AGENDA #5c
MEMORANDUM
TO: Mayor and Town Council
FROM: W. Calvin Horton, Town Manager
SUBJECT: Report on Automated Local Evaluation in Real Time (ALERT) and Other
Storm Warning Systems
DATE: November 27, 2000
This report discusses automated storm warning systems used to notify citizens and community emergency personnel of potentially dangerous flooding conditions resulting from rainfall events.
In August of this year, Mr. Herschel Slater petitioned the Town Council suggesting that an automated storm warning system should be considered to provide local information warning of impending flooding due to severe weather in and around Chapel Hill. Mr. Slater mentioned a system developed by the National Weather Service called Automated Local Evaluation in Real Time (ALERT). This report discusses the ALERT system and similar systems for possible use in Chapel Hill.
The ALERT system includes two principal elements consisting of hardware used to collect rain and stream flow data and software to analyze the data. The typical hardware for this system is made up of stream gauges, rain gauges, radar, communications devices, computers and public address systems. The computer software analyzes the data in real-time, predicts flood events, and recommends when flood warnings should be issued.
Rain gauges collect data about the amount and intensity of rainfall, and stream gauges monitor the level of flow within the streams. This data is transmitted to a computer where software analyzes the rainfall/runoff relationship and predicts the height (depth) that water will rise at given locations in the watershed. If the computer determines that water is anticipated to rise above certain predetermined levels, it automatically notifies authorities who may decide to issue a flood warning to the public. The public warning may include calling pre-determined phone numbers, activating sirens, and/or notifying emergency personnel.
DOPPLER RADAR
Doppler radar can play an important role in an ALERT system. Doppler radar is able to look inside a storm and determine the intensity of rain that is falling. Based on this intensity, the computer software can predict the amount of runoff the rainfall is likely to generate. This data can be used to predict the height (depth) the streams will reach at given points of interest in the watershed.
The National Weather Service in Raleigh uses the Doppler radar to analyze rainfall in order to issue flash flood “watches” and “warnings”. We believe that this is the best existing flood warning method available to Chapel Hill residents and businesses. A “watch” means that conditions are favorable for flooding, and a “warning” means that flooding is imminent within a region. The Doppler radar is most useful when it can provide a “warning” with enough lead time for people to take necessary precautions. As we experienced in the July 2000 storm, this may not always provide sufficient information or lead time to the public.
The National Weather Service can provide a two-hour lead time for an area of approximately 300 square miles, which is about the size of an average county. They are only able to predict with some certainty that the storm path will cross all or a portion of that area. They do not have the ability to specify precise locations within an area or watershed that will be impacted during a given time period.
EXISTING ALERT SYSTEMS
The City of Milford, Connecticut (population 49,000) is located on Long Island Sound along the Wepawaug River. Fifty-seven percent of the population lives within the 100-year floodplain. Milford suffered from four (4) major flood events during the 1990’s. In 1993, the city installed an ALERT system along the Wepawaug River. Their ALERT System can provide the residents with as much as five hours of lead-time to make preparations prior to a flood. The initial cost of the system was $130,570. The Wepawaug River has a very large drainage basin, and the considerable time it takes for water to flow from the upper reaches to Milford accounts for the five-hour warning time.
When drainage basins are large or storms cover a large area, significant flood warning time can be provided for people to protect themselves and their belongings. There is a multi-county ALERT system in use in Western North Carolina, where the drainage area is large enough to allow warnings with sufficient lead-times to be issued concerning pending flooding.
CHAPEL HILL ALERT SYSTEM
The three primary watersheds within Chapel Hill; Morgan, Booker, and Bolin Creeks are relatively small. While technology is available that could be used to issue flood warnings in Chapel Hill, the lead-time for those warnings would be short, allowing only limited time to implement necessary action in the community to prepare for a flood.
As an example, if the Town were to place an ALERT system on Booker Creek at Piney Mountain Road, it would likely only allow for approximately ½ hour lead-time prior to flooding at Eastgate. ALERT could be somewhat more practical for the larger Bolin or Morgan Creeks drainage basins, depending on the area of interest requiring a flood warning for evacuation or protection of property.
The exact cost of installing an ALERT system on some or all of our basins would require an analysis by a consultant to determine how many locations would need to be monitored, rainfall/runoff relationships, stream characteristics, monitoring locations, and notification methods. We estimate that it would cost between $150,000 and $200,000 to develop and install an ALERT system on Morgan, Bolin, and Booker Creeks. The actual cost would depend on the selected system components. Because of the relatively modest size of the watershed areas, warning lead times would be limited.
The ALERT automated storm warning system has proven to be useful for certain areas, generally those that have drainage basin characteristics that allow for warning lead-times of two hours or more. The basins within Chapel Hill are not large enough to allow adequate lead time to effectively notify people of pending flooding.
The National Weather Service issues flash flood “watches” and “warnings” for all areas, including Chapel Hill. Currently, these notifications give the most accurate information with the greatest amount of lead time possible. However, they are not specific to local sites.
One could argue that the ability to know thirty minutes ahead of a flood event is not worth $150,000-$200,000. One could also argue that even one half-hour could provide effective warning and is a worthwhile expenditure. Irregardless of how one sees the balance, there is no clear source of funding now. We believe that if the Council establishes a stormwater utility, it may be practical to fund and operate such a system at that time.
1. Herschel Slater letter dated August 30, 2000 (p. 4).