Trenches Added To Flood Program

Federal officials have decided to dig contour trenches on about 800 to 1,000 acres of the 15,300 acres burned last month by the Eighth Street Fire in the Boise Foothills.

The decision was announced Monday by the Forest Service and Bureau of Land Management after meetings with local authorities on how to minimize the potential for devastating floods and debris flows in Boise’s North End.

Contour trenches will be installed in portions of the upper Crane Creek and Hull’s Gulch drainages to capture soil and water runoff on steep slopes where other treatment options are less effective, the agencies said.

The trenching is expected to begin Friday and should take about six weeks.

The plan includes provisions to reduce the visual impacts of trenching. Small track-mounted backhoes, called miniexcavators, will be used to dig the trenches instead of large bulldozers. The miniexcavators will dig narrow trenches rather than the wide roadlike terraces constructed in the Foothills in the 1960s.

The decision also will require vegetative screening to reduce visible scarring, and provides the option of restoring the trenches to natural grade in several years after the need for emergency flood control has passed.

The trenching decision was based on an interagency fire rehabilitation team’s finding that the kind of rainstorm that occurs in the area every two to five years on average could trigger larger floods and mud flows down the charred hillsides than ordinarily would occur from a 500-year storm.

The study found that floods and mud flows could threaten lives and affect more than 4,700 homes and commercial buildings worth at least $500 million.

Besides trenching, about 5,000 acres in the Foothills are being lightly tilled to improve water absorption.