Exploring the Feasibility of Restoring the Han River Sandy Beach Through Dynamic Beach Nourishment Techniques in Coastal Engineering: Once a Source of Everlasting and Cherished Memories for Seoul Citizens

Due to extensive heavy rainfall in the summer, the Han River used to appear as a dry river, with much of its riverbed exposed during all seasons except the rainy season. As a result, the Han River once boasted vast sandy beaches covering 3 million square meters, creating a beautiful waterfront beloved by many people. However, the Han River Comprehensive Development Project, carried out in the 1980s to address frequent flooding issues, led to the disappearance of most of these sandy beaches. The implementation of the Singok and Jamsil submerged weirs, part of a project to add a reservoir function to a segment of the Han River that crosses central Seoul, changed the river's character. It became a "Green River," filled with water year-round, rather than the "White River" it used to be. Additionally, the construction of the Riverside and Olympic expressways along the northern and southern banks of the Han River has made access for the general public more difficult, and the Han River is currently used to a very limited extent. However, with the completion of the Gyeongin Canal on May 25, 2012, which restored access to the West Sea via the Han River—blocked for political reasons since the end of the Korean War—Seoul now resembles port cities like New York, Tokyo, and Sydney. This development brings new opportunities for more diverse and dynamic cultural experiences. Unlike the other cities mentioned, Seoul is unique in having a river over 1 km wide running through its center. In the near future, the once-idle Han River will be filled with beautiful yachts, large boats, and cruise ships sailing through the wind. A closer look at the Han River waterway before its development reveals vast sandy beaches along the inner riverbank on the downstream side of the river bends, such as Hangang, Sapyeong-ri, Ttukseom, and Jamsil. Most of these beaches disappeared due to dredging during the comprehensive development of the Han River in the 1980s. The formation of an accretion-friendly water environment along the inner riverbank on the downstream side of river bends results from the quasi-dead zone along the inner riverbank. This is aided by water rushing towards the outer riverbank due to inertia, maintaining linear motion. This phenomenon creates a secondary flow from the outer bank to the inner bank, driven by the difference in water levels between the inner and outer riverbanks. Even after the completion of the Han River Comprehensive Development Project, it is believed that this secondary flow still persists. This perspective suggests that if sand is initially introduced into upstream areas where erosion is dominant, it will be carried by the river flow to areas where siltation is prevalent. Therefore, periodic dynamic beach nourishment could potentially restore beautiful white sandy beaches close to their natural state (Brunn: 1962; Dean: 1973; Dean and Dalrymple: 1991, 2002). Restoring a sandy beach with golden sand gleaming under the setting sun to the Han River waterway, which reconnects to the West Sea through the newly built Gyeongin Canal, will further enhance Seoul's cultural offerings. This study aims to explore the possibility of restoring a sandy beach on the Han River in detail. To achieve this objective, the author closely examined recent bathymetric survey results from the Han River Flood Control Center to assess the impact of the reservoir aspect added by the Singok and Jamsil submerged weirs on the river's topography. This analysis revealed that the wavelength of the ripples on the Han River's riverbed is on the same scale as the river's width. These values are lower than those typically found in natural rivers, indicating that the newly added reservoir aspect of the Han River and periodic discharges from the upstream dam significantly affect the river's topography. However, the quasi-dead zone along the inner riverbank on the downstream side of the river bend, which bypasses the main water stream and facilitates the formation of the wide sandy beach, was clearly visible on the topographic map. This area exhibited the presence of the quasi-dead zone through its shallow water depth. These observations suggest that the accretion-friendly water environment still exists along the inner riverbank on the downstream side of the river bend. Therefore, it appears feasible to restore the sandy beach, filled with gleaming golden sand that remains only in our memories, through dynamic beach nourishment. This accretion-friendly environment is likely created by the secondary flow from the southern riverbank, combined with the quasi-dead zone along the inner riverbank that bypasses the main water stream. Building upon these observations, the author conducted a numerical simulation to validate the aforementioned perspective. The objective was to ascertain the spatial extent of the quasi-dead zone at the Han River bend and the characteristics of the secondary flow against flood waves with a 50-year return period. To accomplish this, the author employed interFoam, a multiphase solver, within the OpenFOAM-based toolbox. The simulation results revealed the presence of a quasi-dead zone approximately 50 meters wide, diverting the main flow stream on the northern riverbank before and after Dongjak Bridge. Additionally, a large eddy, known as the primary driving mechanism for sand waves on the riverbed, was observed. Furthermore, the presence of a secondary flow from the outer riverbank to the inner riverbank around the river bend was identified. The water level difference between the outer and inner riverbanks across the river, caused by water rushing towards the outer riverbank due to inertia—the driving mechanism for this secondary flow—was simulated to be around 0.6 meters. When exposed to a flood wave with a 50-year return period, the bottom shear stress increased to 0.001 N/ρ on the northern riverbank before and after Dongjak Bridge. Converted to Shields number, this value equals 0.3641, surpassing the critical Shield’s number of 0.05. However, the sands saltated from the riverbed are entrapped by the large eddy mentioned above and are presumed to return to the bottom of the northern riverbank when the flood wave subsides.