(Refer to Handbook, Section 6 )
1. During road construction, reasonable
measures should be undertaken to control water runoff and erosion in order to
avoid sedimentation of water bodies.
2. Drainage ditches should be provided
whenever necessary to effectively control water entering and leaving the road
area.
3. Cut and fill sections and
road banks should be effectively stabilized so as to prevent erosion or
slumping.
4. In order to prevent
road surface drainage from directly entering water bodies, roads and their
associated drainage ditches, should be located, constructed and maintained so
as to leave an unscarified filter strip, of at least the width indicated in the
following table, between the exposed mineral soil of the road and the normal
high water mark of the surface waterbody or water course:
|
Average Slope of Land Between Exposed Mineral
Soil and Normal High Water Mark (Percent)
|
Width of Strip Between Exposed Mineral Soil and
Normal High Water Mark (Feet Along Surface of the Ground)
|
|
0
|
25
|
|
10
|
45
|
|
20
|
65
|
|
30
|
85
|
|
40
|
105
|
|
50
|
125
|
|
60
|
145
|
|
70
|
165
|
This guideline shall not apply to road approaches to water
crossings.
5. Drainage
ditches for roads approaching a water crossing should be designed, constructed,
and maintained to empty into an unscarified filter strip, of at least the width
indicated in the table set forth in Subsection 4 above, between the outflow
point of the ditch and the normal high water mark of the waterbody. Where such
filter strip is impracticable, other appropriate techniques should be used to
reasonably avoid sedimentation of the waterbody. Such techniques may include
the installation of sump holes or settling basins, and/or the effective use of
additional ditch relief culverts and ditch water turnouts placed so as to
reasonably avoid sedimentation of the waterbody.
6. Cross drainage culverts or drainage dips
should be installed to get drainage water from the uphill side of the road to
the downhill side before the flow in drainage ditches or roads gains sufficient
volume or head to cause erosion and water sedimentation.
Drainage water on the downhill side of the road should be
diverted from the road ditch into vegetated areas by installation of water
turnouts spaced so as to prevent the flow in the ditch from gaining sufficient
volume or head to cause erosion and water sedimentation.
Such water diversion structures should be located and
constructed as follows:
a. Drainage
dips should be used in place of ditch relief culverts only where the road grade
is 10 percent or less. Such dips should be constructed so that the road slope
is reversed, the base of the dip drains to the outslope, and the surface of the
dip is graveled.
b. On roads having
slopes greater than 10%, ditch relief culverts should be placed across the road
at approximately a 30 degree angle downslope from a line perpendicular to the
center line of the road.
c.
Culverts and drainage dips should also be installed so as to direct drainage
water onto a unscarified filter strip.
d. Inlet end of culverts should extend into
side ditches to intercept ditch flows and should be adequately stabilized by
riprap or other suitable means to reasonably avoid erosion of material around
the culvert.
e. Culverts, drainage
dips, and water turnouts should be spaced along the road at intervals which are
sufficient to prevent the water flow in drainage ditches or roads from gaining
sufficient volume or head to cause erosion and water sedimentation.
As a general guide, culverts, dips and water turnouts should
be space as follows:
|
ROAD GRADE
Percent
|
SPACING
feet
|
|
1-2
|
1,000
|
|
3-5
|
800-500
|
|
6-10
|
400-200
|
|
11-15
|
180-130
|
|
16-20
|
125-120
|
f.
Cross drainage culverts should be sufficiently sized and properly installed in
order to allow for drainage of storm or spring water runoff.
