After a three-year battle to keep their underground job site from flooding, the construction crew at Lake Mead is ready to let the water win. Sometime late next week, workers plan to shut off the pumps keeping the water out and allow it to fill the cavern they have carefully excavated more than 500 feet beneath the shore. The move will mark the latest milestone for the Southern Nevada Water Authority’s low-lake-level pumping station, a $650 million safety net for a community that draws 90 percent of its drinking water from Lake Mead.