Real-Time Stormwater Management Using Depth, Duration, Frequency Thresholds

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Courtesy of Water Environment Federation (WEF)

ABSTRACT
The City of Austin (City) operates a Flood Early Warning System (FEWS) to reduce public exposure to flash flood hazards. The system is operated and maintained by the Watershed Protection and Development Review Department, Watershed Engineering Division. Advanced information and warning of urban storm water flooding is achieved by the FEWS. A critical component of the system is rainfall detection and interpretation. Radar rainfall detection and forecasting also utilizes the existing ALERT (Automated Local Evaluation in Real Time) rain gauge network. An innovative web-based hydrologic information system built upon radar hydrology and implemented in 2004 has proved to be useful during storms producing heavy precipitation and flooding. This presentation will describe the development and use of Depth, Duration, Frequency (DDF) values in near real-time.

BACKGROUND
Central Texas is often called 'Flash Flood Alley' because of its frequent, intense storms. While large events seem to happen every decade, lesser events also cause public safety concerns. Coordination between the Watershed Protection and Development Review Department (WPRDR) and Office of Emergency Management (OEM), both City of Austin agencies, results in hydrologists and emergency managers working together in the Emergency Operations Center (EOC) to warn of and respond to flooding along creeks and at major intersections in the City. Because of the challenge with identification and timely response during high-risk flood events, an integrated warning system is needed so that appropriate actions and deployment of resources can be effectively managed.

Operation of the Flood Early Warning System (FEWS) helps reduce public exposure to flash flood hazards. The system is operated and maintained by the WPDRD, Watershed Engineering Division. The FEWS is used to identify the development of flood hazard conditions that warrant public safety response for roadway closings and building evacuations. The FEWS consists of a variety of field sensors which transmit stream water level and rainfall data to a central data management computer system for decision-making.

The FEWS has been in operation since 1985 and is considered an ALERT system. Portions of the FEWS have been expanded and upgraded with growth in the service area and advancing technology. Currently, the field sensor network consists of 86 rain gauges, 41 stream depth gauges, and ten automated low-water crossing barricades. Information from the sensors is transmitted by a low frequency FM radio to a repeater station. From the radio repeater station, the information is then retransmitted on a different frequency to the base station – the Emergency Operations Center, where it is managed by the Hydrolynx Novastar software system. The FEWS provides rainfall and stream stage information for 26 major watersheds and it extends over 1400 square miles. The gauge components themselves are provided by both Hydrolynx Systems and High Sierra Electronics. Data received from the gauges is pushed to a File Transfer Protocol Site (.FTP) that is maintained by Hydrolynx Novastar. This information is monitored and used by the National Weather Service and the City of Austin’s publicly available internet site. The information is also sent to the City’s rainfall processing contractor Vieux and Associates, Inc. (VAI). Radar-based rainfall data services are provided via customized Internet access operated as a web service by VAI.

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