“The Leat” – Its significance to maintaining the established ecology of Bosham’s Central Watercourse. A Report to Chichester Harbour Conservancy By Dick Pratt, BA, MSc, FRGS. On behalf of Bosham Association August 2016 Bosham’s Central Watercourse This watercourse is responsible for draining central Bosham. When it has become blocked or overwhelmed, a part of Bosham Lane floods as can the area to the south including land now occupied by Oyster Mews. The catchment for this drainage system is land to the east and west of Delling Lane and the large field to the north-west of the Berkeley Arms (Green’s Field). The watercourse supports year round species specifically birds and reptiles. The natural flow is augmented by a flow through a leat, tapping the supply available from Bosham Stream, which is a diverted year-round chalk stream. When there has been no flow available from this field run-off and road drains, the Leat becomes vital in maintaining a water level capable of supporting the established ecology of the Central Watercourse. The Central Watercourse is categorised by the EA as a ‘main river’. Location of the Leat The Leat in question runs from Bosham Millstream at The Manse in Brook Avenue via culverts to the east into the garden of Leat House and thence via the old bakery (now called Loafers) along the north side of Bosham Lane to join with two drainage courses coming from the north and east. It then combines under Bosham Lane with the Central Watercourse and runs south via Benedict’s Pond and the Cemetery pond through a pipe under Canute Road to debouch to the sea via a pipe under Shore Road. Definition The term ‘leat’ dates in written form from 1642 and denotes “an open water course to conduct water for mills, mining works, etc” (Oxford Shorter English Dictionary 1983). Overview Watercourses through Bosham, as in so many other places, have been severely modified by human communities over centuries. These watercourses, whilst initially providing a water supply for a range of human requirements, have in their train created opportunities for nature, unforeseen hazards for humans and challenges for continuing management. Failure to manage them effectively on a day-to-day basis risks any or all of these historic tasks. The depth of water in the Bosham Millstream determines how much flow is released into the leats. Crucially a low level of water in Bosham Mill-stream will give lower flow in The Leat. Low or no-flow in The Leat has disastrous consequences for the ecological health of the Central Watercourse. Historical significance Because Bosham (Mill) Stream is itself a leat, diverting water from a chalk stream springing between Funtington and West Ashling from its shortest course debouching into Colner Creek, the stream runs down toward Bosham Quay on elevated and at times embanked sections. These embanked sections can be seen alongside Ratham Lane, behind the Manor House and lastly adjacent to Quay Meadow. The Conservation Area Character Appraisal (CACA) makes the following observation about Bosham Millstream There is some uncertainty about the date in which the mill leat (at Bosham Sailing Club) was built and the present Millstream may be no earlier than 18th century. However its location next to the church and Manor House is typical of medieval practice and it may be that its current course is merely an 18th century remodelling of an earlier leat. It is likely that the Romans would also have required a supply of fresh water so parts of the leat may even date back to their occupation. (CACA 2007, 2013, page 14) Equally, District Archaeologist, James Kenny has pointed out that following the Roman period, Irish monks were in residence in the seventh century and had pre-existing knowledge of capturing water supply for milling and fisheries. Several secondary leats draw water away from Bosham Stream to supply water to fields, ponds, areas where once stood orchards and the Manor House moat. Importantly Bosham Stream supplies two ancient millponds, one at Broadbridge Mill and one behind the Manor House and Church supplying Quay Mill (now occupied by Bosham Sailing Club). These millponds have associated sluices, by which water-flow to millraces would have been controlled as reservoirs for milling operations. The Domesday Book records 8 mills in the Manor of Bosham, but this would have included mills in West Ashling (1), Old Fishbourne (possibly 2), Ratham Lane (1 or 2), and Cut Mill (1). There is clear evidence surviving today of mills at Ratham Mill, Broadbridge Mill, Mill House and Quay Mill and these are all along Bosham Mill Stream. What controls the flow of Bosham Mill Stream? 1. Rainfall 2. Groundwater conditions 3. Ingress into the north end of the Millstream off Ratham Lane at a recently installed “Penstock”. 4. Leaks from the mill-stream into groundwater and neighbouring land 5. Through various historic leats, both east, but mainly west off the millstream itself 6. The active management or its absence of flows into mill-ponds 7. Outflow at the Bosham Quay Mill sluice – recently and competently replaced by Burhill Golf & Leisure (BGL) 1 and 2. Taking rainfall and groundwater conditions together, Bosham residents have noticed significant periods of downpour producing standing water and flooding periodically. This is not seasonally related and significant flooding of roads has occurred in summer months. Such were the problems in the 1990s that a report was commissioned by the Bosham Association and this was duly completed and published by Coombes, Price and Stern in May 2001. With the completion of the joint WSCC/EA ‘Operation Watershed’, these problems have been significantly reduced. However, it has left a legacy with the EA locally of being vigilant of flood threat, but largely unconcerned about drought conditions. The sketch map overleaf shows the course of the millstream (in dark blue) and significant drainage channels including direction of run-off, ditches, culverts and leats (light blue) as well as notable sites of overtopping or flooding (red-starred). It is locally noted that the only time that the millstream itself overtopped was in 1993. The EA has telemetric devices at the Millstream Hotel in Bosham Lane and under the A27 bridge off Ratham Lane. Low flow is not registered by the EA but causes Bosham Millstream to run very low indeed and the leats that take water from this stream then will run dry for long periods resulting in loss of habitat and displacement or loss of species. 3. Ingress into the north end of the Millstream off Ratham Lane at a recently installed “Penstock” – see photo right. This mechanism was installed and is controlled by the EA. typically should the EA judge that the Millstream could overtop, the sluice can be opened and the surplus flow released. This additional flow then is added to a continuous smaller flow that maintains a year-round flow to the west, following the course of the ‘natural’ river and debouching to Colner Creek through what were once water meadows south of the A259. Very occasionally, Bosham residents living close to Bosham Millstream have alerted the EA to low flow conditions, whereupon the EA has adjusted the penstock to allow more water into the Millstream system. However the ecosystem that is the first to become stressed is that along the Central Watercourse fed by the Leat. The EA’s telemetry does not alert the Agency to low- flow conditions and hence it has no routine monitoring of risks to habitat other than what is reported to it by environmentally aware residents. 4,5&6. Leaks from the stream into groundwater and neighbouring land and through various historic leats, mainly west off the millstream itself and the active management or its absence of flows into mill-ponds The observable leaks are indeed at those locations where leats have historically been controlled through small sluices set into abutments and which have degraded over time. The evidence that these flows were historically regulated for the benefit of maintaining depth of water in the millstream is as follows. It is within living memory that debouching leats at the site of Burnes shipyard and Mill Meadow (on the shore side of Second Meadow) would dry up every summer, but now they are running continuously all year round. The Manor House moat is fed by a leat from a managed sluice within the grounds. It debouches the excess water from the moat via a culvert at Street End. The Millpond to the west of the Manor House and the Millstream is fed by a leat at its north end and had a return sluice at its south end in the garden of Mill House. Hence, most of these sluices need attention so that the flow in the Millstream can be maintained in a way that continues to support the historic ecosystems. 7. Outflow at the Bosham Quay Mill sluice – recently and competently replaced by BGL. This is yet to be used in earnest having been commissioned in late 2015. The original mechanism would have controlled the flow of water over the millrace within the old mill, now occupied by Bosham Sailing Club. Photo left from CACA Page 6. Obviously regulating the most significant amount of flow in Bosham Millstream requires a combined approach to intervention at both the Penstock and the Sailing Club sluice. The Ecology of the Central Watercourse. Being subject to seasonal variation of flow, the ecology is consequently dependent. However, in period of drought, two significant ponds can dry out. These are locally known as Benedicts Pond and Cemetery Pond. When these ponds are dry, so too is the section south of the cemetery.