In lieu fee mitigation plan
Right Bank Wetland Surface Water Elevations, Groundwater Elevations and Wetland Hydrology
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- Right Bank In-Stream Scour Structure
- Left Bank Off-Channel Habitat, Wetland and Buffer
- Right Bank and Left Bank Plant Establishment
- Right Bank Wetland, Riparian Upland, and River Margin Planting
- Left Bank Off-Channel Habitat, Wetland and Riparian Enhancement Planting
- Determination of Mitigation Credit
- WSDOT SR 520 Replacement Project Impacts
- KC SWSS Impact information
- NW Pipeline Impact Information
Right Bank Wetland Surface Water Elevations, Groundwater Elevations and Wetland HydrologyThe design of the right bank wetland was based heavily on surface and groundwater data collected from an array of shallow wells and river stage gauges on and adjacent to the site. The data shows a very strong correlation between surface water elevations in the river and groundwater elevations across the site. Once this relationship was understood, designers were able to use the long record of flow statistics from the Cedar River as a predictive tool for post project surface and groundwater elevations. After reviewing historic flow data, designers selected a target flow rate of 800 cubic feet per second as a bench mark flow. At this flow, the right bank wetland would have static groundwater elevations that range from two and six inches from the finish grade. Looking at the USGS flow data from 1950-2015, this corresponds to the mean monthly flow for April over this period. Although January, February and March are technically considered part of the growing season (per WETS table) and all have higher monthly average flows (1,130 an 1,030 and 891 cfs respectfully) over that same period, the lower rate provided a more conservative basis for design. Based on the analysis of available data, the right bank wetland is expected to have areas of seasonal flooding and experience saturated soil conditions (to the surface) for at least 30 consecutive days during the growing season. Total wetland area established on the right bank of the EBR site is expected to total 2.16 acres. An additional 0.28 acres that is contiguous with the Cedar River Channel and below the OHWM will be considered aquatic area. Approximately 2.65 acres of riparian upland area around the wetland (of which 1.15 acres are credit generating) will be enhanced as well. It is important to note that while reliance on historic flow patterns is appropriate, flow in the Cedar River is variable and dependent upon weather patterns, conditions in the upper watershed and dam operation. All of these factors can affect flow conditions and therefore surface and groundwater elevations at the site. Post project monitoring and subsequent wetland delineations must therefore be aware of and consider flow conditions at the time the work is performed. Right Bank In-Stream Scour StructureThe right bank in-stream scour structure is designed to meet the mitigation obligation for an engineered log jam (or similar wood structure) to form a scour pool(s) suitable for adult salmon holding habitat and to provide LWD features for floodplain roughness. The scour structure consists of an upstream rock deflector and a rack or jam of large wood downstream. The scour structure will be constructed within an alcove excavated into the right bank of the Cedar River. The existing rock bank protection and toe rock will be removed and the channel widened by approximately 22 feet as measured horizontally from the existing toe of bank to the proposed toe of bank at its widest point. The bottom of the proposed alcove will match the elevation of the adjacent channel bed (El. 94’ NAVD 88). Angular rock will be placed on the bank of the new channel and tied into existing bank protection upstream and downstream of the alcove to resist erosion and replace the level of protection provided by the existing revetment. The wood component of the scour structure consists of large coniferous logs secured to wood piles located within the proposed alcove. The piles will be driven through an excavated bench to their final depths. This will allow installation of the piles without working within the wetted channel. Prior to installing the piles, a “pilot pool” will be excavated from the channel bed, just waterward of the pile array. The purpose of this excavation is to provide conveyance capacity for the channel in order to comply with “zero-rise” flood regulations. Once the piles are in place, the logs will be placed and anchored to them per the plans (Attachment 2). The rock deflector structure will be constructed upstream of the scour structure and mostly landward of the existing OHWM. Ten very large rounded boulders, each approximately 8-feet in diameter, will be placed so that they are half submerged in the channel bed. The interstitial spaces between will be filled with rounded cobbles and boulders to eliminate large voids and reduce erosive flow through the structure. The rock deflector functions both as a habitat element creating hydraulic complexity and to deflect recreational river users around the rack of large wood to minimize the risk of entrapment on the large wood. The top of the rock deflector is designed to be overtopped as flows reach Ordinary High Water (OHW) (elevation 98’-NAVD88) in comparison to the top of the wood jam which is designed to approximately match the top of the existing levee (elevation 102’). The rock deflector will be exposed during most summer flow when recreational users are abundant but will be overtopped during channel forming flow above OHW when the taller log jam will be the primary element driving scour. The in-stream scour structure is located on the RB adjacent to the recently constructed RB wetland and downstream of Stewart Creek. This location is on the outside of a bend of the river and is currently occupied by the channel thalweg. The structure is designed to obstruct flows during channel forming flows and drive bed scour in the channel adjacent to and just upstream of the structure. The structure is design to be deformable but persistent as it interacts with the channel over time. Small wood and cobbles will be mobilized by the river and may be replaced by other naturally recruited material that racks on the structure. Scour may cause minor settling of the boulders or movement of the key logs, but these features are designed to be robust and continue to serve their intended function. While the structure is intended to drive scour, it is designed to have multiple benefits including providing channel heterogeneity, hydraulic complexity, high flow refugia, low flow cover for fish due both to the depth of scour pool and the cover within the porous interior of the log jam. These features will be beneficial to both migrating adult fish and juvenile fish rearing or migrating in the river. Left Bank Off-Channel Habitat, Wetland and BufferMitigation actions on the LB include the creation of off-channel habitat (1.02 acre), wetland establishment (0.49 acre), wetland enhancement (0.62 acre) riparian enhancement (0.90 acre), and riparian upland enhancement (3.28 acres total, of which 2.18 acres are credit generating). These mitigation actions on the LB are intended to meet the EBR mitigation obligations as well as offset wetland impacts to existing wetlands during the construction of the off-channel habitat feature and provide additional mitigation credits for the MRP. The LB mitigation measures are designed to maximize the restoration potential of the site and to be compatible with the dynamic nature of the floodplain environment. While these features are designed to stand alone, they are also being developed in consideration of future restoration actions including the acquisition of more parcels, the removal of levees and the further reconnection of the river to the floodplain. Specifically the design of the off-channel habitat feature as a backwater channel is intended to function as backwater habitat, and the design accommodates the potential for them to become side channels connected to the river as part of a future project. While this could be undertaken in the future, the potential effect on the habitat and downstream areas would need to be fully considered. The LB off-channel habitat feature is a branching backwater channel that is connected to the Cedar River at the downstream end of the project area and arcs upstream to parallel the river. About 450 feet upstream from the Cedar the backwater branches into two channels with the main channel continuing another 400 feet upstream roughly parallel to the Cedar and the secondary channel arcing back toward the river another 300 feet in length. The channel cross-section averages about 45 feet wide at the top of bank and 25 feet wide at the toe of slope. A complex compound channel will be created that includes a narrow low flow channel that meanders within this side channel and habitat features including large wood, small wood, and boulders. A minimum of 57 logs with rootwads will be placed in the side channels. These logs will be embedded into the banks of the side channels and will extend across ¼ to ½ of the channel width. These logs will provide habitat complexity, hydraulic complexity and cover. Log structures may also be installed at the mouth of the side channel to promote scour and maintain a year round surface water connection between the side channel and the Cedar River. The LB mitigation actions include the creation of 0.90 acres of riparian buffer planting along the establishment of 0.49 acres of wetlands and the enhancement of 0.62 acres of existing wetlands. The 0.49 acres of wetlands created is intended to meet the EBR mitigation obligations for 0.22 acres of wetland on the left bank for WSDOT impacts and to offset 0.18 acre of wetlands that will be impacted by the off-channel habitat. Right Bank and Left Bank Plant EstablishmentPlant establishment is important to achieving mitigation goals and performance standards. Planting areas are defined for each habitat creation area based on site conditions and planting communities. Planting areas and community types are shown in Table 3 and Table 4. Native plants will be installed in the habitat creation areas as shown on Sheets 11-19 of Attachment 1. In addition to the plantings shown on these plans, all areas disturbed by construction will be planted. Right Bank Wetland, Riparian Upland, and River Margin PlantingPlant species for the right bank wetland, riparian upland, and river margin are shown in Table 3 and on sheet 17 Attachment 1. Planting areas within the riparian upland area include the buffer area, wetland buffer within the stockpile and access road, and the buffer slopes. Planting areas within the wetland include forested, scrub-shrub, and emergent planting areas. Planting areas within the river margin include the river margin staking and river margin emergent areas. Table 3 Right Bank Plant Specifications
Left Bank Off-Channel Habitat, Wetland and Riparian Enhancement PlantingPlant species for the left bank side channel aquatic area habitat, wetland and riparian enhancement are shown in Table 4 and on sheet 18 of Attachment 1. Planting areas associated with the off-channel habitat are backwater channel banks and the riparian planting area, which is the buffer of the backwater channel. Wetland planting areas include wetland establishment and wetland enhancement areas. SignageAt the completion of project construction, the Sponsor will evaluate locations at the perimeter of the site for signs to identify the site as a sensitive habitat area and to discourage public access of the site. The location and content of signs will be discussed with the IRT. Table 4 Left Bank Plant Specifications
See Figure 9 and Figure 10 for planting area maps. Determination of Mitigation CreditThis section describes the unavoidable impacts being mitigated at the EBR Mitigation Project (section VII.A), as well as the proposed project elements generating mitigation credit (section VII.B). If the EBR Mitigation Project generates more credit than is necessary to offset impacts described in this section, KC MRP will consult the IRT to gain approval for use of the EBR Mitigation Project as mitigation for additional impacts. Impact InformationWSDOT SR 520 Replacement Project ImpactsThe original overall goal of WSDOT mitigation measures for the SR 520 Bridge replacement project was to achieve no net loss of habitat functions and values for wetlands and aquatic resources. Under the mitigation approach used by WSDOT, compensatory mitigation was required for unavoidable adverse impacts that exist after avoidance and minimization measures were employed. WSDOT determined that a suite of mitigation sites, located in various key locations in the Lake Washington basin, would mitigate for the temporary and permanent impacts of the project for both wetland and aquatic resource impacts. The EBR Mitigation Project is one of several mitigation sites that WSDOT proposed to implement to offset these impacts. WSDOT purchased compensatory mitigation credit, to be implemented at the EBR Mitigation Project, by payment of a mitigation fee to KC MRP. The purchase of mitigation credit from the KC MRP transferred these specific mitigation obligations from WSDOT to King County as detailed in the ILF Purchase Plan (“ILF Use Plan”) and described below. The ILF Use Plan also identified Mitigation Goals, Objectives and Performance Standards that were based on the WSDOT’s conceptual design and include those in WSDOT’s Final Aquatic and Wetland Mitigation Plans (2011a and 2011b). WSDOT’s conceptual design included only the RB mitigation site. The EBR Mitigation Project design expands on WSDOT’s conceptual design by expanding the mitigation area and by modifying the design to fit additional design constraints and site conditions not included in the WSDOT design. KC has updated these mitigation criteria and developed the mitigation design in consultation with the IRT. This Mitigation Plan addresses the revised mitigation obligations, goals and objectives that are the result of this consultation. The revised commitments are documented by WSDOT in Final Wetland Mitigation Report Addendum 1—SR 520, I-5 to Medina: Bridge Replacement and HOV Project (WSDOT, 2013a) and Final Aquatic Mitigation Plan Addendum 1—SR 520, I-5 to Medina: Bridge Replacement and HOV Project (WSDOT, June 2013b). WSDOT evaluated impacts using a clear distinction between wetland resources and aquatic habitat resources; impacts and mitigation for each were documented in separate mitigation plans (WSDOT 2011a and 2011b). Wetland resources include the palustrine and lacustrine fringe wetlands along the shorelines of Lake Washington and Lake Union out to a depth of 6.6 feet or the extent of floating aquatic bed vegetation. Aquatic resources occur waterward of the extent of wetlands and are primarily evaluated in the context of fish habitat. The impacts result from four primary mechanisms of project construction and operation: filling, clearing, shading, and alteration of aquatic habitat complexity. WSDOT engaged regulatory agencies and other stakeholders in a collaborative forum, the Natural Resources Technical Working Group (NRTWG), to identify and discuss project impacts and build consensus on an appropriate approach for mitigating the types and amount of project impacts. The NRTWG was composed of federal, state, and local regulatory agencies, the University of Washington, and the MITFD and was held on 11 occasions between June 2010 and December 2012. It is important to note that the impact characterization framework was developed in collaboration with the NRTWG and the Project permits and mitigation reports were approved based on that framework described in WSDOT 2011a and 2011b (the original wetland and aquatic mitigation plans). The SR 520 Impact Project is occurring in multiple construction phases, and only a portion of the overall wetland and aquatic habitat impacts will be mitigated at the EBR Mitigation Project. (WSDOT is implementing a number of other mitigation sites.) The mitigation provided at the EBR Mitigation Project is intended to compensate for impacts associated with construction of the West Approach Bridge - North (WABN) portion of the SR 520 project, which connects the westbound lanes of the floating bridge to landings in the Montlake neighborhood of Seattle. There are both wetland and aquatic impacts associated with the construction of temporary work bridges and the new WABN Bridge. Wetland impacts include both temporary and permanent fill and shading on Foster Island and the shoreline of the Montlake neighborhood. Aquatic impacts include both temporary and permanent benthic fill, noise (pile driving), and shading within Union Bay. Per a conversation with WSDOT on February 4, 2015, the status of the WABN portion of the SR 520 Impact Project is as follows:
In fall 2014, construction of work bridges began on Foster Island. In late 2014, work began to drive piles in Union Bay to extend the work bridges. (Note: a portion of the aquatic impacts associated with driving piles is being mitigated at a separate mitigation site). Aquatic work on the WABN portion of the SR 520 Impact Project will continue until the fish window closes in spring 2015. When the fish window reopens, work is expected to resume. The WABN is expected to be open to traffic in 2017. As originally indicated in the WSDOT ILF Use Plan, the impacts from the WABN and the construction of the EBR Mitigation Project would be occurring during the same window of time. During discussions with WSDOT, King County, and regulatory agencies and MITFD prior to the ILF credit purchase/sale, there was general agreement among all parties to strive for construction of the EBR Mitigation Project during the 2015 construction window (summer). The following tables show the impacts to wetlands (Table 5) and aquatic areas (Table 6) associated with the WABN portion of the SR 520 project generating the impacts being mitigated at the EBR Mitigation Project. These are the mitigation obligations WSDOT was given via their original permits that were apportioned to the EBR Mitigation Project. How these obligations were accounted for on the approved mitigation site is shown in a separate table (see Table 1 on page 10 and Table 8 on page 33). Table 5 WSDOT SR 520 Wetland Habitat Impacts Being Mitigated at the EBR Mitigation Project 
Table 6 WSDOT SR 520 Aquatic Habitat Impacts Being Mitigated at the EBR Mitigation Project 
King County’s mitigation obligations at the EBR Mitigation Project are detailed in the ILF Use Plan. The EBR Mitigation Project fulfills WSDOT’s compensatory mitigation needs for 4.78 acres of aquatic mitigation and 2.33 acres of wetland habitat types. On the RB, the mitigation obligations for both aquatic and wetland types are overlapping (i.e., they are “stacked”) so that the same area satisfies the mitigation needs for both impact types simultaneously. The result is that the EBR Mitigation Project obligations for WSDOT total 5 acres as shown in Table 7. Table 7 Total Acreages Proposed in WSDOT MRP Purchase
a Source ID # designates mitigation areas to be purchased by WSDOT and also corresponds to required mitigation acreages in Tables 1-2 and Exhibit 1 of the ILF Purchase Plan. b Also satisfies aquatic habitat mitigation type “Riparian/Floodplain” In addition to the provision of habitat areas, the mitigation obligations related to the WSDOT impacts include these requirements: The backwater channel will incorporate a minimum of 14 clusters of LWD, with two-three pieces each; Build a scour pool forming (wood) structure in the mainstem of the Cedar River to create adult holding habitat with cover; Wetland, floodplain and buffer areas will be densely vegetated to restore a more functional floodplain community; and Additional wood will be added in the form of snags, downed logs and wood piles to further enhance habitat and floodplain roughness. KC SWSS Impact informationThe May Creek Drainage Improvement Project (NWS 2010-158) includes a variety of impacts to aquatic resources which are fully described in the ILF Use Plan on file with the KC MRP. The May Creek impacts being mitigated at the EBR Mitigation Project include approximately 4,674 square feet (0.11ac) of in-stream habitat impacts in May Creek and Long Marsh Creek. This in-stream habitat is being impacted by the project, but the habitat is not being completely destroyed. Though degraded, the habitat will still exist post-project, and the natural processes that create in-stream habitat will continue to function in the long-term in May Creek. The habitat will eventually return to its pre-project function. In addition, over time, the riparian trees and shrubs that are being planted in the buffer of May Creek will provide new sources of woody debris and nutrients to the stream system. For these reasons, the May Creek project sponsor proposed purchasing mitigation credit at a 1:1 mitigation ratio for impacts, rather than a higher ratio that is often typical for offsite mitigation. Using this rationale, the May Creek Drainage Improvement Project resulted in a requirement to purchase from the KC MRP mitigation credit equivalent to 4,674 square feet of off-channel rearing and high-flow refuge salmonid habitat area. The ILF Use Plan states, “…To fully compensate for the function impacted by the May Creek Drainage Improvement Project, the 4,674 square feet of off-channel area should contain habitat features such as log clusters, boulders, and overhanging vegetation, in quantities appropriate for the 4,674 square foot area and in configurations that will create and sustain cover, refuge, rearing, and complex edge habitat. The off-channel area should also be constructed at an elevation that would provide refuge and rearing for salmonids during winter storm flows, but should also be inundated during summer low flow.” NW Pipeline Impact InformationThe South Seattle Delivery Lateral Expansion Project (NWS 2012-574) includes impacts to 1.65 acres of wetlands and waterbodies at 12 systems along the project which are fully described in the ILF Use Plan on file with the KC MRP. The NW Pipeline project will not permanently fill wetlands or wetland buffer. The NW Pipeline impacts being mitigated at the EBR Mitigation Project include approximately 0.31 acre of temporal palustrine forested and scrub-shrub wetland impact and 0.28 acre of permanent wetland vegetation type conversion impacts associated with wetlands located in the temporary construction right-of-way that are outside of NW Pipeline’s permanent easement. Compensatory mitigation requirements for the NW Pipeline project’s temporary impacts to scrub-shrub and forested wetlands (outside Northwest's permanent easement) using re-establishment or creation projects would require 0.18 acre of mitigation. Compensatory mitigation requirements for the Project’s wetland type conversion impacts to scrub-shrub and forested wetlands (within Northwest's permanent easement) using re-establishment or creation or restoration projects would require 0.29 acre of mitigation. The combined total compensatory mitigation requirement for both temporal and wetland type conversion impacts to scrub-shrub and forested wetlands equates to 0.47 acre for reestablishment or creation projects. Because the impacts are temporary and type conversions and not wetland fill, the NW Pipeline project sponsor proposed purchasing mitigation credit at ratios that are ½ and ¼ of permanent impacts. The temporary impacts will be replanted onsite at the impact site. The NW Pipeline In Lieu Fee Use Plan states, “The KCRMP is designing and implementing projects that provide floodplain creation, side-channel habitat creation with downed wood and timber species plantings, and riparian enhancement with large wood and timber along river banks. Such projects are suited to provide the reestablishment and/or creation credits NW Pipeline is seeking.” Areas of the EBR Mitigation Project being used to fulfill mitigation requirements are consistent with the NW Pipeline ILF Use Plan. Download 7.62 Mb. Share with your friends:
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