How the Mill Pond is Managed

The main weir is adjacent to Quay Mill at the end of South Street. This consists of a concrete weir wall (historically the concreted-up lock gate) with a crest height of 2.509m Above Ordnance Datum (A.O.D.). Set in the inner (Pond) face of this wall is a tide flap (invert level 0.160m A.O.D.) that will allow sea water to enter the Mill Pond at high tide through a 900mm diameter pipe. The tide flap is closed during the winter using timber boards on the outer (harbour) face of the wall.

The other outlet to this part of the Pond is the 900mm diameter turbine feed that runs under the mill building and discharges though the Quay wall. This is controlled by an automated penstock. The invert level of this outlet is 0.300m A.O.D.

Near Bath Road at the southern end of the Mill Pond is a more complex structure. A 900mm diameter pipe with an invert level of 1.008m A.O.D. passes through the Mill Pond wall. At the harbour end is a tide flap that prevents water flowing into the Mill Pond at high tide but permits water to drain at low tide. On the inner face of the Mill Pond wall is a penstock that can be closed manually to stop the flow of water at any time. Set inside the Mill Pond is a concrete weir wall with a crest height of 2.272m A.O.D. This weir wall is fitted with a tide flap on its inner (Pond) face and is fitted with chains and levers to allow it to be locked in the open position. When locked open, water will leave the Mill Pond at low tide and this allows an additional means of rapidly emptying the Pond at low tide should this be necessary.

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

Uncontrolled Flows

Water enters the Mill Pond from the West Brook at its northern end. This is a 'critical ordinary watercourse' 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. The entry to the Mill Pond is constrained by its passage through the Bridge Road culvert, with reported flooding events caused by the restriction of this culvert and blocking up of the grill over its entrance. The Council replaced the old double grill with a new structure in 2003, designed to avoid flooding if blockage occurs.

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 fill of the wall is extremely variable with some voids (since repaired) and fill comprising sands and gravels.

It is apparent that the public have two expectations with regard to the management of the water level in the Mill Pond these are:-

That a high water level will be maintained throughout the summer. Ideally there should be no mud exposed at the edges of the pond.
That the water level in the pond should be lowered at times when flooding might occur.

The predicted neap high tide will be 4.100m above Admiralty Chart Datum (A.C.D.) which is the equivalent of 1.370m A.O.D. High Barometric pressure can depress the tide level by up to 600 mm. (Good weather in summer is associated with high barometric pressure) . Thus there can be periods of days where the high tide level in the harbour will not replenish the water in the Mill Pond.

During the Summer the tide flap in the bottom of the main weir is allowed to operate with the stop boards removed. Water will thus flow into the Mill Pond whenever the level of the tide in the harbour is higher than the water level in the pond. Unfortunately during periods of neap tides little replenishment actually occurs and the water levels achieved are not sufficient to completely cover the mud banks.

Spring tides typically will reach a depth of 5.000m A.C.D. which is equivalent to a ground level of 2.270m A.O.D. Low barometric pressure will cause the tide to lift above its predicted height, since a difference in barometric pressure of 13 millibars will cause a change of approximately 100 mm in water level. Wind effects in the harbour may also cause water to surge at the top of the harbour.

It is believed that flooding generally occurs when the level of water in the harbour is higher than the level of the main weir and waves, at least, have been breaking over the promenade. In these conditions the Mill Pond effectively becomes part of the harbour and the Mill Pond water levels cease to be affected by the promenade wall, weirs etc. However flooding at Bridge Road in September 1995 and in September 2000 has shown that this is not always the case if the West Brook is in full stream - on this occasion there was no flooding in Bath Road or Bridgefoot Path adjoining the Mill Pond.

This combination is likely to give rise to flooding due to surface water run-off in and adjoining the areas of Bridge Road, Bath Road, Bridgefoot Path and Victoria Road, with extensive flooding due to the tidal surge affecting the same areas together with South Street and the A259 Havant Road. All these areas become impassable to traffic.

	Flooding at Bridge Road in September 2000.

Certain low lying properties within the Mill Pond or West Brook flood plain, together with those associated with the Mill Pond to the east of Emsworth and the River Ems in West Sussex, are therefore at risk of flooding and their owners and occupiers should pay particular attention to the weather forecasts and especially over the five day period at each Spring Tide, and take suitable steps to avoid flooding of their premises and damage to their property. Residents should register with the Environment Agency's Floodline service (0845 988 1188) to receive automated Flood Warnings.

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