The Oroville Dam is to be soaked by 30cm of rain by Tuesday, ramping up the pressure on engineers who are frantically trying to repair "patch and pray" quick-fixes which led to its near-collapse last week.

Up to 13cm of rain will land on the Northern California dam this weekend in the first storm which will also bring winds of up to 50km/h.

Another 25-30cm will land at the start of next week and bring even stronger winds which may thwart repair works.

Meteorologists said it is that storm which will be a concern to engineers using helicopters to try to secure the dam's emergency spillway which nearly gave way last week to the dam's full reservoir.

Water trickles down as workers inspect part of the Lake Oroville spillway failure. Photo / AP
Water trickles down as workers inspect part of the Lake Oroville spillway failure. Photo / AP

,000 people were evacuated on Sunday when the California Department of Water Resources dramatically announced that an emergency spillway at the dam was an hour away from collapse.

Residents nervously returned home as the evacuation order was downgraded to a warning on Tuesday but they have been told to be ready to flee again at a moment's notice if the volatile situation turns on its head again.

"Wind looks like a concern," Sacramento National Weather Service forecaster Eric Kurth told yesterday.

Engineers were battling gusts of up to 50km/h as they lined the erosion with rocks and boulders.

"It's pretty windy up there. There were reports of some trees coming down in the area too. With sustained winds of 45km/h and gusts of 50km/h, that would make things pretty difficult," Kurth added.

Earlier it was revealed cracks in the main concrete spillway have been included in site reports since 2009 but crews have been resorting to quick-fix concrete solutions to try to stop them from getting worse.

The repairs that have been going on for years were labelled shoddy by Robert Bea, professor emeritus of civil and environmental engineering at University of California, Berkeley.

"It's obvious those repairs didn't work. We don't have details on the repairs, but they put cement into the cracks and troweled it over. I call it 'patch and pray'."

Inspectors with the state agency that both operates and checks the dam, the nation's tallest at 235m, went into the 800m spillway in 2014 and 2015 and did not find any concerns.

In this photo taken October 7, 2009, California Department of Water Resources crews do maintenance repairs on the Oroville Dam spillway. Photo / AP
In this photo taken October 7, 2009, California Department of Water Resources crews do maintenance repairs on the Oroville Dam spillway. Photo / AP

"Conditions appeared to be normal," the inspector wrote in reports from both years. The most recent report which deemed the dam safe was in August last year when a team of inspectors checked it from afar.

The inspection was ordered as California was enduring a five-year drought which meant that until last year, the spillways had gone largely unused.

The Department of Water Resources declined to answer specific questions about the repair work, saying engineers were focused on ensuring public safety.

On Monday, federal regulators told the department it must enlist a group of independent consultants both to assess what went wrong and to recommend long-term fixes.

Documents and interviews show that crews were patching cracks in 2009 and 2013. A water resources department spokesman said it was normal for maintenance crews to be troubleshooting cracks in the channel during dry summer months.

One resident of the region said he saw crews in the spillway at least once a year for the past several years.

"When they have four or five trucks down there, the only thing they have to do is fill cracks," said Don Reighley, a retiree and fisherman who several times a week drives past the channel to launch his boat into the reservoir.

One of the state inspectors who went to Oroville Dam in August said authorities may never know exactly what destabilised the spillway.

"Any type of evidence that might have been there is gone," Eric Holland of the water resources department's dam safety division said. "Everything has been washed away."

It has emerged that the critical document that determines how much space should be left in Lake Oroville for flood control during the rainy season hasn't been updated in 47 years.

The Sacramento Bee reports the outdated document uses climatological data and runoff projections that don't account for two past floods.

Independent experts familiar with the flood-control manual at Oroville Dam say there's no indication the outdated document contributed to the ongoing crisis involving the dam's ailing spillways. Structural failures are the trouble.

But experts say the manual points to larger operational issues that affect most of California's primary flood-control dams.