Mass Wasting and Slope Stability:Jeff Clark, Lawrence University
Intended Audience: Undergraduate course in geomorphology.
This field lab takes place on the western shores of Lake Michigan just north of Two Creeks, WI, in Manitowoc Country (where Route BB dead ends). The site is owned by the Nature Consrvancy so public assess is assured even though there is a nuclear power plant just 2 miles north.
This activity is the field portion of a two-week lab that runs during the third and fourth week of an undergraduate geomorphology class "Physics of the Earth: Surficial Processes." The students will have read about and attended lectures on physical weathering, mass wasting, infinite slope analysis, Mohr-Coulomb theory, and the hydrometer method. Mass movement along the shores of Lake MI are investigated in the field, lab, and through GIS analysis of digitized aerial photography. Students assess strength characteristics of unconsolidated sediment in the field and survey the morphology of the movement, and take samples for laboratory analysis of the fine fraction. Using ArcGIS, they determine the frequency (or rate) of mass wasting in this area using aerial photography. They will use the field and laboratory data to formulate hypotheses on the type and cause of the failure and present their findings in a formal report.
Goals of this exercise include:
- To learn field techniques for determining the relative strength of unconsolidated material.
- To learn laboratory techniques for determining soil texture and the sediment size distribution of fine (<2mm) sediment.
- To use repeat aerial photography to estimate the rate of shoreline retreat.
- To use geomorphic surveys to discern the type of mass movement.
- To infer the failure mechanism based upon field data and observations.
- To work collaboratively within groups and between groups to effectively share data and disseminate results.
Notes and Tips:
This site is the type location for the Two Creeks buried forest which is interpreted to represent the warm interval before the Younger Dryas period. In the 30' banks there are well preserved logs, and boreal forest litter. The purpose of the trip however is to look at the stability of these slopes which fail episodically. The failure mechanism is likely increased pore water pressure within the sandy unit after heavy spring rains. A perched water table forms within the sands that rest on the Cary till. It is not uncommon to see water seeping out of the sandy layers in the spring .When the lake level is low (as it has been for the past 10 years) the toe of the slope is protected so failures are inhibited. High lake levels enable winter storms to clear the beach of colluvium and allow enable fresh failures to take place. It is unlikely that students will think of all of these nuanced factors, but that is part of what makes it such a rich activity. Each group is equipped with a GPS enabled tablet PC computer which has the reference readings. This is also the first lab where I require a formal lab write up. I provide handouts and discuss what this entails and how this is different from previous labs.
Assessment and Evaluation:
The sand layer is invariably weaker than the others. Depending upon the time of the year it may well be saturated as well. In the field I conduct formative assessment by evaluating their field technique, reasoning, skills, and how they handle the logistics of the exercise. I leave much of the methodology open to them so that they must devise their own strategies. Astute students will observe the potential for wave action at the base of the slopes and will include this as a potential driver of mass movement as well. My summative assessment of the write up is based primarily upon their reasoning, how they use their data (observations support inferences) and how this is all organized and presented. I weight the presentation and content equally.
Materials and Handouts:
- Activity Description/Assignment with Instructor's Notes (Microsoft Word 286kB May5 08)
- Handout describing content of a formal lab report (Microsoft Word 29kB May5 08)
Papers and websites on science and pedagogy related to the field trip.