LEVEE VEGETATION MANAGEMENT - Strategy Overview

Transcription

1 LEVEE VEGETATION MANAGEMENT - Strategy Overview The levee vegetation management strategy represents our local preferred approach to levee vegetation management that balances the needs of flood risk reduction with the habitat needs of salmonids and other aquatic species found within our river systems. Currently three of these species are listed as threatened under the Endangered Species Act. The strategy acknowledges the agreement established by Court decree (Case No. C79-269T) entered into between Pierce County and the Puyallup Tribe of Indians relating to vegetation management along the Puyallup River System (Puyallup River Vegetation Management Program, PRVMP). The strategy works within the constraints of that court ordered agreement and the Corps Interim Guidance Policy (Corps, 2014). The levee vegetation strategy is driven by Pierce County s desire to ensure all PL levees within the County s river system meet the Corps inspection criteria to retain PL program eligibility. The strategy provides basic guidelines to establish an appropriate balance between maintaining flood risk reduction structures and habitat considerations. The vegetation management strategy will be implemented annually and monitored for effectiveness and potential negative impacts to fish and wildlife. The program will be adaptively managed to make adjustments as identified through routine annual monitoring. The vegetation management strategy will be performance driven, centered by three main performance considerations: Risk - Flood Risk Reduction Habitat Retain Functional Habitat Maintenance - Maintenance Efficiency Goal: The goal of this vegetation management strategy echoes the goal stated in the PRVMP, to provide for the riparian vegetation habitat requirements of the fish and wildlife resources in conjunction with the basic requirements entrusted to Pierce County of revetment (and levee) integrity and inspection, emergency revetment repairs, river channel capacity, and County road maintenance along tributary streams. (PRVMP, 1985) Objectives: Risk Vegetation management will be performed in a manner to minimize risk to both habitat and flood risk reduction structures. Habitat -Vegetation management will be performed in a manner that avoids or minimizes impacts upon fish and wildlife habitat. Maintenance A vegetation management strategy will be developed that is cost-effective and practical to implement. Pierce County Public Works 1

2 Levee Vegetation Management Strategy - Deliverables The following products (deliverables) are incorporated into the levee vegetation management strategy: Riparian Vegetation Communities Mapping Mapping of the current vegetation communities has been conducted using the County Geographic Information System, and field verified by crews. The mapping was used to inform the development of a vegetation management strategy. ( See Appendix X.x) Risk Analysis - The levee vegetation management strategy considers the level of risk to levee structural integrity posed by the river as well as characteristics of the river. The risk analysis will considers ( See Appendix X.x): Flood Risk Reduction Structures River hydraulics Previous damages Hazard trees Potential Benefits of Vegetation (erosion control, root structure, flood calming) Levee Structure Schematics Five typical levee structure scenario s that are encountered along the river system are listed below (See Section X.x.): Schematic A River Channel Adjacent to Levee Schematic B Silt Bench or Gravel Bar upon Levee Schematic C Concrete Panel Levee with Silt Bench and Unpaved Road Schematic D Concrete Panel Levee with Extended Silt Bench and Paved Trail Schematic E - Levee Setback with Floodplain Bench Vegetation Maintenance Inspection Standards Maintenance specifications standard operating procedures will be developed as an action item for each levee structure type to help direct field personnel in the performance of levee vegetation maintenance. (See Chapter X Action Plan) Vegetation Risk Matrix - Protocols and methodology to manage vegetation that threatens the structural integrity of flood risk reduction structures, i.e. hazard trees. (See Section X.x.) Planting Plan Standards, specifications and protocols for replanting vegetation removed associated with levee repairs. (See Section X.x.) Invasive Species Management Plan A plan will be developed as an action item addressing protocols and a strategy for managing specified invasive species. (See Chapter X Action Plan) Standard Operating Procedures Develop a set of Standard operating procedures to ensure the consistent implementation of the Levee Vegetation Management Strategy, including: Vegetation maintenance, invasive species control, plantings/enhancement, and hazard tree removal. Pierce County Public Works 2

3 General Guidelines A Vegetation Management Zone is provided across the levee structure prism, extending from the upland side of the levee down to the structural toe of the levee. This represents the area of vegetation management influence addressed in this strategy that aligns with the following documents: PRVMP: The provisions of the Inter-Governmental Agreement between Pierce County and the Puyallup Tribe of Indians, regarding the Puyallup River Vegetation Management Program (PRVMP) are incorporated within this interim vegetation management strategy. USACE PL Program: This interim strategy is provided in response to the United States Levee Rehabilitation Program, PL Program Interim Policy for Establishing Eligibility Status for Flood Risk Management Projects. It is understood that the vegetation management strategy developed may need to be revised as necessary to comply with the PL program levee maintenance standards once the Corps releases a new Engineer Technical Letter on Vegetation Management (ETL ). Current understanding is that vegetation management strategies developed through the once approved will be honored through the lifetime of the plan necessary to implement. USACE PL84-99 Interim Policy : The Corps has released an Interim Policy for determining eligibility Status of Flood Risk Management Projects for the Rehabilitation Program Pursuant to Public Law 84-99, dated March 21, The purpose of the interim policy is to allow eligibility determinations in the Rehabilitation Program to continue to be implemented while the new policy is developed, and to do so in a manner that avoids potentially incentivizing public sponsors to take actions that may negatively impact natural resources or tribal rights. Under this new Interim Policy, vegetation on levees is no longer directly a criterion for determining Program eligibility. Indirectly, vegetation may impact the ability to properly visually inspect the levee, provide adequate vehicular access, or may pose a hazard to the structural integrity or operation of the flood control structure facility. Photo Illustration XX.x Levee riparian vegetation along the Puyallup River, near City of Puyallup Pierce County Public Works 3

4 General Vegetation Management Strategies The following strategies are applied as the basic protocols for all vegetation management applied across the levee prism: Vegetation Management Zones: A Vegetation Management Zone will be provided across the levee prism, extending from the upland side of the levee down to the structural toe of the levee. This represents the area of vegetation management influence addressed in this strategy. Levee vegetation management subzones will be established to reflect the various components of the levee structure relative to the adjacent river habitat. Levee Structure Schematics: Levee vegetation management strategies are applied to various levee structure types, each that represent a unique consideration of levee construction and orientation to the adjacent river and habitat. Vegetation Removal: Vegetation will be managed to meet the minimum requirements of the USACE for levee inspections and access. This includes specifications for levee vegetation management allowing visual and physical access onto the levee for inspection purposes. o o o Levee vegetation understory will be thinned to provide visibility and physical access for inspections, retaining clusters of native shrubs and saplings approximately feet in diameter, for recruitment of future understory native vegetation and overstory trees. The cleared area around the native shrubs will be approximately feet between clusters, depending on site conditions and character of vegetation. (See Illustration X.x) Woody riparian vegetation will be preserved as feasible on the riverside levee slope in order to provide moderate riparian functions including bank stability, roughness, cover, shade, wood and nutrient contribution, water quality filtering and moderation of water temperature. Vegetation that provides for shading of adjacent waters will be retained to the fullest extent feasible. Generally, trees over 6 inches in diameter will be retained, unless the tree interferes with minimum inspection, access standards, or considered a hazard tree. The retention of mature trees is a priority and these trees will not be removed unless the tree presents an unacceptable hazard to the levee structural integrity, public infrastructure or adjacent private property. Vegetation maintenance work should be conducted in a way that does not kill or weaken the remaining trees, and retains saplings for continued growth of desirable species. Efforts will be made to identify and retain sapling trees within cluster areas to provide for long term successional growth of trees. Vegetation on silt benches formed upon levee structures will be preserved as feasible, unless site conditions indicate underlying structural concerns necessitate removal. Major vegetation clearing to allow for levee repairs will follow the standards of the Puyallup Tribal Vegetation Management Program. At all times, precautions will be taken to protect trees that might be retained as feasible. This may include flagging trees for retention, protective construction fencing, and working from the riverside in the dry when site conditions allow retention of the larger trees on the upper portion of the levee structure. Areas where vegetation is cleared for repairs or to remove undesirable vegetation (hazard trees or invasive species) will be replanted with native vegetation per plan details Figure XX.x. Pierce County Public Works 4

5 Targeted Invasive Species Removal: Specified non-native Invasive species will be controlled through a programmatic approach of working with resource agencies, tribal biologists, and private property owners. Mature Tree Preservation: Mature Trees may exist in sections of the levee where the general maintenance schematics would suggest their removal. Often these trees are integral to the structural integrity of the levee. These special situations will be evaluated on a case-by-case basis using a Site Specific Levee Vegetation Risk Matrix (See appendix XX.X). Recognizing the importance of retaining large trees within the riparian zone, Mature Trees will be retained to the furthest extent feasible. Hazardous Trees: Trees that pose a threat to levee structural integrity, nearby structures, public infrastructure and/or pose an elevated danger to the safety of maintenance personnel will be surveyed and monitored for degree of hazard based on level of risk (High, Medium, and Low). High Risk Hazard trees will be removed. Medium and Low Risk Hazard Trees will be monitored. Strategies for assessing and addressing potential hazard trees and reducing the impacts of vegetation removal will be developed. (See Appendix XX.X) Habitat Protection/Enhancement: Riparian vegetation along the levees will be maintained and enhanced with native plantings and removal of invasive plants to support habitat functions critical to fish and wildlife resources. Riparian Plantings will be provided to offset the removal of hazardous trees and removal of vegetation necessary to perform levee repairs. (See Chapter XX.x) Long Term Tree Preservation/Conservation: Long term riparian tree preservation will be directed to the area beyond the upland side of the levee, with a focus upon the first 200 feet of properties contained within the riverine shoreline environment. Efforts should be directed to retaining the largest mature trees that provide benefit to thermal cooling, as well as other benefits to habitat. Since approximately only 20 percent of the riparian corridor associated with the system of PL levees is under the control of Pierce County, it will be necessary to work closely with owners of private holdings to provide for additional long term preservation. As opportunities are made available, land may be set aside in conservation easements where existing native trees will be preserved and the area enhanced with appropriate conifer trees. Photo Illustration XX.x Levee vegetation along Ford Setback Levee near City of Orting Pierce County Public Works 5

6 Proposed Levee Vegetation Maintenance Schematics A Vegetation Management Zone is provided across the levee prism, extending from the upland side of the levee down to the structural toe of the levee. This represents the area of vegetation management influence addressed in this strategy. The Vegetation Management Zone is divided into five (5) subzones to guide vegetation removal to five commonly encountered levee structure scenarios. The subzones are described as follows: - Riparian Management Zone This zone is defined as the waterward slope measured from the waterward edge of the crown extending to the levee toe, but never extending waterward of the ordinary high water mark. The width of this zone is variable depending upon the presence of a gravel bar, silt bench, paved trail or levee height. - Clear Zone This zone is defined as the crown of the levee. The Clear Zone will be maintained to insure adequate vehicular access for a minimum width of 15 feet. - Backslope Zone This area is defined as the landward slope measured from the landward edge of the crown extending down to the levee toe. - Upland Zone This zone is defined as the area landward of the landward of the levee toe. - Inspection Zone This zone is defined as the area extending 8-10 feet above the ground between the landward toe and the waterward extent of the Riparian Zone. Figure X.x Levee Vegetation Management Sub Zones: The following Levee Maintenance Schematics (Schematics A-E) depict how vegetation would be maintained on typical levee structure prisms, for each management zone, relative to level of risk and habitat considerations. Pierce County Public Works 6

7 Levee Maintenance Schematic A River Channel Adjacent to Levee (w/o silt bench) This levee x-section depicts levees immediately adjacent to the river without a silt bench or floodplain bench area at the riverward toe of the levee. Type A levees have very limited space for balancing levee stability and safety risks, while maintaining some level of riparian vegetation function. Typically Type A levees are situated on a steep river bank that functions as the levee embankment. The maintenance approach for Type A levee sections allows for the growth of native trees and shrubs in the riparian management zone adjacent to the river. Woody riparian vegetation is preserved as feasible on the riverside levee slope in order to provide moderate riparian functions including bank stability, roughness, cover, shade, wood and nutrient contribution, water quality filtering and moderation Figure X.x of water temperature. Vegetation will be managed to provide for visual inspection of the levee on a rotational basis based on degree of risk exhibited on that portion of the levee segment. (See Levee Visual Inspection Standards - TBD) Mature vegetation ( Mature Trees ) situated upon the levee area,that are not identified as a hazard tree of high risk, nor interferes directly with vehicular access requirements, will be preserved to the greatest extent feasible. A Clear Zone is provided across the crest of the levee to remain clear of woody vegetation for vehicle access for purposes of maintenance work and in times of emergency. The Clear Zone will extend 5-15ft downslope of the Riparian and Backslope Zone as necessary to control growth of invasive plants and branches through routine mowing and limbing of tree branches that may crowd the clear zone. The backslope will be managed to provide visibility for inspection of possible seepage and structural integrity issues. An Upland Zone is provided beyond the landward toe as available for long-term enhancement and conservation protection. Pierce County Public Works 7

8 Levee Maintenance Schematic B Levee with Silt Bench (on levee structure) This levee x-section represents levees immediately adjacent to the river, with a silt bench on the riverward face of the levee. Type B levees also have limited space for balancing levee stability and safety risks, while maintaining some level of riparian function. The maintenance approach for Type B levee sections preserves native trees and shrubs on the silt bench zone of the levee adjacent to the river. The silt benches are typically found in the middle to downstream reaches of the Puyallup River System and have built up over time due to long term sediment accretion along the river bank. Typically the silt benches result in oversteepened river banks that lie above the levee structural prism Figure XX.x embankment. The silt bench is an area where native trees and shrubs have become established on silt deposits along the riverward slope of the levee. The silt provides a rooting zone on top of any riprap which may be present, which better protects the lower levee from potential tree impacts from uprooting. The vegetated silt bench along the levee may provide for further protection of the levee prism during high flow events. Levees with silt benches will be mapped for longer term protection. Woody riparian vegetation on the silt bench (below the levee prism) will be preserved in order to provide moderate riparian functions including bank stability, roughness, cover, shade, wood and nutrient contribution, water quality filtering and moderation of water temperature. Vegetation on the levee above the silt bench will be managed to provide for visual inspection of the levee on a rotational basis based on degree of risk exhibited on that portion of the levee segment (see Levee Visual Inspection Standards). Mature vegetation ( Mature Trees ) situated upon the levee area, above the silt bench, that are not identified as a hazard tree of high risk, nor interferes directly with vehicular access requirements, will be preserved to the greatest extent feasible. A Clear Zone is provided across the crest of the levee to remain clear of woody vegetation for vehicle access for purposes of maintenance work and in times of emergency. The Clear Zone will extend 5-15ft downslope of the Riparian and Backslope Zone as necessary to control growth of invasive plants and branches through routine mowing and limbing of tree branches that may crowd the clear zone. The backslope will be managed to provide visibility for inspection of possible seepage and structural integrity issues. An Upland Zone is provided beyond the landward toe as available for long-term enhancement and conservation protection. Pierce County Public Works 8

9 Levee Maintenance Schematic C - Concrete Panel Levee w Silt Bench/Access Road This levee x-section represents levees that were constructed along the lower reaches of the Puyallup River constructed of local riverine gravel soil mix and armored with concrete panels extending down below OHW with a silt bench formed on top of brush mats lying in front of the levee concrete panels adjacent to the river. The silt bench provides a medium where riverine vegetation grows. A maintenance access road has been established on portions of the levee silt bench. The maintenance approach for Type C levee sections preserves native trees and shrubs on the silt bench zone of the levee adjacent to the river, while allowing for visual inspection of the upper portions of the levee concrete panels above the silt bench. The silt benches are typically found in the lower reaches of the Puyallup River System and have built up over time due to long term sediment accretion along the river bank. The silt bench is an area Figure X.x where native trees and shrubs have become established on silt deposits along the riverward slope of the levee. The vegetation silt bench along the levee may provide for further protection of the levee concrete panels and levee prism during high flow events. Woody riparian vegetation on the silt bench (below the levee prism) will be preserved as feasible in order to provide moderate riparian functions including bank stability, roughness, cover, shade, wood and nutrient contribution, water quality filtering and moderation of water temperature. Vegetation on the levee above the silt bench will be managed to provide for vehicular access and visual inspection of the levee concrete panels. Mature vegetation ( Mature Trees ) situated upon the levee silt bench area, that are not identified as a hazard tree of high risk, nor interferes directly with vehicular access requirements, will be preserved to the greatest extent feasible. A Clear Zone is provided across the crest of the levee down to the lower silt bench access road, to remain clear of woody vegetation for purposes of vehicular access for maintenance work a d in times of emergency. The Clear Zone will extend 5-15ft downslope across the Riparian and Backslope Zone as necessary to control growth of invasive plants and branches through routine mowing and limbing of tree branches that may crowd the clear zone. The backslope will be managed to provide visibility for inspection of possible seepage and structural integrity issues. An Upland Zone is provided beyond the landward toe as available for long-term enhancement and conservation protection. Pierce County Public Works 9

10 Levee Maintenance Schematic D - Concrete Panel Levee w/ Silt Bench and Trail This levee x-section represents levees that were constructed along the lower - mid reaches of the Puyallup River constructed of local riverine gravel soil mix with a silt bench formed in front of the levee structure. In some locations, additional bank erosion controls have been provided along the silt bench, primarily around culvert/pipe discharge pipe outfalls. The silt bench provides a medium where riverine vegetation grows; including large deciduous trees. A levee trail and maintenance access road has been established on portions of the levee silt bench. The maintenance approach for Type D levee sections preserves native trees and shrubs on the silt bench zone of the levee adjacent to the river, while allowing for visual inspection of the levee prism above the silt bench. The silt benches are typically found in the lower mid reaches of the Puyallup River System and have built up over time due to long term sediment accretion along the river bank. The silt bench is an area where native trees and shrubs have become Illustration X.x established on silt deposits along the riverward slope of the levee. Large, Mature Trees are typically found growing upon the wellestablished silt bench. The vegetated silt bench along the levee may provide for further protection of the levee prism during high flow events. Woody riparian vegetation on the silt bench (below the levee prism) will be preserved as feasible in order to provide moderate riparian functions including bank stability, roughness, cover, shade, wood and nutrient contribution, water quality filtering and moderation of water temperature. Vegetation upon the silt bench will be managed for maintenance access and pedestrian safety purposes. Vegetation on the levee structure above the silt bench will be managed to provide for vehicular access and visual inspection of the levee prism. Mature trees situated upon the levee silt bench area and upon the levee prism, that are not identified as a hazard tree of high risk, nor interferes directly with vehicular access requirements, will be preserved to the greatest extent feasible. A Clear Zone is provided across the crest of the levee down to the lower silt bench access road, to remain clear of woody vegetation for purposes of vehicular access for maintenance work and in times of emergency. The clear zone will extend 5-15ft downslope across the Riparian and Backslope Zone as necessary to control growth of invasive plants and branches through routine mowing and limbing of tree branches that may crowd the clear zone. The backslope will be managed to provide visibility for Pierce County Public Works 10

11 inspection of possible seepage and structural integrity issues. An Upland Zone is provided beyond the landward toe as available for long-term enhancement and conservation protection. Levee Maintenance Schematic E Setback Levee with Floodplain Bench This levee x-section depicts levees (known as setback levees) that are 50 feet or more back from the active river channel, with a floodplain bench at the riverward toe of the levee that is at least 50 feet wide. If the floodplain bench (vegetated bar) is less than 50 feet wide, Levee Maintenance Schematic A or B applies. Typically, these floodplain benches are associated with constructed setback levees, and reflect the furthest extent that levees can be constructed under current land use constraints. Upon these vegetated bars (floodplain bench), all woody vegetation, trees and native shrubs are preserved and provide riparian functions including bank stability, roughness, cover, shade, wood recruitment, nutrient contribution, water quality filtering and moderation of water temperature. The levee prism will be maintained in grass for routine maintenance and inspection purposes. This approach recognizes that levees located 50 feet or more from the active channel provide for a greater level of riparian function compared to the other schematics, and can therefore be maintained in grass with less ecological impact. However, over time the river may migrate towards the levee and be less than 50 feet away, and vegetation maintenance may Illustration X.x need to change accordingly. If the river migrates to within 50 feet of the levee toe, the maintenance practices switch to Section A or B methods. If the river migrates further than 50 feet away from the Type A or Type B levee embankment, maintenance practices switch to those for Type E. The levee is maintained for ease of levee maintenance and for inspection purposes. A Clear Zone is provided across the crest of the levee to remain clear of woody vegetation for vehicle access for purposes of maintenance work and in times of emergency. The Clear Zone will extend to the toe of the slope or as necessary down the slope to control growth of invasive plants and branches through routine mowing and limbing of tree branches that may crowd the clear zone and for maintenance efficiency. The backslope will be managed to provide visibility for inspection of possible seepage and structural integrity issues. An Upland Zone is provided beyond the landward toe as available for long-term enhancement and conservation protection. Pierce County Public Works 11

12 Levee Vegetation Management Plan View Details The following illustration reflects the concept of how vegetation will be retained while providing for visual and physical access onto the levee for the purpose of levee inspection, maintenance and flood fighting. Woody riparian vegetation will be preserved as feasible on the riverside levee slope in order to provide moderate riparian functions including bank stability, roughness, cover, shade, wood and nutrient contribution, water quality filtering and moderation of water temperature. Vegetation that provides for shading of adjacent waters will be retained to the fullest extent feasible. Mature trees that provide for significant shade value will be retained as feasible. Additionally, all vegetation that provides shading by hanging over the water will be retained. Vegetation maintenance work should be conducted in a way that does not kill or weaken the remaining trees, and retains saplings for continued growth of desirable species. Vegetation on silt benches formed upon levee structures will be preserved as feasible, unless site conditions indicate underlying structural concerns necessitate removal. Levee vegetation understory within the riparian management zone will be thinned to provide visibility and physical access for inspections, retaining clusters of native shrubs and saplings approximately feet in diameter, for recruitment of future understory native vegetation and overstory trees. The cleared area around the native shrubs will be approximately 8-25 feet between clusters, depending on site conditions and character of vegetation. Backslope zone will retain clusters of understory vegetation around existing mature trees. A continuous buffer will be retained extending from below ordinary high water mark to a minimum 5 feet upland along the levee face. The buffer understory that does not overhang beyond OHW may be Figure X.x Pierce County Public Works 12

13 limbed up to a maximum height of 4 feet above the ground to provide for visual inspection of the levee toe in areas of higher risk to provide for visual inspection of the levee toe. Hazardous Trees The integrity of levees, floodwalls, embankment dams, and appurtenant structures is paramount to the public health, safety, and welfare. The presence of undesirable vegetation can undermine that integrity and lead to failure if not corrected. Trees and other woody vegetation, such as shrubs and vines, can create both structural and seepage instabilities, prevent adequate inspection, and create obstacles to maintenance and flood-fighting/flood-control activities. (ref.: ETL , Apr 30, 2014) It is recognized that vegetation on levees can impede inspection and introduce uncertainty for levee performance. Trees that become unstable can damage levees if they fall over and remove part of the levee prism as the tree rootball is overturn. Once a tree on a levee falls, the pit created by the rootball is susceptible to erosion and will likely result in damage to the levee if it is not stabilized. Cutting the tree before it falls will reduce the need for a future repair project. Mature vegetation ( Mature Trees ) situated upon the levee area, that are not identified as a hazard tree of high risk, nor interferes directly with vehicular access requirements, will be preserved to the greatest extent feasible. Strategies for assessing and addressing potential hazard trees and reducing the impacts of vegetation removal will be incorporated into standard operating procedures for levee condition assessment and vegetation work. Personnel will be trained to utilize a standard form for identifying potential trees that may pose a hazard to the operations and performance of the levee system Trees that pose a threat to levee structural integrity, nearby structures, public infrastructure and/or pose an elevated danger to the safety of maintenance personnel will be surveyed and monitored for degree of hazard based on level of risk (High, Medium, and Low). High Risk Hazard trees will be removed. Medium and Low Risk Hazard Trees will be monitored. Trees that may pose a threat to the structural integrity of the levee structure will be reviewed by an engineer to determine the level of risk to the levee structure. Trees that pose a high risk will be removed. Trees that appears to pose a moderate threat to neighboring properties, general public or nearby structures will be reviewed by a trained arborist to determine if the tree is healthy and stable or should be removed due to the health of the tree and risk imposed to neighboring properties. (See Vegetation Risk Matrix, Table XX.X) Hazard trees removed will be documented, indicating tree species, height, diameter, and orientation to the river. Trees removed will be offset with trees to be planted on the upper 1/3 of the levee riparian management zone, back slope zone, or the upland side of the levee upon County controlled land for long term preservation. Pierce County Public Works 13

14 Photos of common concerns of impacts posed by hazard trees upon levees. Trees can topple, damaging bank levee armoring Tree roots can penetrate levee embankment causing seepage and instability problems. Trees can direct river flows to induce scour. Tree rotation can damage levee slopes (Source, Charles Ifft, USACE Seattle District) Pierce County Public Works 14

15 Vegetation Risk Matrix The following protocol has been developed to establish a methodology to manage vegetation that threatens the structural integrity of flood risk reduction structures, i.e. hazard trees. Pierce County Public Works 15

16 Levee Plantings Planting native vegetation upon the levee system has been a component of the levee maintenance program for several decades. With the Puyallup Tribal Vegetation Management Agreement in 1989, replanting of the levees with willow plants became the standard practice following a levee repair project. Planting typically occurs during the winter months when plants are in a dormant period. Initially, rip rap levees were prepared with soil then willow livestakes were placed between the rocks. Once the plant roots establish and find their way between the rip rap then there is assurance the plant will survive long term. In most cases this program resulted in a productive riparian vegetated corridor. The program evolved to most recently supplementing the soil with mulch material to better retain moisture necessary for the plants to thrive in the harsh setting of planting amongst rip rap material. Levee vegetation is established on the system of levees by natural propagation as well as through the levee vegetation planting program following a levee repair. Native bare root plants are now preferred since the plants have a jump start on their root structure necessary to carry them through the first summer. Nisqually Levee prepared with soil for planting following repair work, 2013 Nisqually Levee native plants (red osier dogwood, willow, ninebark, snow berry, etc) installed, 2013 Pierce County Public Works 16

17 Planting Plan The following establishes the standards, specifications and protocols for replanting vegetation removed associated with levee repairs. Planting plans will be designed to allow inspection of levee structures and sufficient access for performing levee maintenance work and flood fighting operations, when needed. Access and inspection requirements will be incorporated into the planting plan following specifications for clustering and long term tree retention illustrated in Figure X.x. Plantings shall be installed to complement existing vegetation communities, and to promote a functional vegetation riparian zone over time. Plantings of native vegetation is integrated 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 figure XX.x. 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. Photo illustration X.x Technique 1: Bioengineered brush layering Pierce County Public Works 17

18 Cottonwood poles can also be used for similar applications fitted between rip rap near the OHWM. The siting of the planting units to maximize plant-able area above the ordinary high water mark (OHWM) will help insure the success of plantings. 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 long-term 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 the photo below, is instability of the toe from high water (flood events) that leads 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 illustration X.x: Technique 2 Planting over topsoilcovered rock riprap Pierce County Public Works 18

19 The loss 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 XX.x illustrates 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 XX.x - Rock placement and planting toe 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. The filter layer shall be comprised of clean quarry pit run material, with the intent of filling the voids between the rip rap rock and providing drainage for the upper planting medium. Riparian Plant Selection To improve upon the ecological services provided by riparian buffers, Pierce County will manage the growth of vegetation, promoting both a functional riparian buffer contributing to the benefit of fish and wildlife, while also insuring structural integrity through proper plant selection and adaptive management. Vegetation positively affects the mass stability of slopes through root reinforcement of the soil, helping to offset the scour effects of the rivers by calming river flow and lessening the effects upon the levee structure. Pierce County Public Works 19

20 Plantings will be comprised of a mix of native vegetation typically found within a northwest riparian environment (see Figure XX.x). The Lower 2/3 rd of the levee face will be typically planted with a mix of willow including Hookers Willow (Salix hookeriana), Pacific Willow (Salix lucida), Scouler s Willow (Salix scouleriana ), and Sitka Willow (Salix sitchensis). Additional native plant shrubs will be incorporated into the plantings to provide greater diversity, food source, and habitat. Compost material will be placed around the plantings to provide for moisture retention and erosion control. See Figure XX.x for list of plants. Willow Plant Species (typ.) Hookers Willow (Salix hookeriana) Pacific Willow (Salix lucida) Scouler s Willow (Salix scouleriana ) Sitka Willow (Salix sitchensis) Pierce County Public Works 20

21 The upper 1/3 rd of the levee will be planted in a mix of native shrubs, willows, and taller growing trees, such as big leaf maple and conifer trees. The upper portion of the levee is typically above the high flows of the river, insuring higher probability of planting success. Additionally, soil medium will be prepared to allow for natural tree regeneration from seed broadcast from adjacent sources where available. Observation has found that vegetation growth from natural propagation tends to have a higher level of success. Emphasis will be placed upon planting taller growing trees in the upper 1/3 rd or back side of levee where the tree can provide maximum benefit for shading sun exposure (typically on the south side of an eastwest oriented river reach). Trees planted on the levee face that promote a root structure that is beneficial to levee structural integrity will be chosen. 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 Big Leaf Maple (Acer macrophyllum), Grand Fir (Abies grandis), Western red cedar (Thuja plicata), and Western hemlock (Tsuga heterophylla). Native tree species such as pacific alder and black cottonwood that grow from natural propagation will be accommodated in the planting area. These saplings grow at a fast rate and will provide additional shading of the understory planted shrubs and shading of the river banks in short time; while also providing increased diversity and nutrients to the riparian planting area. Volunteer tree saplings will be thinned overtime to conform to the levee planting plan view details depicted in figure XX.x. Pierce County Public Works 21

22 Native Tree Species List (typ.) Figure XX.x Pierce County Public Works 22

23 Native Shrub Species List (typ.) Species Bareroot Plant Specification Figure XX.x Red Osier Dogwood Cornus sericea (C. stolonifera) -, Red 18-36" Osier Dogwood Pacific Ninebark - Physocarpus capitatus " Pacific Willow - Salix lucida 36"+ Sitka Willow - Salix sitchensis 36 + Rubus spectabilis - Salmonberry 6-12" Nootka Rose - Rosa nutkana 18-36" Snow Berry - Symphoricarpos albus- Snow Berry 12-18" Pierce County Public Works 23

24 Further reading: King County, 2016 King County Interim System Wide Improvement Framework. King County website. Adams, Ashley N Determining the effects of vegetation on levee structural integrity on the Green River in King county, Washington. University of Washington. Battelle Memorial Institute (2008), Revised Final Independent Peer Review Report for U.S. Army Corps of Engineers Vegetation Policy for Local Flood Damage Reduction Systems, Columbia, OH. California Department of Water Resources (2011), Effects of U.S. Army Corps of Engineers Policy on Levee Vegetation in California, a useful web site. Hanak, E., J. Lund, A. Dinar, B. Gray, R. Howitt, J. Mount, P. Moyle, and B. Thompson, Managing California s Water: From Conflict to Reconciliation, Public Policy Institute of California, San Francisco, CA, 500 pp., February Haselsteiner, R. (2010): Woody Vegetation on Small Embankments. 8th ICOLD European Club Symposium, From research to design in European practice, 22th 23th September 2010 in Innsbruck, Austria (and a nice ppt presentation) Kelley, R Battling the Inland Sea. Berkeley: University of California Press. MBK Engineers (2010), Natomas Levee Improvement Program, Phase 3 Project, Appendix B Levee Vegetation Management, for Sacramento Area Flood Control Agency, Sacramento, CA. TAW (1999), Grass Cover as a Dike Revetment, Technical Advisory Committee for Flood Defence in The Netherlands. USACE (2009), Guidelines for Landscape Planting and Vegetation Management at Levees, Floodwalls, Embankment Dams, and Appurtenant Structures, ETL , US Army Corps of Engineers. USACE (2007), Treatment of Vegetation within Local Flood Damage Reduction Systems USACE (n.d.), Issues Regarding Vegetation Management On Levee Embankments, REMR Technical Note Ei-M-1,4, US Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS. USACE (n.d.), Vegetation and the Structural Integrity of Levees: Results Of Field Investigations, REMR Technical Note Ei-M-1,3, US Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS. USACE (n.d.), Questions and Answers on Variance from Vegetation Standards for Levees and Floodwalls Policy: Guidance Letter. USACE (2009), Temporary extension of P.L Rehabilitation Eligibility for Non-Federal Sponsors. Implementing System-wide Improvements, Memorandum from Steven Stockton, USACE Director of Civil Works, April 9, Pierce County Public Works 24