Background and Research Questions
This project explores stream bed mobility in the HJ Andrews experimental forest in relationship to peak discharge events and channel geometry. The HJ Andrews Experimental Forest is a Long Term Ecological Research (LTER) Site located east of Eugene on the western slope of the Cascade Range. The site has been managed since the 1950s for ecological and forestry research, and the largest stream in the forest has been gauged since 1950. From 1978 to 2011, researchers conducted repeated cross sectional surveys on five reaches in three different creeks in the forest. An analysis of these cross-sectional profiles will help researchers and managers gain a better understanding of how, when, and where stream beds responds to extreme hydrologic events.
Water flowing through streams exerts stresses on the bed material. Whether or not these stresses have the capacity to mobilize sediment and change the shape of a stream bed depends on the amount of water moving through the stream along with other factors. As with many geomorphological processes, bed sediment transport is dominated by movement during extreme events. Although cross sectional changes overall appear to be strongly related to the magnitude of greatest flow between measurements, I am interested in investigating confounding factors including the extent of temporal and spatial autocorrelation in the data set.
I would like to explore (a) if and (b) in what manner aggradation or erosion in one cross section might be related to changes in adjacent cross sections. I would also like to know if aggradation or erosion in one cross section in one year are related to changes in the same cross section in an adjacent year.
Data
I am analyzing cross sectional measurements of five reaches within the Andrews Forest, which range from 12 m to 55 m in horizontal extent and 1.2 to 7.3 meters in vertical extent. The cross sections are variously spaced along an along-stream dimension, and they are were surveyed every one to five years over a period of 30 years between 1978 and 2011. The vertical precision of the data set is roughly 1 cm (though the data are unlikely to be accurate to 1 cm) and the horizontal precision varies from 1 cm to several decimeters.
The cross sectional change between two adjacent pairs of years at one cross section is shown below. Areas of erosion are shown in red and areas of aggradation are shown in green.
Hypothesis and Approach
I predict that there will be a relationship between changes at one cross section during one year and the same cross section at another year. Portions of the stream bed that have been recently scoured or contain newly emplaced sediment may be less armored than undisturbed portions of the stream bed. These less armored portions of the stream bed may be more susceptible to future disturbance. Alternately, changes at a cross section may represent longer-term processes related to channel geometry: e.g. a series of cross sections could show continued incision of a cut bank over multiple years.
I do not expect to see a detectable relationship between changes at adjacent cross sections because I expect that the most important geographic controls on channel change are either smaller (e.g. pools) or much larger (e.g. along-stream variation in discharge) than the distance between cross sections.
I want to use this class as an opportunity to explore statistical relationships, but I don’t know yet what kinds of analyses are best suited to this problem. I’d like to learn more in general about how to handle spatial autocorrelation, and when it is and isn’t a statistical issue.
Justification
This project could be scientifically useful for improving our understanding of sediment transport in Pacific Northwest mountain streams. Resource managers may also have an interest in sediment transport because it relates to stream channel mobility (“How much can we depend on this creek staying in the same place?”) and ecology (“How vulnerable is stream life to disruption via bed transport?”). From a resource management perspective, it is becoming increasingly useful to study peak flow events because downscaled climate models for our region predict increased frequencies of large storms.
Preparation
I feel good about my experience with spatial technology, and I’m most interested in learning about how to use that technology to answer statistical questions. I am highly proficient with Arc software. I have TAed one undergraduate class and independently taught another short undergraduate class in ArcGIS. I have a working knowledge of ArcPy, but I still need to use references regularly to write code. I conducted some undergraduate remote sensing research using ArcPy and used ArcPy for research at a government science agency. I work extensively in R. I’ve done image processing using Arc software, Python, ENVI, ImageJ, and raster tools in R, but it’s a very broad field, and I definitely think I could learn more.