Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
5.6.5 INFILTRATION RATES
Infiltration is the ability of the soil surface to absorb water from rainfall, snowmelt, irrigation,
or road drainage. A high infiltration rate indicates that the soil surface can transmit high rates
of water; a low rate indicates that the surface has low capability of absorbing water. When
infiltration rates are lower than the rate of water input, runoff or overland flow will occur. Under
these conditions, runoff can detach and transport soil particles, resulting in soil erosion and,
in many cases, off-site water quality problems (Figure 5-49). Overland flow can also remove
sown seeds.
The size, abundance, and stability of soil surface pores determine the infiltration rates of a
soil. Large stable pores created by worms, insects, and channels created by decaying root
systems will absorb water quickly and have high infiltration rates, while surfaces that have
been compacted, have had their topsoil removed, or are low in organic matter will have poor
infiltration rates.
Under undisturbed conditions, infiltration rates are usually high, especially where a litter and
duff cover exists. When soil cover is removed, the impact from raindrops can seal the soil
surface, creating a crust that will significantly reduce infiltration rates. Infiltration rates are
also reduced when soil is compacted by heavy equipment traffic.
Infiltration Rates – How to Assess
The most accurate equipment for measuring infiltration rates is the rainfall simulator. This
equipment simulates rainstorms of different intensities under controlled conditions and
measures how the soil surface responds. Infiltration rates are determined at the point when
runoff occurs. The amount of runoff water is measured at the bottom of the plot to calculate
runoff rates and sediment yields (Figure 5-50). While most rainfall simulators were developed
for agricultural operations, several have been developed specifically for wildland conditions.
These simulators were built for transportability and conservation of water since construction
sites are often in remote locations and far from water sources. The “drop-forming” rainfall
simulator, developed for wildland use, delivers rainfall at the drop size or impact velocity
determined for the climate of the project site (Grismer and Hogan 2004).
Using the rainfall simulator in revegetation planning is expensive, yet it is an important tool
and should not be discounted because of the cost. Specifically, rainfall simulation used to
compare the effects of different mitigating measures, such as mulches or tillage, on runoff
and sediment production takes the guess work out of whether such measures are effective.
This quantitative evaluation of erosion control methods might be essential in areas where
water quality objectives are critical.
In most cases, infiltration rates are estimated under routine field investigation by inference
from site conditions. Infiltration rates can be considered high when the soil surface has not
been disturbed and has a high percentage of cover. With compaction and loss of surface cover,
infiltration rates are proportionally reduced. It is often assumed that construction activities
that remove surface cover or disturb the topsoil will have very low infiltration rates that will
create overland flow under most rainfall intensities.
Mitigating for Low Infiltration Rates
Avoid Compaction
Where possible, avoid operating heavy equipment on undisturbed soils or soils that have been
tilled. Coordinate with project engineers to verify that site preparation does not compromise
the compacted roadbed prism which is engineered as a structural entity.
Figure 5-49 | Infiltration rates
When precipitation exceeds infiltration
rates, water collects on the surface of
the soil and begins to move downslope,
causing erosion. On this site, litter and
duff layers that typically protect the
surface from rainfall impact have been
removed, causing low infiltration rates.
B
A
C
Figure 5-50 | Portable
rainfall simulator
A portable “drop-forming” rainfall simulator
developed by scientists at the
University of California Davis (Grismer
and Hogan 2004) delivers water droplets
through hundreds of needles (A).
Pressure is increased or decreased to
simulate rainstorm events. Droplets hit
the soil surface within a plot frame (B) and
runoff water is collected at the bottom
of the frame (C) to measure runoff rates
and sediment production.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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