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|Flood control glossary
Here are some common words used to discuss flood control projects.
You can find many more on the California Department of Water Resources’ web site at http://cdec.water.ca.gov/glossary.html
Acre-feet are how engineers measure the volume of water in a lake or reservoir. One acre-foot of water would cover one acre of land one foot deep. An acre-foot contains 43,500 cubic feet. It’s the amount of water the average family of five uses each year.
The Corps is investigating the feasibility of using weather forecasts to make releases from Folsom Dam before a large storm actually arrives. The result is more storage space behind the dam and lower flood flows in the lower American River.
Cubic feet per second (cfs)
This is how engineers measure the amount of water flowing in a river or stream. One cubic foot of water is about 7.5 gallons. The computer monitor sitting on your desk (unless you have a flat screen monitor) is roughly one cubic foot. So, if the flow is 10,000 cfs, that’s about 10,000 computer monitors going by every second.
A dike is an embankment that fills in an area around a lake rim that is lower than the top of the dam. There are eight dikes around Folsom Lake. See Levee.
Flood of record
The flood of record is the highest observed river stage or discharge from a dam at a given location during a particular period. The period could be in days, months, or years.
This is the lowland area that borders a river or stream and is subject to flooding when the river or stream goes over its banks.
Engineers measure the risk of flooding as 1 chance in a particular number, say, 1, 10, 50, 100, 500 or some other number. What people commonly call a 100-year flood has a 1 in 100 chance of occurring in any one year. Storm data shows that the largest storms of record have occurred over the last 50 years. With each large storm, engineers have more information to help determine the flood risk. See One-hundred-year flood.
Freeboard is the vertical distance between the normal maximum level of the water in a river or reservoir and the top of the levee or dam. Engineers design freeboard into a levee or dam to keep waves from going over the top of the structure.
A landside berm is a smaller levee placed on the landside of a larger flood control levee to strengthen it.
People build levees to keep rivers out of their towns, farms, and to confine the water to where they want it to be. Most levees are made of soil. A concrete levee is called a flood wall.
This is the size of a flood with a 1-in-100 chance of occurring in any one-year period. It’s possible to have more than one 100-year flood in any one year. It’s really a statistical measurement. Storm data shows that the largest storms of record have occurred over the last 50 years. With each large storm, engineers have more information to help determine the flood risk. Here’s another way to look at a storm or flood with a 1 in 100 chance of occurring in any given year: There is a 25 percent of having a 100-year flood in a 30-year period, and a 50 percent chance it will happen in a 75-year period.
See Flood risk.
A parapet wall is a concrete wall added to the top of the dam to allow dam operators to store (surcharge) additional floodwater during large storms. See Surcharge.
Engineers use relief wells on the landside of flood control levees to relieve the pressure on the levee from high flows in the river. The water coming through the wells goes into a drainage ditch.
A river gage measures the level of a river or stream at a particular place. Engineers use this information to determine how much water is heading downstream.
This is the naturally occurring slope that borders a river. Looking in the direction the river is flowing, the left bank is on the left and the right bank is on the right. Pretty simple, huh.
Slurry cut-off wall
A slurry cut-off wall creates a watertight barrier to prevent seepage through and under levees during high flood flows. The barrier is constructed by excavating a 3-foot-wide trench up to 80 or more feet deep along the length of the levee and filling the trench with slurry, which is a mixture of soil, cement, clay and water. See Jet grouting.
Around bridges and utility lines, where a trench cannot be dug to place a slurry cut-off wall, a process called jet grouting is used. Crews insert a drill into the levee near the bridge or utility, and inject the slurry mixture into the levee using high-pressure nozzles. See Slurry cut-off wall.
A spillway is the part of a dam over which excess floodwater flows. It’s separate from the normal flood control outlets. The spillway may be part of the dam or separated from it. At Folsom Dam, the spillway is part of the dam.
Also called an energy dissipater, a stilling basin slows fast-moving water from a dam’s spillway in order to prevent erosion of the downstream channel. A new stilling basin will be included in the modifications to Folsom Dam.
Surcharge is the word dam operators use when they temporarily store floodwater above the normal floodwater storage capacity of a reservoir. It’s a safe way to get a little extra storage for a short period of time.
A watershed, also called a drainage basin, is the area surrounded by mountain or hill ridges that drains into a river system.