Environmental Engineering Reference
In-Depth Information
The storm flow depth was calculated using the SCS storm flow equation (5.5), where Q is
defined as the direct runoff response to a given rainfall event, and consists of both surface
runoff and subsurface flows, but excludes base flow or a delayed subsurface response.
(5.5)
where Q = storm flow depth (mm), which can be converted to volume by introducing
catchment area; P = rainfall event depth (mm); I
a
= 0.1 S = initial losses prior to
commencement of the storm flow, comprising of storage, interception and initial
infiltration (mm); S = potential maximum soil water retention (mm) = index of the
wetness of the catchment's soil prior to the rainfall event.
The total net runoff volume for the study period for each specific point was evaluated
as the sum of the storm flow volume for each rainfall event and this cumulative value,
including runoff from up-stream points was used to estimate the pollution fluxes in the
subsequent section.
It is obvious from the procedures described that the manual estimation of net runoff
volumes is a tedious and time consuming procedure, therefore for a regular monitoring
and control practice, the implementation of GIS systems and their incorporation in a
model for pollution loads and fluxes evaluation would lead to a more precise and user-
friendly procedure.
4.3
Pollution fluxes evaluation
The study area under consideration (Figure 5.4) could be used as an example to illustrate
the procedure for pollution fluxes evaluation along a river or a stream. Such evaluation
could help to differentiate the contribution of different pollutant constituents, generated
by different types of land use practice and to prioritize the corresponding abatement
measures. From the previous section, the concentrations of the selected parameters were
not evaluated as EMCs but as composite samples taken after the rainfall event, due to
financial, transport and manpower restrictions during the study period. This would reflect
on the accuracy of the determined pollution fluxes, especially with respect to parameters
as COD, TP and metals, which were expected to be associated to sediment particles. The
ratio between TP and ortho-P (Table 5.3) shows that 50-65% of the TP was present in
dissolved form and this fraction of the TP should not be affected considerably. For the
purpose of this example, pollution fluxes with respect to ammonia were determined,
because this parameter is usually present in dissolved form and its concentration should
not vary considerably during the storm event.
4.3.1
Methodology
The fluxes were evaluated based on the flow volume during the study period and the
median values of the ammonia concentrations at the different sampling locations. The
background flux was calculated as the product of the base flow, measured during the dry
period at a location close to point F and the ammonia median concentration at the control
point (Table 5.3). The base flow was measured on two occasions during typical dry
season conditions by the velocity-area method (Mays 2001).
