Engineers in China must struggle to divert the flow of the countrys mighty rivers for new dams, but their counterparts in California seem to be having an easy time of it.
The new Eastside reservoir just outside Los Angeles will be in a dry valley. There is no river to divert and when the reservoir is complete, water will have to be pumped into it for storage before it is supplied to the city.
But although this cuts out the need for river diversion works, the scheme is still on a massive scale and represents one of the biggest civil engineering projects under way in the US. When it starts operation it will almost double the surface storage capacity for southern California.
Two main goals lie behind the construction of this new reservoir. The first is to meet the regions endless thirst for more water on a day to day basis. Existing supplies are already stretched and as the city sprawls further, demand rises.
In addition to this, however, the Metropolitan Water District of Southern California needs
the new reservoir to fulfil its obligations for earthquake preparedness. Should an earthquake sever supply lines to the reservoir, there would be enough stored water to supply the MWD area for six months long enough for repairs to be carried out.
While the dam will be large by US standards, it does not compare to the massive facilities currently being built in China. Eastsides 986M.m3 capacity is just a tenth of the Xiaolangdi scheme (NCE 15 January).
According to MWD construction manager Chuck Nichols, a group of Chinese engineers from the Xiaolangdi dam recently visited the Eastside project. When we told them that the reservoir would serve a population of 16 million, they were not impressed, he recalls. We got out of that conversation as quickly as possible!
The scale of the $1.9bn (1.18bn) project has led to construction management on the scheme being provided by a fully integrated joint venture team, with US consultants Parsons Corporation and Harza Engineering Company being brought in to join up with engineers from the MWD. Design of the reservoir scheme was carried out by US consultant Raytheon Infrastructure Services of New York.
Creating the reservoir is fairly straightforward. Two earth dams are being built, one at each end of the ring of hills which will enclose the water.
There are five main contracts which will provide the major features of the new reservoir, along with two smaller packages. Under a 240M contract, a joint venture of US contractors Atkinson Construction Company, Washington Construction Company and HB Zachry is building the west dam while the east dam joint venture contractor is Kiewit Granite.
AWZs contract also includes the saddle dam which fills a low part in the northern hills, and the forebay structure.
Almost complete now, the forebay is a large holding pond through which water will pass on the way into the reservoir, and which lies on the downstream side of the west dam. Water will be piped into the forebay from the San Diego canal or the Colorado River aqueduct, then pumped through the pumping station, via a tunnel through the hills to the inlet/outlet tower and into the reservoir. On its return journey it will follow the same route but under gravity.
At full capacity, says Nichols, the pumping station can handle 60,000m3 each second a swimming pool of water, which is enough to fill the reservoir in just six months. This enables it to take advantage of times of heavy rainfall. Under average rainfall conditions it will take between two and three years, he says.
Construction of the pumping station is being carried out by local firm Advanco Contractors under a 39M contract. Work started in February 1997 and is due to complete in the middle of 1999, says Advanco project manager Steve Earpe.
Also under construction is the inlet/outlet tower which links the tunnel into the reservoir. This 80m high tower will have nine levels with valves, and a full-height wet well behind, linking into the tunnel.
The hefty concrete structure has 3.6m thick walls at the base and a special concrete mix with low heat of hydration is being used to eliminate cracking. The concrete is also being cooled to counteract the temperature difference of 20C between the middle and the outside of the concrete walls. Reinforcement is heavy, with three layers of steel mesh as well as large diameter bars making up a dense cage.
Work on the main dam contracts started in 1996 with completion due in 1999.