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History PmWiki / OrovilleDamAndSpillwayFAQ Disclaimer: I am an engineer (born and trained), but am not a dam or civil engineer. The opinions and hypotheses expressed below are based my own observations of all available evidence, and using as much primary source material (original documentation/reports) as possible. I have no agenda other than to try to clarify questions for which there may be some confusion surrounding available facts/evidence. Glossary of abbreviations and terms

MSW = Main Spillway

ESW = Emergency Spillway

HPP = Hyatt Power Plant

DP = Thermalito Diversion Pool

TDD = Thermalito Diversion Dam

Tailrace = generally a pool of water into which turbines outlet their water. See below for relationship between Tailrace and DP levels.

RVOS = River Valve Outlet System

Invert = With tunnels the invert just means the lowest point of the tunnel. So if you see "tunnel invert at 230ft" it means the bottom of the tunnel is at 230ft elevation.

DWR = California Depart of Water Resources Q: Is the Oroville Dam going to collapse?

A: Available evidence suggests, No. Q: Really? I heard it had 1. seepage, 2. a massive wet spot on the face, 3. piping, 4. broken/clogged drain pipes, 5. a huge crack in its core ...

A: In turn All dams have some seepage (even all concrete ones), but the dam contains drainage pipes behind the impervious layer which catches water and runs it to water monitors.

Recent inspection reports have indicated that the seepage is within historical norms for this dam.

https://www.documentcloud.org/documents/3458959-Oroville-Dam-Inspection-Report-June-2014.html

https://www.documentcloud.org/documents/3458958-Oroville-Dam-Inspection-Report-2015.html

https://assets.documentcloud.org/documents/3458956/Oroville-Dam-Inspection-Report-August-2016.pdf 2014 Inspection report noted a small area of increased green vegetation where it appears water had seeped out in the past. At the time of the report in 2014, and 2015, and 2016 the spot was bone dry, but inspector recommended setting up monitoring there to see if it ever seeps again after normal non-drought year and whether it carries sediment.

Seepage alone is not generally an issue, unless it starts carrying sediment with it which can lead to piping. Piping is most often caused by some external influence (animal tunnels, rotten tree roots, etc) which leads to a new flow channel under a dam or weir and then as water flow through that channel increases it carries sediment with it, increases the size of the piping, which can lead to destruction of the dam. There is no evidence of piping at Oroville Dam. There is evidence to suggest that there may be piping under the Oroville MSW (see below). The broken/clogged drain pipes folks are most often aluding to are the original two Diversion Tunnels, created for the purpose of diverting the Feather River below the construction area of the dam itself. At completion of the construction, Tunnel #1 was purposely plugged with concrete at its inlet, and again in at its middle. Tunnel #2 was plugged in the middle, but through that plug are two 6 foot diameter pipes, each with an on/off ball valve and a variable flow cone valve. These valves are part of the RVOS, which was in fact broken in 2009, and repaired and operated in 2014 (See RVOS section below) Earthen gravity dams, as the name implies mostly consist of earth (really highly engineered soil) and are literally held down by their own weight and the weight of the reservoir water on their low angled front face. However to prevent them from ever moving horizontally over time there is usually what's called a core block. This is a big irregular shaped block of concrete that is anchored to the bedrock and above this block the central impervious layers are built up vertically or at an angle. During construction they were piling compacted materials on top of the core block at a greater rate than they were placing earth on the reservoir side of it, and this caused a lateral imbalance of forces which tried to tilt core block, which then caused a crack to develop. They filled the crack with grout and continued with a more even distribution of loading. The core block sits at the very bottom of the dam and is not directly responsible for blocking water flow through the dam. That is the job of the impervious layers above it (as well as the impervious layer of the coffer dam they built in front of it, and then filled all around. The cofferdam becomes part of the main earthen dam).



Notice how small the Core block is relative to the total size of the dam. Notice also that there are impervious zones protecting the front of the core block connected to the impervious zone above it, plus the additional impervious zone of the original cofferdam with it's own core block. Q: I heard that now that the water has risen to 900ft level, it has compromised the dam because they never let it get that high. It's still going to fail right?

A: No. They run res level up to 900ft most non-drought years. It is normal for this dam (and most others with a similar dual purpose of flood mitigation and water supply) to run their level right near the top. In non-drought years the normal procedure for operating Oroville Dam is the following throughout a "normal" year. Start of Flood Season (Nov/Dec in Norcal) - Usually the water level is still low following end of summer demands, but starts to rise with rain and low demand.

Middle of flood season - Rain and snow runoff has increased the Res level to around 850-860ft. At this point they will start spilling water over the MSW to keep the level between 850-860ft, to maintain a healthy buffer against big storms.

BIG WARM WET RAINSTORM during flood season (see Jan 1, 1994 or Feb 7, 2017) - The inflow rates increase dramatically and they begin spilling at 50-60K CFS. If storm continues more than a day, they may ramp spill up to 100K CFS. Only in most dire conditions will they go above 100K CFS outflow, but need for this is rare. Note: Generally if net inflow (inflow - outflow) is running +50K CFS over a full day then the res rises about 10ft, so starting at 850ft level provides 5 day buffer against a storm that produces 150K inflow for 5 days with 100K outflow, or 100K inflow for 5 days with 50K outflow. After 2nd day, they'll usually raise outflow to match or exceed inflow. (based on historical CDEC data) End of flood season. Water has been managed between 850-860ft for say.. Jan/Feb/March. By April chances of big wet rainstorms drops dramatically so they'll usually stop spilling, and let the res level rise. The later into spring they go, the less chance they'll spill even if res level continues to rise due to snowmelt. By June the res level may be above 890ft, but inflow is so low that they can hold it there using only the 0-13K CFS outflow from HPP.

Mid summer - Demand is high so HPP runs at full and draws level down

Late summer into fall - demand continues to pull res to its lowest levels. In drought years this can be down in the upper 600ft levels. Non drought years 700-800ft levels.

Rinse/Repeat. This diagram shows the Oroville res level along with inflow and outflow for the last 25 years.



Notice that level is run up close to 900ft more years than not. The small squiggle in the middle of the rise each year represents the short winter flood season when they hold it near 850-860ft. TL;DR - In normal non-drought years, level is managed with spillway at 850-860ft during flood season, and managed near full (890-900ft) during summer water use season. Q: Doesn't the emergency spillway over-top at 900ft?

A: Nope. Top of Emergency spillway is at exactly 901ft elevation. 900ft is the "normal" level of the res per original design specs. Q: What is RVOS and how did it break, and is it still broken?

A: It needs some explaining

RVOS system, purpose, history, failure and repair.

There are three primary ways to release water in a controlled fashion from Oroville Reservoir but each works through different res elevations and at different rates, and for different purposes. (note, emergency release is not covered here)

MSW releases water at 0-150K CFS from res elevation 901ft (top of ESW) down to absolute minimum of 813ft which is the bottom of its gates (they would never normally run MSW below 835ft to prevent scouring).

HPP releases water at 0-13K CFS (with only 5 of 6 turbines operational) from 901ft level down to the bottom of its inlets at 650ft (where max flow rate would be much reduced).

If the water level drops below the 650ft level then both MSW and HPP are non-functional, and this is where the RVOS takes over.

The RVOS inlet is at 230ft level (about 20ft above original flow level of the Feather River before dam existed) and has a secondary inlet at 340ft (in case primary is ever clogged by sediment).

RVOS releases water at 0-5400 CFS with res elevation at 900ft, max of 4000CFS with res elevation at 650ft and less below that.

It pulls from the very bottom of the res with an inlet tunnel invert at 230ft, and is used primarily to maintain at least a 2000CFS flow rate into the Feather River to satisfy minimum downstream demands during drought conditions. It is also used to introduce cold water into the Feather River during drought conditions (when both res and river water warms up). RVOS is not meant for flood control, as its release rate is too small, and can't be run at full capacity along with the HPP (which releases at a higher rate anyway). Think of MSW as being for flood control

RVOS is for drought control and temperature regulation

HPP for long term res elevation trim and power generation

ESW only for extreme emergencies (only used once in lifetime of dam) RVOS failure in 2009

Because there is up to 300psi of pressure on the water that comes through the RVOS valves, it is released at a very high velocity even at low flow rates which can cause problems with the way the water flows in the half filled Tunnel #2. To minimize these problems there is is an angled steel faced ring around the circumference of Tunnel #2 just downstream of the RVOS cone valves which catches the high speed water flow and deflects it back inward at a lower velocity which in turn helps reduce the overall flow velocity through Tunnel #2 outlet. It's appropriately called an energy dispersion ring or baffle.

Due to general decay of the energy dispersion ring and some damage caused early in the life of the dam, they removed it early in 2009 (presumably with intent to replace at a later date) For reasons unknown, late in 2009 Oroville Dam management decided to conduct a test of the now compromised RVOS system.

According to an article which described a report of the incident

https://www.newsdeeply.com/water/articles/2017/03/08/key-oroville-drain-plugged-as-heavy-storms-pounded-the-reservoir "DWR removed the baffle ring in April 2009, four months before the test. It did so without consulting its Division of Dam Safety, which should have approved that action first. The test was ordered because DWR “wanted to determine the destructive effects” of opening the valves to 100 percent, according to the investigation."

Besides the original design specs calling for RVOS to never be operated without the energy dispersion ring (following *extensive* scale testing), they had been previously warned to never run Oroville's RVOS at 100% due to pre-existing damage to ring early in dam's history. Whatever the reason, 5 people went into the RVOS's manual valve control chamber, and started to ramp the RVOS flow up to 100%.

At about 85%, a massive vacuum was formed at the head of the tunnel where the RVOS cone valves outlet their water.

Side Note: Suspected reason for vacuum - The original design specs for the RVOS showed that without an energy dispersion ring in place, high velocity water flow in the tunnel would cause a hydraulic jump some distance downstream. This is where the water goes from flowing very fast along the floor of the tunnel to suddenly piling up into a huge standing wave that fills the tunnel all the way to the top. Tunnel #2 has a venting system designs to equalize air pressure between the RVOS valve outlet chamber and tunnel downstream of the turbine outlets in case the tunnel was every blocked by a wave between them. I suspect that the hydraulic jump formed immediately underneath the downstream vent opening in the ceiling, but due to high flow rate water it caused a siphon pump effect evacuating air from that vent hole (if you drill a hole in the side of a hose and run water through it at max pressure, instead of the water coming out of that side hole it will pull air into it. This is a siphon pump.). The vacuum in the vent tube extended back to the vent tube opening in the RVOS cone valve outlet chamber. There is a steel pressure relief wall between the RVOS manual valve control chamber and the RVOS cone valve outlet chamber designed to fail only if the RVOS valve control chamber were ever to flood and exert at least 15ft of head pressure on the back of that wall (which prevents the water from backing up so far that it floods the access tunnel and ultimately the turbine room at the other end of that tunnel). Instead of that, the extreme vacuum on the downstream side of the wall ripped a very large hole in it and sucked it into the high velocity RVOS water flow. The 5 people in the valve control chamber now experienced hurricane force winds at their backs as it pulled air from the access tunnel behind them (ultimately leading back to the turbine room) where doors were either slamming shut or being ripped off their hinges and hurled into the access tunnel along with various other debris. One person was seriously hurt. Admist this chaos someone managed to get the RVOS turned off. RVOS system was now almost totally broken. RVOS Repair and operation for 2014 drought

In 2012-2013 in the middle of the California drought with no end in sight, they realized that res elevation might drop below the 650ft level which would stop the flow of water into Feather River. So they set about fixing the RVOS.

In 2014 they repaired the energy dispersion ring, tunnel damage, re-manufactured and replaced the two cone valves, and replaced the pressure relief wall with a better design. They installed remote controls for all the RVOS valves, and remote monitors in the RVOS valve control room and RVOS valve outlet chamber and placed the electronic control panels and monitors all the way back on the floor of the turbine room.

http://www.nwhydro.org/wp-content/uploads/events_committees/Docs/2015_Annual_Conference_Presentations/04-Tuesday/4-Anderson.pdf

Note: 125K acre-feet released through RVOS in 2014, and Lake Oroville dropped to 647ft in Nov 2014 due to drought as expected. The RVOS is functional (can be operated), but they currently are not using it for two reasons.

1. It's not meant for flood control, and won't be run with HPP at full capacity.

2. Due to an unrelated fire in the control room at Thermalito Dam, they have institute new fire mitigation practices which need to be applied to the RVOS's own controls in the turbine room. Q: Why is one outlet tunnel entirely below water and the other only half filled?

A: Most hydroelectric power plants are installed either just inside the downstream walls of, or just outside the walls of a dam and their outlets vent into open air. This provides the maximum pressure differential between the incoming water (with dozens to hundreds of psi of pressure at the inlet) and to 1atm of pressure at the outlet.

Also the outlet of a turbine cannot be flooded in such a way to fill a tunnel with high velocity water, or it can cause cavitation or the same type of conditions that led to the 2009 RVOS failure.

Hyatt Power Plant is special because it was carved into bedrock under the hillside a few hundred feet below the south side of the dam, roughly under the center line of the dam. This means the turbine outlet water at its tailrace still has about 2000ft feet to go to get out from under the dam into the Thermalito Diversion Pool.

The original Diversion Tunnel #1 was set right at the level of the Feather River to provide free flow of river water under the construction area.

It was planned that following construction they would create the Thermalito Diversion Pool which would raise the level of the river water above that of diversion tunnel #1 outlet, flooding the entire tunnel.

So they built Diversion tunnel #2 about 20 ft higher than tunnel #1 such that the eventual diversion pool level would reach back into tunnel #2 half filling it with water. During construction Diversion Tunnel #2 was rarely used, because it was well above the river flow, but during flood conditions the water would rise up behind the construction cofferdam and the excess water was then handled by Tunnel #2.

Following construction both tunnels were purposely plugged in the middle, and re-tasked to become the turbine tailraces.

As such, four of the six turbines outlet into the submerged tunnel #1, and two of them into the half submerged tunnel #2.

Half of the 6 turbines are also capable of pumping, so when they're operating in this mode, they draw water from the tailraces (pulling water from the DP) to pump back up into the reservoir.

There are pressure equalization tubes between tunnel #2 and tunnel #1 which prevents cavitation from occurring in both tunnels as long as tunnel #2 is never allowed to to fill to the top at any point along its length. Think of the combination of tunnel #1 and tunnel #2 as simply one big pool of water open to the air. They are in fact a direct extension of the Thermalito Diversion Pool and Tunnel #2's level is regulated by changing the level of the DP. Q: What's the relationship between Tailrace level and Diversion Pool level? I see both values in the daily incident report, and they're not the same.

A. Per above, both HPP and RVOS can only be operated at full capacity if Tunnel #2's level remains roughly half full. That level is 225ft (centerline of the tunnel) give or take a few feet. The diversion pool level is controlled by release of water 3 miles downriver at the Thermalito Diversion Dam. That is where the DP level appears to be measured. For a given HPP outflow rate the water in the tailrace will back up and the only way to lower the tailrace level is to lower the DP level to making the water flow out of the tunnel a little faster. That's why the tailrace level will always be higher than DP level. Q: Lately DP tailrace level has been a lot higher than DP level. Why is that?

A: This is due to the addition of the debris dam across the DP below the broken MSW and a backflow effect of MSW itself. The deeper and wider the DP is, the more consistent the water level is from one end to the other. The narrower and shallower the DP is, the more the water backs up at upper end, then flows fast (forms a small river rapid) over the debris dam, to a lower level at the bottom of the DP.

Following the February emergency spill and loss of the lower half of the MSW and huge chunk of the hillside, it deposited 1.7 million cu yd of material into the DP, damming it up quite a lot. It raised the DP level above the debris dam by over 20ft which completely flooded tunnel #2 making it impossible to operate HPP or RVOS, and had it continued to rise could have threatened the turbine room itself (another 10-15ft higher). Before the March spill event 1.2 million of the 1.7 million cu yds of the debris dam had been removed, but the spilling at 40K CFS carried some more debris into the DP (DWR estimated about 170K cu yds) and that combined with the constant 40K CFS outflow from MSW caused a backwater effect which raised the upper part of the DP about 7-8 feet above the desired 225ft level. That's why those incident reports listed Tailrace level at nearly 233ft, while DP level (measured down at the diversion dam) at 222ft. All they could do to prevent it from going too high was to limit the number of operational turbines during the spill event. They ran 2 turbines during most of the spill, then 1, then shut HPP off entirely for a couple days before the end of the spill for maintenance, before ramping up to all 5 turbines at the conclusion of the March spill event. Q: How did it all go so wrong in early Feb 2017? Note: This does not include speculation about what they may or may not have known, been told, or suspected beforehand. The assumption here is that DWR thought that both their spillways could handle a large flood.

Here's CDEC data for 300 hours in February https://cdec.water.ca.gov/cgi-progs/queryF?s=ORO&d=17-Feb-2017+00:45&span=300hours covering most of the event.

Following is a rough timeline of events. Feb 5 - Res level well regulated at 849ft and they're spilling at 30K CFS, matching or slightly exceeding inflow. It starts to rain. Feb 6 - Res level 849ft. By 3pm an inch of rain has fallen and inflow has increased into the 40K CFS range, so they increase outflow to 50K Continues to rain Feb 7 - midnight - Res level 850ft, and another 1.16 inches of rain (and probably more in the mountains). Inflow at 67K and rising. 9am - res level 853ft. another 0.4in rain, outflow still 50K but inflows are now just over 100K CFS. somewhere between 9am and noon they see the hole opened up in the MSW noon - Res level 855ft. no local rain, but inflows at 130K CFS. MSW shut off completely so they can examine the hole. Outflow drops to two turbines at HPP (5K CFS) Feb 8 - 4pm - res level 870ft, +1 inch of new rain, and inflows which have been consistently over 75K CFS again increase to just over 100K CFS. Note: Every +50K net inflow over a 24hr period raises the res level about 10ft. 4pm - They are forced to turn the MSW back on despite the hole. Outflow increases to 30K, inflow 103K They run MSW for only 3 hours 7pm - Res level 871ft. They see the hole in MSW rapidly head cutting upstream and have to shut off MSW(or reduced rate) again. Outflow 12.7K which is probably HPP at max. Inflows still over 100K CFS. Nothing in the forecast looks good. 11pm - Res level 874ft. no choice but to run the MSW again, so they bring outflow rate back up to 32K. inflow 120K CFS. Feb 9 - 6am Res level 878ft. still raining, inflow 110K and they turn MSW off briefly to examine the extent of the head-cutting. HPP at 12.5K Note: I know the MSW was turned off for some period on the 9th, because there's a photo of it off with an enlarged hole. noon - Res level 881.6ft - inflow 121K CFS. They bring MSW back on for total outflow of about 47K CFS. Bounces between 47K, 44K 37K 41K over next few hours. 7pm - Res level 887ft. +2 inches of rain in last 12 hours inflow peaks at 190K CFS. outflow only 42K CFS Feb 10 - 1am - Res level 891.5 ft. Inflow at 170K CFS. Outflow increased to 55K CFS. 3am - Res level 892.5 ft. Inflow at 155K CFS. Outflow increased to 65K CFS. Inflow never drops below 105K CFS rest of the day. Outflow is dropped to 55K CFS. Falling behind rapidly Feb 11 2am - Res level 899.88ft. Inflow 106K CFS, Outflow still 55K CFS.. 8am - Res level tops 901ft, and starts flowing over the ESW. Inflow around 90K, but inflow numbers become unreliable here because ESW is uncontrolled release, so no longer can track net outflow, and inflow is a calculated value that depends on res elevation versus res capacity minus known outflow. inflow rates while starting with a 750K acre foot buffer capacity so you'd never really see that full rate go over the spillways, but either way the designers thought that the ESW could handle *a lot* of water. Erosion on the hill was always expected below ESW if it were ever to be used, but ESW itself was designed solely to prevent total dam failure, not to prevent downstream flooding. Let's pause here to understand why DWR didn't originally think there was a big problem when it overtopped ESW. The original design specs for the dam called for use of the Emergency Spillway during a hypothetical maximum probable flood (10K year event) with 624K CFS inflow. The brass info plaque on Oroville dam, https://3.bp.blogspot.com/-20k2iJvIQkI/WKNQFPMxw3I/AAAAAAAAEtA/6YBnIeVTuecmtxqACT_dUaA3aNzawOmsACLcB/s1600/IMG_4328.JPG claims the two spillways together can accommodate 650K CFS. I personally think the plaque misinterprets the design specs which referred to maximumrates while starting with a 750K acre foot buffer capacity so you'd never really see that full rate go over the spillways, but either way the designers thought that the ESW could handle *a lot* of water. Erosion on the hill was always expected below ESW if it were ever to be used, but ESW itself was designed solely to prevent total dam failure, not to prevent downstream flooding. Feb 11 - 3pm - Res level 902ft. There's now over a foot of water flowing over the emergency spillway. MSW+HPP is still only going at 55K CFS, and true total inflow and total outflow are unknown (maybe someday someone will calculate the flow rate of 1+ft over the ESW of a given length).

Feb 12 4pm - Res level 902ft. For 24 hours, a foot of water has been going over the ESW, and the downhill erosion has caused head cutting which is rapidly moving uphill toward the base of the ESW.

Note: The very first time I saw the ESW (before it over-topped), I said to myself "Why don't they have a longer concrete run-out at the base of the ESW?" Almost every other ESW I've seen has a long horizontal run out often terminating in a massive pile of rocks to slow the flow and prevent erosion. Note: By this time the lower portion of the MSW is pretty much destroyed, a new flow channel has been formed, but there is a significant piece of good news. They've detected no significant additional head cutting at the end of the broken MSW. When the hole was smaller, the water would churn around in the hole eroding upstream. Once the MSW had broken off and the high flow rate created a giant deep plunge pool the water simply flew off the end of the break in the MSW into and even over the plunge pool and stopped head cutting back under the end of the MSW itself. DWR fearing that the ESW may be undercut which would cause a potentially catastrophic failure now triggers two significant actions. Local emergency services initiate a mandatory evacuation order for Oroville area DWR increases outflow rate from MSW to 100K CFS, calculating that destruction of the upper MSW has lower odds (due to minimal observed head cutting) than destruction of the ESW (which is showing rapid head cutting). Feb 12 9pm - Res level 900.7ft, just below top of ESW. Controlled outflows 100K. Inflows unknown.

DWR almost immediately starts work filling holes below the ESW and other nearby flow channels with rocks, first with helicopters and then over the next few days starts building roads into the hillside area so they can begin covering ground with shotcrete and later regular concrete.

Feb 13 9pm - Res level 893ft. outflow 100K. Inflow 40K

As long as MSW doesn't deteriorate further and it doesn't rain again, situation is stabilized a bit.

Feb 16 9am - Res level 868ft. Outflow 97K and droping. Inflow 16K.

Feb 17 midnight - Res level 863ft. Outflow steady at 80K CFS. Inflow risen to 25-30K due to half inch of additional rain.

Feb 18 Res level 856-853ft. outflow dropped to 55K

Feb 19 Res level 853-850ft. outflow ramped back up to 60K due to 1.5in rain on previous day

Feb 20 Res level 850-850ft. outflow 60K. Inflow averages about 60K for the day due to another 1.25 inches of rain on top of snow melt

Feb 21 Res level 850-852ft. outflow 60K. Inflow averages almost 77K for the day. additional 0.84 inches of rain

Feb 22 Res level 853-852ft. outflow 60K. Inflow 55K. Still handling rain from previous day.

Significant rain finally stops and inflow drops from 55K down to less than 20K over next few days.

Feb 24 Res level 847ft. outflow dropped to 50K

Feb 27 Res level 839ft. outflow droped to zero.

From Feb 28 to March 17th

DWR does most of rest of work armoring the hillside immediately below ESW

Crews perform following work remove 1.2 million cu yds of debris from the DP

Shotcrete the slope immediately below the break in the end of the MSW

drill, grout and anchor the MSW to the rocks near the end of the MSW

Cut some of the hillside back immediately below the MSW to limit erosion

Patch cracks in the MSW March 17

Res reaches 865ft after a couple weeks of relatively good weather with mostly snow melt and some light rain.

They begin second spill, ramping MSW up immediately to 40K CFS, then 50K CFS, then back to 40K CFS for remainder of spill.

HPP is taken mostly offline then at first, brought back online with 2 turbines running for most of the spill, then one, then off for day or two before end of spill for maintenance.

March 27

Res level 836.4ft. End of spill they perform the following steps Ramp down from 40K to 35K starting at noon, and hold for two hours.

Ramp down from 35K to 30K and hold for two hours.

Ramp down from 30K to 0K, while simultaneously bringing HPP up to full capacity (about 12K CFS) and also increasing outflow rate from Thermalito Afterbay so that downstream river release ramps down gradually over a couple days to 12-13K CFS. Presumably the flow tests at 35K and 30K were done to see if they could safely run the MSW at these lower levels during the next spill event without causing additional erosion on the slope immediately below the break in the MSW. They knew the water cleared it at 40K flow, but would probably like the option to run at a lower flow. Q: Don't they want to drop the res elevation as fast as possible? Why would they want to run at lower flow levels?

A: No. What they want to do is minimize the number of startup/shutdown cycles during which the maximum erosion below the break occurs. They also want to handle the maximum volume of snow melt they can during any given spill event. They know they will exceed the snow melt inflow rate by a wide margin while spilling at 40K, so the res level drops quickly to 835ft when they must shut off the MSW to prevent excessive scouring at the inlet of the MSW gates (shallower the water there, the fast it moves). So what they'd like to do is be able to run MSW at a lower rate for a longer time, so that more total snowmelt occurs during the spill event. Ideally they'd handle the majority of it during only one more spill event, rather than having to do two. That means they'd like to see warmer temps, more snow melt now with higher inflows during spill, or a longer spill at lower outflow (slow and steady wins the race). Q: A MSM news story quoting an AP story (cited no primary source) claimed that Dam experts said "Damaged Oroville Spillway Posing Significant Risk" and that they said Spillway must be entirely fixed or replaced by Nov 1 2017.. Is that possible?

A: No. And it's not really what the experts said.

DWR brought in a small group of dam experts who created a report which was published on March 17.

https://cbssacramento.files.wordpress.com/2017/03/oroville-dam-document.pdf

It's a very good read, and I highly recommend reading the whole thing, now. What they said was, "A very significant risk would be incurred if the Gated Spillway is not operational by November 1".

There's a big difference between "operational" and "fixed" or "replaced".

What they're saying is that there must be a way to do controlled spills by next flood season, but that that may mean some sort of temporary fix would be in place. DWR confirmed at March 27 press conference that preliminary plans may include only a temporary solution and they're ok with that for next season while they continue work on a permanent solution.

The dam experts make a couple recommendations themselves, but my own best guess at a viable plan looks something like this. Drill, fill, patch, and anchor the remaining upper MSW quite thoroughly. Use ground penetrating radar and drilling to find and fill voids under the deck. Patch cracks, holes etc. The rock anchors they use work in conjunction with grout so you drill, then add grout through the inside of the bolt which expands into cracks and voids. Can see if this has a positive effect by running a test spill and see if flow from the side drains is significantly reduced. Cut the softer red rock slope immediately below the break in the MSW back to a much steeper angle, and then construct a proper smooth sloped concrete wall (or steep spillway depending on how you look at it) from bedrock up to the lip itself. If done properly this would allow them to run the MSW at much lower flow rates (15-30K) without incurring additional head cutting. Cut the red rock cliffs walls on the south side of the flow channel back to be less vertical, away from the top edge of the grey bedrock channel a little, and then shotcrete about 15-20ft up the sides above the bedrock. What we've seen from overflight vids is that at 30-40K CFS the water is already mostly contained inside the bedrock flow channel. At 50K+ it starts to lap up onto the red rock cliffs which erode more quickly. If they can be armored they can probably get the max sustainable flow rate up to about 60K, and run it up to 100K for emergencies knowing there will be some additional erosion, but not bring the entire hillside down into the DP. Those three remediation steps could get em through the 2017-2018 flood season while they simultaneously work on a more permanent replacement.

The following is total speculation, but I suspect that a permanent fix would start by removing what remains of the lower MSW that is still anchored to the grey bedrock slab, and then they can start building the new MSW from the bottom up across that bedrock.

After flood season in 2018, they would then work on either tearing out and replacing the entire upper half of the MSW, or filling the plunge pool and linking the old upper MSW to the new lower section. Maybe they would attempt both at once, but I think there's still an option here to do this over 3 off seasons instead of 2.

Somewhere in this period they need to bring the res level down below 813ft so they can do some work on the MSW gates (at minimum replace leaking rubber seals, and maybe replace the gates entirely). Q: Can't they just use big pumps to move water out of the res?

A: No. There don't exist any portable water pumps in the world big enough.

The main spillway at flood levels runs at 40-100K cubic feet per second, where 1 cu ft is 7.4 gallons so 40K CFS is nearly 300K GPS and 100K CFS is nearly 750K GPS.

A typical large portable pump runs about 28,000 gallons per minute which is only 62 CFS.

It would take 645 of those pumps to get to 40K CFS.

The largest water pump in the world is still being constructed in New Orleans at the new Gulf Intracoatal Waterway West Closure Complex and it does a whopping 150K gallons per second.. which is still only 20K CFS.

This is a pump complex the size of a large building that only has to lift the water a few feet. Q: Can't we just start a siphon in a big pipe?

A: No. The vacuum in a siphon tube can only lift water a maximum of 32ft. Beyond that the weight of the water is too great.

The only way to lift water greater than that height is to put a pump near the water and push it uphill. Q: Can't those those massive canals and tunnels in the California Aquaduct system handle that volume of water?

A: Nope. They max out around 10-11K CFS. Q: 40-100K isn't really that much water is it?

A: Flow rate of Niagra falls, about 84K CFS

Flow rate of the Nile river, about 100K CFS

Normal flow rate of Colorado River through the Grand Cayon, about 15-20K CFS.

40-100K is *a lot* of water. It is a big river, in full flood stage.

The MSW was designed to manage a controlled flow of up to 150K CFS. Q: Why doesn't DWR put a live web cam on the main spillway?

A: I can only speculate that it's because their job is to build/operate/maintain/fix/replace dams and other water works, rather than do the job of the media. Q: Why doesn't Parks & Rec who run the web cam behind the dam put one on the front?

A: Their cam sits at their visitor's center and looks out over the recreational side of the dam. They have no reason to look at the spillway side of a dam. Q: Isn't DWR hiding all information from us?

A: There may be some specific info you're interested in they haven't released, but they do release Daily incident reports at http://www.water.ca.gov/recent_news.cfm

A new drone video every day or two on YT (33 vids so far) https://www.youtube.com/playlist?list=PLeod6x87Tu6eVFnSyEtQeOVbxvSWywPlx

A regular stream of new photos on https://pixel-ca-dwr.photoshelter.com/galleries/C0000OxvlgXg3yfg/G00003YCcmDTx48Y/Oroville-Spillway-Incident

some or all of which they often reference on their Facebook page https://www.facebook.com/CADWR/ Q: Why won't DWR show us that X thing that everyone is talking about?

A: I don't know. Q: What is Cavitation?

A: Cavitation is when a solid mass of water (having little to no air in the water) is forced by momentum to pull away from a hard surface leaving a vacuum between the water and that surface. Q: Why is cavitation bad?

A: That vacuum will eventually pull the water back in to collapse the vacuum bubble often smashing the water against the hard surface with more power than that of the original moving water. A practical example of this can be demonstrated at home with an empty long necked glass beer or pop bottle, with adult supervision.

Fill it about 90% full. Hold it by the neck firmly with one hand somewhere where you can safely make a glass mess. Then slam your other hand down over the open top of the bottle with moderate force. This drives the bottle downward. The water temporarily remains motionless and pulls away from the bottom of the bottle leaving a vacuum pocket. The vacuum pulls the water back to collapse the vacuum bubble with tremendous force, and it smashes into the glass so hard, it'll usually break the bottom of the bottle out. Q: Why is cavitation an issue for dam spillways?

A: They move a lot of water and if it ever moves in a smooth laminar flow (not mixed with air), then cavitation bubbles can form downstream of any type of irregularities in the surface of the spillway. The most famous example of this is the Glenn Canyon Dam Spillway Failure of 1983.

The first time the emergency spillway tunnels were used, at only 20% of their maximum rated capacity, cavitation bubbles formed below a small irregularity in one of the tunnels. The collapse of the vacuum bubbles caused pitting of the concrete a few inches beyond the initial irregularity. The pitting disrupted the water flow causing a larger cavitation bubble, which caused greater pitting a few inches further along, and this effect repeated and grew until dozens of feet downstream it ripped the concrete walls of the spillway apart, and dug a large cavity into the solid sandstone foundation. See Parts 1/2/3 of this video series. https://www.youtube.com/watch?v=dHpKvQ9XHV4 which shows the problem, symptoms and eventual massive repair operations. Q: Did cavitation cause the Oroville Dam Main Spillway Failure?

A: Probably not. Cavitation requires that the water be relatively free of air. If there's air in the water then it flows quickly into the area where a cavitation vacuum bubble started and raises the pressure well above vacuum, so when the bubble collapses the air acts as a cushion. The water flowing down Oroville's Main Spillway is relatively well aerated due to turbulent flow. Q: So what exactly caused the Oroville Dam MSW failure?

A: May never know exactly but the March 17 report has some ideas.

https://cbssacramento.files.wordpress.com/2017/03/oroville-dam-document.pdf

They suggest that there is excessive water flowing beneath the Spillway deck, and that this compromised the foundation, possibly causing piping along existing deteriorated clay drainage pipes which carried away enough sediment to cause a large void, into which the spillway deck eventually collapsed.

They note that the side drains in the spillway walls always flow fairly heavily when the MSW is in use, and they stop when it is off and stop unevenly when the last bit of flow is directed to only one side of the deck (which they did to initiate repairs). They hypothesize that this means the majority of the water under the deck is coming directly through cracks in the deck itself, and not leakage under the gate structure. I have an extension to this hypothesis to explain why it failed where it did and perhaps why it's been a problem area before. It is unknown to me what type of rock the upper section of the deck is built on, but the lower north section was anchored directly to bedrock, and the middle section (now missing) was clearly built on top of the fractured red rock soil (which has all been scoured away). This type of rock is relatively stable (sort of like lego building blocks) as long as you don't force water into the fractures in the rock.

My suspicion is that a relatively moderate flow of water into cracks in the deck saturated the ground in the middle section, getting into those fractures in the rock and carrying away their "glue". This caused a small bit of vertical settling of the deck (doesn't have to be more than an inch or two) just uphill of where it was more firmly anchored to the bedrock. This would create a vertical lip between slabs or at a crack where the downstream edge of the lip is higher than the upstream edge. Fast moving water hitting this lip would be directed both upwards and downwards through the crack at high velocity into the soil, rapidly increasing erosion and piping. Once the piping found a route out to the side of the spillway it quickly carried away enough material to form a large void into which the deck started to collapse. Q: Is the grey bedrock below the break in the main spillway eroding away?

A: The solid grey bedrock appears to be eroding quite slowly if at all. There's a few things that support this conclusion.

I created a photo album with various borrowed pic which shows the original position of the hole on Feb 7th, then again how it had grown on the 9th, and later after everything had eroded away.

https://goo.gl/photos/NPWDc8KVYfSSzfRm9

Matching up the positions in each pic, it appears that the grey bedrock was already visible on the left side of the hole in the first couple pics, and despite a couple weeks of running at 50-100K CFS in Feb after the initial failure, and 10 days at 40-50K during 2nd spill, the grey bedrock that was exposed in the original hole, has not changed or eroded significantly. It has simply been scoured clean of the red rock that was on top of it, along with most other debris including most of the lower MSW.

Comparing DWR's MArch 2 drone video from before the March spill

https://www.youtube.com/watch?v=TpXtMZwpDqs

with the March 27 drone vid immediately after the spill

https://www.youtube.com/watch?v=ixTg5Tgzeus

the main differences are, that where the new flow channel had lots of jagged corners and a bunch of massive debris on March 2, by the end of the March spill, most of that debris is gone and a lot of the sharp corners have been rounded off. The lower section of the flow channel got a little deeper, mostly by removing loose debris, but overall the bedrock channel is pretty stable. Q: But I've seen water cut glass, and steel and stone.. and the Grand Canyon..

A: Water cutters require an abrasive material to be added to the water before it can cut anything more than cardboard or foam.

This includes cutting rivers through rock to form canyons. They pick up particulate matter from upstream and use it as an unrelenting abrasive.

The water coming off the end of the broken MSW is clean with little to no particulates, so its cutting effect against the bedrock slab near the plunge pool is minimal. It may pick up some particulates on the way down the flow channel, but not long enough to have a major cutting effect. Q: What are they doing with the ESW? Isn't it beyond repair?

A: The ESW wall itself is sound. It carries a relatively low pressure, as there's only about 30ft of water behind it.

The only risk to the wall came from undercutting its base from erosion immediately downhill.

They've now armored the heck out of the hillside below the ESW, and built it up a bit so that it will collect water into a channel.

They've also added crosswise weirs in the flow channel which slow the water down as it goes from one step to the next.

The ESW in its current state could have easily handled the overflow rate it saw in Feb.

If MSW were completely out of commission for a Feb sized event and ESW had to handle 50-100K CFS or more by itself, that would probably still be a pretty dire situation, but should buy enough time for another mass evacuation. No dam can handle every possible flood, but it can buy time to get away. Interesting references and quotes

Current operational status of RVOS from Jan 1 2016.

From https://cwc.ca.gov/Documents/2016/01_January/January2016_Agenda_Item_7_Attach_1_SWPReview_FinalDraft.pdf

"The Commission was also updated on the River Valve Outlet System (RVOS) at Oroville Dam. Following an accident at Oroville Dam’s low level outlet works in 2009, DWR concurred in a 2012 Agreement with both CalOSHA and the International Union of Operating Engineers (representing DWR Trades and Crafts staff) not to operate the RVOS until the system was completely refurbished. DWR embarked on an accelerated refurbishment program to respond to concerns about operational needs during the ongoing drought. Personnel from DWR’s Division of Operations and Maintenance and Division of Engineering, along with DWR contractors, successfully refurbished, tested, and commissioned the RVOS for 2014 drought emergency operations which commenced in August 2014. DWR is currently utilizing the RVOS for Feather River fishery water temperature management; if the drought continues the RVOS may be the only method available to release cold water into the Feather River." http://www.westcoast.fisheries.noaa.gov/publications/Central_Valley/Sacramento%20River/2016_12_05_oroville_ferc_bo_final_signed.pdf

"Although not part of the original design intent of the River Valve Outlet System (RVOS) at Oroville Dam, the RVOS had been used for water temperature control in a handful of years over the 48-year operational history of Oroville Dam. This is because the valves provide access to cold water from the bottom of Lake Oroville under essentially any Lake Oroville water surface elevation condition. The Hyatt Powerplant intake was designed to control water temperatures taken into the plant and released to the Feather River. Through agreements the temperature criteria at the Oroville Facilities have been lowered over time. In some conditions where Lake Oroville is below about 700 feet elevation, the RVOS has be en used to blend colder water with the Hyatt Powerplant releases to meet downstream temperature requirements. " Original design specs and construction history of the Oroville Complex including all supporting dams.

https://archive.org/stream/zh9californiastatew2003calirich#page/n5/mode/2up

If you download the PDF, you can usually rotate the view 90 degrees to more easily view the diagrams.

Of special interest are

Page 82 - Diversion Tunnels which later become Turbine Tailrace tunnels and RVOS outlet.

Page 88 - Core Block Access Tunnel and drain pipes/pumps

Page 92 - Spillway (includes both MSW and ESW).

Take special note of the Standard Project Flood and Maximum Probably Flood discussion at the bottom of the page. Observed performance of Dams During Earthquakes

http://www.ussdams.org/wp-content/uploads/2016/05/EQPerfo2_v3.pdf Dynamic FEM Model of Oroville Dam

http://www.water.ca.gov/damsafety/docs/OrovilleDYNFEM.pdf

This document talks about the monitoring capabilities at Oroville Dam and the goal of building a computer model to match the actual dam's performance.

It also includes a great description of the original Embankment Materials including the specs of the engineered soil that went into the Impervious Zone. DWR Timeline of Major Events

http://www.water.ca.gov/oroville-spillway/pdf/2017/Lake%20Oroville%20events%20timeline.pdf "FLOOD!" describing the 1964 flood of the Oroville basin during construction of the dam

https://archive.org/details/x5flooddecember196161calirich

At one point the two diversion tunnels were releasing a total of 156K CFS and the coffer dam was at risk of overflowing.

Mitigating this type of flood is why Oroville Dam was built.

2014 RVOS document with many photos describes failure, testing and repair, and subsequent use during 2014-2015 drought emergency

http://www.nwhydro.org/wp-content/uploads/events_committees/Docs/2015_Annual_Conference_Presentations/04-Tuesday/4-Anderson.pdf

125K acre-feet released through RVOS in 2014, and Lake Oroville dropped to 647ft in Nov 2014. Article describing the 2009 RVOS accident

https://www.newsdeeply.com/water/articles/2017/03/08/key-oroville-drain-plugged-as-heavy-storms-pounded-the-reservoir

'DWR removed the baffle ring in April 2009, four months before the test. It did so without consulting its Division of Dam Safety, which should have approved that action first. The test was ordered because DWR “wanted to determine the destructive effects” of opening the valves to 100 percent, according to the investigation.' Had it before, but can't find it now.

Original RVOS design/testing document showing what happens with various different types of energy disspersal devices. Normalized Daily CDEC data from 5/1992 to 4/2017 including various graphs showing res elevation versus capacity, elevation versus inflow, outflow and elevation versus rain level, inflow and outflow.

For earlier years some of the data was missing.

https://docs.google.com/spreadsheets/d/14FG0PlDnxXJiqpcoPso9FVQ4JG_50xoSSraViWxMuzE/edit?usp=sharing California Nevada River Forecast Center rain/snowmelt/flood stage forecast for Middle Fork Feather River

http://www.cnrfc.noaa.gov/graphicalRVF.php?id=MFTC1

http://www.cnrfc.noaa.gov/graphicalRVF.php?id=MRMC1

What's interesting about this forecast is that it shows forecast for rain + snow melt.

Can also put in past dates, to compare a fast forecast against actual results. I finally figured out where the various MSW and ESW flow ratings come from.

This is a diagram from the original design and construction specs linked above and is supported by other text in the same document

The bottom left part of the curve shows the max flow rate of the MSW at various res elevations.

When res is below 813ft, MSW can't function.

Flow rate increases to the rated 150K CFS "controlled" spill at roughly 865ft level.

As the elevation rises, the max possible flow rate of the MSW increases, even though they would rarely choose to do so (has only been run at 160-156K CFS once).

When res elevation is at 901ft, MSW can spill a max capacity of about 270K CFS.

Above 901ft ESW starts to spill with an initial flow rate of zero, but increases with each foot above 901ft, while MSW capacity continues to increase above 270K.

For the "Maximum Probable Flood" (a 1 in 10,000+ year event with peak 770K inflow), the combined capacity of MSW and ESW is about 650K CFS, and at 920ft, the dam itself is overtopped.

Generally the assumption is that a massive flood event will start with some buffer capacity in the res (ideally 750K acre feet). Notes from April 6th DWR Press Conference.

On further review - made some changes below

5 foot increase in res elevation from two forecasted storms

inflows may get up to 39K CFS then come back down to 18K

small bump in inflows from 2nd storm

Water 197% of average. #2 wettest in 100 years.

Rain year through Oct 1, so can easily break record of 88in total. At 84 in now.

Snowpack 140%.

HPP max is around 12500 discharge, but will increase/decrease HPP outflow to keep running efficiently.. blah blah blah (no specifics) Plan dates

4 pre-qualified bidders

Design is not final

Little less than a 60% design proposals expected

Bids are due on April 12th

Execute a contract on April 17th Emergency response work ongoing

Sealing the cracks, rock bolting, pulling back the slopes to minimize erosion.

Emergency spillway - emergency response side is done.

Waterproofing HPP if DP levels were ever to go high again (as they did during Feb spill with huge debris dam) Plan, repair, recover, replace damaged structures.

Multiple phases to allow for contingency plans

First season Restore the upper gated control structure to handle nearly twice historical flow

Give upper MSW ability to handle 270K CFS. 160K is historical max usage Current plan for upper MSW, remove and replace drains, slabs and walls as necessary, and dig through minimal to moderate amount of foundation rock to ensure solid foundation.

Designed to modern standards.

Replace as much as possible in one season, but implication is may not replace all slabs first season, in which case reinforce to handle design flow through next season, then finish repair/replace next construction season

Lower concrete MSW will be demolished and replaced.

Eventually return lower MSW to original geometry

Replace concrete, with stronger reinforced

RCC = Roller compacted Concrete. Placed using large compactors.

Fill the plunge pool with RCC with intent to build and connect to a lower MSW chute capable of handling 100K CFS

Two contingencies plans were mentioned by Mr. Croyle and Ms. Cuttle. Cuttle: Buttress end of break in upper MSW with RCC wall down to plunge pool, reinforce plunge pool and natural flow channel.

Croyle: Attempt to connect upper MSW to lower MSW with lower MSW having only 100K capacity, but if they need to spill 150K CFS through MSW next season, then let it flow over the wall into the natural eroded spill channel

Total completion most likely in two construction seasons. Emergency Spillway - emergency response completed.

Much of work done will likely be removed and replaced.

Large concrete "cutoff wall" downstream of the existing weir anchored to bedrock.

Intended to stop head cutting.

Place large RCC "Buttress" against existing ESW weir, to help reinforce that wall.

RCC splash barrier between buttress and cutoff wall.

More RCC to guide water into central channel. Current work

Drilling upper MSW to gather info

Concept design (overview above) but need more Geotechnical information

Some core holes going 4-5 feet through concrete before they get to rock so they're finding more concrete than original design called for. Q: What are we shooting for by Nov 1?

Concept: Upper MSW should handle 270K CFS by replacing all/part of Deck and walls of MSW.

Lower chute: Croyle: "Fill up plunge pools" and duild up lower chute to handle 100K CFS for 2017/2018.. with contingency plan to overflow the engineered chute into the existing eroded channel, if they need to fly more than 150K. Q: How will downstream handle 270K CFS?

Downstream levees only able to handle 150-160K CFS.

Flood control structure + ESW designed to take extreme flood event (10K year event) total of 750K CFS to save the dam, not to prevent downstream flooding.

(Goal during extreme flood event is controlled downstream flooding versus uncontrolled wall of water due to destruction of dam).

(In 1965 when dam was built, it was designed to handle Maximum Probable Flood (10K year event) of 624K CFS inflow starting with a 750K acre feet buffer. Sounds like estimate of "maximum probable flood" has has been increased to 750K.) Q: about erosion below ESW

Distance to cutoff wall below ESW is yet to be determined.

Anchor to Blue metamorphic rock (what we commonly refer to as "solid bedrock", as opposed to the easily fractured red rock).

Water flows over ESW wall, over buttress, over RCC, over cutoff wall anchored to bedrock, and don't really care how much it erodes below cutoff wall, as long as it doesn't head cut behind cutoff wall. When can RVOS be used?

RVOS control systems (fire control mitigation) in place by May. Can be used then. (Note: RVOS was repaired and operational in 2014 and subsequent years, but temporarily taken out of commission to do fire control mitigation work on control system, after lessons learned from Thermalito power plant control room fire) Future DOC reports will not be released to public except possible details through Sheriff

First DOC report was never supposed to be released. Details of specific construction plans will not be released to public

Falls under classification of Critical Infrastructure https://en.wikipedia.org/wiki/Critical_infrastructure and therefore is considered classified material. Update as of 4/17/2017 Independent Root Cause Analysis of Oroville Spillway Failure with a ton of really informative photos.

https://www.documentcloud.org/documents/3673031-OrovilleSpillway-RootCauseAnalysis.html

It is clear that there was plenty of warning that there were problems with the MSW, had DWR been paying attention.

They ignored the fact that there was a tremendous amount of water draining through cracks in the spillway being collected and exiting out the side drains, but more damning is that there were two non-functional drains indicating water would not be able to escape, thus could build up pressure lifting a slab (which is most likely what happened). Update as of 4/25/2017 Mildly redacted BOC reports

http://www.water.ca.gov/oroville-spillway/bocreports.cfm Update 4/28/2017

Takeaways and analysis of DWR's April 27 Community meeting Some notable takeaways from this meeting. They stated that RVOS will be back to operational status in first week of May and can provide up to 4K CFS of additional capacity to be used in conjunction with HPP (we don't know if they ever run them both together at full, but they do sometimes run at same time).

They may start the next spill before res elevation rises to 860ft, but may also wait till 860ft.

They expect the res level may rise a bit following the end of the next (and last of the season) spill during the period they start construction, but are expecting that HPP+RVOS will be able to draw down the level through rest of summer, and expect the res elevation to drop below 835ft and stay below that through summer and into fall.

Their stated intention is to not be storing any water against the MSW gates during this period.

Their stated intent is to be able to pass up to 150K CFS through the upper gated spillway (MSW) by Nov 1. Lower MSW may only pass 100K, and any excess will drop into the natural flow channel. There was a rather contentious question by an audience member based on their flawed understanding of why DWR is running the MSW at *only* 35K CFS and not higher. This person asserted that the reason must be that they feel the MSW is so damaged that they fear running at a higher level. That is not the reason.

They also questioned how they expect to handle the total volume of expected snow melt water given a relatively small storage volume between 860 and 835ft. This is also based on the same flawed understanding. Let's work through the answer to both.

Q: Why is running MSW at a lower rate better than a higher one?

A: You can pass more total volume of water

Intuitively one would think that running the MSW at its highest rate to drop the res elevation as quickly as possible would be desirable. It is not. DWR's goal is to pass the greatest total volume of snowmelt water through the spillway during a single spill event and to minimize the total number of spill events. If they run the MSW at a very high rate (far exceeding inflow #s), then the res elevation drops quickly from 860ft down to 835ft and then they have to shut it off.

Let's say they could hypothetically drop the level from 860ft to 835ft in less than a day.

Res capacity at 860ft is 2.94M acre feet, and at 835ft 2.615M acre feet, so in that short period they'd push a little more than 325K acre feet over the spillway.

But what happens if they slow down and spill at 35K CFS for a full 14 days?

That's 35K CFS x 14_days x 86400_s/day = 42,336,000,000 cubic feet which is 971.9K acre feet of water of the spillway.

Thus by running MSW at a slower rate for a longer period of time they pass almost 3x as much water during the same 860->835ft drop.

This is not intuitive, and unfortunately Bill Croyle hasn't even tried to explain it, but I do think it might benefit them to try (maybe on the DWR web page).

Bill has repeatedly alluded to the same issue when he remarks that the temps are rising and snow melt increasing and says "that's a good thing".

He's right. If inflows were to rise closer to 35K CFS for a while, then it would allow them to run the spill even longer, and pass a greater total volume of water. Forensics Team Preliminary report

http://www.water.ca.gov/oroville-spillway/pdf/2017/Memorandum_050517.pdf

Mainly they're just identifying *possible* contributing factors, and have not yet laid blame on any specific combination of them.

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