Monday, March 2, 2015

Seiche and Tsunami: Geology Hazard Regulations

I have been doing some volunteer work on the Whatcom County update of the County's Critical Areas regulations. Critical areas are mandated by the Washington State Growth Management Act to be regulated. Critical areas include wetlands, flood areas, wildlife habitat, aquifer recharge areas and geologic hazard areas. I am on the technical committee due to the later part - those pesky geology troubles.

One geologic risk that we recently addressed were tsunami and seiche hazard areas. Wikipedia has a good description of seiche current Whatcom County code lumps seiche hazards in with tsunami hazards. I took the view that they should be pulled apart since we have tsunami hazard models for at least part of the marine shore areas of Whatcom County (see map here) and can build specific tsunami hazard regulations based on those model run results. The areas outside the tsunami hazard modeling still are likely at some risk - we simply do not have modeling done in those areas, but it is probable that there is a risk of tsunami in Birch Bay, Drayton Harbor and Point Roberts and for low lying areas in those areas it was suggested that using a 10-foot elevation design would be warranted.     

For the seiche hazard the following new separate category was proposed:   

Seiche and Landslide Generated Wave Hazard Areas.  Seiche hazard areas and landslide generated wave hazard areas include lake and marine shoreline areas susceptible to flooding, inundation, debris impact, and/or mass wasting as the result of a seiche or landslide generated waves. No specific areas of seiche or landslide generated wave hazard areas have been identified in Whatcom County.

Based on my limited understanding of how seiche events can be generated, the risk is very low at the marine and lake shores of Whatcom County. Seiche events from earthquakes are bit more difficult to ascertain and are limited to a bit of speculation. Could one of the lakes in Whatcom County start sloshing around in perfect timing with seismic waves arriving from some earthquake? Lake Union in Seattle has a bit of history of this type of action ( Apparently the way the Lake Union is shaped makes it susceptible to developing seiche waves that can be damaging. 

Landslides can generate localized large waves - in some cases incredibly large waves (see 1958_Lituya_Bay_megatsunami). For this hazard, one needs to think through the question Just where might this happen? and Are there any known potential areas? Alas I am not aware of any, but that does not mean it could not happen. Perhaps a steep mountain slope above Ross Lake or Baker Lake. That risk is mitigated by the minimum development along those lake shores. In areas I am more familiar, I think the odds of a big wave being generated are very low. That said, I am aware of an estimated 7- to 8-foot wave generated by a landslide in 1997 on Hood Canal. That landslide was very large and fast and was in a bay that reduced the amount of dissipation over distance.

So what we came up with at least for now regarding seiche and landslide generated waves:  

Standards for Seiche and landslide generated wave hazards will only apply if the hazard area is mapped by the United States Geologic Service or the Department of Natural Resources Geology Division or other credible source approved by Whatcom County. If a mapped hazard is present, the standards of WCC 16.16.320 and 16.16.350 shall apply. For residential development within a mapped seiche and landslide generated wave hazard areas, the proposed development should be designed to withstand the mapped hazard. If the risk of the event is less than 0.1% on a yearly basis, development standards may not be required, but notice on property title will be required.

The regulations are only at the earliest stage and might get tweaked a bit more based on input from others. The next step will to go through the Citizens Advisory Committee, then the Planning Commission and finally the County Council. Plenty of time to make changes dependent upon the whims and policy lenses.

1 comment:

ebaer said...

Landslide Generated wave hazards are not trivial in Lake Roosevelt. Yet I don't know that they would fall into your CAO regulation.