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Frequently Asked Questions on Flooding
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Q: Why is the City prone to flooding?
A: 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.
Q: Why doesn’t the City dredge the channel of Issaquah Creek to reduce flooding?
A: Current Federal and State environmental regulations make it extremely difficult to justify stream channel dredging as a means to control flooding. While it can be done, it is very costly and time consuming to propose such work. While the City was successful in a limited dredging project on Tibbetts Creek (see the Tibbetts Creek Greenway Project description), this project was primarily a restoration project serving multiple objectives, and Tibbetts Creek does not support Chinook salmon, an endangered species. Dredging streams to control flooding is also a very expensive means for controlling flooding, which the City cannot afford given the relatively minor benefits that would result in reduced flood damages. (The total amount of flood insurance claims during the period 1990-2003 was about $2 million. It would take many times that amount of money in capital projects to significantly reduce those losses).
Q: How is the City dealing with flooding?
A: Following the serious flood of February 1996, the City embanked on a multi-year program to implement flood control projects in the City to reduce flood impacts from major floods. Methods that are being employed to improve the flood carrying capacity of the stream, enhance the fish and wildlife habitat of the corridor, and reduce flood losses include:
- Purchase and removal of homes in the floodplain, and purchase of undeveloped lots
- Channel and bridge improvements
- Flood proofing of homes (property owner responsibility)
- Flood insurance (property owner responsibility)
In recent years this program has resulted in several capital improvement projects, including the construction of the Gilman Area and Pickering Area Channel Improvements, the Tibbetts Creek Greenway Project, and many bridges replacements. Washington State Department of Transportation is also helping flooding on Tibbetts Creek by replacing the culverts under I-90 with a bridge, and improving the channel downstream in the State Park. This work currently out to bid and should be completed during 2004. In the coming years the NW Dogwood and NW Juniper bridges will be replaced, improving flooding conditions in the area. Acquisition of several flood-prone properties along Issaquah Creek has also been completed. However, available funding for purchasing property is very limited and is subject to annual budget appropriations.
With regards to stream improvements, no other major projects are proposed in the near future. Minor projects will be pursued on City-owned land and on private property if there is a benefit to multiple properties. Property acquisition is the preferable approach for reducing long-term flood damage costs.
Q: After the 1996 flood, why did the City construct the flood improvements in the commercial area of town, but did nothing in residential areas?
A: Funding of Gilman channel project was assisted by $895,000 in FEMA hazard mitigation grants in response to the 1996 federally declared disaster. The Federal grant funds were justified because of a favorable cost-benefit ratio (i.e., the cost of flood improvements was greater than the predicted long-term flood damage costs in the immediate area). The City tried to obtain Federal grant assistance for other areas of the City impacted by that flood, but was unsuccessful because flood damages were significantly less and a favorable cost-benefit could not be demonstrated.
Q: Flooding seems to be getting worse. Is development responsible?
A: Flooding affects large areas of Issaquah, which until only a few decades ago was farmland, impacting commercial and residential properties alike. The pattern of increased flooding, and associated damages, in Issaquah is due to several factors:
- Continued development within the floodplain, which makes floods more likely to cause greater damage due to closer proximity and greater numbers of structures near the stream channel.
- Increased flood elevations due to reduced channel capacity, which is caused by sedimentation in the stream channel and fill in the floodplain and along stream banks. Natural streams deal with excess sediment by meandering of the channel, creation of bars, and deposition in the floodplain. If a channel is fixed between two fixed banks, the channel cannot move to avoid the accumulated sediment and deposit sediment in former channel bends.
- Larger peak flows due to additional impervious surface area within the watershed. Hydrologic modeling conducted by King County for the Issaquah Creek Basin Plan concluded that current levels of urbanization has caused flood peaks to increase by 8%, and under future unmitigated land use the flood magnitudes could increase by 33% over forested conditions if stormwater detention is not provided at new development. However, future land use would include stormwater detention that would mitigate a large portion, but not all, of the predicted future increase; and
- Larger peak flows caused by greater total storm precipitation in recent years, the product of an apparent upward trend in the long-term cycle of weather patterns. In fact, several large floods occurred early in the century, followed by a long period of reduced flood activity, which was again followed by increased flood activity starting in about 1975.
Q: Why can’t I remove fallen trees from the stream? It seems they will cause flooding if not removed.
A: 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. In those situations verbal permission must be granted by WDFW before logs and other debris can be removed. Call WDFW’s North Puget Sound Regional Office at (425) 775-1311, or for off-hours call the 24-hour "hotline" (360) 902-2537. The City cannot grant such permission nor assist private property owners in removal (unless it threatens a bridge or other public property).
Q: Why can't I build a flood wall around my property?
A: Floodwalls are illegal under the Issaquah Municipal Code. The reason for this is because floodwalls alter flood patterns, causing impacts to neighboring properties. However, floodwalls are allowed immediately adjacent to buildings, to protect structures only.
Q: What does it mean to have a house in the floodway?
A: There are many requirements in City code that apply to all properties in the 100-year floodplain, such as no net fill and no blockage of floodwaters. Under State requirements floodways are more restrictive in basically one way: no new construction or substantial improvements to existing structures (i.e., more than 50% of the value of the existing structure). A house would existing within a floodway only if it was built prior to 1980, when floodway restrictions were first adopted. If an existing structure in the floodway is damaged by more than 50% by any cause, including fire, it cannot be rebuilt. The only exception to this rule is a recent change in State law (soon to be incorporated into City code) that allows substantial improvements on an existing building footprint if the flood velocity in the floodway at the site is less than 3 feet per second. The Flood Insurance Study that documents the basis of floodplain maps has the information to make this determination.
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