This is a particularly important project relative to many of the DEF Water Groups aims and objectives associated with our concerns to maximise returning water to the aquifer whilst incorporating safeguards against pollution. The Highways Agency were in regular discussions with the Environment Agency as the detailed design of the drainage proposals for the widening scheme have been developed.
As there are very few natural outfalls for this drainage the design has therefore had to rely heavily on the use of balancing/settling ponds and soakaways. The motorway crosses the river Ver on its route between Markyate and Redbourn and its tributary, and the River Red, actually receives water from the settling ponds between Redbourn and The Aubreys hotel. A ditch was constructed with the original M1 in the 1958/9 and runs to an existing soakage pond along the disused Featherbed Lane to the east of the motorway. Hence the water going entering the new overflow facility, will also be returned to the groundwater. The overflow facility was provided for this pond because the specific geology in this location restricted the capacity of the new balancing pond and soakaways. Elsewhere on the scheme, the geology gives the soakaways higher capacity so all the runoff is returned to the ground via the soakaways around the pond.
Surface water run-off from all roads has been considered an environmental concern for many years since the invention of the internal combustion engine. Catchpits have been provided throughout the drainage system for the collectionof silt and Petrol/Oil Interceptors are being provided between the motorway and the balancing pond, together with a concrete line ditch with penstock on the outlet which is to provide containment of spillages in an emergency. The interceptor also has provision for collection of silt. These silt traps and interceptors will require periodic maintenance and emptying to ensure their optimal operation. The forebay will only require very occasional cleansing, hopefully.
The environmental aspects of the ponds have been developed in accordance with current best practice and the sediment forebay itself is left clear from planting as it acts as a settlement facility. A gabion wall spreads the flow across the width of the pond so there is no concentrated flow path from one end of the pond to another, but instead, a diffuse flow across the central planted area. The central area of the pond has an undulating bed and pools of different depths and will be planted with a range of aquatic species, including Phragmites Australis, which will provide vegetative treatment to the run-off. The outlet to the pond is designed to retain water at low levels for an extended period of around 48 hours to maximise the treatment being provided.
The following photographs show recently planted phragmites reeds to contribute natural filtering for the water before being received by the aquifer. The phragmites will slowly spread across the whole lagoon.
(NB information courtesy of the Highways Agency)