Development of initial options
The consultants identified opportunities to improve the management of flood risk within the Park and focussed on improving the hydraulic conductivity of the Brook by altering or removing the culvert in which it flows. A new channel could replace the two sharp angles that the culvert currently includes, to enable more efficient flows through the P, and by providing greater capacity within in the channel help mitigate ‘the area’s typical high intensity, short duration rainfall events.’
‘Any opportunity to restore Norbury Brook should not be considered solely on the basis of flood risk reduction but also on the advantages and justification for the improvement of the habitat, ecology and amenity of the park. Any options for further consideration will need to accommodate these criteria, as communicated in the project’s objectives in Section 1 of this report.’
The consultants key finding was that there was ‘potentially’ an ‘ insignificant ratio of flood damage reduction …. compared with any potential scheme cost’ and would not meet the then current benefit cost ratio of 5:1 required as a minimum. ‘To this end, it is clear that any range of options will need to incorporate features that provide more than just a reduction of flooding benefit to the wider community in order to obtain sufficient funding from other sources.’
The consultants ruled out the option to implement a large flood storage area across Norbury Park because it will:
- be heavily constrained by the nature of the flows in the Brook
- be ‘prohibitively expensive’
- be ‘unlikely to benefit a sufficient number of properties to be economically justifiable.’
- comprise the provision of a sustainable environmental and amenity improvement to the area.
The consultants therefore recommended that ‘in recognition of the potential for flood storage, that any new channel proposed should include a sufficient cross section that increases the capacity of the brook. Any excavated material should be utilised to provide a bund at the lower western end of the park to ensure flood water is routed back into the channel where possible.’ (p.15)
1 No active intervention
2 Minimal intervention
3 Remove top section of culvert
4 Replace culvert with new straight channel
5 Replace culvert with new meandering channel (150m short stretch)
6 Replace culvert with new meandering channel (200m medium stretch)
7 Replace culvert with new meandering channel (300m long stretch)
8 Channel improvements in the downstream allotments
9 Creation of a new channel in the upstream allotments
10 Works within the existing channel in the upstream allotments
11 Works to enhance the flood storage capacity of the park
The consultants costed the three options they most favoured: Option 5: £180,000; 6: £165,000; and 7: £195,000.
‘The construction and maintenance costs associated with each option are likely to be within a similar range. The environmental enhancement opportunities are, however, maximised when channel length is increased.’
As ‘Option 7 has the lowest cost per metre of enhanced river’, it ‘provides the greatest opportunities for enhancement (when assessed against the key project objectives) for the lowest proportional expenditure.’
The consultants therefore recommended that Option 7 be adopted as the preferred option.
They also suggested that three of the additional scheme options offered ‘considerable opportunities to deliver the key project objectives’:
- Option 8: Channel improvements in the downstream allotments
- Option 10: Works within the existing channel in the upstream allotments
- Option 11: Works to enhance the flood storage capacity of the park and prevent any increase in flood risk from surface water
They recommended that these Options be considered in addition to 7.
The Preferred Option – No. 7
The consultants explain the advantages of Option 7.
Morphology: ‘The preferred option will consist of a new meandering channel between the downstream end of the allotments and the upstream end of the culvert in the north western corner of the park. The channel will have a multi stage design, with a small low flow channel and a series of low riparian units which will be inundated during periods of higher flow. The channel will meander within the confines of a wider bank line. The channel profile and bank line will be varied and non-uniform along the length of the channel. The channel will have a gravel bed, and, if possible, incorporate natural pool and riffle sequences. This will reflect the channel configuration found in many natural river channels, and provide opportunities for the development of a range of in-channel and riparian habitats.’
Biodiversity: ‘As described above, the preferred option will incorporate a wide variety of morphological conditions. This will allow a range of in-channel and riparian habitats to develop in the river corridor. In addition, this option also provides the opportunity for the establishment of BAP priority reed bed and standing water habitats within the park, potentially in the form of riparian reed fringes and off line ponds.
Amenity: ‘The preferred option will provide an area of open water and improved habitat, which will improve the amenity value of Norbury Park. In addition, the river banks will, in places, be gently profiled in order to promote public interaction with the river channel and encourage “natural play”. Public access across the park will be maintained by the provision of one or more bridges across the new open channel.’
Flood risk: ‘The preferred option will provide a new channel with a capacity at least equal to that of the existing culvert and concrete channel. It will not, therefore, increase flood risk to Norbury Park and surrounding properties. Furthermore, the multi-stage channel design will provide opportunities for additional flood storage within the river corridor.’
Policy delivery: ‘The preferred option will provide opportunities to contribute towards the delivery of London river restoration policies by:
- Creating a 350m natural river channel (London Plan, South London River Restoration Strategy);
- Enhancing riparian biodiversity and providing the opportunity for the development of BAP habitats (London Plan, London BAP, London HAPs);
- Improving the natural environment and encouraging people to explore the recreational value of open water (London Plan, Improving Londoner’s Access to Nature, Creating a Better Thames); and
- Removing a culvert and helping to reduce flood risk (South London River Restoration Strategy, Thames CFMP, Croydon CFMP).
- WFD: The preferred option will provide a range of new morphological and habitat conditions. It therefore offers the possibility to improve the ecological potential of Norbury Brook. (p.38)
The additional options
Option 8: ‘Channel improvements in the downstream allotments. These will consist of debris clearance works in the river channel, removal of broken bank protection, and targeted clearance of scrub vegetation. This will improve the morphology of the river and allow natural habitats to develop, without impinging on the adjacent land use.’
Option 10: ‘Works within the existing channel in the upstream allotments. A number of flow deflectors will be installed within the existing channel, to increase flow diversity and allow sediment to build up naturally. This will promote the development of in-channel habitats.’
Option 11: ‘Works to enhance the flood storage capacity of the park. A low bund, with an indicative height of 1 m and width of 10 m will be constructed around the edge of the park, using material excavated during the construction of the new channel. This will help to contain any surface flood waters that leave the confines of the open channel.’ (p. 39)
The preferred option will contribute towards a considerable increase in the morphological, biodiversity and amenity value of the park and as such will help to fulfil the following key aims of the grant scheme:
- Increase the number and diversity of people benefiting from the natural environment;
- Allow people to learn about the natural environment; and
- Increase the number of wildlife-rich natural places that are sustainably managed and meet the needs of the local community. (p. 49)
‘The culvert in Norbury Park should be replaced with an open, meandering channel. This channel should follow a multi stage design, with a small channel to contain low flows and adjacent riparian units which can become inundated during periods of higher flow. The channel should meander within the confines of a larger bank line.’
As explained in Part 1 the above is a summary of a 2008 study.
Part 1 explains what the Environment Agency is now doing
to progress options towards an implementable scheme.