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Hat Yai flood prevention strategy opts for InfoWorks RS and FloodWorks

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The southern Thai city of Hat Yai lies 50km north of the Malaysian border. This important commercial city began the journey to its present urban sprawl in the early 1970s and today is southern Thailand's principal town, serving as a popular tourist centre for Malaysian and Singaporean visitors from the south. Thailand's climate is equatorial in the extreme south, while the centre and north experience a tropical monsoon climate.

Most of the country has abundant but not excessive rainfall, mainly between May and October when the weather is dominated by the southwest monsoon. This blows from the Indian Ocean, bringing warm, humid air and banks of cloud. Rainfall in the region of Hat Yai is around 2m annually.

The city of Hat Yai is low-lying, sitting at the base of a roughly circular valley. It is also prone to inundation from the Utaphao river, which flows through its heart, often in concrete channels that provide limited drainage. The Utaphao rises at Sadao, on the border with Malaysia, and flows to Songhkla lake, a coastal lagoon just downstream of Hat Yai that is hydraulically connected to the Gulf of Thailand.

The river suffers from regular flash flooding during the monsoon season – serious flooding occurred in 1988 and again in 2000, due to unusually heavy rains that were attributed to the La Nina weather phenomenon. During a La Nina, normal trade winds become stronger, and cold, nutrient-rich waters occupy much of the tropical Pacific Ocean, increasing rainfall over South East Asia.

During the 2000 La Nina downpours, most of Hat Yai was inundated and flood depths of two to three metres were recorded. Many basements were flooded for up to one week, and there were over 40 fatalities. Such serious flooding has a major impact on livelihoods, as well as seriously affecting the city's economy and infrastructure.

Hat Yai's existing flood forecasting system dated back to 1996. However, the original software did not predict the 2000 floods, demonstrating the weakness and lack of robustness of a manually-operated telemetry system. This failure triggered a project to review the city's flood forecasting capability.

A comprehensive flood mitigation study of the Utaphao river catchment and Hat Yai city areas was undertaken between 2001 and 2003 as a result, with the study awarded to Kasetsart University for its client, the Royal Irrigation Department (RID) of Thailand.

The study utilised Wallingford Software's InfoWorks RS, an integrated software solution for simulating flows in rivers, channels and on flood plains, to undertake the detailed flood mitigation modelling. High-resolution calibrated InfoWorks RS models were developed and used to assess existing flood prone locations and possible future flood mitigation options. The options suggested included numerous bypass channels, reservoir storage, land use guidance in the surrounding area of the city, and a real-time flood forecasting system.

The study culminated in a high profile public hearing held in Hat Yai by the RID in early 2003. One of the report's major recommendations was that a real-time flood forecasting system be established, and that the system should be based on Wallingford Software's InfoWorks RS and FloodWorks, the software's widely-used, modular, dynamic flood-mapping system. Design of the system began in 2003.

Both parts of FloodWorks, the configuration and operational sides, utilise audit control, which enables users to readily track changes in models – it is possible to see who has made edits and run flood simulations. The software also has tools for quality validation and data management, and runs on a client-server system that allows clients at any location with a connection to the server to access the system. The system then generates warnings when event thresholds are passed.

The new system has been operational since June 2004, with the FloodWorks server based at RID's southern regional headquarters in Hat Yai and Event Manager clients in RID headquarters in Bangkok, the Hat Yai local basin office and the Songkhla Province administrative offices in Songkhla. Operational flood forecasting rooms for the city are situated outside the flood risk area. Around 80% of project costs were spent on automated telemetry, which is essential to make the system work.

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