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DATE: - - 8/28/00
1. - - The following attended a field/site visit on 8/23/00 to view the locations of potential flood control alternatives and some potential habitat restoration sites along the Chehalis River, near Centralia. Chris Runner, USACE; Marie Garrett and Matthew Boyle, Pacific International Engineering; and Harry Gibbons, Sara Townsend and Merri Martz, Tetra Tech, Inc.
2. - - From the Mellon Street bridge in Centralia, we viewed a portion of the area proposed for channel widening (overbank excavation), both upstream and downstream of the bridge. The riparian zone, in this reach of the river, ranges from about 5 to more than 75 feet wide with some good mature deciduous trees along the riverbank over much of the reach and some sparse LWD in the channel. At least one agricultural field upstream of the bridge has been cleared of most vegetation all the way to the top of bank, leaving only a narrow strip of shrubs and herbs below the top of bank. Overbank excavation would likely entail the removal of all riparian vegetation and excavation of the banks up to 700 feet. The flood reduction benefits of channel widening will be constrained by the bridge, requiring structural modification, and several houses and other structures which are located within the excavation footprint. The bridge will act as a constriction on flood flows.
Some potential impacts from channel widening in this area may include: loss of the riparian zone, increased water temperatures from loss of shade and increased surface area, increases in sediment loading, aesthetics, loss of cover, loss of woody debris recruitment to channel, simplification of channel, and relocation of houses or other structures. Other impacts may be possible, depending on channel hydraulics, such as increased/decreased scour and erosion. A plan to avoid or minimize environmental impacts might be to create an overflow channel or channels rather than removing the existing riparian zone. However, retaining trees and shrubs may significantly reduce the conveyance of the channel so revegetation could be limited to grasses and some shrubs.
3. - - We then viewed the fairly large area of floodplain, currently in agricultural production below Scheuber Road. Scheuber Ditch currently drains this agricultural land, and may have formerly been part of an old river channel. A few old oxbows still remain, with some water still in them, amidst the fields. Scheuber Ditch has very limited habitat value currently; it is very channelized with no riparian zone (only blackberries and reed canary grass). This area currently floods, although would need modeling to determine with what frequency. There may be some additional flood storage benefits to be gained in this area, especially below the 100 year flow. Some habitat enhancement opportunities include reconnecting the oxbows to the Chehalis River and potentially to Scheuber Ditch as well, to provide off-channel habitat for juvenile salmonid rearing and winter cover. Some of this area is proposed for the placement of a very large berm under the overbank excavation alternative. It is unclear what impacts that may have on the usability of the agricultural fields or what opportunities would exist for upland habitat creation.
4. - - We then viewed the existing airport levee and some portions of the proposed levee alignment. Levees that are set back from the river by at least 100 feet would have significantly reduced impacts versus levees directly on alongside the river banks. Setback levees could allow the retention of all existing riparian vegetation. The proposed levee alignment will roughly parallel I-5 along the mainstem of the Chehalis River. The currently proposed levee system is quite extensive, but could potentially be reduced if off-channel flood storage is incorporated into the alternative wherever possible. Potential impacts of levees include channelization and simplification of the river channel, loss of riparian zone (unless set back), increased scour of channel bed, and increased water temperatures if riparian zone is removed and not replaced with trees.
5. - - We then viewed several potential restoration or mitigation sites. Salzer Creek has been identified as a system that could be significantly enhanced. The lower end of the creek runs through agricultural fields and under several roads. The riparian zone is generally poor, with sparse Oregon ash trees, otherwise dominated by reed canary grass and blackberries. Salzer Creek could also be connected to an old oxbow near its confluence with the river. Near the City of Centralia closed landfill, Salzer Creek continues through agricultural fields with a narrow riparian zone. The City has planted a six-acre site with Oregon ash to develop a forested wetland area. Levees may be required in this area as well. It would be best to tie the levee into the landfill, if feasible, to reduce the placement of fill material through existing wetlands. Existing vegetation that may be removed as a result of levee construction includes Oregon ash, spirea, reed canary grass, burreed, and snowberry. Limited erosion protection of landfill materials may preclude use of this area, since only a top liner is currently present. We walked across an agricultural field presently used for spray disposal of a vegetable and fruit processing operation that holds the potential to serve as a good restoration site between the creek and Centralia landfill property. Depending on the configuration of potential structural features, there are opportunities in the lower portion of Salzer create a wetland complex, a wider riparian corridor, and a more diverse habitat area.
6. - - Downstream of the Skookumchuck River confluence, some additional overbank excavation is proposed. The existing riparian zone in this area (near boat ramp) ranges from 20 to more than 50 feet wide with some mature deciduous trees in a healthy gallery forest. Species include cottonwood, Oregon ash, and serviceberry. There are several existing historic structures in the footprint of the overbank excavation in this area. Potential impacts in this area will be similar for the reach near Mellen Street, but would likely have adverse impacts to recreation use as well.
7. - - Levees may be required along the lower portion of the Skookumchuck River, including in and adjacent to Wilbur Perkins Park. The riparian zone is approximately 50 feet wide through this area and the river has moderately good habitat for fish spawning and rearing. There are many locations of riprap or other erosion control materials along the river. Once again, a setback levee would avoid most of the adverse impacts in this area.
8. - - Several of the smaller tributaries to both the Chehalis and Skookumchuck Rivers provide restoration opportunities. We briefly viewed China Creek, which is a ditch parallel to Little Hanaford Road, with no riparian vegetation. Hanaford Creek could also benefit from a restored riparian zone, but water quality in the creek appeared very poor currently. The upper portions of Salzer Creek provide good opportunities for flood storage of several hundred acre-feet of water. Existing roads which cross the floodplain could be raised to act as flood barriers, thereby reducing flood peaks downstream and slowly letting water out through existing culverts or bridges. The areas proposed are currently in hay production or pasture land. Providing several locations of flood storage in the basin could dramatically reduce the levee or channel widening requirements, potentially reducing both the adverse impacts and costs of the project.
9. - - Additional flood storage has been proposed for Skookumchuck Dam. It appears that raising the spillway elevation could fairly easily provide additional flood storage. We did not access above the dam, but much of the land appeared to have been clearcut and may not provide very good habitat. There will likely be some adverse impacts from raising the reservoir elevation depending on the frequency of inundation.
10. - - Further development of the alternatives will be required to fully assess the potential environmental impacts and the requirements for mitigation.
- - - - - - - - - - - - Merri Martz
- - - - - - - - - - - - Senior Biologist
- - - - - - - - - - - - Tetra Tech, Inc.
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