AN EXAMPLE OF IN-STREAM PEAK FLOW REDUCTION TARGETS BASED ON HABITAT AND GEOMORPHIC INDICES
Abstract:Hydrology is such a strong determinant of benthic habitat structure that flow regime can be a very strong predictor of benthic macroinvertebrate health. In sand and gravel bed streams, peak flow control targets based on surface sediment median grain size (D50) mobilization thresholds can function as surrogate targets for benthic habitat and macroinvertebrate health in highly impacted or urban streams. In the present example, we found it was also reasonable to assume that the peak flow associated with the D50 mobilization threshold is probably indicative of pre-development bankfull peak flow. For this analysis, the bankfull event, defined as the event doing the most work on an annual basis to move sediment, was assumed to be the 1.5-year recurrence interval rain event. The peak flow target for the bankfull event is set at the threshold of motion for the existing surface sediment D50. This threshold is based on the ratio of applied shear stress to the shear stress at the estimated threshold of motion using a Shields number approach.
At a moderately entrenched reach of the urbanized Millers Creek in Ann Arbor, Michigan, macroinvertebrate sampling since 1995 has consistently yielded low species variability. Because this tributary of the Huron River has such as steep gradient, particularly in relation to regional averages, the expected variation of species is very high. This disparity emphasizes the extent of habitat degradation. Meeting the bankfull peak flow target here requires a reduction of more than 90% of existing peak flows. The probability of significant bed mobilization for the bankfull event on any given day has risen by two orders of magnitude, from 0.14% to 6.5%, since development of the watershed. Before development, bed mobilization was likely occurring once every year or every other year and but can now occur as many as twelve (or more) times per year. With the implementation of proposed improvements in the watershed the frequency could be reduced to six times per year. Getting back to pre-development flows, based solely on implementing watershed improvement practices, is not feasible. These changes in peak flow recurrence intervals are similar to the results from a Wisconsin watershed evaluated between the period of 1960 and 1998 when the watershed was transformed from primarily agricultural and undeveloped land to a built-out condition (Novotny, et.al., 2001).
Document Type: Research Article
Publication date: January 1, 2004
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