1.3 Flood control space
Overview - flood control space
Flood control reservoirs collect water at times of high flow and then release it slowly over the course of the flood and during the subsequent days or weeks. In order to ensure the reservoir maintains the capacity to attenuate a design flood, planners set a requisite volume of flood control space in the reservoir which must remain empty during the flood season (Figure 1.3a).
Figure 1.3a. Example diagram of reservoir space allocation to meet different objectives
Designating flood control space (historical)
While it is essential to test final project designs through a detailed operation study, there are approximate methods for estimating the flood-control volume needed to attenuate the design flood without causing downstream damage, which were used historically. The 1975 Hydrologic Engineering Methods for Water Resources Development provides an approximate graphical method illustrated in Figure 1.3b, which was historically used as a starting point for approximating requisite flood storage space. The method uses results from a flood frequency study to develop a curve of runoff-versus duration at the design flood level (see OSU Stream tutorial for flood frequency methods). The slope of the tangent line represents the objective release and the intercept signifies the required flood control space. Thus, Figure 1.3b represents a dam with a 100-yr design flood and 30,000 unit/day objective release rate. The dam requires 155,000 units of active flood control storage to protect against a 100-yr flood.
Figure 1.3b. Graphical method to determine requisite flood control space
To more accurately determine the flood control space required to regulate the design flood size, planners route a hydrograph (synthetic or observed) through a hypothetical reservoir with unlimited storage capacity and flow releases at the objective rate. In the simplest case, the flood control space is designated as the maximum storage volume plus some safety factor.
Consider a 200-year design flood with a 3-day average discharge of 163,000 cfs (Figure 1.3c hydrograph) and an objective flood release rate of 115,000 cfs (comparable to the planning conditions at Folsom reservoir). Starting with an empty flood control space (volume = 0 AF), press the ROUTE button to see how flood water could accumulate in the reservoir during a flood (light blue line). Reservoir operating manuals dictate the amount of discharge (purple line) released before, during, and after floods. The release level is based on reservoir and downstream stage, and the capacity of the reservoir outlets. The routing shown below follows typical rules for flood control operations.
Figure 1.3c. 1996-1997 New Years flood scaled to a 3-day average discharge of 163,000 cfs