Canadian Dam Association Bulletin - Summer 2015 - (Page 12)

Boundary Dam total Dissolved Gas mitigation measures 1. introDuCtion Seattle City Light (SCL), a department of the City of Seattle, owns and operates the Boundary Hydroelectric project (Boundary Project), located on the Pend Oreille River in northeastern Washington State. The Boundary Dam is a 340 ft high arch dam with two spillways and seven low level sluiceway discharge gates. The discharge facilities have been shown to produce elevated Total Dissolved Gas (TDG) concentrations in the river reach downstream of the dam. TDG is the amount of air held in saturation in the water and is quantified in the standards in terms of percent of saturation pressure relative to ambient barometric pressure. These high gas levels can be detrimental to the local fish species. SCL has worked with Hatch Associates Consultants Incorporated (Hatch) and their team that includes Alden Laboratories Incorporated (Alden) since 2005 to explore a wide range of solutions to address TDG and satisfy their agreements with state and federal authorities. One of the techniques employed by the Hatch team to mitigate TDG production involves the addition of roughness 12 elements to the downstream end of the chute on Spillway No. 2 on the right abutment of the dam. (See Figure 1) Detailed engineering studies were undertaken to design and construct a series of super-cavitating blocks or roughness elements (blocks) that could be added to the end of the chute on Spillway No. 2. The purpose of the blocks is to accelerate the breakup of the spillway jet prior to its plunge into the downstream pool and thereby minimize plunge depth and gas transfer. Construction and initial testing of the blocks, was completed during the fall of 2014. This paper describes the design process followed by SCL and the Hatch team that led to the detailed block arrangement and geometry selected for Spillway No. 2, the unique structural design details of the blocks, and their construction. 2. BaCKGrounD The Boundary Project is located on the Pend Oreille River in northeastern Washington State, 1 mile south of the Canadian border. It is the third of five dams on the Pend Oreille River - Seven Mile and Waneta dams are located downstream in Canada, Box Canyon Dam is immediately upstream of the Boundary Reservoir, and Albeni Falls Dam is 50 miles farther upstream near Newport, Washington. Boundary Project consists of an arch dam, reservoir, and underground powerhouse. The dam has a variableradius arch with a structural height of 340 ft, a crest length of 508 ft, and a total length of 740 ft with both spillways included. The gross head of the project for power generation purposes is 261 ft. On each abutment there is a 50-ft-wide overflow spillway and these two spillways have a combined total maximum discharge capacity of 108,000 cubic ft per second (cfs). In addition, there are seven low-level sluices through the dam that under a design head of 190 ft can provide 252,000 cfs of discharge capacity. The powerhouse flow capacity is approximately 55,000 cfs, through six generating units, units 51 through 56 (two equipped with 200-MW Francis turbines and four equipped with 160 MW Francis turbines). Flows through the powerhouse discharge into the tailrace immediately below the dam. SCL operates the project in a loadfollowing mode, generating power during peak-load hours and curtailing generation during off-peak hours. view past issues of the CDA Bulletin online at www.naylornetwork.com/cda-nXT/ http://www.naylornetwork.com/cda-nXT/

Table of Contents for the Digital Edition of Canadian Dam Association Bulletin - Summer 2015

Board of Directors
President's Message
Boundary Dam Total Dissolved Gas Mitigation Measures
CDA Conference 2015
New Bulletin on Emergency Management
CDA Publications Wrap Up
CDA Directors & Officers – Call for Nominations
Notice of AGM
2015 CDA Awards – Calll for Nominations
Time to Renew Your Membership
Buyers’ Guide and Trade List

Canadian Dam Association Bulletin - Summer 2015

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