When compared with
the costs of traditional lawn preparation practices the enhancement of
native soil with soil amendments may have increased upfront costs.
However, the cost of using amended soils can be at least partially
offset by reductions in the volume of stormwater ponds or other
detention or retention practices. Tilled Compost-Amended Turf (TCT)
practices, besides requiring greater site preparation, require larger
volumes of material to be delivered to the site as well as methods to
ensure that the amendments are well mixed with the existing soil. In
some cases, additional soil analyses may be required to determine the
optimum amounts of compost and other nutrients required. The following
is a case study, from the Seattle, Washington area, that demonstrates
the costs of a typical residential TCT project.1
This study
found that the greatest costs of TCT practices are those associated
with:
Soil and site preparation
Costs of curb side delivery of soil
amendments
Hydroseeding application
The following
cost estimates are based upon 1996 prices in the Seattle,
Washington metropolitan area. Potential soils analysis costs are
not included, but can cost as much as $125 per sample. The costs
for TCT practices and ingredients can vary considerably from region
to region, time of year and for various on-site soil conditions.
This is
applicable for a shallow, up to 8” depth, soil amended project
incorporating in-situ soils along with the soil amendments and
no underdrain system present. Labor provided by hand or
machinery and the disturbed area is small enough to avoid
additional permits and fees.
Soil and
site preparation:
$0.60 - $0.75 per square foot, depending upon subsoil
density, whether the site can be amended by hand or machine,
and if a ripper must be installed.
Delivered
curb costs and application of soil amendments:
$0.50 - $2.00 per cubic yard, depending upon mulch type and
application method, i.e., blower application versus standard
spreading techniques. Cost of chemical nutrients or pH
stabilizers is not included.
Turfgrass
and hydroseeding application:
the same costs for amended and nonamended soil. Hydroseeding
application preferred over turfgrass application on amended
soils.
TOTAL ESTIMATED
COST:$1 to $3 per square foot
In the above example, the payback period
for TCT practices were estimated as follows for TCT-seed and
seeded-turf:
5 to 6 years
when compared to topsoil-seed.
Within the
first year when compared to topsoil-sod.
Within 6 to 7
years when compared to minimum-seed.
2 to 3
years when compared to minimum-sod.
Detention Facility
Costs
Due to the
increased moisture holding capacity of compost amended soils they
have the potential to attenuate peak flows and decrease overall site
runoff, thus providing stormwater management savings over those of
conventional methods. In a 1996 study performed in Redmond,
Washington by the Stormwater Utility staff, computations were
performed to determine the estimated storage volume reductions of
stormwater management detention facilities, as related to the
storage capacity available in soil amended residential lawns for the
6-month 24-hour storm. Hydrology calculations were based upon
existing pre-amended soils and then upon amended soils with their
newly increased water holding capacity. Based upon these
calculations the detention volume required for an on-site stormwater
management facility dropped by an estimated 7 percent of the
original detention volume. This potential reduction in volume for
the stormwater facility resulted in a potential cost reduction of
$8,640 for this particular example.1