Real-time Flood Forecasting
We’ve learned to predict typhoons. What is required to predict the floods they bring?
When geography teachers instruct their students about the great rivers of the world, the Amazon, Nile, Yangtze, Mississippi and Yellow usually head the list. Those are truly large rivers, but they earn their distinction by length. From a hydraulic engineer’s perspective, however, neither length nor even total discharge is the most important characteristic. When it comes to flood control and prediction, peak discharge per unit area of watershed (specific peak discharge) is the essential criterion, because it describes a river’s volatility.
The five rivers that lead in this category may be unfamiliar to you. All are found on the island of Taiwan (Figure 2). The specific peak discharge of the Shinano River in Japan is outsized compared to the other storied rivers, yet it is dwarfed by the river systems of Taiwan. The explanation for their singular flows is relatively simple: Taiwan is very steep and very wet. More than 30 percent of the terrain is mountainous, with elevations above 1,000 meters, and more than 60 percent consists of hills and plateaus 100 meters or more above the remaining plains. In other words, much of the country consists of channels to watersheds. Annual rainfall is 2.5 times the world average at 2,500 millimeters. Most of the rain arrives between May and October, and most of that arrives with typhoons, of which there were 350 in the past century.
We have become accustomed to accurate weather forecasts, trusting them even several days into the future. If the weather announcer says bring an umbrella on Tuesday, we are wise to listen. In the Pacific region, the weather includes typhoons, and these tropical cyclones bring not just rain but floods. In Taiwan the floods originate in the mountains as huge amounts of water channel toward the river basins. The channels converge in the race to the ocean. The changes in the volumes of flow in just a few moments can be spectacular. Can these flows be predicted? The question is a critical one for the millions of people who live in Taiwan’s lowlands.
Between 2001 and 2005, we worked on a project to create a real-time flood-forecasting model for complex river systems, with the specific and immediate goal of forecasting the torrents that arrive in the Tamsui River Basin. The Tamsui River system in northern Taiwan is a main geographical and economic artery of the greater metropolitan Taipei area. It has signature features that make it unique, beautiful—and dangerous (Figure 3). We undertook the development and implementation of the real-time forecasting model as members of the Hydroinformatic Research and Development Team (HIRDT) of the National Taiwan University, sponsored by the 10th River Management Bureau of the Water Resources Agency of Taiwan. As we will describe, our efforts were rewarded by gratifyingly accurate results when our model was challenged, barely after we had begun our work, by an epic storm.