| The purpose of this study was to investigate and characterize the fine sediment concentration of urban runoff in the Lake Tahoe Basin. Urban stormwater runoff samples were collected at several locations throughout the Tahoe Basin continuously for water years 2003 and 2004 as part of the Lake Tahoe Total Maximum Daily Load technical development. The results show that there are several types of urban runoff (e.g. runoff off due to rain, runoff due to snowmelt) which have differing levels of fine sediment concentration, loads, and runoff volume. Runoff from rain-on-snow consistently introduced the majority of runoff volume when compared to the other event types. Runoff volumes from thunderstorm and rain, though not observed as frequently, did introduce large runoff volumes and fine sediment loads. Postevent snowmelt was not responsible for large runoff volumes. No relationships were found which related the total runoff volume or peak runoff discharge to the amount of fine sediments in urban runoff. Nor was there a relationship between fine sediment concentration or fine sediment load and time of the year. Measurements of fine sediment concentrations show that runoff from rain-on-snow, thunderstorm, and event-snowmelt had the highest fine sediment concentrations, in that order. Furthermore, the mass loading of fine sediments was on average the highest for the rain-on-snow event type, although thunderstorms and rain also introduced large loads. The urban runoff characteristics of two sites (with similar drainage areas) SY and SB, which are located at the southern and northern end of the Tahoe Basin, respectively, were examined in detail. SY, which is a commercial and highly residential area, introduced larger fine sediment loads than SB, which consists of mixed land use but is mostly residential area. At both of these sites, the majority of fine sediment load over the two water years was conveyed in several (5--10) of the largest runoff events. Analysis of these results showed a strong relationship between turbidity (not in situ) and fine sediment particle concentration. Between the two aforementioned sites, the turbidity and fine sediment particle concentration were not markedly different. Additionally, strong relationships were seen between total TSS load and the 0.49--63 microm fraction of TSS load. |
