Challenges encountered in dam removal and underwater valley restoration projects

Built in 1913 to provide the earthquake-devastated San Francisco with fresh water, the Hetch Hetchy Reservoir has given rise to significant political dispute over decades, with environmentalists and nature lovers opposing its construction. And while San Francisco's citizens are nowadays discussing the possibility of its removal, other areas across the U.S. are taking action with very interesting feedback. 

Even though each dam requires a different approach, the first step in every case, would be to lower the water level of the reservoir behind the dam. In the case of the Hetch Hetchy Reservoir, this would be an easy task, due to the tunnel and a series of aqueducts already in place. Engineers would only have to open the pipes and keep them open, until total drainage of the stored water. The environmental restoration specialist and vice chairman of Restore Hetch Hetchy, Mark Cederborg, estimates that the particular process would take approximately three years. However, in the case of the Glines Canyon dam, in Washington state, the spillways located in the upper part of the dam, required the pounding of the upper 17ft of the dam's concrete down to the level of the spillway gates to allow water drainage.

The above time-consuming process, would be disclosing a flat bottom valley, being underwater for about a century and waiting to be revived. Unlike other dam removal projects, the Hetch Hetchy valley is expected to be covered in only two inches of sediment, as a result of the slow erosion process of the granite riverbed of Tuolumne River. It is estimated that the demolition of the Glines Canyon Dam, being the largest dam removal project in the U.S. history, has released about 34 million cubic yards of sediment and debris downstream. University of Washington scientists are still studying and evaluating the environmental impacts of this massive sediment release in the delta of Elwha River.

Apart the sediment handling, restoration measures would be required directly after the reservoir drainage. Stabilization works of both the reservoir slopes and floor, as well as erosion control measures to prevent sediment creation after rainfall can accelerate the natural process of re-vegetation. According to Cederborg, it would take about two years for meadows and seedlings to appear, while 5 to 10 years past dam removal, inundation sings would no longer be visible, stressing out nature's amazing restoration capability.