Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
angle. This is calculated by measuring the number of feet rise over a 100-foot length. Slope
gradient as expressed in degrees is not commonly used.
Slope gradient can be measured in the field using a handheld instrument called a clinometer.
This equipment reads slope angle in percent slope and in degrees. Readings from a clinometer
can be converted to rise over run notation using the chart in Figure 5-52. Road construction
plans display the slope gradients for every cross-section corresponding to road station
numbers (see Figure 3-3, in Section 3.2.3, Cross-Section View). Using these cross-sections,
slope gradients can be identified on the plan map by color coding the run:rise for cut and fill
slopes. For instance, 1:1 cut and fill slopes might be highlighted red for areas of concern, while
those areas with gradients 1:3 or less might be light green, favorable areas for mitigation work.
This exercise can quickly identify areas with the highest risk for soil erosion and difficulty in
establishing vegetation.
Slope angle plays a key role in revegetation planning because of its potential limitation on the
types of mitigation measures that can be implemented. Figure 5-52 shows which practices are
generally limited to gentle slopes and which can be implemented on steep slopes.
200 ft
250 ft
100 ft
50 ft
Slope
V:H
% Slope
Degrees
2:1 200 65
1:1 100 45
1:1.5 66 34
1:2 50 25
1:3 33 18
1:5 20 11
50 ft 100 ft
A
V:H
% Slope
Degrees
2:1 200 65
1:1 100 45
1:1.5 66 34
1:2 50 25
1:3 33 18
1:5 20 11
Revegetation Methods
Hay/Straw Mulch
Wood Strand
Rip/Chisel
Mechanical Planting
Hand Planting
Cat Track
Top Soil
Hand Seedling
Blown Compost
Hydromulch
B
Figure 5-52 | Revegetation methods and slope gradients
For engineering work, slope is generally expressed as the rise
(V) over run (H). For slopes flatter than 1:1, (45° or 100%), slope
gradient is expressed as the ratio of one unit vertical to a number
of units horizontal. For example a 1:2 slope gradient indicates
that there is one unit rise to 2 units horizontal distance. For
slopes steeper than 1:1, it is expressed as the number of units
vertical to one unit horizontal (e.g., 2:1 indicates that there is
a 2 unit rise to 1 unit horizontal distance). In general to avoid
confusion, it is wise to notate the ratio by indicating the vertical
(V) and horizontal (H), when defining gradient (e.g., 2V:1H).
Range and forest sciences use % slope gradient to describe slope
angle. Slope gradient refers to the number of feet elevation
rise over 100 feet. A 66% slope gradient indicates that for every
100 feet, there is a 66 foot vertical distance rise. Slope gradient
controls what type of revegetation treatments can be used (B).
The steeper the slope gradient, the fewer tools are available.
Vertical Distance from Ditch
60 ft
30 ft
A
B
Roadbed
Ditch
1:1
C
30 ft
60 ft
Horizontal Distance from Ditch
D
1V:1.5H
1V:2H
Natural ground 1V:2H
E
Figure 5-53 | Effects of steep and gentle
gradient slopes on revegetation
The trade–off between designing steep cuts that are difficult to
revegetate or creating gentle slopes that disturb more area is
demonstrated in this example. The centerline (A) of a new road
intersects the 1V:2H natural ground (A to D). Fifteen horizontal
feet of material must be excavated from the center of the road
(A) to the ditch (B) to create the road bed. The resulting road
cut will have varying lengths depending on how steep it is
designed. A 1:1 cut slope will expose a 25 ft cut from the ditch
(B) to the top of the cut slope (C). A 1V:1.5H slope (B to D) will
lengthen this exposure threefold to approximately 80 ft. A
1V:2H slope (B to E) is not achievable since it remains parallel
to the natural ground slope.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
109