1. Quantifying Mixing in Low Velocity Vegetated Ponds
Development of Laboratory Evaluation and Field Applications
Diffuse pollution—from agriculture and highways accounts for the majority of riverine contamination. There is an increasing use of wetland and pond environments in the remediation of aquatic pollution.
Wetlands slow down and spread contaminated water, detaining it before entering major channels. Further, vegetation found in these environments promotes biochemical degradation by micro-organisms. The
dispersion of contamination is also altered by the hydrodynamic conditions; which are, in turn, affected by the type and distribution of vegetation. The degree of Dispersion helps to assess a system’s
performance and ability to reduce downstream concentrations.
Patrick West, James Hart, Ian Guymer, University of Warwick
 Initial LIF images appear
promising (Fig. 3).
 LIF system permits high
frequency, high resolution data
acquisition
 Point source instantaneous
injection allows longitudinal and
transverse properties to be
characterised
 High spatial resolution describes
mixing across heterogeneous
vegetated flows
LIF RESULTS
Fig. 5 Profile view of the various aquatic environments under
investigation. A dye injection and potential velocity profile are shown.
 Spatially heterogeneous flow field requires
multiple point measurements.
 Trace “meandering” and poor mixing demand
more precise measurement technique.
 Laser Induced Fluorometry (LIF)
increases spatial resolution and
provides a non-intrusive detection
method (Fig. 1).
 Two-dimensional concentration
distribution recorded with single laser
beam and camera (Fig.2).
LASER INDUCED FLUOROMETRY
Fig. 3 Longitudinal and Lateral LIF distributions
 Artificial Vegetation, controlled flow.
 Three flow regions modelled: free flow,
vegetated flow and mixing layer.
 Fluorescent dye evolution recorded
spatially and temporally to
characterise pollution mixing.
 Variable density and discharge
PHYSICAL MODEL
Fig. 1 Physical modelling using dye trace injections within a partially vegetated
system.
MOVING FORWARD
 Real vegetation (Preferential species, optimum configuration,
Fig. 5).
 Range of flow conditions
 Detailed Velocity Profiles (Ultrasound & Laser Induced
Fluorometry (LIF) )
 Variations with the growing seasons
 Concentration Modelling at interface
APPLICATIONS
 Motorway and urban drainage
 Diffuse agricultural pollution (Pesticides, Nitrates,
Phosphates)
 Sustainable drainage systems (SuDS)
 Wetland and Environmental regeneration
 Industrial point source discharges
 Vegetation alters mean residence
time.
 Short circuiting an apparent problem
(Fig. 4)
FIELD RESULTSRESEARCH AIMS
To develop a method for quantifying the dispersion of contamination in
complex vegetated aquatic environments. Laboratory tests are
combined with field studies to provide extensive results. The effects of
local mixing on residence time in regions of vegetated patches, borders
and open flow need to be quantified. The influence of growth, density
and discharge also need to be investigated.
FIELD STUDY
 Fully vegetated constructed wetlands
 Tracer studies are conducted to measured
residence time distributions
 Auto-sampling records water
quality i.e. turbidity.
 Full field and vegetation
survey conducted
 Travel times compared.
Fig. 2 LIF results
LIF Recorded Concentration x = 100cm
Time (s)
Lateral Distance
Pixels
(1m
m
=
2.5pixels)
QUANTIFYING MIXING
Dxx = 3cm2
s-1
u = 0.85cms-1
Fig. 4 Nominal and mean residence time distributions for
a) minimal vegetation and b) full pond vegetation.
a
b
Upstream
Downstream