Two of the nation’s flagship military hospitals will be combined into one on Saturday, the culmination of a costly and controversial decision six years ago to close Walter Reed Army Medical Center and expand the Bethesda National Naval Medical Center.

A parade of ambulances is scheduled to carry the last 30 patients from Walter Reed the seven miles to Bethesda on Saturday morning, effectively closing the historic Army hospital in Northwest Washington. The move, initially planned for Sunday, has been accelerated by one day in the hopes of beating Hurricane Irene.

As of Monday, the Bethesda facility will be known as the Walter Reed National Military Medical Center and will be the U.S. military’s largest medical center.

The move is part of a larger consolidation of military medicine in the Washington area stemming from the 2005 Base Realignment and Closure decision to shutter Walter Reed, which stunned the Washington region as well as Army medicine, particularly coming in the midst of wars in Iraq and Afghanistan.

Four military hospitals in the Washington area — at Walter Reed, Bethesda, Andrews Air Force Base and Fort Belvoir — have been merged into two large facilities: a vastly expanded medical center at Bethesda and the new Fort Belvoir Community Hospital in Fairfax County.

The Department of Defense estimates that the consolidation will save $140 million a year by eliminating redundancies. But the cost of consolidation has been far greater than the $988 million originally estimated by the Pentagon and now stands at $2.8 billion. The size and scope of the consolidation have increased in response to concerns that the new facilities would not provide the best care for wars’ wounded.

The consolidation has been meant as a test bed to integrate military medicine across the services. But it has been beset by tensions between the Army and Navy over control of the new facility.

While the Joint Task Force National Capital Region will have command over both hospitals, the Navy will oversee the administration at Bethesda, as will the Army at Fort Belvoir. Both are slated to become joint commands but no date has been set until differences can be settled.

“There will be unity of command,” said Lt. Gen. Eric Schoomaker, the Army surgeon general.

Merging two hospitals with storied histories and their staff with deep institutional cultures has not been easy. “This is a historic transformation, given that these are two icons of military medicine,” said Rear Adm. Matthew Nathan, the commander at Bethesda. “You can imagine the brand loyalty at Walter Reed and Bethesda.”

“My goal is not to homogenize those cultures,” Nathan added. “I want there to be friendly rivalries.”

Army doctors and nurses from Walter Reed are treating patients alongside Navy counterparts, although it is difficult to tell them apart once they put on their scrubs.

“In the OR, you have to ask people what service they are,” said Army Col. Charles Callahan, who was previously at Walter Reed and Fort Belvoir and is now the deputy commander at Bethesda.

The bustling Bethesda base has added $1.3 billion worth of new construction, including a dozen buildings, and now totals 2.5 million square feet. The hospital, with 345 patient beds — including a 50-bed intensive-care unit — also has new centers for treating soldiers who have lost limbs or are suffering from traumatic brain injury.

Despite the increased costs, the Army’s top medical officer predicts that consolidation will prove worth it by providing better and more efficient health care to wounded troops as well as more than 400,000 recipients of military health care in the Washington region. “We should see improved outcomes in care, health and readiness at a reduced cost to the taxpayer,” Schoomaker said.

An independent panel warned in 2009 that the original plans fell short of a congressional mandate to provide “world-class” medical facilities at Bethesda and Fort Belvoir, saying they lacked enough operating rooms and that the operating rooms were too small to accommodate surgical advances. In response, the Department of Defense spent an additional $800 million to meet the standards.

“We are obliged to be world-class,” said Vice Adm. John M. Mateczun, commander of the Joint Task Force National Capital Region Medical, who made his comments at a recent briefing at Fort Belvoir. “This is world-class.”

The 20 operating rooms at Bethesda — some of which are still being renovated — and 10 at Fort Belvoir are large enough to accommodate new technologies, Mateczun said. The 30 operating rooms, though 10 fewer than existed before the consolidation, are “more than adequate” to meet projected demand, hospital officials said. But patients have been asked to put off elective surgery or seek treatment in private facilities to ensure that there is enough space for casualties during the transition.

Walter Reed, named after the Army doctor who identified mosquitoes as the carrier of yellow fever, was established a century ago, treating soldiers from every conflict since World War I and serving as the center of Army medical research.

Its reputation was badly tarnished in 2007 when a Washington Post series disclosed that soldiers recovering from war wounds were living in substandard conditions and tangled in bureaucratic red tape. The problems stemmed in part from the decision two years earlier to close the hospital, as attention focused on preparing for the move rather than maintaining the existing facility.

The resulting outcry led to repairs and better supervision of recovering soldiers. Congress passed legislation requiring that “world-class” facilities be ready at Bethesda and Fort Belvoir before Walter Reed closed.

Bethesda, with its iconic Art Deco tower based on a design sketched by President Franklin D. Roosevelt, has established its own proud history since opening during World War II, treating wounded sailors and Marines as well as many U.S. presidents.

Bethesda and Walter Reed have been on a war footing for a decade, treating many of the most seriously wounded from Iraq and Afghanistan.

Walter Reed handled the bulk of the patients, including most of the complex trauma cases involving amputations. Bethesda took care of most of the serious head injuries requiring neurosurgery. “Now there is complex, comprehensive care under one roof,” Nathan said.

The number of patient visits at Bethesda is expected to jump from 600,000 to about 1 million a year.

The 515,000-square-foot America Building, now the military’s largest outpatient medical facility, already has corridors crowded with doctors and patients. A soldier in a gray Army T-shirt and two artificial legs buzzed down the hallway on a Segway.

Several dozen recovering soldiers worked with physical therapists and family members in an advanced training center, equipped with state-of-the-art rehabilitation equipment for amputees.

Marine Sgt. Maj. Ray Mackey, who lost his legs in Afghanistan, exercised his arms on an upper-body cycle. Some of the troops who have been at Walter Reed are “a little apprehensive” about the move, he said. But he said concerns have been allayed since their arrival at Bethesda. “Everybody’s really impressed with the facility,” he said.

The America Building houses a prosthetic lab with double the capacity for making artificial limbs as the lab formerly at Walter Reed. “This is a big improvement,” said Mike Stevens, a lab supervisor. The new facility includes six fitting rooms where patients can try on prosthetics in private — an idea that came from wounded troops at Walter Reed.

Last weekend, about 200 recovering troops from Walter Reed moved into Tranquility Hall, a new wounded warrior barracks at Bethesda with 153 two-bedroom suites equipped with kitchenettes, laundry machines, computers and flat-screen televisions. Some of the troops were wary about making the move to the Navy base. “After they see this, they don’t complain anymore,” said one of the new residents, Army Sgt. 1st Class Manuel Rodriguez.

At Fort Belvoir, patients from the old DeWitt hospital are set to be put in ambulances for the two-minute ride to the new hospital Wednesday. The Malcolm Grow Medical Center at Andrews is being converted into an outpatient clinic.

After the final patients leave Walter Reed on Saturday, the hospital’s flag will be lowered and a bugler will play taps.