An aerial view of the Village of Dundee with overlays of the rights-of-way (red lines) and a number of locations of GPS shot for elevation (blue dots), which indicate partial flood levels Making More Informed Decisions Continued from page 27 implemented better treatment measures to meet the newer requirements. However, while the community was expanding, many older residential areas along the river had CSO-era and earlier systems that contributed excess rainwater to the river. This water went to the treatment plant, rather than the storm water system. Recently, the village made aesthetic improvements to the riverbanks—with DEQ approval—by adding a small number of structures along the banks that altered the river's flow, direction, and magnitude. These changes, both man-made and natural, modified the river and its flow enough that the floodplain map needed to be reevaluated. The amount and flow of water are the bases for 28 ArcUser Winter 2010 calculating the floodway and floodplains. In early 2008, an unexpected problem developed with the storm water system. Large amounts of rainfall diverted from homes to the river brought the problem to light. Older homes were diverting more water into the sewer system than had been anticipated. This caused the treatment plant to overflow into the river and eliminated its effectiveness as a water treatment plant. These homes returned excess rainwater directly to the sewer system via house foundation drains, roof downspouts, and sump pumps. Because periods of high rainfall could overflow the treatment plant, the village began fixing the remaining portions of the storm water system. However, there was no list to indi- cate which houses remained partially connected to the sanitary sewer system. The problem reoccurred in early 2009 when three days of rainfall caused the village to experience approximately a 100-year flood as defined by the most recently revised hydraulic analysis for river flow by the Federal Emergency Management Agency (FEMA). The water reached levels recorded in the village only twice before by the National Oceanic and Atmospheric Administration (NOAA). This event raised a serious question: During a severe weather event, how can the village determine which residences and businesses are in the predicted floodplain so it can warn them? To solve this problem and more easily www.esri.com