Storm & Flood Damage Recovery in Vancouver, WA

That is almost always a fine-grained lens within the outwash. This ground was laid down by enormous floods and it is not uniform: coarse gravel that drains freely can sit above a silt layer that does not. Water infiltrates until it meets that layer, then perches and moves sideways. It surfaces or arrives at a foundation somewhere downslope of where it fell, which is why the wet spot and the source are often not in the same place.

Two things commonly cause that here. Either it was built into a part of the profile with a fine-grained layer beneath, in which case it never had the capacity it was assumed to have, and it fails once the winter rain becomes continuous. Or it has silted up over years, which is ordinary and fixable. A test during the wet season, rather than in August, tells you which one you are dealing with.

Because the rain is continuous rather than intense. Nothing gets a dry interval between November and March, so moisture accumulates in crawl spaces and framing week after week without any single event to mark. By late winter the ground is at its wettest and the accumulated moisture in the building is at its highest. Inspecting a crawl space in February tells you far more than inspecting it in summer.