Fall-Spring 2018-19 Powderstash Run Failure: Analysis and Stability Report Submitted in partial fulfillment of the requirements for the degree of Allison Green, Maisy Snyder, B.S. GEOLOGICAL ENGINEERING Michele Reaume, Paige Courtney, MICHIGAN TECHNOLOGICAL UNIVERSITY Virginia Cistaro Department of Geological & Mining Engineering & Superior Geotech Solutions Sciences Fall-Spring 2018-19 May 6, 2019 Under the advisement of: Dr. Rudiger Escobar Wolf & Dr. John Gierke ABSTRACT Slopes failed across two ski runs at Mont Ripley Ski Hill as a result of an extreme rainfall event occurring in Michigan’s Keweenaw Peninsula in June 2018, locally known as the “Father’s Day Flood”. Hydrological and slope analyses were performed to understand the likely failure mechanisms/events and to consider mitigation methods to restore the ski runs and protect infrastructure downslope of the failures. The failure area was examined by comparing digital elevation models (DEMs) derived from aerial imagery obtained prior to and after the flood and included field observations. The DEMs were used to evaluate the slope stability using two geotechnical models. Slope hydrology was considered in the analysis. The TR-55 Method, Rational Method, and Manning’s equation were utilized in order to find time of concentration, rainfall intensity, and flood-stage height. These hydrological inputs were used in an excel-based Bank Stability and Toe Erosion Model Static Version 5.4 from the USDA, a two-dimensional profile model using Rocscience Slide Version 8.015, and an aerial surface stability analysis using Scoops3D. The propensity for erosion was analyzed using the Hjulstrom Erosion Model approach. The volume of material moved by the flood event was calculated using the before and after DEMs in ArcGIS. It is hypothesized that the heavy overland flow of water, associated with the flooding event, travelling down the ski hill was the primary cause of failure and the failure area was minimally impacted by toe erosion at the base. Currently, the failure area is unstable and continues to erode with snowmelt and spring rain events. It is proposed that inserting a steel culvert, larger than the one existing pre-flood, will redirect overland flow to Ripley Creek, located below the failure, and allow for the continued use as ski runs. Debris deflectors and rip-rap will also be utilized to minimize erosive potential along the open channel ditch running down the ski hill that would empty into the proposed culvert. 1 TABLE OF CONTENTS Abstract .................................................................................................................................................. 1 1. Project Background ........................................................................................................................... 7 1.1 Purpose ....................................................................................................................................... 7 Rain Event Description ................................................................................................................... 7 1.2 Location and Site Characterization ............................................................................................. 8 Geology from Published Sources ................................................................................................. 10 Groundwater Conditions ............................................................................................................... 10 1.3 Objectives & Scope ................................................................................................................... 11 2. Approach ......................................................................................................................................... 12 2.1 Data Acquisition ........................................................................................................................ 12 2.2 Drone Survey ............................................................................................................................ 12 2.3 DEM Generation ........................................................................................................................ 18 2.4 GIS Analysis .............................................................................................................................. 18 3. Geohydrology Study ....................................................................................................................... 22 3.1 TR-55 Method ........................................................................................................................... 22 3.2 Rational Method ........................................................................................................................ 25 3.3 Manning’s Equation ................................................................................................................... 29 Parameter Disscusion ................................................................................................................... 29 Derivation of Stage Height ............................................................................................................ 30 Derivation of Velocity from Equation ............................................................................................ 30 4. Geotechnical Study ......................................................................................................................... 32 4.1 Toe Erosion Model .................................................................................................................... 32 4.2 Scoops 3D Model ...................................................................................................................... 33 4.3 Rocsience Slide Model .............................................................................................................. 34 4.4 Volume of Material Moved ........................................................................................................ 37 4.5 Erosion Model ........................................................................................................................... 38 5. Results ............................................................................................................................................ 40 5.1 From Geohydrology................................................................................................................... 40 Storm Event Results ..................................................................................................................... 40 Future Snowmelt Results ............................................................................................................. 40 2 Stage Height Results .................................................................................................................... 41 Velocity over Powderstash Results .............................................................................................. 45 5.2 From Geotechnical .................................................................................................................... 45 Toe Erosion Eesults ...................................................................................................................... 45 Scoops3D Results ........................................................................................................................ 47 Rocscience Slide Results ............................................................................................................. 48 Hjulstrom Diagram Results ........................................................................................................... 55 5.3 Discussion of Method of Failure ................................................................................................ 56 6. Recommendations .......................................................................................................................... 58 6.1 Solutions .................................................................................................................................... 58 6.1.1 Culverts ............................................................................................................................... 58 6.1.2 Slope Stability ...................................................................................................................... 59 6.2 Recommended Future Work ..................................................................................................... 63 7. Conclusions ..................................................................................................................................... 64 Acknowledgements .............................................................................................................................. 65 References ........................................................................................................................................... 66 Appendices .......................................................................................................................................... 68 3 List of Figures Figure 1. Annotated Map of Failure Locations on the Mont Ripley Ski Hill ........................................... 7 Figure 2: Location of Ripley, Michigan and Mont Ripley Ski Hill ........................................................... 8 Figure 3: Location of slope failure on Mont Ripley Ski Hill ...................................................................