- 12th October 2018
- Posted by: Sophie Bunker
- Category: Projects
The Institute of Fisheries Management (IFM) annual conference takes place next week and will focus on the theme of ‘Thriving or Surviving – Creating Resilient Fisheries.’ Jon Whitmore, Lead in Hydroecology, and Ryan Jennings, Assistant Analyst, will be there highlighting our poster on the project outlined below – Evaluating and delivering fish passage improvement measures at tidal pointing doors on the River Ancholme at South Ferriby.
JBA Bentley was appointed to investigate the impact of a trial soft closure mechanism (SCM) on the ability of eels to migrate upstream past the existing pointing door arrangement on the River Ancholme at South Ferriby Complex. There are two key barriers to fish passage from the River Humber into the River Ancholme at South Ferriby. These include:
- Tidal timber pointing doors, which generally close on a rising tide and open on a falling tide
- Guillotine sluice gates, which can be mechanically opened and closed to control upstream water levels for navigation and flood risk management purposes.
The first investigation involved installing time-lapse cameras facing the pointing doors on the upstream and downstream side. These cameras were set to record the closure of the pointing doors on the River Ancholme during neap and spring tide events.
Photographs were taken at 10-second intervals to determine the duration of the closure event. The duration of the closure event was determined to be the difference between the time prior to any obvious commencement of closure and the next photograph showing the pointing doors in a closed position.
The results show that the pointing doors closures events varied between 40 seconds and three minutes depending on the speed of the tidal rise. The SCM was activated for less than 30 seconds during each closure event.
Anecdotal observations indicate that the SCM reduces the ferocity of pointing door closure; this may have benefits from the perspective of the longevity of the pointing doors themselves.
Baseline Passability Modelling
To give context to the SCM monitoring results, upstream and downstream stage and sluice gate position data was obtained from the Environment Agency (EA) and a desk-based assessment of the baseline peak velocities beneath the sluice gates on the Ancholme was undertaken.
An ISIS model was constructed of Sluice Gate 1 to determine the discharge passing under the gate at a range of sluice gate opening scenarios. The model included the sluice gate and the pointing doors (modelled as an orifice to prevent backflow). The upstream and downstream boundaries of the model were controlled by the water level data supplied by the EA, as was the position of the sluice gate.
The velocity of water passing beneath the sluice gates was then calculated using the discharge outputs from the model and geometry of the structure, taking account of the position of the sluice gate.
Using SWIMIT v3.1, estimates of swimming speed of an assumed 7cm long juvenile eel were derived. The median burst speed was estimated at 0.46m/s and the median sustained swimming speed estimated at 0.072m/s.
A more theoretical approach to the impacts of the SCM was also undertaken. This was done by estimating discharges below which passable velocities occur for a range of sluice gate openings and pointing door gapes at Humber stages that are more representative of water levels observed during closure events.
Use of time lapse cameras to quantify the duration of closure event (between 40 seconds and three minutes) and SCM deployment (up to 30 seconds) worked well. It also limited the need to deploy staff in exposed locations for any length of time. Without a measurable reference however, they could not be used to capture speed of closure in detail.
The pointing door and sluice arrangement is only effectively passable by juvenile eels migrating upstream from the Humber for a very small proportion of the time. It worked out at less than 0.004% of the available data points for Sluice Gate 1 from 2012 to 2018 when the sustained swimming speed threshold is used, and rising tides only are considered.
Back calculation of passable velocities using a range of structural arrangement details allowed the production of theoretical operating rules which could be used to revise structure operation to maximise the passage window for juvenile eels.