Keyhaven Marsh, Hurst Castle, Hampshire

[SZ 315 905]

V.J. May

Introduction

The Keyhaven saltmarshes (see (Figure 10.1) for general location) are important for the range of geomorphological features they display, particularly the intricate pattern of saltmarsh creeks. The site is an important research area for examining the relationship between creek dynamics, tidal processes and sedimentation. The western part of the saltmarshes forms an integral part of the Hurst Castle Spit system (see GCR site report in Chapter 6), a classic site for the study of coastal geomorphology.

Description

Hurst Castle Spit protects a large area of saltmarshes, known as 'Keyhaven Marshes' (Figure 10.11). They are drained by an intricate pattern of creeks dominated by three major creeks — Mount Lake, alongside the spit, Keyhaven Lake and Hawker's Lake. The first two merge and drain into the Solent after being diverted by the modern recurves of the spit. Marsh-edge beaches ('cheniers' — see GCR site reports for St Osyth and Dengie above) are formed of shells and shingle. Their sand content is very low. Low-relief cheniers have developed along the marsh edge and provide some protection against erosion. Much of the saltmarsh edge is being eroded rapidly (6 m a⁻¹ over the past 50 years: Pye and French, 1993), resulting in some patches of mud mounds. The upper intertidal zone is characterized by steep micro-cliffs and a strong concave upward profile within the upper part of the intertidal zone. The upper marsh lies at about 2.4 m OD with a seaward marsh edge at about 2.0 m OD. The elevation of the upper tidal flats is typically about 1.0 to 1.5 m OD (Pye and French, 1993). The seaward cliffs vary in height but are typically 0.7–1.5 m. The marsh surface varies in level by about 0.4 m. The surface of the marshes is characterized by a high proportion of eroded marsh, saltpans, and broad channels. There are only small areas of higher-level, species-rich salt-marsh, located mainly dose to the spit and on its older recurves. Sea purslane Atriplex portulacoides, common sea-lavender Limonium vulgare, sea plantain Plantago maritima, sea meadow-grass Puccinellia maritima, common seablite Suaeda maritima, glasswort Salicornia spp., and sea aster Aster tripollum are common throughout these higher marshes. In contrast, the more extensive lower marshes are species-poor and dominated by common cord-grass Spartina anglica. The intertidal area dose to the spit is often a stony mud. Before the late 19th century, much of this marsh stood as much as 1 m lower and was dominated by eelgrass Zostera. Colonization by Spartina anglica following its hybridization from the native Spartina maritima and the introduced Spartina alterniflora in Southampton Water led to a rapid buildup of the saltmarsh surface. The area of Spartina-dominated saltmarsh reached a maximum about 1930, after which the area declined (Bradbury 1996). As the recurves of the modern spit have extended into the westernmost creek, they have increased local accretion of mudflats.

Bradbury (1996) describes the rapid short-term morphological and ecological evolution of the western Solent saltmarshes that include this site. There have been substantial losses of intertidal flat. Ke and Collins (1993) estimated the average annual loss of saltmarsh in the western Solent as 3.6 x 10⁴ m² a⁻¹, at the same time as the saltmarsh surface is accumulating sediment at between 2 and 5 mm a⁻¹. Average erosion of the marsh edge was 3 m a⁻¹ between 1992 and 1994, less than the open coast retreat but more than the fringing edge retreat of 1 m a⁻¹ since 1950. Dyer (1980) showed that between 1950 and 1973 reduction in intertidal width varied between 180 and 360 m (7.8 m a⁻¹). There was a strong correlation between wind-generated wave-attack and the rate of erosion. Tidal range is 2.5 m on spring tides, but meteorological surges may raise waters levels by up to 50%. The upper marshes at Keyhaven are typically formed in sandy silts, becoming silty sand on the upper tidal flats.

Interpretation

These saltmarshes are remarkable for their rapid vertical accretion and areal extension with the arrival of Spartina anglica in the late 19th century. Their subsequent reduction in altitude and area was almost as rapid during the mid-20th century and is related to die-back of Spartina described in a series of papers (Braybrooks, 1957; Goodman, 1957, 1960; Goodman and Williams, 1961; Goodman et al., 1959), which showed that it was associated with exceptionally poorly drained saltmarsh soils. Die-back occurred, however, both along channels and within the central parts of the marshes. In the latter, 'pan die-back' may have been associated with the restriction of drainage by rapid accretion around the edges of marshes. In the former, however, other factors, including algal mats, possibly resulting from local eutroph-ication and cloaking the surface, may have led to more extensive die-back. As channels widened, erosion of the marsh edges appears to have accelerated, although in many parts of the saltmarsh, die-back resulted in a lowering of the marsh surface rather than wholesale retreat of the marsh cliff. The saltmarshes that shelter behind the beach are also liable to damage from recreational use, as well as local pollution.

Conclusions

The development of saltmarsh in the lee of Hurst Castle Spit was limited until the arrival of common cord-grass Spartina anglica at the end of the 19th century. The geomorphological interest of this site lies in the rapid sedimentation and saltmarsh development associated with Spartina followed by an equally rapid decline and loss of saltmarsh area. Unlike the saltmarshes and cheniers of the Essex coast, those of the Keyhaven marsh are very recent in origin.