University of Virginia Library

LECTURE 8 - DRAINAGE

An Engineer in the Park Service used to say, "... remember three
things about roads: the first is drainage, the second is drainage,
and the third is drainage!"

Road construction requires that runoff be intercepted in roadside
ditches before the water that falls onto the watershed reaches the
road prism. It is imperative that drainage be provided to prevent
ice forming in the roadbed and prevent soft spots from developing
under the road surface.

Keep in mind that ditches placed in most soils will erode at
gradients steeper than two percent; therefore, a series of drop
inlets is necessary along the ditchline at intervals not greater than
300 feet. Ditches on steep grades are sometimes paved with asphalt
or concrete to prevent scouring; rock lining is used to reduce velocity
of flow. Woven fabric is sometimes used in swales to encourage
the growth of turf.

One of the chief reasons for erosion of ditches and subsequent
sloughing of the slopes is the practice of shaping the ditch with
the blade of a patrol grader. The blacksmith at Yellowstone fashioned
a shoe made from steel plate which is attached to the blade
end; the form of the shoe produces a rounded ditch which is aesthetically
superior and more stable than the V-ditch.

We will review the Rational Formula which you learned in your
Grading course, and then Manning's Formula for the design of ditches.

Should the ditchline be a swale? a V-shaped channel? a rock
lined gutter? a rounded turf swale? Although the formulas dictate
the volumes of water to be accounted for, it is the immediate
setting that determines the kind of ditch you will build.

Subsurface drainage systems can be quite complex in areas where
the groundwater is close to the surface, or where adjacent soils
are subject to saturation and freezing and thawing in the Springtime.
The following details are taken from the Virginia Highway's book,
Road Design and Standards:

Drainage Structures are critical visual elements deserving
special design consideration where the structures are obvious to
the visitor. Standard details are minimums: we notice the ends
of culverts, the faces of drop inlets, the ends of bridge scuppers.
Design decisions are needed wherever drainage structures are conspicuous:
is design needed? is design not needed?

DIVERSION DITCHES:

Diversion ditches are frequently built above the cut face of
the slope to intercept surface water. The gradient of the diversion
ditch, being steeper than the roadside ditch requires a velocity
check before it joins the roadside ditchline or culvert. See the
case at Michie Tavern where paved ditches are joined at the toe of
slope; the need for a velocity check structure is obvious.

In situations where the centerline of the road is level, it is
necessary to draw a profile of the ditchline to insure flow of
water. Constitution Avenue in the District of Columbia is level;
the gutters are undulating with highpoints placed between drop
inlets. In developed areas water is collected in gutters rather
than ditchlines; therefore, the drop inlets and underground pipe
systems are designed as closed systems. Recall the exercise you
did in Grading for a closed system.

CULVERTS:

Early culverts called "pole drains" were probably nothing more
than poles laid in stream beds; these were simply voids under the
fill. Timber and masonry structures were developed to an art in
the construction of canals before Portland cement became a common
material. One of the culverts on the C and O Canal has a rifled
liner designed to increase velocity thereby reducing the area of
the opening to accommodate the volume of water reaching the culvert.
In some parts of the storm water system in the District of Columbia
we find pipes fabricated from wood staves bound by metal hoops.
Brick was extensively used for storm sewers and drop inlets. It
was not until the development of corrugated metal pipe that culverts
could be made cheaply; from the various shapes and sizes
available we can select the pipe which satisfies the flow requirements.

Concrete pipe has several advantages: the friction
coefficient is lower, the crushing strength is greater, and its
service life is longer than metal pipe. Terra cotta pipe is still
used where acid is prevalent in waste water. Cast iron is used
where heavy loads must be accommodated.

Culvert size is a function of volume, roughness and slope. Our
primary interest is in the soil surfaces at the ends of the pipe
where appearance is especially troublesome. Conspicuous culvert
openings may require masonry or concrete headwalls; outfalls may
require special velocity checks or provisions to spread the water
over a paved apron.

BOX CULVERTS:

Box culverts built of timber or concrete are designed to carry
loads on a slab placed over parallel walls; water is spread over
a broader surface than with round pipe. Box culverts over 20 feet
wide are classified as bridges and are therefore regarded as major
structures. These are often provided to connect cattle pastures
and to provide routes for wildlife to move from one habitat to
another. Bridges usually cause a lesser impact on the land than the
fills associated with box culverts. Multi-box and multi-arch culverts
are used to reduce the depth of fills necessary to accommodate
larger single pipe sizes.

CAUSEWAYS:

Causeways are bridges in effect; the roadway is supported either
on a long earth fill over a series of culverts or on a timber or
concrete slab resting on piling. Earthen causeways are inappropriate
in tidal areas and marshes where the ebb of water is critical
to the quality of aquatic habitat. Causeways make up most of the
Chesapeake Bay Bridge-Tunnel and are used to carry roadways across
Lake Pontchartrain and to connect the Florida Keys. The structures
provide a beneficial reef effect for fish populations; adversely,
they impose a narrowing of navigation channels. I do prefer pilings
to earthen causeways.

SELECTION OF STRUCTURES:

The consequences resulting from construction will determine
the choice of drainage structures:

1.

• The impact of the changes on wildlife habitat on the lower
  watershed.

2.

• The constriction and consequent impounding of runoff above
  the structure.

3.

• The visual encroachment of the structure itself.

4.

• The views outward from the roadway.

Vegetation along elevated structures will be different obviously
from vegetation grown on fill slopes; the former will favor wet
site species; the latter will favor dry site species. Our attitude
is on the side of preservation of natural vegetation provided that
sight distance not be obscured by vegetation.