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''''.-.".'''''' GOJ4G8 DEVELOPMENT OF A FLASH FLoOD DETECTION AND WARNING SYSTEM ,FOR BOULDER COUNTY, COLORADO. INTEGRATION OF FLOOD DETECTION TECHNOLOGIES WITH PUBLIC SAFETY OECISION MAKING .' .~ ....'.. 'Dona1d G. Van Wle Boulder County Sheriff's Department Emergency Services Division Boulder, Colorado 1. INTRODUCTION Boulder County encompasses about 650 square miles In north central Colorado. Its western boundary. Is the Continental Divide, with elevations ranging from 12,000 to.14,OOO feet. The eastern boundary of-the county lies on the high plains, at an elevation of 'approxlmate1y 5,000 feet. The western two-thlrds'of the. County consists Of foothll1s.and high mountains, transected by. seve,ra1dratnage .systems which generally flow from west to.east.wlth mainstream slopes of 3% to 10~.' The 'streams are subjec.t to flooding from snowmelt augmented by rainfall In the months of May and June, and to flash flooding from orographic thunderstorms.from'May through September.' '. . Boulder County. has a population of about 220,000 people, gO% of whom reside on the plains of the eastern third of the county.. The cttles of Boulder and Lyons are built on the banks of Boulder and St. Vraln. Creeks, resp~ctlvely, with the c!.ty limits extending dlrectly to: the mouths. of the canyons.. In ,1~1~,;.the .Corps of>.Englne~r.s es.t Imated that damage fraD flood to the City .of, ' . Boulder could exceed 22'nji1l1iindollars,.';l'- figure which maYeasl)y.. be, d~ubledby now. Tom Downing, working at the Institute of Behavtoral Sciences at the Un1verslty of Colorado,' estimated that gO or more 1tves would be ~ost In Boulder In a 100 year flood. In July, 19'6,;131 lives were lost In the Big Thompson Canyon flash flood about 25 miles north of Boulder County. Rain' .whfch produces life-threatening flash floods falls from thunderstorm cells generally situated over the foothills In the middle third of the county. Storm cells may be comparatively small but Intense, producing . rainfall which Is highly variable over the basin. When the Infiltration capacity of the shallow mountaln soil ls exceeded, runoff can produce flood peaks In the long, narrow canyons In about three-quarters of an hour. Little attenuatton of these peakS occurs as they are translated downstream toward the plains; travel times of these peak' flows depends on stom.:' location, and ranges from a few. minutes to over two hours. .Typlcally, peak flows are expected ln the downtown Boulder area wlthln 1.5 hours of peak ralnfall. The storm event producing the 100 year flow Is estimated to release 2.5 to 3 Inches average precipitation In three hours over the portion of the basin below 9000 feet. Several severe floodfhave been experienced since the first. settlement of the area In the 1860's, and peak flows have been meosured or esttmated for several of these. .Howe.er, precipItation. .records for"the 'foothll1s are vlrtuall:L....c~__. non-existent except for the'last 30 years; during which major floods have'not occurred. Therefore.. hydro10glc_delllng of the basin must proceed with minimal data relating: rainfall Intensity' to basin responsei 2. WARNING NEEDS The primary goal of Boulder County's flood detection and warning system Is to provide TIMELY and ACCURATE warning' to the public to reduce or prevent the loss of life from . flooding. To Induce people 'to take action to protect themselves, the warnings must contatn sufficient Information to be convincingly credible, must be given' with sufficient lead time for hId 1 vldua Is - toover'come - their' reluctanceto.act, and' must be' repeated In substantially' the same form'by all' methods of dissemination to produce a mutually reinforcing saturation effect," . . Warnings Issued at the local level are much more drastic and Invasive than flash flood warnings Issued 'by the Notional Weather Service (NWS). Once the decision' to'warn Is made locally, Public Safety agencies are.utlllzed to broadcast the warnjng message throughout the flood prone area using publiC address'systems on police and 'fire vehicles, affected schools are . closed and evacuated, high hazard roads are blocked, c1v1l .defense sirens are sounded, and local TV and radio programs are lnterrupted t~ carry the warning message. The message ls also broadcast on a local public safety frequency which can be monitored by citizens having tone alert radlos activated from the flood command center. These radios are located In major faclllttes In the flood plain, such as schools, hospitalS and Industrial plants, and are now a requlrement for new construction ln flood prone areas of the City of Boulder.