Monday, December 5, 2011
Where do they get this stuff from?
Today's Peninsula Daily News ran a front page story on the most recent gravel nourishment project on Ediz Hook. Now let me first be clear that I love my local paper. I am a loyal subscriber to the PDN, and proudly read it every morning. But for this article I feel that I know too much. So let me ask the question, "Where do they get this stuff from?". It appears from the Army Corps of Engineers, who should know better.
The story that we are told is that the dams and the placement of rip rap have reduced sediment supply to Ediz Hook and that, as a result, Ediz Hook would go away without the engineering solutions provided by the ACOE. I want to examine an alternative hypothesis though - that Ediz Hook is, by its very nature, a mobile feature, a pile of sediment moving in response to a complex interaction between sediment supply, waves and sea level. The "erosion" observed on Ediz Hook MIGHT have less to do with these relatively small changes to sediment supply (I will discuss this assertion later) and more to do with the simple fact that Ediz Hook is heavily developed. It simply can't be allowed to move. Let me say in advance that this alternative hypothesis doesn't suggest that the engineering on Ediz Hook is the wrong approach. Regardless, Ediz Hook is important and there is a strong case to be made that it needs to be kept in place. I just think that the story backing it up should be right.
Let me start with the mythology that permeates this story: "Wave erosion and a lack of new sediment feeding the spit caused bank failure". In their 1971 "Report on Survey of Ediz Hook for Beach Erosion and Related Purposes" on page 13 the Army Corps forward the idea that, I think, persists to this day: "the system was able to maintain equilibrium". The idea is that the damming of the Elwha River combined with the placement of rip-rap to the west of Port Angeles under tall bluffs restricted the alongshore transport of sediment enough to create a severe deficit, "upsetting the balance" of sediment supply and loss on Ediz Hook. Erosion was the result. Interestingly, this story contradicts the geomorphic model of Ediz Hook published in Appendix B of the same report (and subsequently republished by John Downing in his 1983 book and again by Galster and schwartz in a 1990 Journal of Coastal Research manuscript, and shown here) which suggests that Ediz Hook has evolved and moved landward continuously since about 5000 years ago. The notion of equilibrium in such an extraordinarily dynamic environment - marked by changes in sea level, punctuated sediment supply and variable wave climate, is a difficult one for me to stomach.
The second question is, "How much has sediment supply been reduced?". The PDN quotes a figure suggesting that the removal of the dams will increase sediment supply by 35%. This figure comes from the National Parks Draft Environmental Impact Assessment for dam removal, published in 1996. This estimate was sourced to personal communication and was subsequently struck from the final EIS (though it appears in ACOE documents published as recently as June of 2011). Interestingly, this figure contradicts the Army Corps own estimates of how much sediment supply has been reduced by dam emplacement. In the same 1971 report mentioned above, a post-dam reduction of sediment delivery to the coastal zone exceeding 90% is stated. This would imply that after dams are removed that sediment delivery should increase 9-fold from its present delivery. What actually made it into the final environmental impact statement for the dam removal project (from 2005) was a nearly complete (98%) reduction in sediment delivery to the coastal zone, from an estimated 280,000 cy/yr to about 5000 cy/yr - figures that are cited to a 1993 Federal Energy Regulatory Commission Report. My own research suggests that the dams have probably reduced the supply of coarse sediment to the coastal zone by, on average, about 50 to 90%. But it is important to note that this supply is extraordinarily variable and seems to be related primarily to the magnitude of winter floods.
The key thing though, is to move away from percentages. A good estimate for the annual average historic (i.e pre-dam) coarse sediment supply is probably about 100,000 m3/year (based on what has accumulated in the reservoirs). Some of my work suggests that the river is delivering, on average, between 15,000 and 45,000 m3/yr. so the volumetric deficit would be on the order of 55,000 to 85,000 m3/yr. This then begs the question - how does this fit in to the whole sediment budget for the drift cell. And that question, well, nobody knows...my guess is that, relative to the budget for Ediz Hook, that that volume reduction is relatively insignificant.
Finally, I want to consider another key argument made first by the Army Corps of Engineers in their 1971 report, that erosion from the bluffs to the west of Ediz Hook has been significantly arrested by rip-rap at their base. I am no fan of this rip-rap, lets be clear, but I have noted that the bluffs give every sign of being active (steep, unvegetated) and that the base of the bluffs is now, on average, about 70 feet from the rip-rap. Photos taken at the time of the placement of the rip rap (1929) suggest that the water line and protection structures were placed right at the base of the cliff. Bluff erosion is on-going. Doing the math in a rough fashion suggests that, at a linear erosion rate of about 1 foot/year the bluffs deliver on the order of 25,000 cy/yr of sediment to the coastal zone. This equates roughly with what the ACOE estimated in their 1971 report as the contemporary (i.e. post-rip-rap) supply rate. But has it decreased over time? Figures published by the ACOE in 1971 suggest that the answer is yes, but they provide none of their methodology and no estimation of uncertainty in their measurements. Based on my rough calculation using their erosion figures, the bluffs would have been eroding at a mean annual rate of about 2.5 meters, or about 8 feet, per year to deliver the estimated volumes. Given that the Dungeness bluffs seem to erode at a mean annual average rate of around 1 foot year I view the high estimate as suspect. This is an area that is well worth additional research - finding the old surveys used by the ACOE, revisiting them, working out their uncertainties, etc.
There is more to cover here. Notably I haven't even delved yet into observations published by the ACOE that they used to "show" that the hook was shrinking. Maybe my next post. In conclusion, though, it should be clear that I am not yet sold on the story of Ediz Hook's erosion. Its worth noting again, though, that this doesn't mean that I am, by default, opposed to the engineering done on the hook. Even if its not about a sediment deficit and more about migration, the fact of the matter is that a strong case can be made that Ediz Hook needs to be kept in place - I just want the story to be right.