Advancing Deltas

[USGS Diver Deploying a Transect Marker at the Mouth of the Elwha River in the Strait of Juan de Fuca.  “The primary goal of the dive survey is to learn how underwater plant and animal life react and adapt to the downstream effects of dam removal and provide scientists a more detailed and complete picture of the ecological restoration.]

Washington’s Elwha and Glines Canyon Dams were recently unplugged and their reservoirs are in the early stages of being drawn down in preparation for removal of both dams.  A massive experiment in landscape reconstruction (over two decades in development and political process) is about to commence. As the U.S. Geological Survey reports:

“More than 19 million cubic meters of sediment, enough to fill 11 football fields the height of the Empire State Building, has accumulated behind the Elwha River dams. As the dams are removed, a portion of this sediment will be carried downstream, changing the structure of the riverbed below the dams.”

[Images of early reservoir drawdown (early August, 2011) behind Elwha Dam, exposing accumulated sediments in the upper reaches of Lake Aldwell.  Images courtesy of the Seattle Times]
[Glines Canyon Dam, Lake Mills, and the constructed delta of sediment]

Hydroelectric dams tend to reorder the material and spatial dynamics of the rivers upon which they span.  A river course that formerly might have taken a drop of water a single day to pass through can take weeks or more to reach the sea once dams are in place.  These extended hydrologic residence times and other shifted flows create upstream depositions of earth and a scarcity of such sediments below the dams.  Essentially all the distribution mechanisms of the system become shifted and its materials migrate.  Over time the river morphs to the new set of forces, so much so that if one attempts to return the river to its pre-dammed state after nearly a century of it becoming something else, a new and very dynamic condition of entropy is activated; sluggish residua slung into fast-forward.  ‘Sediment management’, a relatively obscure and modest discipline, entails designing with such transformative states, seeking to assess (or astutely guess) where all this mobile landscape is going to find its new equilibrium and with what affects.

[Diagram of Post Dam River Processes, via Wikipedia Images]

As the National Park Service describes the process of its restoration project “As dams are removed, lateral movements of the river will redistribute most of the delta sediments forward into the receding reservoir pool.  This advancing delta will slowly move north towards the dam, leaving the entire length and width of the reservoir floor covered with 10 to 20 feet of coarse sediments, burying the layers of fine sediment currently on the reservoir floor. Once dam removal is completed, the river channel will incise down through the layers of coarse and fine sediments, leaving stepped terraces along the reservoir edges.”  This advancing delta will be most prominent at the Lake Mills Delta where most of the sediment has accumulated (13 million cubic yards is trapped behind Glines Canyon Dam, 5 million cubic yards is trapped behind the Elwha Dam).

Its been claimed that physicist Albert Einstein warned his son not to become a river engineer because the physics of aggregated sand particles and riparian sediment transport are too complex to really be known or calculated (mentioned in Sand: the Never-Ending Story).  Yet we persevere and attempt to design with them:

[Physical scaled model of Lake Mills sediment erosion (above and below) by Bromley, C. T. Randle, G. Grant, and C. Thorne, 2005. The bottom picture shows a predicted topography after dam removal is completed, with terraces that have formed along the reservoir edges, terraces that at a 1: 1 scale may be anywhere between 20-60 feet above the original valley bottom.]

Prior to these scale models, a full size experiment was conducted in 1994, in which the Lake Mills reservoir was drawn down 18 feet over the course of one week, then allowed to remain at that level for an additional week for observation.  “As the water level dropped, the river began to cut into the existing delta, creating a deep and narrow channel. A new delta began to form at the new, lowered lake level and the river channel moved laterally along the existing delta. The test indicated that the gradual lowering of the water level would allow for efficient erosion and movement of the delta sediment load downstream.”

Exactly how the efficient erosion of 19 million cubic meters of particulated mobile landscape happens is soon to be tested and revealed.

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