REUNGOAT, D., LUBIN, P., LENG, X., and CHANSON, H. (2019). "Tidal Bore Hydrodynamics and Sediment
Processes: 2010-2016 Field Observations in France." Coastal Engineering Journal, Vol. 60, No. 4, pp. 484-498 (DOI:
10.1080/21664250.2018.1529265) (ISSN 0578-5634).
1
Tidal bore hydrodynamics and sediment processes: 2010-2016 field observations
in France
by
David REUNGOAT (
1
) (
*
), Pierre LUBIN (
1
), Xinqian LENG (
2
) and Hubert CHANSON (
2
)
(
1
) Université de Bordeaux, I2M, Laboratoire TREFLE, 16 avenue Pey-Berland, Pessac, France, CNRS
UMR 5295, Pessac, France
(
2
) The University of Queensland, School of Civil Engineering, Brisbane QLD 4072, Australia
(
*
) Corresponding author: E-mail: reungoat@enscbp.fr
Abstract
A tidal bore is a compressive wave, advancing upstream in an estuary when the flood tidal flow starts. It is
observed when a macro-tidal flood flow enters the funnel shaped river mouth with shallow waters. Its upriver
propagation impacts the natural system, with sediment scouring and suspension. The tidal bores of the
Garonne and Sélune Rivers in France were extensively investigated between 2010 and 2016. Instantaneous
velocity measurements were conducted continuously at high-frequency (50 to 200 Hz) during each bore
event. In the Garonne River, instantaneous sediment concentration data were obtained and the sediment
properties were systematically tested. The nature of the observations was comprehensive, regrouping
hydrodynamics and turbulence, sedimentology and suspended sediment transport. The key outcomes show
that the tidal bore occurrence has a marked effect on the velocity field and suspended sediment processes,
including a sudden flow deceleration and flow reversal during the bore passage. The turbulent Reynolds
stresses present large instantaneous amplitudes, with rapid fluctuations, during the tidal bore. The sediment
flux data imply considerable mass transport rates during the first hour of flood tide. This unique review of
field data further shows a number of common features, as well as the uniqueness of each individual event.
Keywords: Tidal bores, Field measurements, Hydrodynamics, Turbulence, Sediment processes, France.
1. INTRODUCTION
While laboratory experiment could generate specific "unnatural" flow, nature can create some unlogical flow
phenomena that may occur under specific conditions. Tidal bore is one of this famous unexpected
phenomena: a wave front that flow agains the initial mean stream of a river. A tidal bore is basically a water
surface discontinuity, i.e. compression wave, propagating upstream in the estuary (Fig. 1). It may take place
under large tidal range conditions, in a funnelled shaped estuary which bathymetry amplifies the energy of
the tide flowing upstream against the river flow. A hydrodynamic shock then occurs, generating a bore
REUNGOAT, D., LUBIN, P., LENG, X., and CHANSON, H. (2019). "Tidal Bore Hydrodynamics and Sediment
Processes: 2010-2016 Field Observations in France." Coastal Engineering Journal, Vol. 60, No. 4, pp. 484-498 (DOI:
10.1080/21664250.2018.1529265) (ISSN 0578-5634).
2
propagating inland as the river flow reverses upstream, behind the compression wave (LYNCH 1982,
CHANSON 2011a). Historically, the best documented tidal bores have been those of the Seine River
(France) and Qiantang River (China). Although it no longer exists in its most energetic and spectacular form,
the "mascaret" (tidal bore) of the Seine River was documented first during the 7th and 9th centuries AD
(MALANDAIN 1988). Records of the Qiantang River bore may be traced back to the 7th and 2nd centuries
BC, with further writing from the 8th century (MOULE 1923) (Fig. 1A & 1B). Another famous tidal bore is
the "pororoca" of the Amazon River (Brazil), first reported by PINZON and LA CONDAMINE during the
16th and 18th centuries respectively (LA CONDAMINE 1745, MANZANO MANZANO and MANZANO
FERNANDEZ-HERDIA 1988). The Hoogly bore (Gange River in India) was described in 19th century
historical nautical reports and is still active. Smaller tidal bores occur on the Severn River near Gloucester
(UK), on the Garonne and Dordogne Rivers (France), and in the Bay of Fundy (Canada). Worldwide, about
300 to 450 shallow-water bays, estuaries and rivers are influenced by tidal bores (BARTSCH-WINKLER
and LYNCH 1988, CHANSON 2011a).
The upstream propagation of tidal bores may span over long distance and has a significant influence on the
natural systems. Figures 1A and 1B illustrates the impact of tidal bores on artificial structures. While tidal
bore surfing has become an extreme sport in a number of rivers (Fig. 1E & 1F), other bores have had
ominous reputations, e.g. the Seine and Qiantang River bores. Along the Qiantang River, numerous tidal
bore warning signs are erected (Fig. 2), with over 80 drownings in the bore for the past two decades (PAN
and CHANSON 2015). The French novelist Jules VERNES described the terrifying impact of tidal bore:
"pororoca, this terrifying tidal bore that, for three days before the new or full moon, takes only two minutes,
instead of six hours, to rise the flood tide water elevation of about 4 to 5 m above the low water level. It is a
real tsunami, terrible among all." (VERNES 1881). The bore propagation provokes intense sediment
transport, linked to erosion, solid suspension and upwelling (GREB and ARCHER 2007, CHANSON et al.
2011, KEEVIL et al. 2015, FURGEROT et al. 2016) as well as interactions with sediment bed forms
(KEEVIL 2016, REUNGOAT et al. 2017b). The flow field behind the tidal bore advects very high
suspended sediment concentrations (SSCs), with observed concentrations in excess of 40 kg/m
3
(FAN et al.
2014, REUNGOAT et al. 2017). In cohesive sediment estuarine and coastal zones, the erosional and
scouring processes leading to landforms modifications are functions of the rheological characteristics of the
sediment deposits (FAAS 1995, KEEVIL et al. 2015). Field observations in tidal bores hinted a two-stage
bed scour mechanism at each event: first surface bed erosion in the form of surface peeling, followed by
delayed mass erosion (POUV et al. 2014, REUNGOAT et al. 2017).
At present, in situ tidal bores measurements are relatively rare and scarce data is available. Early
observations in China, France and UK mentioned the bore shape and arrival time. The earliest detailed
measurements are possibly those of PARTIOT in the Seine River, including water elevations and float
speeds (PARTIOT 1861, BAZIN 1865). In the last 50 years, a limited number of field studies were
conducted. Table 1 presents a summary. All studies emphasised the massive impact of the bore passage. In
REUNGOAT, D., LUBIN, P., LENG, X., and CHANSON, H. (2019). "Tidal Bore Hydrodynamics and Sediment
Processes: 2010-2016 Field Observations in France." Coastal Engineering Journal, Vol. 60, No. 4, pp. 484-498 (DOI:
10.1080/21664250.2018.1529265) (ISSN 0578-5634).
3
several cases, substantial losses of equipments and damage to instrumentation were reported (KJERFVE and
FERREIRA 1993, SIMPSON et al. 2004, WOLANSKI et al. 2004).
The tidal bore of the Garonne River (France) was comprehensively investigated between 2010 and 2016 as
part of the Project Mascaret ANR-10-BLAN-0911. In addition, a field study was investigated in the Sélune
River (France). The nature of the field observations was thorough, including hydrodynamics and turbulence,
suspended sediment transport and sedimentology. The aim of the paper is to review the key outcomes of the
project, spanning over several years of observations and measurements, as well as developing a basic
understanding of tidal-bore-affected estuarine zones in terms of temporal evolution of suspended sediment
processes correlated to hydrodynamics, based upon large-scale field observations. A review of field data
further shows a number of features common to all tidal bores, as well as some unique characteristics of each
individual event.
2. FIELD DEPLOYMENTS
2.1 In situ experiments in macro-tidal estuaries
Field measurements were conducted in two different systems in France, with semi-diurnal tides: the Garonne
River upstream of Bordeaux, and the Sélune River at Pointe du Grouin du Sud, Bay of Mt St Michel. The
first site was the Arcins channel (Fig. 3A) about 102 km upstream of the river mouth. The Arcins channel is
a 1.8 km long and 70 m wide, and the water depth is about 1.1 m to 2.5 m at low tide (Fig. 4A). The second
site was located at the end of the Bay, about 500 m downstream of the confluence of the Sélune and Sée
Rivers. The channels in the Bay change with time. During the field study, the main channel is about 35 m
wide, with less than 0.5 m water depth at low tides, while the flood tide flow submerged all the sand banks,
the channel width exceeding 2 km during the flood and early ebb tide (Fig. 3B & 4B). Figure 3 presents
photographs of the sites and Figure 4 shows the surveyed cross-sections, looking downstream.
Field observations took place predominantly during spring tides. All field deployments took place between
June and November, when the river water levels were relatively low. Table 2 summarises the site, dates and
tidal range (2nd, 3rd and 4th column). In Table 2, the initial water depth is listed in the 9th column (prior to
the tidal bore passage).
2.2 Instrumentation
Free surface elevations were recorded using a survey staff, placed 2 m away from the velocimeter to
minimise any form of interference. During each bore passage, a video camera was used to record the rapid
change in water levels. The instantaneous velocities were recorded with an acoustic Doppler velocimeter
(ADV) sampled continuously at a frequency of 50 to 200 Hz depending upon the field deployment. The
ADV recordings started more than 1 h before the the bore and continued for more than 1 h after the bore
passage. All the ADV data were rigorously post-processed to eliminate any wrong and spurious data.
REUNGOAT, D., LUBIN, P., LENG, X., and CHANSON, H. (2019). "Tidal Bore Hydrodynamics and Sediment
Processes: 2010-2016 Field Observations in France." Coastal Engineering Journal, Vol. 60, No. 4, pp. 484-498 (DOI:
10.1080/21664250.2018.1529265) (ISSN 0578-5634).
4
In the Garonne River, water samples were collected at low tide on most days, for sediment bed and sediment-
laden investigations. The bed material was composed of about 80% silt. Samples were analysed in terms of
material density, granulometry, and rheometry using a well-established protocol (CHANSON et al. 2011,
KEEVIL et al. 2015). The ADV unit was calibrated as a function of suspended sediment concentration (SSC)
against known, artificially produced diluted concentrations of material collected on bed site and thoroughly
mixed. The SSC estimates were successfully compared to water sample data (KEEVIL et al. 2015,
REUNGOAT et al. 2017).
3. OBSERVATIONS
In the Arcins channel, the tidal bore formed at the entrance of the channel, i.e. the northern end (Fig. 3C).
The bore propagated as a breaking surge across the entire river width, initially, as it advanced over a very
shallow bar consisting of gravel, sand and mud. As the bore advanced, it became an undular bore owing to
the deeper bathymetry (Fig. 3A) and propagated for the full length of the channel. At the sampling site
(marked in Figure 3C), the tidal bore was undular: a series of free-surface undulations, with a wave period
close to a second, followed the leading front and lasted for several minutes. The bore appearance is observed
to differ in the Bay of Mt St Michel. The tidal bore occurred about 15 minutes before reaching the sampling
location. The breaking bore presented a marked roller with a curved shape, viewed in elevation (Fig. 3B). In
the far background towards the left side of the channel (Fig. 3B), the bore front propagated and overtopped
on the dry sand banks and the waters were dark.
A key feature of all tidal bore is the rapid rise d in free-surface elevation, when the bore front passed.
Typical data are reported in Table 2 (12th column). The equations of conservation of mass and momentum
may be developed in their integral form across the bore front. The result is an analytical solution in terms the
ratio of conjugate cross-section areas A
2
/A
1
as a function of the Froude number Fr
1
and cross-section shape
(CHANSON 2012):
B
'B
B
'B
2Fr
B
B
B
'B
8
B
'B
2
2
1
A
A
2
1
1
2
1
2
(1)
where A
1
is the initial cross-section area, A
2
is the new cross-sections area: A
2
= A
1
+ A immediately
behind the bore front, B
1
is the initial free-surface width, B and B' are characteristic dimensions defined as:
21
21
AA
A
B
dd d
(2)
REUNGOAT, D., LUBIN, P., LENG, X., and CHANSON, H. (2019). "Tidal Bore Hydrodynamics and Sediment
Processes: 2010-2016 Field Observations in France." Coastal Engineering Journal, Vol. 60, No. 4, pp. 484-498 (DOI:
10.1080/21664250.2018.1529265) (ISSN 0578-5634).
5
2
12
A
A
2
)dd(
2
1
dA)zd(
'B
2
1
(3)
with d
1
and d
2
the initial and new water depths. The tidal bore Froude number Fr
1
is defined as:
1
1
1
1
VU
Fr
A
g
B
(4)
with V
1
being the late ebb flow velocity (positive downstream), U being the bore celerity (also positive
upstream), and g being the acceleration of gravity. Field observations are reported in Figure 5A. The present
data are compared to Equation (1) (open circles) and previous field data. In addition, the Bélanger equation is
included:
1Fr81
2
1
A
A
2
1
1
2
(5)
although Equation (5) is only valid for a smooth rectangular channel. Figure 5A shows a satisfactory
agreement between Equation (1) and in situ observations. The data are further reported in Table 2.
All field observations showed that the bore shape was related to the Froude number (PEREGRINE 1966,
CHANSON 2011a). A bore with a breaking front and marked roller was observed for Fr
1
> 1.5 to 1.7. For
1.3 < Fr
1
< 1.5-1.7, the bore was undular with limited breaking: it was also called a breaking bore with
secondary waves. And an undular bore, without breaking, was observed for Fr
1
< 1.3. A key aspect of
undular bores was the smooth front followed by some secondary wave motion. Dimensionless wave
amplitude and wave length data are plotted (Figures 5B and 5C), as well as listed in Table 2. In Figures 5B
and 5C, the present data are compared to past field data and analytical solutions: i.e., linear wave theory
(LEMOINE 1948) and Boussinesq equations (ANDERSEN 1978). All undular bore data showed that the
dimensionless wave amplitude a
w
/(A
1
/B
1
) increased with increasing Froude number for 1 < Fr
1
< 1.3 to 1.4
(Fig. 5B). The maximum wave amplitude corresponded to the appearance of wave breaking at the first wave
crest. For Fr
1
> 1.3 to 1.4, the observations presented a trend with decreasing wave amplitude with increasing
Froude number. For all Froude numbers, the undular wave length decreased with increasing Froude number,
as seen in Figure 5C. Overall the entire field data were relatively close, and the data trend was consistent
with laboratory data and theoretical solutions.
A key feature of all field measurements conducted as part of the project MASCARET (Table 1) was the high
temporal resolutions with continuous high-frequency water surface elevation and velocity measurements,
starting prior to the tidal bore and extending for at least one hour after the bore passage. The results showed a
sharp flow deceleration associated with the bore passage, together with rapid and large fluctuations during
and after the bore. Figure 6 presents a typical example for the Garonne River undular bore, and Figure 7
presents some observation in the Sélune River breaking bore. Figure 6A shows about a three-hour record of
