Observer le mascaret sur la Dordogne à Saint-Pardon/en

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The 2013 tidal bore experiment

The 2013 tidal bore experiment was organized around observation, measurement and modeling of the tidal bores between 18th and 21st of september 2013 at Saint-Pardon on the Dordogne river.
WIKHYDRO pages, which concern this experiment are the following (in French):

• Organisation d'une campagne d'observation du mascaret en Dordogne
• Vivre,_observer, mesurer, modéliser le mascaret à Saint-Pardon en_Dordogne
• Observer le mascaret sur la Dordogne à Saint-Pardon
• Observer la propagation du mascaret sur la Dordogne à Saint-Pardon à partir des berges

Context

This experiment "mascaret 2013" was dedicated on the observation, measurement and modelling tidal bores produced in Saint-Pardon on Dordogne river between 18 and 21 September 2013.
The main observations were collected inside the tidal bore by surfers ans kayakists, by the drone operated by IFSTTAR and by the qualitative observations made by the master 2 "Engineering, water management and Environment" group of students from the university of Limoges.

Surfing tidal bore : a source of first hand information

The following video was recorded on Saturdy 21 September 2013 on the Dordogne river in the Saint-Pardon area. The tidal coefficient was 101 (maximum is 120) and the Dordogne discharge was 118 m3/s.
The 3 windows in the video are synchronous and correspond to the camera located on the left bank (half lower portion of the screen), to a GO PRO camera on the top of a stick of 1m high (left upper portion of the screen) operated by a person in the front of a kayak completed by an other GO PRO camera hanged on the top of the helmet of the driver of the kayak (so approximately 1m above the water). You can follow the kayak trajectory (upper right of the screen).

Considering this video, we can outline some qualitative elements, which could be confirmed by student observations and we canproduce some rough quantitative evaluations.

WIKHYDRO : mascaret Dordogne à Saint-Pardon 21... par Wikhydro

Qualitative observations

To observe of a tidal bore is not a so easy task that it can appears at first sight. This physical process is not easily approached for different reasons. It corresponds to a wave train, composed of several successive waves, which have not necessary the same characteristics, which propagate upstream during time and which are submetted to common behaviour of waves: mainly refraction of the bathymetry, but also diffraction and reflection.
The main first observations are the following:

• the shape of the waves are very dependent of the bathymetry. It seems to increase when the wave train arrives in front of Saint-Pardon (the pilot of the kayak told the surfer that the wave height will increase).
• the wave length seems greater at the beginning with the high bathymetry than after in front of Saint-Pardon)
• at the exit of the bend of "la rivière" location, the wave front seems to be divided in two parts :

• a wave front whose left wing is breaking along the left bank
• a wave front whose right wing "travels" the river and propagates straight away to Saint-Pardon : this is the wave preferred by the surfers, familiar with the site.

• the conditions of breaking on the crest of the waves vary in space and time
• the banks of the river are submitted to wave breaking and also to erosion
• the wake produced by the surfers propagates downstream
• the protection wall of Saint-Pardon, which receives the wave coming from the right bank produces a reflection and has an effect of destructuration of the wave train in front of Saint-Pardon. The phenomenon is amplified in the high bathymetry area along the left bank located upstream Saint-Pardon.
• the tidal bore generates a high level of sediment suspension : the brown color of water is clearly visible as well as the thin layer of clay deposed on the surfer diving suit

Some rough evaluations

From these observations, we can deduce some quantitative points :

• the duration of the total descend was 8 mn 45s and the length was 2,6 km, which gives a speed of the tidal bore of 18 km/h = 5 m/s
• however, the speed of a small wave propagating on small depth follows the law $ V=\sqrt{gH} $ where H is the water depth. We can deduce a theoretical mean water depth along the path of the tidal bore H = 2,5 m, which corresponds to the small depth upstream after the bend of the river (lower than 2 m) as well as downstream of Saint-Pardon. In the center of the river, the depth is higher.
• in front of Saint-Pardon, we can estimate the wave lenght at about 5 kayak lenght ie 10 m
• the mean wave curvature is given by H/L between 0,25 (depth of 2m) et 0,5 (depth of 4m), lower then the Miche criteria (H/L=0,8), which explains the relatively low length of the surf zone crests: the surf zones are mainly due to surfers wakes.

Recommendations

This qualitative approach was possible due to the synchronous measure of 3 cameras. This can be improved :

• the areal monitoring by drone is able to give more precisely the propagation - deformation in space and time. An improvement should be a 3D restitution in real time of the surface of the tidal bore.
• the measures in the vicinity of the banks can only give a local indication of the process, very influenced by local conditions (surf zone, wave reflection...)

Comparison of the 4 tidal bores of 18, 19, 20 and 21 September 2013

Qualitative exploitation of the comparative images comparatives of tidal bores recorded at Saint-Pardon on the Dordogne river at the four following dates :

• 18 September 2013 ; tidal coefficient 97 ; discharge Dordogne : 137 m3/s
• 19 September 2013 ; tidal coefficient 104 ; discharge Dordogne : 111 m3/s
• 20 September 2013 ; tidal coefficient 105 ; discharge Dordogne : 133 m3/s
• 21 September 2013 ; tidal coefficient 101 ; discharge Dordogne : 118 m3/s

These 4 tidal bores were shot from the wharf of Saint-Pardon and compose the four parts of the film below. A qualitative analysis is thereafter proposed

WIKHYDRO : comparaison de 4 mascarets à Saint... par Wikhydro

wave amplitudes

The tidal bore 97 produces a wave train of less amplitude than those of higher coefficients

wave crest breaking in the center of the river

In front of Saint-Pardon, the wave crests do not break : only the wake of the surfers produce surface perturbations

Right bank refraction

If we examine in details tidal bores 104 and 105 (top of the video), it appears that the wave crest line, straight in the middle of the river is bent towards the left side, which represents a delay in the propagation. This results of the high bottom, generating a lower wave velocity compared to the middle of the river where the water depth is higher (see formula before). This wave along the bank which propagates with very small depth is breaking (before reaching Saint-Pardon).

Bottom refraction along the left bank and effect of the descent slope of the surfboards

The camera turned towards the river shows several phenomena:

• The small depths along the left side bank produce wave breaking, but only for the first waves
• Downstream, the water surface is turbulent but waves do not break, in opposite of what appends in the center of the river where the waves maintain a well formed pattern : the surfers have difficulties to maintain their equilibrium on their boards
• If we start the video at time 1:26, the wave along the bank, first propagating behind the wave crest (due to a propagation speed lower in small depths) is reconstituting "in advance" after the descent slope. It "sleeps" rapidly on this small structure which is very inclined. This process is more apparent starting from the second wave. This second wave rank, which propagates with an angle towards the middle of the river, destroys very rapidly the wave train (2:00)
  

Something to notice

• The water surface is the most turbulent for coefficient 105 : although the zoom is different for the 3 coefficients, the wave heights seem more important for the bigger coefficients
• The entire disappearance of a kayakist (2:16 coeff 101) inside the wave shows that the wave heights are bigger upstream Saint-Pardon (in the common direction of the river)

Recommandations

To understand the propagation - deformation process of the wave train, one should install in several places along each bank of the river some cameras to take crosswave videos The measurement devices placed in the near proximity of the banks inside the river give information on water level evolution, but these locations are submitted to high turbulence (upper right of the screen) in a refraction area near the wall and works of Saint-Pardon. The most relevant information should be measured in the middle of the river by drones (see before) or by ULM completed by topographic time restitution of the wave train. This last point is still related to research.

IFSTTAR drone survey

This video was recorded by the drone of l'IFSTTAR on 19th Septembre 2013, with the following conditions :tidal coefficient 104 and Dordogne discharge : 111 m3/s. The drone operated in the vicinity of Tressac. The video was recorded by a GOPRO HERO 3 camera, showing downstream where the tidal bore comes from.

WIKHYDRO : survol du mascaret à Saint-Pardon... par Wikhydro

Image processing

• in the far downstream direction, the water surface is very perturbed due to high elevation of the river bottom. The wave front is hardly discernable.
• an important dissymmetry is apparent between the two banks

• left bank with a very perturbed surface with breaking waves
• right bank, a more smoothy surface

• perturbations seems being formed and propagate from banks and develop toward the center of the river, in a V shape. These two waves brake the wave train and the high turbulent state of the water surface indicates a non linear wave area.
• along its propagation, the wave train is distinguishing and takes some advance compared to the high perturbed area, which propagates slower. This probably results of the deeper bathymetry in the center of the river.

• left bank : the bathymetry creates a wave refraction - reflection but weaker than before. A reflection wave appears and propagates to the center (3:15)
• right bank : the tidal bore is indiscernible

• the well formed wave train in the center of the river counts more waves

Some rough estimations

• localization: 250 meters downstream "port de Tressac" (lat 44.929343° Long -0.320711°) ;
• stationary drone ;
• vertically oriented GOPRO ;
• altitude drone : 70 m ;
• vertical distance "star shaped photo device" - down the drone : 30 m ;
• distance between the extremity cameras of the "star shaped photo device" : 1,80 m.
• result : λ = 11,20 m

to be improved...

Recommandations

• compared with what is observed in Saint-Pardon, this "downsteam" part is also interesting to be investigated because the wave train is well established dowstream Tressac river bend;
• unfortunately, drone survey is missing above this river bend ;
• complete the embedded system with a vertical shot camera and/or video camera (in addition to the existing GOPRO ;
• improve the star shaped device ;
• use a altimeter telemeter.

Student's Observations

The students from the university of Limoges were dispatched on 7 sites Their missions consisted to record videos of the tidal bore during its progression in front of their observation site as well as water elevation. All videos are available on this page : Observer la propagation du mascaret dur la Dordogne à Saint-Pardon à partir des berges

Le créateur de cet article est Jean-Michel Tanguy Note : d'autres personnes peuvent avoir contribué au contenu de cet article, [Consultez l'historique]. Pour d'autres articles de cet auteur, voir ici. Pour un aperçu des contributions de cet auteur, voir ici.