By Kimberly Arsenault
Contributor, EDM Digest
Recent storms that have produced record snowfalls and intense rains have ended the years' long drought in California. Now, residents of the Golden Gate State are facing new extremes from heavy rains, flash floods, mudslides, and widespread and intense flooding that has impacted area dams, levees, and bridges.
Just what has been causing such intense storms throughout the state? In many cases, it has been a phenomenon known as atmospheric rivers. What's making the situation worse is that these systems have been stalling out across regions vulnerable to floods, dumping copious amounts of precipitation that has led to the increased risk of flooding in watershed areas.
According to NOAAs National Weather Service (NWS), an atmospheric river (AR) exists outside the tropics, moves water vapor horizontally, averages 400-600 kilometers wide, and strong ARs can transport water vapor amounts anywhere from 7.5 to 15 times the average flow of liquid water at the mouth of the Mississippi. ARs account for anywhere from 30-50% of the precipitation that occurs on the west coast, usually from just a few of these events. ARs are a crucial part of the global water cycle and contribute to water supplies but affect flood risks in areas vulnerable to flooding, especially if they stall over those regions.
There are many shapes and sizes of ARs, but the ones that contain the strongest winds and largest water vapor amounts can be particularly devastating if they stall over watersheds prone to flooding. When stalled, the intense rains and high winds in the ARs often cause extreme rainfall amounts, filling up lakes and overflowing rivers and creeks. This often causes widespread flooding and high lake levels that can weaken levees, along with other potential catastrophic damage including downed trees, toppled power lines, and mudslides. One known strong AR, the "Pineapple Express," brings tropical moisture from the Hawaii area to locations along the west coast.
Recent Storm are Strong ARs
Recent storms in California have been a result of these ARs and have wreaked havoc on the state and its water control methods, including its dams and levee systems. Rivers and creeks have swollen to record high levels, overflowing their banks and filling up reservoirs as spillways have struggled to manage the surge in the area water supplies. The heavy rainfall has also saturated soils and affected area infrastructure, including bridges.
Impacts of Recent Storms
Here is a sampling of the impacts from the recent AR driven storms to hit California:
- A main spillway failure at the Oroville Dam ultimately resulted in the evacuation of 188,000 people last week after officials feared increased damage to the emergency spillway would likely result in a catastrophic failure.
- A levee broke along the San Joaquin River in San Joaquin County, in Manteca, California forcing the mandatory evacuation of 500 people.
- Santa Clara County's Andersen Reservoir reached capacity and poured over its spillway into Coyote Creek, causing flooding in San Jose, California, in the Rock Springs neighborhood. Officials failed to order evacuations soon enough, causing firefighters to rescue many residents from their homes as water levels rose too high for them to leave.
- Residents of Lake County, California experienced widespread flooding when Clear Lake overflowed its banks early this week. Officials warned that evacuations may be required if lake levels rose any higher. Extreme winds and driving rain also felled trees, downed power lines, and caused other serious damage within the county. Lake County declared a local emergency as a result of the storm damage.
- Water releases began earlier in the week at the Don Pedro dam and reservoir, prompting concerns of flooding for residents living near the Tuolumne River in California. Residents were cautioned to be prepared to seek higher ground. The lake was at 97 percent capacity.
- Rising waters in the Carmel River in Monterey County resulted in an evacuation order for individuals living alongside the river.
- Residents in a Salinas neighborhood and nearby Royal Oaks, a rural location in central California near the Santa Rita Creek, were issued an evacuation order due to a mudslide.
- Bass Lake in Madera County had to increase water discharges which threatened to overflow area rivers so a pre-evacuation advisory was issued for an at-risk community for potential flooding.
- Shasta Dam engineers made an historic water release for 15 minutes on Thursday from the upper gates of the dam to test operational functionality as the lake is at 93 percent capacity. Releases of 70,000 cubic feet of water per minute are flowing into the Sacramento River in preparation for incoming water flows to Lake Shasta from Sierra Nevada and the surrounding foothills after recent storms. Although the dam is functioning normally, downstream officials are working on contingency plans should anything change.
- A levee along the San Joaquin River in the western part of Fresno County was damaged from the heavy rains that have created high flows in the river. A disaster declaration was issued by the Fresno County officials as a precaution. Crews have attempted to repair the damage but repairs have been stalled due to the soft soil and remote location of the levee. A full breech would likely flood the towns of Tranquility, Mendota, and Firebaugh.
- Landslides below the Pfeffer Canyon Bridge in Big Sur, California sheared off a section of the hillside near one of the bridge's anchored column supports. This allowed the bridge to move six or seven inches, creating a massive crack that caused engineers to render it beyond repair, requiring a new bridge to be built.
— NWS Bay Area (@NWSBayArea) February 22, 2017