During the first phase of the Year 2 work package, Royal Smals were commissioned to use their Cutter Suction Dredger (CSD) “Phoenix” to dredge approximately 12,000m3 from Brightlingsea Creek South Channel. This material was pumped up to 1.7km and used to fill the St Osyth borrow pits. These twenty three (23) pits, approximately 30x30x1.5m in size, were originally excavated in the 1960’s in order to strengthen the local coastal flood defences following the 1953 floods. A series of connecting excavations, dams and drop-board sluice gates were installed by subcontractors, Miles Water Engineering, whilst a series of “Y-“ unions and valves allowed the close control of the pumped dredge material, in order to achieve maximum fill and storage volumes.
The dredging and restoration works were carried out over three successive neap tide periods, to limit the potential for erosion during high springs, with Exo providing site management services throughout. This included coordination of works between the harbour, dredging and restoration teams, liaising with both contractors and members of the public to ensure understanding and encourage support of the project and to confirm all works and mitigation measures put in place were adhered to.
Inevitably, whilst undertaking a complex operation in a logistically challenging and high risk intertidal environment, problems can arise as a result of unforeseen or uncontrollable circumstances. We employ an adaptive management strategy to our dredging and restoration projects, as recommended by the Central Dredging Association (CEDA), to combat and allow the project to remain robust to such uncertainty. Exo’s site management and expertise allowed any minor issues to be solved efficiently enabling the project to be delivered on time, within budget and with zero harm to people and the environment.
Exo Environmental were appointed Principal Designer and Project Manager of a 4 year dredging and restoration project by the Brightlingsea Harbour Commissioners. This project aimed to remove 53,000m3 of accumulated sediment from within the historic Brightlingsea Harbour and beneficially reuse the majority of the arising material to restore 5ha of local intertidal and saltmarsh habitat.
Due to the quality and extent of the local habitats and the rich biodiversity they support, the site is located within several national and international designated sites, including a Marine Conservation Zone (MCZ) that specifically targets the preservation of the local Native Oyster (Ostrea edulis) populations and the biogenic reef habitats this species creates.
Under the conditions of the marine licence, the temporal impact of the dredging works on benthic fauna community composition, particularly with regards to monitoring recolonisation and recovery of the local populations within the dredge areas, required monitoring and assessment. To aid interpretation and to monitor rates of sediment accumulation within the Creek, particle size distribution (PSD) was also assessed.
To achieve this, Exo conducted repeated surveys of the Year 1 dredge areas immediately following cessation of the dredging works and at regular intervals over a 32 week period using a Day Grab deployed from the Exo Surveyor survey vessel. Although species richness and abundance decreased following dredging activity as expected, recovery of benthic populations was observed over the survey period and corroborated recovery times of estuarine species recorded in existing literature.
The River Parrett and River Tone on the Somerset Levels are well known for their high risk of flooding, with prolonged flood events becoming increasingly worse in recent years. In 2012, it was estimated that the dredging of approximately 160,000m3 of sediment would significantly reduce the risk and magnitude of flood events and their associated impacts. However, the initial cost calculations were deemed too large and ineligible for standard national funding schemes, thereby rendering the project not unfeasible.
Exo Environmental provided advise to both the IDB and the EA on innovative dredging solutions that could potentially provide significant cost saving and reductions in levels of nuisance. Following the latest significant flood event dredging was carried out with significant political pressure.
Exo Environmental continued after 2014 with providing advice on dredging the River Brue in relation to dredging methodology, licensing, cost saving methods and delivery.
Waterside Marina is situated within a high end riverside residential complex, within Brightlingsea Harbour on the River Colne Estuary in Essex, UK, and provides moorings and shore side facilities for recreational vessels and boat owners. In 2015, 11,000m3 of accumulated sediments were removed during a major dredging campaign, with the arising sediments being dispersed on the ebb tide. Following this work, the Brightlingsea Harbour has taken responsibility for the site, which includes undertaking regular maintenance dredging to prevent the marina from silting up in the future.
To monitor siltation rates and identify target areas to focus maintenance dredging efforts, a survey was undertaken. However as the season was in full swing with high numbers of moored vessels present which limited access throughout the marina, whilst the surrounding sheet piling and high rise residential blocks satellite reception of the bathymetric and topographic GNSS positioning system was not 100%.
To overcome this issue, a combination of bathymetric and topographic survey techniques were employed to achieve the best possible results. An initial bathymetric survey was undertaken from the Exo Surveyor survey vessel in all accessible areas, subject to satellite coverage. To obtain data in areas where satellite reception was available but access was limited, a centimeter accurate GNSS topographic system on a sounding rod was deployed. In areas where both access and satellite reception were limited, a manual measurement using a graduated rod was taken to ensure 100% coverage of the marina.