As a result of these factors, compaction and drainage are
the two major problems of urban soil which greatly affect hydrology. Highly
compacted soil and poor infiltration means that the soil is not able to
absorb water. This causes an increased amount of surface water runoff on
the site, or puddling in low places on the site. The amount and time of
runoff from the site will vary, depending upon the topography of the site.
This increased runoff overburdens the existing water/sewer system and results
in flooding in low places around the neighborhood, including the basements
of buildings.
Compacted soil and poor drainage has a profound effect
on vegetation. Urban soil is generally nutrient poor, due to a lack of organic
material. The high degree of disturbance from the original "natural"
soil to its current status creates a patchwork of different kinds of soil,
which results in inconsistent growth of vegetation. The highly compacted
characteristic of urban soil leads to limited root growth. Many trees, for
example, do not reach their full maturity in urban environments and have
a maximum growth and life span that is significantly less than in a non-urban
setting. Urban soil requires a certain amount of remediation in order to
promote healthy plant growth.
In addition to its effect on the hydrology and vegetation,
urban soil affects micro climates. The temperature of soilis altered in
an urban environment, and will affect the vegetation. Depressions in compacted
soil will hold water, and create various micro-climates within a given site.
Urban soil therefore, more than the original soil
of the site, could significantly affect the form of your design. In particular,
knowledge of the condition of soil is critical to how well you manage water
on the site, and the long term success of plantings. | 