Representation by East Suffolk Internal Drainage Board (East Suffolk Internal Drainage Board)
- Date submitted
- 30 September 2020
- Submitted by
- Non-statutory organisations
This representation is written on behalf of East Suffolk Internal Drainage Board (ESIDB), a Member Board of the Water Management Alliance. The development area is partially within the Internal Drainage District (IDD) of the East Suffolk IDB and the site is wholly within the watershed catchment of the aforementioned and the Board are the regulator for several elements of the proposed works which require consent as per the Land Drainage Act (including the Board’s Byelaws). The Board are concerned by the possible impacts the Sizewell C project may have on flood risk both within the Internal Drainage District (IDD) and the wider watershed catchment area. The Board is therefore an interested party due to the potential impact of the project on the Board’s ability to carry out its statutory functions. The main issues the Board wishes to highlight can be summarised as follows: 1. Watercourse Diversion The project proposes to realign two ordinary watercourses known to the Board as Leiston 2 and Leiston 1 (DRN163G0202 and DRN163G0201) but collectively referred to in the submitted documents as the “Sizewell Drain”. These watercourses are both designated by the Board as ‘Adopted Watercourses’. The status of an ‘Adopted Watercourse’ is an acknowledgement by the IDB that the watercourse is of arterial importance to the IDD and will normally receive maintenance from the IDB in accordance with water level management requirements. Major drainage works such as the realignment of an adopted watercourse (and the installation of associated control structures) must be done in liaison with, and subject to the approval of, the relevant Drainage Board so as to ensure that the works do not adversely impact the wider catchment (and in this instance to ensure the realigned watercourses retain their current arterial function). Resultantly, the Board is the regulator for the proposed works to alter the Watercourses as per Section 23 of the Land Drainage Act 1991. Additionally, please note that consent is required to do works controlled by the Board’s Byelaws (made as per Section 66 of the Land Drainage Act 1991). Specifically the principle aim of Byelaw 10 is to ensure watercourses can be maintained by the Board now and in the future without restrictions being placed on the Boards access, and to ensure operatives are aware of third party structures when undertaking maintenance. To summarise, the proposed works to alter the adopted watercourses has the potential to impact water levels within adjacent land parcels and within the wider catchment and I am concerned that insufficient information has been presented regarding the proposed realignment. 2. Hydrological Impacts and Surface Water Discharge The Board are concerned that the project has not fully considered the holistic hydrological impacts of the development or the associated drainage strategy. Changes to water levels within the wider catchment may be associated with potential adverse impacts across a wide spectrum including but not limited to flood risk and drainage management as well as ecology due to the way the drainage systems in the area are currently optimised for breeding and overwintering bird populations. Specifically, the Board is concerned regarding the following: • Changes to the hydraulic connectivity (including surface water to groundwater interaction) resulting from the cut-off wall may lead to a change in water levels within the adjacent watercourses. The Board does not feel that sufficient research into the possible impacts of these changes have been considered despite the Board previously raising with the applicant that we were concerned regarding the impact of an increase (or decrease) in surface water flows. • The submitted Flood Risk Assessment (FRA) and Environmental Statement (ES) only assess the impacts of an infiltration led drainage strategy, despite the feasibility of the scheme remaining unknown. The Board therefore believes that the FRA and ES should consider and assess the impacts of discharging additional surface water (including volume where the rate is reduced to a greenfield figure) to watercourses, where the feasibility of infiltration has not been evidenced. • The Board is concerned that in maintaining the option to discharge surface water to the sea, due consideration has not been given to the impact of potentially decreasing the volume of surface water into the drainage network, relative to the greenfield scenario. • The potential infilling of the Borrow pits with new materials is likely to lead to a change in infiltration rates and a possible result of this could be a change in the surrounding water levels or risk of flooding. 3. Water Management Zone (WMZ) and Detailed Drainage Design: There is a consistent and continuous lack of detail and evidence in the available drainage strategy which makes it very difficult to assess how the various proposals intend to manage surface water as part of the development and how feasible they are. Our concerns include, but are not limited to, the following: • The timeline for Water Management Zone (WMZ) implementation is currently unclear. This could result in substantial flood risk if development takes place before the drainage infrastructure within a Water Management Zone is not ready to operate. • We understand that the currently planned attenuation basin in WMZ 1 may be subject to changes at detailed design due to encroachment on a wildlife habitat. If these changes result in the abandonment of this basin, it is possible that all discharges from WMZ 1 (as well as those from WMZ 7,8 and 9 before the Combined Drainage Outfall is ready) would either be discharged to a watercourse or to WMZ 2. The impacts of these potential scenarios have not been investigated and subsequently could increase flood risk. • On occasion (e.g. WMZ 6) it appears that where an infiltration system cannot be designed to accept flows from the entirety of a WMZ catchment, discharge is to be directed to a watercourse with no attenuation. Any unattenuated discharge is strongly discouraged by the Board, the NPPF and National Best Practice without appropriate justification. • The available information states that some of WMZ 8 will drain ‘naturally’ to the adjacent environmentally sensitive Marsh, but has not advised whether this is through the means of infiltration, discharge to a watercourse or overland flows. I am concerned that no drainage strategy has been provided for the increased impermeable area and therefore increased volumes (rather than ‘natural’) of surface water flows. • The drainage strategies for WMZ groups 1 and 2, as well as WMZ 10, the Northern and Southern Park and Ride Sites, the Two Village Bypass, the Yoxford Roundabout, the Link Road and the Freight Management Facility have not been evidenced as being feasible due to the lack of infiltration testing. • Where infiltration has been deemed impossible from geological surveys, nearby watercourses/rivers have been identified but no direct discharge location has been proposed. At this stage, the Board is not able to comment on these discharge locations due to the lack of information provided. • The Board has been unable to locate a drainage strategy for the Depot South of the WMZ 10. 4. Minsmere Sluice While the Environment Agency are the most appropriate Risk Management Authority to comment in detail on the assessment of the impact on the Minsmere Sluice, we wish to highlight the importance of the Sluice in relation to surface water drainage form the catchment (and therefore from the proposed development area). It is our understanding that the Sluice may be nearing the end of its useful life (a milestone which will have a significant impact on surface water drainage, and may result in the requirement for a pumping station) and therefore any impact on the Sluice may act to accelerate this process. We therefore have the following concerns: • With the changes in water-level and discharge volumes that are likely to result from the development the Board is concerned that the existing pressure on the sluice will increase thus furthering any adverse impacts discussed in section 2 of this representation. The applicant has indicated that they might consider placing a temporary pump upstream of the sluice to redirect water for re-use within the catchment when needed. While this might prove useful in the short term, the drainage impacts of the development will persist beyond its operational timeframe and a temporary solution is therefore not sustainable. • As the applicant has identified, changes to long term sediment flow off the Coast because of the hard-coastal defence will likely lead to accretion to the North of the development. I am concerned that this could hamper future discharge to the sea from the gravitational drainage system at Minsmere and that this has not been sufficiently considered. • The Board feels that a suitable long term solution to the above problems may be the installation of a new pumping station to replace the gravity sluice. Any such structure would likely be managed by an appropriate Risk Management Authority on completion. 5. Development Consent Order – Protective Provision As a Flood Risk Authority responsible for several areas within the development boundary and a large section of the surrounding hydrological catchment the Board would welcome the consideration of the benefits of including a protective provision for drainage and flood risk authorities (including Internal Drainage Boards) within the Development Consent Order. We consider that it is possible that such a provision may act to avoid conflict between the planning process and the Board’s regulatory regime and consenting process (as per the Land Drainage Act 1991 and the Board’s Byelaws) while assuring the Board that their interests and ability to undertake their statutory functions are subject to due consideration.