Insights into the Oroville Dam 2017 Spillway Incident

Abstract

In February 2017, a failure occurring in Oroville Dam’s main spillway risked causing severe damages downstream. A unique aspect of this incident was the fact that it happened during a flood scenario well within its design and operational procedures, prompting research into its causes and determining methods to prevent similar events from reoccurring. In this study, a hydroclimatic analysis of Oroville Dam’s catchment is conducted, along with a review of related design and operational manuals. The data available allows for the comparison of older flood-frequency analyses to new alternative methods proposed in this paper and relevant literature. Based on summary characteristics of the 2017 floods, possible causes of the incident are outlined, in order to understand which factors contributed more significantly. It turns out that the event was most likely the result of a structural problem in the dam’s main spillway and detrimental geological conditions, but analysis of surface level data also reveals operational issues that were not present during previous larger floods, promoting a discussion about flood control design methods, specifications, and dam inspection procedures, and how these can be improved to prevent a similar event from occurring in the future

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geosciences
Communication
Insights into the Oroville Dam 2017 Spillway Incident
Aristotelis Koskinas 1, Aristoteles Tegos 1, 2, *, Penelope Tsira 1, Panayiotis Dimitriadis 1,
Theano Iliopoulou 1, Panos Papanicolaou 1, Demetris Koutsoyiannis 1and Tracey Williamson 3
1Department of Water Resources and Environmental Engineering, National Technical University of Athens,
Heroon Polytechniou 9, Zografou, GR-15780 Zographou Athens, Greece; tel9021@yahoo.gr (A.K.);
tsira_p@hotmail.com (P.T.); pandim@itia.ntua.gr (P.D.); theano_any@hotmail.com (T.I.);
panospap@mail.ntua.gr (P.P.); dk@itia.ntua.gr (D.K.)
2Arup Group Limited, 50 Ringsend Rd, Grand Canal Dock, D04 T6X0 Dublin 4, Ireland
3Arup, 4 Pierhead Street, Cardiff CF10 4QP, UK; tbulley1@gmail.com
*Correspondence: tegosaris@yahoo.gr
Received: 9 December 2018; Accepted: 7 January 2019; Published: 11 January 2019


Abstract:
In February 2017, a failure occurring in Oroville Dam’s main spillway risked causing severe
damages downstream. A unique aspect of this incident was the fact that it happened during a flood
scenario well within its design and operational procedures, prompting research into its causes and
determining methods to prevent similar events from reoccurring. In this study, a hydroclimatic
analysis of Oroville Dam’s catchment is conducted, along with a review of related design and
operational manuals. The data available allows for the comparison of older flood-frequency analyses
to new alternative methods proposed in this paper and relevant literature. Based on summary
characteristics of the 2017 floods, possible causes of the incident are outlined, in order to understand
which factors contributed more significantly. It turns out that the event was most likely the result of a
structural problem in the dam’s main spillway and detrimental geological conditions, but analysis of
surface level data also reveals operational issues that were not present during previous larger floods,
promoting a discussion about flood control design methods, specifications, and dam inspection
procedures, and how these can be improved to prevent a similar event from occurring in the future.
Keywords:
Oroville Dam; spillway; incident; flood control; flood-frequency analysis; dam operation
1. Introduction
Dam construction and operation across the centuries has resulted from major and multipurpose
human needs and is linked to the development of human wealth, health and the growth of
civilization [
1
–
4
]. Spillways are a crucial aspect of dams, as their most important function is to
discharge excess flows during severe floods to prevent dams from failing due to overtopping [
5
,
6
].
The International Committee on Large Dams (ICOLD) has suggested that nearly a third of dam related
incidents is linked to this cause of failure. These cases are usually brought about by extreme weather
conditions that exacerbate faulty or incomplete spillway designs, leading to significant damage [
6
,
7
].
As such, reducing the risk of spillway failures is a topic that promotes continuing research and
improvement. Ongoing studies have attempted to investigate these structures both from a hydrologic
approach [
8
,
9
] and by looking into the various available design methods and materials [
10
–
12
].
The former studies found several cases of spillways built using outdated data and formulas which
are now obsolete, and proposed alternative methods to calculate design flows taking into account
the effects of long-term persistence on floods [
2
,
7
,
8
,
13
]. On the other hand, works that focused on
the more practical aspects of building spillways analyzed the core elements of these structures [
5
,
14
],
and determined scenarios where they can be undermined even when not under extreme conditions,
much like the case of Oroville Dam itself. Studies of previous similar dam failures [
9
,
14
,
15
] reveal
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Geosciences 2019,9, 37 2 of 24
multiple aspects of a typical dam failure due to a problem developing in its spillway, and prove that
usually when such an incident occurs, several different factors likely contributed to it.
In their publication “Lessons from dam incidents”, ICOLD summarizes some 500 incidents from
1800 to 1965. Several of these have been the subject of major investigations and have a substantial
literature [
16
]. Another useful reference in USCOLD’s “Lessons from Dam Incidents USA”, which lists
over 500 incidents in the USA [
17
]. A report published by the UK’s Environment Agency offers
insights and information on lessons learnt from over 100 national and international dam incidents and
failures [
18
]. Many of these include incidents caused by overtopping, internal erosion and foundation
failures such as those related to the Oroville incident.
Analyzing scenarios like these leads to preventing future disasters, and the Oroville Dam incident
is yet another case study that promotes varying topics of dam risk assessment, flood control analysis,
as well as offering insights into design and operational procedures regarding these crucial structures.
2. Materials and Methods
2.1. Feather River Basin Characteristics
Oroville Dam’s catchment, the Feather River Basin, lies between the north end of the Sierra
Nevada range and the east side of the Sacramento River Valley. It is bounded by Mt. Lassen to the
northwest, and by the Diamond Mountains to the northeast. Documented results of geological studies
in the vicinity [
19
,
20
] suggest that the area immediately in and around Lake Oroville composed mostly
of what is called the “Bedrock Series”. This consists mostly of metavolcanic and pyroclastic rock,
such as amphibolite. Above this bedrock lie various younger sedimentary rocks such as shales,
dolomites, Quaternary alluvium, playas, terraces, glacial till and moraines, and finally various
marine and non-marine sediments [
21
]. In general, the Feather River Basin is considered an area
of low seismicity [
20
]. The Feather River Basin as well as the city of Oroville are characterized by
a Mediterranean climate. Precipitation in the Feather River basin occurs most usually during the
cooler months, in rare yet intense events. On average, there are only 57 days of precipitation per year,
and 36 of those are liquid. Large floods in the Feather River basin occur due to severe winter rain
storms, in some cases augmented by snowmelt. A typical event may last several days, not being a
single storm, but a sequence of smaller individual storms in quick succession. In these cases, runoff
can produce high-peak intense flows downstream with a variety of flood characteristics [22].
A report [
23
] contains unregulated, annual maximum flow data for the Feather River at Oroville
station resulting from rainfall for 1-day, 3-day, 7-day, 15-day, and 30-day durations as provided by
the US Army Corps of Engineers. Each n-day period is useful for different aspects of reservoir
management [22–25]. The most intense floods from this analysis can be found in Table 1.
Table 1. Historical maximum 1-day and 3-days floods, Feather River at Oroville [23].
1-day 3-day
Water Year Date Flow (m3/s) Date Flow (m3/s)
1903–04 24-February 3001.59 18-March 2501.23
1906–07 19-March 5295.25 18-March 4256.87
1908–09 16-January 3879.41 14-January 3643.53
1927–28 26-March 3544.42 25-March 3139.77
1937–38 11-December 4501.81 10-December 3004.98
1939–40 30-March 3815.98 27-February 3055.67
1955–56 23-December 5140.36 22-December 4160.03
1964–65 23-December 5055.97 22-December 4683.32
1979–80 13-January 3896.96 12-January 3032.73
1985–86 17-February 6145.32 17-February 5295.53
1994–95 10-March 3799.78 9-March 3221.98
1996–97 1-January 8860.14 31-December 6923.04

Geosciences 2019,9, 37 3 of 24
2.2. Oroville Dam Characteristics
Oroville Dam is a zoned earth-fill embankment structure with a maximum height of 235 m above
river excavation as shown in Figure 1. The dam embankment has a volume of approximately 61 million
m
3
and comprises an inclined impervious core on a concrete foundation, supplemented by zoned
earth-fill sections on both sides.
Geosciences 2019, 9, x FOR PEER REVIEW 3 of 24
2.2. Oroville Dam Characteristics
Oroville Dam is a zoned earth-fill embankment structure with a maximum height of 235 m above
river excavation as shown in Figure 1. The dam embankment has a volume of approximately 61
million m3 and comprises an inclined impervious core on a concrete foundation, supplemented by
zoned earth-fill sections on both sides.
Figure 1. Aerial view of Oroville Dam [26].
This dam has a large catchment, with an area of approximately 9342 km2 and reservoir surface
area of approximately 64 km2. The reservoir capacity (up to the main spillway sill level) is 3427
hm3, whereas the maximum operating volume is stated to be 4364 hm3 (up to the emergency
spillway sill level). Further pertinent data on the dam, including a stage-storage capacity curve can
be found in related design reports released shortly after the dam’s construction [19,22]. An
important additional note is that this dam is not the only one that operates in the Feather Basin; it
is part of a network that includes several upstream reservoirs and diversion pools [27,28].
Oroville Dam’s spillway is located on a natural ridge adjacent to right abutment of the main
embankments. It consists of two independent structures, a combined flood control outlet and an
emergency weir. The former consists of an unlined approach channel with walls in such a way as
to make flows smoothly transit into an outlet passage, a headworks structure, and a concrete lined
chute, approximately 929 m in length. The headworks structure is comprised of eight top-seal
radial gates, 17.78 cm thick and 5.18 m wide by 10.06 m high. At the end of the lined chute, chute
blocks help absorb some of the energy from the outgoing flow before it pours into the Feather
River.
The main concept behind designing the flood control outlet was to limit Feather River flow
to 5094 m3/s in the occurrence of a flood event known as the Standard Project Flood (SPF). For
Oroville Dam, the peak inflow of the SPF was estimated at 12,700 m3/s, and is claimed to have a
return period of 450 years in related design documents [19]. In order to meet this criterion, the
flood control was designed for a 4245 m3/s controlled release, and a flood control reservation
volume of 925.11 hm3 was deemed necessary. This volume is also mentioned in the official manual
for flood control operation of Oroville Dam [19,22].
According to references [19,29], the combined capacity of the main and emergency spillways is
17,472 m3/s, which corresponds to a peak inflow of 20,160 m3/s. The event that would cause this inflow
corresponds to what has been known as the Probable Maximum Flood (PMF). Given the known
design capacity of the main spillway, this would set the design capacity of the emergency spillway
to approximately 9900 m3/s in order to meet the combined outflow required by the PMF.
Blasting was used for almost 90% of the main spillway chute foundation, in order to reach grade.
The remaining amount consisted of the removal of several seams of clay located in the foundation,
Figure 1. Aerial view of Oroville Dam [26].
This dam has a large catchment, with an area of approximately 9342 km
2
and reservoir surface
area of approximately 64 km
2
. The reservoir capacity (up to the main spillway sill level) is 3427 hm
3
,
whereas the maximum operating volume is stated to be 4364 hm
3
(up to the emergency spillway sill
level). Further pertinent data on the dam, including a stage-storage capacity curve can be found in
related design reports released shortly after the dam’s construction [
19
,
22
]. An important additional
note is that this dam is not the only one that operates in the Feather Basin; it is part of a network that
includes several upstream reservoirs and diversion pools [27,28].
Oroville Dam’s spillway is located on a natural ridge adjacent to right abutment of the main
embankments. It consists of two independent structures, a combined flood control outlet and an
emergency weir. The former consists of an unlined approach channel with walls in such a way as
to make flows smoothly transit into an outlet passage, a headworks structure, and a concrete lined
chute, approximately 929 m in length. The headworks structure is comprised of eight top-seal radial
gates, 17.78 cm thick and 5.18 m wide by 10.06 m high. At the end of the lined chute, chute blocks help
absorb some of the energy from the outgoing flow before it pours into the Feather River.
The main concept behind designing the flood control outlet was to limit Feather River flow to
5094 m
3
/s in the occurrence of a flood event known as the Standard Project Flood (SPF). For Oroville
Dam, the peak inflow of the SPF was estimated at 12,700 m
3
/s, and is claimed to have a return period
of 450 years in related design documents [
19
]. In order to meet this criterion, the flood control was
designed for a 4245 m
3
/s controlled release, and a flood control reservation volume of 925.11 hm
3
was
deemed necessary. This volume is also mentioned in the official manual for flood control operation of
Oroville Dam [19,22].
According to references [
19
,
29
], the combined capacity of the main and emergency spillways is
17,472 m
3
/s, which corresponds to a peak inflow of 20,160 m
3
/s. The event that would cause this
inflow corresponds to what has been known as the Probable Maximum Flood (PMF). Given the known
design capacity of the main spillway, this would set the design capacity of the emergency spillway to
approximately 9900 m3/s in order to meet the combined outflow required by the PMF.
Blasting was used for almost 90% of the main spillway chute foundation, in order to reach grade.
The remaining amount consisted of the removal of several seams of clay located in the foundation,

Geosciences 2019,9, 37 4 of 24
and a few areas where the slope failed [
19
]. The slopes in the flood control outlet section were of a
lower quality rock than initially presumed and several large seams ran parallel with the main spillway
chute. The countermeasure that was applied was the replacement of planned anchor bars with grouted
rock blots, pigtail anchors and a chain-link covering the area’s surface [19].
2.3. Annual Maxima Rainfall Analysis
Until now, known flood control studies for Oroville Dam and the Feather Basin have attempted
to determine the Probable Maximum Flood (PMF) for Lake Oroville, based on the Probable Maximum
Precipitation (PMP). The most recent existing study available detailing PMP calculations in California
is Hydrometeorological Report No. 59 or HMR 59 [
30
]. In brief, the computational procedure includes
tracing an outline of the drainage basin, placing this outline on top of a given PMP 10-m
2
, 24-h index
map, then determining depth-duration relationships and areal reduction factors, and finally conducting
temporal distribution of incremental depths extracted from a given curve.
While this method is simple to use, and the analysis involved in creating these PMP index maps
undoubtedly contains valuable information, it would be better to adopt a probabilistic approach to
precipitation analysis, where instead of assuming a deterministic, theoretical upper limit, a return
period would be assigned to any precipitation and flood value. This would be achieved by studying
existing precipitation data and extracting a return period for the already calculated 24-h index depths,
for every sub-area of the Feather River Basin, as determined by the California Department of Water
Resources [27]. One of the possible methods to achieve this is exposed below.
The 24-h index PMP depth essentially describes a daily maximum precipitation value. If the
distribution of daily rainfall for a given area is known, one can assume that the annual maxima of
daily rainfall would resemble one of two limiting types: type I, known as Gumbel distribution
or type II, known as Fréchet distribution. The Generalized Extreme Value (GEV) distribution,
which comprises these types by way of its shape parameter (as well as type III, known as reversed
Weibull, which however is not recommended for rainfall maxima [
31
]) can be fitted to a series of
annual maxima of daily rainfall.
In accordance with References [
13
,
32
,
33
], the GEV distribution using the method of L-moments is
fitted to various precipitation data gathered from the Feather Basin [
34
,
35
]. A map of the basin with
the measurement stations used in this analysis can be found in Appendix A[34–36].
To improve accuracy, a filter is applied to the data, i.e., only years with 300 or more daily
measurements are taken into account, roughly equivalent to at least 25 days with measurements per
month. After discarding stations with data suspected of containing erroneous measurements that
could not be cross-referenced with floods around the same time period, four significant precipitation
measurement stations were selected for this analysis. Then, annual daily maxima time series are created.
The process is simple: First, select the maximum daily precipitation value of every year, then rank them
in descending order. Obviously, the highest value is the most important one, so it is imperative that
it is cross-referenced with multiple sources to confirm its validity. Finally, the GEV-max distribution
with the method of L-moments is fitted using the “Pythia” statistical tool of the HYDROGNOMON
open software, which follows the exact principles stated in the related literature [13,32,33].
2.4. The 2017 Event
During the first few days of January 2017, two small rain storms occurred just over Oroville
Dam’s reservoir [
34
,
37
]. The first rain storm was short, lasting only 4 days, peaking at 90 mm on
January 3, and the second was a stronger 6-day event, peaking at 136 mm on January 10. These rain
storms quickly led into a large increase of inflows into Lake Oroville. Two inflow peaks occurred:
The primary one was 4839 m
3
/s on January 8 at 21:00 p.m., and a secondary peak of 3079 m
3
/s,
occurring on January 10 at 22:00 p.m. These inflows are definitely significant, yet expected during a
typical wet season. However, outflows from Lake Oroville at the same time were very low, almost
zero, as there was a sharp water storage increase in Lake Oroville, as well as a significant rise in its

Geosciences 2019,9, 37 5 of 24
surface elevation. Lake Oroville’s surface elevation initially exceeded the flood control minimum on
January 12, 2017 at 17:00 p.m. Around that time, outflows from Oroville Dam’s main spillway were
increased to compensate for this fact and return the surface elevation to below the minimum. Overall,
the Oroville Dam operator was able to return the surface level to below the flood control limit on
February 3, 2017 at 17:00 p.m, just in time for an upcoming February rain storm.
Thereafter, according to CDEC, a rain storm over the Feather Basin began on February 2, 2017,
and ended around February 11. The largest flood value occurred on February 9 at 19:00 p.m., and was
5392 m
3
/s. This value is significantly lower than the highest recorded floods to ever occur in the
Feather Basin. Under normal circumstances, Oroville Dam should have been able to deal with this
event without trouble. On February 6 at approximately 13:00 p.m., outflows from Lake Oroville were
raised in order to prepare for incoming inflows to 1500 m
3
/s. However, the next day, February 7,
at approximately 10:00 a.m., workers at the Oroville Dam site noticed a discoloration in the water
flowing through the main spillway. Outflow from the main spillway was immediately halted, in order
to detect the source of this discoloration, revealing a large hole in the main spillway chute, seen in
Figure 2.
Geosciences 2019, 9, x FOR PEER REVIEW 5 of 24
increased to compensate for this fact and return the surface elevation to below the minimum. Overall,
the Oroville Dam operator was able to return the surface level to below the flood control limit on
February 3, 2017 at 17:00 p.m, just in time for an upcoming February rain storm.
Thereafter, according to CDEC, a rain storm over the Feather Basin began on February 2, 2017, and
ended around February 11. The largest flood value occurred on February 9 at 19:00 p.m., and was 5392
m3/s. This value is significantly lower than the highest recorded floods to ever occur in the Feather
Basin. Under normal circumstances, Oroville Dam should have been able to deal with this event
without trouble. On February 6 at approximately 13:00 p.m., outflows from Lake Oroville were raised
in order to prepare for incoming inflows to 1500 m3/s. However, the next day, February 7, at
approximately 10:00 a.m., workers at the Oroville Dam site noticed a discoloration in the water flowing
through the main spillway. Outflow from the main spillway was immediately halted, in order to detect
the source of this discoloration, revealing a large hole in the main spillway chute, seen in Figure 2.
Figure 2. 7 February 2017. Front view of the initial main spillway chute damage [38].
At this point, the main spillway is already severely damaged, and any discharges at that point
would rapidly amplify this erosion and move entire parts of the concrete chute and walls
downstream. However, Lake Oroville’s surface elevation is already past the flood control minimum,
and inflows from the February rain storm are imminent. After brief consultation with various dam
safety agencies, the operators decided to release test flows into the main spillway and monitor the
damage. These small flows ranged hourly from around 300 m3/s to 900 m3/s over the course of
February 8th. On the very next day, February 9, the hole in the main spillway had increased in size.
A worrying aspect of the spillway damage is that it was moving uphill. This is a typical sign of
a failure known as headcutting (or undercutting), which is what happens when water flowing across
a hard surface falls onto a softer surface below.
With the ever-increasing inflows dangerously raising the reservoir surface level, which is
already above the minimum flood control elevation, there was no time to quickly repair the main
spillway. At this point, the Oroville Dam operators were facing a tough dilemma; either continue to
release flows through the already damaged chute and cause further erosion, or risk using the untested
auxiliary spillway. However, as the latter structure is ungated, if unchecked the dam itself would
make that choice for them, as water would flow over the emergency spillway as soon as the surface
elevation surpassed its crest, at 274.62 m. As such, a plan was formulated to continue letting small
Figure 2. 7 February 2017. Front view of the initial main spillway chute damage [38].
At this point, the main spillway is already severely damaged, and any discharges at that point
would rapidly amplify this erosion and move entire parts of the concrete chute and walls downstream.
However, Lake Oroville’s surface elevation is already past the flood control minimum, and inflows
from the February rain storm are imminent. After brief consultation with various dam safety agencies,
the operators decided to release test flows into the main spillway and monitor the damage. These small
flows ranged hourly from around 300 m
3
/s to 900 m
3
/s over the course of February 8. On the very
next day, February 9, the hole in the main spillway had increased in size.
A worrying aspect of the spillway damage is that it was moving uphill. This is a typical sign of a
failure known as headcutting (or undercutting), which is what happens when water flowing across a
hard surface falls onto a softer surface below.
With the ever-increasing inflows dangerously raising the reservoir surface level, which is already
above the minimum flood control elevation, there was no time to quickly repair the main spillway.

Geosciences 2019,9, 37 6 of 24
At this point, the Oroville Dam operators were facing a tough dilemma; either continue to release flows
through the already damaged chute and cause further erosion, or risk using the untested auxiliary
spillway. However, as the latter structure is ungated, if unchecked the dam itself would make that
choice for them, as water would flow over the emergency spillway as soon as the surface elevation
surpassed its crest, at 274.62 m. As such, a plan was formulated to continue letting small flows
pass through the main spillway, while also preparing the area around the auxiliary spillway in case
it would have to be put to use. To that end, workers began clearing the area downstream of this
secondary structure, as well as placing large rocks at its foot to mitigate possible erosion. At this point,
the inflows into Lake Oroville increased tremendously, reaching the aforementioned peak of 5392
m
3
/s. On February 11, at 8:00 a.m., surface elevation at Lake Oroville surpassed that of the emergency
spillway crest, meaning that for the first time in the dam’s history, water would pour over it. According
to data from CDEC, water poured over this ogee weir for just over 37 h in total, as the surface level
dropped below its crest elevation again on February 12 at 21:00 p.m.
A noticeable fact is that there is a parking lot just next to the emergency spillway, which is at a
lower elevation, and thus is flooded by design whenever water pours over the weir. Furthermore,
an access road located just below the structure was also subsequently flooded and quickly destroyed,
as seen in Figure 3.
Geosciences 2019, 9, x FOR PEER REVIEW 6 of 24
flows pass through the main spillway, while also preparing the area around the auxiliary spillway in
case it would have to be put to use. To that end, workers began clearing the area downstream of this
secondary structure, as well as placing large rocks at its foot to mitigate possible erosion. At this
point, the inflows into Lake Oroville increased tremendously, reaching the aforementioned peak of
5392 m3/s. On February 11, at 8:00 a.m., surface elevation at Lake Oroville surpassed that of the
emergency spillway crest, meaning that for the first time in the dam’s history, water would pour over
it. According to data from CDEC, water poured over this ogee weir for just over 37 h in total, as the
surface level dropped below its crest elevation again on February 12 at 21:00 p.m.
A noticeable fact is that there is a parking lot just next to the emergency spillway, which is at a
lower elevation, and thus is flooded by design whenever water pours over the weir. Furthermore, an
access road located just below the structure was also subsequently flooded and quickly destroyed, as
seen in Figure 3.
Figure 3. 11 February 2017. Image of the flooded parking lot and access road located next to the
emergency spillway [39].
Unfortunately, erosion downstream developed much more rapidly than anticipated. While the
emergency spillway was only active for a very brief duration, and peak discharge did not exceed 400
m3/s; large boils occurred downstream, destroying the access road below and threatening to damage
the spillway crest itself by failure due to headcutting. The exact extent of the damage was not clearly
visible when water was still pouring over the downstream hill on February 12, however, and thus
local authorities, fearing the worst outcome, were forced to spring into action and order the
evacuation of Oroville and other areas downstream of the dam, including Yuba City and Marysville.
The California Department of Water Resources responded to the evacuation order by immediately
increasing outflow releases from the main spillway to 2830 m3/s. This would drastically lower the
surface elevation and stop flows over the emergency spillway and any resulting erosions there, at the
cost of causing irreparable damages to the main spillway. Luckily, despite the conditions, the upper
portion of the main spillway was able to release these discharges without causing further upstream
erosion. However, the hill downstream of the initial hole would be quickly eroded away from high
velocity flows.
3. Results
Based on the evidence gathered, it is possible to make several hypotheses for the possible causes
of failure for both spillways.
Figure 3.
11 February 2017. Image of the flooded parking lot and access road located next to the
emergency spillway [39].
Unfortunately, erosion downstream developed much more rapidly than anticipated. While the
emergency spillway was only active for a very brief duration, and peak discharge did not exceed
400 m
3
/s; large boils occurred downstream, destroying the access road below and threatening to
damage the spillway crest itself by failure due to headcutting. The exact extent of the damage was
not clearly visible when water was still pouring over the downstream hill on February 12, however,
and thus local authorities, fearing the worst outcome, were forced to spring into action and order the
evacuation of Oroville and other areas downstream of the dam, including Yuba City and Marysville.
The California Department of Water Resources responded to the evacuation order by immediately
increasing outflow releases from the main spillway to 2830 m
3
/s. This would drastically lower the
surface elevation and stop flows over the emergency spillway and any resulting erosions there, at the
cost of causing irreparable damages to the main spillway. Luckily, despite the conditions, the upper
portion of the main spillway was able to release these discharges without causing further upstream
erosion. However, the hill downstream of the initial hole would be quickly eroded away from high
velocity flows.

Geosciences 2019,9, 37 7 of 24
3. Results
Based on the evidence gathered, it is possible to make several hypotheses for the possible causes
of failure for both spillways.
3.1. Emergency Spillway
It is much easier to determine the cause of the near failure of the emergency spillway due to
the fact that it was actuated for a very brief duration under constant supervision, as authorities were
already alerted of the situation. While water was pouring over the concrete weir without a problem,
it was the surrounding conditions that posed a threat. Already from the documents describing Oroville
Dam’s construction, the following facts are known:
•
The emergency spillway was untested, even in the model studies conducted by the US Bureau of
Reclamation [29].
•The area downstream of the emergency spillway was not cleared [19].
•
The emergency spillway foundation excavation continued 3 m—deeper than expected—in order
to reach the foundation rock that met the design criteria [19].
While it is known that this concrete overpour weir was built on a solid foundation, no effort
was made to secure that the downstream ridge would be able to accommodate flows passing over
it without significant erosion occurring as a result. This could have been acceptable if this structure
was truly used as an emergency measure (i.e., any outflows from it not being factored into hydrologic
design calculations, using only the main structure’s design capacity instead), but this is not the case.
According to Reference [
40
], a high risk structure such as Oroville Dam should be able to withstand
the PMF. All PMF analyses so far [
22
,
27
] have included the emergency spillway in their calculations,
and in fact, in the event of the PMF, the emergency spillway is expected to reach outflow discharges
of around 10,000 m
3
/s. Seeing as erosion threatened to cause structural failure at less than 420 m
3
/s,
the spillway’s ability to withstand PMF-level discharges is questionable. In any case, this warrants
the need for the structure to be properly armored with concrete and considered to be an “auxiliary”
spillway, not an “emergency” one. This has been repeatedly requested by the community [
41
–
43
],
and has yet to be fully implemented.
3.2. Revisiting the Minium Flood Control Elevation
When posing the question of why Oroville Dam was capable of withstanding the previous
devastating floods of 1986 and 1997, and not the 2017 event, one is prompted to also examine the
surface elevation levels prior to each flood. Thus, an attempt is made to compare Oroville Dam
reservoir surface levels shortly before and after each of the three recent flood events, occurring in 1986,
1997 and 2017 [
34
]. In Figure 4, the vertical axis represents surface elevation in meters, whereas the
horizontal axis represents time, up to 240 h (10 days) before and after peak inflow. Hour 0 is the hour
during which peak inflow occurred for each event.
Naturally, no two flood events are the same and they all impact Oroville Dam in subtly different
ways, but this comparison contains clues on what went wrong during the 2017 spillway incident.
Notably, while the 2017 peak inflow is the lowest of the three major flood events, its surface elevations
are the highest. This is due to two factors. First, as is clear from the graph, shortly prior to peak inflow,
the surface elevation during the 2017 event was higher than in previous floods. Already, this has a
negative impact on flood management. While this elevation is below the minimum limit specified
by the flood control manual [
22
], the 2017 flood is actually harder to manage than previous events.
This is partly why, despite not being a record flood, this event came close to causing severe damages to
Oroville Dam’s key structures once the main spillway failed.

Geosciences 2019,9, 37 8 of 24
Geosciences 2019, 9, x FOR PEER REVIEW 8 of 24
Figure 4. Comparison of Oroville reservoir hourly surface elevations 10 days before and after the peak
inflow of the 1986, 1997 and 2017 flood events.
Naturally, no two flood events are the same and they all impact Oroville Dam in subtly different
ways, but this comparison contains clues on what went wrong during the 2017 spillway incident.
Notably, while the 2017 peak inflow is the lowest of the three major flood events, its surface elevations
are the highest. This is due to two factors. First, as is clear from the graph, shortly prior to peak inflow,
the surface elevation during the 2017 event was higher than in previous floods. Already, this has a
negative impact on flood management. While this elevation is below the minimum limit specified by
the flood control manual [22], the 2017 flood is actually harder to manage than previous events. This
is partly why, despite not being a record flood, this event came close to causing severe damages to
Oroville Dam’s key structures once the main spillway failed.
According the flood control manual [22], the additional following restrictions are applied to
flows from the main spillway:
• Any water stored in the designated flood control space should be released as quickly as possible,
according to a given flood control diagram. Flows from Feather River should not exceed 4245
m
3
/s.
• During extreme flood events, releases greater than 4245 m
3
/s may be required in order to
minimize uncontrolled spillway discharges.
• Releases from Oroville Dam are not to be increased more than 280 m
3
/s or decreased more than
140 m
3
/s in any given 2-h period.
While levels were within the flood control manual standards [22], the fact that they were close
to the limit made dealing with the February 2017 inflows a much more daunting task once the main
spillway failed. Thus, it would seem reasonable to request a lowering of the minimum flood control
elevation level for Lake Oroville. Lowering the minimum flood control elevation level has its
downsides, mainly due to losing reservoir capacity and its valuable resource, but it could be adjusted
5 m lower or even further down at the spillway’s sill elevation at 248 m without significant losses in
efficiency. Alternatively, it can be maintained even lower by utilizing draw-offs and other outlets to
Figure 4.
Comparison of Oroville reservoir hourly surface elevations 10 days before and after the peak
inflow of the 1986, 1997 and 2017 flood events.
According the flood control manual [
22
], the additional following restrictions are applied to flows
from the main spillway:
•
Any water stored in the designated flood control space should be released as quickly as possible,
according to a given flood control diagram. Flows from Feather River should not exceed
4245 m3/s.
•
During extreme flood events, releases greater than 4245 m
3
/s may be required in order to minimize
uncontrolled spillway discharges.
•
Releases from Oroville Dam are not to be increased more than 280 m
3
/s or decreased more than
140 m3/s in any given 2-h period.
While levels were within the flood control manual standards [
22
], the fact that they were close
to the limit made dealing with the February 2017 inflows a much more daunting task once the
main spillway failed. Thus, it would seem reasonable to request a lowering of the minimum flood
control elevation level for Lake Oroville. Lowering the minimum flood control elevation level has its
downsides, mainly due to losing reservoir capacity and its valuable resource, but it could be adjusted
5 m lower or even further down at the spillway’s sill elevation at 248 m without significant losses in
efficiency. Alternatively, it can be maintained even lower by utilizing draw-offs and other outlets to
a greater capacity. However, this method of managing the flood risk would need to be considered
together with other economic and ecological factors to ensure the benefits are balanced with any other
detrimental impacts.
Therefore, taking all of the above into account, it would seem logical to request a small reduction
in the minimum flood control level. In their 2006 statement [
41
], FOR et al., had requested an additional
185 hm
3
of surcharge storage be added to the 925 hm
3
control pool in order to compensate for the
never constructed Marysville Dam. This was a project that was included in the flood control pool
calculations, yet was never completed. If this measure were to be implemented, according to the flood

Geosciences 2019,9, 37 9 of 24
control manual, the new minimum flood control elevation would be 255 m. Under these conditions,
according to Reference [
29
], the flood control outlet’s release capacity is approximately 1274 m
3
/s.
By chance, this was the Oroville Dam reservoir’s surface elevation just before the 1997 flood [
44
],
and the spillway performed adequately even when outflows briefly exceeded the designed discharges.
Following additional analyses of other flood events, it may be considered that this small lowering of
the flood control level could reduce risks to an acceptable level [5,8,45].
3.3. Main Spillway
Attempting to detect what caused the initial failure of the main spillway is a much more
complicated task, as due to the nature of the incident, very few pictures are available showing
the initial chute hole that was spotted on February 7. Any physical evidence that could have been
gathered from the scene at the time has been likely washed away from the subsequent discharges
that eroded away the bottom half of the chute and much of the downstream ridge. Simply looking at
pictures of the February 7 chute damage is not enough, and can lead to forming biased conclusions.
Thus, prior to studying these pictures, further background research is required.
A dam inspection guide [
40
] lists potential incidents that can occur on spillway concrete chutes
and possible causes based on studies of previous similar events. More specifically, the following
defects mentioned in the guide are directly related to the Oroville Dam main spillway chute.
•
Cracking of concrete in floor slabs. Visible on casual inspection when concrete is dry, possibly
caused by temperature changes or inadequate reinforcement.
•
Damaged concrete. Possibly caused by cavitation or erosion due to irregularities or rough surface.
•
Lifted slab panels. Indicated by vertical offsets in joints, possibly caused by poor drainage under
slabs, and/or inadequate anchoring of slab to foundation.
Furthermore, the geological conditions below the spillway chute are also considered.
3.4. Structural Flaws
Based on previous inspection reports and other sources [
19
,
27
,
46
,
47
], it is known that cracks had
previously occurred in the main spillway chute’s floor slabs, just above the herringbone drains.
Aside from cracking, removal of joint filler and spalling have been also been documented as
mechanisms that cause damages. Attempts to repair the structure took place in 1977, 1985, 2009,
and 2013. These efforts usually included a simple removal of spalled concrete and patchwork with
the intent of simply restoring flow surface. By 2017, cracks remained present above the drains, which
likely allowed water to flow through the chute’s concrete slabs whenever the spillway was actuated.
Unfortunately, due to the nature of the incident and the measures that had to be taken to ensure
Oroville Dam’s safety, if the initial cause of the main spillway chute’s failure was slab uplift due to a
fault in the drain system, the only available evidence can be found in pictures taken shortly before and
after the February 6 chute hole was spotted, as any physical evidence was subsequently eroded away
by the February 12 outflows. However, by conducting background research, the following factors are
discovered about the main spillway’s drain system and the concrete chute slabs [19,46,48,49]:
•
The invert slabs have a minimum thickness of 380 mm, are anchored to rock with grouted anchor
bars, and are provided with a system of underdrains [19].
•
The initial drain system plan ended up being significantly altered during construction. After a
recommendation by the Oroville Dam Consulting Board, the original horizontal pipe drains under
the chute were enlarged and placed in a herring-bone pattern. The collector system operating in
line with the chute was also enlarged and modified so as to enhance its capacity and self-cleaning
ability [19].
•
The last official inspection of the main spillway chute’s full length took place in 2015 [
49
]. At the
time, no structural deficiencies were detected. An additional inspection took place in 2016 [
50
],

Geosciences 2019,9, 37 10 of 24
but the spillway chute was only examined from the top of the FCO outlet structure, not up close
like in 2015. A reason for this is not specified.
In addition, a comparison of pictures of the spillway shortly before the February 6 hole was
discovered yield additional clues. Figure 5is a comparison of two pictures of the main spillway chute,
taken shortly before the February incident. The first was taken on 11 January 2017 and the second on
27 January of the same year.
Geosciences 2019, 9, x FOR PEER REVIEW 10 of 24
• The invert slabs have a minimum thickness of 380 mm, are anchored to rock with grouted anchor
bars, and are provided with a system of underdrains [19].
• The initial drain system plan ended up being significantly altered during construction. After a
recommendation by the Oroville Dam Consulting Board, the original horizontal pipe drains
under the chute were enlarged and placed in a herring-bone pattern. The collector system
operating in line with the chute was also enlarged and modified so as to enhance its capacity
and self-cleaning ability [19].
• The last official inspection of the main spillway chute’s full length took place in 2015 [49]. At the
time, no structural deficiencies were detected. An additional inspection took place in 2016 [50],
but the spillway chute was only examined from the top of the FCO outlet structure, not up close
like in 2015. A reason for this is not specified.
In addition, a comparison of pictures of the spillway shortly before the February 6 hole was
discovered yield additional clues. Figure 5 is a comparison of two pictures of the main spillway chute,
taken shortly before the February incident. The first was taken on 11 January 2017 and the second on
27 January of the same year.
Figure 5. Views of the Oroville Dam main spillway chute. (a) taken on 11 January 2017, and (b) taken on
27 January 2017. A red arrow points to the location of the initial chute failure [38,51].
While these pictures were only taken within 16 days of each other, there are significant
differences in the spillway chute. A center section of the chute’s concrete floor appears dry on the
right-hand picture, despite flows passing over the rest of the structure. This indicates possible
irregularities among the floor slabs. Furthermore, the fact that this dry patch is not visible in the photo
taken earlier, could possibly mean that a possible slab uplift occurred near the red arrow’s location,
diverting small water flows around it instead of over it.
Furthermore, by looking at the drain system more closely, two clues are revealed: First, water is
coming out of the drains under pressure, which is not according to design specifications, and
secondly, discharge from these drains significantly increased in a short time, once flows from the
January flood filled up the Oroville Dam reservoir. This is a telltale sign of a buildup of excess water
occurring beneath the spillway, which could apply significant forces to the concrete slabs from below
and cause them to uplift [40]. Additionally, the January 27 photograph shows the drains on the
opposite wall operating under pressure as well.
Figure 5.
Views of the Oroville Dam main spillway chute. (
a
) taken on 11 January 2017, and (
b
) taken
on 27 January 2017. A red arrow points to the location of the initial chute failure [38,51].
While these pictures were only taken within 16 days of each other, there are significant differences
in the spillway chute. A center section of the chute’s concrete floor appears dry on the right-hand
picture, despite flows passing over the rest of the structure. This indicates possible irregularities
among the floor slabs. Furthermore, the fact that this dry patch is not visible in the photo taken earlier,
could possibly mean that a possible slab uplift occurred near the red arrow’s location, diverting small
water flows around it instead of over it.
Furthermore, by looking at the drain system more closely, two clues are revealed: First, water is
coming out of the drains under pressure, which is not according to design specifications, and secondly,
discharge from these drains significantly increased in a short time, once flows from the January flood
filled up the Oroville Dam reservoir. This is a telltale sign of a buildup of excess water occurring
beneath the spillway, which could apply significant forces to the concrete slabs from below and cause
them to uplift [
40
]. Additionally, the January 27 photograph shows the drains on the opposite wall
operating under pressure as well.
3.5. Possible Cavitation—1-D Water Surface Profile Analysis
One of the possible causes of the initial damage to the concrete chute floor is cavitation. In order
to better understand this cause, extensive examination of the USBR hydraulic model study of the
main spillway [
29
] is required. Furthermore, comparing this data to a simple mathematical model of
the main spillway chute could help find possible clues. A simple mathematical model is constructed
in a spreadsheet software which uses an iterative procedure to simulate 1-D steady open-channel
flow, known as the standard step method [
52
]. In order to construct this model, some additional
assumptions must be made, which are analyzed below.
Based on the USBR main spillway chute profile, its main rectangular concrete section is 54.46 m
wide, begins at Station +13 00 (past the beginning of the approach channel) and ends at Station +43 00,

Geosciences 2019,9, 37 11 of 24
just before the terminal structure with the concrete chute blocks. As such, this main section is exactly
914.4 m in length, and only this part of the main spillway is modeled. To avoid confusions between
the USBR calculations and those of the model, the entire model is constructed using American unit
measurements (distance in feet, discharge in cfs, etc.).
To calculate flows, Manning’s n coefficient is additionally required. Unfortunately, there is no
mention of the specific coefficient used for the hydraulic calculations of the final chute in Reference [
29
].
However, a profile drawing of an earlier model describes a lined concrete channel with an nvalue of
0.013. Based on this and the HEC-RAS manual specifications, an nvalue of 0.014 was selected for the
model. Furthermore, in the interest of time and with the intent of keeping the mathematical model as
simple as possible, critical flow depth was assumed at the chute’s beginning for every discharge profile,
instead of the true depth which is partially controlled by the flood control outlet gates. However, as is
evident later, this did not have a significant impact on the results.
Four discharge profiles were created, in accordance with those of the USBR model study: 20,000 cfs
(566 m
3
/s); 50,000 cfs (1416 m
3
/s); 100,000 cfs (2832 m
3
/s); and finally 277,000 cfs (7484 m
3
/s), which
is the main spillway’s design capacity. A water surface profile view of the chute for the latter discharge
is plotted in Figure 6, with an additional data label at the exact point where the 2017 hole occurred
(Station +33 00). Figures for the other surface profiles can be found in Appendix B.
Geosciences 2019, 9, x FOR PEER REVIEW 11 of 24
3.5. Possible Cavitation—1-D Water Surface Profile Analysis
One of the possible causes of the initial damage to the concrete chute floor is cavitation. In order
to better understand this cause, extensive examination of the USBR hydraulic model study of the
main spillway [29] is required. Furthermore, comparing this data to a simple mathematical model of
the main spillway chute could help find possible clues. A simple mathematical model is constructed
in a spreadsheet software which uses an iterative procedure to simulate 1-D steady open-channel
flow, known as the standard step method [52]. In order to construct this model, some additional
assumptions must be made, which are analyzed below.
Based on the USBR main spillway chute profile, its main rectangular concrete section is 54.46 m
wide, begins at Station +13 00 (past the beginning of the approach channel) and ends at Station +43
00, just before the terminal structure with the concrete chute blocks. As such, this main section is
exactly 914.4 m in length, and only this part of the main spillway is modeled. To avoid confusions
between the USBR calculations and those of the model, the entire model is constructed using
American unit measurements (distance in feet, discharge in cfs, etc.).
To calculate flows, Manning’s n coefficient is additionally required. Unfortunately, there is no
mention of the specific coefficient used for the hydraulic calculations of the final chute in Reference
[29]. However, a profile drawing of an earlier model describes a lined concrete channel with an n
value of 0.013. Based on this and the HEC-RAS manual specifications, an n value of 0.014 was selected
for the model. Furthermore, in the interest of time and with the intent of keeping the mathematical
model as simple as possible, critical flow depth was assumed at the chute’s beginning for every
discharge profile, instead of the true depth which is partially controlled by the flood control outlet
gates. However, as is evident later, this did not have a significant impact on the results.
Four discharge profiles were created, in accordance with those of the USBR model study: 20,000
cfs (566 m
3
/s); 50,000 cfs (1416 m
3
/s); 100,000 cfs (2832 m
3
/s); and finally 277,000 cfs (7484 m
3
/s), which
is the main spillway’s design capacity. A water surface profile view of the chute for the latter
discharge is plotted in Figure 6, with an additional data label at the exact point where the 2017 hole
occurred (Station +33 00). Figures for the other surface profiles can be found in Appendix B.
Figure 6. Oroville Dam main spillway chute water surface profile, discharge 7484 m
3
/s.
Figure 6. Oroville Dam main spillway chute water surface profile, discharge 7484 m3/s.
From the chute flow analysis, it is clear that the initial assumption of critical flow depth at the
chute’s beginning does not negatively impact the results significantly, as due to the chute’s design,
flow depth quickly approaches normal depth with a standard S2 curve for supercritical flow [
52
].
For low discharge profiles, normal depth is reached fairly quickly, and only when the spillway is
running at maximum capacity, 7484 m
3
/s (277,000 cfs), does the flow reach normal depth close to the
chute’s end. No surface flow irregularities are immediately apparent from this analysis, indicating that
cavitation is probably not the initial cause of the of main spillway’s failure. However, as this model
assumes hydrostatic pressure, to confirm this assumption one could use the method recommended in
Reference [53], that allows for the detection of cavitation despite one-dimensional flow assumptions.

Geosciences 2019,9, 37 12 of 24
A more thorough analysis was carried out as part of the Forensic study and noted that the
cavitation was not a contributor to the failure of the service spillway chute. In fact, following our
calculations (Appendix C) at the area of failure, one can make an estimate of cavitation number for
the 100,000 cfs release of February 12. The channel velocity was around 95.5 fps and the flow depth
around 5.9 ft providing a cavitation number around 0.275. This is much higher than 0.15–0.20 which
according to Figures 3–8 of Reference [53] could have caused damage after 100 h of operation.
3.6. Geological Conditions Beneath the Main Spillway Chute
The fact that the main spillway chute was built on rock that required blasting to excavate would
mean that the rock is suitably hard to serve as a foundation for the concrete chute sections. However,
pictures of the initial spillway failure reveal more information about this foundation rock.
Based on Figure 7, it appears that the foundation rock is indeed composed of the metavolcanic
materials mentioned previously. However, this particular section of bedrock appears highly fractured
and heterogeneous. There is a significant variance of color in the formations, indicating different
degrees of weathering. Furthermore, due to the orientation of the seams, the rock is expected to erode
away in large chunks, not in sheets. It is also possible that water was able to seep through cracks in the
weaker, more weathered sections of rock and undermine the chute from below.
Geosciences 2019, 9, x FOR PEER REVIEW 12 of 24
From the chute flow analysis, it is clear that the initial assumption of critical flow depth at the
chute’s beginning does not negatively impact the results significantly, as due to the chute’s design,
flow depth quickly approaches normal depth with a standard S2 curve for supercritical flow [52]. For
low discharge profiles, normal depth is reached fairly quickly, and only when the spillway is running
at maximum capacity, 7484 m3/s (277,000 cfs), does the flow reach normal depth close to the chute’s
end. No surface flow irregularities are immediately apparent from this analysis, indicating that
cavitation is probably not the initial cause of the of main spillway’s failure. However, as this model
assumes hydrostatic pressure, to confirm this assumption one could use the method recommended
in Reference [53], that allows for the detection of cavitation despite one-dimensional flow
assumptions.
A more thorough analysis was carried out as part of the Forensic study and noted that the
cavitation was not a contributor to the failure of the service spillway chute. In fact, following our
calculations (Appendix C) at the area of failure, one can make an estimate of cavitation number for
the 100,000 cfs release of February 12th. The channel velocity was around 95.5 fps and the flow depth
around 5.9 ft providing a cavitation number around 0.275. This is much higher than 0.15–0.20 which
according to Figures 3–8 of Reference [53] could have caused damage after 100 h of operation.
3.6. Geological Conditions Beneath the Main Spillway Chute
The fact that the main spillway chute was built on rock that required blasting to excavate would
mean that the rock is suitably hard to serve as a foundation for the concrete chute sections. However,
pictures of the initial spillway failure reveal more information about this foundation rock.
Figure 7. 7 February 2017. Side view of the initial spillway chute failure [38].
Based on this photograph, it appears that the foundation rock is indeed composed of the
metavolcanic materials mentioned previously. However, this particular section of bedrock appears
highly fractured and heterogeneous. There is a significant variance of color in the formations,
indicating different degrees of weathering. Furthermore, due to the orientation of the seams, the rock
is expected to erode away in large chunks, not in sheets. It is also possible that water was able to seep
through cracks in the weaker, more weathered sections of rock and undermine the chute from below.
Figure 7. 7 February 2017. Side view of the initial spillway chute failure [38].
3.7. Annual Maximum Rainfall Analysis—Results
After consulting the 24-h PMP index depth maps in Reference [
30
] and comparing them to
those specified in Reference [
27
] for the subareas of the Feather River Basin, it is possible to use
these distribution fits to estimate the annual daily maximum precipitation value with a 10,000 year
return period and find the return period of the stated probable maximum precipitation index depths.
The results of this analysis are summarized in Table 2.

Geosciences 2019,9, 37 13 of 24
Table 2.
10,000-year annual daily maximum precipitation forecasts, compared to the 24-h PMP index
depths and their return periods, based on the GEV-Max distribution fit.
Station ID
Available
Daily Record
(years)
10,000-year Daily
Maximum
Precipitation (mm)
HMR 59 Avg.
PMP 24 h Index
Depth (mm)
GEV-Max Return
Period of PMP
(years)
BRS 1986–2017 688.6 800.1 33,333
USC00041159 1959–2016 529.7 647.7 50,000
USC00044812 1913–1967 414.8 635.0 >100,000
QCY 1989–2017 481.1 431.8 4348
The PMP usually has a return period that is extraordinally high, which increases safety. However,
it would be an error to assume that designing with the PMP method removes risk entirely simply
because it generates large values. This is why assigning a return period to a design precipitation value
is better for representing the associated risks, which are inevitable in engineering.
Furthermore, the PMP method evidently does not always generate overly high values. In the
case of the Quincy station (QCY), the distribution of the annual maximum rainfall results in a daily
maximum precipitation value with a 10,000 year return period that is above the PMP 24-h index depth
for the same region. That same probable maximum value has a corresponding return period of only
4348 years, which, while still being very high, leads to the conclusion that the PMP method is not
risk-free as some would expect.
3.8. Flood Frequency Analysis
Instead of assuming a fixed “worst case scenario” flood that supposedly cannot be exceeded,
which is what the PMF suggests, it is possible to assign a return period to existing design floods by
using customary flood frequency analysis methods. The record of unregulated, annual maximum flow
data for the Feather River at Oroville station resulting from rainfall for a 1-day duration provided by
USGS [
23
] is an ideal input time series for this purpose, and further cross-examination with known
extreme floods such as the 1964, 1986 and 1997 events as mentioned above confirms its accuracy.
Using HYDROGNOMON, two distributions are fitted to the data, namely the Log-Pearson III with the
method of maximum likelihood estimators and the GEV distribution using the L-Moments method,
according to References [13,54,55]. The results of the distribution fitting can be found in Appendix C.
From this analysis, it is possible to extract the 10,000 year floods for each of the distribution fits.
For the Log-Pearson III fit, the 10,000 year flood is estimated to be 32,000 m
3
/s and for the GEV fit,
the same value is 24,464 m
3
/s. Furthermore, it is possible to assign return periods to existing calculated
inflows such as the Standard Project Flood and various PMFs that can be found in References [
19
,
22
,
27
].
The results are documented in Table 3.
Table 3. Return periods in years for various floods, as generated by the distribution fitting process.
Flood Event Peak Inflow (m3/s) Return Period LP3 Fit (years) Return Period GEV Fit (years)
1986 Flood 6145 50 97
1997 Flood 8860 75 150
2017 Flood 5392 20 33
Standard Project Flood 12,459 250 610
PMF 1965 20,388 1360 4500
PMF 1983 33,046 >10,000 33,300
PMF 2003 (HMR 36) 25,202 3500 11,100
PMF 2003 (HMR 59) 20,530 1500 4800
The Standard Project Flood is mentioned to have a return period of 450 years [
19
], which is close
to the average of the two distribution fitting results. However, the return period of the probable
maximum flood is supposed to exceed 10,000 years, yet only the 1983 PMF achieved this for both
distribution fits. Notably, the most current PMF was calculated in 2003 based on HMR 59 [
27
,
54
],

Geosciences 2019,9, 37 14 of 24
and its return period does not exceed 5000 years for both distributions. Furthermore, according to this
analysis, the return period of the 2017 flood is only 20 years for the LP3 fit and 33 years for the GEV fit.
It should be noted that these flood figures are overall peaks, whereas the input for the fit is the slightly
lower daily averages given by Reference [
23
], so these estimates are on the conservative side. In any
case, these return periods should be viewed more as guidelines than as exact results. Nevertheless,
lowering of the flood control elevation could allow Oroville Dam to still withstand these floods.
4. Discussion
Based on the above analysis, and after consulting dam inspection manuals [
40
] and reviewing the
on-site investigation report [56], the following points stand out:
•
From a structural standpoint, the main spillway chute appears to have initially failed due to uplift
of its concrete floor slabs, caused somewhere between Stations +33 00 and +33 50 (2000 and 2050
feet of its rectangular section length, respectively). This uplift appears to have been caused by
water accumulating below the chute floor, which was unable to be routed through the underdrains.
This is evidenced by photographs showing them operating under pressure, which should never
occur under design specifications. The authors findings agree with those of the Independent
Forensic Team memos released recently [47,56].
•
The rest of the damage to the main spillway was caused by high velocity flows due to the
large amount of water that had to be routed through it to avoid erosion downstream of the
emergency spillway.
•
The fact that the Lake Oroville’s surface elevation was at the nominal minimum flood control level,
which was above that during previous major flood events, resulted in more severe conditions,
even though the February 2017 inflows were not at a record high. Thus, a lowering of the
minimum flood control level to 255 m is recommended. FOR et al. [
41
] revealed that this actually
would not be a new requirement, but an adaptation to outdated assumptions made in the 1970
flood control manual [
22
]. Based on the main spillway rating curve [
29
], it would be feasible to
maintain the dam reservoir at this level during wet seasons. Further research into the Feather
River’s Environmental Flow Components could also reveal crucial details of how floods impact
the local ecosystem and how lowering the minimum flood control elevation might affect the
existing balance [
4
]. Future research may use the available inflow and outflow data for this
incident, as well as the most intense previous events to create spillway operation scenarios of
their own and develop a more robust strategy for flood control.
•
The current PMF value for Lake Oroville has a return period of less than 10,000 years based on the
above analysis. It is recommended to either calculate a new 10,000 year flood for Lake Oroville
using a probabilistic method, or use the 1983 PMF value which is suitably large. However, it is
important to assign a return period to any resulting flood, as the whole concept of “probable
maximum flood” is problematic [32] and its scientific content is disputed [31].
•
The California Department of Water Resources’ quick response to the incident and initiation of
a full scale repair and reconstruction of the Oroville Dam spillways are admirable. However,
under current design, the dam is only capable of withstanding the Standard Project Flood with a
return period of 500 years without sustaining significant damage. In order to withstand a flood
with a return period of 10,000 years without causing significant erosions to the downstream areas,
the emergency spillway needs to be redesigned and fully armored with concrete. This has been
repeatedly requested by local interest groups [41,43].
•
In the United States, many are using this incident as an example of severe issues the country
has with maintaining the gigantic number of high-risk structures it has built over the past
century [
42
,
57
]. Indeed, the Oroville Dam itself has been around for half a century. Until a major
problem occurs at a critical facility like this one, it is easy to get complacent and avoid or postpone
critical maintenance procedures like routine inspections and small repairs. Rven when larger

Geosciences 2019,9, 37 15 of 24
problems or design flaws are pointed out [
41
], it is difficult to convince the authorities to fund
large-scale repair projects. However, one would argue that such repair projects actually conserve
money in the long run. The new Oroville Dam spillway is estimated to cost around $1.1 billion [
58
],
which is significantly more than what would have been required for a full concrete armoring of
the emergency spillway back in 2006.
In addition, the PMP–PMF analysis has several flaws. From a theoretical standpoint, the PMP
suggests that there exists a theoretical upper limit of precipitation, which is simply not true. Nature
is not bounded by numerical constraints, and the study of a brief history of available data cannot
generate a true possible maximum value of precipitation. According to Reference [
45
], the only merit
of the PMP value is that it is a large one. However, in some instances, this precipitation has been either
exceeded shortly after it was published, and in others it has been considered absurdly high upon
reexamination. On the other hand, constructing input timeseries of annual daily maxima from the
available daily precipitation data is not a foolproof method either. As the daily maximum precipitation
is a single value for each year, the resulting time series of annual maxima can be sensitive. For this
reason, Appendix Dcontains the annual daily maxima series used as input for the distribution fit to
promote further research and allow for cross-examination.
The concept of the Probable Maximum Flood is also highly controversial, for much of the same
reasons as the PMP. Indeed, the fact that over the years various PMF studies for Lake Oroville have
found largely varying values of probable maximum inflow and outflow does indicate that a true
mathematical upper flood limit does not exist. Therefore, even the PMF is again associated with a
certain degree of risk, however small. Especially due to the extent of the Feather River Basin and the
large number of smaller reservoirs within it above Oroville Dam, it is difficult to generate a reliable
design flood without taking multiple factors into account. At the very least, it is possible to assign a
return period to existing design floods by using customary flood frequency analysis method.
5. Conclusions
The Oroville Dam 2017 spillway incident presents an interesting case study, as it is a failure of a
dam’s key structure that occurred under standard operating conditions, yet at an unfortunate time.
It raises very interesting questions from a dam operator’s perspective: What does one do when a
spillway, a structure built to deal for emergency situations, fails just when it is needed? And in the
specific case of Oroville Dam, is the auxiliary spillway a feature, or a mark of a critical flaw in its
design? While it would indeed save the main dam from overtopping in the extraordinarily high
flood event, in doing so it would likely not be able to hold for long, while its failure would flood an
enormous area with more than 180,000 permanent residents. Furthermore, what has been thought of as
“probable maximum flood” seems more probable then presumed, and it is definitely not a maximum.
An independent forensic team tasked with determining the causes of the spillway incident recently
published summaries of their findings [
47
,
56
]. With the ability to conduct an on-site investigation,
they were able to confirm some of the causes mentioned in this study as well as outline new ones.
Namely, the redesign of chute’s underdrain system apparently led to an inconsistent thickness in the
concrete floor slabs, which resulted in cracks above the herringbone drains, allowing water to pass
through the slabs and also potentially led to concrete spalling. Furthermore, the anchorage of the
concrete to the foundation was in some places developed in weathered sections of rock, leading to
pullout strength lower than the intended design.
After the incident, the California Department of Water Resources seems to have taken a different
stand on the issue, being more open to suggestions about the construction of the new spillways [
59
].
Still, this response came at a rather late time and is being met with some criticism [
42
,
43
]. However,
their stance on providing free access data to the public and attempting to communicate and cooperate
with local residents and interest groups is definitely a step in the right direction. It must be stated that
this study would not be possible without the large amount of digital information available directly
from the Department of Water Resources and related websites.

Geosciences 2019,9, 37 16 of 24
If there is a lesson that must be learned from this incident, it is that even when a critical structure
like Oroville Dam seems to operate up to standard, one small flaw can emerge at any time and result
in a severe failure due to the sheer scale of the facilities and the conditions they are expected to
consistently work under. While routine official inspections by the dam operators and independent
authorities are a necessity, they are simply not enough as time goes by. Informal inspections of all
related facilities must be conducted by dam operators on a weekly or bi-weekly basis, in accordance
with existing guidelines [
40
], not with the intent of writing official reports, but simply to detect the
telltale signs of imminent failure before the potentially worst outcome becomes a reality. If the dam
operators had noticed the differences in the main spillway chute’s floor slabs between mid and late
January they might have been able to repair it in time and avoid the incident from occurring entirely,
or at least mitigate its results.
Furthermore, this incident shows a possible lack of regulatory requirements based around the
prevention of failures that could occur during normal operating conditions such as what happened at
Oroville Dam. Even though no lives were lost as a result of the incident, some consequences on the
local environment, economy, and communities might be felt in the years to come. In the end, while it
takes a great amount of knowledge, research and responsibility to build a large dam, it takes much
more to consistently operate one and protect it from damage.
Author Contributions:
Conceptualization, A.K., A.T. and D.K.; methodology, A.K.; software, P.D.; validation, P.T.,
A.T., P.P., T.W. and D.K.; formal analysis, P.D. and T.I.; investigation, A.K.; resources, P.D., P.P., T.W. and D.K.;
data curation, P.D. and T.I.; writing—original draft preparation, A.K.; visualization, A.K.; supervision, T.W. and
D.K.; project administration, A.T. and P.T. All authors read and edited the paper before submission.
Funding: There was no funding for this work.
Acknowledgments:
We are grateful to the three anonymous reviewers for their constructive comments which
helped us to improve an earlier version of this manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
Appendix A
Geosciences 2019, 9, x FOR PEER REVIEW 16 of 24
If there is a lesson that must be learned from this incident, it is that even when a critical structure
like Oroville Dam seems to operate up to standard, one small flaw can emerge at any time and result
in a severe failure due to the sheer scale of the facilities and the conditions they are expected to
consistently work under. While routine official inspections by the dam operators and independent
authorities are a necessity, they are simply not enough as time goes by. Informal inspections of all
related facilities must be conducted by dam operators on a weekly or bi-weekly basis, in accordance
with existing guidelines [40], not with the intent of writing official reports, but simply to detect the
telltale signs of imminent failure before the potentially worst outcome becomes a reality. If the dam
operators had noticed the differences in the main spillway chute’s floor slabs between mid and late
January they might have been able to repair it in time and avoid the incident from occurring entirely,
or at least mitigate its results.
Furthermore, this incident shows a possible lack of regulatory requirements based around the
prevention of failures that could occur during normal operating conditions such as what happened
at Oroville Dam. Even though no lives were lost as a result of the incident, some consequences on the
local environment, economy, and communities might be felt in the years to come. In the end, while
it takes a great amount of knowledge, research and responsibility to build a large dam, it takes much
more to consistently operate one and protect it from damage.
Author Contributions: Conceptualization, A.K., A.T. and D.K.; methodology, A.K.; software, P.D.; validation,
P.T., A.T., P.P., T.W. and D.K.; formal analysis, P.D. and T.I.; investigation, A.K.; resources, P.D., P.P., T.W. and
D.K.; data curation, P.D. and T.I.; writing—original draft preparation, A.K.; visualization, A.K.; supervision,
T.W. and D.K.; project administration, A.T. and P.T. All authors read and edited the paper before submission.
Funding: There was no funding for this work.
Acknowledgments: We are grateful to the three anonymous reviewers for their constructive comments which
helped us to improve an earlier version of this manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
Appendix A
Figure A1. Map of Oroville Dam catchment and selected precipitation measurement stations [34–36].
Figure A1. Map of Oroville Dam catchment and selected precipitation measurement stations [34–36].

Geosciences 2019,9, 37 17 of 24
Appendix B
Geosciences 2019, 9, x FOR PEER REVIEW 17 of 24
Appendix B
Figure A2. L-Moments GEV-Max distribution fit to annual daily maxima of precipitation
measurements, Brush Creek station (BRS).
Figure A3. L-Moments GEV-Max distribution fit to annual daily maxima of precipitation
measurements, station USC00044812.
Figure A2.
L-Moments GEV-Max distribution fit to annual daily maxima of precipitation measurements,
Brush Creek station (BRS).
Geosciences 2019, 9, x FOR PEER REVIEW 17 of 24
Appendix B
Figure A2. L-Moments GEV-Max distribution fit to annual daily maxima of precipitation
measurements, Brush Creek station (BRS).
Figure A3. L-Moments GEV-Max distribution fit to annual daily maxima of precipitation
measurements, station USC00044812.
Figure A3.
L-Moments GEV-Max distribution fit to annual daily maxima of precipitation measurements,
station USC00044812.

Geosciences 2019,9, 37 18 of 24
Geosciences 2019, 9, x FOR PEER REVIEW 18 of 24
Figure A4. L-Moments GEV-Max distribution fit to annual daily maxima of precipitation
measurements, station USC00041159.
Figure A5. L-Moments GEV-Max distribution fit to annual daily maxima of precipitation
measurements, Quincy station (QCY).
Figure A4.
L-Moments GEV-Max distribution fit to annual daily maxima of precipitation measurements,
station USC00041159.
Geosciences 2019, 9, x FOR PEER REVIEW 18 of 24
Figure A4. L-Moments GEV-Max distribution fit to annual daily maxima of precipitation
measurements, station USC00041159.
Figure A5. L-Moments GEV-Max distribution fit to annual daily maxima of precipitation
measurements, Quincy station (QCY).
Figure A5.
L-Moments GEV-Max distribution fit to annual daily maxima of precipitation measurements,
Quincy station (QCY).

Geosciences 2019,9, 37 19 of 24
Appendix C
Geosciences 2019, 9, x FOR PEER REVIEW 19 of 24
Appendix C
Figure A6. Log-Pearson III distribution fit to annual unregulated maximum 1-day inflows at Oroville
Dam (m
3
/s).
Figure A7. L-Moments GEV-Max distribution fit to annual unregulated maximum 1-day inflows at
Oroville Dam (m
3
/s).
Figure A6.
Log-Pearson III distribution fit to annual unregulated maximum 1-day inflows at Oroville
Dam (m3/s).
Geosciences 2019, 9, x FOR PEER REVIEW 19 of 24
Appendix C
Figure A6. Log-Pearson III distribution fit to annual unregulated maximum 1-day inflows at Oroville
Dam (m
3
/s).
Figure A7. L-Moments GEV-Max distribution fit to annual unregulated maximum 1-day inflows at
Oroville Dam (m
3
/s).
Figure A7.
L-Moments GEV-Max distribution fit to annual unregulated maximum 1-day inflows at
Oroville Dam (m3/s).

Geosciences 2019,9, 37 20 of 24
Appendix D
Table A1. Annual daily maximum precipitation (mm), Brush Creek (BRS) station.
Year Annual Daily Maximum (mm) Year Annual Daily Maximum (mm)
1986 217.4 2002 211.8
1987 121.9 2003 122.4
1988 2004 113.8
1989 89.4 2005 162.8
1990 93.2 2006 124.5
1991 122.4 2007 109.5
1992 89.4 2008 100.8
1993 170.2 2009 86.4
1994 76.5 2010 206.5
1995 141.5 2011 87.9
1996 179.1 2012 163.8
1997 285.2 2013 56.9
1998 115.1 2014 147.1
1999 94.5 2015 92.2
2000 130.3 2016 218.4
2001 108.0 2017 135.6
Table A2. Annual daily maximum precipitation (mm), station USC00044812.
Year Annual Daily Maximum (mm) Year Annual Daily Maximum (mm)
1913 127 1941 116.8
1914 44.5 1942 114.3
1915 129.5 1943 162.1
1916 125.5 1944 88.9
1917 96.5 1945 92.7
1918 68.6 1946 61.5
1919 96.5 1947 113.8
1920 107.2 1948 64.3
1921 76.2 1949 70.4
1922 116.1 1950 144.8
1923 61.5 1951 95.3
1924 101.6 1952 104.1
1925 97.3 1953 115.8
1926 132.3 1954 109.2
1927 80.3 1955 189.2
1928 76.2 1956 125.5
1929 123.2 1957 119.9
1930 57.7 1958 89.7
1931 97.8 1959 77.5
1932 57.2 1960 66.8
1933 77.7 1961 79.5
1934 63.5 1962 239
1935 114.3 1963 133.9
1936 98 1964 156.2
1937 191.5 1965 88.9
1938 85.1 1966 87.4
1939 81.8 1967 134.9
1940 194.6

Geosciences 2019,9, 37 21 of 24
Table A3. Annual daily maximum precipitation (mm), station USC00041159.
Year Annual Daily Maximum (mm) Year Annual Daily Maximum (mm)
1959 98.8 1988 132.8
1960 168.4 1989 130.6
1961 96.8 1990 91.7
1962 271.8 1991 106.2
1963 125.5 1992 98.3
1964 254.5 1993 103.6
1965 117.9 1994 119.9
1966 109.2 1995 210.8
1967 142.5 1996 145.5
1968 97.3 1997 61.5
1969 121.9 1998 174
1970 115.1 1999 100.3
1971 100.3 2001 105.4
1972 50.8 2002 119.4
1973 160.5 2003 101.6
1974 102.4 2004 102.4
1975 82 2005 153.4
1976 66.8 2006 94.5
1977 70.4 2007 57.4
1978 136.1 2008 106.9
1979 135.9 2009 86.6
1980 157.5 2010 85.3
1981 154.9 2011 95.5
1982 218.4 2012 119.9
1983 117.9 2013 37.8
1984 88.9 2014 112.5
1985 86.6 2015 88.1
1986 123.2 2016 66.8
1987 88.4
Table A4. Annual daily maxima of precipitation (mm), Quincy (QCY) station.
Year Annual Daily Maximum (mm) Year Annual Daily Maximum (mm)
1989 121.9 2004 93.7
1990 80.3 2005 77.0
1991 95.5 2006 101.3
1992 38.6 2007 49.3
1993 201.7 2008 104.1
1994 43.7 2009 90.2
1995 2010 54.1
1996 239.8 2011 61.5
1997 86.4 2012 106.7
1998 96.5 2013 31.5
1999 46.2 2014 117.1
2000 65.5 2015 76.5
2001 63.5 2016 110.7
2002 110.5 2017 123.7
2003 74.7
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