How is Water Level Controlled? | Uncontrolled Flows | Water Level Management | Flooding   

 

Questions about the operation of the Mill Pond, including water level management, should be directed to the Environment Agency on 08708 506506. Havant Borough Council can help with questions about maintenance of the promenade, and about events held on or next to the Pond - call Customer Services on 023 9244 6019. The Opening Schedule for the Mill Pond (dates when it is opened to allow tidal change of water) is available through this link >>.

 

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Emsworth Mill Pond was originally built to serve the Quay Mill at the junction of Bridgefoot Path and South Street. The Quay Mill and its associated land are now used as a sailing club. The Mill Pond and its promenade wall, together with Bath Road and Bridgefoot Path, form an area of recreation and amenity used by both the sailing and non sailing communities.

 

 

Before the Mill Pond wall was constructed in the 18th century, the area presently known as the Mill Pond was a tidal creek similar to the 'rythes' on Hayling island. When the Mill Pond was first enclosed, its tidal nature would have been largely retained. Wooden gates next to the Quay Mill opened at high tide to let the Pond fill, and let vessels into the pond which loaded or emptied cargoes from the malthouse.

 

Emsworth Quay Mill - the bridge spans the old lock, the original entrance to the Mill Pond.

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The water level in the pond would have fallen quite rapidly when the mill was working since the impounded water was released to drive the turbine under the mill building.

 

In 1925 the Mill Pond was purchased by Warblington District Council as it was no longer needed to supply the motive power for the mill. At this time the earth bank that impounded the pond was improved, stone facings were constructed and it was surfaced to form a promenade. The Mill Pond was used as a swimming pool and there was a bathing hut on the Promenade near to the mill building, with a short bridge ending in a ladder used by bathers to enter the water. Bathing fell out of fashion when it was found that the water in the Mill Pond was brackish due to the flow of fresh water from the West Brook.

 

Mill Pond looking south at Nile Street, with Bridgefoot Path on the left.[Click on the image to view full size (61 kb ) - use your browser's 'Back' button to return to this page.]

The Mill Pond is now administered by Havant Borough Council as a local amenity, with the Mill Pond wall used as a promenade for walkers, forming as it does part of the Solent Way and Wayfarers Walk long distance footpaths. The water level is managed by the Environment Agency (the Pond is classified as 'main river') and is kept in the Pond at low tide to avoid the exposure of unsightly mud banks (bearing in mind the extensive mudflats in the harbour outside the Mill Pond at low tide, smell is a minor consideration).  On a regular basis (typically every second or third Spring tide cycle, apart from the summer holiday period) the control structures are opened for a few days to allow the Pond water to rise and fall tidally, so as to avoid it becoming brackish (Schedule here >>).  This also allows the edges of the Pond to be litter picked. During the July 2005 draining, a more intense clean of the Mill Pond basin was carried out by hovercraft - see pictures of this operation from the links below.

 

Hovercraft carrying out litter picking and heavier clean in the drained Mill Pond, 21st July 2005.

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Rubbish removed from the Mill Pond during the cleansing operation by hovercraft in July 2005.

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Water level in the Pond is controlled by a mixture of 'fixed' and 'dynamic' controls.

 

The level of the promenade is approximately 3.000 m AOD , but does not act as a sea-defence structure.

 

The main weir is adjacent to Quay Mill at the end of South Street. This consists of a concrete weir wall (actually the concreted-up lock gate) with a crest height of 2.509 m Above Ordnance Datum (AOD). Set in the inner (Pond) face of this wall is a tide flap (invert level 0.160 m AOD ) that allows sea water to enter the Mill Pond whenever water level in the Harbour (that is, around high tide) is higher than whatever water level is in the Pond. Water enters through a 900 mm diameter pipe. The tide flap is sealed during the winter using timber boards on the outer (harbour) face of the wall to reduce the amount of water entering the Pond on the higher tides.

 

With the installation of the new automated controls (see below) it has been decided to seal the flap earlier than normal to allow monitoring and 'fine tuning' of the new water management regime to be carried out. It is possible, given the automated controls, that the flap valve may no longer be required - a decision will be made later.

 

A 900 mm diameter turbine feed runs under the Quay Mill building and discharges though the Quay wall. This is controlled by an automated penstock. The invert level of this outlet is 0.300 m AOD .

 

At the southern end of the Mill Pond, near Bath Road, is a more complex structure. A 900 mm diameter pipe with an invert level of 1.008 m AOD passes through the Mill Pond wall. At the harbour end is a tide flap that prevents water entering the Mill Pond, but allows water to drain at low tide. On the inner face of the Mill Pond wall is an automated penstock (the 'wall penstock') that can be closed to stop the flow of water at any time. Set inside the Mill Pond is a concrete weir wall with a crest height of 2.272 m   AOD. This weir wall has another automated penstock (the 'weir penstock') on its inner (Pond) face which is usually in the closed position.

 

Normal summer operation will be for the wall penstock to be open, but the weir penstock to be closed. This will allow water to leave the Mill Pond if its water level is above 2.272 m AOD and if water level in the open harbour is lower than this.

 

When both penstocks are opened, water will leave the Mill Pond at low tide and this allows an additional means of quickly emptying the Pond at low tide if this is necessary.

 

The following diagram shows the three modes of operation at Bath Road (southern end of the Pond).

 

 

 

Water enters the Mill Pond from the West Brook at its northern end. This is a 'Main River' draining an area to the north of Emsworth. It reacts in a "flashy" manner to storm events within the catchment and may go from base flow to flood flow in the space of 30 minutes. Its entry to the Mill Pond is limited by the size of the culvert from Bridge Road, with reported flooding events caused by the restriction of this culvert and blocking up of the grill over its entrance by floating vegetation. The Council replaced the old double grill with a new structure in 2003, designed to avoid flooding if blockage occurs, and the Environment Agency further improved the performance of this critical infrastructure in 2007, adding telemetry which allows remote monitoring of water levels in the Pond and the Brook.

 

Water leaks out of the Mill Pond through the gravel beds under the Mill Pond promenade wall. Piling has been placed to the seaward of the promenade wall in the 1970s but does not completely cut off this flow.

 

Water leaks out through the structure of the Mill Pond promenade wall. A radar survey in 1995/6 revealed that the filling of the wall is extremely variable with some spaces (since repaired), and fill comprising sands and gravels.

 

Water also leaks into and out of the surface water sewer running north to south under the bed of the northern part of the Mill Pond.

 

 

Please note that management of the water level in the Mill Pond is the responsibility of the Environment Agency. Havant Borough Council plays no part in controlling water levels in the Pond.

 

The public generally have two expectations regarding the management of water levels in the Mill Pond. These are, firstly, that a high water level will be maintained throughout the summer, with no mud showing around the edges of the Pond. Secondly, an expectation that the water level in the Pond should be lowered at times when flooding might occur.