Boggs Mountain Demonstration State Forest Draft ... - Cal Fire

Boggs Mountain Demonstration State Forest Draft Management Plan, June 2008
during the winter period. The waterbar should be extended to the cutbank to intercept all ditch
flow and extend beyond the shoulder of the road. A berm must block and prevent ditch flow from
continuing down the road during flood flows. The excavated waterbar should be skewed 30
degrees to the ditch-line with the excavated material bermed on the downhill grade of the road.
Water should always be discharged onto the downhill side on a stable slope protected by rip-rap
or vegetation. The cross ditch depth and width must allow vehicle cross-over without destroying
the function of the drain.
4. Critical Dip (CD)
Critical dips are broad swales excavated into the bed of the maintained road in order to eliminate
the potential for stream diversion if the culvert plugs during a storm or flood. At stream crossings
with a high diversion potential, floodwaters back up behind a plugged culvert, flow onto the road
surface (or into the ditch) and flow down the road. In a crossing with no diversion potential,
floodwaters emerging onto the road surface travel across the road prism and back into the
channel on the lower side of the crossing potentially washing out the fill.
Critical dips may be excavated over the top of the crossing, provided the culvert is deeper than
the proposed excavation. If the culvert is located shallow in the crossing fill, the excavation for the
critical dip may be made immediately down-road from the crossing site. The excavation work can
typically be performed with a crawler tractor in 1 to 1.5 hours.
5. Rolling Dip (RD)
Rolling Dips are simply breaks in the grade of a road. They are sloped either into the ditch or to
the outside of the road edge to drain and disperse road surface runoff. Rolling dips are installed in
the roadbed as needed to drain the road surface and prevent rilling and surface erosion. They are
most frequently used on outsloped roads.
Excavation for a rolling dip typically begins 50 to 100 feet up-road from where the axis of the dip
is planned. Material is progressively excavated from the roadbed, slightly steepening the grade
until the axis is reached. This is the deepest part of the excavation, with the overall depth being
determined by the slope of the road. The steeper the road, the deeper the dip will have to be in
order to reverse grade. To effectively direct runoff to the side of the road, the axis of a rolling dip
should be angled about 30 degrees to the road alignment. On the down-road side of the rolling
dip axis, the road bed slope should actually rise slightly to ensure that runoff cannot continue
down the road surface. This is called a “grade change”.
The rise in grade is carried for about 10 to 20 feet before the road surface begins to fall again at
its original slope. This transition from axis bottom, through rising grade, to original falling grade is
achieved in a road distance of 15 to 30 feet. The rolling dip should be broad and shallow enough
to permit low-boys, log trucks and other equipment to pass without slowing traffic excessively or
causing them to scrape bottom.
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