The frequency and extent of hydrologic disturbances in streams in the Puget Lowland, Washington
Konrad, Christopher P. (Christopher Peter)
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Hydrologic changes resulting from urban development degrade the biological conditions of stream ecosystems by modifying annual and inter-annual stream flow patterns. In urban streams, discharge is less than mean annual discharge on more days of the year, discharge exceeds the magnitude of frequent floods for a shorter duration of time, and the peak discharge rate of the annual maximum flood is less variable than in suburban streams. These hydrologic changes may cause increases in the frequency and extent of disturbances in urban streams.Floods and drought are common forms of disturbance in stream ecosystems. The biological effects of these hydrologic disturbances depend on their spatial extent and frequency. The extent of seasonal drought was documented in 59 Puget Lowland stream basins. Streams draining 1.2 km2 had a 50% probability of being dry during summer base flow conditions (ephemeral). The length (km) of perennial streams in a basin varied as a linear function of drainage area (km2), L = 0.4 A + 0.8 with a root mean square error of 0.04 km. While urban development did not influence the extent of perennial streams in the basins, it may reduce the period of continuous flow during winter and spring in ephemeral streams.The spatial extent of bed material entrainment during floods was documented at seven gravel bars in three Puget Lowland streams using bed tags, which are metal washers placed in the stream bed. Partial entrainment (PEbar ), which is the fraction of a bar's surface disturbed in a flood, was estimated by PEbar = 12.5(tau0* - 0.045) with a root means square error of 0.099, where tau0* is the total boundary shear stress applied by the flood divided by the product of the median of the particle-size distribution of the surface material on the gravel bar and its buoyant specific weight. Frequent and extensive flood disturbance is likely in urban and other gravel-bed streams where the magnitudes of floods are greater than the magnitude of longer-duration intermediate flows, represented by the discharge exceeded 5% of the time, that control the strength of the stream bed.
- Civil engineering