«DRAFT ENVIRONMENTAL ASSESSMENT ENVIRONMENTAL ASSESSMENT FOR THE COLUMBIA STOCK RANCH SECTION 536 ECOSYSTEM RESTORATION PROJECT The environmental ...»
April 22, 2016 Page 22 Specific to the CSR project site, drainage within the site originates from local runoff that is carried by the remnant channel of Tide Creek and terminates in a 24-inch culvert with a flap gate in the levee before flowing into the Columbia River. There are several wetted areas on the project site fed by groundwater seepage, as well as local precipitation. These areas are not directly connected to the Columbia River or any other conveyance channels because the site is isolated from the mainstem river. During large interior flooding events, surface waters from Deer Island Slough can spill into the CSR property.
Flooding in the Deer Island complex landward of the levee coincides with high flows on the Columbia River, which halts gravity drainage through the flap gates at Tide Creek and Deer Island Slough. When the flap gates are closed, rainfall runoff, groundwater flux and seepage accumulates behind the levees and contributes to interior flooding. During the wet season, the pump station is currently operated to prevent nuisance flooding, and while not used frequently, pumping can occur for weeks at a time.
The pumping history between 2005 and 2013 is documented by the Deer Island Diking District log book, which reports that pumping was initiated at least four times to combat winter flooding and three times for spring freshets over the eight-year period of evaluation.
Documented pumping shows that the pumps are used when runoff accumulates in the Deer Island watershed in response to prolonged rainy periods coincident with high Columbia River water levels. All permanent structures currently on the CSR project site and on Deer Island are located on relatively high ground with a minimum ground surface elevation of approximately 18 feet (NAVD 88).
Environmental Consequences No Action Alternative If no action were taken, the hydraulics and hydrology of the CSR project site would remain relatively unchanged from current conditions. The area would remain disconnected from the mainstem Columbia River, experiencing no tidal influence or connectivity to daily inundation patterns. Furthermore, current conditions are not anticipated to degrade during the period of analysis, nor are they anticipated to markedly improve with regards to habitat structure and function. Water control structures (culverts, tide gates, flap gates, pump stations, etc.) would remain in place and continue to limit or restrict floodplain inundation and surface water exchange. Operation of the pump station at Deer Island Slough would be maintained by the diking district to address incidental flooding associated with high flows and large rainfall events. Water levels within the Deer Island watershed and floodplain would not change as a result of the No Action Alternative and there would be no change to surface water elevations in the project site or adjacent properties.
Additionally, no off-channel habitats would be protected or restored and there would be no change in flood storage capacity and conveyance of waters during high flows under the No Action Alternative. Ponded waters west of the railroad would be disconnected from the Columbia River and would therefore be unable to support riverine and estuarine processes for the benefit of fish and wildlife.
Proposed Action Following implementation of the Proposed Action and reconnection with the Columbia River, there would be effects to hydrology and hydraulics in the CSR project site. Where the existing levees would be breached or lowered, the project site would be inundated and hydrologic processes which have been disconnected for decades would be restored almost April 22, 2016 Page 23 instantaneously. The direct effects of this action would include localized increases in turbidity during and immediately following the initial breach, but it is assumed that these effects would be short-term and last no longer than the duration of construction. Due to these temporary impacts, the timing and method of construction would be coordinated to minimize potential adverse effects to water quality, nesting birds, and other environmental factors. Over time, the restoration of hydrologic connectivity and inundation at a project site would support the restoration of natural processes contributing to habitat establishment and development, fish and wildlife usage, and would influence structural and functional dynamics at the project site.
The hydrologic effects resulting from excavation of tidal channels to mimic historic conditions includes a change in the frequency, duration, and spatial extent of intertidal inundation on the floodplain and channel habitat compared to current conditions. Re-establishing surface connectivity via levee breaching and channel excavation would restore tidal exchange in the excavated channels and marsh habitats on a daily basis during normal or low flow conditions and throughout most of the project site during flows higher than the 2-year flood event (15.8 feet NAVD 88). These actions support the restoration and long-term maintenance of ecological processes and estuarine habitats in the lower Columbia River and estuary. The size and shape of the tidal channel networks is designed to support the full tidal prism and allow for sediment erosion and accretion to equilibrate over the course of several years and a variety of hydrologic conditions. Restoring the tidal prism and hydrologic connectivity has indirect effects on the project site, including increased support for ecological processes influencing fish and wildlife production, water quality, sediment transport processes, nutrient cycling, primary production and food web dynamics, and water storage to attenuate the impacts of high flow and flood events.
In addition to the ecological effects of the Proposed Action, the Corps evaluated potential impacts to adjacent properties in response to alterations to the downstream boundary of Tide Creek from its current configuration. These changes include altering drainage patterns and a reduction in the total drainage area and storage capacity within the Deer Island watershed by 3 percent. Seepage and groundwater exchange influence interior flooding on Deer Island, and both processes are associated with a deep, highly permeable layer of uniformly graded, clean sand. Following implementation of the proposed restoration actions, namely construction of the setback levee, seepage rates and groundwater exchange are assumed to remain similar to existing conditions because there would be no effective change in the total length of the flood protection levee and subsurface materials would remain unchanged. Because the drainage area and storage capacity of the Deer Island watershed would be reduced following implementation, there could be increased water levels and interior flooding during intense rainfall events and high flows on the Columbia River. In response to these potential impacts, the Corps conducted a hydrologic and hydraulic (H&H) analysis to assess the range of impacts resulting from implementation of the Proposed Action.
The H&H analysis assessed maximum water levels, the Deer Island pump station, and corresponding impacts to adjacent properties during flood events. Conclusions were drawn from model simulations, field observations, and general understanding of the hydrology on Deer Island. During normal and low flow conditions when the pump station is not operated, surface water levels may be consistently higher due to the introduction of a fish-friendly tide gate replacing the gravity outlet at Tide Creek, but water levels would remain in-bank and there would be no flooding. Introduction of a fish-passable tide gate in Tide Creek would result in a daily tidal signal in Tide Creek during most of the year. Under normal conditions when the pumps are running and inflow does not overwhelm the pump station, there would be virtually no rise in maximum water levels due to normal operation of the pumps; large storms could April 22, 2016 Page 24 result in instances when inflow exceeds the pumping capacity at Deer Island Slough, and water levels could rise 0.2 feet and result in nuisance flooding until the pump station catches up with the inflow. During extreme events, water levels could increase upwards of 0.5 feet. The probability of flood conditions resulting in a 0.5 feet rise is less than 1 percent (100-year recurrence interval).
Based on the results of the H&H analysis, operation of the pump station is not expected to change considerably. Simulations of large rainfall events did not change the duration of pumping, but rather altered the window during which the pumps were turned on (i.e. the pumps were turned on earlier in an event). Because the pumps are usually operated for days to weeks during flooding events, this change would have no noticeable impacts in the operation of the pumps compared to current conditions. During a moderate flood event that is driven by seepage from high flows on the Columbia River, pump capacity could exceed inflow volumes, resulting in rapid dewatering of Deer Island Slough. In this case, one pump would likely be operated with frequent on-off cycles to reduce nuisance flooding until the Columbia River recedes to lower flows, resulting in an increased frequency of shorter pump cycles, which ultimately could increase pump maintenance and repair needs.
An overflow channel was incorporated into the design of the Proposed Action to maintain the current pattern of drainage between Tide Creek and the Deer Island Slough. Model results were used to optimize the channel geometry and invert elevation (the elevation at which the overflow channel connects to Tide Creek) of the overflow channel. It should be noted that without the overflow channel included as part of the restoration actions, model results showed water surface elevations would increase by as much as 1.0 foot upstream of the CSR project site during common flood conditions and throughout the entire Deer Island complex during major flood events. Additionally, replacement of the gravity outlet in Deer Island Slough would support evacuation of flood waters during high flow events. Replacement of this water control structure would not minimize the maximum extent of inundation; rather, increasing the capacity of this outlet would increase the rate of evacuation, minimizing the time agricultural areas landward of the setback levee are inundated during high flow events.
4.2. Geology, Topography and Soils The CSR project site is generally characterized as a flat, low lying historic floodplain with pockets of wetlands, channel bars, and numerous existing and abandoned channel features.
Construction of the Deer Island Levee in 1942 interrupted the natural interaction of Tide Creek and Deer Island Slough with the Columbia River, disconnecting the CSR project site from flood events. Embankments and levees constructed over the past 100 years have further fragmented the site, including the railroad alignment bisecting the CSR property.
Site elevations east of the Hwy 30 range from 4 to 40 feet (NAVD 88), with the majority of the site ranging from 10 to 20 feet (NAVD 88). Some isolated low lying elevations fall below 10 feet, but these are confined to ponded areas and existing channels. Land use practices and anthropogenic activities have fundamentally altered the topographic landform of the CSR project area. In the project area, the top of the railroad embankment is between 20 and 28 feet, which is approximately 10 feet above adjacent ground elevations. The portion of the levee bordering the eastern boundary of the project site tops at 30 to 32 feet, which is approximately 20 feet above the adjacent ground level. State Highway 30, located at the western boundary of the CSR site, is between 30 and 40 feet, and is situated approximately 20 feet higher than the adjacent grade. Consequently, the existing landform at the project site has many areas that are higher in elevation than historic conditions.
April 22, 2016 Page 25 Soils consist primarily of silty loams deposited during frequent historic flooding from the Columbia River (NRCS 2012). Geologic mapping by the U.S. Geologic Survey show the CSR project site is underlain by Holocene and Pleistocene, unconsolidated alluvial deposits5.
Generally, these deposits are mostly silt and fine sand, ranging from 65 to 100 meters thick around Deer Island; surface soils consist of silty alluvium between 1 and 15 feet thick.
Additionally, the Natural Resources Conservation Service (NRCS) listed soils located in the
project site as:
rafton silt loam rafton silt loam, protected Sauvie silt loam Sauvie silt loam, protected Wauld very gravelly loam, 30 to 70 percent slopes Wauld-rock outcrop complex, 5 to 30 percent slopes Xeropsamments, nearly level Xeropsamments, nearly level, protected.
Sediment samples were collected by the Corps’ Sediment Quality Team on June 11, 2015, to screen CSR project soils and sediments for contaminants. Section 404 of the Clean Water Act (CWA) (40 C.F.R. § 230.60-230.61) requires sediment testing when there is reason to believe contaminants may be present within dredged or fill material that are discharged into waters of the United States, including wetlands. The Corps uses the 2009 Sediment Evaluation Framework for the Pacific Northwest (SEF) to comply with the CWA sediment testing regulations (Corps 2009a). Soil samples were evaluated for a multitude of compounds, but semi-volatile organic compounds, polychlorinated biphenyls (PCBs), organotin compounds, total petroleum hydrocarbons (TPHs), and dioxin and furan congeners were not analyzed.
Based on the current and historical land use, there is no reason to believe that these contaminants would be onsite, however TPHs may be present at low concentrations in the farmstead area (e.g., where tractors and farm equipment have been stored throughout the years).
Three composite samples were collected from the CSR project area and laboratory results were compared to the 2015 freshwater benthic toxicity screening levels6. All samples were composed mostly of fine silts and clays, and total organic carbon ranged from 1.2 to 2.0%. No pesticides were detected above laboratory limits of quantitation in any of the three samples.
Metals concentrations were below their respective screening levels, except for arsenic, which was slightly elevated. The reported arsenic concentration is not substantially different from the screening level and the reported concentration is less than 10% above the screening level7.
This difference is within the margin of error associated with the laboratory's instrumentation.