Authors: Cline, D; Stevens, C; Paulson, E; Crouse, P; Gargeya, D

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Cline, D, Stevens, C, Paulson, E, Crouse, P & Gargeya, D 2012, 'Guidance on estimating peak flood flow frequency in ungauged watersheds', in AB Fourie & M Tibbett (eds), Mine Closure 2012: Proceedings of the Seventh International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 785-799,

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The Holden Mine is an inactive underground copper mine located in a remote area on the eastern slopes of the Cascade Mountains in the State of Washington. Operated between 1938 and 1957 by Howe Sound Company, mine operations covered approximately 40 ha along a 16 km reach of the Railroad Creek watershed. During the mine life, portions of Railroad Creek were straightened and realigned to make room for additional tailings. Rio Tinto is currently developing a mine reclamation plan that includes restoring Railroad Creek to improve aquatic habitat and ensure long-term hydraulic stability of the tailings piles. This paper discusses the hydrologic analysis performed to determine peak discharges for the ungauged Railroad Creek watershed needed to design the new channel restoration features required for final mine site reclamation and closure. The assessment of water resources in remote regions is often challenging due to limited data availability at the location of interest. In addition, mine closure often occurs many years after the mine was active, requiring new flood hydrologic analyses based on current data and standards of practice. The most common method of predicting stream flows for ungauged sites is the transfer of hydrologic information from gauged (donor) sites to ungauged (acceptor) sites based solely on unit-area characteristics. While this method might work for similar sized basins, there is significant uncertainty with using an area-only methodology. Over-predictions or under-predictions of stream flows might result in improperly zoned floodplains, over- or under-designed hydraulic structures, and an inaccurate understanding of flood risk. To reduce uncertainty in data transposition for ungauged Railroad Creek at the Holden Mine site, a two station comparison method coupled with Log Pearson Type III (LPIII) procedures was used. The Railroad Creek at Lucerne stream gauge served as the donor watershed for the Holden Mine Railroad Creek reach. Since the Lucerne stream gauge was discontinued in 1957, the Stehekin River at Stehekin stream gauge in the same watershed was used to compare and extend the data record for the Lucerne site. The results of this regression analysis were used to estimate the annual peak flows at the Lucerne stream gauge for the 1958–2010 period. The estimated annual peak flows were subjected to frequency analyses using Water Resources Council procedures, followed by an LPIII parameter analysis to obtain revised peak discharges for the Lucerne site. These peak discharges were then transposed to the Holden Mine location in the Railroad Creek watershed using regression equations based on drainage area and mean annual precipitation. In addition to magnitude and frequency of annual peak discharges, design flows for the fall season (September–December) were computed to assess flood risk during the construction period for specific site remediation facilities.

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Water Resources Council (1982) Guidelines for Determining Flood Flow Frequency, Bulletin 17B, Hydrology Committee, Washington, D.C.

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