4.2 Principal River Floodplains

Key characteristic features

• Flat alluvial floodplain on valley floor follows north-south alignment of principal river valleys.
• Relatively small-scale pastures with rectilinear fields; subject to seasonal flooding.
• River typically flows within open, grassed embankments.
• Extensive areas of ponds, reedbeds and marsh.
• Drainage channels and canals.
• Relatively few hedgerows; many fields enclosed by wire fencing or ditches.
• Patchy willow and alder scrub around ponds and marshes and, sporadically, within pastures.
• Major roads often mark boundary between floodplain and valley slopes; railways typically built within floodplain on embankments.
• No settlements (sited on edge of floodplain within the principal chalk valleys).
• Pylon lines.
• Extensive, open views within valley.

Landform and context

The principal river floodplains are found on the flat valley floor of the valleys of the Arun, the Adur, the Ouse and the Cuckmere rivers.

The floodplains broadly follow the north-south alignment of the rivers and average 0.5km in width. There is often an abrupt break of slope at the edge of the floodplain, marking the boundary between this and the principal chalk valley landscape on the valley slopes.

The river floodplains are formed on alluvial heavy silts and clays deposited by the river. More generally, river terraces occur in places along the edges of the floodplain, at points where the river has tended to deposit rather than erode material.

The rivers meander across the floodplain in broad loops, which tend to increase in size towards the south. In each case tidal lengths of the river are tightly enclosed by embankments; there are often abandoned isolated remnants of curving embankments elsewhere on the floodplain, indicating its former alignments. Many of the rivers show signs of an artificially straightened course; the Arun, the Cuckmere and the Ouse are all examples of rivers with a history of canalisation. Winding minor tributaries often link with the straight man-made drainage ditches on the floodplain before joining the main river. Some are restricted by the embankments and there are often marshy pools on the outer edge of the embankments. A more extensive area of pools and marshes on the edge of the Arun floodplain is an important wildfowl reserve.

The floodplain landscape of the Cuckmere valley is particularly scenic, reflecting the tranquil, unspoilt character of this valley and its relatively intimate scale. The Cuckmere River is the only one with an estuary within the AONB and Cuckmere Haven, at the mouth of the meandering river, is therefore a very special, relatively unspoilt landscape.

Land use and landscape pattern

These wide river floodplains are predominantly used as seasonal pastures for cattle, but there is also some horse grazing and arable fields sometimes occur in areas with gravelly river terrace soils on the edges of the floodplain. Grazing may be restricted during winter months due to water-logging and flooding.

There is typically a strong contrast between the relatively open, flat pastures of the floodplain and the undulating, well treed village/mixed farming landscape pattern on the lower valley slopes. The contrast is emphasised by the curving linear strips of woodland which often mark the boundary between the two landscapes. These woodlands also tend to screen roads and development, giving the floodplain landscape a more secluded character.

The floodplain pastures are small or medium sized fields of various shapes: some are laid out in planned, rectangular plots, while others have irregular, wandering boundaries. Most are bounded by narrow drainage channels, which in places break down to form odd, abandoned ditches and areas of hummocky, marshy ground. Marshy areas and pools of water on the outer edges of the river embankments often have extensive reed beds. The drainage ditches themselves are reedy and often form strong contrasts of texture with the grazed pasture, particularly where there are no hedgerows to interrupt views across the ground plane.

Hedgerows provide a strong, visual structure in some areas but the network is sporadic and discontinuous. In places it is non-existent and the fields are bounded by wire fencing or left unenclosed and separated only by drainage ditches. There are a few widely-scattered, small patches of woodland within the floodplain, but the landscape is relatively open and the strips of woodland marking the edge of the floodplain therefore have a strong enclosing influence.

The river embankments are open, grassy slopes. There is typically little immediate river-side vegetation, although groups of willows and alders occur sporadically alongside the channel and are locally important in views along the river. Patches of wetland scrub are often too low in relation to the embankments to have any visual influence in the wider landscape.

Settlement

There is no settlement on the river floodplain. The many villages and farms on the edge of the floodplain (within the principal chalk valleys), and valley-side landmarks, such as Lancing College and chalk quarry sites, are focal points in local views.

Footpath routes typically follow the open river embankments and provide popular access to the floodplain.

The character of each of the river floodplain varies markedly, depending on the degree of urban influence. The floodplains of the Arun and the Cuckmere valleys are relatively unaffected, although a railway line crosses the Arun floodplain on a low embankment. However the floodplains of the River Adur and the Ouse are crossed by major transmission lines and have a more open character which is strongly influenced by the development and industry adjacent to the floodplain. The major road junction on the Adur floodplain, marking the southern boundary of the AONB, is particularly dominant and visually intrusive.

Subjective response

The meandering river gives the floodplain landscapes a strong, rhythmic focus, although it is usually hidden from local views.

This is a domestic, gentle, landscape which often seems secluded, despite the proximity of busy roads and nearby settlement. The enclosing valley slopes give a sense of scale and security. In the more developed valleys, the character of the floodplain landscape is swamped by views to more dominant visual features on the valley sides, in particular, the many scars of chalk pits and active quarries.

Sensitivity to change

These linear floodplains are one of the most visible landscapes within the AONB. They are particularly subject to pressures related to communications, such as roads, railways and transmission lines. These large-scale linear developments are visually intrusive and can be a dominant influence.

Three elements within the principal river floodplains landscape are particularly vulnerable to change. The river itself is a visual focus and it is important that the meandering, tranquil character of the river channel is not disrupted by insensitive engineering works. Pressures for drainage to improve traditional water meadows, convert pastures to arable fields on the terrace gravels towards the edges of the floodplain, or to increase the proportion of horse paddocks should be resisted so as to conserve the pastoral qualities of the floodplain landscape.

The strong contrast between the floodplain landscape and that of the principal chalk valleys on the valley sides is often a distinctive visual feature and one which may be threatened by changes to communication routes along this boundary. The curving, linear strips of woodland, which emphasise the contrast between the two landscape types, should be conserved and it is important that the floodplain landscape in this area remains unified and uncluttered by development or atypical land uses.

Key issues for landscape management

Refer to brooks pastures - landscape guidelines, for key management issues in this areas as they are broadly similar. The following issues supplement those mentioned in this earlier section.

Farmland mosaic

• Conserve varied scale and form of the existing floodplain landscape pattern, particularly in those areas with a patchwork of small, irregular fields.

Hedgerows

• Conserve and manage all existing hedgerows. There is scope to extend the hedgerow network in some parts of the river floodplain, particularly where the local landscape is visually degraded and lacking in identity and where visually intrusive development or activities has a dominant influence. Belts of hedgerow trees should be carefully located towards the edges of the floodplain to provide screening as necessary, but without impinging on the open character of the floodplain landscape.

Trees and woodlands

• Woodlands are not usually a characteristic of the floodplain landscapes and it is important that the relatively open character of this landscape is retained. However, small-scale woodland planting in small, curving, linear belts towards the edges of the floodplains would help to screen visually intrusive development such as caravan sites, sewage works or car parks, while also helping to give the floodplain a more distinctive visual structure in areas where it is degraded. Such new woodland planting should be concentrated towards the edges of the floodplain and should only be undertaken following careful visual analysis to identify key views and visually intrusive features which would benefit from screening. Only native broadleaved species which tolerate waterlogging are suitable.
• Additional river-side planting would help to frame sequences of views from the embankment footpaths and would give the river more visual reference points within the floodplain landscape. Such planting should be on a very small-scale and should be designed to give visual emphasis to key landscape features such as strong bends in the river, bridging points, footpath links to villages, adjacent marshy areas etc.

Wetlands

• Examine the potential for management of local water tables to allow the extension of existing areas of marshland to create relatively large-scale areas of wetland which would have both visual and nature conservation value.

Priorities for action

• Conservation and management of drainage ditches and water levels.
• Conserve varied landscape pattern, in particular smaller, more irregular pastures.
• Carefully design new small-scale hedgerow and tree planting schemes to screen visually intrusive features and enhance overall landscape setting of valley, whilst retaining its open character.

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