Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
Mitigating for High Rainfall Interception
It is important to consider the water-holding capacities of the mulches or soil covers to be used,
especially on arid sites. Highly decomposed, fine-textured composts have high water-holding
capacities compared to coarser-textured composts and hold more moisture after a rainstorm.
Coarser materials, such as shredded wood, bark, wood chips, and wood strands, hold less water,
allowing more rainwater to enter the soil. In addition, because fine-textured composts hold
more water than course mulches or soil, they are good growing media for desirable native
plants as well as undesirable weed species. The question that should be asked when selecting
a soil cover is whether it is to be used as a mulch or a growing media. If a mulch, then a coarse
material should be used; if growing media, a fine compost should be used.
5.2.3 INFILTRATION
Infiltration is the ability of the soil surface to absorb water from rainfall, snowmelt, irrigation,
or road drainage. When infiltration rates are slower than the amount of water applied to the
surface of the soil, runoff will occur and this water will not be available for plant uptake. In
addition, runoff can detach and transport soil, causing soil erosion, decreased water quality,
and increased peak flows. See Section 5.6.5, Infiltration Rates, for a discussion of infiltration
rates on surface stability.
The size, abundance, and stability of soil aggregates in the surface soil determine the infiltration
rates. Large stable pores created by worms, insects, and channels left behind from decayed
roots will absorb water quickly and have high infiltration rates; soils that have been compacted,
had their topsoil removed, or are low in organic matter will have poor infiltration rates.
Under undisturbed conditions, infiltration rates are typically high, especially where a litter
and duff cover exists. When soil cover is removed, the impact from rain splash can seal the
soil surface, creating a crust that will significantly reduce infiltration rates. Infiltration rates are
also reduced when the soil is compacted by heavy equipment or traffic.
Rainfall Absorbed (inches)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
3" thick
2" thick
1" thick
0
0 25 50 75
Moisture Holding Capacity (% of dry weight)
Figure 5-7 | Moisture holding
capacity of mulch or litter
The amount of rainfall intercepted by
soil cover (e.g., mulch or litter) is dependent
on its water- holding capacity
and thickness.
Infiltration – How to Assess
The most accurate method to measure field infiltration rates the rainfall simulator (Figure
5-50). This equipment is calibrated to simulate the appropriate drop size and impact velocity
of many rainfall events (Grismer and Hogan 2004). The rainfall simulator is expensive to operate
and is not routinely used by the designer. The most common application for this technology
is in comparing different mitigating measures, such as mulches and tillage methods, on
infiltration capacity.
Without conducting rainfall simulation tests, infiltration rates must be inferred by measuring
soil strength using a soil penetrometer, bulk density measurements (see Section 5.3.3, Soil
Structure), and from site characteristics such as visual observation of compaction and the
percentage of soil cover. For most construction activities that remove surface cover or disturb
the topsoil, it can be assumed that infiltration rates will be reduced to levels that will create
overland flow under most rainfall intensities.
Mitigating for Low Infiltration Rates
Minimize Compaction
Driving heavy equipment over soils causes compaction and reduces infiltration rates. After
sites have been prepared for seeding or planting, heavy equipment must not be driven over
soils. Such practices that are often recommended for erosion control, such as trackwalking,
can actually decrease infiltration rates and adversely affect the establishment and cover of
native plants. These practices may not be appropriate on all soil types and should be assessed
on a site-specific basis (Grismer and Hogan 2007).
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
63