Bear Swamp: OutRun the River
The design criteria for most major dam and powerhouse projects are based on record recorded rainfalls during the past 100 years. This information is also used when designing temporary construction facilities such as river crossings, drainage of work areas, cofferdams, and diversion structures. The Deerfield River ran through the Bear Swamp Project. The underground powerhouse was adjacent to the river. The lower portions of the powerhouse, the tailrace tunnels, the tailrace gate structures, and intake excavations were all lower than the river. A cofferdam is a temporary dam constructed to protect work areas during construction. Prior to excavating the tailrace tunnels we built a cofferdam along the side of the river. The height of the cofferdam was determined by 100 year flood data. The only road to all of the construction of the powerhouse crossed the river over three twelve foot diameter culvert pipes designed to take 100 year flood flows.
The previous 100 year record floods occurred in Nov 1927 as remnants of a tropical storm dumped on Vermont. Late in June of 1973 two frontal systems converged on Vermont with sustained rains of up to six inches in twenty four hours. The second day of rains the Deerfield River was coming up a foot per hour. There was water running in places you never imagined it would run. We immediately mobilized every needed piece of equipment to raise the height of our cofferdam. Every mobile crane on the job, along with every flatbed truck, was put into service evacuating crew shacks, welding machines, and supplies out of the tailrace intake area. The square steel stop logs designed for future maintenance of the tailrace tunnels were lowered into the gate guides by crane. If we lost the cofferdam the stop logs would minimize the amount of water flowing into the tunnels as they became submerged. We used burlap bags and wood wedges to chink the cracks.
Part of my crew was on standby with equipment at the river culvert crossing. If the river overflowed the crossing it would destroy the huge culvert pipes and severely delay our construction schedule while a new crossing was installed. A meeting earlier in the day had determined the maximum safe river elevation for the culverts. It was ten o’clock at night when the water hit the high mark. We started downstream and excavated a wide looping ditch around the culverts and across the road. Soon a third of the rivers flow was turned through our ditch. The ditch saved our crossing. We out ran the river as the two fronts exceeded the 100 year records. It was a great sigh of relief a few days later when the river started to recede to normal levels. The state of Vermont three miles upstream was declared a federal disaster area.
It was challenging to defeat a 100 year record flood……..but how sad for the people who lost so much.