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Surveillance networks are best designed using procedures that give a random design in order to
avoid bias. The Environment Agency’s River Surveillance Network is based on a truly random GRTS
(Generalized Random Tessellation Stratified) design.

GRTS monitoring network design Population of rivers
and streams
What to monitor/measure • Digitised river map
• WFD compliant assessment methods
• Water quality and biology (except fish)
• Morphology • Co-located Network design
• Equal probability
Sub populations of each site being
• Scales of data use Surveillance selected
assessment - are • More sites in more
our rivers getting widespread types
better or worse?

YEAR
1 2 3 4 5 6 7 8 9 10
Fixed panel - all years 100 100 100 100 100 100 100 100 100 100
Rotating panel 1 100 100 100 100
Rotating panel 2 100 100 100 100
Panel design Rotating panel 3 100 100 100 100 Network size
Rotating panel 4 100 100 100 100
• Pattern of site visits Rotating panel 5 100 100 100 100 • 500 sites per year
Rotating panel 6 200 200
Rotating panel 7 200 200
Rotating panel 8 200 200
Rotating panel 9 200 200
Rotating panel 10 200 200
No. of sites this year 500 500 500 500 500 500 500 500 500 500
No. unique sites 500 800 1100 1400 1600 1600 1600 1600 1600 1600
Figure 2.2
An overview of the GRTS monitoring network design of the Environment Agency’s River Surveillance Network
Source: Environment Agency Training Slide (unpublished)
Table 2.1
The Environment Agency’s River Surveillance Programme design: a fixed panel of sites are monitored every year; a rotating
panel of sites are sampled for two consecutive years every 5 years and another rotating panel of sites is sampled once in
every 5 years.
YEAR
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Fixed panel - all years 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Rotating panel 1 100 100 100 100 100 100 100 100
Rotating panel 2 100 100 100 100 100 100 100 100
Rotating panel 3 100 100 100 100 100 100 100 100
Rotating panel 4 100 100 100 100 100 100 100 100
Rotating panel 5 100 100 100 100 100 100 100 100
Rotating panel 6 200 200 200 200
Rotating panel 7 200 200 200 200
Rotating panel 8 200 200 200 200
Rotating panel 9 200 200 200 200
Rotating panel 10 200 200 200 200
No. of sites per year 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500
No. of unique sites 500 800 1100 1400 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600

Investigative surveys that do not rely on status classification The Environment Agency’s River Surveillance Network also
can happen at any time of the year and do not necessarily changes from year to year, but in a pre-planned way that
rely on samples collected in spring and autumn. Pollution, includes a combination of new and previously surveyed
drought, floods, and other perturbations can happen at any sites to balance the need to measure change and have a
time of the year so investigations to assess their impact widespread coverage (see Table 2.1).
cannot be planned. The operational and investigative
monitoring programmes vary each year and sites are The Environment Agency’s operational monitoring is
sampled depending on the requirements of investigations, managed by local Area teams responsible for managing
or risk of water bodies failing their environmental quality individual catchments and water bodies, but its River
objectives. Surveillance Network is managed nationally.

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