The Hollis Reservoir can be assumed to also function as a flood control structure by attenuating peak flows in the Quare River and thus reducing the frequency of flooding in the town of Valencia and surrounding areas. The capacity of that river as it passes through the town is 80 m³/s.
Consider a 10-hour hydrograph (see Table 4) that occurred when the reservoir stage was just at the crest of the weir, at 619.75 ft (MSL). It can be taken that below this depth, the water is available for water supply. That is to say that effectively for flood control, this stage represents zero storage.
Time (h) 0 1 2 3 4 5 6 7 8 9 10
Flow (m³/s) 1 6 18 37 60 75 48 30 15 6 1.5
Two tributaries flow into Quare river and contribute 28 m/s to the flow from the spillway. You are to determine whether the reservoir has been able to attenuate the inflow hydrograph to avoid flooding in Valencia.
Recall that the crest of the weir is at Elev. 619.75 ft (MSL).
Use the stage-storage relation in Table 5, and the flow-head relation for the weir to develop the relation between the discharge, O, and the quantity 2s/∆t + 0 for a 1 hour time step.
The flow per unit width, Q, over the spillway is given by: Q = 1.83H^1.5.
The weir width is 44 m.
Table 5. Stage-storage relation of Hollis Reservoir.
Stage (ft MSL) Storage (m³) Stage (ft MSL) Storage (m³)
619.75 4630682 622.00 5090909
620.00 4681818 622.5 5192827
620.50 4784091 623 5301947
621.00 4886364 623.5 5412193
621.50 4988636 624 5523565