On undisturbed ground the leaves of grass and other plants act as shock absorbers, but when rain bombs fall on bare ground, they displace soil particles and destroy soil structure. As the rain continues, displaced particles move about, clogging larger soil pores that normally help absorb water. If it keeps raining, the water begins to pond, then to flow downhill - carrying displaced soil particles and dislodging more. Construction makes matters worse because the heavy equipment packs the soil and diminishes its ability to soak up excess water. Despite the clear lapses in implementation, it's hard to be too critical of the builders, simply because the conservation professionals are a little contemptuous of their own techniques.
Silt fences get no respect, and straw bales even less.
For example, silt fences will filter particles down to 50 microns, but the real damage comes from silt and clay 5 to 10 microns in size. They flow right through the defenses. Correcting this problem is the primary reason for our visit to Southbridge. At the edge of the site are two plots that, with the cooperation of developer Don Tierney, may help to upgrade the science of soil conservation. For now their kingdom is one of red mud and curious devices.