On the Optimal Design of Deep Stormwater Storage and Drainage Tunnels to Address Urban Flooding in Korea Due to Climate Change
Authors: Cho Yong Jun 
Published: July 2024

Numerous factors influence the design of deep stormwater detention/drainage tunnels, which have emerged as the primary countermeasure against severe flooding in urban areas of Korea due to climate change. These tunnels typically consist of an entrance reservoir, a circular reinforced concrete culvert with a flat slope, a downstream reservoir, and an outlet to the river. In this configuration, the pressurized flow within the circular reinforced concrete culvert is designed to be solely driven by the momentum carried by the free-falling stormwater runoff. Lately, this aspect has raised some concern regarding its drainage capacity in the Korean water resources engineering community despite the substantial falling height of around 30m. Among these factors, intensified and heavily localized rainfall, and the abrupt shift from raging flood waves to pressurized flow that fully saturates the circular concrete culvert forming the deep stormwater/drainage tunnel stand out most, along with trapped air pockets occurring during this transition ultimately compromising the drainage capacity of such tunnels. Considering the inherent uncertainties associated with these factors, the design of stormwater detention/drainage tunnels of great depth appears to be fraught with considerable challenges. Despite these uncertainties, conventional design approaches have typically been deterministic, leaving significant room for improvement. Consequently, the spatial scope and diameter of the circular concrete culvert, which directly influence detention and drainage capacity, have often not been optimized, leading to its poor performance. This study aims to establish the necessary database for the reliability-based design optimization of stormwater detention/drainage tunnels of great depth. To achieve this objective, 3D numerical simulations were conducted using interFoam, a multiphase solver among the OpenFOAM-based Tool Boxes. This solver could accurately describe the air pocket formation process and its subsequent interaction with the flow until pressurized flow is fully developed within the circular culvert. It should be noted that this aspect significantly influences the detention and drainage capacity but has not been fully integrated into the current design platform of deep stormwater tunnels in Korea, mainly due to the intrinsic difficulties involved in the analysis. First and foremost, the numerical simulation results in this study demonstrate that the deep stormwater detention/drainage tunnels, equipped with circular reinforced concrete culverts featuring a flat slope, can provide the necessary drainage capacity to convey excess stormwater runoff from rainfall events expected to occur once every 30 years. This finding could address some concerns raised in the Korean water resources engineering community regarding the flat slope. Additionally, the numerical simulation results indicate that the drainage capacity of the deep stormwater tunnel reaches its maximum when utilizing the smallest diameter of the circular culvert, provided that this diameter ensures the maximum permissible flow velocity in the concrete culvert is not exceeded. With the smallest diameter, the drop in flow velocity accompanying the shock waves, defined as the stagnation portion of the flow rapidly expanding upstream, is much weaker due to the substantial inertia carried by the swiftly moving pressurized flow developed in the circular concrete culvert. This inertia, stemming from the smaller diameter, quickly dampens out shock waves, leading to the rapid development and ensuing stabilization of the pressurized flow within the circular concrete culvert, thereby enhancing drainage capacity. It is worth mentioning that these simulation results contradict the empirical perception underlying the current design platform for deep stormwater tunnels in Korea, which suggests that a larger diameter of the circular concrete culvert provides superior drainage capacity. Besides, the simulation results indicate that the drainage capacity of deep stormwater tunnels is influenced by the spatial scope of the air pocket trapped within the circular concrete culvert forming the tunnel. As the diameter of the culvert increases, the spatial scope of the air pocket trapped within the circular culvert expands, thereby reducing the drainage capacity of the tunnel. Furthermore, the simulation results demonstrate that an air chamber installed within a deep stormwater detention/drainage tunnel can effectively control the size of the trapped air pocket. However, if the air chamber is positioned near the downstream end of the circular culvert, where the shock wave originates, the high pressure accompanying the formation of the shock wave would cause the air chamber to fill with stormwater. This can impede its ability to extract the trapped air pocket from the tunnel. Additionally, a larger diameter of the circular concrete culvert results in a greater drop in flow velocity and higher pressure accompanying the shock waves, ultimately compromising the effectiveness of the air chamber as it becomes filled with stormwater. In conclusion, the following recommendations could be drawn based on the previously discussed simulation results: Firstly, even with the deployment of an air chamber, it is imperative to design deep stormwater tunnels in a way that minimizes the spatial scope of trapped air pockets within the circular concrete culvert. Secondly, in the current design platform for deep stormwater tunnels in Korea, there should be a relaxation of the emphasis placed on detention capacity. If the circular concrete culvert can efficiently convey the target stormwater runoff without exceeding the maximum permissible flow velocity, and it maintains the required detention capacity throughout the transient period—from the incipient entry of the flood wave into the upstream reservoir of the deep stormwater tunnel to the fully developed pressurized flow within the circular concrete culvert—there may be no need to secure extra detention capacity. Pushing for excessive detention capacity beyond the previously discussed standard norms not only enlarges the trapped air pockets but also risks compromising the water drainage capacity by nullifying the effectiveness of the air chamber. Thirdly, adjusting the inlet channel to create a spiral flow within the entrance reservoir could mitigate the impulsive force acting at the bottom of the entrance reservoir of the deep stormwater tunnel, leading to lower maintenance costs for these massive structures. Additionally, this adjustment could stabilize the flow within the circular culvert forming the deep stormwater tunnel more rapidly, thereby improving its drainage capacity.