«DRAFT ENVIRONMENTAL ASSESSMENT ENVIRONMENTAL ASSESSMENT FOR THE COLUMBIA STOCK RANCH SECTION 536 ECOSYSTEM RESTORATION PROJECT The environmental ...»
Acquisition of lands, easements and rights of way from land owners, diking district, utilities, Portland & Western Railroad and the Oregon Department of Transportation for all actions associated with staging, access and construction Removal of a residential home, associated outbuildings, and fences Construction of temporary haul road to dredged material placement site Construction of setback levee, seepage berms, and installation of tide gate at Tide Creek Modification to the existing flood control levee, including three (3) breaches and lowering the remaining elevation to approximately 15 feet (NAVD 88) Installation of two (2) bridges in the railroad embankment, spanning existing waterways Excavation of tidal channels, marsh and wetland habitats and an overflow channel Grading select portions of upland areas below 2-year flood elevation and filling an agricultural drainage ditch Replace existing gravity outlet in Deer Island Slough adjacent to the pump station with large structure to increase capacity Removal of existing culvert, tide gate and channel-spanning cattle grates in Tide Creek Removal of culvert in existing access road and grade road to match adjacent topography Invasive species removal, and planting native vegetation Structures and Staging Areas, Access and Haul Roads Residential buildings (houses, barn, out-buildings), cattle grates, and fences would be removed from the CSR project site as part of the initial construction activities.
It should be noted that the state-owned dredged material placement site at Deer Island was not included in the wetland delineation conducted in 2015. It is assumed that the area between the placement site and the Columbia River Levee consists of some emergent wetlands, but the extent of wetland coverage is unknown. The National Wetlands Inventory shows the area as containing emergent and forested wetlands. It is assumed that gravel used for the haul road would temporarily fill any wetlands existing between the levee and the placement site.
However, the location and dimensions of the haul road has not yet been identified and would be field verified and coordinated with the Oregon Department of State Lands to minimize impacts to wetlands.
April 22, 2016 Page 7 Figure 2: Proposed Action and design features for the CSR project site April 22, 2016 Page 8 There is a natural gas pipeline that crosses the project site (east – west) and Northwest Natural Gas needs to maintain access to the site for maintenance purposes. Currently, an access road passes between the western half of the project site and the eastern half atop a 36-inch corrugated metal culvert in Tide Creek. The proposed plan includes removing the culvert to restore the channel in Tide Creek. Year-round access to the pipeline would be maintained atop the setback levee connecting to Hwy 30. During the dry season, access would occur via the current road from Hwy 30 which would be lowered to adjacent grade of the floodplain.
Levee Modifications, Construction of Setback Levee and Tide Gates The Columbia River Levee runs a total length of approximately 10 miles along the project site and Deer Island. Adjacent to the CSR project site, the levee runs approximately 7,100 feet.
Approximately 3,000 feet of the existing levee would be modified by lowering the surface elevation to 15.0 feet (NAVD 88), representing a reduction in 42 percent of the levee along the project site. The levee, both the portion lowered and the remnant levee would be planted with a mix of shrub and tree plant communities to support riparian habitat development in the project area. Construction of the setback levee would functionally replace the existing levee and provide flood protection to adjacent properties landward of the levee.
In addition to lowering the existing levee, the levee would be modified by breaching the levee at three locations to open the floodplain to tidal inundation and Columbia River flows and permission from the diking district for the breaches would be required. The breaches would be sized to accommodate National Oceanic and Atmospheric Administration, National Marine Fisheries Service (NOAA Fisheries) fish passage guidelines for access and egress into the project site via Tide Creek and accommodate the full tidal prism. Approximately 600 feet of the levee would be breached in total, representing approximately 16 percent of the total levee along the project site. If additional fill material is needed for construction of the setback levee, the overall
breach length could be widened. Breach locations are currently proposed for three locations:
Breach 1, Breach 2, and Breach 3 (see attached design map). Breach 1 is the western most breach nearest to the Portland & Western Railroad, Breach 2 is located where Tide Creek passes through the existing levee via a gravity flap gate, which is currently non-functional. The tide gate would be removed and a replacement tide gate would be installed in the setback levee.
Breach 3 is located east of Breach 2 and connects tidal channels on the northern portion of the site directly to the Columbia River via excavated channels and existing waterways riverward of the existing levee. The current elevation of the Columbia River Levee is 31.5 feet (NAVD 88).
Each breach would be approximately 200 feet in width, and excavated to match topography of the adjacent floodplain and channel thalwags. No armoring would be used to stabilize the breach openings, as it is intended for the openings to destabilize naturally over time.
A setback levee and seepage berms would be constructed on the southern and eastern portion of the CSR project site to provide similar flood protection to adjacent properties that the existing levee currently provides. The setback levee would have a top width of 13 feet, a riverward side slope of 3 horizontal feet for 1 foot change in vertical elevation (3H:1V) and landward side slope of 4H:1V. The location of the setback levee is based on existing topography and ties in to high ground at the southern portion of the CSR project site near the railroad and Hwy 30. In addition, the setback levee would also serve as an access road to the CSR project site east of Tide Creek for the landowner and utility companies. To construct the setback levee and access road on the levee, acquisition of easements and permissions from utilities and Oregon Department of Transportation, Rail Section related to this construction would be required.
Construction would also require the utilities to install river weights on utility lines (primarily the gas pipeline) and relocate manholes. Geomorphic evaluation of the project site identified April 22, 2016 Page 9 underground seepage as a concern with regards to construction and long-term stability of the setback levee. As a result, seepage berms would be constructed at the toe of the landward side of the levee to minimize the risk of seepage and provide additional levee stability (see Figure 3).
Further analysis will determine the final dimensions of the berms, but initial estimates assume a maximum of 5 feet in depth, extending outwards up to 250 feet from the levee.
Figure 3: Setback levee cross-section
All offsite drainage would be routed through the setback levee via a side-hinged tide gate (with float control) in Tide Creek and an overflow channel connecting to Deer Island Slough (see Figure 4). The tide gate would close at 9 feet (NAVD 88) to reduce flood risk and minimize the potential for fish stranding landward of the setback levee during high flows. In general, during high flows (above 9 feet), Tide Creek and all off-site drainage would be routed through the overflow channel (discussed in greater detail below) to the existing Deer Island Slough (and pump station) for evacuation to the Columbia River. To decrease the amount of time adjacent properties are inundated during high flow events, the existing gravity outlet in Deer Slough adjacent to the pump station would be replaced with a larger structure to increase capacity and the rate of evacuation. The replacement structure would be a gravity outlet (culvert with flag gate), providing one-way flow out of the flooded areas. Alternatively, existing freshwater inlets installed at various locations along the levee could be replaced to increase evacuation rates.
Figure 4: Draft plans for tide gate located in setback levee at Tide Creek
April 22, 2016 Page 10 Channel-Spanning Structures: Railroad Bridges The Portland & Western Railroad embankment bisects the property and disconnects the western portion of the property from the Columbia River and crosses a remnant channel and wetland. Two bridges would be installed in the embankment to provide hydrologic connectivity across the entire project site. Each railroad crossing would consist of three 40 feet spans, for a total opening width of 120 feet at each bridge. It should be noted that coordination with railroad personnel and the Oregon Department of Transportation have confirmed that, while additional coordination and permitting are required, making a connection within the rail embankment is feasible. Installation of the two railroad bridges would require a channel improvement easement and permission from the railroads to allow construction within the railroad right-of-way.
Excavation and Grading of Tidal Channels, Marsh and Wetland Habitats, and Tide Creek Overflow Channel Tidal fluctuations for the Columbia River at river mile 76.5 (immediately adjacent to the CSR project site) are 4 feet. Elevations for mean higher-high water (MHHW), mean high water (MHW), mean tide level (MTL), mean sea level (MSL), mean low water (MLW), and mean lowerlow water (MLLW) have been interpolated based on observations of tidal fluctuations from NOAA’s Longview and St. Helens gauges and the results are summarized in the table below.
To support inundation of the tidal channels and marsh habitats during the daily tidal cycle for fish access and use, tidal channels on the northern portion of the site would be excavated to 3 feet (NAVD 88) and tidal channels on the southern portion of the site where the channel ties into Tide Creek would be excavated to 5.5 feet (NAVD 88). Water levels in the excavated marsh would fluctuate in response to Columbia River water levels and tidal fluctuations. The excavated areas would provide daily wetting and drying for most of the year, and prolonged inundation during the rainy winter months and spring freshet. During the summer dry months when river and groundwater levels are lower, the tidal channels on the southern portion of the project site would drain between diurnal tides and not be fully wetted during the high tide. An existing irrigation channel spanning the project site between Tide Creek and Deer Island Slough would be filled to maintain the structure of the tidal channels and ensure the irrigation channel does not re-water during tidal inundation from the Columbia River.
The proposed restoration action incorporates the existing channel morphology of Tide Creek into the design of the proposed tidal channels. The bottom elevation of Tide Creek varies between approximately 3 feet (NAVD 88) near its terminus at the existing Columbia River Levee and 5.5 feet (NAVD 88) near the southern portion of the property where the setback levee is April 22, 2016 Page 11 proposed for construction. Consequently, the tidal channels are designed to match the existing Tide Creek thalwag to prevent fish stranding between tidal cycles and high flood events. The bankfull width of the proposed tidal channels vary between 75 feet where they intersect with Tide Creek or the Columbia River, decreasing up to 24 and 19 feet bankfull width at the upper, terminal ends. Similarly, bottom widths of the proposed tidal channels vary between 68 feet to 12 feet.
Excavation of marsh and wetland habitats adjacent to the tidal channels is intended to target a gradient of low and high marsh elevations for the establishment of intertidal wetland habitats across the project site. “Low marsh” areas would be excavated between 6.3 feet and 8.3 feet (NAVD 88) before grading up to the adjacent floodplain, supporting the establishment of wetland vegetation between MLLW and MTL. Annual tidal fluctuations indicate that these areas would remain semi-permanently flooded for the majority of the year, drying out between August and October when river elevations are lowest. The “low marsh” habitat would extend 100 feet on either side of a tidal channel. It should be noted that “low marsh” habitat would only occur on the northern portion of the project site because the existing topography and channel grade of Tide Creek would require extensive excavation to support low marsh habitat on the southern portion of the project site and low marsh habitat was also deemed unsustainable over the long-term (see Figure 5).
Figure 5: Schematic for low and high marsh elevations on northern portion of CSR project site “High marsh” areas are proposed for both the northern and southern tidal channel (see Figure 5 and Figure 6). High marsh elevations promote the establishment of intertidal habitats and would be excavated to 8.3 feet (NAVD 88), an elevation between MTL and MHHW providing added resilience to anticipated changes in water surface elevation as a result of climate change.