Lower Itchen Options Appraisal

Description

The River Itchen SSSI/SAC is an internationally renowned chalk stream, and despite being highly designated, is in unfavourable condition. One reason being the condition of the physical channel, and another linked to the status of the Itchen Atlantic salmon population, now deemed to be in crisis and at high risk of extinction. The Lower Itchen, at Woodmill, Southampton, has been significantly modified historically to manage water for navigation, milling, and fishing activities. As a result of this, there are several water control structures: a sluice gate, 2 penstocks, and a sea door that are near the end of their design-life, and a technical fish pass (known as a Denil plain baffle fish pass). In addition, upstream, the true right bank, retaining the river in an artificial, perched state through Riverside Park, is increasingly permeable and has developed large breaches. This allows a significant proportion of the river to drain from its main (artificial) channel to follow a more natural course through the floodplain. There is an increasing risk that the whole river will continue to change course over time, and this will be expediated through bank failure. There are multiple partners with both interests and legal requirements to protect, restore & enhance, and increase the resilience of the designated features of the Lower Itchen. Southampton City Council (SCC) own the structures, and a large extent of adjacent wetland and parkland; and they own Fishing Rights on the river from the Salmon Pool to Gaters Mill. SCC lease some of the wetland area (including a lake) to Active Nation for educational, water sports and fishing activities. The Environment Agency (EA) has an interest in the management and condition of the river, particularly regarding abstraction, flood risk, biodiversity, water quality and fish passage. Southern Water (SW) have an interest in the availability of water for abstraction, particularly during low flow and drought conditions. SW have a requirement to put in place a package of mitigation measures to offset environmental impacts from abstraction pursuant to a Lower Itchen Drought Order. Portsmouth Water also abstract for public water supply at Gaters Mill, upstream of the A27. There have been several technical studies undertaken to date, including a Woodmill Project (Options Appraisal) study, 2016 (Appendix B), which primarily focused on easing fish passage. However, several changes since this date, for example, the Monks Brook weir (a major impediment to fish passage) has collapsed; 50% of flow is now going through the main breach (on the true right hand bank); sea level rise predictions have changed, the status of Itchen Salmon has deteriorated; climate-change impacts are increasingly evident through storms, surges, flooding, low flows and drought; and the number of small breaches in the true right bank are increasing in size.