FAQs • Issaquah, WA • CivicEngage

What is a 100-year flood?: The term "100-year flood" is misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1 percent chance of being equaled or exceeded each year, based on a statistical analysis of streamflow records. Thus, the 100-year flood could occur more than once in a relatively short period of time, and since it is based on past flood history, this estimate can be wrong if the climate shifts. The 100-year flood is used by the National Flood Insurance Program as the standard for floodplain management and to determine the need for flood insurance. The 100-year floodplain corresponds to the area inundated by the 100-year flood, as determined by state-of-the-art hydraulic modeling and aerial topographic mapping.

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Flooding

1. What is a 100-year flood?: The term "100-year flood" is misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1 percent chance of being equaled or exceeded each year, based on a statistical analysis of streamflow records. Thus, the 100-year flood could occur more than once in a relatively short period of time, and since it is based on past flood history, this estimate can be wrong if the climate shifts. The 100-year flood is used by the National Flood Insurance Program as the standard for floodplain management and to determine the need for flood insurance. The 100-year floodplain corresponds to the area inundated by the 100-year flood, as determined by state-of-the-art hydraulic modeling and aerial topographic mapping.

2. Why is the City prone to flooding?: Flooding along Issaquah Creek, Tibbetts Creek and other nearby streams is a natural phenomenon due to the fact that much of the City is located in a floodplain. Historically, areas within Issaquah that are located along streams and prone to flooding were developed as farmlands. As such, the farm properties were only minimally affected by high water. Early newspaper accounts generally wrote of flooded farmlands washed out roads, and an occasional flooded basement.

3. Why can't I build a flood wall around my property?: Floodwalls are illegal under the Issaquah Municipal Code. Floodwalls alter flood patterns, causing impacts to neighboring properties. However, floodwalls are allowed immediately adjacent to buildings, to protect structures only.

4. Can I remove fallen trees from the stream since they may cause flooding?: Logs in streams provide habitat for fish. The State Department of Fish and Wildlife, which manages state fisheries and has jurisdiction over all activities that physically affect streams, strictly prohibits removal of logs from streams. An exception to this policy is if a log creates an imminent hazard during a flood. The City cannot grant such permission, nor assist private property owners in removal (unless it threatens a bridge or other public property).