By Ray Gronberg

gronberg@heraldsun.com; 419-6648

DURHAM -- Acting in part due to prodding from state legislators, County Commissioners on Monday voted 3-0 to tighten Durham's erosion-control rules.

The City Council also has to act on the revised regulations and is expected to do so on Dec. 7, said Aaron Cain, senior planner in the City/County Planning Department.

The most significant of the rules approved by commissioners Monday spell out the standards developers and engineers have to meet when designing catch basins and other measures to limit how much soil washes off construction sites.

At the General Assembly's order, Durham's rules will now say such traps have to handle the runoff from a 25-year storm -- the sort of heavy rain that has a 4 percent chance of happening in any given year.

Durham's so-called "unified development ordinance" previously required that traps and basins handle a 10-year storm -- a smaller, less-intense rainfall that has a 10 percent chance of happening each year.

The state-mandated changes to the ordinance also included provisions that spell out how efficient basins should be in trapping soil and how well channels should deal with heavy flows.

They further set deadlines ranging from seven to 14 days for developers to place ground cover on sites after clearing them.

The deadline tightens as the slope of land increases. Durham law already required cover but allowed builders up to 21 days to place it on steep slopes.

The state mandates came down as part of a bill the General Assembly passed in August to set an early 2011 deadline for regulators to propose new rules to protect water quality in Falls Lake, the city of Raleigh's only source of drinking water.

Raleigh officials have complained about sedimentation and other runoff-caused pollution problems in the lake.

The rules commissioners acted on Monday also included proposals submitted by a group of business, environmental and neighborhood leaders that reviewed Durham's land-use rules at the behest of local elected officials.

The most significant of those changes lowered the threshold for requiring builders to make an erosion-control plan part of their permit applications. The limit will change from its present one acre of land disturbance to 20,000 square feet.

The local proposals also require developers to install "skimmers" in catch basins, devices that ensure water drains from the surface down so soil has more time to settle out.

Furthermore, if the excavation necessary to install erosion controls itself has the potential to cause erosion, developers have seven days to install or plant ground cover to prevent it.

Monday's vote came on a night when two commissioners -- Becky Heron and Brenda Howerton -- were absent. The board had a light agenda in the run-up to the Thanksgiving holiday.