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Washouts like this one from heavy rainfall contribute to bunker sand contamination. Each successive washout carries underlying silts and clays into the sand in the bottom of the bunker. This is a big reason the bottoms of the bunkers tend to firm up. Deep cultivation is often required to improve playability.
Fortunately when doing new bunker construction, there are now many choices available to seal off the underlying soils. Most methods use polymers to lock the soil into an impervious shell, sort of like a swimming pool bottom.
This photo shows the darkening in the bottom of the bunker where the silts and clays have been deposited. Over time, these will inhibit drainage and create puddling from even the slightest rainfall or irrigation event. Unfortunately, the only way to completely correct this contamination is by removal of the sand, sealing the bottom of the bunker, and then adding clean new sand.