Western Rother Fish Pass North Mill Gates, Midhurst Site Visit and Field

Western Rother Fish Pass North Mill Gates, Midhurst Site Visit and Field Study The key aspect of this project is to look for opportunities to improve fish passage given the issues and constraints on the site. The main questions that you need to address are: 1) The current fish passage is not adequate so how could you design-in improved fish passage? - You will need to illustrate your answer with some sketch diagrams? 2) What are the implications of your design in terms of future maintenance, sedimentation, hydrology, flood risk, aesthetics , current drainage, any issues related to structural stability and cost? 3) See also questions on sediment in section 4 What additional data would you need to collect? The following information provides some additional data that may help you to define your ideas. See also maps of the site in section 5 1. Background The site is located on the western part of the River Rother at the mill structure at Midhurst where the River Rother flows under the A272, SU 88881 22086. There is a long term issue with fish passage for both salmonids and coarse fish over the structures associated with North Mill weir at Midhurst. The tilting gate fish pass on North Mill weir consists of a Denil design on the downstream face of a tilting weir. There is a notched pool at the base of the fish pass. Flood risk is an important consideration at this location as there are two towns, Midhurst and Easebourne, close to the river. Modifications to the watercourse to improve fish passage might alter the volume of water passing down the channel and might also affect sediment transport and deposition rates and location. To ensure that flood risk is not increased any suggested works will probably require a hydraulic model of the proposed options and modifications. 2. Current data available There are two locations where fish surveys have been undertaken: Upstream located at Woolbeding Bridge approx 3km upstream of North Mill, surveyed 2002 to 2008. Downstream at Cowdray (Moor Farm) approx 4km downstream surveyed 2002 to 2007 except 2004. The closest gauging stations, all continuous flow measuring stations, are: Iping weir upstream around 7.5km away. Fittleworth Mill downstream approx 17km however data at this site is somewhat variable and The more reliable gauging may be at Hardham around 20km away. These are all continuous flow measuring stations. The structure at North Mill is managed by the EA. It is allegedly an automatic gate but it is unclear how reliably it works and it may be manually adjusted at times of high flow. There appears to be no survey or model data but LiDAR data should be available. 3. Current fish pass structure and EA recommendations for fish passage gradients The structure of the main fish pass itself is estimated to be slightly steeper than the maximum 1 in 4 gradient recommended for a Denil channel fish pass, particularly when upstream river levels are high. There is also a large drop (traverse) between the notched pool at the bottom of the weir and the river below, particularly at times of low flow. It is recommended that the traverse in a pool and traverse fish pass should not exceed 0.45m, whereas the height of the pool at North Mill is estimated to be around 1m with a fall of about 1:3.5. It is therefore likely that the traverse at North Mill is too large for fish to enter the bottom of the fish pass and gain access to the main fish pass itself, particularly at low flows. When flows are higher and the traverse height is reduced, the increased volume of water cascading over the rim of the notched pool might further impede fish passage (Figure 1). Where an artificial river channel is constructed as a fish pass, the Environment Agency state that typical gradients should range from 1% to 3%, with gradients of up to 5% being acceptable if energy-dissipating structures such as rock ramps or baffles are built into the channel. Such shallow gradients will allow the passage of juvenile and smaller coarse fish, for which other types of fish pass may be unsuitable. This recommendation should be taken into account when planning the modifications Figure 1. The notched pool at the base of the fish pass 4. Sand deposition and its implications for flood risk There is an issue with sand deposition below North Mill weir at the location of the A272 road bridge, where large sand bars build up and these sand deposits can completely block the left hand road bridge arch through which the river used to flow (Figure 2). It is thought that this blockage has resulted partly from a change in sediment maintenance and it is understood that the sand is transported from upstream sections of the river. Questions to think about: 1. Do you think there are catchment wide issues that may be having an impact on sediment loading? 2. Do you think that the current deposition pattern is having a major impact on flood risk? 3. Can you think of any options to reduce the sediment impact if you feel this is necessary? 4. What additional data do you think you may need to confirm the answers to the above? ( note the data currently available – see above Figure 2a and 2b. Sand deposition prevents the river from flowing through the A272 road bridge arch 5. MAPS OF SITE .

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