Logo Department Name Agency Organization Organization Address Information United States Forest Monongahela National Forest 200 Sycamore Street Department of Service Elkins, WV 26241 Agriculture 304-636-1800 File Code: 1900; 2700 Date: December 23, 2016 Kimberly D. Bose, Secretary Federal Energy Regulatory Commission 888 First St., N.E., Room 1A Washington, DC 20426 Dear Ms. Bose: Subject: Forest Service Analysis of Landslide Data from a Recent Flood Event on the Monongahela National Forest OEP/DG2E/Gas 4 Atlantic Coast Pipeline, LLC Docket No. CP15-554-000 and CP15-554-001 The Forest Service submits a landslide analysis conducted by Forest Service staff. The analysis utilizes locations of landslides that occurred during the flood event of June 23, 2016 to identify properties of slope, geology, and soil where the landslides occurred. The analysis also compares the results of the properties associated with the landslide analysis with the properties identified along the proposed route of the Atlantic Coast Pipeline (ACP) Project on the Monongahela National Forest. The Forest Service provides this analysis to illustrate the potential for similar high-intensity precipitation events to cause slope stability problems along the proposed ACP route. Thank you for your attention to this matter. Please direct any questions to Jennifer Adams, Special Project Coordinator, by phone at (540) 265-5114 or by email at [email protected]. Sincerely, CLYDE THOMPSON Forest Supervisor cc: Atlantic Coast Pipeline, LLC Caring for the Land and Serving People Printed on Recycled Paper Landslide Analysis Monongahela National Forest Flood Event (June 2016) December 19, 2016 Introduction Following the June 23, 2016 flood event, heavy rain resulted in numerous landslides across southern West Virginia, some of which occurred within the Monongahela National Forest (MNF). The MNF Engineering staff collected point data (Latitude/Longitude) from 48 landslides. These landslides were found and documented randomly as they were located during road system checks post-flood event. While this point data does not encompass all of the landslides that occurred on the MNF during the flood event, it does offer a “snapshot” of some of the mass movements on forest. As mentioned above, almost all of the landslides used in this analysis were located along road systems. Consequently, it is reasonable to assume that many of these landslides were in part triggered by previous disturbance (i.e. the road system). Although these conditions are not natural, these instances depict how the landscapes in this region respond after they have been disturbed. The GPS location of these landslides can be used to ascertain environmental conditions present at the area of each landslide. MNF soils staff used the point location of these landslides in conjunction with slope, geology and soils maps to determine the types of environmental conditions present at these landslides. Methods, results and subsequent management considerations of this analysis are described in the paragraphs below. Objective: Use site specific, fine resolution data (landslide locations) to observe properties where these slides occurred. Methods Data The GPS coordinates of 48 landslides were used as point data in this analysis. Geospatial data layers including a slope layer created from 3m DEMs, geology map, and soils map were also used for this analysis. The metadata on the slope, geology, and soils layers used in this analysis are summarized in Table 1. Table1: Metadata on slope, geology, and soil layers used in analysis. Layer Source Scale/Resolution Soil Series NRCS Web Soil Survey 1:24,000 USGS Geology Map USGS 1:24,000 Slope Created from DEMs 3 m As seen in the table above, the environmental variables used in this analysis are of varying, but relatively coarse scale (as compared to the landslide point data). The MNF acknowledges that there may be some mapping errors as the result of the relatively coarse scale of the environmental data. However, no order 1 soil survey is available for use in this study area, and the environmental variables used in this analysis are of the finest resolution available. 1 Results Geology The geologies where the landslides occurred are listed in Table 2 and summarized below. Attachment A and B display the geology of the northernmost and southernmost recorded landslide points used in this analysis respectively. Attachment C displays the path of the storm. The Mauch Chunk formation is formed from grayish-red shale, siltstone, sandstone, and some conglomerate. This formation produces soils with shrink-swell clays (3:1:1 vermiculite dominated clays) that are prone to mass movement. The Mauch Chunk formation has the highest potential for slippage of any geology found on the MNF. The Brallier formation is formed from micaceous shale, siltstone, and sandstones. Chemung formations consist of non-calcareous sandstones and shales. The Pottsville formation is predominately gray sandstone and conglomerate which also may contain thin beds of shale, claystone, limestone, and coal. The Pocono group is predominately gray massive sandstones with some shale. Table 2: Geology of landslides Geology Number of Slides Present on Formation Mauch Chunk 2 Brallier 5 Chemung 13 Pottsville Group 26 Pocono 1 Quaternary Alluvium 1 Total Number of Slides Analyzed: 48 Due to the intensity of the precipitation, landslides occurred on geologies which were previously thought to be relatively stable. The majority of landslides occurred on Pottsville and Chemung as opposed to Mauch Chunk, which is historically known to be at high risk for mass movement. The distribution of landslides should not be interpreted as evidence that landslides are more likely to occur on Pottsville and Chemung than the other geologies. Compared to the other geologies, Pottsville and Chemung covered large portions of the area affected by the stormed (Attachments A and B). However, the fact that many slides occurred on these geologies indicates that they are susceptible to slides during extreme conditions such as those that occurred during the June 23 flood, especially when the soils have been subjected to prior disturbance (prior road construction in this case). Soil Series Soil series present at identified landslides locations can be seen in Table 3 below. For complete typical characterization of the soil series described below, locate their Official Series Description (OSD) online. The descriptions below focus on the management concerns typical for these soil series. Ernest and Laidig are colluvial soils that can be very deep (> 60 inches) with poor drainage in the subsoil. Shouns series share similar characteristics to those of Ernest and Laidig but are even more susceptible to mass movement due to the fact that Shouns forms from Mauch Chunk geology (and Hampshire Formation) with shrink-swell clay minerals. Potomac soils are very deep and cobbly, floodplain soils. Weikert, Lily and Gilpin are residual soils that are 2 typically very shallow to moderately deep with high rock fragment contents. These soils are typically classified as non-sensitive to earth disturbing activities. Table 3: Soil series of landslides Soil Series Number of Slides Present on Soil Series Ernest Silt Loam 1 Gilpin-Laidig Association 13 Laidig Channery Silt Loam 4 Lilly Loam 1 Potomac Very Gravelly Loam 1 Shouns Silt Loam 1 Udorthents 10 Weikert Channery Silt Loam 17 Total Number of Slides Analyzed: 48 Based on Table 3, the majority of landslides were recorded on soils that formed from material weathered from interbedded gray and brown acid siltstone, shale, and sandstone. This coincides with the finding from Table 2. As mentioned previously, these soils are not typically associated with management restrictions, but due to the intensity of the storm, these soils became saturated and resulted in mass movement. Due to the poor drainage in the subsoil and the topsoil saturation during the rain event, Laidig also experienced mass movements. This observation also shows that 21% of landslides occurred on Udorthents (human-altered), even though Udorthents typically cover a very small percentage of the overall landscape. These soils have been previously disturbed and as a result are more susceptible to intense rain events due to the instability resulting from altering the natural hydrologic drainage patterns. Forest Plan standards (SW07) limit the use of wheeled and/or tracked motorized equipment on soil types that are susceptible to landslides—“Use on slopes greater than 15 percent with soils susceptible to downslope movement when loaded, excavated, or wet is allowed only with mitigation measures during periods of freeze-thaw and for one to multiple days following significant rainfall events. If the risk of landslides during these periods cannot be mitigated, then use is prohibited.” Slope Slope classes present at identified landslides are located in Table 4. The Natural Resource Conservation Service classifies slopes between 20-60% as steep, and anything over 45% classifies as “very steep.” Based on this analysis, the majority of landslides were recorded on slopes of 41-50%. The second most frequent amount of landslides were recorded on 1-10, 51-60, and 61-70% slopes. The likelihood of landslides occurring on each slope class cannot be evaluated because the proportion of the landscape contained in each class is not known. However, the information shows that landslides occurred across all slope classes, including steep slopes, very steep slopes, and slopes that are not