Friday, September 17, 2010

The rain and floods

The rainy season has begun in earnest.  Here it's from mid-September to probably May, and the long-term forecast this year is that we'll get a lot of rain.  For me, it's a time to look at the items on the farm and start preparing for a flood. 

Ebey Island, where my farm is located, is entirely surrounded by a 15' tall dike.  I'm close enough to the ocean that the tides push the river water up 8' each high and low tide on the outside of the dike, and the dike does break from time to time. 

The last time it broke was on an outgoing tide, so there was a 6  hour period where the waters were receding because of the tide that allowed the hole to be patched.  If the break had occurred during a rising tide I suspect that the hole would not have been plugged, and the entire island would have flooded. 

When Ebey island was diked in the 1920s they dug a network of ditches across the island to drain it, and a set of giant pumps to pump out the water.  There are also tide gates that open during minus tides to remove the water, and close when the tide comes in.  This draining of the island has causes the land inside the dike to subside about 6' from historical elevations.  So a flood event is more like a bathtub filling up than a traditional flood. 

Here's the sequence that will cause me problems: 
  1) heavy rain for a few weeks to saturate the ground
  2) heavy snowfall in the mountains to build up some water
  3) warm rain on that snow to release the water all at once
  4) high tide

And there's a 5th factor that has nothing to do with the weather. 
  5) various government agencies that are not allowing the diking district to do the same sorts of maintenance on the dike that has allowed it to last all these years.  This list includes the Washington Dept of Fish and Game, The army Corps of engineers and the Snohomish county planning and development service, along with the snohomish county surface water management district.  Each of these agencies has the potential to, through requirements or permitting changes, or the refusal to issue permits, make the dike more susceptible to failure than it has been.

The first 4 I knew about when i bought this land.  #5 has been an unpleasant surprise. 

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