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A highly respected ecologist (who probably deserves the title "Father of the Herring River Restoration Project"), Dr. Portnoy has assured the public – particularly the community that derives its livelihood from shellfishing – that the shellfish beds in Wellfleet Harbor are safe from being smothered by silt resulting from restoration of tidal flow to the Herring River estuary because sediment movement is "flood-dominant." Yet, he has not provided scientific evidence negating the opposite alternative – that silt flowing on the ebb tide could settle in the harbor to threaten shellfish beds.
In his email below, Dr. Portnoy asserts that "Big rivers like the Rhone carry the bulk of sediment downstream, simply due to the force of gravity on river water. But relatively small tidal rivers like Herring River are fundamentally different: the volume of seawater flowing in during flood tides is so much more than, and overwhelms, fresh river flow. That's why net sediment flow in the Herring River estuary is (perhaps counter-intuitively) upstream, carried by the upstream force of the semi-diurnal tidal wave."
To this assertion I point out that my concern about silt being deposited in Wellfleet Harbor is not related to net flow or sediment flowing during the flood tide (which Dr. Portnoy addressed), but silt carried downstream during ebb tides, when the tide and the river are flowing in the same direction." And so, the stalemate remains unresolved.
RG note:
- The most recent email is at the top of the thread below.
- A particularly compelling image of silt carried downstream to the Rhone River delta appears near the bottom of this page.
From: Gabel, Ronald A.
Sent: Saturday, May 9, 2020 8:32 PM
To: 'John Portnoy'
Cc: Martha Craig; Carole Ridley; Tim Smith; Bob Prescott; Spear, Stephen - NRCS, West Yarmouth, MA; Hillary Greenberg Lemos; Barbara Brennessel
Subject: FW: FW: Sediment
John:
Does your week-long silence signify that you agree (perhaps reluctantly) that the assertion "sediment flow is flood-dominant" tells only half the story – and that fine sediment flows downstream on the ebb tide and settles when flow rate slows to a critical level?
My request for scientific evidence still stands. If credible peer-reviewed scientific studies show that fine sediment does not behave as characterized above, please send me the citations.
Thank you.
Ron Gabel
_______________________________________________________
From: Gabel, Ronald A.
Sent: Saturday, May 2, 2020 7:23 AM
To: 'John Portnoy'
Cc: Martha Craig; Carole Ridley; Tim Smith; Bob Prescott; Spear, Stephen - NRCS, West Yarmouth, MA; Hillary Greenberg Lemos; Barbara Brennessel
Subject: RE: FW: Sediment
John:
Thank you for your prompt and detailed response to my request. Every time I ask you to comment on an issue related to salt marshes, you reward with an amazing outpouring from your font of knowledge. Your comments on sediment follow true to form.
I am particularly intrigued with your description of the ebb and flow of sediment with incoming and outgoing tides, including the influence of tidal duration on sediment deposition. (The marine origin of silts and clays dominating New England coastal marsh peat is also fascinating.) Of direct relevance to my concern is your reference to "net upstream transport of fine sediments," which implicitly acknowledges that downstream transport exists (and is consistent with your narrative about the ebb and flow of sediment).
The ebb phase of fine sediment flow is at the heart of my concern. We know that when the rate of flow of moving water decreases below a critical point, the force keeping fine sediment particles suspended is reduced to the point where the particles settle. We also know that when the route of flow substantially widens, as when a river flows into a harbor, the rate of flow suddenly falls.
This is the location of my concern about fine sediment flow in the Herring River – a concern that has persisted since a hydrologist who has modeled the Herring River revealed that his model could not rule out the settling of fine sediment in Wellfleet Harbor as a consequence of increasing tidal flows in the Herring River.
Please explain why my concern about the deposition of fine sediment in Wellfleet Harbor is unwarranted. And please buttress your explanation with scientific evidence. Your suggestion that I investigate expert consensus on these matters does not resonate with me because I am painfully aware from my experience with writing medical practice guidelines that policy makers resort to expert opinion when scientific evidence is unavailable to guide best practices. Therefore, to be convinced that increasing tidal flows in the Herring River will not result in the settling of fine sediment in Wellfleet Harbor, I need to see the scientific evidence, preferably in peer-reviewed scientific articles. However, even hydrodynamic modeling of sediment flow and deposition provided by reputable sources (preferably more than one) could begin to assuage my concerns. Can you refer me to such sources?
If I could be convinced by compelling scientific evidence that increasing tidal flows in the Herring River will not threaten the oysters in Wellfleet Harbor, my voice on this matter will fall silent.
Thank you.
Ron Gabel
_______________________________________________________
From: John Portnoy <jp.wellfleet@gmail.com>
Sent: Friday, May 1, 2020 7:22 PM
To: Gabel, Ronald A.
Cc: Martha Craig <mcraig@herringriver.org>; Carole Ridley <cr@ridleyandassociates.com>; Tim Smith <tim_p_smith@nps.gov>; Bob Prescott <rprescott@massaudubon.org>; Spear, Stephen - NRCS, West Yarmouth, MA <stephen.spear@usda.gov>; Hillary Greenberg Lemos <Hillary.Lemos@wellfleet-ma.gov>; Barbara Brennessel <bbrennes@wheatonma.edu>
Subject: Re: FW: Sediment
The widely recognized conclusion that many (but not all) of the estuaries of New England, e.g. Herring River, and throughout the world are flood-dominant certainly doesn't originate from me, but from the observations and analysis of physical oceanographers. Flood dominance, i.e incoming tides flowing faster on average than ebbing tides, of Herring River has been confirmed by extensive observations of tidal-water velocities throughout the tidal cycle. It can also be logically concluded given that the duration of flood tide is several hours shorter than the ebb and, with nearly the same volume of water going into the river above the dike as coming out, flood tidal currents have to average faster. Thus net sediment transport must be upstream. Particle tracking using the hydrodynamic model confirms this.
Strong evidence for the net upstream transport of fine sediments in particular is in the inorganic composition of New England coastal marsh peats: they are dominated by marine-derived silts and clays - typically 2-4 meters thick. In a nutshell, salt marshes in New England have depended on the tidal import of fine sediment over the past 4000 years of their development. The Herring River dike's blockage of fine sediment transport from the marine environment is a major factor, along with desiccation and consequent pore-space collapse and peat oxidation, causing severe wetland subsidence.
In general, I think that this notion of the Herring River "flushout" sediment comes from a confusion of tidal rivers with real rivers, like your example from Lake Geneva in landlocked Switzerland, obviously lacking tides. Big rivers like the Rhone carry the bulk of sediment downstream, simply due to the force of gravity on river water. (I spent two years working in the Mississippi Delta so I do get this.) But relatively small tidal rivers like Herring River are fundamentally different: the volume of seawater flowing in during flood tides is so much more than, and overwhelms, fresh river flow. That's why net sediment flow in the Herring River estuary is (perhaps counter-intuitively) upstream, carried by the upstream force of the semi-diurnal tidal wave.
I suggest that you carefully investigate expert consensus on a subject outside your professional experience before forming conclusions from a small sample of studies that may or may not be physically applicable. We're lucky in having a rich supply of world-class expertise in coastal science right here within the Cape scientific community.
I'm copying some colleagues who are devoting massive amounts of time and mental energy to getting this project right.
_______________________________________________________
On Fri, May 1, 2020 at 4:18 PM Gabel, Ronald A. wrote
John:
I believe you were a silent participant in yesterday's public hearing on the Herring River project; so you probably heard my comment in response to Carole Ridley's assertion that "sediment flow is flood-dominant." Because I believe you are the origin of that oft-quoted statement, I would be interested in your comments on my elaboration below on why I believe that statement is misleading.
Thank you.
_______________________________________________________
From: Jonathon Idman <jidman@capecodcommission.org>
Sent: Friday, May 1, 2020 9:58 AM
To: Gabel, Ronald A.
Subject: RE: Sediment
Good morning Dr. Gabel,
Thank you for the email: as I have done with your previous correspondence, I will include it in the public comment record for the Project and forward a copy to the applicant. The individual who made the request to you yesterday is Bob Prescott, who is apparently on the board of the Friends of Herring River (he is also the former long-time executive director, recently retired, of the MA Audubon Wellfleet Bay Wildlife Sanctuary).
Regards, Jon Idman
Jonathon Idman
Chief Regulatory Officer
Cape Cod Commission
3225 Main Street, P.O. Box 226
Barnstable, Massachusetts 02630
508/ 744-1260
_______________________________________________________
From: Gabel, Ronald A.
Sent: Friday, May 1, 2020 9:31 AM
To: Jonathon Idman <jidman@capecodcommission.org>
Subject: Sediment
Mr. Idman:
A participant in yesterday's virtual public hearing on the Herring River Restoration Project requested that I provide a scientific source for my assertion that sediment is not monolithic and that the oft-quoted statement that [all] "sediment flow is flood-dominant" is simply not true. I believe he asked me to provide scientific support for my statement that fine sediment sometimes flows downstream.
One article providing the information the questioner requested is titled "What regulates sedimentation in estuaries?" and is available at https://niwa.co.nz/sites/niwa.co.nz/files/import/attachments/estuaries.pdf. The following statements address the issue of the flow of fine sediment downstream:
"Fresh river water floats over seawater. So when sediment-laden floodwaters enter an estuary the finer particles that stay in suspension may be flushed out to sea quite quickly. But heavier particles sink to the bottom as the flow meets salt water. This is why sediment deposition is greatest near the upper reaches of an estuary."
That same article presents a simulation model of a New Zealand harbor in which most of the sediment flow is ebb-tide dominant, while only a small fraction is flood-tide dominant:
Another scientific publication titled "Sediment Transport and Deposition," which is available at https://www.fondriest.com/environmental-measurements/parameters/hydrology/sediment-transport-deposition/, contains the following image and caption that should lay to rest the misconception that all sediment flow is flood-dominant and that fine sediment is unlikely to flow downstream:
Sediment can be carried downstream.
(Photo Credit: NASA Visible Earth, via USGS)
Another image from the same article is even more compelling:
Sediment transported down the Rhone River into Lake Geneva
(Photo Credit: Rama, 2007, via Wikimedia)
This article also supports my assertion that sediment is not monolithic but is composed of different components having different behaviors. The authors state:
"Sediment particles come in different sizes and can be inorganic or organic in origin. These particulates are typically small, with clay defined as particles less than 0.00195 mm in diameter, and coarse sand reaching up only to 1.5 mm in diameter. However, during a flood or other high flow event, even large rocks can be classified as sediment as they are carried downstream. Sediment is a naturally occurring element in many bodies of water, though it can be influenced by anthropogenic factors."
A broad view of sediment transport and deposition is the foundation for my concern that increasing tidal flows in the Herring River might cause fine sediment, either laden with toxic chemicals or not, to smother or poison shellfish in Wellfleet Harbor. In a discussion with a scientist from the Woods Hole Group a couple of years ago following his presentation on the hydrodynamics of the Herring River, he admitted that the model he presented did not rule out the possibility that fine silt washed downstream on an outgoing (ebb) tide could settle out in Wellfleet Harbor.
Stay well!
Ron Gabel
P.S. Please share with me the name and organizational affiliation of the person who requested my sources. Thank you.
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