Environmental Engineering Reference
In-Depth Information
available, the determination of the runoff coefficient could be more accurately and
quickly determined.
2.4
Diffuse pollution loads from urban areas
The pollutant loads from urban areas are strongly affected by the drainage volume. The
lowest pollutant loadings are typical of sub-urban areas with a so-called “natural surface
drainage”, as well as from parks, playgrounds, and other developed open spaces. The
highest pollutant loadings are emitted from highly impervious densely populated or
heavily used urban centres with separate or combined sewers. A basic classification
regarding the link between the land use pattern and the expected level of diffuse pollution
is given below:
• Low pollution loads. - This category includes low and medium density residential land
uses anddry-process industrial activities.
• Medium pollution loads - typical examples in this category include high-density
residential andcommercial land use.
• High pollution loads - typical examples include medium and high intensity industrial
uses (wet pro-cesses) and densely populated urban centers with a high level of
impervious surfaces.
Different land use patterns could account for a different magnitude of the “unit load” in
respect to pollution constituents. This term could be defined as follows:
A
unit load,
sometimes referred to as the export coefficient, represents the mass of
pollution with respectto any water quality characteristic, exported from a unit
drainage area per unit time. Usually, it is expressed in kg or ton/ha per annum
(season), and is related to a specific land use practice.
To illustrate this definition, we could consider a given drainage area, with a relatively
constant land use pattern, during a specified time period, e.g. one year, which discharges
its runoff in a storm water channel. Then the annual unit pollution load with respect to
TSS would be equal to the annual volume of runoff during the stipulated period,
multiplied by an averaged EMC of TSS during this period and the product divided by the
area under consideration. The value obtained could be used for planning and simulation
purposes.
Unit loads allow for the calculation and prediction of pollution loads from a catchment
basin, by adding the contribution of all individual sub catchments with characteristic land
use practice and corresponding unit pollution loads. However, it should be emphasized,
that when applied at a larger scale, the pollution loads estimated are determined with
respect to a natural water course, and then they would represent not only the runoff
characteristics from the drainage area, but the conditions in the stream as well. In such
cases, the unit loads might include the combined effect of the surface runoff load, as well
as the load from the base flow and background pollution. In most cases, when the term
“unit load” is applied to perennial water streams, it would refer to the combined effect,
while unit loads determined in ephemeral streams reflect the surface runoff pollution load
only. It should be noted that unit loads are highly site-specific and depend on numerous
demographic, geographic and hydrologic factors.
