Managing fall floods in the Lower Skenna region, British Columbia
Creators: name: Urquhart, James Michael
Publisher: name: University of British Columbia
Year: 2010

description: This study investigates flood characteristics and management strategies in the Lower Skeena River Region. Rivers in the region exhibit two annual flood seasons in which different types of floods occur. Typically, rivers tend to flood during spring freshet in May and June as a result of snowmelt runoff within the Skeena basin. However, intense and sustained rainstorms contribute to another type of flood in the fall months in the Lower Skeena Region. Although the most extensive and largest flood of record occurred as a result of regional snowmelt runoff in the spring of 1936, "the flood inflicting the greatest damage occurred in the fall of 1978 as a result of three days of continuous heavy rain. In contrast with the remainder of the Skeena basin where spring freshet floods are critical, fall floods in the Lower Skeena occur more frequently than spring floods and for the same return period are greater in magnitude. Despite the implementation of flood damage prevention measures in the Lower Skeena Region, flood damage continues to increase and this is largely the result of fall floods. Analysis of the meteorologic and hydrologic features of fall floods indicates important differences in the duration and pattern of flooding throughout the Lower Skeena as compared to spring freshet floods. The interval between the time when the possibility of a flood is known and when it actually occurs is shorter for fall floods, different properties are subject to flooding and the frequency of flooding of most flood prone areas is greater. This indicates the need for a different strategy to manage the flood problem. The current program of flood damage prevention measures in the Lower Skeena is based on the physical characteristics associated with spring freshet flooding. The existing approach in the region is part of a blanket strategy toward managing floods province-wide. The strategy relies almost entirely on nonstructural measures of flood forecasting, floodplain regulation and floodproofing. The design flood, with a 1 in 200 year return period adopted as a basis for the current strategy is derived from the features of the 1936 spring freshet flood. These measures provided no assistance in reducing damages during the fall flood of 1978. A framework for developing a comprehensive flood management strategy to handle fall floods is applied to New Remo, a Lower Skeena flood prone community. The strategy entails the following five steps: I Define the Spatial Distribution of Flood Damages II Design a Flood Forecasting Service III Design an Emergency Plan for Action During Floods IV Assess Remaining Practicable Alternatives for Reducing Flood Damage V Develop a Financing Policy for the Program Potentially feasible adjustments to the flood hazard in New Remo include; (l) flood forecasting, (2) emergency action, (3) floodproofing and (4) permanent evacuation of some homes. However, to develop an optimum combination of alternative adjustments will require site specific information on flood damage risk, which currently is not available for properties in New Remo. The flood management strategy developed for New Remo would be applicable to other communities in the Lower Skeena, prone to fall flooding. However, regional application of the approach requires tailoring the strategy according to the physical, social and economic features within each community affected by fall floods.