Tidal exchange systems

Design Guidance

Existing Guidance

Numerous guidance documents are currently available for the purpose of Managed Realignment, all of which cover the physical and biological factors that need to be considered when adopting this mitigation measure.

Guidance documents of specific relevance to the design of Tidal Exchange techniques are:

Coastal and Estuarine Managed Realignment: Design Issues (CIRIA, 2004)

Section 2.8.3 of this document specifically relates to ‘Tidal Exchange Systems’.  This section defines what is meant by regulated tidal exchange, describes the situations to which it is suited and suggests how sensitive issues can be overcome.

Section 5 of this document provides useful information on ‘Designing and Implementing Managed Realignment’ in general and covers issues such as:

  • Physical processes and morphology (tidal levels, tidal range, tidal prism, tidal discharges, tidal current velocities, tidal asymmetry, waves, sediment dynamics and morphological responses, including the stability of breaches.
  • Engineering requirements (site boundaries, new ‘backstop’ defences, new intertidal zones (including topographic levels, creek systems, vegetation establishment, plant and access, dealing with existing buildings, structures and services, decommissioning, and health and safety).

Whilst dealing with Managed Realignment in general, much of the above is directly relevant to Tidal Exchange.  In addition, Section 5.5.7 of this document specifically relates to ‘Tidal Exchange Design’.  This section covers how these systems can be used to regulate tidal inundation to a site. 

A range of case studies are available in the The Online Managed Realignment Guide (ABPmer, 2009 ) provides some case studies.

Scheme Design

Site Selection

There are several essential requirements of potential sites for tidal exchange:

  • An existing sea defence such as a seawall into which a pipe, sluice or tidegate could be integrated;
  • An area that can be flooded without flooding adjacent farmland (may require a bund to be constructed behind the primary defence);
  • A nearby source of sea water to permit saltwater flushing. Sea water should ideally have enough suspended sediment to enable accretion at a higher rate than sea level rise;
  • The site must be no less than 0.1m lower than sea level at the highest part of the tidal cycle;
  • The site must have a tidal range of at least 3m;
  • Impermeable underlying geology, not prone to erosion (i.e. not peat or chalk); and
  • Gradients of at least 1-6%, this will determine the ratio of saltmarsh to mudflat.

Types of Tidal Exchange

There are several approaches/techniques used to control the flow of sea water and not all techniques are suitable at all sites; some are more suitable for creating mudflats than saltmarsh. The main techniques include:

  • An open culvert, with no tidal flap through the sea wall. Tidal water will flow in and out on every tide as long as the invert level is around the mean low water mark. A variation on this method is to have a drop board on the landward side to prevent water flowing out of the culvert, creating a permanently flooded area.
  • Lowered crest level and spillway allowing inundation during the top part of the tide.
  • Culverts with manually operated flaps that let water through into an impoundment at high tide over several high tides, until desired water level is reached. Where tidal inundation of a site is by a spillway, a means of draining the site at low water may be required unless a tidal lagoon is desired.
  • Tidal flaps with apertures included as part of the design. Case Study 38 (p. 30) from the Environment Agency’s Working with Natural Processes Guidance demonstrates Tidal Flap Replacement Dark Water in Hampshire.
  • Self regulating tide gates (SRTs) have one moving part and an adjustable float system, allowing the SRT to stay open and float on flooding and ebbing tides until the specified desired water level has been reached, at which point the SRT will close and stay closed. When the tide recedes on the outside of the site, the SRT automatically reopens, allowing the impounded water to flow out. Please click here for EA/Defra guidance on self-regulating tidegates: Self-regulating tidegate - design facilitates (EA, 2009)

Goosemoor in Devon is an example of a successful tidal exchange system which used a floating flap design to allow inundation landward of the embankment to create saltmarsh, saline lagoon and saltmarsh. A 1.2m diameter self regulating tidegate was attached to a culvert of similar diameter through the river wall and a channel was excavated to link the River Clyst with the SRT and pipe.

Self regulating tidegate imported from the USA

Source: http://www.saltmarshmanagementmanual.co.uk

The Environment Agency have developed a new design based around a rotary gate, that is closed at low tide to prevent ingress of freshwater; opens at mid-tide when the salt wedge underlying the fresh has reached the gate and then closes again to prevent flooding from high tides. It has been installed on the Axe Estuary in Devon (tidal range approximately 3m) where the cut through the embankment is 900mm diameter but it is proposed that it could be possible to use for sizes between 300-1200mm diameter. Further development may also replacement with traditional flapped outfalls to open up access for fish.

  • Electronically operated tide gates. Flow is regulated by a vertical lift, rectangular tide gate on the seaward side that opens and closes electronically at desired water levels, which are monitored by pressure sensors. The gate is normally open for a short period on each rising and falling tide.

Site design

To create the type of habitat proposed in the project aims, the land elevations and creek systems need to be designed accordingly for example, the work at Goosemoor comprised excavation of existing drainage ditches to create a new creek system, re-profiling of ground levels to create intertidal habitat areas, creation of a secondary embankment. The type of habitat will also need to take into account the functionality of drainage from the site, for example where tidal inundation of a site is by a spillway, a means of draining the site at low water may be required unless a tidal lagoon is desired. More information can be found on different types of habitat requirements in general guidance documents such as http://www.ukbap.org.uk/habitats.aspx.org.uk.

Internal creek network at Goosemoor

Reference: http://www.saltmarshmanagementmanual.co.uk/Management/ManCaseStudies RegTidExch.htm

Managing erosion around structures

Consideration to impact of flows exiting the control structure should be given as part of the design to ensure the outlet is not over-engineered whilst managing localised hydraulic scour. More detail is given in the in Manage intertidal erosion.

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