Do the observed settlements result from changes in the fabric of the clay layers?
Some good articles:
FixCAwater's own opinion is that the principal need is to make much more use of the aquifers in the San Joaquin Delta in order to provide storage for dry years and droughts, but that Fiorini is correct that selected additional surface storage might be appropriate, and that the fossilized bureaucracy should stop beating the dead horses at Sites and Temperance Flat and take a fresh look at other possible surface storage sites.
These comments were prepared by Bill Kier who writes "Before attempting to prepare testimony for PCFFA-IFR for those 2010 Delta through-flow needs proceedings I consulted with USFWS’ Patricia Brandes at some length . My question to Patricia was ‘how has the science on the matter completed since 1987 altered what we know of the Delta through-flow requirements of Sacramento River fall-run juvenile chinook salmon?’ Patricia went on at length how much better the estimation methods are these days – but, at the end of our conversation, I could only conclude that the situation is today as it was in 1987 – that if the goal is to meet the PFMC’s Sacramento River fall-run chinook salmon escapement goal it will require downstream migration flows past Rio Vista to the western end of Chipps Island of between 20k and 30 cfs for the critical weeks in spring. For the medium length version of the original recommendation, go to Kjelson and Brandes and for the longer version go to Kjelson.
Bill Kier also writes in response to a question posed by FIXCAWATER.COM: You ask what effect the development of Shasta Dam had on salmon. Well, for starters it blocked migration to several hundred miles of essential habitat, like the cool spring-fed streams needed by winter-run chinook salmon which enter freshwater in late winter/early spring and hold during the run-up to their late-summer spawning in the home-streams. That volcanic, spring-producing cold water habitat – the McCloud River in particular – was lost to winter-run. [the only thing like the McCloud available to winter-run today is Battle Creek – a $23 million, two-year (BurRec’s 1999 estimate) restoration project that BurRec has now stretched to 15 years and $133 million – the all-new BurRec’s custom of fondling environmental restoration projects to death] But, ironically, fall-run chinook salmon blossomed below Shasta Dam. I recall as a headquarters DFG biologist taking a delegation of Israeli scientists to Redding to see the magic we’d wrought – Shasta Dam was completed well ahead of the completion of the many irrigation facilities – delivery ditches, laterals, etc that would eventually take the reservoir’s water, so Shasta Lake would fill, be held during the summer and then the water above the flood control-reservation level would simply be dumped down the river after Labor Day. Absolute heaven for salmon - for a while. Now that Shasta’s customers have their distribution facilities in place, however, the drawdown of the lake in summer threatens the downstream water temperatures required for salmon spawning (particularly that for the late-summer spawning of winter-run chinook salmon) – which led to construction a dozen years ago of a $100 million temperature control device on Shasta Dam – something that had been justified in/argued for strongly in the then-U.S Bureau of Fisheries’ 1940 Special Scientific Rpt No. 10, ‘An Investigation of Fish-Salvage Problems in Relation to Shasta Dam’.
Probably not. The dramatic change in fabric from a flocculated structure to a dispersed structure that is shown in the SJMN graphic are familiar to us from CE273 which Prof. Jim Mitchell taught at UC Berkeley for many years. However, we think that it is likely a simplification of reality since we don’t see anything that is quite equivalent in Jim’s book “Fundamentals of Soil Behavior”, John Wiley and Sons, Second Edition, 1993. Nonetheless, this is the mechanism that leads to “quick clays” which result from leaching of clays deposited in a marine environment by fresh water and “changes in monovalent/divalent cation ratios”. That behavior is illustrated in the figure to the left. The specimen on the left of the photo has strength that results from a card-house type flocculated structure but it is unstable and when shaken collapses to the liquid on the right. According to Mitchell’s book “consolidation under pressure usually strengthens the structure through decrease in porosity and the formation of stronger inter-particle contacts. However, in some soils that possess bonding and cementation in their initial states, consolidation stresses greater than some critical value can break down the structure, thus causing weakening and collapse”. However,that applies more to silts and sands and is not so relevant here. In any case, you don’t have to call on these mechanisms to explain the observed settlements in the San Joaquin Valley.
Sediments like those that fill in the Central Valley usually have been deposited in complex cycles of deposition and erosion. Thus, it is hard to know what the maximum past pressures that have been felt by various soil layers might be without careful sampling and testing. However, in the areas of greatest subsidence, which correlate well with known high rates of extraction of groundwater, it is likely that the overburden pressures have been increased in the deeper soil layers past their maximum past pressures. The shallower soils, including the Corcoran clay, are likely be cycling under pressures that are less than their maximum past pressures. Down to at least three hundred feet below present day sea level the sediments would have been high and dry 105,000 years ago at the peak of the last ice age, when sea level was that much lower than it is at present. The Corcoran clay extends beyond that depth, but it is 600,000 years old and would have been subject to some degree of overconsolidation in each of the five ice ages. So, likely the observed ground settlements do result largely from compression of deeper clay layers, but not so much because they are thinner and consolidate more quickly as suggested by Sneed et al. and more because their maximum past pressures are now being exceeded.
The correct answers, more or less, are YES and NO!
But the otherwise excellent report on land subsidence along the Delta-Mendota Canal by Michelle Sneed and others from the US Geological Survey and the also excellent newspaper article by Lisa M. Kreiger in the San Jose Mercury News California Drought: San Joaquin Valley sinking as farmers race to tap aquifer, from which the graphic to the left is taken, do not get the mechanisms involved quite right. Hence the following clarifications.
Graphic from San Jose Mercury News
More detailed analysis of relevant issues.
This sounds ludicrous, and probably is, but the idea has been treated seriously in Overgrown Sierra forests gulping water that could flow to Valley by J.N. Sbranti in the Modesto Bee:
Numerous billion-dollar proposals to create more water storage in California are competing for attention and funding during this third year of drought. But there may be a less-expensive way to increase water flows into the Central Valley: Start thinning out the overgrown Sierra Nevada forests. Cutting down trees may not sound environmentally friendly, but researchers from UC Merced and elsewhere think that may be just what’s needed to restore forest health and increase water runoff. “It’s one of the lower-cost options (to increase California’s water supply) … and it also would reduce the probability of big destructive fires,” said Roger Bales, a UC Merced engineering professor who specializes in mountain hydrology. “There could be measurable and significant gains” – a hypothesized 9 percent increase in snowmelt runoff – if the forests are properly thinned.
Here's a thoughtful response from John Buckley, executive director of the Central Sierra Environmental Resource Center in Twain Harte, CA:
Over the past two years I have been at four different presentations or talks given by Dr. Roger Bales and/or Dr. Martha Conklin, the two researchers who are the focus of so much current media publicity. Before folks with strong views about forests or water attend the highly publicized day-long event on the topic of "Thinning For Increased Water,” it may be helpful to know a few basics that have come out of Bales and Conklin’s various presentations.
First, I believe that everyone will openly acknowledge that if there are fewer trees sucking up water out of the soil there will be some degree of increased water seeping downslope or flowing into streams. Second,, and if trees are thinned so that they spaced just far enough apart that snow lands and accumulates on the ground, rather than landing on the trees, more snowpack will accumulate between the trees than directly under the trees. (Think about tree wells at the base of trees when you go skiing or snowboarding).
So the basic claims are correct. Some amount of increased water can be created by doing a very careful thinning across a forest mountainside that spaces trees far enough apart that snow accumlates between, yet spacing the trees close enough together so that the trees provide some shade that limits the melting of the snow during the winter season.
But what Dr. Bales has admitted at meetings is (a) even if you aggressively thin a broad belt of forest to produce increased water supply, the increase is relatively small (sometimes described as 9-10%, sometimes more), and you only get that increased water supply if you ALSO treat all the brush, groundcovers, grasses, etc. in between the trees. Otherwise those plants still draw water from the soil, add to evapotranspiration, and reduce the benefits of doing all the logging to thin the trees. I heard him mention herbicides as one treatment, but however it is done, to remove much of the soil-holding vegetation between the trees is both expensive and potentially negative for watershed health, water quality, and wildlife. (b) When I talked with him about frequency of treatments, Dr. Bales acknowledged that to get anything but very short term benefit for water supply from thinning, you need to come back in within a few years to thin again because the crowns of the trees simply fill the gaps created by logging. So to get water supply increases, it would take a massive, massive increase in forest management expense to not only thin a huge percentage of a watershed (which would be needed to make any measurable difference in water supply). And it also would be necessary to come back in just a few years later and thin the forest again, and in the meantime, keep spraying groundcovers and brush with herbicides or keep shredding and masticating brush. Our environmental center has very strong concerns about widespread use of hercidides because the forest web of life is so complex and so delicate. But even going past the issue of herbicides … the cost of keeping vegetation controlled in between millions of trees across mountain slopes would be staggering.
Yet despite the major flaws in the strategy, the bottom line is that thinning is highly positive for opening up forest stands to get sunlight down to the forest floor, to benefit hardwoods and sun-loving conifers such as ponderosa pine and sugar pine, and to break up fuel continuity to reduce the rate of wildfire spread. Thinning without highly controlling all the rest of the forest vegetation can also slightly increase water supply. But to measurably produce more water in comparison to a control similar watershed, Bales and Conklin both acknowledge that it takes intensive thinning to end up with trees just the right distance apart. It takes treatment of surface fuel vegetation in between the trees. It takes coming in every few years to thin again. And even then, I heard Dr. Bales say that the amount of extra water produced may not necessarily be as much of an “increase” as the natural variation that happens every year based on variations of rainfall.
So, thinning is not a low cost, highly productive way to increase the water supply. But thinning can have some minimal positive benefit for water supply objectives if done on a large enough portion of a landscape with the right strategic prescription.
Last, at a recent water conference I attended with Dr. Bales, it is very clear that like any good researcher, one of his key goals is to find new funding that can allow him and his team to continue to do more research. Certainly that is a predictable expectation for someone who is enthusiastic about his current water study.
Do the observed settlements mean that there is a big decrease in the capacity of the aquifers?
No. If the observed settlements are mostly caused by consolidation of the deeper clay layers, which are aquitards bounding the acquifers, and are limited in thickness relative to the acquifers, then the observed settlements will have little impact on the volume of the void space in the sand and gravel layers that are actually the acquifers. There may be some compaction of the sand and gravel layers both as a result of loading past their maximum past pressures and as a result of cycling at lower load levels, but for every 10 feet of settlement it is likely that no more than about a foot is in the sand and gravel layers. It may be that for other reasons people find that they cannot recharge an acquifer as easily as they can pump the water out of it, but the pore space should still be there.
The 2013-2014 drought has shown that the Water Board's usual requirement of a Delta outflow of 11,00 cfs in order that X2 be kept in the vicinity of Chipp's Island is about right. During March 2014 when Delta outflows have been less than that, X2 has moved eastwards towards Sherman Island. But this "standard" speaks only to water quality and not to the flows that are required to move juvenile salmon out of the Delta and into Suisun Bay and hence through the Bay to the ocean. A nice summary of that problem is provided in theCLOSING STATEMENT OF THE PACIFIC COAST FEDERATION OF FISHERMEN’S ASSOCIATIONS AND THE INSTITUTE FOR FISHERIES RESOURCES CONCERNING THE DELTA THROUGH-FLOW CRITERIA NECESSARY FOR THE PROTECTION OF THE PUBLIC TRUST RESOURCE REPRESENTED BY SACRAMENTO RIVER FALL-RUN CHINOOK SALMON - submitted into the California State Water Resources Control Board’s 2010 informational proceeding to develop the flow criteria needed to support the public trust resources of the San Francisco Bay-Delta estuary. That testimony concludes:
PCFFA and IFR respectfully recommend – again, based upon science that your Board has already deliberated at length – that your Board adopt all of the following criteria necessary to protect the public trust salmon resources of the Delta ecosystem:
The more dramatic land subsidence is usually not reversible because it results from loading soils, both sands and clays, past their “maximum past pressure”, that is, the greatest pressure that they have ever been subjected to in the past. This puts the soil back on the “virgin consolidation line”. If you then unload and reload from a point on that line the soil will behave, more or less, in a nonlinear elastic fashion and the volume change for a given change in pressure is much less, say a tenth, of what it is for the same change in pressure on the virgin consolidation curve. We say “more or less” because repeated unloading and reloading cycles at less than the maximum past pressure will cause some settlement, but again, in relatively small amounts. All this applies to both sands and clays, although clays are much more compressible than sands. See figure to the right from Lambe and Whitman, “Soil Mechanics”, John Wiley & Sons, 1969. For clays there is another consideration, as you correctly pointed out in your talk – it takes time for the pore pressures in a clay to readjust to changes in the load so that in a fat clay layer the changes in loading caused by changes in the position of the water table might not be felt right away, and if the water table is cycled quickly, they may not be felt at all. But, once a clay is consolidated under stresses higher than its maximum past pressure, reduction of the applied stresses by recharging the groundwater will recover only a small fraction of the settlement that has occurred. This is the big problem in Venice for instance. The problem there is compounded by rising sea levels, but the basic problem is mostly irreversible settlement due to excessive extraction of groundwater.