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Hermitage Stream Catchment

TOPOGRAPHY - The Hermitage Stream catchment of 19 square km comprises mainly Tertiary deposits of London Clay and Reading Beds with some Upper Chalk outcropping on the coast. The area extends from the Queens Inclosure and Havant Thicket in the north to Langstone Harbour in the south. The Hermitage Stream catchment includes three branches, being the Park Lane Stream, the Riders Lane Stream, and the geographically separate Brookside Road stream at Bedhampton (which serves the eastern end of Portsdown Hill).

HERMITAGE STREAM - The head of the main river is at the point where the Stream leaves the Queens Inclosure in Cowplain. This heavily wooded area has altered little in the last three decades except for its increased usage by the public as a recreation and amenity area.

The stream runs from east to west along the southern edge of the Inclosure and acts as a natural barrier between the woodland and the developed areas to the south. Where it turns south at Highfield Avenue it is joined by a minor watercourse from Park Wood. Continuing southwards, the watercourses runs through a natural steep sided valley leading to the Hulbert Road, partly culverted beneath the dual carriageway section of this major transport corridor. Approximately 80% of this area is now developed and almost all of the existing woodland has been lost. There are various groundwater and land drainage problems of a minor nature associated with the local geology, terrain, and interruption of the natural land drainage due to the development of the area which gives rise to flooding from springs and groundwater movement.

An improvement scheme up to Ramsdale Avenue in Leigh Park was completed in 1977 to allow for an in bank flow of up to 24 cumecs (at the down stream end). The improvement scheme consisted of the construction of some 820 metres of earth channel, 3.2 kilometres of concrete lined channel with bank revetment in concrete blocks, various alterations and additions to highway and private bridges and structures, and the provision of an additional culvert under the Portsmouth - Havant railway line. The channel is designed to take predicted 1 in 50 year catchment flows. The lower reaches (below Barncroft Way) are also designed to accept a transferred flow of up to 3 cumecs from the adjoining catchment's main river, the Lavant, via the pipe link from Crossland Drive.

The EA recalculated the Hermitage Stream catchment in 1998, and the results were used when redesigning the central reach of the river through Leigh Park (between the Riders Lane stream confluence and Middle Park Way) as a natural open channel. This was completed in 1999 in a project jointly funded by HBC and the EA. Information about the restoration work can be found on the School resources page. To give early warning of potential flood events caused by blocked / obstructed river grillages, the EA monitors water levels upstream of New Road bridge.

PARK LANE STREAM - This main river originally had its source at Padnell Cuts Wood, now the Hazleton Way Estate. The stream passes through a natural valley running under the A3M and south to Blendworth Common and The Warren, from where it is enmained. Its lower reaches are culverted through St. Clares Avenue open space, Sunwood Road and Overton Crescent before issuing into the Hermitage Stream just downstream of Middle Park Way. The watercourse drains the housing conurbation of the Warren and the western area of Leigh Park.

HBC purchased St. Clares open space (which includes the Warren) from Portsmouth City Council in April 1992 and so became the responsible landowner of the watercourse between Calshot Road and the Warren Dam. This transferred to the EA upon the enmainment of the Stream in 2005. They carry out a programme of planned maintenance to meet its land drainage obligations to provide for the free flow of water in the stream and to clear the grill protecting the entrance to the Dam.

Giant Steps Area

At the foot of the open section lies the 'Giant Steps' area, named after the now-removed Warren Dam. This was a set of weirs with a flow control penstock. This structure was removed by the Environment Agency in 2009, although the replacement land form and grillage arrangements provide an equivalent level of flood protection. The culverted section of watercourse downstream of the Dam remains in the ownership of Portsmouth City Council, until it reaches Middle Park Way and ultimately outfalls to the Hermitage Stream.

RESERVOIRS ACT 1975 - THE WARREN DAM – This structure was designed to control the discharge of flood waters from the northern part of the catchment into the culverted section south of St Clares open space, and comprised an earth embankment with weirs, through which passed a culvert controlled by a penstock gate. Due to the amount of water it could impound, annual inspections are a requirement under Section 10 of the Reservoirs Act 1975.

Due to considerable vandalism and dereliction to the weir during the late 1990’s, the penstock winding gear was been removed and the penstock itself was fixed half open to control the down stream flow. Maintenance work in 2002 and 2003 resulted in an easier to maintain structure. Inspection of the dam passed to the EA upon enmainment in 2005, and they brought forward proposals to remove it and replace it with a much more open landform, whilst still providing the same flood protection to neighbouring properties.

RIDERS LANE STREAM- This main river has its head in a fan of minor issues between the Avenue, Longwood and Furzy Plain within Havant Thicket, and Hammond Hands Copse to the east of Leigh Park Lake (Staunton Park). The two main arms of these sub-tributaries join just north of Middle Park Way (from where the main river designation starts, since 2005) flowing south and west through Great Copse to Dunsbury Way, then south and east via Riders Lane / Purbrook Way to join the Hermitage Stream at Corhampton Crescent.

There are persistent problems associated with the grills protecting the culverts under Middle Park Way, Dunsbury Way and Purbrook Way which require a programme of regular cleansing by the Environment Agency. This problem is especially acute in the autumn if the heavy leaf fall associated with the indigenous hard wood trees is accompanied by periods of heavy rainfall. In particular the blinding of the triple grills protecting the triple 24" diameter culverts at Purbrook Way causes flooding of the adjoining highway, even though a realignment of the stream including a flood retention area were provided north of Purbrook Way at Riders Lane open space in 1983. A possible long-term solution to this problem could be to increase the capacity of the culvert under Purbrook Way to a size which would include for the removal of the grills. This would have to be carried out along with improvements to the down stream section of the Riders Lane Stream between Purbrook Way and the Hermitage Stream to ensure that there would be adequate provision for any increased flow.

Flooding can also occur upstream of the brick arched bridges in the northern part of Great Copse and adjoining Dunsbury Way (the latter causing overtopping and closure of Dunsbury Way on a number of occasions on 2000/2001), and at the southern end of Great Copse where an undersized culvert provides a vehicular and pedestrian access across the stream from High Lawn Way. The headwaters of this Stream are likely to be incorporated into the proposed Havant Thicket Reservoir currently being promoted by Portsmouth Water Company on land north of Middle Park Way.

BROOKSIDE ROAD STREAM - This main river has its source in the syncline valley to the north east of Portsdown Hill in an area of Upper Chalk deposits the remainder being predominantly London Clay. The open downland falls steeply to the east crossing under the A3M - where the main river designation starts - and on to Brooklands Road where it enters culverts which outfall at Brookside Road. The original stream no longer exists between Brooklands Road and Brookside Road, having been enlarged and piped to deal with the increased flows generated by the urbanisation of the area. Although the original pipe was designed to take the combined flows of both the adjoining housing conurbation and the stream it was necessary to duplicate it for part of its length in an attempt to alleviate flooding to property in Brooklands Road and Brookside Road. From Brookside Road the stream continues in open channel south under Bidbury Lane and then separates into two branches as it crosses open land, one being the original stream and the other a man made irrigation ditch controlled by a private sluice gate. Rejoining north of the railway the stream is culverted under the railway and Mill Lane issuing in Portsmouth Water Company land where it joins the Hermitage Stream at the Old Mill Dam.

Images and text courtesy of Havant Borough Council
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