Effects of August 1995 and July 1997 Storms in the City of Charlotte and Mecklenburg County, North Carolina

Prepared in cooperation with the City of Charlotte and Mecklenburg County

EFFECTS OF THE AUGUST 26-28, 1995, AND JULY 22-24, 1997, STORMS ON WATER QUALITY

In addition to the rainfall and streamflow data-collection network, the USGS operates a network of small-basin stormwater-runoff monitoring sites for the City of Charlotte and Mecklenburg County. Land use within each of these small basins generally is uniform, and samples are collected during storms throughout the year.

Water-quality samples were collected at four sites during the August 1995 event and at nine sites during the July 1997 event (table 4; Robinson and others, 1998). During a runoff event, increased streamflow may dilute concentrations of constituents. However, the total amount of material transported by the stream, referred to as load or export, may increase dramatically because load is a function of the concentration and the streamflow.

Instantaneous loads, in pounds per acre per day, were computed for baseflow conditions and a typical summer rainfall event, which was sampled in the summer of 1996 (table 4). Water quality samples were collected near the time of maximum streamflow in selected small basins during the August 1995 and July 1997 flood events, and an instantaneous load was computed for total nitrogen and suspended sediment.

It is apparent from table 4 that most of the export of nitrogen and sediment occurs during storm events. Nitrogen export appears to be greatest from residential land-use basins, and high runoff during the floods had a greater effect on nitrogen export in these basins than in other basins. Sediment yields generally were much larger for the two flood events than for a typical summer runoff event.

The effect of basin size on material load also is evident from these data. In the small basins, essentially all of the material that washes off the land surface reaches the basin outlet, so the load is high. In the large basins, runoff material travels farther and load decreases as material settles to the streambed, undergoes instream chemical transformations, and is diluted by inflows.