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49 DAM,BREAK MODELING breach hydrograph. The dam breach develops when the water level at the dam reaches a preselected elevation. The outflow hydrograph is computed with a hydrologic (storage-continuity) method, or with a hydraulic method used for streamflow routing. Instantaneous flow through the breach is approximated by broad-crested weir equa- tions for triangular, rectangular, or trapezoid shapes. The breach develops during a specified time period and pro- gressively takes one of these shapes to a specified max- imum breach bottom width and elevation. The broad-crested weir equation is used to compute spillway and gate outflow. A discharge through outlet pipes also can be specified. The second part is routing the outflow hydrograph through the stream. The routing is mathematically accomplished with the Saint Venant flow equations and a nonlinear implicit finite-difference method. If the stream has segments where the state of flow is different, then the flood wave is completely routed through one segment before being routed through the next. 'Ib accurately represent the conveyance and storage areas of a cross section, the cross-section subareas can be specified as being active or inactive flow areas. ~ '" / ~ 3 Rfuer 2 DATA REQUIREMENTS AND ASSUMPTIONS The model requires initial and boundary conditions, as well as a physical description of the lakes, dams, and stream-ehannel reach. The model layout shown in figure ,45 indicates the location of the segment numbers. cross ,sections, and computational nodes. Computational nodes, . or nodes, refer to the location where discharge and water- surface elevations are computed inside the model. These data can be either measured or estimated. For this study, most of the data were available, particularly for calibrat- ing the model with observed data to evaluate its capa- bilities on high-gradient streams. Hydraulic routing of the dam-break hydrograph through the stream channel requires a physical descrip- tion of the channel and other flow characteristics. The geometry of the channel was defined by 12 cross sections (Supplemental Cross-Section Data). The channel was sub- divided into segments to allow for an evaluation of sub- critical or supercritical flow in each segment, primarily based on channel slope (fig. 2). Generally, if the average slope of the segment was less than about 5 percent, EXPLANATION X Cross section 1 Segment number o Segment node 4.75 Miles downstream from tawn Lake Dam 4 !5 ::'-' Olympus dam FIGURE 45.-Dam-break model layout for the lakes-stream system.