104 26 kt) to 115 km/h (72 mph; 63 kt). Monitors measuring winds greater than 121 km/h (75 mph; 65.2 kt) might have been ren- dered inoperative by hurricane force winds. Additionally hurri- cane dynamics are complex and tornadoes, wind bursts or ante- cedent rain might have more of an influence on debris production than does maximum sustained wind speed (Everham and Brokaw 1996). Also, as hurricanes move inland, wind speeds are reduced but are still capable of causing substantial levels of damage to trees and infrastructure. A substantial increase in tree damage from branch failures has been reported by Luley et al. (2002) to begin around 81 km/h (50 mph; 43.4 kt). Francis and Gillespie (1991) report tree damage to begin around 60 km/h (37mph; 33 kt) with damage increasing rapidly to approximately 130 km/h (81 mph; 70 kt), but not worsening at higher gust speeds. Results of urban forest damage as expressed by debris generation from this study are consistent with other studies, which found that tree and stand characteristics were the best predictors of damage while storm meteorology was of sec- ondary importance (Kupfer et al. 2008; Oswalt and Oswalt 2008). The relationship between wind speed and debris gen- eration was found to be significantly related, while there are clearly other factors, in addition to wind speed, that are im- portant in debris generation. Assuming that all other effects are at their average over the range of the independent vari- ables, this study indicates that tree canopy cover and density, wind speed, and percent of urban development had complex relationships with debris generation (Figure 1a, Figure 1b). Increased tree density in a community and high tree canopy cover with increasing winds were two interactions that resulted in decreased debris generation (Figure 1a, Figure 2a). This can possibly be a result of urban forest structure, management, and the effects of past hurricanes. Everham and Brokaw (1996) in- dicate that uniform canopy heights of wind-impacted forests are less susceptible to windstorms. Because of this, tree cano- pies collectively reduce turbulences and shed winds. Addition- ally, the authors report that past and current management ac- tivities might selectively remove susceptible trees and change species and size composition to a more wind-resistant structure. This study’s results also suggest that models and meth- ods that use tree cover or wind speed alone might not be con- sidering important factors that influence tree debris generation (FEMA, 2007a). However, other factors such as urban forest structure and impact of prior hurricane events warrant further investigation and could improve the reliability of multivari- ate models (Everham and Brokaw 1996; Kupfer et al. 2008). This study found that debris generation varied from a low average of 0.59 m3 (0.77 yd3 for low damage, 3.4 m3 (4.44 yd3 17.47 m3 (22.85 yd3 ), per 30.5 m of street segment ) for moderate damage, and ) for high damage levels. Given the vari- ability of cubic meter debris production in the sampled com- munities, these damage level averages are plausible and within range of past post-storm tree debris production events (Rankin 2000). These averages can be applied by a Florida commu- nity, who can use street lengths to obtain a preliminary esti- mate of potential debris generation from their urban forest. It is interesting to note that tree removal rates were surpris- ingly low in the communities that reported these data. One rea- son may be that trees needing removal were treated as debris and removed during debris removal activities. Alternatively, it is possible that standing trees were not removed regardless of the ©2009 International Society of Arboriculture Escobedo et al.: Hurricane Debris and Damage Assessment Figure 2a Figure 2b Figure 2. Graphs depicting two-way wind speed interaction of debris generation using low and high values of (a) percent tree canopy cover (%Canopy Cov) and (b) percent developed urban cover (%Dev Urb Cov). Low and high values used were 10% and 30% tree canopy and 50% and 90% developed urban cover, re- spectively. level of damage. Tree pruning rates were more consistent with ice storm events in the northeast United States (Bloniarz et al. 2001). An important difference between ice storm damage cleanup in the northeast US and hurricane storm damage clean up in our sample is the use of unit costing for tree removals and pruning, or the inclusion of tree removal and pruning costs in debris cleanup costs. This is suggested by the absence of reported tree removal and pruning data in PWs, the absence of itemized (e.g., diam- eter class) data for these costs when calling communities, and the common use of a unit costing approach in PWs. We concluded from our limited data that itemization of costs for tree pruning and removal is rare for hurricanes in Florida, despite FEMA’s request- ing this approach in their formal documentation (FEMA, 2007a). Limitations of this study included the few data that were re- ported in PWs or available from communities are related to the direct exploration of any relationships between specific costs, tree removal and hazard pruning rates, and wind speed. In fact, we only obtained usable pruning and removal data from two communities, which had data by tree as required by FEMA. Since communities apparently use unit costing or report removal and pruning costs with debris clean up, these data will be more difficult to obtain than debris removal estimates. Additionally, recent and state-wide Flor- ida urban forest cover and structure data are unavailable. Clearly, it would be desirable to find additional data such as these so that more robust numbers may be used for vegetation debris estimates.
March 2009
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