Stuck in the mud: investigating a tidal bore

My right foot is six-inches deep in mud – thick, smelly, sticky mud – and with every wiggle I sink deeper still. Levering myself free with my left foot is no good: the slightest pressure and it too will slip into the quagmire that lines the steep sides of the river. Muttering a few choice words that should not be committed to print, I lunge artlessly towards the nearest clump of reeds. A few inelegant moments later and I am just about free, my foot pulling out from the mud with a loud, rasping squelch.

Tidal bore of the Great Ouse, 18th September 2013. Photo: Peter Sheehan

FIGURE 1. Tidal bore of the Great Ouse, 18th September 2013. Photo: Peter Sheehan

I scramble to the top of the bank, grasping onto reeds and patches of grass in order to prevent myself from sinking in again. Reaching the top, out of breath and warmer than is comfortable when wearing waders, I join my supervisor. He hasn’t really noticed that I’ve spent the last few minutes calf-deep in mud: he is staring intently about half a kilometre downstream, at a point where the river turns into view as it passes under a narrow road bridge. The instruments – two current meters and two water-level recorders – that we have spent the afternoon placing in the centre of the river are waiting to measure the curious phenomenon that will soon be passing by.

The Great Ouse, the fourth-longest river in the United Kingdom, flows steadily through the Midlands and East Anglia, before entering the North Sea at King’s Lynn. It drains a large part of the table-flat farmland of the Fens; in many places its main channel has been canalised to increase its discharge. So far, so ordinary. But the Ouse is one of the few rivers in Britain – indeed, in the world – that hosts a tidal bore.

The names given to this phenomenon vary around the world. In France, it’s generally known as a mascaret; on the Amazon it’s called the pororoca; the largest in the world, found on the Qiantang river in China, is called the Silver Dragon. In the UK, while tidal bore has become the generic term, on a few rivers – for instance, the Trent – it’s called an aegir.

After some ten minutes of waiting, a small fleck of white appears on the surface of the water on the far side of the bridge. A few moments later and the rest of the bore, a half-metre-high wave that stretches across the breadth of the river, comes into view. The initial wave front, rounded in the centre of the channel but sometimes breaking and white-capped towards the very edges, is followed by a train of progressively smaller undulations (Figure 1). Water, flowing downstream on the ebb (outbound) tide in advance of the wave front, about turns with the passage of the bore: behind it, the flood (inbound) tide rushes up the river.

Picture a wave approaching a beach. Out in the deep, it exists as a gentle rise in the sea surface. Closer to shore, the wave begins to slow down – in shallow water, a wave’s speed is proportional to the square root of depth. As the crest is higher than the preceding trough, and thus is in deeper water, the crest travels marginally faster and so steepens the wave front Eventually, the wave reaches a critical height and breaks – the crest tumbles over the trough and the wave’s energy is released as turbulence and noise.

It is the same mechanism that produces a tidal bore, only the breaking wave is the incoming tidal wave rather than a wind-driven surface wave. In shallow, converging estuaries where the tidal range exceeds 5 – 6 m (Chanson, 2011), the leading edge of the tidal wave progressively steepens as the flood tide surges up the river. Eventually, it becomes an abrupt jump in the water level, a so-called hydraulic jump in translation that propagates upstream (Officer, 1976; Cun-Hong et al, 2007).

Figure 2. Time series (hours) of water column height (m) in the Garonne river at Podensac, France, when tidal range at the mouth is (left) 4.4 m, (centre) 5.1 m and (bottom) 6.3 m. Adapted from Bonneton et al (2011).

Figure 2. Time series (hours) of water column height (m) in the Garonne river at Podensac, France, when tidal range at the mouth is (left) 4.4 m, (centre) 5.1 m and (bottom) 6.3 m. Adapted from Bonneton et al (2011).

Of course, not all estuaries host tidal bores. In most, the tidal range – the difference between high and low water – is too small; bores are only found in rivers with the very largest tidal ranges, and even then only on spring tides. The effect of increasing tidal range on bore formation during the spring-neap cycle can be seen in Figure 2. For a tidal range of 4.4 m, there is no bore, although the steepening of the wave front is sufficient to produce a highly asymmetrical tide (short, strong flood tides followed by longer, slower ebbs). Only when the range is 5.1 m and above is a bore produced: notice the sudden jump at the start of the flood tide. The estuary also needs to narrow steadily with distance from the sea so that the energy of the incoming wave is not dissipated too quickly.

Figure 3. Top panel: time series (s) of water column height (m) in the mid-channel of the Great Ouse, United Kingdom. The black line is actual height, and the blue line is a 20 s running mean. Bottom panel: time series of velocity at the same point. The black line is along-channel velocity (positive downstream), the blue line is transverse velocity (positive to the left when looking downstream), and the green line is vertical velocity (positive upwards). Data from Rob Hall, reproduced by kind permission.

Figure 3. Top panel: time series (s) of water column height (m) in the mid-channel of the Great Ouse, United Kingdom. The black line is actual height, and the blue line is a 20 s running mean. Bottom panel: time series of velocity at the same point. The black line is along-channel velocity (positive downstream), the blue line is across-channel velocity (positive to the left when looking downstream), and the green line is vertical velocity (positive upwards). Data from Rob Hall, reproduced by kind permission.

The observations we made of the bore on the Great Ouse are shown in Figure 3. The bore’s shape can be clearly seen in the measurements of water column height. The water level recorders were sampling eight times a second, giving an incredibly high-resolution picture of the shape of the bore as it passes the study site – a mid-river spot at the gloriously named Wiggenhall Saint Mary Magdalene, a small Norfolk village some 18 km upstream from the river mouth. The sharp wave front – the hydraulic jump – can be seen very clearly, as can the train of waves, known as whelps, that follow gentle, undular bores such as this one.

The velocity observations also reveal the passage of the bore. A rapid longitudinal flow reversal accompanies its passage: in systems with a bore there is no period of slack water (Simpson et al, 2004). At the same time, the vertical velocity switches from negative to positive as the water level quickly begins to rise.

Despite their power and the danger that they can present to people – the Qiantang bore, among others, has claimed countless lives over the centuries – bores are actually a fragile phenomenon. Dredging, diversion and construction can drastically alter an estuary’s characteristics and so prevent bores from forming. The highest-profile example is probably the bore of the Seine, which did not survive the 1960s. Observing bores is therefore a crucial part of understanding and protecting the rare and complex river systems that depend on these magnificent waves.

The bore having passed, I clamber back into my waders. Water level recorders don’t haul themselves out of rivers. Indeed, the first time we deployed the instruments, the bore and the turbulence of the wave train stirred up so much sediment that the frames were almost irretrievably buried. Clutching onto reeds, and not expecting to make it home any time soon, I slide down the bank to rising water.

 

References

Bonneton, N, Bonneton, P, Parisot, JP, Detandt, G, Sottolichio, A and Crapoulet, A, 2011. Structure verticale des courants associés à la propagation de la marée dans la Garonne : impact du mascaret [Vertical structure of tidal currents in the Garonne river: impact of a tidal bore.] 20ème Congrès Français de Mécanique, 29th August – 2nd September 2011, Besançon, France

Chanson, H, 2011. Current knowledge in tidal bores and their environmental, ecological and cultural impacts. Environmental Fluid Mechanics, 11, 77 – 98

Cun-Hong, P, Bing-Yao, L and Xian-Zhong, M, 2007. Case study: numerical modelling of the tidal bore on the Qiantang River, China. Journal of Hydraulic Engineering, 133, 130 – 138

Officer, CB, 1976. Physical Oceanography of Estuaries (and Associated Coastal Waters). John Wiley & Sons, London, United Kingdom

Simpson, JH, Fisher, NR and Wiles, P, 2004. Reynolds Stress and TKE production in an estuary with a tidal bore. Estuarine, Coastal & Shelf Science, 60, 619 – 627

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P.Sheehan@uea.ac.uk'

Peter Sheehan

I am a PhD student at the University of East Anglia doing research into the variability and driving processes of the Fair Isle Current, one of the main inflow routes for Atlantic water into the North Sea.
P.Sheehan@uea.ac.uk'

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