The construction of Herring Bridge at Great Yarmouth has sparked discussions about its impact on the river's ability to flow freely to the sea. For some, the bridge has been an easy target to blame for the flooding in the Broads this winter.
In the busy world of social media, lots of different ideas on this subject have been shared, but they are not always correct or complete. For example, the new bridge has reduced the river's width by one-third, but rarely noted is the fact it is situated at one of the widest points in the river and, more importantly, the channel not far upstream at the Victorian Haven Bridge is much narrower.
Discussions and claims often overlook the complex interplay between river channel width, depth, and flow dynamics. They also ignore the absence of publicly available flow data for the river at the point in question. Jumping from measurements on Google Earth to making guesses about river flow without any recorded data muddles the discussion and shifts attention away from the real problems faced by the Broads this winter.
This article looks into the effects of river channel restrictions at the Herring , Haven and Breydon Bridges, to understand their comparative impact on the river system.
River Scouring and Channel Narrowing
River scouring occurs when moving water erodes the riverbed. It's the way rivers naturally shape their course when the bed or banks are made of soft or mobile materials like silt, sand and gravel. Only rivers with hard rock beds are not affected by scouring.
When trees fall into rivers, they reduce the water volume (or capacity) at that point, causing it to move faster and wear away the sides or bottom of the river, creating a depression to replace the lost volume. Once the water slows down to its normal speed, it stops wearing away the riverbed. This same process affects man-made objects like shipwrecks, oil platforms, sea defences, and wind farms in the North Sea, so engineers must consider scouring when designing supports for these structures. Its why there are little pools at the end of groynes on our beaches.
The channel of the River Yare, which is soft, shows signs of scouring along its whole length. This is why the outer parts of river bends are deeper; the fast-moving water on the outside of a curve scours the bottom away.
We use sonar imaging to find scours, measure their depth, and see what the riverbed is made of. Side imaging sonar tells us the size and shape of a scour. But sophisticated equipment isn't always necessary to observe how water is behaving. Since water doesn't compress, any changes beneath the surface can create visible patterns on top that we can spot with the naked eye.
Many bridges on the Broads reduce the river's width, affecting its flow dynamics. We see these during times of strong tidal flow at places like Reedham rail bridge and Potter Heigham. When water flow is forced through narrows in a river or stream, several signs can be seen on the surface.
Increased Water Velocity: When water is forced through a narrow channel, its greater velocity is easily visible when compared with water upstream or downstream. Bubbles, debris, floating objects all act as indicators.
Turbulence and Eddies: The restriction creates turbulence as water flow is disrupted and redirected, resulting in swirling eddies on the surface, especially downstream of the restriction.
Changes in Water Level: Water levels may rise upstream of the restriction as flow is backed up but fall again immediately downstream due to the accelerated flow. This creates a “crease” or step in the water that is clearly visible.
When water flows through a river channel without any restrictions or is not affected by shapes or obstacles, it often has a glass-like appearance, appearing very flat and uniform. For those who navigate boats, these differences are not just visual but can also be felt by an experienced skipper, through delicate movements in the hull or through the helm.
Observations of Flow Dynamics and Scouring
Even when flow data is unavailable, observation of river behaviour and sonar measurements can offer a good understanding of what is happening. On January 6th, 2024, the Broads Society conducted a thorough examination of the river channel at Great Yarmouth during the morning ebb tide. The observations were carried out both visually and with the help of Garmin Side Imaging technology.
Here are the summarised findings:
Breydon Bridge: Despite its minimal constriction in a wide part of the river (175m), the presence of distinct creasing near the supports and scouring of the riverbed 3m deeper than surrounding areas highlight the complex interactions between the bridge and river flow. The east section's 2m scour and more even bottom suggest localised effects of flow acceleration and pressure differentials on sediment displacement and deposition patterns. The echo returns from the imager determined the bottom composition to be soft throughout the approach and the scour.
Haven Bridge: Positioned 0.7km seaward of the Breydon Bridge, Haven Bridge narrows the channel from 67m to 49m, leading to much-increased flow velocity between the bridge supports, evidenced by the significant crease on the water surface and the turbulent current affecting navigational stability when off throttle. The observed scouring depth of about 3m, with a noticeably uneven riverbed, underscores the bridge's substantial impact on the hydraulic conditions. Sonar imaging indicated a firm bottom within the scour, transitioning back to soft upon exiting the scour area.
Herring Bridge: 2.75km seaward of Haven Bridge, the reduction of the river channel from 83m to 55m at the Herring Bridge, while notable, does not appear to introduce severe flow restrictions or scouring effects when compared to the other bridges. There is no sonar-visible scour and the riverbed remains flat and uniform. The calmer water surface and slower tide observed near this bridge indicate much less hydraulic stress on the riverbed, possibly due to the influence of upstream restrictions on flow velocity and sediment transport dynamics before reaching the Herring Bridge. The echo returns from the imager determined the bottom composition to be soft throughout the approach and the scour.
River Levels: From Great Yarmouth to Norwich and even Hickling Broad, tide gauges have recorded river heights many times per hour for more than a decade. When we sift through this long-term record, it doesn't show any significant shifts in tidal patterns following the construction of the new bridge. Of course, environmental variables like rainfall and locked tides introduce complexity, but the underlying consistency of the tides is echoed by reports from those who live and work along the lower stretches of these rivers, who have not observed any fundamental changes.
Conclusions
The riverbanks at both the Haven and Herring Bridges are reinforced with piling that resists being worn away by river flow, but the riverbed is soft and can be significantly affected. Breydon, Haven and Herring Bridges all restrict the river channel to some degree, and scouring is observed at both Breydon and Haven Bridges, but it's clearly the Haven Bridge that narrows the river most significantly, has the deepest scouring and imposes the most substantial constraint on flow.
The absence of scouring on the soft riverbed at Herring Bridge indicates that the river is not compensating for any volume displacement caused by the narrowing channel at this point. Furthermore, the lack of visible turbulence, creasing, or eddies suggests that the bridge's support design avoids a significant increase in water flow at that part of the river channel.
This study by the Broads Society demonstrates how important it is to gather facts and study evidence before rushing to conclusions. Public attention was drawn to the Herring Bridge simply because it was new, but the Haven Bridge has constrained the river flow since it was built nearly a century ago.
To understand the impact of Great Yarmouth’s bridges, including the Herring Bridge, measurements and observations need to be continued regularly over time, but to find the cause of the recent winter floods greater focus should be given to climate-induced sea-level rise and high winter rainfall, and perhaps to flow bottlenecks at other points on the rivers.
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