summary

 
©Crown Copyright. All rights reserved. Hertfordshire County Council, LA 076678, 2001

LOCATION

The Mimram valley between Digswell Water and Welwyn.

 
©Crown Copyright. All rights reserved. Hertfordshire County Council, LA 076678, 2001

LANDSCAPE CHARACTER

River valley intertwined with modern road network and partly confined by housing development. Similar in character to the river valley to the east, but with stronger urban influence.

KEY CHARACTERISTICS

• wetland vegetation screens river from view, so not major feature within the landscape
• parkland aspects, especially around Lockleys school (waterfall)

DISTINCTIVE FEATURES

• Digswell Viaduct
• impact of roads and housing

Mimram Valley aerial view from Digswell Viaduct (HCC Landscape Unit)

assessment

PHYSICAL INFLUENCES

Geology and soils. Stoneless mainly calcareous clay soils over river alluvium (Thames series)

Topography. Shallow south-facing slope of river valley.

Degree of slope. 1 in 7 on steepest valley slopes, with as fall of 1 in 1500 along this reach of the river.

Altitude range. 61m to72m

Hydrology. The Mimram is a relatively fast-flowing, gravel-bedded chalk stream within a mainly rural catchment. The water quality is suitable for species such as trout and grayling and the dominant species is the brown trout, although sections upstream of Digswell have been dry in recent years. In some sections terrestrial vegetation has taken over from aquatic, but channel improvements have been carried out at Digswell and Whitwell to prevent impoundment and remove accumulated silt. The riverbanks are mostly less than 1m high, with good shallow margins and varied marginal flora.

Land cover and land use. Mainly pastoral, with extensive wetland vegetation.

Vegetation and wildlife. Mainly willow and alder, which make a significant contribution to the local landscape. There is some classic riverside pasture with limited improvement. The more interesting areas for wildlife are the riverside margins and damp areas.

HISTORICAL AND CULTURAL INFLUENCES

The Digswell Viaduct is apparent at the eastern end of this area, as is the 17.5 acres of ornamental parkland now run by a local society. It comprises woodland and ornamental trees around a small lake, made in 1810 as part of Digswell House Park. It is marred by the noise of the nearby A1000 and A1(M). The western section of this area lies within the parkland around what is now Lockleys School

Field pattern. There is no apparent field pattern in this area, due to current and historic land use as amenity, pasture and parkland.

Transport pattern. The river valley in this area is contained by the road network, with the A1000 weaving across the river and a continuation of the B1000 forming a boundary to Digswell village. The river disappears beneath the A1(M) but is a notable feature within old Welwyn village.

Settlements and built form. This section of the river valley is confined by the southern edge of Digswell and mid-20th century development within Digswell Park, now part of the development of Welwyn Garden City. Digswell Viaduct was built in the late 1840s to carry the new railway, out of bricks manufactured on site. It is a Grade II listed structure 450m long and 25m high, with 40 arches. Originally built of red brick, it was refaced with blue engineering bricks in 1935.

evaluation

VISUAL AND SENSORY PERCEPTION

The overall parkland character of this area has been masked by 20th-century development, in particular the road network, which has destroyed its former tranquillity and adds constant noise to the intermittent rush of trains over Digswell Viaduct. It is a rather confined area, with a sense of offering a retreat from its urban surroundings.

Rarity and distinctiveness. There are several unusual features in this area . Digswell Viaduct is 'probably the most widely recognisable structure in the county' (Landscape Strategy Vol.1)

VISUAL IMPACT

The Viaduct is gloomy and oppressive, as well as impressive, but is matched for impact by the road network around i.

ACCESSIBILITY

Footpath from Digswell, otherwise quite inaccessible, especially the western section. There is unrestricted access to the parkland around Digswell Lake for memebrs of the Digswell Lake Society

COMMUNITY VIEWS

This area includes some valued elements (D).

 

CONDITION
Land cover change:	localised
Age structure of tree cover:	mixed
Extent of semi-natural habitat survival:	fragmented
Management of semi-natural habitat:	not obvious
Survival of cultural pattern:	declining
Impact of built development:	high
Impact of land-use change:	moderate
ROBUSTNESS
Impact of landform:	apparent
Impact of land cover:	prominent
Impact of historic pattern:	interrupted/relic
Visibility from outside:	locally visible
Sense of enclosure:	confined
Visual unity:	incoherent
Distinctiveness/rarity:	unusual

guidelines

STRATEGY AND GUIDELINES FOR MANAGING CHANGE: IMPROVE AND CONSERVE

• promote the value of biodiversity and the use of locally native species
• promote the maintenance of semi-natural habitats along the river, with buffer zones where necessary to prevent agricultural run-off
• encourage the retention of ditches running into the river
• promote the maintenance of pasture within the river valley
• promote the planting of trees as a buffer between rural and urban zones in this area, to provide visual and noise screening and to absorb pollution from road traffic.
• encourage the use of native species in rural areas and the retention of ornamental and semi-ornamental species for urban situations
• promote an increase in public access to the river, especially from the urban areas
• ensure that the value of veteran trees and dead wood is fully recognised and that appropriate management techniques are implemented and maintained
• encourage a diversification of mowing regimes in areas of open space, to increase biodiversity, create visual interest and reduce maintenance costs
• manage ponds and lakes so as to avoid enriching habitats with debris
• avoid damage to riverbanks and over-widening of the open water channel as a result of agricultural or other practices
• do not c lear vegetation from river bank, but retain shrubby vegetation, where there is no conflict with public rights of way, for perching and nesting.
• should a second viaduct be constructed near the existing Digswell Viaduct, care should be taken to ensure that the design of the new structure is informed by the proximity and style of the existing. Should lighting of either structure be considered, care should be taken to ensure that bat colonies are not disturbed. Consideration could be given to low-UV lighting to highlight this notable local landmark.

Digswell Viaduct with grazing meadow (HCC Landscape Unit)