1.2 Objective flood release
Overview of objective flood release
The objective release designates the maximum reservoir release rate during a flood, as designated in the operations manual. It is set based on the conveyance capacity of channels below the dam in order to ensure that discharge at a downstream point(s) (referred to as a control point, impact point, or damage center) remains below the project flood stage, a level which could cause damage. In general, the project flood stage was established at a minimum of 3 feet below the top of a levee or bank to include a freeboard as a safety precaution to account for error (see Figure 1.2a).
Figure 1.2a. Cross-section of a leveed river reach with a 3 ft freeboard above the flood stage during the design flood (http://www.ccdpw.com/description.html).
Designating Folsom's objective release (historical)
The river channel below the Folsom dam site can convey 152,000 cfs at the minimum freeboard of 3 feet (see Figure 1.2a). However, downstream landowners, primarily agricultural lands, desired more protection than that provided by the minimum freeboard and requested that the USACE base design and operations considerations on a 5 foot freeboard, such that the channel could only convey 115,000 cfs. Further downstream, about 23 mi (37 km) from the dam, the American River flows pass the State Capitol City of Sacramento, where it can convey 180,000 cfs at the H Street gauge. 4
Based on the information about the historical landuse below Folsom dam and the river's conveyance capacity at different levee freeboards, what objective release would you specify for Folsom dam? Below are some questions you might consider when trying to determine an appropriate level for the objective flood release:
Questions to Consider:
- How might the different landuses and conveyance capacities downstream of the dam site influence your decision (agriculture directly below the dam and urban area further downstream with higher conveyance capacity)?
- What are the advantages and disadvantages to basing design considerations on a 5 foot freeboard rather than a 3 foot freeboard?
- What are the advantages and disadvantages of using a standard freeboard height to account for all uncertainties and potential errors, as opposed to specifying freeboard height based on local conditions?
- What additional factors may be important that are not presented here?
Based on your answers, suggest an objective flood release for the dam:
Designating Folsom's objective release (present)
Since the planning stages of Folsom Dam in the 1940s to the present, Sacramento County witnessed an eight fold increase in population, from 170,000 to over 1.4 million residents in 2010. The population growth, along with expanding infrastructure and increased land value in the floodplains, has multiplied the risk to life and property if a flood were to occur. Considering that risk is the product of the probability of an event occurring and the magnitude of damages from that event, it is clear that risk can increase even in the absence of changes to the flow regime.
Taking these modern demographic and landuse changes into consideration, do you think it is necessary to adjust the objective flood release for the dam?
You will learn more about incorporating future climate change and population growth projections into objective release decisions, visit: Addressing current and future flood challenges (Module 4.1).