The hydraulic design of ditches shall establish that the proposed
ditch is sufficient to carry the design ten-year peak flow and has
the type of lining necessary to prevent scour or undesirable sedimentation
in the ditch.
Drainage ditches, swales, and channels shall be located and
aligned in order to alter the original drainagecourse as little as
possible. However, it shall generally be considered desirable: to
eliminate bends, to cross existing and future roads normal to the
road, and to eliminate channels running through the center of a property
where relocation near or on a property line is feasible. The shape
and size of all ditches shall be so designed as to create the most
economically efficient and scour-resistant channel possible. The use
of ditches and swales with 3:1 or flatter slopes is encouraged; however
the maximum side slope permissible is 2:1 in existing ground and 1:1
in rock. The minimum center-line slope of roadside ditches shall be
1%.
The maximum ditch design velocity shall be taken from Plate
SD-7 of the Calvert County Construction Standards for Roads, Streets,
and Incidental Structures containing ditch design velocities, per
the ten-year design storm.
Particular attention should be given to outfall situations for
all storm drainage systems including single culverts. Outfall velocities
shall be reduced sufficiently to avoid erosion in the downstream channel,
stream or ground. Stormwater should be conveyed to the lowest point
possible prior to release at an outfall. Reduction of outfall velocities
can be accomplished by the use of ungrouted riprap or other approved
methods. An outfall analysis complete with cross sections shall be
submitted for all outfalls. The engineering professional representative
shall certify on the plans that there shall be no adverse impact on
downstream properties.
