The Oroville Dam Spillway Failure

As I’ve mentioned in some of my previous posts, this has been an exceptionally stormy winter for much of California. While these storms have been crucial in eliminating California’s drought, they have also caused a variety of weather-related hazards in the Golden State. And no hazard has been bigger than the crisis on Northern California’s Oroville Dam. Thankfully, the situation has improved over the past week, but there are still a variety of problems ahead. In this post, I’ll provide an explanation of what’s been happening on the Oroville Dam and offer some brief concluding thoughts about dam safety and preventing these types of things from happening in the future. I’ve included many pictures from the California Department of Water Resources. To see these pictures and hundreds more, click here.

The Oroville Dam is located in Butte County, Northern California, and impounds the Feather River to form Lake Oroville. Lake Oroville is the second-largest reservoir in all of California, and at 770 feet, the dam is the highest dam in the United States. After an extremely wet January and a series of intense storms to begin February, operators of the Oroville Dam opened the main concrete spillway to lower the lake level in early February. The spillway is used when the hydroelectric station at the dam is operating at full capacity and additional water needs to be released to prevent the lake from rising further.

On February 7, water flowing down the spillway formed large a crater, and as a result, water escaped the main spillway and began to erode a large channel to the left (viewer’s right in the pictures below). The main spillway continued to erode over the next several days even as the dam operators reduced the flow of water onto the spillway, causing the lake to rise.

On February 11, with the main spillway continuing to erode and the lake continuing to rise, the emergency spillway was prepared for usage for the first time since the dam opened in 1968. While the main spillway is composed of concrete along its entire length, the emergency spillway only has a 30-foot concrete wall at the very top and is simply an earthen slope for the rest of the spillway. The spillway soon began to erode the earth around the concrete wall, threatening its collapse and the subsequent release of a 30-foot wall of water into the Feather River. An evacuation order was issued on 2/12/2017, resulting in the evacuation of 188,000 people downstream of the dam.

With the emergency spillway eroding more rapidly than anticipated, dam operators made the decision to release even more water onto the main spillway to lower the lake level and prevent water from spilling onto the emergency spillway. Though this additional increase in flow eroded the main spillway even further, the failure of the emergency spillway was a much greater threat, and with heavy rains anticipated later in the week, it was imperative that the dam operators lowered the lake level.

Water stopped overflowing the emergency spillway on the evening of the 12th, and on the 13th, sandbags and rocks were added to the base of the concrete wall to repair the damage at the top of the spillway. By the 14th, the lake level had lowered enough to allow the mandatory evacuation order to be lifted.

As of 2/20/2017, the Lake Oroville is at 79% capacity and continuing to lower as the main spillway lets more water downstream. Work continues on repairing the emergency spillway, and once water no longer has to be directed through the main spillway, repair work can begin there too. Unfortunately, it is only February, and the highest reservoir levels, on average, occur in June. Sierra snowpack is over 200% of normal, so I expect the damaged spillway will see heavy usage through the spring.

All of the Sierra Nevada have seen exceptional amounts of precipitation this year, but the Northern Sierra Nevada in particular have been hammered, with the 2016-2017 water year on track to be the wettest winter ever by a wide margin. Out of the 47 reservoirs listed on the California Department of Water Resources website, only five have below average reservoir levels, and only two have below 90% of their average. Lake Oroville is close to “Brush Creek” on the map below.

Over the past week, Lake Oroville has managed to decrease levels from over 100% to 79%, primarily by releasing water over the damaged main spillway. This, along with the repairs on the emergency spillway, are the primary reasons why the mandatory evacuation order has been lifted.

However, other reservoirs impounding the Feather River upstream of Lake Oroville are holding as much water as they can to lessen the inflow into Lake Oroville. In fact, one of them, Antelope Lake, is currently at 109% capacity. Though the most serious threat from the Oroville Dam is now over, huge challenges still lie ahead, including dealing with spring runoff and the worsening erosion along the main spillway.

There are some important lessons from this. Even though hydroelectricity is generally safe, dam failures are catastrophic and more common than many realize. The collapse of the Banquiao Dam due to heavy rains from Typhoon Nina in 1975 killed 171,000 and displaced 11 million, and just last May, the collapse of an iron-ore-tailings-dam in Brazil owned by miners Vale and BHP Billiton killed 17 and has been cited as the worst environmental disaster in Brazil’s history. Even though the Oroville Dam itself is not predicted to fail, the failure of the emergency spillway would have caused immense damage to locations downstream.

Since dam failures are so catastrophic, we should make sure that our dams are kept in good shape and can handle whatever Mother Nature throws at us. The emergency spillway of the Oroville Dam was found to be unsafe in 2005, and a 100-million-dollar renovation project proposed by local communities was denied by the federal government.  I hope that what has happened on the Oroville Dam will spur federal investment to maintain dams across the U.S.

To end on a happy note, here’s a picture of Lake Oroville in January 2014. While the situation on the Oroville dam is very frightening, we can rejoice knowing that California’s drought is over – at least for the time being.