Birch Hall Park Pond Project

The project works have progressed well and, as of the 29 September, the works are complete.

• The haul track has been constructed from the entrance down to the pond
• The spillway has been built
• The access track along the dam crest is completed
• Ditch clearance works on Loughton Lane (a planning condition) have been completed
• The works to install leaky dams have now all been completed

Birch Hall Park is located in the northeast of Epping Forest in the lands to the south of Coppice Row, and west of Loughton Lane, near Theydon Bois, CM16 7DR. It is in Theydon Bois ward of Epping Forest District Council.

Birch Hall Park pond is approximately 9,800m2 in area. The pond is fed by natural surface runoff from a catchment of approximately 8.05 hectares to the general west of the pond.

Works are proposed to the pond (and its access route) within Birch Hall Park, which is also known locally as the Deer Sanctuary.

A map is available at the bottom of this page.

The pond forms part of a designed parkland landscape which was once connected with Birch Hall. The pond stores water by impounding the natural topography with an earth dam which is thought to be over 100 years old.

The Parkland was acquired in 1959 as Epping Forest’s first element of Buffer Land to provide a sanctuary for Epping Forest’s melanistic (dark coated) Fallow Deer.

As part of the City of London Corporation’s regular reservoir inspection programme, the need for works to stabilise the existing dam structure and to provide more formalised overflow provisions were identified.

It has been found that the earth dam structure has areas of significant internal leakage. There is also a risk that when the pond is full there is the possibility of the water “overtopping” the dam in an uncontrolled manner especially, if the water inflow during storm events exceeds the capacity of the current outfall pipe (which is of limited capacity). If left, both internal leakage of the dam or external uncontrolled overtopping could potentially lead to significant erosion and possible collapse of the dam which would result in the sudden escape of large volumes of water, which would cascade towards Loughton Lane.

There are a number of measures proposed that will ensure the pond safely conveys storm water flow in a more controlled manner and to stop it leaking. These are detailed below. A map is also available at the bottom of this page.

Construction of a reinforced grass / concrete block spillway to the earth embankment dam

This would allow for controlled release of excessive water in the pond, allowing for storm events, without overtopping the dam in other areas. The spillway will be in the northern area of the dam, close to the existing outfall pipe.

A permanent lowering of water levels in the lake, to mitigate leakage issues

Monitoring over several years has indicated that the dam currently leaks when the water level is above 69.50m Above Ordnance Datum (AOD), which is approximately 300mm below the current outfall pipe invert level. Investigations have proved unsuccessful in identifying the precise source of the leak, but it is thought to be caused by decayed tree roots providing leakage paths in the top 900mm of the dam embankment.

If left, the leak could cause internal erosion of the embankment, and in the worst case, possible failure of the dam. To mitigate this, we will lower the water level to 69.50m AOD (the level at which the water stabilises after heavy rainfall and stops leaking) by setting the new spillway at this level

Re-grading the varying dam crest levels to a common datum and constructing a gravel emergency access track along the dam crest for construction and future maintenance vehicles

This will provide a common “freeboard” of approximately 900mm between the crest of the dam and the spillway invert level, which will prevent overtopping of the dam during storm events, as the flow passes through the new spillway. The dam embankment is approximately 180m in length, and over this length the level currently varies by 40cm.

The gravel access track along the dam will allow maintenance plant access all year round which will facilitate any access that may be needed in the event of a potential emergency or extreme weather event. Currently vehicles can only access the dam in the drier months of the year.

Construction of an approximately 700m long gravel haul track through the site, for the purposes of construction access and future maintenance access to the Pond and the dam

Currently, the dam can only be accessed for maintenance in the summer months of the year, which has significant risks if access to the dam is needed during an emergency which occurs in the wetter months. The haul track has been sympathetically designed to follow the contour of the land from the entrance of the Sanctuary to the Pond, as far as possible, to minimise the visual impact it has in the site.

Planning consent was granted from Epping Forest District Council in February 2021.

The construction works have been tendered and we have selected a Kenson Highways as contractor.

The works will start in early August and be completed by the end of September 2022.