Landcom Book 4 Maintenance - WSUD

Water Sensitive Urban Design
5.1 Performance Monitoring
Requirements
Surrogate methods
can often be
equally effective
in evaluating the
adequacies of the
operation, and thus
performance, of
WSUD elements.
Performance monitoring of WSUD
elements can be undertaken through
detailed water sampling and laboratory
analyses for contaminant concentrations or
through the use of surrogate performance
indicators. With the former, it will be
necessary to set up field monitoring sites
to undertake the water quality sampling at
inlet and outlet of systems. The following
points should be considered when setting
up a sampling program:
••
Auto-sampling with partial or full
composite samples is most cost
effective
••
Monitoring should be accompanied by
continuous flow and depth observations
••
Design of monitoring set up and
sampling intervals is site dependent
••
20 events should be monitored as a
minimum to obtain typical performance
••
The following key water quality
parameters should be analysed in
registered laboratories:-
––
TSS analysis should be
undertaken
––
TP (with occasional filter of
sample on-site with 0.45um filter
to test for Orthophosphate)
––
TN (with occasional speciation to
organic and inorganic nitrogen
An implicit assumption made when water
quality improvements are measured by
comparing observed water quality at the
inlet and outlet of the system, is that these
water qualities represent the same ‘parcel’
of water. Occasionally, especially with
wetlands and ponds, negative or very low
pollutant removal results for a system are
observed. These results are common when
the volume of an inflow event is less than
the permanent pool volume of the wetland
or pond. In the case of bioretention
systems, accurately accounting for the
volumetric balance of inflow, outflow,
and soil moisture replenishment, is the
main analytical problem that may lead to
erroneous performance assessment.
Surrogate methods can often be equally
effective in evaluating the adequacies of
the operation, and thus performance, of
WSUD elements. They are often more cost
effective and involve the monitoring of the
hydrologic and hydraulic performance of
these systems. The implicit assumption
with surrogate methods is the premise
that if the WSUD elements operate in
accordance to the design hydrologic and
hydraulic characteristics, it follows that
these systems can be reasonably expected
to deliver the pollutant reduction as
determined from laboratory and field
experiments.
Key hydrologic and hydraulic operation
characteristics define the detention
time of WSUD elements. Monitoring of
the following operation of bioretention
systems, wetlands and ponds, can
provide important insights on the likely
performance of these WSUD elements in
pollution reduction:
••
Flow pattern (most relevant to wetlands
and ponds), to identify the presences
of short-circuiting that may inhibit the
uniform distribution of inflow to this
system
••
Duration of inundation (most relevant
to wetlands and bioretention systems)
to assess the operating detention
time of these systems and to highlight
potential clogging of soil media
(bioretention systems) or the outlet
structure (wetlands) that would have
a direct impact on its performance in
water treatment
••
Turbidity of inflow and outflow which
are good surrogates for suspended
solids, total phosphorus, and metal in
urban stormwater
20 Book 4 | MAINTENANCE