Climatological Perspectives on the Rainfall Characteristics Associated with Landslides in Western North Carolina Christopher M. Fuhrmann, Charles E. Konrad II, Lawrence E. Band, Martin W. Doyle Department of Geography University of North Carolina at Chapel Hill IGU Conference on Water Sustainability 15 August 2007 Asheville, North Carolina, USA

Peeks Creek Debris Flow Initiated during Hurricane Ivan 30 feet deep 250 feet wide 33 miles/hr 5 fatalities, 15 homes damaged Landslide Hazards in the Southern Appalachians Over 200 fatalities since early 1900s Thousands of acres of destroyed forest Recurrence intervals for individual landslides are highly variable – Considerations of scale – Location

Landslides in Western North Carolina CAUSES Underlying geology Geomorphology Hydrology Weather and climate Deforestation TRIGGERS Earthquakes Blasting/excavation Freeze-thaw periods Precipitation/storms Dam failure Source: North Carolina Geological Survey [ CAUSES Underlying geology Geomorphology Hydrology Weather and climate Deforestation

Excessive point rainfall totals alone cannot be used to adequately determine the potential for flooding or excess rainfall… Must also consider the timing and spatial distribution of the rainfall (Hirshboeck et al., 2000; Konrad, 2001) What are the return intervals for these storms and how do they rank in the context of other heavy rainfall events in western NC? What antecedent period(s) are tied climatologically to landslides? What are the storm types associated with landslides in western NC? Heavy rainfall associated with landslides in western NC (Witt, 2005): 1. Short-lived, intense localized storms 2. Long-lived, regional-scale storms 3. Multiple short or long-duration storms that “train” Excessive point rainfall totals alone cannot be used to adequately determine the potential for flooding or excess rainfall…

Landslide Inventory for Western North Carolina (North Carolina Geological Survey, 2006) : 30 events comprising 221 individual landslides TN NC SC AppalachianEscarpments

Objective 2: Identify the predominant storm types associated with landslides in western NC Objective 1: Estimate the concurrent and antecedent rainfall amounts over each landslide location and determine how they rank against other heavy rainfall events in western NC Methods: 1. Construct a heavy rainfall climatology for western NC (i.e., the heaviest 55 rainfall events between 1950 and 2004) 2. Determine the antecedent rainfall at various time scales for all rainfall events (8718 events) and identify the most significant “wet periods” 3. Rank the concurrent and antecedent rainfall totals associated with each landslide event in the context of the heavy rainfall climatology and antecedent wet periods 4. Identify predominant storm types using the manual classification scheme developed by Maddox et al. (1979) for flash flood events

Study Area: Western North Carolina Shaded relief overlain with locations of rainfall stations from the COOP network

Time Period Median Ranking Percent Heavy Median Ranking Percent Heavy 2-day9247%7850% 4-day5353%5909% 7-day5153%22627% 14-day3979%31718% 21-day4384%20927% 30-day3684%10145% 60-day2684%5936% 90-day2095%5145% What is the percentage of landslide events connected with heavy concurrent and antecedent rainfall? Antecedent Rainfall Soil Events Rock Events n=19n=11n=19n=11

What types of weather events are associated with landslides in western North Carolina? 5/19 7/19 5/19 2/19 42% of soil events associated with an antecedent tropical cyclone *Only soil events examined

What percentage of landslide events are associated with isolated heavy rainfall? Determine the ratio of local to regional scale rainfall High ratio = locally heavy rainfall over the landslide area Weather Type MeanMaxMin Synoptic Cyclonic Isolated Tropical Cyclone Local:Regional Rainfall Ratios *Only soil events examined

November 1977 Bent Creek Debris Flows near Asheville (Pisgah National Forest and Coweeta Forest) Where do some of the more notable landslide events in the past 55 years rank in the context of heavy concurrent and antecedent rainfall? Time Period Rank 2-day22 4-day 7-day 14-day38 21-day49 30-day42 60-day5 90-day10

Hurricane Opal (October 1995) Blackstock Knob Debris Flows onto the Blue Ridge Parkway (just northeast of Asheville in Buncombe Co.) Time Period Rank 2-day 25 4-day 7-day 14-day 21-day 30-day 60-day35 90-day42

December 2003 (and December 1990) One fatality and one house destroyed in the Maggie Valley Apartment Complex (Haywood Co.) Why develop on a debris fan? Time Period Rank 2-day 4-day 7-day 14-day 21-day38 30-day14 60-day21 90-day18

July 1997 Pigeon River Gorge Rockslide onto US Interstate 40 Infiltration of water along weakened fracture planes Time Period Rank 2-day 4-day 7-day 14-day 21-day30 30-day39 60-day44 90-day35

Hurricanes Frances and Ivan September 2004 Time Period Rank 2-day6 4-day1 7-day2 14-day1 21-day1 30-day1 60-day1 90-day1 Rank 2-day1 4-day2 7-day3 14-day2 21-day6 30-day8 60-day2 90-day3 Frances (9/6)Ivan (9/16)

Summary 1.Landslides in western NC can be broken down into “rock events” and “soil events” based on the contribution of heavy rainfall to slope movement 47% of soil events connected with heavy concurrent rainfall; 0% of rock events connected with heavy concurrent rainfall 2.An important connection established quantitatively (median rankings) between landslide activity and antecedent rainfall in western NC 3.Landslide events are generally associated with heavy isolated rainfall embedded in a regional-scale rainfall event (positive local:regional rainfall ratios) 4.An important connection established between landslide activity and tropical cyclones Trigger event – 80% associated with heavy concurrent rainfall Primer event – 42% of landslides associated with an antecedent tropical cyclone 5. Longer antecedent time periods connected more strongly to landslide activity Landslide data acquired through a grant from the US Forest Service awarded to L. Band and M. Doyle Rainfall data acquired through a grant from the National Science Foundation awarded to C. Konrad (BCS )