FLOOD MITIGATION AND ADAPTATION STRATEGIES
Flood mitigation and adaptation strategies are infrastructures, projects, programs, or any actions to reduce or eliminate the risk generated by flooding. They can be classified into 3 major categories: 1) Gray, 2) Green, and 3) Nonstructural.
Gray strategies are human-made structures using hard building materials (Szönyi, M. and Svensson, A., 2019). Gray strategies involve physical changes to natural features and are also known as hard or structural strategies (WWF, 2017). Some examples in Tillamook include tide gates, dikes, levees such as Shillo levee, the east bank of the Nehalem River levee, and levees within the Agricultural and non-agricultural lands, etc.
A dike is a Dutch word that originally referred to the bank of a body of water but now refers to a wall or embankment that holds back or prevents flooding. Dikes have been used to hold back ocean waters and drain land for agricultural and other human use.
There are several diking districts in Tillamook County that manage drainage on approximately 2,216 acres. Diking districts are located in the Trask, Tillamook Bay, Wilson River, and Kilchis River watersheds (Runyon et al., 2017). Dick Point Dike is a cultural feature in Tillamook County with an elevation of approximately 7 feet. It is located near the Tillamook River, and close to Rock Point.
Tide gates are located in areas subject to tidal inundation. There are 41 tide gates within Tillamook County’s Agricultural Lands and 40 within non-agricultural lands, concentrated in North Fork Nehalem River Watershed, the Little Nestucca River Watershed, Tillamook Bay Watershed; and the Tillamook River Watershed (Runyon et al., 2017). Two side-door tide gates aim to provide more effective drainage near the confluence of Anderson Creek and the Tillamook River. They are fish friendly, made of aluminum, and installed during low-tidal periods (Cameron, 2017). The difference in water levels on the gate's downstream and upstream sides causes tide gates to open and close. In order to prevent backflow during flooding, it can only open in one direction, allowing water to flow exclusively downstream (Giannico, G., and Souder, J., 2005)
An example of a gray strategy is a levee. A levee is a barrier that blocks water from overflowing from a river or other body of water. They are used to prevent flooding from storm surges. Levees are also used to divert bodies of water to drain land for agricultural and other human use.
The Shilo levee was built to protect Highway 101 and Wilson River Loop Road business (Meyer, G., 2019). The levee along the east bank of the Nehalem River separates the Nehalem Bay Wastewater Treatment and the Nehalem River. It is located in the flood hazard zone (FH), farm zone (F-1) and estuary conservation zone (EC 2) (Tillamook County, 2022). There are 25 miles of levees within Tillamook County’s Agricultural Lands, and 38 miles of levees on Non-Agricultural lands, both primarily concentrated in the Tillamook Bay Watershed and the Tillamook River Watershed (Runyon et al., 2017).
Green strategies are natural solutions that mimic nature by soaking up and storing water. They use vegetation, soils, and natural processes to manage flooding, and provide ecosystem services such as green spaces, habitats, and cleaner air and water (EPA, 2014). Green strategies are also known as natural and nature-based or soft strategies (WWF, 2017). Some examples in Tillamook include Corvallis tidal wetland habitat restoration, conservation of barrier islands such as Bayocean Spit, natural drainage restoration such as reconnecting the Blind Slough to Hall Slough, etc.
Corvallis tidal wetland habitat restoration project has a surface of 443 acres in Tillamook Bay, OR. It was completed in 2017 with an inversion of $11.2 million and designed to reduce flooding and improve salmon habitat. The restoration has led to water quality improvements, flood mitigation, salmon habitat improvements, increased carbon storage, added recreation opportunities, and increased home values.
Flood reduction on Highway 101 in Tillamook’s business corridor may result in savings of up to $7,200 in travel delays per flood event. The restored wetlands may store as much as 27,000 tons of carbon, at a value of up to $736,000 based on current social costs of carbon. Recreation activities such as kayaking and wildlife viewing have an estimated value of $60 to as much as $471 per person per day.
Since the project was completed, Tillamook has only experienced two minor, five-year flooding events, and in both cases, the flooding was less severe than previous similar floods. In both cases, the flood was 9 inches less than previous floods, the city flooding was deleted up to 2-3 hours, and the floodwaters receded 2-3 hours sooner than they would have previously (Klampe, 2021).
Source: Klampe, 2021
The Bayocean Spit is a diverse stretch of dunes and trees between the Tillamook Bay and the Pacific Ocean. The Bayocean Spit shields Tillamook Bay from harsh Pacific storms and provides a healthy habitat for a unique coastal ecosystem (Sawyer, A., 2015).
Waves, currents, and the two jetties at the mouth of Tillamook Bay interact on Bayocean Spit (Malone, S., 2014). This kind of ecosystem prevents flooding by breaking waves offshore, slowing inland water transfer, and reducing the impact of storm surges.
Reconnecting wetlands and floodplain areas with the rivers improves the absorption of flood flows. Therefore, providing a larger opening between Hall and Blind Sloughs showed significant flood reductions in the south bank of Wilson River (Levesque, P., 2013).
Blind Slough and its tributaries are the primary channel system for the North and Middle wetland zones (Brown et al., 2016).
These are actions that do not involve physical interventions (engineering or ecological). Nonstructural strategies can be categorized into 2 groups, depending on the nature of the interventions:
1. Governance changes
These include modification or introduction of laws, regulations or organizational procedures, land use planning, flood monitoring, etc. (WWF, 2017)
2. Community and household practices.
These aim to promote behaviors that contribute to the prevention, mitigation, or adaptation of floods in the community and households (WWF, 2017). They include flood insurance, relocating housing, floodproofing, community flood awareness and preparedness, etc. (Reed, M., 2015).
Tillamook County participates in the FEMA National Flood Insurance Program (NFIP) which makes available flood insurance for all structures, whether or not they are located within the floodplain. Flood insurance may be required for any property within any Special Flood Hazard Area (floodway or 1% annual chance floodplain). Additional information can be found on the Department of Community Development website and current FEMA flood maps can be found and viewed on the NFIP website.
The National Flood Insurance Program (NFIP) policy covers direct physical losses to structures and belongings. For example, damage caused by a sewer backup is covered if the backup is a direct result of flooding. If the sewer backup is not caused directly by flooding, the damage is not covered. The FEMA methodology distributes premiums across all policyholders based on the value of their home and the unique flood risk of their property.
Floodproofing is any combination of structural and non-structural additions, changes, or adjustments to structures that reduce or eliminate flood damage. It makes the buildings more flood tolerant, improves the functionality of the building during floods, and improves drainage, infiltration and temporary water storage in the compound (WWF, 2017).
According to the Tillamook County Land Use Ordinance (LUO), floodproofing is required for structures in A, AE or A1- A30 zones and AO.New constructions, residential and nonresidential buildings, and improvements to any commercial building must be floodproofed.
LAND USE PLANNING:
The Department of Community Development is in charge of land use planning in Tillamook County.
Land use planning is done at different administration and policy levels, and is essential in flood risk management. It involves zoning, which identifies the geographical distribution of different land uses, and what should be permitted or prohibited in specific locations. In urban areas, zoning can help minimize flood damage (e.g., allowing flood buffer zones free of buildings along a river) and enabling efficient flood evacuation (WWF, 2017).
Additional information about the Tillamook Land Use Ordinance (LUO) (Zoning Ordinance) can be found here.