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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) 157dvk@aber.ac.uk, 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-

ARTICLE IN PRESS

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 entry

into 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 contributions

were 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 to

increase 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 to

coincide with wavefront propagation and during the

quasi-stready flow following the passage of the wave

(Fig. 2).

(v)

The FIO concentration remained elevated relative to

pre-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

ARTICLE IN PRESS

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

ARTICLE IN PRESS

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 (or

near) mean flow velocity;

(iii)

elevated post release concentrations (indicating a

continued 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-

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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 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, with

wave 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 not

drawn 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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