For this EarthCache, you will be visiting an area of Pershing State Park to make some observaions about River Bank Erosion along the Higgins Ditch. There is parking available at the trailhead for the Locust Creek Covered Bridge. All observations for this EarthCache can be done from the middle of the walking bridge near the trailhead.
LOGGING REQUIREMENTS
In order to log this EarthCache, send me your answers to the following questions either through email or messaging from my profile page.
1: Estimate the height of the exposed bank.
2: Estimate the width of the river upstream from the bridge.
3: Estimate the width of the river just downstream from the bridge where there is the most erosion.
4: Has anything been done to combat the erosion? If so, is it working?
5: Post a pic of you or a signature item with the bridge in the background.
WHY HERE? -THE RECENT LOCAL HISTORY
Prior to the 1990's, the Higgins Ditch drained water from farm fields during heavy rains and flood events. It lies less than three quarters of a mile west of Locust Creek and eventually flows into Locust Creek. Locust Creek has many meanders which became the locations of many small logjams. These small logjams were seen as part of the river's history and had not caused significant issues. In 1993, the first major logjam developed on Locust Creek. The park management grappled with public opinion on removing the logs, as large logjams affect flooding and farmland in the larger watershed. During that investigation period, the creek adjusted in sections of the logjam on its own—it meandered around or sometimes through the logjams, and deposited sediment on top of the logs which quickly filled in with vegetation, and formed a new channel.
By 1995, with more flooding impacts, more logjams accrued further north in Locust Creek. Although the creek adjusted to the logjams and sediment loading in some sections, other areas became more severely plugged and did not have any clear path for the water to run around or through. During a high flow event in the creek, log jams rerouted overflow directly through the bottomland forests and into the wet prairie. The high flow event filled the sloughs and other wet depressions with sediment. In 1996, the park staff began managing logjams.
Early on, logjam management was primarily conducted with bank-packing. Throughout the 1990s and into the early to mid-2000s, bank-packing was the method the park used to address logjams. But the logjams kept coming, new channels kept forming, and more sediment was depositing. Managing logjams became a major challenge. Additionally,
sedimentation was causing the creek channel to rise. These two facets of ecological issues applied pressure to upstream flows, slowing down the upstream flow. Consequently, Locust Creek naturally did what any creek would do
when faced with a massive plug — it reroute to an easier path. This reroute took Locust Creek and much of its flow to Higgins Ditch.
Higgins Ditch, which previously only carried water from heavy rains and occasional floods, now has all the characteristics of rapidly flowing river.
RIVERBANK EROSION
Riverbank erosion is the removal of material from the banks of rivers when flowing water forces exceed bank resisting forces by the soil and vegetation. Riverbank erosion primarily comprises corrasion (abrasion) and mass wasting. River energy, the primary driver for erosion, differs between, along and seasonally within river systems. The speed at which failed sediment masses are mobilised as fluvial sediment load affects the rate of exposure of the riverbank to further erosion. Consequently, riverbank erosion is a discontinuous process, strongly associated with higher energy events such as flooding.
There are two main types of riverbank erosion.
FLUVIAL EROSION
Fluvial erosion is the direct removal of soil particles by flowing water. The rate of fluvial erosion is determined both by the force of the flowing water and the resistance of the bank material to erosion.
MASS FAILURE
Mass failure occurs when the weight of a bank is greater than the strength of the soil, causing the bank to collapse. This process is dependent upon a number of factors including the internal strength of the soil, soil-water content, and vegetation.
These two erosion processes are linked as fluvial erosion of the bottom of the bank creates a steeper bank angle or overhanging soil blocks which are more unstable and likely to collapse.
references
https://www.preventionweb.net/
https://en.wikipedia.org/
https://www.nwk.usace.army.mil/
https://mdc.mo.gov/
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