Sedimentation rhythms and hydrodynamics in two engineered environments in an open coast managed realignment site

Abstract

Managed Realignment (MR) schemes are considered by many coastal managers and engineers to be a preferable method of coastal flood defence and compensating for habitat loss, by creating new areas of intertidal saltmarsh and mudflat habitat. Monitoring of MR sites has tended to focus on short term ecological factors, resulting in a shortage of high frequency, high resolution long term measurements of the evolution of the sediment erosion, transportation, deposition and consolidation cycle (ETDC) in newly breached sites. This is particularly true of analysis of the formation and preservation of sedimentary rhythmites and evaluations of sedimentation rates (and their variability) in newly inundated intertidal environments. This study provides an evaluation of sedimentation rhythms and hydrodynamics from two contrasting sites within the Medmerry Managed Realignment scheme, the largest open coast realignment in Europe (at the time of site inundation). Bed sediment altimeter data highlighted different sedimentation patterns at the two sites; near constant deposition of sediment occurred near the breach resulting in 15.2 cm of sediment being accreted over the one year monitoring period, whereas periodic accretion and erosion of sediment occurred inland leading to 2.7 cm of net accretion. Differences in the relationship between suspended sediment concentrations and site hydrodynamics were observed on a semi-diurnal to annual scale. This study highlights the need for further consideration of the sedimentation processes in MR schemes in order to enhance the design and construction of these sites. Advancements in the understanding of these processes will increase the success of MR schemes in terms of the evolution of the sediment regime and the ecosystem services provided, particularly as they are more widely accepted as a form of coastal flood defence and intertidal habitat creation method.

The evolution of embryonic creek systems in a recently inundated large open coast managed realignment site

“…There has been a large amount of accretion in the vicinity of the creek networks forming at this site; Dale et al (2017) reported over 15 cm of accretion in the borrow pit adjacent to the area monitored here over a 1 year study period during the second year following site breaching. However, at Medmerry, creek formation was already occurring due to a difference in hydraulic head and sub-surface drainage, prior to the level of accretion reaching the critical 20-30 cm depth proposed, albeit site specifically, by Watts et al (2003).…”

“…This site was selected as it allowed for analysis of creek development in a rapidly evolving nearbreach environment. Tidal data, reported by Dale et al (2017), indicated that the bank would typically be inundated for 2.5 h by approximately 0.70 m (spring tides) and 0.30 m (neap tides) of water per tidal cycle (Fig. 2).…”

“…Grain size was relatively coarse, with increased concentrations of fine-grained sediment during the summer. This is probably due to reduced inputs of sandy sediment as a result of more quiescent conditions and therefore less erosion of the relict hedgerows surrounding the site (recognised by Dale et al (2017) to be the source of coarse-grained sediment to this site). Analysis of the sediment in the creek beds, sampled in July 2016 and analysed using the same procedure as the surface samples, indicates they developed on top of a less organic layer (loss on ignition, 2.80%) with a higher concentration of fine-grained sediment (d 50 = 22.50 μm; mud content, 54.59%) compared to the surrounding bank.…”

“…For this study, measurements were taken from the bank of a near-breach former barley field (Fig. 1b), in front of an infilling borrow pit (Dale et al 2017), named Site 5 by Burgess et al (2016), where embryonic creeks had developed following site breaching. This site was selected as it allowed for analysis of creek development in a rapidly evolving nearbreach environment.…”

“…6), repeated sUAS-SfM surveys of this nature would allow for the analysis of accurate volumetric changes across the site. This would then allow for the net changes in sediment accretion and erosion to be established on a spatial scale beyond the capabilities of methods currently being utilised in these environments (e.g., Ni et al 2014;Dale et al 2017), and in intertidal wetland environments generally. This technique may also allow for advanced and effective drainage network mapping considering changes pre-and post-site breaching, over the temporal and spatial scales Fig.…”