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I I I I I I I I I I I I I I I I I I I HISTORIC FLOODING The residential, public, and agricultural properties in and adjacent to the drainageway in Direct Flow Area 0054 are subject to periodic flooding from high intensity and/or high volume rainfall events and, more recently, from nuisance flows generated by newly constructed residential developments. Before the early part of the decade, long time area residents contend that there were no nuisance flows and larger, less frequent floods were not of the magnitude that they are today, but that damage to the irrigation canals and agriculture lands did occasionally occur. Only within the last few years have the drainage problems in DFA 0054 become serious enough to local residents that attention, analysis, and corrective measures are being undertaken by local officials. There are no rainfall or runoff gages in the basin, but there is some recorded information and a few eyewitness accounts of floods on 0054, specifically along the southernmost tributary at 112th Avenue, Holly Street and Riverdale Road. The occurance of the problems at these areas has become more frequent in recent years and has prompted some of the long time residents to seek legal council to bring about solutions to the local flooding problems. One of the first documented floods on this drainageway occured on July 21, 1977, after a thunderstorm dropped approximately 1-3/4" of precipitation. The flow generated by this storm was of sufficient magnitude and duration to cause the embankment at Holly Street to be breached and subsequently severly eroded. At the time of the flood, the detention ponds upstream from Holly Street had not been constructed. Figures 1a and 1b are photographs of the problem areas after this flood. On April 20, 1978, one inch of rain reportedly fell within an 18-hour period on the basin and caused shallow flooding on the properties downstream of Holly Street. In this storm, the embankment at Holly Street was not overtopped, but Riverdale Road was overtopped and shallow flooding occured in the fields downstream from Riverdale. Similarly, the storm of May 17, 1978, caused the partially constructed detention facility at Holly to fill and finally overtop the road embankment. Partial failure of the embank- ment occurred as heavily silt laden waters flowed over Holly Street and spread over the fields downstream. -5- la. Holly Street Embankment Failure - July 21, 1977 , lb. Holly Street Embankment Failure - July 21, 1977