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During the heavy
rain on October 20, areas downstream from Sea-Tac Airport
got a taste of possible conditions
after third-runway construction - if there is inadequate
stormwater mitigation. Failure to provide full replacement
for lost wetlands can lead to sudden, disastrous
flooding of Miller, Walker, & Des Moines Creeks, similar
to what has just been experienced.
 
Walker Creek at 1st Ave. after October
20
While the storm was treated as
an unusual event, a "hundred-year
storm", some sort of climate change, perhaps the onset
of global warming, seems to produce "hundred-year
storms" every couple of years or so.
If the third runway is built without
stormwater mitigation, then damage like that experienced
during this big storm, & shown
in the accompanying photos, could occur regularly. The
streams will have continuing trouble in recovering from
such conditions. If such damage occurred regularly, the
streams would likely not recover in our lifetimes.
Problems around inadequate culverts
are especially worrisome. First Avenue South, the border
between Normandy Park & Burien,
has small culverts that cannot handle big storms. Even
with the repair program now underway, these culverts remain
as ongoing barriers to fish moving upstream to spawn, & choke
points that lead to flood damage to roadways & surrounding
terrain. To replace an existing culvert with a fish-friendly
structure that is also able to handle expectable flooding
will cost at least $500,000 each, & estimates go up
from there. The Port of Seattle is happy to shift these
costs on to the cities downstream.
The area around the culvert for Walker Creek, east side
of First Avenue So. and So. 172nd is especially vulnerable,
for the ground underlying the road is mostly sand.
Since the major modification of
the Port's NPDES permit, issued in June 2000, large new
amounts of stormwater are
released from the Port's Pond F into the headwaters of
Walker Creek, through the new 36-inch pipe under SR 509.
This pipe was put in place to handle run-off accumulated
East of the freeway, resulting from construction of the
special off-ramp (now hardly used) for fill-hauling trucks.
No environmental impact statement was prepared for this
work, & unfortunately, the near-by communities were
not prepared to challenge the failure to do a proper environmental
assessment.
Because the soils are mostly sandy
loam, the creek bed of Walker Creek & downstream
culverts, together with the duck pond at Normandy Park's
'The Cove', have continued
to fill with sand swept downstream by run-off. This requires
frequent digging-out by the City of Normandy Park.
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