Hydroclimatology and debris flows in the Front Range
The Colorado Front Range is dominated by differing precipitation patterns varying from the high elevation areas to the lower. Most precipitation above 2,300 m (7,500 ft) occurs as snow and the hydrograph peak is dominated by spring snowmelt.
Below 2300 m, the most abundant component of annual precipitation is summer rainfall generated by convective storms. These precipitation patterns influence erosional processes and bank stability.
Debris flows occur when super-saturation destabilizes hillsides and causes failure. Debris flows are a common landscape-forming feature below 2300 m (Costa and Jarrett 1981), whereas glaciation has the most pronounced impact on the current landscape above 2300 m (Caine 1984, 1986).
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Fire and Debris Flows
Fire indirectly influences the balance between shear stress and shear strength on the hillslope. For example, fire accelerates rates of ravel and surface erosion, depositing sediment in hillslope hollows, a common initiation point for debris slides (Dietrich et al. 1982). Debris flows can be initiated in two ways—either from surface runoff or from debris slides and both types of initiation events have been documented for debris flows in Colorado (Wondzell 2003). Numerous studies have documented increased frequency of debris flows following wildfires (Wells 1987, Wohl and Pearthree 1991, Meyer et al. 1992 ). Debris flow initiation from rilling is common on steep hillslopes where vegetation has been recently burned by wildfire. Debris flows may continue long distances down relatively large streams. Wherever debris flows finally stop, they typically construct large jams of sediment and wood which can block the stream channel and create zones of sediment deposition immediately upstream. During major floods, debris jams can impound water and, in some cases, may fail catastrophically, releasing flood surges downstream (Nakamura et al. 2000).
The frequency of high intensity convective storms during summer in the Colorado Front Range increases the probability of rainfall on dry soils and, when combined with generally less rapid vegetation recovery, substantially increases the likelihood of overland flow and surface erosion following fire (Cannon et al. 2001).
The time frame of susceptibility to accelerated erosion following fire is relatively long in Colorado. Immediately following the fire and for several years thereafter, sites are susceptible to overland runoff and related surface erosion and debris flow occurrence. Areas are also susceptible to debris slide and related debris flow activity about 5–10 years following fire (Martin and Moody 2001).
Debris flows are not the prevalent response of burned basins (Cannon et al. 2001). Those that do occur are frequently the initial response to significant rainfall events immediately following intense fire. Specific geologic and geomorphic conditions control the generation of fire-related debris flows. Debris flows are typically triggered via runoff and in arid regions (Cannon et al. 2001). The greatest increase in runoff and erosion occurs within the first 1 or 2 years after burning, although this is affected by the timing of high intensity rainfall events (Benavides-Solorio and MacDonald 2001). Debris flows can occur under appropriate conditions, but they are not a given in all circumstances.
