Flooding solutions a moving target
DAVE SEABROOK
Recent high-water events remind us that our best efforts at flood control are not always adequate or permanent.
The Puyallup River flooded this past January, and the Election Day flood of 2006 was the highest on record at some places. Additionally, decertification of our levies by the Army Corps of Engineers reveals an expansion of the likely flood plain on the lower Puyallup River.
As a result, dredging has recently resurfaced as a solution to the flooding problem, as reported April 8 by Melissa Santos (“County weighs action on rising river sediment”).
However, river flooding and our response to it require a broader and more realistic reaction. We must also remember that high flows and the corresponding redistribution of gravel in the river are beneficial to many species, including ourselves. Gravel redistribution, in the riverbed itself, creates the diversity of habitat that fish need for spawning and refuge from predators.
The truth is that as mere humans, we cannot prevent flooding, but we can reduce its chance of occurrence and the risks of damage when it does happen. We can reduce the impact of high flows if we do not build our homes and our businesses in the path of the river.
Recent events have shown that it is smartest to set our levees farther back so the river can move freely within reasonably well-defined boundaries. However, for years we have condoned this “path of the river” approach by narrowly confining our rivers and permitting, in both the legal and social sense, construction of our homes, businesses and recreational facilities right up to the edge of such rivers.
This has led to regular rescues of property and people, whose costs are borne by the community. We do not intend to abandon community members in distress, so we must find ways to live with the river’s behavior rather than assume we can simply control it in whatever manner we might choose.
A concern often associated with flooding is the assumption that sediment is accumulating in the streambed. As a glacial stream, the Puyallup River moves huge quantities of sediment. All rivers that do that are subject to changes over time. A river will move from side to side and up and down due to the deposit and removal of sediments.
If a river channel is capable of carrying a high flow without coming out of its levees, the question is “Will it always be able to carry that much water?” As long as the width and depth of the channel remain the same, the answer is yes.
If the channel sides are constrained by levees, then changes in the bottom elevation will cause changes in the depth. If a river deposits sediment in the bottom of a particular length or reach of the stream, this will raise the bottom elevation and reduce the area to carry water. High flows will then overtop the levee and become a flood.
So any particular high flow may be a flood in one instance and not in another.
Dave Seabrook of Tacoma is chairman of the Puyallup River Watershed Council and a member of the Pierce Conservation District board.
