Flooding in the Chehalis River Basin

Issue Paper

Chehalis Basin Watershed Planninng

Flooding Issue Paper - Lori Morris: f.lori.morris@usace.army.mil

What is the Issue?

The Chehalis River Basin, with the largest drainage area on the west slopes of the Cascade Range, responds directly and relatively quickly to rainfall events. The largest of these occur typically in the fall and early winter months, causing flooding. Flood-caused damage to private and public property and periodic closure of critical transportation routes has been an ongoing problem. Recent heightened environmental awareness and the potential listing of area aquatic species as threatened and endangered have resulted in a need for increased focus on development of flood control alternatives that minimize environmental impacts and that incorporate environmental features to mitigate adverse impacts to fish and wildlife and their habitats.

What is the background to this issue?

Flooding has been a persistent problem in the lower watershed - urban and rural areas -- at least since records began to be kept in the early 1900's. Contributing causes include:

• Filling in of tidelands,
• Establishment of urban and suburban areas along tidal estuaries,
• Continued development of the floodplains of the Chehalis River and major tributaries
• Bridge construction and railroad trestles that cause some obstruction to flood flows
• Low levees associated with road construction in the floodplain
• Modifications to river channels that can speed river flows downstream
• Forest harvest practice

While flooding has caused damage to public and private facilities, periodic flooding is important to the ecology of a stream. Flood flows maintain the natural hydrology of a river by allowing the river to meander unconstrained; these high waters can also improve access to habitat that is otherwise inaccessible in the upper reaches of the tributaries.

Traditional flood control efforts have focused on controlling the river's natural tendencies of channel shifting and over bank flow during floods. It is often more cost-effective in the long term and more environmentally sound to accommodate these natural river processes, rather than attempting to control them.

What are some possible solutions?

Local counties have identified a wide array of possible options to address flooding; they include a mix of structural and nonstructural methods. The following list of options is based on the plans developed by Grays Harbor, Thurston and Lewis Counties. Structural alternatives tend to address problems that have already been identified while nonstructural measures refer to land use regulations and policies that exist or may be adopted to reduce damages related to flooding.

Nonstructural Options

• Continue enforcement of existing land use regulations and permitting processes. This alternative includes ensuring that existing land use regulations and permitting processes continue to be strictly enforced. Floodplain management regulations, land use regulations and subsequent permitting processes can be used to ensure that development occurs in a manner that not only protects citizens and property from flooding, but also does not contribute to increased flooding.
• Continue Inter-Jurisdictional Coordination. For effective flood hazard management, it is important to coordinate flood hazard planning and regulatory enforcement with other jurisdictions within the same watershed to ensure consistency.
• Develop Floodplain Conservation Easement Program. Floodplain conservation easement programs are a cost effective means of protecting land within the floodplain from property losses and damages.
• Provide Educational material on Flood Hazard management. Developing posters, maps, pamphlets, and other materials to inform residents of the flooding issues throughout the basin helps property owners understand land use regulations and permitting processes for development activities within the floodplain.
• Improve Flood Monitoring System. Installing new water gauges on several major rivers within the basin would improve the river monitoring system that notifies the National Weather Service and NW River forecast Center of impending floodwaters.
• Use New Design, construction and maintenance Standards. Utilize environmentally sensitive design elements in river repair projects (i.e., bank stabilization projects)
• Join the National Flood insurance Program Community Rating System Program. By joining this program, more homeowners and renters in flood-prone areas can purchase flood insurance, and this may also reduce flood insurance by 5 to 45 percent. Design the entire flood plan to address the program needs of the National Flood Insurance Program, Community Rating System.
• Provide flood proofing guidance to residents.
• Improve existing development regulation implementation by providing more accurate mapping (i.e., 100-year floodplain, high groundwater and wetlands).
• Expand stormwater utility rate boundaries to include all unincorporated areas.
• Adopt development regulations for high groundwater areas.
• Work with landowners and others to establish reforested corridors along river and stream shorelines.
• Encourage research into bioengineering and other techniques which provide streambank protection an improve fisheries through the use of large woody debris. Support local demonstration projects, which could provide such research.
• Focus on the cause of flood damage. Flood damage can be related to upstream land management and development in flood-prone areas. Recognizing that flooding is a natural process, and only becomes a problem when people develop in areas that flood, is an important concept.
• Consider the entire watershed, not just local conditions. Because watersheds do not respect political boundaries, local flood management activities impact downstream jurisdictions.
• Incorporate public participation and coordinate among all affected agencies. Because flood hazard reduction affects most people in the county and overlaps with the responsibilities of other governmental agencies, it is necessary for these groups to be involved in the planning process. Without involvement from these groups, it is nearly impossible, in the end, to get support from them.
• Incorporate other resource protection goals. Coordinating flood hazard reduction measures with other resource protection programs is the best use of financial resources.
• Install additional river gauging stations. Current river monitoring provides flow information for a large portion of the Chehalis River; however, flood responsiveness could be increased with additional gauge sites. Flood preparation lead time would be increased with gauge installation within the upper reaches of the Chehalis drainage. Additional telephone-linked gauges would reduce personnel needed to visually inspect river levels. New gauges are recommended for the ungauged sections of the upper Chehalis River, the South Fork of the Chehalis River, and for major tributaries in the Centralia/Chehalis region. The Newaukum gauge near Chehalis should be updated to provide telephone-linked capabilities.
• Establish regional coordination on flood forecasting. Lewis County, Chehalis, and Centralia currently each have independent efforts for flood forecasting. Combining resources for flood forecasting is recommended.
• Establish a forum for coordination between Lewis County, Chehalis, and Centralia flood officials. These officials should meet regularly to discuss flood issues. Through this forum they can maintain consistency among all flood programs and share ideas and resources.
• Adopt stormwater management ordinance and technical manual to help jurisdictions throughout the basin deal with stormwater more effectively

Structural Options

• Biostabilization and other Engineered Solutions. Use existing guidance manuals for using biostabilization techniques to stabilize embankments.
• Consider Capital Projects in areas with repetitive damages.
• Move vulnerable activities out of the floodplain.
• Ensure that development activities take place outside the floodplain of rivers in the basin. Consider moving people out of the floodplain if a cost-benefit analysis shows repeated flooding is more costly than moving people. Industrial, commercial, and residential development in the floodplain should be restricted by local planning or development authorities and types of land use that are more appropriate to frequently flooded areas (agriculture, for example) should be encouraged.
• Cluster densities outside of the floodplain, instead of within the floodplain.
• Functioning floodplains, complex stream channels, wetlands, and riparian areas all contribute to retaining runoff locally and/or improving the infiltration of precipitation, which reduces the flashiness of flood waters. Local jurisdictions should make it their goal to contribute to the protection and restoration of these natural systems. Natural flood storage areas should be identified and protected. Where the connection between the river and its floodplain has been severed through levees or berms, opportunities should be examined to open up flood storage areas through levee setbacks or removal. An analysis of areas within the basin, which historically had wetlands should be undertaken to determine if restoration or creation of wetlands in these areas might help increase natural flood storage. Riparian vegetation, which helps with infiltration of precipitation, should be protected where it exists and restored where it has been removed. Large woody debris and numerous logjams historically acted to create complex, meandering stream channels that could hold more water than simplified, channelized streams. Large wood should not be removed from streams and rivers.
• Further analysis within the basin should be done to determine positive restorative actions that might be taken to improve the natural functioning of floodplains, wetlands, and riparian areas, including reconnecting rivers to their historic floodplains, streambank rehabilitation and conversion of land uses (i.e., buyouts, easements, etc).
• Actions that would help retain storm runoff in the upper and middle watershed would include removal of agricultural drain tiles, wetland restoration and creation, and the addition of large woody debris and log jams. In addition, opportunities exist to remove levees throughout the basin or to set existing levees back to allow more flood storage.
• Local jurisdictions should work with the state and federal authorities to develop floodplain management plans that integrate land use planning, current knowledge of the extent of flooding, and an understanding of naturally functioning rivers and floodplains. Areas with a history of flooding should be identified and protected to provide valuable flood storage, ecological values, and potential restoration. These areas should be targeted for buyouts, easements, or other programs that offer incentives to landowners.
• Place flood elevation poles and staff gauges along major rivers and within chronic groundwater flooding areas.

Analysis

• Cost effectiveness - see language with the flood hazard management plans.
• Political/policy factors - FEMA guidelines
• Technical issues - see flood hazard management plans for why these are done and when they are applicable.

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