«DRAFT ENVIRONMENTAL ASSESSMENT ENVIRONMENTAL ASSESSMENT FOR THE COLUMBIA STOCK RANCH SECTION 536 ECOSYSTEM RESTORATION PROJECT The environmental ...»
In the northern portion of the project site, high marsh habitat would be excavated to 8.3 feet and graded up to 11.0 feet (NAVD 88). In the southern portion of the project site, the high marsh habitat would be excavated to 8.3 feet and graded up to a 9.6 feet (NAVD 88) to increase April 22, 2016 Page 12 topographic diversity between MTL and MHHW. Annual tidal fluctuations indicate that these areas would be wetted on a daily basis, but could dry out during low tide for most of the dry, summer season. During the April – June freshet, low tide would likely stay above the high marsh elevation, temporarily inundating this area for weeks or months. The “high marsh” area would be excavated as a buffer around the tidal channels in the southern portion of the project site, creating a micro-basin around the tidal channels. High marsh areas would tie into the existing grade with a gradual slope to mimic natural conditions and minimize the potential to strand fish as water recedes and would be planted with a mix of native wetland plants to support habitat needs for fish and wildlife.
Figure 6: Schematic for low and high marsh elevations on southern portion of CSR project site As noted above, an overflow channel is needed to direct off-site drainage from Tide Creek into Deer Island Slough during high water events to minimize interior flooding landward of the setback levee. The overflow channel would have trapezoidal cross section with a bottom width of 10 feet and sides gradually sloping up to the adjacent floodplain elevation. Bank elevations would tie into the nearest adjacent grade and top width could vary along the length of the channel. The diversion channel would be aligned with existing low spots along the setback levee and utilize swales where they occur on the landscape. Acquisition of 13.4 acres of Deer Island by the Columbia Land Trust would also be required for the overflow channel and Corps or BPA would then acquire a right-of-way for construction and a channel improvement easement on this parcel.
Additional excavation would occur in two locations adjacent to Tide Creek where the existing floodplain topography is higher than the ordinary high water elevation, which is the same as the 2-year flood elevation (15.8 feet NAVD 88). These areas would be excavated to 15 feet (NAVD
88) to increase the areal extent of inundation during a 2-year flood event to support fish and wildlife habitat and promote natural ecological processes. Any excess material not used for
Invasive Species and Native Vegetation Currently, the CSR project site is largely dominated by non-native pasture grasses that were promoted for cattle grazing and agriculture and invasive species that have become established throughout the project site. The proposed restoration includes removal and control or treatment for invasive species, as well as planting native vegetation to increase the composition of native plant communities and overall biodiversity. Invasive species control is proposed by spraying herbicides formulated for aquatic environments (glyphosate and imazapyr) via a backpacker sprayer and a boom sprayer mounted on an ATV. Timing of herbicide application would depend on species present and season of activity (spring and fall). It is assumed that the proposed restoration project would be implemented over two (2) field seasons and invasive species control measures could be implemented up to four (4) times during construction activities: fall of 2016, spring and fall of 2017, and spring of 2018 before the final levee breach is completed. Following reconnection to the Columbia River, invasive species control on the project site would be timed to coincide with seasons of low water when the project site is not flooded.
The setback levee would be constructed during the summer and fall months, before weather conditions make effective construction prohibitive. The contractor would return the following year to complete channel work, vegetation measures, and finally the removal of segments of the existing levee. If necessary to establish vegetation, the final phase may occur in a third construction season.
The proposed planting plan for low and high marsh habitats include broadleaf arrowroot, or wapato (Sagittaria latifolia), common spikerush (Eleocharis palustris), yellow water-lily (Nuphar lutea), and slough sedge (Carex obnupta). Silverweed cinquefoil (Potentilla anserine) and water parsley (Oenanthe sarmentosa) were included to provide bank stability and attract waterfowl. Common or soft rush (Juncus effuses), spike bentgrass (Agrostis exarata), common velvetgrass (Holcus lanatus), fowl bluegrass (Poa palustris), fowl mannagrass (Glyceria elata), and tufted hairgrass (Deschampsia caespitosa) are target species for the higher marsh elevations. Many of these species are sod- or bunch-forming grasses and their establishment would help control invasion by reed canary grass (RCG) (Phalaris arundinacea). Riparian scrub-shrub habitat would provide a transitional zone between high marsh habitat and floodplain forests and upland grasslands. Vegetation in this zone would be dominated by trees and shrubs less than 15 feet in height, including Douglas spirea (Spiraea douglasii), Scouler’s willow (Salix scouleriana), Pacific willow (S.
lucida), Pacific dogwood (Cornus nuttallii), thimbleberry (Rubus parviflorus), evergreen huckleberry (Vaccinium ovatum), American elder (Sambucus rubra), and nootka rose (Rosa nutkana). Native asters, yarrow, and milkweed would be seeded in the understory to attract butterflies, moths and other pollinators.
Construction Sequence and Schedule Implementation of the Proposed Action requires that flood protection must be maintained throughout the duration of the project in compliance with the authorized intent of the existing levee. While the Proposed Action could logistically be constructed in one calendar year beginning in April, this would be highly dependent on weather conditions and soil moisture content. To reduce the uncertainty of project delivery based on winter and spring rainfall amounts, a two year construction schedule is recommended. Table 2 provides a schedule that April 22, 2016 Page 14 can be used for construction of the ecosystem restoration project over two construction seasons.
Construction would begin in spring of 2017 and is expected to be complete in late fall 2018. In order to maintain flood protection to adjacent property owners, the following construction sequence is proposed for project implementation following acquisition of real estate interests
in 2016 and 2017 (see Table 1):
1. Access project site and establish traffic signage, work staging areas, job facilities, and initial erosion control measures.
2. Demolish existing ranch structures and fences, prepare construction staging area, and decommission existing septic systems, water wells, power lines and agricultural drainage.
3. Demolish radio control airport structure and decommission access driveway to airport.
4. Strip, clear, and grub vegetation along proposed setback levee and setback levee access road to Highway 30, stockpile for incorporation as wood elements in marsh areas.
5. Excavate and stockpile topsoil along alignment of proposed setback levee and marsh areas.
6. On landward side of existing levee, excavate tidal channels, marshes, scrape-down areas, and overflow channel and haul material to proposed setback levee.
7. Construct setback levee using excavated site material to an elevation of 25.0 ft NAVD 88.
8. Stabilize and winterize site with erosion control measures, demobilize equipment as necessary.
9. Mobilize equipment and build setback levee to final elevation of 32.0 ft NAVD 88.
10. Build setback levee access road connection from railroad crossing to Highway 30.
11. Install tide box culvert and levee closure structure at railroad embankment tie-in location.
12. Place large wood habitat complexity throughout marsh areas.
13. Place stockpiled topsoil on proposed setback levee.
14. Excavate existing levee breaches and channels to the Columbia River and transport material to new setback levee for seepage blanket on landward side of levee.
15. Seed and plant site including sod cover on levees and wetland plants within excavated channels.
16. Remove existing Tide Creek culvert crossing at existing access road near ranch headquarters.
17. Install railroad bridges at crossing 1 and 2 on existing rail line.
18. Regrade the existing access road down to the surrounding ground elevation for slough and habitat connectivity.
19. Demobilize and restore construction staging areas.
July Access project site, establish signage, work staging areas, job facilities, erosion control measures.
Demo existing structures and fences, and prepare construction staging area.
Demo RC airport structure and decommission access driveway to airport.
Clear and grub vegetation along setback levee and access road to Hwy 30, stockpile large wood.
Excavate and stockpile topsoil along alignment of proposed setback levee.
Excavate channels, marshes, scrape-down areas, overflow channel; haul material for setback levee.
Construct setback levee using excavated site material to 25.0 ft NAVD 88.
Stabilize site with erosion control measures for winter conditions, demobilize equipment.
Mobilize equipment and build setback levee to 32.0 ft NAVD 88 and setback levee access road.
Build setback levee access road connection from railroad crossing to Highway 30.
Install tide box culvert and levee closure structure at railroad embankment tie-in location.
Place large wood habitat complexity throughout marsh areas.
Excavate breaches and channels to Columbia River, transport material for seepage blanket.
Seed and plant site including sod cover on levees and wetland plants within excavated channels.
Remove existing Tide Creek culvert crossing at existing access road near ranch headquarters.
Install railroad bridge crossing 1 and 2 on existing rail line.
Regrade existing access road down to the surrounding ground elevation.
Demobilize and restore construction staging areas.
3.3. Alternatives Considered but Dismissed from Further Evaluation Along with the No Action and Proposed Action Alternatives selected for impact analysis, other alternatives were considered for further evaluation. The Action Agencies follow an established process to estimate the relative benefits of ecosystem restoration projects in the Columbia River estuary to improve the survivial of ocean- and stream-type juvenile salmonids. The process assigns survival benefit units (SBUs) by scoring for three factors: (1) certainty of success; (2) potential benefits for habitat access/opportunity; and (3) potential benefits for April 22, 2016 Page 16 habitat capacity/quality. Scoring criteria were developed for each of these metrics and an expert regional technical group (ERTG) reviews proposed habitat restoration projects and assigns SBUs to individual projects and alternative designs. The ERTG is comprised of regional experts in estuarine, riverine and ocean ecology, fisheries biology, and restoration science. The CSR project site was identified as a potential site for restoration and scoring with SBUs. The project site was evaluated as a stand-along location and not compared against alternative locations.
The Corps evaluated restoration opportunities in terms of maximizing restoration potential across the CSR project site. The project site was divided into four quadrants and four conceptual designs were evaluated for potential project benefits and impacts, including the No Action and Proposed Action described below. All conceptual designs included construction of a setback levee and adjoining seepage berms to minimize adverse impacts to adjacent properties, modifications to the existing levee to reconnect the Columbia River to the floodplain, as well as removal of the residential buildings, outbuildings, cattle grates and fences lines. All alternatives incorporated aspects of the final proposed design to increase restoration potential.
The simplest alternative (Alternative 1) restored hydrologic connectivity via modification of the existing levee, but did not actively restore habitat conditions across the site;
following construction, much of the site would remain dominated by pasture grasses and non-native species. Under Alternative 1, the majority of the fill material needed for construction of the setback levee and adjoining seepage berms would be imported from offsite. Alternative 2 consisted of breaching the existing levee and excavating tidal channels on the southern portion of the project site to create wetlands across 42 acres. Excavation of tidal channels and wetlands had the added advantage of providing some of the material needed for construction of the setback levee, but additional material would be needed to construct the setback levee. Because the restoration potential of the CSR project site was not maximized under Alternatives 1 or 2, these alternatives were not further evaluated because they did not meet the goals and objectives of the project.
Alternative 3 included additional excavation of tidal channels and 14 acres of marsh habitat on the northern portion of the project site, connecting directly to the Columbia River via a second modification to the existing levee approximately 500 feet east of the levee breach at Tide Creek.
Excavation of the channel network and marsh habitats in Alternative 3 had the added benefit of reducing imported materials required to construct the setback levee and adjoining seepage berms, all pervious and impervious material needed for levee and berm construction would originate from this increment. However, this alternative was not considered further because restoration across the project site was not maximized, as much of the site west of Tide Creek was not restored by these actions.
Alternative 4 included partial restoration of shrub and emergent wetlands west of Tide Creek by modifying the existing levee 2,250 feet west of the breach at Tide Creek and additional channel excavation to facilitate direct connection to the Columbia River. Because Alternative 4 did not maximize restoration potential of the CSR project site west of the railroad, it was not further evaluated.