Surge Response and Double-Peak Formation Mechanism in a Throttled Surge Tank with a Lateral Chamber

Lateral chambers increase surge-tank storage, but horizontal free-surface motion can delay their participation in transient water redistribution. A water–air volume-of-fluid model was used to investigate throttled surge tanks without a lateral chamber and with 40 m and 80 m chambers. Volume-equivalent shaft levels and sectional depths were combined with a quasi-static reference at equal instantaneous water volume, water-volume balances, and proper orthogonal decomposition (POD). Under identical imposed discharge histories, the 80 m chamber reduced the maximum shaft level by 6.15 m relative to the configuration without a chamber, but lowered the subsequent minimum by 3.56 m. The 40 m case exhibited two peaks with a secondary-to-primary amplitude ratio of 0.734. Between the intervening minimum and the secondary peak, chamber release supplied 231.1 m3 against 90.1 m3 of net external outflow, producing an actual main-domain gain of 146.3 m3. Chamber storage maxima lagged the shaft peaks by 9.4 s and 10 s. The first two POD modes represented 89.7% and 86.6% of depth-fluctuation mean-square content. These results link surge-waveform formation to delayed storage and return flow. The findings provide a physical basis for understanding the transient behavior and hydraulic design of throttled surge tanks with lateral chambers.