A detailed study of landslide and debris flow source areas in the northern Colorado Front Range Arana-Morales, Ashlyann 1, Baum, Rex 2, Godt, Jonathan 2 (1) Dept. of Geology, University of Puerto Rico Mayaguez, and RESESS Internship at UNAVCO, Boulder, CO [email protected] (2) U.S. Geological Survey, 1711 Illinois Street, Golden, Colorado` 80401 [email protected], [email protected]
Introduc on Using informa on obtained in the GIS database the two pie charts below illustrate the vegeta on and geomorphic se ngs Landslides contribute to erosion and landscape evolu on of the landslides in the area of Boulder and Eldorado Springs.
and cons tute common natural hazards and ac ve 0% Figure 1: Example of debris flows landslide processes. In September 2013 nearly con nuous rainfall cause by the rainfall, September 11-13. 3% occurred causing flooding and widespread landslides and Located in the west part of Boulder in a dip slope area. debris flows in the northern Colorado Front Range. Sparse Trees Whereas previous studies have iden fied erosion as the Burn most common process leading to debris flow in the 37% mountains of northern Colorado, nearly all of the debris 53% Grass flows mapped in this event began as small, shallow Heavy Trees
landslides. 7% Above treeline
Methods
Figure 5: This pie chart shows the vegeta on that tends to surround the Exis ng maps of landslide loca ons in the Boulder and landslides in this part of Colorado; sparse trees and grass are the most Eldorado Springs quadrangles were improved by adding common vegeta on types.
!(!( details to show outlines of sources areas as well as transport !(!(!(!( !(!( !(!(!(!(!(!( !( !( !(!( !( !(!( !(!( !(!( !( !( !( !( !( !(!(!( !( 1% !( !(!(!( !(!( !(!( !(!( !( !(!(!( !( !(!(!( !( !(!( !(!(!(!( !( !( !(!( !( and deposi on zones. !(!( !( !(!(!( !( !( !(!(!(!(!( !(!( !( !(!( !( !( !( !( !( !( !(!( !(!( !(!( !( !( !( !(!( !( !( !( ¯ !(!( !( !( !(!(!( !(!(!(!(!(!( !(!( !( !( !( !( !(!(!(!( !( !(!(!(!(!(!( !(!(!( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !( !(!(
!( !(!(!( !( !( !( !(!( !( !( !(!( !( !(!(!( !( !( !(!(!( !(!(!(!( !( !(!(!(!(!( !( !(!( !( !( !(!(!(!( !(!(!( ¯ !( !(!( !( !(!( !(!(!( !( !( !(!( !( 25% !(!(!(!( !( !(!( !(!( !( !( !( !( !( !(!( !(!(!(!(!(!( !( !(!(!( !( !( !( !(!(!(!(!(!(!(!( !( !(!( !(!( !(!(!( !(!(!( !( !( !( !(!(!(!(!( !(!( !(!( !( !(!( !(!(!( !(!( !( !( !( !( !(!(!( !( !( !(!( !( !(!(!( !( !(!(!( !( !( !( !( !( !(!(!( !(!(!(!( !( !( !( !(!(!( !( !(!( !(!( !( !(!( !( Boulder !( !(!( Open Slope !( !( !( !(!( !( !(!( !( !( !(!( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !(!( !( !( !(!( !(!( !(!( !(!(!(!(!( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !(!( !( !( !( !( !(!( !( !( !( !(!( !( !( !( Swale !(!(!(!(!( !(!(!(!(!( !( 74% !(!(!( !( !( !( !(!(!( !(!(!(!( !(!( !( !( Bould!(er!(!( !( !( !( !( !(!(!(!( !( !(!( !( !(!( !( !(!(!(!( !(!(!(!(!( !( !(!(!( !( !( !(!( !( !( (a) (b) Channel !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !(!( !( !( !( !( !( !( !(!( !(!( !( !( !( !( !(!( Figure 3: Fieldwork was conducted in northern Colorado to measure dimensions and record characteris cs of some of the landslides source !(!( !( !(!( !( !( !(!(!(!( !(!( !( !( !(!( !(!( !( !(!( !(!(!( !( !(!( !( !( !( !(!(!( areas shown on the maps. Google Earth images showing the loca on of the landslide that we studied in Boulder and Eldorado Springs !( !( !( !(!( ¯ !( quadrangle. (a) Loca on of landslide studied in the Boulder area. (b) Landslides loca on visited in the Eldorado Spring area. !( !(!( !(!( !( !( !(!( !(!( !( !( !(!( !(!( !( !( !(!( !( !(!(!( !(!( !( !( !( !( !(!( !(!(!(
!( !( !(!( !(!( !(!( Eldorado Spring!(s !( !( Figure 6: This second pie chart shows the frequency of geomorphic !(!(!( !(!( !( !(!( !(!( !( !( !( !( Eldorado Springs !(!( !( !(!( !( !(!( !( se ngs where the landslides occurred. In this case they are more likely !( !(!(!(!( !( !( !(!(!( !( !( !(!( !( !( !( to occur on a open slope. !(!(!(!( !(!( !(!( !( !( !( !( !( !( !(!( !(!( !( !( !( !(!(!(!(!( !(!(!( !( !(!( !( !( !(!( !( !(!(!(!( !( !( !( !( !(!( !( !( !(!( !( !(!( !( !(!(!( !( !( !(!( !(!( !( !(!( !(!( !( !( !( !( !( !(!(!(!(!(!( !(!(!( !(!(!( !( !( !( !( Discussion !( !( !( !( !( !(!(!( !( !(!(!(!( !( !( !( !( !( !( !( !( !(!( !( !( !( Although we observed no obvious differences between the Figure 1: The exis ng maps had points at the top of landslide scarps and (a) (b) (c) (d) travel lines showing how far they ran out. We started with a database of !( source areas and surrounding non-landslide areas, we noted Figure 4: Observed characteris cs included topographic curvature, proximity to rock outcrops, and geologic materials exposed in the landslide 1350 landslides points where 265 point were located in el Boulder source areas. In addi on, observa ons in non-source area of similar slope and storm rainfall were conducted to record characteris cs in those several characteris cs that the source areas have in common. quadrangle and 150 were located in the Eldorado Springs quadrangle. areas, and determine the differences between source areas and nonsource areas. Subsequently, the observa ons were analyzed and Slope of the source areas ranged from 28° to 35° and most summarized to indicate which detailes observed in the field might explain why landslides occurred where they did and why they did not occur Mapping was performed using a Geographic Informa on occurred on planar or slightly concave slopes that were in other similar areas. (a) Measuring the length of the landslide. (b) Determining the width of the landslide. (c) Quan fying the depth of the vegetated with grass, small shrubs and few trees. The source System (GIS) with satellite imagery and topographic data. landslide. (d) Making observa on of the geological material exposed in the surface of the landslide. areas were shallow and elongated downslope: width ranged High-resolu on satellite imagery acquired shortly a er the !(
!( !( !( from 4 to 9 m, length from 6 to 40 m and depth ranged from 0.7 September 2013 floods along with pre-event and post-events !( Results topographic map data. to 1.2 m. Colluvium was the source material for all of the We studied the source areas of six debris flows in northern Colorado. Four were in the Pinebrook Hills area, on the Boulder debris flows and bedrock was exposed in the basal surface of ¯ ¯ 7.5-minute quadrangle, and the other two were near Gross Reservoir on the Eldorado Springs 7.5-minute quadrangle. Each the source areas. We observed no evidence for concentrated of the debris flows started as shallow landslides. surface runoff upslope from the sources. The characteris cs !( Dimensions Landslide 1 Landslide 2 Landslide 3 Landslide 4 Dimensions Landslide 5 Landslide 6
La tude 40.04553 N 40.0475 N 40.0475 N 40.0481 N La tude 39.9461 N 39.9478 N and dimensions of the shallow landslide source areas of these
!( !( Longitude 105.29856 W 105.2988 W 105.2988 W 105.2988 W Longitude 105.3481 W 105.3478 W debris flows are consistent with either localized weakness of
!( Landslide Type Shallow transla onal Shallow transla onal Shallow transla onal Shallow transla onal Landslide Type Shallow transla onal Shallow transla onal the colluvium, localized pore-pressure build-up during rainfall Boulder Eldorado Springs Geological materials Siltstone and shale Sandstone Siltstone and shale Shale Geological materials Granite Fine to medium-grained granite !( !( !( Vegeta on Grass Grass Grass Grass infiltra on, or both. Local topographic and geologic details and Vegeta on small shrubs, aspen trees and sparse grass Shrubs, aspen and pine trees and grass !( Length 10.6 m 7.1 m 6.45 m 5.63 m !( Length 16.3 m 40 m colluvium heterogeneity probably contributed to these. Width 7.5 m 6.5 m 7.05 m 4.47 m Width 8.9 m 8.3 m Depth 1.2 m 1.1 m 0.78 m 0.77 m !( !( Depth 0.69 m 1.1 m Acknowledgements !( Slope angle 28° 35° 33° 33° !( !( Slope angle 34° 32° !( !( Source material Colluvium, contain poorly Colluvium Colluvium Colluvium !( sorted sand and angular Source Material Colluvium derived from local bedrock Colluvium gravel
Geomorphology Open dip slope Open and dip slope Open dip slope Open dip slope, slight swale Geomorphology Open slope at the mouth of a broad swale Open slope RESESS, UNAVCO, USGS, ExxonMobil, NSF (a) (b) Shape elongated downslope, Circle Irregular circle Irregular rectangle Shape elongated, irregular rectangle elongated, irregular rectangle rectangle Figure 2:The satellite imagery had previously been orthorec fied so that (a) (b) References objects visible in the imagery appeared n their correct spa al loca ons. Polygons were made around each landslide to outline the source and Table 1:(a) Observa ons of the four landslides studied in the Boulder quadrangle. (b) Observa ons of the two landslides studied in the Godt, J.W., Coe, J.A., Kean, J.W., Baum, R.L., Jones, E.S., Harp, E.L., Staley, D.M., and Barnhart, W.D., 2014, Landslides in the deposi on areas. (a) Example of polygons created in the Boulder quadrangle. Eldorado Springs quadrangle. Northern Colorado Front Range caused by rainfall, September 11-13, 2013: U.S Geological Survey Fact Sheet 2013-2014. (b) Some of the polygons created in Eldorado Springs quadrangle
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