Less than 5 feet. That was the difference in water level that occurred between the emergency on Sunday night and level of Lake Oroville early Monday morning. According to the Sacramento Bee, “Water flows over the emergency spillway when the lake level is 901 feet. At its highest, at 3 a.m. Sunday, the lake level was 902.59 feet.” As of 4 a.m. Monday morning, the lake level had fallen to 898 feet. The drop of 4.59 feet is good news, but the area is far from out of danger.
A spillway is a structure used to provide the controlled release of flows from a dam. The Oroville Dam has two spillways, and both were in use earlier this week.
The dam is structurally sound, but the concrete spillway is eroding, so water was diverted to the emergency spillway. The 48-year-old emergency spillway had never been used. As water went over the top of the emergency spillway barrier for the first time, it pounded the unpaved surface below and washed away some of earth at the base of the spillway wall. If the spillway wall were to fail, a 30′ wall of water could flood the valley downstream.
This caused the evacuation of nearly 200,000 people as the lake behind the dam reached its maximum capacity. CNN reported, “California towns flee as Oroville Dam threatens to release ‘wall of water’.” The National Guard was brought in to help reinforce the emergency spillway used to draw down the level of the lake behind America’s tallest dam.
While residents have since been allowed to return and authorities are cautiously optimistic, this event is a reminder of the stress record rains can cause.
Analyzing data from a documented spillway failure
Spillway breaches have caused flooding in the past. In December 2010, the Montedoglio Dam on the Tiber River in Italy suffered a failure that flooded the surrounding area. This failure was due to a partial collapse of the spillway. Fortunately, there were no causalities.
Researchers obtained data from the Water Agency for Umbria and Tuscany Region. The data included the physical characteristics of the breach and a discharge hydrograph. The river cross-section geometry was also available, as was data recorded at two downstream gauge sites during the flooding event. The information on the actual downstream flooded areas was also collected. The researchers noted that it was unusual to have access to so much data from an actual flooding event.
Their analysis of the data was shared in their technical note, Spillway Collapse of the Montedoglio Dam on the Tiber River, Central Italy: Data Collection and Event Analysis, which was published in the American Society of Civil Engineers Library in 2014. The goal of the research was to simulate the breach evolution and the downstream propagation of the outflow using the data collected at the dam and at two river gauges downstream of the breach.
According to the researchers, “… when a dambreak event occurs, it represents a great opportunity for collecting data for the breach and corresponding outflow, flood routing, and impacts on the downstream area.”
Based on the data collected, the spillway collapse was investigated by simulating the breach formation and the consequent flood wave. MATLAB was used for calculations to simulate the outflow at the time of the breach and the propagation of the outflow hydrograph. The researchers concluded, “The approach adopted analyzing the partial failure event of the Montedoglio surface spillway is simple and provided reliable results. It can be easily transferred to other cases wherein flooding occurred.”
This was one of the first times data was available from a dam breach and subsequent flooding. This research is a great example of how data collection and analysis can improve our understanding of historical events. it enabled accurate simulations to show how a dam breach could affect a downstream area in future events.