2019_KittitasCountyHMP_Volume 1_For Adoption

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CHAPTER 11. FLOOD 11-2 flood. Corresponding water-surface elevations describe the elevation of water that will result from a given discharge level, which is one of the most important factors used in estimating flood damage. 11.1.2 Floodplain Ecosystems Floodplains can support ecosystems that are rich in quantity and diversity of plant and animal species. A floodplain can contain 100 or even 1000 times as many species as a river. Wetting of the floodplain soil releases an immediate surge of nutrients: those left over from the last f lood, and those that result from the rapid decomposition of organic matter that has accumulated since then. Microscopic organisms thrive and larger species enter a rapid breeding cycle. Opportunistic feeders (particularly birds) move in to take advantage. The production of nutrients peaks and falls away quickly; however the surge of new growth endures for some time. This makes floodplains particularly valuable for agriculture. Species growing in floodplains are markedly different from those that grow outside floodplains. For instance, riparian trees (trees that grow in floodplains) tend to be very tolerant of root disturbance and very quick -growing compared to non-riparian trees. Natural floodplain functions can be summarized in the following categories: – Biology: conserve biological resources and support highly productive ecosystems – Hydraulics and Hydrology: conserve the quantity, quality, and flow of water in the floodplain – Geomorphology: Conserve channel and coastal stability – Historic and Cultural Resources: Avoid or minimize adverse effects to sites possessing historic and cultural significance, including archaeological resources – Cultivated Resources: protect stream banks and vegetative buffers to reduce runoff from entering water sources – Natural Hazard Protection: protect human life, health, welfare, and property from flood hazards The 2016 Kittitas County Shoreline Master Program update outlines policies and management practices consistent with the protection of these natural floodplain functions. The plan also classifies floodplain areas by environmental designation. These classifications and corresponding maps were reviewed to identify the areas of Kittitas County most critical to protecting natural floodplain functions. The vast majority of these areas are the floodplains surrounding streams, rivers, and lakes in unpopulated areas of the county. A smaller number of floodplain areas are located in smaller neighborhoods along rivers or in the incorporated communities of Cle Elum, South Cle Elum, and Ellensburg. 11.1.3 Effects of Human Activities Because they border water bodies, floodplains have historically been popular sites to establish settlements. Human activities tend to concentrate in floodplains for a number of reasons: water is readily available; land is fertile and suitable for farming; transportation by water is easily accessible; and land is flatter and easier to develop. But human activity in floodplains frequently interferes with the natural function of floodplains. It can affect the distribution and timing of drainage, thereby increasing flood problems. Human development can create local flooding problems by altering or confining drainage channels. This increases flood potential in two ways: it reduces the stream’s capacity to contain flows, and it increases flow rates or velocities downstream during all stages of a flood event. Human activities can interface effectively with a floodplain as long as steps are taken to mitigate the activities’ adverse impacts on floodplain functions. 11.1.4 Federal Flood Programs National Flood Insurance Program The NFIP makes federally backed flood insurance available to homeowners, renters, and business owners in participating communities. For most participating communities, FEMA has prepared a detailed Flood Insurance Study (FIS). The study presents water surface elevations for floods of various magnitudes,