Hydrographical characteristics of an urban stream flowing through the Seoul metropolitan, Korea

Hydrograph analysis provides very important information to make decisions for achieving a sustainable urban watershed, especially for urban streams sustained by stream flow augmentation activities. However, it can be complicated by human activities to control flood. The purpose of this study was to understand the hydrological features of an urban stream from its hydrograph using public data and to suggest points that should be considered during its analysis. Public hydrograph data of the Yangjae stream flowing through the Seoul, Korea were studied using the web-based hydrograph analysis tool (WHAT) system with Eckhardt filter. The normal stream discharge at the outlet of watershed with area of 62.2 km² is 0.42 m³/s, owing to stream flow augmentation activities. The Yangjae stream periodically fluctuates with tides and is significantly affected by precipitation. The lag time for our study period (Jan 2016–Oct 2018) was 2.9 ± 1.4 h at the outlet and direct runoff comprised over 60% in total stream discharge: the Base Flow Index (BFI) value of 0.388 with the optimized BFI_max value of 0.5083. Because of flood control activities, the stream discharge showed anomalous peak flows during extreme events (July 2016, July and August 2017, and May 2018) which caused overestimation of base flow in hydrograph analysis using long-time data. Further, the user should be careful in the use of the filtering method when analyzing data with resolution finer than the hourly one. Our results can contribute an increase in feasibility of hydrograph analysis using public data in urban watersheds.