The Impact of Land Development on Runoff and the Analysis of Runoff Adaptation Resilience: A Case Study of Tainan City, Taiwan

Urban surface runoff is intricately linked to the spatiotemporal distribution of rainfall and surface water flow dynamics. Therefore, when conducting simulations of urban surface runoff, it is essential to account for the hydrological and physiographic conditions within the study area. This research analyzed the terrain and landforms of the study area, arranged computational cells, and selected appropriate flow equations to simulate surface water movement. Using the concept of quasi-two-dimensional flow, a flood simulation core model was established and applied to simulate rainfall-runoff processes within metropolitan areas. Adjacent grid cells were connected using the continuity equation and suitable flow laws derived from quasi-two-dimensional flow theory to assess water levels and flow rates between cells.

A physiographic drainage-inundation model (PhD model) employed in this study utilized unstructured cells constructed based on physiographic conditions. The cells were designed and calibrated in accordance with current land use, spatial planning functional zones, or post-implementation urban planning zoning. The model encompassed five major river basins in Tainan City (Bazhang River, Jishui River, Zengwen River, Yanshui River, and Erren River), covering a total area of approximately 2,446.62 square kilometers and divided into 30,500 computational cells.

The analysis is based on a geomorphic scenario using current land use for runoff analysis, incorporating scenarios with 10-year return period rainfall, a quantitative torrential rainfall event (350mm/24hr). Both scenarios utilized the 10-year return period tidal levels along Tainan’s coastal areas as downstream boundary conditions. The results identified flooding hotspots near the Xiaying Interchange, Shinshih Interchange, and Rende District.

To assess the impact of future development areas on Tainan's flood risks, the study adjusted the CN (Curve Number) values of corresponding cells in PhD model to simulate flooding under the 10-year return period rainfall scenario. The findings revealed that future developments would exacerbate flood risks in Tainan, with significant increases in flooding depths observed in areas near Shinshih, Gueiren, and Rende Districts. The maximum increase reached up to 0.15 meters.

Finally, the study explored integrating runoff allocation plans into spatial planning to enhance urban flood resilience. Using the Zengwun-chi Drainage Plan as a case study, the simulation assessed the flood mitigation effects of implementing runoff detention and storage measures. Results indicated that areas with larger flood storage capacities exhibited more substantial flood reduction effects, with maximum reductions in flooding depth reaching 0.13 meters, while areas with smaller capacities showed limited effects.

In conclusion, this study established a reliable physiographic drainage-inundation model and simulated the impacts of various rainfall scenarios and future developments on flood risks in Tainan City. The findings serve as a valuable reference for governmental authorities to evaluate potential disasters associated with regional development and formulate mitigation strategies during urban planning processes.