Methods and Equations for Estimating Peak Streamflow Per Square Mile in Virginia’s Urban Basins
Models are presented that describe Virginia urban area annual peak streamflow per square mile based on
basin percent urban area and basin drainage area. Equations are provided to estimate Virginia urban peak
flow per square mile of basin drainage area in each of the following annual exceedance probability
categories: 0.995, 0.99, 0.95, 0.9, 0.8, 0.67, 0.5, 0.43, 0.2, 0.1, 0.04, 0.02, 0.01, 0.005, and 0.002
(recurrence intervals of 1.005, 1.01, 1.05, 1.11, 1.25, 1.49, 2.0, 2.3, 5, 10, 25, 50, 100, 200, and 500
years, respectively). Equations apply to Virginia drainage basins ranging in size from no less than
1.2 mi2 to no more than 2,400 mi2 containing at least 10 percent urban area, and
not more than 96 percent urban area. A total of 115 Virginia drainage basins were analyzed.
Actual-by-predicted plots and leverage plots for response variables and explanatory variables in
each peak-flow annual exceedance probability category indicate robust model fits and significant
explanatory power. Equations for 8 of 15 urban peak-flow response surface models yield R-square values
greater than 0.8. Relations identified in statistical models, describing significant increases in urban
peak stream discharges as basin urban area increases, affirm empirical relations reported in past
studies of change in stream discharge, lag times, and physical streamflow processes, most notably those
detailed for urban areas in northern Virginia.
Streamflow, Water Quality, and Aquatic Macroinvertebrates of Selected Streams in Fairfax County, Virginia, 2007–12
Efforts to mitigate the effects of urbanization on streams rely on best management practices (BMPs) that are implemented with the intent of reducing and retaining stormwater runoff. A cooperative monitoring effort between the U.S. Geological Survey and Fairfax County, Virginia, was initiated in 2007 to assess the condition of county streams and document watershed-scale responses to the implementation of BMPs. Assessment of the data collected during the first 5 years of this monitoring program focused on characterizing the hydrologic and ecological condition of 14 monitored streams.
Hydrologic, chemical, and macroinvertebrate community conditions in the streams monitored were found to be consistent, overall, with conditions commonly observed in urban streams. Hydrologically, the monitored streams were found to be flashy, with flashiness positively related to road cover in the watershed. Typical pH values of streams throughout the network centered around neutrality (pH = 7) with strong daily fluctuations apparent in the continuous data. Patterns in specific conductance were largely representative of anthropogenic disturbances—watersheds having the greatest percentage of open space and estate residential land-use had the lowest typical specific conductance values, and specific conductance variability was less than what is observed in watersheds that are more intensively developed. In watersheds having greater road coverage, and more development in general, increases in specific conductance over several orders of magnitude were observed during winter months as a result of the application of de-icing salts on impervious surfaces. Dissolved oxygen conditions were typically within the range required to support healthy biological communities, although occasional departures during summer months at some sites fell below the impairment threshold for streams in Virginia.