Built on land reclaimed with hydraulic fill, the lower end of downtown Olympia adjoins a tidal arm of Puget Sound called Budd Inlet. Downtown Olympia is an average of 18 to 20 feet above mean lower low water, and tides in the inlet have reached 18 feet.
The city released a report on impacts of predicted rises in sea level on the city, including effects on flooding by creeks routed through culverts under downtown. Flooding of low-lying streets occurred occasionally by water backing up storm drains.
The city council built on that first study by requiring incorporation of more detailed processes, such as tidal, wave, and surge impacts on flooding elevation in the face of future sea-level scenarios. Defense against flooding was the objective; retreat was not an option in this case.
The city of Olympia retained Mott MacDonald to develop new flood maps, identify the flooding probability of critical infrastructure and transportation routes, conceptualize the most practical means of controlling flooding and their approximate costs, and determine the phasing for implementing the flood control structures for the most efficient use of public funds.
Our study established areas of flooding and flooding depths corresponding to 10-, 50-, 100-, and 500-year return periods for combined storms in Budd Inlet and tributary areas feeding creeks routed through downtown, and for increments of sea level rise up to 50 inches.
A geographic information system (GIS) was used to create inundation maps showing the extent of potential flooding for various scenarios and identify street drains that become flooded. This was critical for combined sewers that were pumped to the sewer treatment plant.
The study identified the following:
- Infrastructure to be modified
- Locations where flood barriers should be installed
- Types of flood barriers applicable for each vulnerable area
- Elevations of barriers needed to protect against a 100-year overtopping event
- Appropriate tidal gates and pump stations to be installed
- Specific sea level rises at which certain responses should be implemented
Mott MacDonald's analysis is an important element of the City’s effort to defend its downtown area from flooding. Our recommendations included the following:
- Protecting the downtown and Port of Olympia with barriers specific to their location, including vegetated earthen berm, armored slope earthen berm, sheet pile, and temporary barrier
- Implementing near-term measures such as raising the shoreline elevation at low-lying western areas, improving erosion protection on the peninsula shoreline, and installing more valves on outfalls connected to catch basins on Water Street
- Installing tidal gates on consolidated outfalls to protect against backflow to the upland
- Ensuring that pump stations to discharge stormwater are in operation by the time the tidal gates are installed