Glossary of Key Phrases
Flood Control Requirements - The Corps of Engineers, specifically Water Management, is responsible for defining and monitoring Flood Control Space within both its own reservoirs, as well as within Section 7 (SC7) reservoirs. Corps' Water Management is also responsible for the minute by minute operation of the flood control projects which the Corps owns. Part of this monitoring responsibility entails the daily determination of how much of the reservoir's total storage space should be used for Flood Control Space. Therefore, the Corps computes a Top of Conservation Pool value for every reservoir under our jurisdiction which has authorized flood space. The reservoir storage space below the Top of Conservation Pool value may be used for any of the project's non-flood control authorized purposes. Space above the Top of Conservation Pool value is reserved to store flood waters for later release and is monitored (SEC7 projects) or managed (Corps projects) by the Corps. The rate (i.e., how quickly) at which flood water stored in the reserved flood space is released depends upon many factors. Some of these factors are the downstream channel conditions, the stage or flow at downstream control points, the magnitude of the storm which produced the flood water storage, the forecast of the timing and magnitude of future storms, and the current flood season. There are typically two overlapping flood seasons for projects in the West, namely the Rainflood Season and the Snowmelt Flood Season. Briefly, flood waters above the Top of Conservation Pool are managed as follows:
- Rainflood Season (October - May) During the Rainflood Season, water stored above the Top of Conservation Pool Level is normally evacuated at a rate which will result in 75% of the Rainflood Space used while operating for the last storm being recovered within a 7-10 day period. The Corps, based on the forecast, channel conditions, and other factors, regularly determines and recommends other release rates and Rainflood Space evacuation times.
- Snowmelt Flood Season (April - July) During the Snowmelt Flood Season, water stored above the Top of Conservation Pool Level does not necessarily need to be evacuated. The objective in managing the Snowmelt Flood Space is to release, over an extended period of time, a constant snowmelt flood release in addition to the normal water supply release. This allows the project to fill while avoiding an uncontrolled spill, and keeps the total release during the snowmelt flood release period below channel damage levels. Water Control Engineers utilize snow survey volume forecasts, runoff timing forecasts, and water supply demand forecasts to achieve this objective. In summary, when the Snowmelt Flood Space is shown as encroached, it is simply an indicator that a supplementary release may be required, NOT that the Snowmelt Flood Space must be evacuated.
River Stage Definitions
For Non-Leveed Stream:
- MONITOR STAGE - The Stage at which initial action must be taken by concerned interests (livestock warning, removal of equipment from lowest overflow areas, or simply general surveillance of the situation). This level may produce overbank flows sufficient to cause minor flooding of low-lying lands and local roads.
- FLOOD STAGE - The Stage at which overbank flows are of sufficient magnitude to cause considerable inundation of land and roads and/or threat of significant hazard to life and property.
For Leveed Stream:
- MONITOR STAGE - The Stage at which patrol of flood control project levees becomes mandatory, or the Stage at which flow occurs into bypass areas from project overflow weirs.
- PROJECT FLOOD STAGE - The Stage at which the flow in a flood control project is at maximum design capacity (U.S. Corps of Engineers "Project Flood Plane"). At this level there is a minimum freeboard of 3 feet to the top of levees.
- DANGER STAGE - The Stage at which the flow in a flood control project is greater than maximum design capacity and where there is extreme danger with threat of significant hazard to life and property in the event of levee failure. This is generally 1 foot above project flood stage.
Source: CDEC http://cdec4gov.water.ca.gov/stageInfo.html