Flood Retention Facility Options
A main focus in the Chehalis Basin Strategy is to reduce damage from catastrophic flooding. The measures already underway include raising homes, buying out frequently flooded properties, working with governments in the basin to assure that local land use policies reduce the risk of future damage. Also under consideration are two options to reduce peak flood levels during major floods. One of these alternatives is a Restorative Alternative proposed as a non-structural option. The other alternative for reducing peak flood levels is a water retention structure on the main stem of the Chehalis River above Pe Ell. Three types of retention facilities have been proposed and are described below. All three options have the same flood reduction benefit. The Flood Retention Only - Expandable facility type has been recommended by the Chehalis River Basin Flood Control Zone District Board of Supervisors, however, it has not yet been adopted by the Office of the Chehalis Basin as the preferred alternative.
Flood Retention Only
This type of facility would be designed to temporarily hold back water during major floods; there would not be a permanent pool of water. The river would flow normally during regular conditions or smaller floods.
Flood Retention/Flow augmentation
This type of facility would be designed to have the ability to retain a permanent reservoir all year. In addition to reducing flood damage during the winter, the water from the reservoir could be released in late spring to early fall to provide more water and cooler water temperatures in reaches of the river downstream of the facility.
Flood retention only - expandable
The expandable type facility would initially be constructed to operate similarly to the Flood Retention Only facility, in that no permanent pool of water would exist, and would temporarily store water during major floods. It would be constructed with a different type of foundation that could allow future generations the ability to expand the water retention capacity of the facility and, if found to be beneficial, augment stream flows. There may never be a need to expand the capacity. This solution was proposed in an effort to address the uncertainty in the climate and hydrology of the future. Future expansion of the facility would need to be completed with the required environmental review, permitting, and design details taking into account the best available science at that time.