Fitzgerald Health Education Associates

Fitzgerald, Evan P., William B. Bowden, Samuel P. Parker, and Michael L. Kline, 2012. Urban Impacts on Streams Are Scale‐Dependent with Nonlinear Influences on Their Physical and Biotic Recovery in Vermont, United States. Journal of the American Water Resources Association (JAWRA) 48(4): 679‐697. DOI: 10.1111/j.1752‐1688.2012.00639.xAbstract: The physical and biological conditions of stream reaches in 16 watersheds within the Lake Champlain Basin of Vermont, United States, were assessed and analyzed for a response to total impervious area (TIA) at multiple spatial scales. Natural gradients (e.g., channel slope) and human impacts to channel boundary conditions (e.g., bank armoring) were considered to ensure a robust test of the Impervious Cover Model for upslope TIA. The response of geomorphic stability and sensitive macroinvertebrates to TIA was nonlinear and significant (p < 0.001), decreasing rapidly at 5% TIA. The effect of urbanization on stream condition was shown to interact significantly with drainage area and channel slope using the analysis of covariance (ANCOVA) (p < 0.05). Hydraulic geometry regressions for urban and rural watersheds and ANCOVA were used to describe a significant watershed scale‐dependent response of channel width to urbanization (p = 0.001). The analysis of macroinvertebrate data from reaches in different stages of channel evolution indicated that stable reaches supported greater richness of pollution intolerant species (p < 0.001) and overall taxa richness (p < 0.01) than unstable reaches, and that biotic integrity improves as channels regain stability during their evolution into a state of quasi‐equilibrium. We conclude that macroinvertebrate communities can respond positively to channel evolution processes leading to natural channel restabilization.

The use of concentrically braced steel frames to satisfy lateral force requirements is a common design practice in regions of strong seismicity. They provide a very efficient means of fulfilling the dual objectives of earthquake resistant design, that is, damage control and collapse prevention. While tensile yielding and inelastic buckling of bracing elements provide the basic energy absorbing mechanism, there are inherent problems with member behavior under multiple cycles of inelastic deformations. Inelastic energy dissapation, however, may also be provided by friction resistance in slotted bolted connections, thus eliminating the need for inelastic member buckling. Limited laboratory tests indicate that this concept may be a viable alternative for use as a lateral force resisting system for both new construction and the seismic upgrading of existing structures.