting program recommendations presented at Stakeholder Meeting 4, which took place December 16, 2015. CURRENT PLANTING TECHNIQUES The County has integrated native vegetation planting into levee repair work using two different techniques: 1) bioengineered brush layering; and 2) planting over topsoil-covered rock rip-rap. Technique 1: Bioengineered Brush Layering One example of the bioengineered brush layering is shown in Photo 1. This technique occurs at the top of rock revetments, and includes lifts of topsoil (which may or may not be wrapped in a biodegradable geotextile fabric), and then placing live dormant branches in layers between these topsoil lifts. Because livestake/live branch plants are limited to a few species (three species of willows and red osier dogwood are commonly used in this application), the planting areas are not diverse. Cottonwood poles can also be used for similar applications. Also, the siting of the planting units to maximize plant-able area above the ordinary high water mark (OHWM) could be improved upon. Overall, this technique is successful as the vegetation appears to establish well, the depth of topsoil installed does not appear to be limiting plant growth, and there are no observed issues with the toe stability at \\fuji\anchor\projects\pierce County\SWM Engineering On-Call\Pierce County SWIF\Task 3 Stakeholder and Public Support\Veg Management\Planting Program Memo_PH_ha.docx
Page 2 the lower edge of the planted area. This technique also requires that plant installation occur during the dormant (fall/winter) season due to the use of dormant live branches. Photo 1: Technique 1 bioengineered brush layering Technique 2: Planting over Topsoil-covered Rock Riprap The second technique, an example of which is shown in photo 2, has had less success in longterm plant establishment and overall stability, but does include a more diverse plant species mix. This technique occurs after the rock riprap armor layer has been placed over the repaired levee. Topsoil is then placed on top of the rock riprap and plants are installed. Plants may be container grown or bareroot, depending on the time of year planting occurs. The main limiting factors for plant success with this technique over time are: soil depth and soil stability. Soil instability appears to be due to three potential causes. The first observed cause, as shown in Photo 2, is instability of the toe from high water (flood events) that leads
Page 3 to unraveling of the topsoil mass. Photo 2 shows a distinct thinning and absence of vigorous vegetation of the lower portion of the soil/mulched area, as well as small scarps where the topsoil is less stable, indicative of the first cause of instability. The second cause is soil piping through the gaps within the underlying rock riprap due to void spaces. A third cause could be inconsistent and insufficient soil depths due to the irregularity of the underlying rock surface. All three of these causes increase the likelihood of installed plants mortality after a few growing seasons when soil depths can t accommodate the growing root systems and the plant s water and nutrient needs. Photo 2: Technique 2 planting over topsoil-covered rock riprap RECOMMENDATIONS Both the brush layering and planting over riprap techniques are viable options to revegetate levee segments; however, some improvements can be incorporated. The following section
Page 4 provides recommendations for refining these approaches and presents two other vegetation planting and recruitment techniques that the County may add to their planting technique toolbox. Refining Existing Techniques Brush Layering Refinements Technique 1 is successful in many respects, but it can be improved by increasing the diversity of the planted areas, and by maximizing the planted area through careful delineation of the OHWM. To improve upon the ecological services provided by riparian buffers, the County can manage the development of these habitats over time, through stewardship and adaptive management. Installing shade-tolerant conifer plantings within the initial bioengineered installation area will increase the biodiversity of the site, adding species that will grow much larger than the willows. Over the long term, the conifers will provide year-round shading and potentially greater over-hanging vegetation, and large wood contribution benefits. In addition, planting conifers after the initial establishment of the willows, or other commonly used deciduous species, will diversify the plant palette. The willows will provide shade to conifer species that might have been too exposed to sun if they had been installed as small container plants initially. These species could include Grand Fir (Abies grandis), Western redcedar (Thuja plicata), and Western hemlock (Tsuga heterophylla). Technique 1 can also be improved by considering carefully the location of the toe of the site in relation to the OHWM, in order to maximize the planted areas in extent, and increase their benefits as overhanging vegetation to aquatic species. State regulations define the OHWM as: that mark that will be found by examining the bed and banks and ascertaining where the presence and action of waters are so common and usual, and so long continued in all ordinary years, as to mark upon the soil a character distinct from that of the abutting upland, in respect to vegetation as that condition exists on June 1, 1971, as it may naturally change thereafter. (Ecology 2015)
Page 5 There is a fair amount of interpretation inherent in this definition, necessary because it applies to a wide variety of stream and lake types. The Washington Department of Ecology offers training in determining OHWM delineations, and we recommend using trained delineators when establishing OWHM locations in the field. Of these primary indicators, changes in vegetation characteristics may be the most applicable for levee segments, where slope and sediment may be more engineered than on natural banks. Typically, woody vegetation is found above the OHWM and only herbaceous species (without a persistent woody stem above ground) or very sparse or immature woody vegetation is found below this mark. More detail on OHWM delineation can be found through the 2014 USACE guide (Mersel 2014) and the Washington Department of Ecology Website (http://www.ecy.wa.gov/programs/sea/sma/st_guide/jurisdiction/ohwm.html). Photo 3 illustrates how one brush layering application was installed with the OHWM below the installation toe. The blue line highlights the lower limit of planting and shows that at least 2 feet of planted space, for woody riparian vegetation, was lost. It also shows that woody vegetation growing beyond the repaired area appears to have rooted several feet lower on the levee bank.
Page 6 Photo 3: Limit of planting (dashed blue line) and adjacent wood vegetation lower on bank beyond repair area indicating Ordinary High Water Mark We recommend having trained personnel delineate the on-site OHWM line based on vegetation and other characteristics, and then taking elevation measurements that can be applied to design drawings, which will be most useful as a reference point for establishing lower limits of planting associated with levee repairs for design and construction. Planting over Topsoil Covered Rock Riprap Refinements Technique 2 has had less long-term success compared to the brush layering applications, due to limited topsoil depth, as stated above. The depth of soil recommended for long-term plant health is a minimum of 18 to 24 inches, with the smaller end of this range more suitable for shrubs rather than tree species. Current County topsoil placement practices may often result in less soil depth than those depths recommended. In addition, it appears, as described
Page 7 above, that after placement topsoil is unstable and lost via two pathways: the first through the lack of any toe protection to the planted slope, and the second through piping in between the riprap base. These losses of topsoil depth can be addressed through the use of a stabilized toe, and the placement of a filter layer between the rock riprap and topsoil. Figure 1 shows how these materials would be placed conceptually. The figure indicates that a 2-foot terrace in the rock riprap is needed in conjunction with a coir log or wood log to stabilize the toe as a lip to protect the topsoil from high water events. Figure 1: Rock placement and planting toe The following coir log product and installation specifications are recommended: Coir Logs shall be manufactured from 100 percent un-dyed, coconut fiber (coir). Coir logs shall be rolls of netting with maximum 2-inch openings with knotted conjunctions. Coil logs shall be a minimum of 12 inches in diameter (+/-1 inch), 20 feet long (+/-2 feet), and weigh approximately 100 pounds (+/-10 percent). Anchoring of coir logs or wood logs over rock will require securing them to the underlying rock riprap using anchor bolts and chain or cable. Other options for the lip feature include a wider rock terrace, and/or additional rock extending above the terrace.
Page 8 The filter layer material specification is as follows: Filter layer material shall be composed of angular quarry rock. Rock shall be tough, durable, clean particles, adequately free from thin, flat and elongated pieces, and shall contain neither organic matter nor soft, friable particles in quantities considered objectionable (by the County s Engineer or other County inspection personnel). The aggregates shall meet the quality requirements of ASTM C33/C33M. Gradation shall conform to the following requirements: U.S. Standard Sieve Size Permissible Limits Percent by Weight, Max. Passing Filter Layer 8 inches 100 3 inches 40 0.75 inch 10 Levee Repair Vegetation Planting and Recruitment Techniques In addition to refining the lower elevation of planting and stabilizing topsoil, other tree installation and native plant recruitment techniques are recommended. Sonotubes for Tree Planting Sonotubes integrated into the rock armoring can be used for tree planting, to provide a contained soil connection between the levee rock riprap surface and the underlying soil core of the levee. This detail applies to a wide variety of native tree species, including conifers and deciduous trees. We recommend that the County try a range of tree species using this planting technique, and considering rooting characteristics. Figure 2 provides an example detail of this technique, and Photos 4 and 5 provide an example of an installed application.
Page 9 Figure 2: Sonotube planting foundation detail Photo 4: Sonotube placement during construction (Port of Seattle Terminal 25)
Page 10 Photo 5: Red alder, madrone, and cottonwood plantings on armored slope using Sonotube technique (Port of Seattle Terminal 25) Engineering considerations need to be applied to Sonotube design to make sure that the diameter and density of the Sonotubes does not create too large or too many gaps in the levee armoring to affect its structural stability. We recommend the following Sonotube product specifications: An X-inch diameter Sonotube (cardboard tube) and metal stake shall be provided to facilitate planting in the levee armor. The Sonotube shall be finished so that it degrades over 3--5 years as plants installed in it grow. Sonoco Builder s Tube or approved equal shall be used, and is available at: http://www.sonotube.com/products/sonotubeconcreteforms/builderstube.aspx
Page 11 Natural Colonization/Recruitment by Native Vegetation Finally, preserving native vegetation on the landward side of the levee is another technique to promote natural colonization and recruitment. This technique is fairly passive and in essence allows nature to do most of the work. Photos 6 through 9 were taken at the upstream end of the Carbon River levee system, where substantial native vegetation exists on the landside face of the levee. This vegetation provided an effective seed source when the river side of the levee was rebuilt, and plantings (assumed to be Technique 2-planted topsoil over rock riprap) by the County did not establish. The seed source resulted in colonization by native deciduous and coniferous trees. Photo 6 shows mature trees near the levee toe adjacent to the upland area. Photos 7 through 9 show how seed sources from this nearby vegetation are producing Red alder (Photo 7), Big-leaf maple (Photo 8), and Douglas fir (Pseudotsuga menziesii) seedlings. Photo 6: Mature trees on landside of levee (left) toe providing natural seed source for natural colonization on river side of levee (right) following unsuccessful attempt at replanting
Page 12 Photo 7: Naturally recruited Red Alder and Douglas Fir plants Photo 8: Naturally recruited Big-leaf maple seedlings
Page 13 Photo 9: Naturally-recruited Douglas fir seedlings and Red Alder trees In order to continue to provide this seed source, we recommend preserving as much of the mature native vegetation on the landward side of levees as possible. If paved recreation trails include fall and winter leaf blowing/removal maintenance, the County might consider placing this material at the upper slope of the river-side of the levee on site or on another levee segment, rather than disposing of these materials elsewhere. The leaf material from this maintenance can provide mulch benefits, and seed pods can have an even better chance of finding suitable levee soils for establishment. This maintenance technique saves in labor and disposal costs and is useful for deciduous tree species, as they tend to drop their seeds in fall through mid-winter.
REFERENCES Rob Wenman Page 14 Ecology (Washington State Department of Ecology), 2015. Definition of Ordinary High Water Mark. Available from: http://www.ecy.wa.gov/programs/sea/sma/ st_guide/jurisdiction/ohwm.html Mersel, M.K. and R.W. Lichvar, 2014. A Guide to Ordinary High Water Mark (OWHM) Delineation for Non-Perennial Streams in the Western Mountains, Valleys, and Coast Region of the United States. ERDC/CRREL TR-14-13. Vicksburg, MS: U.S. Army Engineer Research and Development Center. Available Online: http://acwc.sdp.sirsi.net/client/search/asset/1036027