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Processes driving the episodic flux of faecal indicatororganisms in streams impacting on recreational andshellfish harvesting waters
Jeremy Wilkinsona, David Kayb,�, Mark Wyerb, Alan Jenkinsc
aFlinders Research Centre for Coast & Catchment Environments, Faculty of Science & Engineering, Flinders University of South Australia, GPO
Box 2100M, Adelaide SA 5001, AustraliabRiver Basin Dynamics and Hydrology Research Group, IGES, University of Wales, Aberystwyth, Ceredigion, SY23 3DB, UKcCentre for Ecology and Hydrology, Wallingford, UK
a r t i c l e i n f o
Article history:
Received 7 August 2004
Received in revised form
4 September 2005
Accepted 1 November 2005
Keywords:
Water quality modelling
Entrainment episodes
Wave propagation
Faecal coliform
Particulate transport
nt matter & 2005 Elsevie.2005.11.001
uthor. Tel./fax: +44 (0) [email protected], dave@cr
A B S T R A C T
Understanding the process controls on episodic fluxes of faecal indicator organisms (FIOs)
is becoming increasingly important for the sustainable management and accurate
modelling of water quality in both recreational and shellfish harvesting waters. Both
environments exhibit transitory non-compliance with microbiological standards after
rainfall episodes despite significant expenditures on control of sewage derived pollutant
loadings in recent years.
This paper demonstrates the role of wave propagation in the entrainment of FIOs from
river channel beds as a contributor to episodes of poor microbial water quality. Previously
reported data is reviewed in the light of relationships between wave and mean water travel
velocities. High flows and rapid changes in river flow, driven by releases of bacterially pure
reservoir water, resulted in elevated FIO concentrations and transient peaks in concentra-
tion. The new interpretation of these data suggest three modes of entrainment: (i)
immediate wave-front disturbance, (ii) wave propagation lift and post-wave transport at
mean flow velocity, and (iii) stochastic erosional mechanisms that maintain elevated
bacterial concentrations under steady high flow conditions. This is a significant advance on
the previously proposed mechanisms. Understanding these mechanisms provides an aid to
managing streams intended for recreational use and emphasises the need to control the
timing of high flow generation prior to use of the water body for e.g. canoeing events. In
addition the processes highlighted have relevance for the protection of shellfish nurseries,
drinking water supply intakes and episodes of poor bathing water quality, and associated
health risks.
& 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Rainfall driven episodic fluxes of faecal indicator organisms
(FIOs) in streams are a growing concern because of their
effects on the compliance of recreational waters and shellfish
r Ltd. All rights reserved.
0 423565.ehkay.demon.co.uk (D. K
harvesting areas (CEC, 2002; DEFRA, 2002, 2003; WHO, 1999,
2003). The relative importance of this pollution loading has
increased as programmes for the control of point source
anthropogenic pollution from sewage systems have been
completed in many developed nations (DEFRA, 2002, 2003).
ay).
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WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 5 3 – 1 6 1154
Previous work has demonstrated the highly episodic nature of
FIO transport in drainage basins which is dominated by
movement during high flow events. Thus, models focused on
point-source inputs to rivers during baseflow conditions are
of little operational utility for those concerned with, for
example, the recreational use of riverine environments and or
riverine inputs to shellfish harvesting and recreational
waters. Integrated management of such episodic pollution,
which, in rural catchments at least, is derived principally
from diffuse sources, is central to new Directives such as the
Draft European Bathing Water Directive, which suggests that
the Water Framework Directive is the appropriate tool for
achievement of ‘good’ recreational water quality (CEC, 2000,
2002, 2004).
That FIOs are stored in river channels and may be entrained
into the flowing water when disturbed has long been known
(e.g. Jenkins, 1984; Jenkins et al., 1984; Kay et al., 1999;
McDonald and Kay, 1981; McDonald et al., 1982). The
mechanisms for this entrainment, however, are poorly
represented in the literature. This paper seeks to highlight
processes contributing to this entrainment. McDonald and
Kay (1981) and McDonald et al. (1982) and Jenkins et al. (1984)
were among the first to investigate the fluvial dynamics of
faecal indicator bacteria, demonstrating that in-channel
sources could produce bacterial peaks of similar magnitude
as those occurring in response to rainfall-runoff events.
Wilkinson et al. (1995a, b) provide an early interpretation of
part of the data presented below. This paper addresses the
significance of wave propagation processes as a key to
entrainment of FIO from within-channel sources. A model
based on these findings incorporating model components for
diffuse contamination and long term variation in faecal
indicator concentrations driven by riverine and meteorologi-
cal variables is presented in Wilkinson (2001).
2. Methods
2.1. Study sites
Three study sites were used and four experiments conducted.
The two key study sites were; the Afon Clywedog in the
headwaters of the River Severn, Wales, UK and the River
Washburn, a tributary of the River Wharfe north of Leeds,
Yorkshire, England, UK. At the Yorkshire study site, Thru-
scross Reservoir provides regular white-water releases for
canoeing events in the River Washburn. Opening of the
control valves at the reservoir outlet creates a steep fronted
wave that propagates down the channel into a 1.9 km reach of
pool and riffle sequences with a bed-slope of 0.011 which was
sampled between NGR SE15705695 and SE16605540. In Wales,
the effect of step changes in flow in Afon Clywedog in a
3.6 km reach (NGR SN91408675 to SN94398553) downstream of
Llyn Clywedog reservoir was monitored. The study reach is
topographically confined and comprises a series of step-pools
rapidly changing to a pool and riffle sequence with an
overall bed-slope of 0.0079. The third study site was in the
Rheidol catchment in Central Wales, UK. Controlled releases
were provided from the Rheidol Hydroelectric Scheme,
operated by Powergen plc. The river was sampled on a
straight reach at Blaengeuffordd (NGR, SN64008053) 8.9 km
downstream of the Cwm Rheidol reservoir in the catchment
flood plain. The reach is characterised by partially confined
irregularly meandering pool riffle sequence with a bed slope
of approximately 0.0029. A daily programme of releases is
made in the Rheidol to provide peak and off-peak power for
export to the national grid. A consequence of this regime is
that the system is well flushed and sediment movement
through the system is limited by the various impoundments
that comprise the system. Any organisms flushed by the
experimental hydrograph might, therefore, be expected to
represent inputs to the study reach in the period between
releases.
2.2. Sampling and microbial enumeration
Manual aseptic grab sampling of the experimental river
reaches was carried-out. Samples were collected prior to the
experimental releases in order to establish pre-release con-
centrations, and additional samples of the release water were
taken at, or just downstream of, the reservoir outlets in order
to characterise water entering the study reaches. A fixed
sampling interval was adopted to facilitate finite difference
approximation modelling. Stage, temperature and conductiv-
ity were recorded at each sampling interval. A series of 400 ml
grab samples were collected at, or as near as possible, to the
centre of the channel at approximately 0.6 depth, using sterile
plastic containers. Containers were held at the base and
plunged to into the flow with the neck pointing upstream to
avoid contamination by the sampler or from bed disturbance
caused by the sampler wading into the channel. Samples
were stored in the dark prior to transportation to the
laboratory for analysis. Standard UK methods were used
(HMSO, 1983, p. 46). Thermotolerant coliform enumeration
followed HMSO (HMSO, 1983, p. 46). The count at 18 h is
technically a faecal coliform organism or thermotolerant
coliform count (HMSO, 1983, p. 45). Triplicate enumerations at
multiple dilutions were made in order to capture high and low
concentrations and narrow the confidence interval about
each estimate. Triplicate enumeration produces a 1.73 fold
improvement in accuracy (Fleisher et al., 1993; Fleisher and
McFadden, 1980). The results are expressed as colony forming
units (cfu) per 100 ml.
2.3. Reservoir releases
These experiments were designed to complement the work of
McDonald et al. (1982) and Kay and McDonald (1982) in
providing detailed event data sets intended to capture
fluvially driven dynamic variations in faecal coliform con-
centration:
(i)
Afon Rheidol, 17 February 1993.The release was designed to simulate a natural hydro-
graph. The hydrograph had a maximum flow of
14.1 m3 s�1, and rose from 1.72 m3 s�1. For ease of
implementation the hydrograph was produced with half
hourly steps. The response to these steps prompted the
more exaggerated steps of subsequent releases, high-
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WAT ER R ES E A R C H 40 (2006) 153– 161 155
lighting the entrainment mechanisms discussed in this
paper.
(ii)
Afon Rheidol, 7 April 1993.Following the initial experiment a more complex dis-
charge profile was designed, in order to examine specific
effects. Similar characteristics to those produced by
experiment iii (below) were observed, however, these
were obscured by rainfall runoff influences. The results of
the Washburn (iii, below) and Clywedog (iv, below) best
exemplify the processes discussed in this paper, there-
fore no further discussion of these initial experiments is
presented here.
(iii)
River Washburn, 26 May 1993.Opening of the reservoir valves increased the discharge
from 0.058 to 1.57 m3 s�1. The release generated a steep-
fronted wave with a depth increase of 0.3 m just down-
stream of the dam, increasing to 0.8 m by the time it had
propagated the 1.9 km to the downstream site. The time
taken for the full 0.8 m depth transition was less than
30 s. Samples were taken every 5 min prior to the arrival
of the release-wave and every 2.5 min thereafter.
(iv)
Afon Clywedog, 28 May 1993.A stepped hydrograph was generated. The initial dis-
charge was 1.47 m3 s�1. Four discharge increments, re-
sulting in approximately 0.1 m stage rise, were made and
each was held for 30 min. The peak discharge was
12.1 m3 s�1. The hydrograph recession comprised 9 steps
down to the initial discharge over a period of 5 h. Samples
were taken every 5 min on the rising limb of the
hydrograph and then every 10 min on the recession.
2.4. Wave and bacterial peak travel times
Wave travel velocity was estimated by dividing the reach
length by the time taken for corresponding wave features to
travel from upstream to downstream sites. Wilkinson (1945)
found that the mid-points of rise or fall stages were best for
determining the velocity of an observed wave, these timings
are used here. For the Afon Clywedog, the centroids of
bacterial pulses were assumed to represent the arrival of
water travelling at mean water velocity at each quasi-steady
flow. The timings of these arrivals, relative to the correspond-
ing wave rise at the upstream site, were used to estimate
mean velocity.
3. Results
Previously reported statistical summaries and descriptions of
the faecal coliform concentration data for the four experi-
ments can be found in (Wilkinson, 2001; Wilkinson et al.,
1995a,b). General comments about the nature of the re-
sponses observed in the four experiments are as follows:
(i)
The reservoir release waters sampled at the point of entryinto the stream or river channel were found to have low
FIO concentrations. This indicates that these waters were
not a major source of FIO in these experiments.
(ii)
Channel reaches without point source FIO contributionswere chosen, and sampling was carried-out during dry
weather periods (excepting experiment ii, above). Conse-
quently, the elevated FIO concentrations were derived
from within channel sources.
(iii)
FIO pulses of enhanced concentration were found toincrease with propagation downstream indicating accu-
mulation of entrained organisms from within the chan-
nel (Figs. 1 and 2).
(iv)
Pulses of elevated FIO concentrations were found tocoincide with wavefront propagation and during the
quasi-stready flow following the passage of the wave
(Fig. 2).
(v)
The FIO concentration remained elevated relative topre-release concentrations following the passage of
wave-front and post wave FIO pulses. This suggests a
mechanism that maintains an input of entrained
organisms during periods of elevated but steady flow
(Figs. 1 and 2).
Specific descriptions of the FIO behaviour in response to
changing flow, that are key to explaining the proposed
entrainment mechanisms, are as follows:
3.1. River Washburn, 26 May 1993
The transit of the propagating wave in the River Washburn
was audible some minutes before it arrived at the down-
stream sampling site. The water of the wave-front was visibly
very turbid. The high turbidity declined rapidly with passage
of the front. The peak in faecal coliform concentration
coincided with the transition from low to high flow (Fig. 1).
3.2. Afon Clywedog, 28 May 1993
Various peaks in faecal coliform concentration were indicated
by the samples collected during the artificial hydrograph in
Afon Clywedog (Fig. 2). These peaks coincided with increasing
and decreasing increments in stage and occurred simulta-
neously with the wave-fronts, as well as, during the periods of
quasi-steady flow following each change in stage/flow. The
first increase in discharge (up to 5.8 m3 s�1) observed at Site 2
(the downstream end of the 3.6 km reach) comprised the two
initial flow increments; the faster travelling second wave is
assumed to have caught-up with the first. Two minor faecal
coliform peaks at 11:15 a.m. and 11:20 a.m. coincide with
these flow increments (Fig. 2). The significance of these initial
bacterial peaks is uncertain since they are defined by one
sample; however, the use of triplicate filtration increases
confidence in the enumeration by a factor of 1.73 (Fleisher et
al., 1993; Fleisher and McFadden, 1980). The FIO concentration
peaks during the periods of quasi-steady flow occurring
between flow increments were of greater magnitude than
those coinciding with wave fronts (Fig. 2). The first of these
had its sampled peak at 11:37:30 a.m. during the flow of
5.8 m3 s�1 and subsequent peaks during periods of quasi-
steady flow occurred at 12:10 p.m. and 12:50 p.m. A final peak
in concentration was detected following the onset of the
artificial hydrograph recession limb. This peak was sampled
at 13:05 p.m., and followed the first step reduction in
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0
500
1000
1500
2000
FC
(CF
U /
100
ml)
0
2
4
6
8
10
12
14
16
18
Dis
char
ge (
cum
ecs)
FC2Qo
Downstream12:5012:10
11:5511:37.5
11:20
11:1513:05
0
500
10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:000
15FC1 QiUpstream
Fig. 2 – Faecal coliform responses observed at upstream (Bryntail) and downstream (Cribynau) ends of a 3.6 km reach of the
Afon Clywedog downstream of the Clywedog Dam.
0
200
400
600
800
1000
1200
FC
(cf
u / 1
00m
l)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Sta
ge, m
FC2
h2
Downstream response
0
200
400
16:00 16:30 17:00 17:300.3
0.6
0.9
1.2
FC1 h1Upstream response
Fig. 1 – Time series of faecal coliform concentration associated with the passage of a rapid flow transition in the River
Washburn.
WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 5 3 – 1 6 1156
discharge. This suggests that rapid reductions in flow may
also result in pulses of entrained FIO and associated patho-
gens.
The interpretation and discussion below examines
these observations in the context of the arrival timings
of waves and FIO pulses at downstream locations,
wave hydraulic theory, and related observations from the
literature.
4. Interpretation and discussion
4.1. Channel sinks/sources and entrainment
FIO accumulate in river channels under suitable conditions,
their distribution within the channel is heterogenous. That
FIOs are heterogeneously distributed within a stream channel
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WAT ER R ES E A R C H 40 (2006) 153– 161 157
is not of key importance here, because fluvial mixing
processes will tend to integrate-out the impacts of entrain-
ment from many spatially distributed sources.
Free and particle associated FIO settle and attach to surfaces.
Gannon et al. (1983) and Auer and Niehaus (1993) found that
90% of settled organisms were associated with particles of clay
or silt size (0.45–10mm diameter) with estimated settlement
velocity 1.2 m d�1. Graham (1990), studying the siltation of clay-
size particles, suggested that particles ‘must deposit everywhere
in a stream under all conditions of water velocity and turbulence’ and
Reynolds (1979) proposed a model for deposition from
turbulent flow based on water depth and stokian (still-water)
settling velocity. Particles and associated FIO, if we infer an
analogous process, will only accumulate where the flow
conditions, or a ‘sticky’ substrate (McCave, 1984; Hoagland et
al., 1982), precludes their immediate resuspension. In addition,
Reynolds (1979) suggested that, rivers maintain a spatially and
temporally diverse array of microhabitats, which collectively
offer an almost infinitely ‘patchy’ environment. Indeed,
Jenkins (1984) found this to be the case for FIO.
Epilithic algal growths on hard surfaces are difficult to
remove even when flow velocities are approaching 2 m s�1
(Reynolds, 1979). Abrasion by moving objects such as pebbles
and stones is more likely to break-up epilithon and release
material gradually. Reynolds (1992) refers to episammic and
epipelic algal groups. These attach to sand grains and fine
sediments respectively where flow conditions permit and
represent a further source of FIO for entrainment.
It is also clear that large-scale flow structures, such as dead-
zones, act as preferential storage areas for fine particles and
associated contaminants (e.g. Carling et al., 1994; Tipping et
al., 1993; Barillier et al., 1993). The potential for replacement
of low flow dead-zone features by volume equivalent new
dead-zone features at higher flows, may offer the potential for
entrained material to settle-out during elevated flow and
Table 1 – Times of travel and calculated velocities for variousWashburn 26 May 1993.
Reach length ¼ 2000 m
Wave-front rise mid-point 16:37:24.5 h (ck)
Bacterial peak 16:40:00 h
Mid-point of minor bacterial increase 16:52:30 h
Mean velocity from kinematic wave speed, v ¼ 3/5ck
Table 2 – Summary of wave speeds and bacterial peak travel-vthe Afon Clywedog
Dischargeincrement to Q0
(m3 s�1)
Time ofCorresponding
FC peak
Wave speed c3,from time of
mid rise
3.67 11:15 1.20
5.82 11:37:30 1.50
8.67 12:10 1.78
11.47 12:50 2.12
hence potentially maintain within channel sources that did
not exist at lower flows (e.g. Barillier et al., 1993).
Bank material has been investigated as a potential source of
FIOs and soils may contain faecal indicator bacterial in excess
of a few hundred per gram (Hunter and McDonald, 1991).
McDonald et al. (1982) found that bank erosion was not a
major contributor to observed reservoir release Escherichia coli
concentrations in the Washburn, although this cannot be
ruled-out in other locations.
4.2. Timing of bacterial peaks
The timing of the FIO concentration peaks fits well with open
channel flow theory. The experiments show the occurrence of
peaks in FIO concentration simultaneously with the passage
of wave-fronts and lagging behind the wave-fronts (during
the period of quasi-steady flow between changes in flow).
These later bacterial peaks either precede or coincide with the
arrival of the body of water travelling with mean flow velocity.
The coincidence of the initial faecal coliform peaks and wave-
fronts can be related to ‘kinematic wave’ theory (see Chow,
1959; Martin and McCutcheon, 1998; Dingman, 1984). A
kinematic wave will tend to steepen initially, as faster
travelling deeper components of the wave catch-up with
shallower ones, further steepening tends to be arrested by
dispersion and attenuation effects and the wave ultimately
takes on a stable form (Henderson, 1966). In natural channels
the relationship between mean flow velocity, v, and kinematic
wave speed, ck, varies according to channel geometry
(Henderson, 1966). Chow (1959) reported wave speed relation-
ships for triangular channels and wide parabolic channels to
be v=ck ¼ 0:752 and v=ck ¼ 0:694, respectively.
The relationship between the velocities of waves and FIO
peaks in the Washburn and Clywedog experiments were
consistent with those in the literature (Tables 1 and 2). In
features of the reservoir release response of the River
Travel time (min) Velocity (m s�1)
22.4 1.488
23.75 1.404
37.5 0.889
37.3 0.893
elocities in m.sec-1 for the reservoir release experiment on
Wave speedc ¼ fit mid-rise
Velocity, v, frombacterial peak
Ratio, v/c
1.22 0.84 0.70
1.47 1.00 0.67
1.80 1.17 0.66
2.12 1.39 0.65
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0.5
1.0
1.5
2.0
2.5
3 7 11
Discharge, Qo cumecs
Vel
ocity
(v)
, wav
e sp
eed
(c)
m/s
c1 (pre-rise)
c2 (first rise)
c3 (mid-rise)
c = fit [mid-rise]
c = fit [c1,2,3]
v (FC peaks)
v' = 3/5c(fit[mid-rise])
v =fit [FC peaks]
Wave speeds, c
Flow velocities, v
5 9
Fig. 3 – Summary of wave speed and velocity of bacterial peaks in the Afon Clywedog (see Table 2).
WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 5 3 – 1 6 1158
the Washburn channel, the peak bacterial concentration
occurred in an impulse associated with the wave-front.
Similar responses have been observed in the Afon Tryweryn,
Wales (to stage increases of around 0.3 m) and also during an
earlier release in the Washburn (McDonald et al., 1982). If a
kinematic wave velocity ckE5/3 v were assumed, water of
mean flow velocity could be expected to arrive at the
sampling point 15 min after the wave-front (Table 1), this
coincides with the minor rise in response at around 16:52
(Fig. 1). The timing of the wave-fronts and major (quasi-
steady flow) bacterial peaks observed in the Clywedog
support the argument that these peaks are indeed arriving
with water travelling at around the mean flow velocity.
Table 2 and Fig. 3 present the wave-speed and velocity data
for the features shown in Fig. 2. A clear relationship between
the wave speeds and assumed flow velocity is apparent
(Fig. 3). The mean of the ratios of wave speeds to bacterial
peak velocities is 0.66, this falls within the range of values
(0.752–0.6) for artificial channels given in the literature
(e.g. Chow, 1959).
4.3. Entrainment mechanisms
On the basis of the observations presented here, and those in
the literature, three (there maybe more) entrainment me-
chanisms appear occurring in some combination and result
in:
(i)
FIO peaks coinciding with flow wave-fronts;(ii)
FIO peaks following (i.e. lagging behind) wave fronts at (ornear) mean flow velocity;
(iii)
elevated post release concentrations (indicating acontinued supply of organisms into the release water
(Figs. 1 and 2)).
The first two responses are transient and give the impres-
sion of a finite supply of organisms for a given flow increment
(as suggested by Wilkinson et al., 1995a, b). Jeje et al. (1991)
observed similar responses to multiple storm-flow peaks and
the entrainment of sediment where each flow peak resulted
in a peak in sediment concentration. Studies of reservoir
release impacts (Beschta et al., 1981; McDonald et al., 1982;
Milhous, 1982) have observed concentrations to peak on the
sharp rise of the hydrographs and decline very rapidly
following the passage of the wave-front. Barillier et al. (1993)
studying the passage of a major artificial wave in the Seine,
France, demonstrated that the passage of the wave disturbed
bed material contributing periphytic algae and rich in
nutrients and organic matter which resulted in a period of
dissolved oxygen depression. This was a much longer
duration, larger-scale event to those reported here. Milhous
(1982) proposed a simple conceptual model for the release of
fines from the matrix of gravel-bed rivers and Wohl and
Cenderelli (2000) and Beschta et al. (1981) found differential
disturbance of fine sediments and bed-load.
It is accepted here that river bed environments may be
infinitely patchy, and that a variety of mechanisms may be
contributing FIO to entrainment. As stated earlier, the
integrative effects of channel mixing will mean that the
nature and variety of the source areas and release mechan-
isms will be less significant to the observed FIO pulses than
the forces driving that entrainment. A further examination of
wave theory suggests that severe changes in stage result in a
zone suction as the wave propagates, this is implicated in the
FIO pulses reported in this paper.
The types of wave produced in the Washburn and Tyweryn
(Kay and McDonald, 1982) were severe step changes, in the
Clywedog the changes were not as severe, this may have
determined the nature of the entrainment observed. Rouse
(1946) showed that the nature of the wave-front depended on
the severity of the change in water depth. He used the relative
ARTICLE IN PRESS
Table 3 – Summary of wave height and overrun characteristics for the discharge increments in the River Washburn andAfon Clywedog
Q (m3 s�1) DQ (%) Upstream (Site 1) Downstream (Site 2) Qr Qr/Q1 yc
y (m) Dy/y1 DP (%) y (m) Dy/y1 DP (%)
Washburn 0.058 0.4 0.3
1.57 0.8 1 1.1 2.67 1.05 18.10 0.48
Afon 1.81 0.32 0.70
Clywedog 3.67 103 0.42 0.31 31.3 1.12 0.60 22.1 1.66 0.92 0.65
5.82 58.6 0.51 0.21 10.6 1.25 0.12 16.5 2.74 0.75 0.91
8.67 49.0 0.61 0.20 7.5 1.38 0.10 13.6 4.67 0.80 1.30
11.47 32.3 0.69 0.13 �0.4 1.48 0.07 5.0 6.02 0.69 1.55
WAT ER R ES E A R C H 40 (2006) 153– 161 159
depths of the initial flow, y1, and the wave height,
Dy ¼ y2 � y1, where y2 is the new water depth following the
passage of the wave, to indicate the likely nature of the wave.
For Dy=y1o1 a smooth undular wave is formed, for Dy=y141,
the wave-front breaks resulting in a sharp discontinuity in the
water surface (such as that observed in the Washburn). An
alternative measure of the likely nature of a wave is the
overrun discharge, Qr (Chow, 1959; Henderson, 1966) and is
the rate of inflow into the zone of suction created by the
propagation of the wave. The wave has the cross sectional
area of the new discharge rate, but is travelling over water of
the initial discharge, resulting in a zone of suction and fluid
rushes-in to equalise this pressure difference. The overrun
discharge Qr ¼ ðc� v1ÞA1 ¼ ðc� v2ÞA2, and the overrun critical
depth, yc ¼ Q2r=g
� �13. If yc4y1 (y1 is the downstream depth),
then the wave-front will have a near-vertical ‘shock’ front
(Chow, 1959; Henderson, 1966). The greater Qr, relative to the
actual discharge, the greater the suction and hence lift and
turbulence as water is drawn-in to equalise the drop in
pressure. Rouse (1946, pp. 144–146) demonstrates this turbu-
lence to great effect. Table 3 summarises the characteristics
of the stage increments in the experimental channels. In the
Washburn yc4y1 and Dy=y141 and Qr=Q1418, i.e. there was a
doubling in stage and the overrun discharge was 18 times
greater than the initial discharge. These conditions are
consistent with the development of ‘roll waves’ (Rouse,
1946) or shock fronted waves (Chow, 1959; Henderson, 1966).
For the Afon Clywedog, the information provided by these
measures does not give a clear indication of whether each
stage increment would produce a kinematic shock; the
condition of yc4y1 is satisfied for the upstream stage read-
ings, but not at the compound weir downstream (the weir
causes a distortion of the natural variation in water depth).
Rouse’s Dy=y141 condition is not met for any of the stage
increments, and the ratio decreases with increasing discharge
as y1 is greater at each increment. Qr=Q1 also decreases with
discharge indicating that the overrun discharge is smaller
relative to the total discharge as discharge increases. The
results indicate that the stage increments on the Clywedog
were shock-fronted, but not so steeply that breaking roll
waves occurred.
The final bacterial peak at 13:05 p.m. on the falling stage of
the Clywedog hydrograph (Fig. 2) was appears to have been
produced by a similar effect to that occurring on the increases
in stage. The step reduction in flow was similar in nature to a
negative surge (e.g. Chow, 1959). Such waves tend to dissipate
since the deeper wavelets travel faster than the shallower
ones and the transition is less severe than for a rising wave
(Chow, 1959), although the wave still creates an overrun as the
wave velocity is greater than the mean flow velocity and a
positive pressure results as water is forced away from the
region the wave is passing over.
The maintenance of elevated FIO concentrations during
enhanced flows and following the passage of waves and FIO
pulses may be caused by the kind of process observed by
Garcia et al. (1996). They observed a sporadic stochastic
entrainment of sediment particles caused by turbulent
bursting. Additionally, Reynolds (1992), referring to upland
streams, suggested that the principal means of disturbing
algae, during high flows, was through the movement of
stones (i.e. bed-load), and compression of the boundary layer
exposes more prominent particles to increased mechanical
stress (Reynolds, 1992). Wohl and Cenderelli (2000) and
Beschta et al. (1981) found that bed-load motion was a more
continuous, albeit somewhat erratic, process than the rapid
entrainment of fine sediment.
These results suggest that rapid changes in stage from
initially very low stage are more likely to cause turbulent
rolling waves and carry entrained FIO at the wave-front.
Where the water is initially deeper there may be insufficient
overrun to produce roll waves, an undular kinematic wave
may result. This will cause entrainment due to the suction
process, but the entrained FIO (or material of interest) is only
likely to be lifted into the mean flowing water and arrive
behind the flow peak. With a knowledge of channel hydraulic
characteristics it should be possible to estimate the timing of
bacterial peaks and design flow changes that produce the
desired result be it either flushing or non-disturbance of the
channel bed. In addition channel designs that attenuate
waves will also reduce FIO peak formation.
5. Summary and conclusions
The evidence of the data and processes discussed above
implicate the following processes in the episodic release of
faecal coliform indicator bacteria and maintenance of ele-
ARTICLE IN PRESS
WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 5 3 – 1 6 1160
vated concentrations in response to rapid changes in flow and
continued high flow in two upland UK river channels:
1.
Wave front entrapment: where a steep-fronted wave, withwave height much greater than the preceding water depth,
effectively sucks and holds disturbed organisms in the
turbulent wave-front. The disturbed material travels at the
wave speed.
2.
Wave front disturbance: organisms are lifted but are notdrawn into the wave overrun. The wavefront may be less
steep and the wave height small or not greater than the
initial water depth. This mechanism was also indicated for
falling waves.
3.
Steady-flow stochastic erosion: of bed and/or bank sources,resulting from high flow turbulence. The combined effect
of numerous small and irregular disturbances of bed and
bank maintaining faecal coliform concentrations elevated
above those encountered at lower rates of flow.
These processes may contribute to episodic loadings of
microbial contaminants and other important water quality
vectors e.g. organic pollutants, sediment, trace metals and
radio nuclides. (Berndtsson, 1990; Foster et al., 1995; Neal
et al., 1999; Rowan, 1995). A model incorporating these
entrainment processes is presented in Wilkinson (2001)
which reproduces the observed bacterial entrainment peaks
and incorporates a series of tools of relevance to episodic
microbial contamination as well as longer-term variations
in microbial indicator (faecal coliform) concentrations.
This series of model components offers further potential
for drinking water supply protection and recreational
safety management, especially in streams where high flow
events are generated for immersion sports activities such as
canoeing. Even without detailed data describing the hydro-
morphological characteristics of a river intended for such
events, a ‘rule of thumb’ can be implied from this work to
ensure that immersion is minimised during changes in flow
conditions.
Acknowledgements
The early stages of this work were funded by the UK
Department of Environment (PECD No. 7/7/385) and the
National Rivers Authority (now the Environment Agency).
Peter Joyce at Yorkshire Water plc. Blubberhouses depot and
Tim Harrison at Severn Trent Region of the Environment
Agency provided invaluable assistance in organising the
reservoir releases in the Washburn and Clywedog channels,
respectively.
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