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, 1 . . · Inappropriate development in the floodplain (e.g., buildings too low, too close to the channel, or bloc1dng flood flows), · Development in the watershed that increases flood flows and creates a .. larger floodplain, or · A combination of these two. Here is a typical example of how riverine flood problems develop: A watershed of 10 square miles drains to Small Creek. The watershed originally had many natural depressions and wetlands that stored rain and snowmelt. For the last 1,000 years, the creek has flooded periodically onto the flood- plain. When the area was first settled, the floodplain was only used for, farming~e farmers would lose a crop every 20 to 30 years due to a flood. During the 19508 and 1960s the area began to be developed as suburban subdivisions grew out from an expanding metropolitan area. Bridges were built across Small Creek, but their openings were only large enough to carry the lO-year flood. The channel was moved and straightened to make way for development The people who had owned the land since the 1940s had not seen any floods. There was a general sense that dry land would stay dry, so parts of the floodplain were converted from farming to residential and commercial uses. Parts of the floodplain were built on, using normal construction practices. The first floors were only a foot above ground level. The watershed was gradually changing. Lands that were once forests and farm fields were covered with streets and buildings. Less rain water and snowmelt could soak into the ground, so more of it ran off into the stream. To get the extra surface water away from the buildings, the developers built storm sewers and drainage ditches. The wetlands were drained or filled to allow more buildings or larger farms. Now more THE FLOODPLAIN OF SMALL CREEK. 1950 I I CHANNEL FLOODPLAIN THE FLOODPLAIN OF SMALL CREEK. 1990 ".: I,~~: ':~:: ~'~~~; ~~ --.J L CHANNEL FLOODPLAIN 7