DISCUSSION

The most dramatic change associated with the MR site is related to its drainage pattern. Over the first month following the breaching of the embankment, there was a marked reduction in the length of time the site remained submerged. During the ebb phase of the tide (especially initially) the drainage, within the natural creeks connected to the channels in the breaches, resulted in discharge throughout LW; under natural conditions they would have been empty. In response to these anthropogenically-induced flow conditions, overbank flow occurred at particular locations along the creeks. This process caused overland flow over the adjacent intertidal flats; this, in turn, has led to the increased development of a creek system (this is described in more detail, in Chapter 7).

The MR site became infilled with water more rapidly, following the first month of

inundations. At this time, the flood phase started to mimic closely the tidal curve over the adjacent saltmarsh. After some 20 months, the ebb phase became similar to that on the saltmarsh. The initial dimensions (1 m deep, 2 m wide) of the artificial channels, within the breaches, were insufficient to accommodate the volume of water entering and leaving the MR site. As such, the channel bed within Breach 1 experienced erosion during Deployment 1, to a depth of 0.5 – 1 m over 3 tides; these were some 1 m lower in elevation than the maximum spring tide. During these first tides, severe erosion occurred in all of the channels, to permit larger quantities of water to flow in and out of the site. Elsewhere, in the UK and Europe, similar erosion has been experienced at other sites; (a) at Tollesbury, Essex, UK, a 1 m wide breach eroded, over a few days, to a pre-determined (modelled) width of 60 m (Alsop et al., 2004); (b) at Pagham Harbour, southern England, UK, following a natural breach in an embankment, a 160 m wide channel developed, causing significant erosion and reworking of the sediment around the harbour entrance (Cundy et al., 2002); and (c) in the Westerschelde, Holland, within 85 years of the re-flooding of a series of enclosures, a creek network

developed that reached back some 12 km from a main channel - this had grown at its mouth to a width of ~ 750 m (Allen, 2000b). The development of the channels, within the breaches will be discussed in more detail in Chapter 6.

The tidal currents inside the MR site flowed landward during the flood phase of the tide; this switched, just after HW, to flow out of the site in the opposite direction. The tidal current speed peaked during the first phase flood; it then remained relatively stable throughout much of the ebb phase of the tide. During high spring tides, the current speed was higher and the flood tide was shorter than the ebb. During lower spring tides, the tidal currents were more symmetrical, with a flood current speed only slightly higher than that on the ebb. This difference was related to the topography of the MR site and the adjacent saltmarsh. During

high spring tides, the water level was higher than the base of the breached embankment, and this caused the water to flow both through the channels within the breaches as well as over the banks of the channels. The difference in levels between the channel banks and the MR site caused flow acceleration inside the MR site, creating high tidal current speeds and a rapid rise in water level. Subsequently, drainage of the site was restricted to the channels within the breaches; this caused the emptying of the site to last for longer than the filling. During the lower spring tides, flood and ebb flows were restricted to the channels within the breaches, and were of the same duration.

Wave activity inside the MR site was found to be minimal; this was due to the site itself having a fetch for locally wind-generated waves, whilst the adjacent saltmarsh dissipated much of the wave activity, approaching from different parts of The Wash (Section 9.3.2). The relationship shows, when satisfied, that the oscillatory motion of the waves is felt at the seabed (Soulsby, 1997). Only during the first and last phases of the tide did the measurements satisfy this relationship; so when the water depth was less than 0.25 m, the wave conditions were capable of eroding the bed. As such, the modified and prevailing wave climate is not deemed to have been an important factor in the development of the site. In contrast, the internal embankment at the MR site at Northey Island, Blackwater Estuary, UK, suffered erosion from wind waves generated within the 0.8 hectare site (ABP, 1998); in the absence of vegetation, small locally generated wind waves can cause noticeable erosion within the MR site.

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Since the initiation of the MR scheme, the site has accreted at a similar rate to the natural saltmarsh prior to the land reclamation in 1980. The rates were highest near the breaches in the seaward embankment; they were lower around the strengthened landward embankment. Areas adjacent to the breaches are likely to have experienced higher rates of accretion, as they were submerged during most of the tides which entered the MR site; this, in turn, provides a greater opportunity for suspended sediment to settle, during slack water. Consequently, the rates appeared highly dependent upon the frequency of submergence; this is in agreement with the findings from Tollesbury (Chang et al., 2001). For example, the peak rate of accretion (5 cm yr-1) in The Wash was located between Breaches 2 and 3, whilst the area between Breaches 1 and 2 experienced only 2 cm yr-1 (Fig. 5.13); this indicates that the local topography was important and the sediment inputs from each breach may have varied and affected the accretion rates. These measured rates of accretion were high compared with those observed over other sites: (a) at the historical set-back sites in the Severn Estuary, UK, the annual accretion rates varied from 2.8 to 15 mm yr-1 (Allen, 2000a); (b) within the

Crouch, Medway and Blackwater Estuaries, UK, maximum rates extended up to 80 cm, over a 100 year period (Crooks and Pye, 2000; and Pethick, 2001); (c) for the Blyth Estuary, Suffolk, UK, the most exposed area eroded at an average rate of 10 mm yr-1, accreting in the more sheltered areas at rates of between 7 and 16 mm yr-1 (French et al., 2000); and, finally (d) at Pagham Harbour, southern England, UK, the rates of accretion were 8 to 10 mm yr-1, since a natural breach in the embankment in 1910 (Cundy et al., 2002).

Vegetation had naturally colonised much of the site, some 12 months after the breaching of the embankment, with the main pioneer species being Salicornia spp. and Suaeda spp.; rapid natural colonisation of vegetation was also witnessed at the MRs at Northey Island and Orplands Marsh, Blackwater Estuary, UK (ABP, 1998). The vegetation cover varied around the site; it was found to be predominantly dependent upon the frequency of inundation, with the maximum rates of vegetation cover along each profile located at the zone where the duration of submergence was most suited to vegetation colonisation. The vegetation cover has become denser, based upon observations during sequential field visits; this will enhance the entrapment of suspended sediment, causing the MR site to accrete more rapidly (French et al., 1995; Roman et al., 1997; Brown, 1998; and Cahoon et al., 2000). As the vegetation causes frictional deceleration of the tidal currents (Neumeier and Ciavola, 2004), the rate of accretion will increase; the properties of the surficial sediment will change to become similar to the saltmarsh, and this will enable more species to colonise (Pethick, 1984).