<p>In this study, a second-order accurate temporal discretization scheme is developed and implemented within an incompressible Smoothed Particle Hydrodynamics (ISPH) framework to enhance numerical stability and accuracy in highly transient free-surface flows, such as mountainous river flows interacting with obstructions. The proposed formulation is first subjected to a systematic particle resolution study and validated against well-established laboratory dam-break experiments. The validation results demonstrate good agreement in terms of flow evolution, leading-edge propagation, and free-surface dynamics, confirming the reliability of the developed model. Building upon this validated framework, an extensive parametric investigation is conducted to examine the effectiveness of rectangular obstacle arrays in damping the kinetic energy generated by dam-break flows, which serve as idealized representations of natural or engineered structures in steep river corridors. The effects of obstacle number, width, and height are analyzed independently using the ratio of total instantaneous kinetic energy to the initial potential energy as a quantitative performance metric. The results reveal that increasing the number of obstacles significantly enhances energy dissipation up to a threshold, beyond which further increases yield diminishing returns. While variations in obstacle width exhibit a relatively minor influence on energy attenuation, obstacle height is identified as the dominant parameter governing kinetic energy damping in mountainous river flows, primarily due to the enhanced pressure drag induced by increased flow blockage against rectangular-shaped obstacles. Overall, the findings highlight the critical role of obstacle configuration in mitigating the destructive potential of flash floods in mountainous rivers and demonstrate the capability of the proposed high-order ISPH model as a robust tool for the design and assessment of energy-dissipating structures in hydraulic engineering applications.</p>