Assignment 6:
Topographic Changes:
The River Feshie
River Feshie, Scotland, UK
11 Mar. 2021
The River Feshie winds its way through northeast Scotland, UK at the edge of Cairngorms National Park. Why River Feshie and not Feshie River? That was a question I had, at least- and answered in this thoughtful blog post on the of evolution of fluvial name conventions from Great Britain to America.
On to the matter at hand: topographic changes. As rivers meander through the landscape they naturally erode and deposit material, leading to changes in the texture of the fluvial channel and surrounding area. The River Feshie is no exception; topographic datasets from 2003-2007 are available for a review of topographic changes over time. The data included herein is analyzed using ArcGIS (ArcMap) software to determine topographic changes in landscape over time. First, change detections are computed between two epochs within the provided datasets. Second, a budget segregation will be conducted to identify mechanisms of topographic change. Enjoy!
Location Map
Fig. 1 shows a vicinity map of the River Feshie, located in the Highlands of Scotland. This section of the river is braided and includes a lot of the geomorphic units we're looking for: bars, channel incisions, bank erosion, and aggradation. Topographic analysis using ArcMap Geomorphic Change Detection (GCD) will give us a better understanding of what is happening here through change detections and a budget segregation.
Change Detections
Change detections were first made between two epochs in the available dataset. The results of each change detection are described below, with Figs. 1 and 2 showing the results of each change detection, respectively.
Change Detection A: 2003 - 2005 >
The first change detection investigated was over a two year period from 2003 to 2005. ArcMap results indicating the incisions (red) and aggradation (blue) in the River Feshie from this change detection are highlighted in Fig. 1. The topography from 2005 is shown below these change detections.
Change Detection B: 2003 - 2007 >>
The second set of change detections covers the whole 4-year time period of the available datasets. This timeframe may provide the most insight on significant changes because over time the incisions and aggradation became more distinct. Fig. 2. shows the change detection from ArcMap with the 2007 topography below.
Fig. 1. Change detection from 2003-2005
Fig. 2. Change detection from 2003-2007
Fig. 3. Budget segregation of a portion of the River Feshie. Areas of channel incision, bar development, aggradation, and bank erosion are highlighted as shown in the key.
Budget Segretation
A budget segregation was performed on dataset A, from 2003 to 2005. Fig. 3 shows an attempt to delineate categories of topographic change in this timeframe, including channel bed lowering (incisions), bank erosion, channel bed rising (aggradataion), and bar development. It is important to note that completing a budget segregation over a span of two years makes it difficult to accurately decide what topographic changes fall into which categories. For example, whether aggradation could be considered bar development from the previous year or just a rise in channel elevation over time.
Channel Bed Lowering (Degradation/Incision)
Incisions in the channel include areas where an original channel (one that existed in 2003) became more entrenched and incised by 2005. Channel bed lowering, labeled incision in the ArcMap results shown in Figs. 3 and 4, accounts for the majority of the topographic changes in the given study area.
Bank Erosion
Bank erosion is a specific form of incision that occurs at the edges of the channel on specific banks. Because the study are focused mainly on the center of the channel, it was difficult to determine whether bank erosion occurred, if at all. The portion of the study area having bank erosion was so labeled because the bank noticeably moved to the right over the two year period, indicating an eroding bank.
Channel Bed Rising (Aggradation)
Aggradation is the buildup of sediment that raises the elevation of the channel bed. The areas I chose to label as channel bed rising (see Fig. 3) were the areas between bar development and towards the sides of the river, where sediment was deposited. These areas show up in blue on the change detection (Fig. 1) and led to a rise in the overall channel bed elevation.
Bar Development
Specific aggradation toward the center of the channel (or between diverging channels) can lead to the development of bars, or islands of sediment that separate the main channel in sections. This study area of the River Feshie is braided and thus has many bars have developed over time. I chose to label only places with more significant aggradation as areas of bar development, as shown in Fig. 3.
Totals of each topographical change are shown in Fig. 4, and Figs. 5-8 show specific volumetric influence of each topographical change in relation to the budget segregation. In Figs. 5-8 the left figure vertical scale is total volume up to 500 m^3 and the horizontal scale shows vertical adjustment by 0.5 m increments. The right side bar graph shows total volumes up to 2500 m^3 in increments of 500 m^3. Fig. 4a illustrates common geomorphic changes in the River Feshie (location unknown).
Fig. 4. Total amounts of (from left to right) channel incision, bar development, aggradation, and bank erosion for the study reach of the River Feshie. (vertical scale: up to 2000 m^3, horizontal scale: N/A)
Fig 4a. This picture of the River Feshie shows channel braiding and bar development as well as potential places where banks have eroded and channels have incised in the landscape. Click here to learn more about geology in Scotland!
In summary, Fig. 9 shows the process of analyzing the River Feshie using ArcMap Geomorphic Change Detection software from topographic analysis and change detection to budget segregation used for analysis.
Fig. 9. Analysis process (from left to right): topography in 2003, 2005, change detection between 2003 and 2005, and budget segregation for a portion of the channel.