Fluvial geomorphic characteristics and their impact on the accuracy of commonly applied bank retreat measuring techniques: A case study in the North Carolina Piedmont Region
Emily D. Brown, PE, ENVSP, CFM
Engineer / PhD Student
Freese and Nichols, Inc.
North Carolina State University, Biological & Agricultural Engineering Department
Winston-Salem, NC
Streambank erosion has become a growing concern among society over the past decade. Development in previously rural environments has led to increases in impervious cover and changes in hydrologic conditions. Precipitation runoff is getting to streams quicker, with less time for runoff to infiltrate into the soil. Streambank stability becomes threatened with the increased flows and over time, erosion occurs, creating potential safety concerns. Depending on native soil properties, watershed characteristics, and anthropogenic influence, the severity of erosion can vary. Often, streambank erosion does not become a concern until infrastructure is threatened. Thus, the ability to determine erosion rates of streambank systems and sediment yield within those systems is important. The objective of this work is to determine the stream geomorphic characteristics (e.g., channel-bed and bank material grain size) that impact the accuracy of commonly applied streambank erosion measuring techniques. A comparative analysis of bank retreats using erosion pins, the JET, and Root Dendrogeomorphology was implemented for this case study. Erosion pins have been around since the early 1930's and are one of the more common methods being analyzed. This method involves driving metal rods into a bank and periodically measuring the length of the exposed rod to determine the rate of retreat. The JET technique involves using a device to direct flow towards a small portion of the streambank to measure the erodibility of the soils. From that, other soil properties are determined, and the estimated bank retreat can be back-calculated based on the soil properties. Recent research has been conducted implementing dendrogeomorphic approaches to estimate bank retreat. Measurements of the exposed root location relative to distance of the root from the riverbank (vertically and horizontally) to the center of the root are recorded. Then, through the analysis of cross-sectional sections of the cut root samples, the time of exposure can be identified through changes in the root cellular structure, which is referred to as Root Dendrogeomorphology. Ten streams across the Piedmont Region of North Carolina were included in this analysis, with each stream having experienced varying amounts of erosion and sediment transport. Multiple lines of data [erosion pin, root dendrogeomorphology, and soil parameters collected through the use of JET (jet erosion test)] have been collected on the streams to document erosion. The case study observes the erosion rates predicted from the three techniques and compares the variation in bank retreat.
About Emily D. Brown, PE, ENVSP, CFM
Emily Brown is a project manager with Freese and Nichols in water and natural resources projects, focusing specifically stream restoration, natural resource investigations, bioengineering, stormwater, erosion control, ecological restoration design, channel stability analysis, streambank stabilization and geomorphic assessments. She has more than 8 years’ experience providing a wide variety of technical services, including stormwater design, river stability assessments, H&H assessments, ecological engineering, stream restoration design, project management, erosion control design and permitting, watershed assessments, and sediment transport modeling. She also has experience in environmental assessments, soil, and water quality testing, fluid mechanics, and the development and use of GIS models. Emily is currently pursuing a PhD in Biological Engineering (Riverine Systems/Sediment Transport) at North Carolina State University under the guidance of Dr. Celso Castro Bolinaga.