Using GIS data and mapping techniques to delineate areas most prone to hazardous rockfall in the state of Colorado Amber Patrick and Erin Manitou Alvarez Department of Earth and Environmental Sciences The University of Texas at San Antonio Abstract This paper presents the construction of a GIS geodatabase to effectively integrate manage and display rockfall hazard ratings for the Colorado Department of Transportation CDOT The intention of this project is to incorporate an existing database into GIS format for improved visualization and comparison of rockfall hazard with the factors suspected of causing rockfall State highways are often constructed in geologic areas where rockfall is common As a result state Departments of Transportation DOTs are faced with the arduous task of maintenance and clean up of these highways The development of a Rockfall Hazard Rating System RHRS has enabled many state DOTs to better assess the rockfall hazard in their state The integration of the RHRS within a GIS will facilitate further preparedness by DOTs and their effective involvement with the travelers that frequent roadways attributed with rockfall hazard Introduction Background Certain areas of state highways are often constructed on steep terrain where rockfall is common A variety of problems can be caused by rockfall including traffic accidents increased maintenance costs blocked lanes and roads as well as other hazards to the public Bateman 2003 In the past state Departments of Transportation DOTs and their maintenance crews simply reacted to rockfall as they occurred by cleaning up the site and installing temporary mitigation structures Russell 2005 The continued occurrence of rockfall has prompted many states to establish a Rockfall Hazard Rating System RHRS enabling state DOTs to categorize the rock slopes according to the degree of hazard present Colorado is one such state Numerous factors contribute to rockfall instability and incidents These factors include climate slope conditions geological characteristics and traffic conditions and are further subcategorized for additional rating features Figure 1 shows the original RHRS developed by Pierson in 1994 The state of Colorado s current RHRS is a slight modification of the original RHRS The original RHRS system used subjective terminology such as Possible Minor Many Low Moderate High Few Occasional Many etc The state of Colorado has modified the original RHRS system to fit their needs by removing such terminology and replacing the scoring parameters with numerical values 3 9 27 81 ranging from low to high risk Russell 2005 In addition to the original RHRS parameters the state of Colorado added five further parameters to their RHRS These parameters include slope character climatic conditions geological conditions discontinuity conditions and traffic conditions These characteristics will be discussed in further detail below 1 Figure 1 CDOT s original RHRS taken from Andrew 1994 Slope Four different parameters establish the slope profile These four variables include slope height segment length slop inclination and slope continuity launching features The total 2 slope height was not changed from the original RHRS Total slope height is measured from the road to the highest point of potential rockfall source Russell 1005 and was not changed from the original RHRS The previous slope inclination was based on the slope angle score However research using the Colorado Rockfall Simulation Program CRSP has led to the adaptation of the slope angel score from degree values to a numerical system based on the consequence of whether a rock is more likely to bounce onto the roadway fall directly into a ditch or roll down hill with enough energy to reach the road Russell 2005 The original criteria for delineating launching features were very subjective Terms such as often many few were used to describe launching features and did not provide a clear description of the features Since the features themselves are heavily related to geology they will be discussed in further detail in the geology characteristics of this paper Climate Conditions It has been argued that precipitation and frost wedging are the most important factors that contribute to rockfall Annual precipitation in Colorado combined with freezing temperatures provides for a vast amount of freezing water The temperature range for the eastern plains region of the state is in its extreme at 115 F to 10 15 F below zero It is difficult to generalize temperatures in the western mountainous area of the state due to its vast elevation differences At the summits there is an average of about 32 F Elevation is also a contributing factor when discussing precipitation in this area The state of Colorado has regions that consist of vast plains and others that are highly mountainous with extreme peak heights These elevation differences greatly affect how the climate influences the area The interplay between prevailing air currents from the Pacific and the mountains is very important in determining precipitation on either side of the Rocky Mountain range that crosses the state These factors cause wide temperature differences to occur within short distances The mountain effect is seen here as storms from the Pacific hit the Rocky Mountains rise cool and dump their precipitation on the mountaintops Western Regional Climate Center website 2007 This map of the annual precipitation shows the highest precipitation indicated by dark blue color precisely where the Rockies transect the state 3 Figure 2 Annual Precipitation Colorado At this point it is necessary to define a freeze thaw cycle One cycle is a 24 hour period which has demonstrated temperature fluctuations above and below freezing conditions and had moisture on the surface that was enough to create freeze thaw conditions Arnold 1996 The map below shows the annual freeze thaw cycle distribution for the state of Colorado This map is helpful because it delineates areas by how many freeze thaw cycles they have experienced in that area in that particular year Using this information in conjunction with other factors we may be better equipped to understand why rockfall incidents occur where they do A dynamic freeze thaw cycle may be more conducive to allowing ice to melt deeper into the rock which in turn causes and increased degree of instability This contributes to rockfall incidents One must also consider the geological type of the surface material This topic will