Date of Award
Doctor of Philosophy
Carol P. Harden
Kenneth H. Orvis, Theodore H. Schmudde, G. Michael Clark
Little is known about the coarse load carried by streams in urban areas or the length of time needed for stream channel adjustments to urban conditions. In this study, I examine the history of urbanization in the basin of Second Creek, the status of the channel, and the sediment load of the creek in recent years.
Second Creek is a small perennial stream whose 18.6 km2 drainage basin is almost entirely contained within the City of Knoxville, Tennessee. Almost all of the drainage basin was developed more than 40 years ago, and is now urban and suburban in character.
For this study, I inspected the channel of Second Creek, measured its dimensions in many places, and recorded the types of materials present and evidence of recent deposition and erosion. My analysis of channel materials included measuring the sizes of more than 100 coarse particles on the streambed at each of several locations. In addition, I measured stream discharge and suspended sediment load near the mouth of the stream during several low and high flows, measured suspended load for a year using rising stage samplers, and estimated bedload by calculating the volume of sediment deposited at the mouth of the stream. Data made available as a result of this study include suspended sediment concentrations from rising stage samplers at five locations for a year of record (October 1998 to October 1999), discharge measurements and suspended sediment concentrations from nine storm events, bedload particle sizes from eight sites, and cross-sectional surveys from 18 sites.
If a stream channel is adjusted to present-day hydrologic and sediment load regimes, little net deposition or erosion is expected to take place, yet my examination of the channel of Second Creek reveals that both deposition and erosion have occurred in recent decades. Sediment deposits in box culverts and concrete-lined channel reaches cannot be more than 30 years old, but I found few signs of present-day deposition. Rather than deposition, much of the channel (where not lined with concrete or thick riprap) shows signs of recent erosion. Therefore, channel erosion appears to have replaced deposition as the dominant process in the last few years. This suggests that Second Creek has not adjusted to the urban conditions of its drainage basin, and that channel enlargement is occurring in many places.
Impervious surfaces and lawns cover most potentially erodible soil in the drainage basin, so the suspended load is expected to be low, yet measurements show it to be high. Channel erosion is likely to be contributing suspended sediment and coarse particles to the stream. Existing basin models generally used for water quality analysis do not include streambanks as sediment sources, and would thus significantly underestimate sediment load in Second Creek and other streams experiencing rapid streambank erosion.
Coarse particles are common in the streambed alluvium, yet my calculations of the volume of sediment in the reservoir at the stream outlet indicate the amount of bedload carried by the creek in the last 50 years to be low. The numbers and/or sizes of coarse anthropogenic particles are sufficient at some sites to alter mean particle diameters and, therefore, stream dynamics. Such particles should not be ignored in fluvial studies.
Urbanization has not been a single, discrete event in the drainage basin of Second Creek. The results of this study demonstrate that a stream in a basin where urbanization began more than 200 years ago and essentially ended 40 years ago is not static, but is continuing to respond to major changes made decades ago and to smaller, more recent changes.
Grable, Judith Laing, "Effects of Urbanization on a Small Perennial Stream: Second Creek in Knoxville, Tennessee. " PhD diss., University of Tennessee, 2003.