Science-Based Landscape Planning in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 500

TABLE 15.3
Matrix Showing Complex Interactions at a Site

Landscape Element Soil Type Soil Water Vegetation Slope Precipitation Interflow

Soil type X

Soil water X

Vegetation X

Slope X

Precipitation X

Interflow X

Context: Placement of a structure on a hillslope.

water, vegetation, slope, precipitation, and interflow. In addition, these interactions need
to be considered within the context of the existing amounts of shear strength and shear
stress present on the slope, and the likely changes to these variables incurred by building
the structure. In complex situations like this one, a matrix is a useful way to organize the
interactions to help identify the data required for subsequent field and bench analyses
(Table 15.3).
The landscape elements are arranged in the same sequence down the rows and across
the columns. The “X” in a cell indicates no interaction, since an element does not interact
with itself. We will use two interactions to demonstrate the procedure, which has four
steps: (1) list an interaction, (2) identify the scientific principles governing the interaction,
(3) specify the data required for collection, and (4) repeat until all interactions have been
covered. Some data items listed for collection may be repeated, since different interactions
may warrant a different sampling strategy.

15.3.5.1 Interaction 1: Soil Type with Soil Water

Principles : The amount of organic content influences the amount of moisture a soil is capa-
ble of holding. Soil pore space size varies by soil texture (clay, silt, and sand). Infiltration
rates vary by soil type.
Data required : Soil profile, soil texture, soil organic content, soil infiltration rate (all sam-
pled at different locations on the slope).

15.3.5.2 Interaction 2: Soil Type with Vegetation

Principles : Slope stability depends on the balance between the shear stress and shear
strength forces present. Examples of shear stress forces include gravity, frost heaving, and
water. Water adds weight to a soil by replacing air within the pore spaces, and the added
mass makes soil more prone to downward movement. Vegetation is a cohesive force on
slopes, and the soil type is one factor determining the types of vegetation present. Roots
of vegetation may increase the porosity of near-surface soil and increase the infiltration
capacity. Vegetation contributes organic matter to the soil, the roots help stabilize soil, and
it can intercept precipitation on slopes.
Data required : Soil organic content (sampled by vegetation clusters), root depth mea-
surements, root density measurements taken at several locations on the slope. Soil type
and texture samples should be collected at locations at and near the proposed structure
location. A full list of the species of vegetation present and their distributions at the site.