A Rapid Assessment of Sediment Sources on the Wensum Above Norwich, with Particular Emphasis on Road Crossings
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A Rapid Assessment of Sediment Sources on the Wensum above Norwich, with particular emphasis on road crossings R Evans, 1 Wentworth Road, Cambridge, CB4 3PN Introduction It is considered the quality of the chalk-fed River Wensum is deteriorating, partly because sedimentation of its bed is affecting fisheries. The Wensum valley is a Site of Special Scientific Interest (SSSI) and Special Area of Conservation, is part of the Catchment Sensitive Area Initiative and is a protected area under the Water Framework Directive. It is also a Demonstration Test Catchment in which an evidence base can be built with regard to sources of diffuse pollution and in which it can be shown how these issues can be addressed. A Geomorphological Appraisal of the main Wensum channel, not its tributaries and headwaters, based on a fluvial audit of the SSSI, was carried out for Natural England. The River Wensum Restoration Strategy (Natural England 2009), based on the Appraisal, has been devised by Natural England in partnership with the Environment Agency and the Water Management Alliance and is looking at the whole length of the River and is trying to tackle a whole range of issues in a connected way. The Strategy has been drawn up so as to give a template for the restoration of the form and function of a Norfolk chalk river by adjustment of river levels at former mill sites, but the Strategy could also be promoted as a pilot - a Demonstration Test Catchment - for improvement of river condition under the Water Framework Directive. However, to improve the status of the River so it meets the criteria of good water quality and ecological condition problems associated not just with the adjustment of the river levels need to be solved but also those related to diffuse pollution by nutrients and sediment. This pollution affects the whole of the catchment, not just the main channel of the Wensum. Here we are concerned with sources of sediment throughout the catchment of the River Wensum There are a number of sediment sources. Runoff from agricultural land carries fine soil particles. Flow down roads and tracks transports soil particles to the Wensum. Sediment can be brought into circulation by erosion of the river channel during floods, as well as by disturbance of the main river channel and its tributaries and the ditches draining to the streams and rivers, for example by the cleaning out of the channels or the cutting of vegetation and trees which can destabilise the banks. Such primary sources of sediment are addressed here. Sediment from the above sources will be deposited in the river channel and will be mobilised by high flows and will be a component, possibly an important one, of turbidity and suspended sediment load. I was asked to make a reconnaissance assessment of sediment sources along the River Wensum, particularly from roads and tracks. There are very many road and track crossings in the whole of the catchment and given the available resources it was decided initially to concentrate on the main course of the Wensum from its headwater near Whissonsett to Hellesdon on the northern outskirts of the city of Norwich. This plan was modified somewhat after receiving data for the years 2006-2010 on turbidity of the Wensum and some of its tributaries. 1 Method Fieldwork was carried out over three days. Routes were devised beforehand (see map, day 1 – yellow, day 2 – orange, day 3 – blue), not knowing what was to be found, with the aim of getting a balanced impression of sediment sources. The routes traversed as many water course crossings as feasible without recourse to backtracking or repeating the journey. On the first day road crossings of the Wensum were examined from near Whissonsett downstream to Great Ryburgh. On the second day, for reasons given below, a similar survey was carried out in the Blackwater catchment and on the third day road crossings of the River Wensum from Attlebridge to Hellesdon were examined. A previous reconnaissance survey (Evans 2009) had made a rapid assessment of sources of sediment from Lyng upstream of Attlebridge to Fakenham upstream of Great Ryburgh. Assessments of sediment sources (marked in red on map) were made at 24 locations along the Wensum and its immediate tributaries (Table 1) as well as at the Lyng crossing examined in 2009, and at 21 locations in the Blackwater catchment (Table 2). At each water course crossing an examination was made to see if there was a direct connection between the road and the river channel, for example water could flow via grated drains directly to the river or via drains or ‘cuts’ made across grassed verges to roadside ditches that drained to the River. The presence of sediment in the river channel was noted. The state of the roadside verges and the presence of soil on roads were also noted. Assessments were also made where roads flanked the River. Photographs were taken to record the state of the river channel at the road crossings and of other features of note. Only a selection of the 236 images are reproduced here. Turbidity, a surrogate for suspended sediment and measured by the attenuation of light through water (in Formazin Turbidity Units (FTU)), was analysed for 7 sites along the Wensum for the period 1989-2010 and for Billingford Bridge on the Wensum for 1989-2005 and for a number of sites on tributaries for the years 2006- 2010 (Table 3). Samples were taken to determine turbidity on the same day at Sculthorpe, Great Ryborough and Billingford Bridge on the Wensum and on the River Tatt at Tatterford Common. Samples were taken at sites on the lower Wensum and its tributaries on different days to those on the upper Wensum. Analysis of turbidity for sites on the Wensum and its tributaries showed that although turbidity often peaked on the same day that was not always the case. On the Blackwater, a left (north) bank tributary, over the last 2 years peaks of sediment did not coincide with those on the Wensum (Figure 1) and so road crossings in this catchment were examined to see if there was any evidence of more recent work on roads or adjacent ditches than in the already examined upper Wensum. Findings Along the Wensum (Table 4), at 15 out of 24 (62.5 %) crossings roadside verges were damaged by traffic and bare soil was exposed (eg Photos 1-3). Verges were damaged mostly in the upper part of the catchment above Hempton (Photo 4) immediately west of Fakenham (at all 11 locations). Soil was rarely seen in the middle of the road (eg Photos 1-2), in only 5 locations, all above Hempton. Because of the prolonged dry 2 spell in late winter and early spring of 2011 the evidence for runoff transporting soil down roads was possibly not as evident as it might have been, in only 6 locations was transported sediment by roadside verges recorded (Photo 5). In only 4 locations (Table 4) did a drain or ditch from the roadside connect directly with the River at the crossing point (Photo 6). In many locations ‘cuts’ in the verges made to trap sediment were doing their job well (eg Photos 7-10), and in only a few instances (6) did ‘cuts’ connect to ditches which led to the Wensum. Ditches are not always present by the roadsides and even when present it is not always easy because of the obscuring vegetation to see if they contain sediment or are eroding. Silt was found in only two ditches (Photo 11). In contrast, silt or sand was deposited in the river channel (eg Photos 12-16) at 20 of the 24 locations (83.3 %) seen in 2011 and at all 5 other locations between Guist Bridge and Lenwade recorded in 2009 (Evans 2009). The sandy deposits were found in 10 locations, and the largest bar was in the channel below the bridge at Taverham (eg Photos 17-18), and below there no sandy deposits were seen. At some locations (15), all below Fakenham, silt was being trapped in quiet waters (Evans 2009, and eg Photos 15, 19-20) or by bank-side vegetation (eg Photos 21-24). Erosion of the channel bed was only noted at Lyng (Evans 2009) but in 14 locations channel banks were eroding (eg Photos 12, 25-41), most of these (10) at and below Lyng. Bank erosion at Lyng, dominantly downstream of the mill, was due to a May storm in 2007 ( see below) but elsewhere was often due to trampling of cattle and at Ringland people keep the right bank bare of vegetation (Photo 38). Bank erosion may have been a problem in the past, the downstream banks at Hellesdon are presently partially protected by old rotting boards (Photo 42). In the Blackwater catchment (Table 5), at 14 of 21 (66.7 %) locations damaged verges were recorded (eg Photos 43-52). Verges were not damaged mostly in the northern and eastern part of the basin where slopes are gentle and roads often wider. Soil was occasionally found (6 locations) in the middle of the road and in only 4 locations was sediment being obviously transported down the road. Much silt and sand was trapped in roadside ‘cuts (eg Photos 53-65), Where there were roadside ditches (10 locations), at 8 locations these connected directly to the main river channel or its tributaries. The ditches were not eroding and in only 3 places were sediments found in them. However, in 13 locations (61.9 %) sandy (9) or silty sediments were recorded in the river channel (eg Photos 66-76), and at two crossings stream flow was slightly turbid (eg Photo 77).