Catchment Fluvial Geomorphological Audit of the Axe Catchment
Detailed Geomorphological Survey (Report B)
October 2004
ENVIRONMENT AGENCY ENVIRONMENT AGENCY Catchment Fluvial Geomorphological Audit of the Axe Catchment
Document review and issue history
Report Title: Catchment Fluvial Geomorphological Auditof the Axe Catchment Detailed Geomorphological Survey (Report B)
Attachments: CD containing · Electronic Copy of this Report · Reach-by-Reach Geomorphological Database · Web-Based Photo Viewer · Geomorphological GIS Data
BBR Project Number 0013499
Revision Status Issued to Date Author Reviewed by number R01 First draft Dave Bartram 29th Oct 2004 D Wishart R Traynor J Brookes
A01 Final report Dave Bartram 7th Dec 2004 D Wishart R Traynor J Brookes
Environment Agency Client: South West Region
Agency Project Manager: David Bartram
Consultants: Babtie Brown & Root
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Contents
1.0 Introduction ...... 2 1.1 Background ...... 3 1.2 Aims and Objectives...... 3 1.3 Detailed Geomorphological Survey - User Guide ...... 4 2.0 Consultation ...... 9 2.1 Introduction ...... 10 2.2 Issues raised ...... 11 3.0 Catchment Overview...... 13 3.1 Topography ...... 14 3.2 Geology, Soil and Hydrogeology...... 14 3.3 Hydrology and Water Resources ...... 17 3.4 Land use ...... 18 3.5 Fauna and Flora ...... 20 3.6 Human Beings...... 22 3.7 Catchment Initiatives ...... 23 4.0 Geomorphological History ...... 26 4.1 Introduction ...... 27 4.2 Catchment Changes...... 27 4.3 Channel Changes...... 34 5.0 Characterisation of Watercourses ...... 38 5.1 Introduction ...... 39 5.2 Watercourse Summary Sheets ...... 39 6.0 Catchment-Scale Geomorphology...... 50 6.1 Introduction ...... 51 6.2 The Sediment Regime...... 51 6.3 External Controls...... 59 6.4 Internal Controls...... 64 7.0 Summary ...... 70 7.1 Catchment Influences...... 71 7.2 Geomorphological Processes ...... 71 7.3 Key Issues...... 73 8.0 References...... 74
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1.0 Introduction
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1.1 Background
The decline in habitat and water quality of the River Axe has become an issue of increasing concern in recent years. The following problems have been identified by the Environment Agency, South West Region:
· Point source and diffuse pollution from nutrients · Unrestricted livestock access to the river · Increased cultivation of maize · Soil erosion · Bankside erosion · Waste management
The Environment Agency South West Region are undertaking work to tackle these issues as part of the wider Cycleau Project, to develop and apply innovative best farming practices and deliver sustainable land management in the Axe and Char catchments. Cycleau is a trans- national project involving the rivers of South West England, Western France and Ireland funded by the EU North West Europe INTERREG IIIB programme – further information can be found at the project website http://www.cycleau.com (www1).
As part of the Axe and Char project, Babtie Brown and Root (BBR) have been commissioned by the Environment Agency to undertake geomorphological studies of the Axe catchment and a section of the River Char downstream of the A35 roadbridge to the river mouth at Charmouth.
The outputs from this geomorphological work are as follows:
Catchment Fluvial Geomorphological Audit of the River Axe Report A – Method Description Report B – Detailed Geomorphological Survey of the Axe Catchment (this report) Report C – Catchment Geomorphological Action Plan
Geomorphological Assessment of the River Char River Char Geomorphological Assessment
This report (Report B - Detailed Geomorphological Survey of the Axe Catchment) is a technically based report aimed at rivers and fisheries managers with an appreciation of geomorphological issues. It is intended to provide detailed technical information regarding the findings of the Catchment Fluvial Geomorphological Audit.
1.2 Aims and Objectives
The Catchment Fluvial Geomorphological Assessment of the Rivers Axe is intended to assist in the achievement of some key objectives of the Cycleau Project, which include: · To deliver Best Farming Practices · To ensure that the River Axe SAC meets favourable conservation status · To protect and improve habitats, species, landscape, archaeology and historic environment · To ensure appropriate management of bankside erosion within the catchment.
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The specific objectives of this project are:
1. To provide a baseline data set, identifying the major geomorphological processes and features occurring along selected watercourses in the River Axe catchment.
2. To identify and report on the key areas of geomorphological activity, determine any important factors affecting this activity and to provide guidance on appropriate methods of control or management.
3. To identify any areas where restoring the natural geomorphology may improve the potential for habitat creation whilst reducing bankside erosion.
4. To provide information to inform the control of the development of in-river works such as outfalls, weirs, and flood defence structures, to ensure potentially adverse effects of these on the geomorphological processes are either avoided or minimised.
In addition to informing the Cycleau Project, improved geomorphological baseline datasets will be of particular relevance to ecological appraisal, water resources and development control. Geomorphological information can be used to foster closer links between geomorphology and deliverance of a suitable flow regime through the Catchment Abstraction Management Strategy (CAMS) process. The data can also be used to inform the control of the development of in-river works such as outfalls, weirs and flood defence structures to ensure that potentially adverse effects on geomorphological processes are either avoided or minimised.
1.3 Detailed Geomorphological Survey - User Guide
The detailed geomorphological survey is intended to achieve objective 1) and the first part of objective 2) of the “Catchment Fluvial Geomorphological Audit of Axe Catchment” project:
· To provide a baseline data set, identifying the major geomorphological processes and features occurring along selected watercourses in the River Axe catchment.
· To identify and report on the key hot spots of geomorphological activity and determine any important factors affecting this activity.
This guide is intended to help familiarise the user with the content of the detailed geomorphological survey and assess how the information it contains is best used and applied.
1.3.1 Catchment Fluvial Geomorphological Audit – An Overview
· Methodology Fluvial geomorphology is the study of riverine forms and the processes by which they are formed over a range of temporal and spatial scales. Catchment Fluvial Geomorphological Audit is a method of gathering relevant desk and field-based data and using this information to characterise the geomorphological behaviour of watercourses within the catchment (Figure 1.1)
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The desk-based assessment involves TASK 1: DESK BASED ASSESSMENT investigating physical catchment characteristics and the Data Collection geomorphological evolution of Consultation Historical Analysis watercourses within the channel, including modification due to past human intervention. The field survey TASK 2: FIELD SURVEY involves segmentation of watercourses Fluvial Audit of the Catchment into geomorphologically homogenous reaches, for which a range of associated geomorphological descriptors are recorded. TASK 3: ANALYSIS An assessment of the geomorphological GIS Based Analysis character and behaviour of watercourse Geomorphological Analysis and the catchment as a whole is Linking field and desk based findings subsequently made, based on both the desktop and field survey data. This TASK 4: REPORTING includes the use of tools such as GIS, databases and spreadsheets to inform Detailed Geomorphological Survey geomorphological analysis. · Watercourse Summaries · Catchment-Scale Geomorphology · Catchment Geomorphological Action (NB: A full account of the development of the Catchment Fluvial Geomorphological Audit Figure 1.1 Overview of the Catchment Fluvial method is provided in the Method Description Geomorphological Audit process (Report A) – see Section 1.1)
· Extent The Axe Catchment Fluvial Geomorphological Audit does not include all of the watercourses in the Axe catchment. The extent of the survey was originally defined and agreed in consultation with the Environment Agency, based on their existing knowledge of river issues within the Axe catchment. Details of the lengths of watercourses that were included in the study are provided in Table 1.1 and Map 1.1.
Table 1.1 Extent of watercourses included in the Axe Catchment Fluvial Geomorphological Audit
Watercourse ID code Start NGR Description End NGR Description Length (km)
River Axe AXE ST412 063 Clapton Bridge SY259 926 A3052 Road Bridge 32.3
River Yarty YAR ST254 057 Bowditch Farm SY283 973 Conf. with River Axe 10.7
River Kit KIT ST294 065 Narfords ST322 015 Conf. with River Axe 6.2
Blackwater River BLA ST371 009 Sadborrow Mill ST324 023 Conf. with River Axe 5.6
Temple Brook TEM ST404 030 Broadwinsor Road ST406 030 Conf. with River Axe 3.7
Total Length 58.5
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·
ENVIRONMENT AGENCY Catchment Fluvial Geomorphological Audit of the Axe Catchment
· Reach-by-Reach Referencing System The Catchment Fluvial Geomorphological Audit utilises a logical reach-by-reach referencing system with which users should familiarise themselves. The system is based on the geomorphologically homogenous reaches identified during the field survey system and forms a framework for reporting and electronic outputs.
Watercourses are consistently referred to by the names shown in Table 1.1. (Users should be aware that some of these watercourses might be referred to by other names in other documents).
Reaches are consistently referred to by a unique identification code. This is formed of three letters and three numbers. The three letters are the Watercourse ID Code (see Table 1.1), while the number is a sequential number assigned to the reaches from upstream to downstream.
AXE-004
Watercourse ID Code Sequential Number
For example, the reach above is the fourth reach identified along the River Axe, working in a downstream direction (Map 1.1).
1.3.2 Components of the Detailed Geomorphological Survey
The detailed geomorphological survey comprises four component parts:
· This Report · A Web-Based Photo Viewer · A Reach-by-Reach Geomorphological Database · Geomorphological GIS Data
The electronic components can be found on the CD attached inside the backcover of this report, together with an electronic copy of the report itself. Tables 1.2 and 1.3 provide details of the content of each of the reporting and electronic components and how it is recommended they are used.
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Table 1.2 Outline of reporting content and how it should be used
Report Section Description Recommendations for Use 1.0 Introduction Introduces the project, its aims and Use this section to discover the (This Section) objectives, and provides a user guide concepts behind the project, what to project outputs. information is available and how it can be used.
2.0 Consultation An outline of the known Use this section as an geomorphological issues identified by introduction to the main staff within key organisations involved geomorphological issues in the in watercourse management within catchment. the catchment.
3.0 Catchment An overview of the Axe Catchment in Use this section to understand the Overview terms of physical character, human holistic context for consideration interaction and management. of geomorphological issues.
4.0 Geomorphological Summary of historical catchment and Use this section to develop an History channel changes underlying current understanding of how the current geomorphological condition of the geomorphological condition of the Axe Catchment. catchment has evolved over time.
5.0 Characterisation of Watercourse summary sheets Use the sheets as a first port of Watercourses highlighting the physical character, call for basic geomorphological historical change, geomorphological information relating to a specific behaviour and significant issues. watercourse.
6.0 Catchment-Scale A description of the prevailing Use this section to understand the Geomorphology sediment regime within the Axe geomorphological function of the catchment and the internal and catchment and provide an external factors that control it. overarching framework for consideration of specific geomorphological issues.
7.0 Summary Summary of contemporary catchment Use this section for an outline of influences, geomorphological the main catchment influences, processes and key issues. processes and issues.
8.0 References Literature and websites used to Full reference list of citations. compile this report.
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Table 1.3 Outline of electronic output content and how it should be used
Electronic Resource Description Recommendations for Use Web-Based Photo Viewer Provides a map interface through The Photo Viewer runs in Internet which a typical photograph of each Explorer and can be used to obtain geomorphologically homogenous an image of each reach to reach can be viewed. complement information provided in the report and other electronic outputs. Reach-by-Reach Provides access to all of the The database runs in Microsoft Geomorphological geomorphological field data Access and can be used to view Database collected during the Catchment and extract data relating to the Fluvial Geomorphological Audit. geomorphological character of any of the geomorphologically homogenous reaches surveyed. The reach-by-reach reference system is described in Section 1.3.1. Geomorphological GIS Provides the data required to The shapefiles provided are Data represent the geomorphological compatible with ArcView GIS field data collected spatially using packages. Basic knowledge of GIS. ArcView is required in order to view and manipulate the data. The reach-by-reach reference system is outlined in Section 1.3.1.
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2.0 Consultation
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2.1 Introduction
The consultation process was designed to outline known geomorphological issues within the catchment. Representatives from a range of organisations, with a role in watercourse management within the catchment were consulted. The consultation focused on gaining an understanding of the historical patterns and rates of fluvial geomorphological change. A primary objective was to gain an insight into the impact of past and present management techniques within the catchment.
The consultees are listed in Table 3.1:
Table 3.1 List of Consultees.
Organisation Name Position/Responsibility Watercourse Inspector - Area Environment Andrew Sweetapple Officer (Fisheries) Watercourse Inspector – Area Environment Andy Locke Officer (Fisheries) Dave Bartram Flood Defence Improvements Team Leader
David Brogden Team Leader Envrionment Management
Emma Rose Herrera Technical Officer Biodiversity.
Kate Bowers Area Hydrologist
Environment Kelvin Broad Fisheries – Technical Specialist Agency, Team Leader Fisheries, Recreation and Devon Area Office Nigel Reader Biodiversity. (Exminster) Paul Sadler Project Manager of the Cycleau Project
Phil Monk Flood Defence Operations Team Leader
Richard Smith Principal Officer – Land Quality Environment Officer – PhD on computer Rob Sweet modelling flood flows and floodplain sedimentation. Steve Moore Development Control
Tina Ainsley Ecological Appraisal Conservation Officer, East Devon and English Nature Amanda Newsome Exeter Farming and Wildlife Advisory Group John Hickey Axe / Char Project Officer (FWAG) East Devon District Charlie Plowden Countryside Officer Council
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2.2 Issues raised
Fine sediment delivery to the river channel was identified by consultees as the most significant geomorphological issue in the catchment. Fine sediment is delivered through a combination of bank erosion, soil erosion, catchment runoff and livestock poaching (trampling) on the riverbanks. Fine sediment can adversely impact on fisheries by filling voids in channel bed sediments (gravels) and clogging spawning gravels preventing the oxygenation of fish eggs. In addition, siltation may reduce channel capacity and contribute to an increase in the incidence of flooding. The intensification of farming during the latter half of the twentieth century is believed to have increased the impact of farming on the river system, although there are variations within the catchment. For example, the Blackwater River sub-catchment is intensively farmed, while the Kit Brook, benefits from less intensive farming practices.
Specific farming related issues raised in the catchment include:
· Harvesting late in the year during wetter periods, which is thought to be contributing to the increase in fine sediment to the river channels.
· Allowing livestock to grazing right up to the riverbank top, which can damage riparian vegetation and contribute to bank erosion.
· Increasing herd sizes and grazing pressure on the catchment.
· Installation of field drains and drainage ditches, increasing both sediment transfer to the river channel and decreasing the time it takes the river to respond to rainfall event.
· The compaction of soils by farm machinery increasing overland flow which both increases the rate of water transfer to the river channel and also increases the vulnerability of the land surface to erosion.
· Ad-hoc channel modifications implemented by farmers within the catchment, such as dredging and tipping to alleviate the perceived risk from flooding and channel migration.
The presence of various, more formal localised channel modifications was also highlighted as an issue in the Axe Catchment. For example, there are a number of bridges consisting of pipes through concrete (“Irish Bridges”) which can act as barriers to fish migration and sediment transport. There are also areas of localised bank protection works and channel straightening and re-sectioning. Channel modifications such as these interrupt natural geomorphological processes and can have an adverse impact on morphological diversity. A site at Wadbrook was highlighted as a specific example where the failure of a sympathetic bank protection scheme has resulted in the use of a hard engineering solution, at the detriment of river quality.
Further issues relating to the riparian zone include the extensive presence of invasive plant species, in particular Himalayan Balsam. Himalayan Balsam out competes native riparian plants during the summer retarding their growth. The Himalayan Balsam then dies back in the winter leaving the river channel margins free from vegetation which increases the vulnerability of the riverbanks to erosion. Diseased alders, suffering from Phytophthora root disease, are also extensive within the Axe catchment.
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3.0 Catchment Overview
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3.1 Topography
The River Axe rises in the Somerset Hills at an elevation of 175 m AOD. Initially the river flows in a westerly direction before turning south and flowing to the tidal limit, a total distance of 45 km, draining a catchment area of 303.8 km2 (Map 3.1). The major tributaries of the River Axe are the River Yarty, the Umborne Brook and Corry Brook. These tributaries rise on the steep slopes of the Blackdown Hills in the west of the catchment. The upper valleys of the catchment are well wooded, this gives way to a landscape dominated by small fields and hedges. The lower catchment is dominated by an agricultural landscape of pasture land, mainly used for dairy farming, with some arable farming primarily for maize cultivation. The valley of the River Axe progressively widens in a downstream direction as successive tributary streams join the main river. The main river valley of the River Axe is relatively wide with a flat valley floor across which the river meanders. Below the tidal limit the Axe joins the sea across an estuary, which although narrower than it once was, contains extensive intertidal areas on its western margin. This area is of ecological importance as it contains mudflats and areas of salt marsh crossed by a network of tidal creeks.
3.2 Geology, Soil and Hydrogeology
The geology of the catchment consists primarily of Mesozoic (Triassic, Jurassic and Cretaceous) strata locally overlain by Tertiary deposits (Eocene). The oldest rocks in the catchment, of Triassic in age underlie the lower reaches of the Axe and the lower and middle reaches of the Yarty and Coly in the western half of the catchment. These consist of silty mudstones of the Upper Keuper Marl Formation with small outcrops of laminated shales and limestones (Rhaetic Strata). The middle reaches of the Axe and Kit Brook are underlain by thinly bedded limestones and silty mudstones (Lower Lias) of the Jurassic. The upper Axe, including the Temple Brook and River Synderford, is underlain by micaceous clay and silt (middle Lias) also of the Jurassic. The higher ground of the upper tributaries and interfluves is underlain by sandstones of the Upper Greensand Formation of the Cretaceous. Occasionally the higher hills of the catchment are capped by a layer of Cretaceous chalk overlying the Greensands. Locally the Cretaceous strata is overlain by clay containing flints and chert dating to the Eocene.
The main river valleys in the catchment are filled with alluvium. Large landslips have occurred in a number of locations within the catchment particularly along valley sides where Cretaceous clays outcrop. The soils of the catchment reflect its geology. A variety of brown earth soils have developed in the upper catchment on the Greensand and Clays, these include argillic and stagnogleyic varieties. In areas adjacent to river and stream channels the gley types (clayey soil rich in organic matter) dominate. In the lower reaches of the catchment the poorly draining floodplains are dominated by palaeo-alluvial gley soils. The lower ground bordering the slopes also contain a range of soils including gleyic argillic brown earth, humic gleys (organic rich) and argillic palaeosols (Sweet, 2004)
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3.3 Hydrology and Water Resources
3.3.1 Precipitation and Flow
The average annual rainfall in the catchment ranges from 1067 mm on the upper catchment in the Blackdown Hills and North Dorset Downs, to 820 mm in the lowland areas of the Axe estuary.
River flow has been recorded at Whitford Bridge gauging station since 1964; this shows a mean daily flow of 5.11m3s-1 (cumecs). Flow at Whitifrod Bridge equals or exceeds 1.24m3s-1 95% of the time. The maximum recorded daily flow of the Axe is 144.72m3s-1 with a maximum instantaneous flow of 244m3s-1 which was recorded on 27th December 1979. Hydrological records indicate that base flow is high reflecting the soils and subsoil and the presence of large aquifers within the catchment (Environment Agency, 2003). The main aquifers in the catchment consist of the Cretaceous Greensand and Chalk strata. In addition the alluvium contained within the river valleys provides minor aquifers where high ground water storage and flow provide recharge during low flow conditions.
Table 2.1 Flow data (m3s-1) recorded at Whitford for the gauging station on the River Axe.
Parameter Data Location SY 262 953 Analysis Period 1964 onwards LTA* daily mean (m3s-1) 5.21 Median (Q10**) daily mean (m3s-1) 11.05 Actual 95% daily mean (m3s-1) 1.217
*LTA = long term average. **Q10 describes the threshold at which 10% of the daily mean flow is equalled or exceeded and provides an indication of high flow
3.3.2 Water Resources
There are 766 abstraction licences in the catchment of which 548 are for surface water and 218 from ground water. Over 80% of the abstracted water is from surface water. In total 29,000 m3 of water is abstracted daily of which 34% is used by industry, 27% provides public water supply and 22% is consumed by agriculture (Environment Agency, 2003). Water is also abstracted locally from the chalk and Greensand aquifers for private water supplies (Environment Agency, 2001). With the exception of a single case of over abstraction along the Umborne Brook, the catchment is not at risk from over-abstraction (Environment Agency, 1999).
3.3.3 Water Quality
In 2000 the watercourses in the Axe catchment were graded good or fairly good for chemical quality and good or very good for biological quality, using the national classification scheme, the General Quality Assessment (GQA). An exception, however, was the Temple Brook which was graded poor for chemical quality. Despite this, all the reaches in the catchment have long term River Quality Objectives (RQOs) of good or very good, as defined by the River Ecosystem (RE) classification scheme.
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However, water quality in the catchment has showed deterioration in recent years. In 1997, two reaches failed to achieve their RQOs. By 2002, the majority of the Axe and Yarty were failing to meet RQOs. This deterioration has been attributed to increased biochemical oxygen demand (BOD) and ammonia occurring mainly as a result of agricultural pollution with sewage treatment works (STW) discharges and storm sewer overflows also causing some problems (Environment Agency, 2003).
High nutrient levels, particularly phosphates, in the catchment have resulted in eutrophication, which is most severe in the lower catchment. The phosphates are derived from a range of sources including agricultural pollution, STW and industrial discharges. Nitrate levels are moderate in the upper catchment, however high nitrate levels in the ground waters in the lower catchment have resulted in Nitrate Vulnerable Zone (NVZ) designation. High suspended sediment levels (>1000mg/l) have also been recorded in the catchment. The mean level of suspended solids at Whitford Bridge in 2002 was 57.7 mg/l, well in excess of the Environmental Quality Standard (EQS) of 25mg/l (Environment Agency, 2003).
3.4 Land use
3.4.1 Landuse Types
The land use reflects the topography of the catchment (Map 3.3) and exerts an important influence of run-off and sediment yield. The land use of the catchment is predominantly agriculture, accounting for approximately 93% of the catchment area (Environment Agency, 1999). The quality of this agricultural land varies within the catchment. Agricultural land quality has been classified by the Department for Environment, Food and Rural Affairs (Defra) according to its potential for cultivation, Table 2.2.
Table 2.2 The Defra Agricultural land quality classification scheme. Grade Description 1-2 Prime agricultural land. 3 Moderate limitations to cultivation – low soil fertility or impeded drainage. 4-5 Severe limitations to cultivation – thin soils, steep slopes, poor drainage.
In the Axe catchment agricultural land is predominantly grade 3 with small areas of grade 2 land restricted to the north-east part of the catchment. The grade 3 land in the catchment consists of good quality permanent grass land. Land adjacent to the water courses of the catchment is generally graded 4, due to the presence of clay in the soil which impedes drainage. This land provides rough pasture with fertiliser and field ditches employed to help maintain the land (Sweet, 2004).
Farming can have a detrimental impact on the watercourses of the catchment through pollution. Pollution occurs from both defuse sources, such as land runoff, and point sources such as drains and tanks. Instances of water pollution tend to be dominated by increases in ammonia discharge during high rainfall periods, representing a defuse source of pollution. Sheep-dip and pesticides are also known sources of water pollution, although the impact of this on the water courses of the Axe catchment has not been quantified (Environment Agency, 1999).
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3.4.2 Landscape and Visual Amenity
The catchment is covered by a number of landscape designations including the Blackdown Hills Area of Outstanding Natural Beauty (AONB), East Devon AONB, West Dorset ANOB, these cover the north, south and east parts of the catchment respectively. The more remote areas of the upper catchment in Devon are designated as an Area of Great Landscape Value and those in Somerset as a Special Landscape Area. This designation, recognised by the local authorities, protects these areas through controls on the development process (Environment Agency, 2001). In addition the Blackdown Hills have been designated as a Natural Area by English Nature. This designation is designated to reflect the unique character of landscapes which can arise through the interaction of wildlife, landform, geology, landuse and human impact (www1). The Blackdown Hills are also subject to an Environmentally Sensitive Area scheme (ESA) managed Defra, here grant aid has been provided to support traditional farming methods (Environment Agency, 1999).
3.5 Fauna and Flora
3.5.1 Environmental Designations
A 13 km reach of the River Axe river the Blackwater confluence to the tidal limit is designated as a Special Area of Conservation (SAC) and contains two Sites of Special Scientific interest (SSSI) (Environment Agency, 2003). Important aquatic species include macrophytes such as Water Crowfoot, and fish species including Bullhead, Brook Lamprey and Sea Lamprey and Salmon. The SSSI designation also recognises the importance of this stretch of river for its abundant flora and fluvial geomorphological interest (Environment Agency, 2001). The SSSIs located close to water courses in the catchment are:
o Deadman ST 233 155 o Long Lye ST 264 121 o Park Farm Meadow SY 293 951 o Bulmoor Pastures SY 296 939 o River Axe cSAC SY 289 979 and SY 217 962
The River Yarty and Corry Brook lie within the Blackdown Hills Environmentally Sensitive Area. This designation is designed to encourage traditional farming methods enabling the ecology and landscape of the area to be protected.
3.5.2 Habitats
There are several important wetland habitats in the Axe catchment resulting from the widespread occurrence of heavy gley soils. Wet woodland is a characteristic feature of the upper catchment and contains species rich flora and invertebrate communities. Land drainage and clearance for agriculture has resulted in a decline in this habitat. Springs rising from the chalk areas provide valuable wetland habitats known as spin line mires. Rhôs pasture (unimproved heathy grazing pasture) is internationally important, species-rich wet grassland, of which 80% of the national resource occurs in Devon. Despite its importance, this habitat has decline in extent due to agricultural improvement and a lack of appropriate management (Environment Agency, 1999).
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The catchment contains a diverse range of in-stream and channel margin habitats and species. In addition to the River Axe SSSI, which supports the Starwort and Medicinal Leech, the catchment contains excellent examples of landforms and habitats created by fluvial process. The river channel is characterised by a meandering planform, a gravel bed with a well developed riffle pool sequence, extensive bars, eroding cliffs, frequent backwaters and ox-bow lakes (Grieve and Clarke, 2003). Eroding riverbanks and cliffs provide important nesting sites for sand martins and kingfishers; these are vulnerable to erosion control measures. Exposed Riverine Sediments (ERS) (sand and gravel bars) provide an important habitat for a range of invertebrate species, in particular beetles. These are vulnerable to flood defence works and channel engineering (Environment Agency, 1999). The only significant Reedbed habitat in Devon is found in the Axe catchment. Reedbeds support a range of bird species and are a particularly important amphibian habitat.
3.5.3 Species
A number of nationally and internationally important species are found within the catchment, these include otter, pipistrelle bat, sand martin, kingfisher, curlew, reed bunting, barn owl, lapwing, snipe, Atlantic salmon, lamprey, marsh fritillary butterfly, medicinal leech, water starwort and invertebrates of exposed riverine sediments. The River Axe SSSI supports an internationally important macrophyte community. Channel vegetation is highly diverse and includes Ranunculus spp., Callitriche spp., Potamogeton spp. Of particular importance is the extent of Callitriche truncate which represents on of the most important populations in the country (Grieve and Clarke, 2003). A survey undertaken in 1999 showed that no losses of important species were occurring nor were there signs of any decline in species richness (Holmes, 2000).
During the last decade non-native invasive species have spread widely along the water courses of the catchment. While Japanese knotweed (Reynoutria japonica) is relatively localised, Himalayan balsam (Impatiens glandulifera) is extremely widespread. Giant hogweed (Heracleum mantegazzianum) has not been reported in the catchment, but is known to occur in the neighbouring Lim catchment. There is also concern about the potential spread of non-native aquatic species used in garden ponds, which are vigorous growers. Species include parrot’s feather (Myriophyllum aquaticum), water fern (Azolla filiculoides), floating marsh-pennywort (Hydrocotyle ranunculoides) and swamp stonecrop (Crassula helmsii). Although no information is available about the extent of these species in the Axe catchment, they have been found in the wild across Devon.
Diseased alders (Pytophthora Disease) are relatively common in the Axe catchment, particularly along the channel margins in the Axe SAC. Tree death can contribute to bank erosion both through toppling into the river channels and break-up of the root structure, which increases the vulnerability of bank sediments to scour.
3.5.4 Fisheries
The Axe is an important sea trout and brown trout fishery. Until 1994 some riparian owners stocked reaches of the River Axe with farmed brown trout. This is thought to have had a detrimental effect on wild populations by creating competition for habitat and food and increasing predation (Environment Agency, 1999). Despite a decline in the trout fishery during the past decades recent evidence suggests that this fishery is now improving. Juvenile trout
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have been recorded throughout the river systems and trout populations are good in most tributaries (Environment Agency, 2003).
Until the mid-1960s the river provided an important Salmon fishery, however this has declined dramatically over the last 30 years with returns from rod fishing falling to zero. In 1990-91 NRA fisheries rehabilitation scheme began and has continued under the stewardship of the Environment Agency including habitat improvements and restocking. As a result salmon and roach populations are starting to show signs of recovery. A smolt-rearing pond has been established on the River Coly and several thousand smolts were released into the Axe in the spring of 1999. Since this time the pond has been used to rear juvenile salmon originating from the Exe catchment (Environment Agency, 1999). It appears that this initiative is having some success with 48 salmon caught in 2002 and a breeding stock now appears to have been re- established on the River Yarty (Environment Agency, 2004).
During the 1970s the middle and lower reaches of the Axe supported large stocks of coarse fish, primarily roach and dace providing a coarse fishery, although there has since been a decline in the size of the populations, particularly of larger fish (Environment Agency, 1999). The lower river has recently been restocked with roach (Environment Agency, 2003)
There are several large weirs in the Axe catchment many of which are passable to migrating fish, such as those on the Axe and Yarty. However, there are a number of weirs which remain barriers to fish migration; these include weirs on the upper reaches of the River Coly, Umborne Brook together with weirs on the Fordton Stream, Whatley Stream and River Axe upstream of the Temple Brook confluence. Fish movement is also inhibited by over abstraction along the Umborne Brook at Wilmington where water abstraction for a trout farm reduces flow each year from the late spring to autumn (Environment Agency, 1999).
There are known to be instances where riparian land owners have excavated the river channel flowing through their land, although in many instances such works do not require Environment Agency approval, they can have negative impacts on fisheries by destroying spawning bed and nursery areas (Environment Agency, 1999).
3.6 Human Beings
3.6.1 Population
There are numerous settlements in the catchment, the largest of which are the towns of Seaton by the Axe Estuary, Axminster in the middle reaches of the catchment and Chard in the upper catchment; however, the majority of settlements consist of small villages and hamlets. The population of the catchment in 1991 was approximately 40,000 (Environment Agency, 2001).
3.6.2 Cultural Heritage, Archaeology and Material Assets
The catchment contains many sites of cultural, historic and archaeological importance. Approximately 40 sites have been recognised in the Axe and Lim catchments as being nationally important and have been designated as Scheduled Ancient Monuments (SAMs) (Environment Agency, 2001). In addition approximately 10 settlements in the catchment have been designated as Conservation Areas in order to preserve and enhance their character.
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The earliest archaeological evidence of human occupation dates back more than 250,000 years and consists of 2500 stone Axes from Broom on the River Axe (Environment Agency, 2001). Important archaeology sites in the catchment also include Stockland and Membury, where evidence for Mesolithic (c8,000-4,000 BC) hunter gatherers has been found. Later sites include round Barrows dating from the Neolithic and Bronze Age. Much of the native forest in the catchment was cleared during the Bronze Age. The catchment also contains several Iron Age Hill Forts and Roman sites including settlements villas and roads. The Exeter Road (Roman road) crosses the floodplain of both the rivers Axe and Yarty (Environment Agency, 2001).
The character of the landscape in the catchment was established by the time of the Domesday Book (1086 AD) through the practice of enclosure creating the characteristic small field of the area. Subsequent change occurred with the creation of estates and large holdings and with the development of roads and railways (Environment Agency, 2001). More recent features of historical interest include the Taunton Stop Line a Second World War Defence and numerous pill boxes (gun emplacements) close to the River Axe. Unfortunately recent bank erosion has undermined many of these pill boxes which is leading to both the loss of historic features and obstructions to flow (Environment Agency, 1999).
3.6.3 Recreation
A range of recreational activities occur in the Axe catchment. The most significant water-based activity is angling which occurs throughout the catchment and in particular along the River Yarty. Additional water-based activities are restricted to the coast and include water-skiing and diving and snorkelling in the Seaton area. Cycling and walking activities occur throughout the catchment, however as most of the riparian land in the catchment is in private ownership this is concentrated into the existing rights of way network and is not expected to become more widespread (Environment Agency, 1999).
3.7 Catchment Initiatives
The main catchment initiative is the River Axe and Char Enhancement Project, part of the Cycleau Project. This is a 3 year European Commission funded programme designed to offer sustainable management of rivers, estuaries and coastlines in parts of Devon, Cornwall, Ireland and France (Environment Agency, 2003). In the Axe catchment the project adopts a partnership approach to address problems associated with pollution from nutrients, unrestricted livestock access to the river, increased maize cultivation, soil erosion, bank erosion and waste management. It is envisaged that this project will improve the future for agriculture, protect and improve species, habitats, landscape and archaeology. A number of additional initiatives have been implemented in the catchment, these are:
· The River Axe Catchment Abstraction Management Strategy (CAMS) has recently been published by the Environment Agency. This is designed to manage pressures on water resources and future water needs in the catchment while protecting the requirements of the river environment.
· A Nitrate Vulnerable Zone (NVZ) action programme has been initiated in the lower catchment aims to reduce nitrate from agriculture, specifically through improvements in agricultural management practices.
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· The upper and middle Axe was the target of a Farm Waste Management Campaign funded by MAFF (now Defra) between September 1999 and March 2000. Farmers were encouraged to prepare waste management plans for the spread of organic waste (mainly manure) to establish the location and quantities of spreading in order to minimise water pollution (Environment Agency, 1999).
· The East Devon AONB and West Dorset AONB have been targeted for funding by the Countryside Stewardship Scheme (Defra) to provide grants to farmers and landowners to improve natural beauty and diversity.
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4.0 Geomorphological History
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4.1 Introduction
River character and behaviour reflects the integrated effect of independent basin controls such as climate, geology, land use and valley form which together determine the hydrological regime and the quantity and type of sediment supplied. Changes in the frequency and magnitude of precipitation events (and climate over longer timescales) in addition to land use and land management change has led to river channel adjustments. A better understanding of geomorphological behaviour of the Axe can be gained by collating historic hydrological and land use data. The following sections describe these catchment changes and are summarised in a time chart in Table 4.3.
4.2 Catchment Changes
4.2.1 Hydrological/Climate Change
Seven large flood events are known to have occurred in the Axe catchment prior to 1950 (Table 3.1). Since 1950 significant flood events have occurred in East Devon in 1952, 1960, 1966, 1967, 1968, 1974, 1979 1985, 1992 and 1993. In the Axe catchment particularly significant floods occurred in December 1965 (2.5% probability event), July 1968, December 1985 and December 1992 (Environment Agency, 2003).
Table 4.1 Documentary accounts of flood events in the Axe catchment prior to 1950. Source: British Hydrological Society Hydrochronology database for the Exe Group of rivers (www2)
Year Day Comments 1630 14 January Great storm of wind and rain experienced in Axminster 1875 14 July “Downpour of rain….the Axe valley in Devon was submerged.” 1881 14 February “The highest flood ever known in Colyton.” 1894 13 November “The record of 3.53 in. of rain in three days, November 11th to 13th, is not often equalled, and those who saw the great expanse of the Valley of the Axe resembling an inland sea, are not likely to forget the sight.” 1900 16 February “Heavy snow fell on February 13th, followed by a sudden rise of temp. and much rain, 2.11 in. falling between 13th and 16th. This caused floods of unusual magnitude over the low-lying districts of Devon and Dorset. The valley of the Axe was under water for a long distance inland. The tides, which were the highest known at Axmouth for fifteen years, ran with such force as to carry away the Woodhayne Bridge of the South Western Railway near Axminster.” 1909 22 December The ground was frozen hard when rain began to fall, which in 24 hours amounted to 2.95 in. Roads were torn up, and bridges, gates and fences were carried away.” 1926 17 July “At Axminster thunder was continuous from 10pm on 17th to 11am on the 18th. The rainfall amounted to 2.5 inches, causing great floods which swept away fields of mown hay and other crops.”
Discharge has been gauged in the Axe catchment at Whitford Bridge (NGR SY 262 953) since 1964. The hydrological response of the catchment is rapid with sharp stage rises leading to high peak flows followed by quick declines (Figure 4.1).
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Figure 4.1 Hydrograph illustrating mean daily flow for the River Axe at Whitford between October 2001 and October 2003.
100
90
80
70 )
-1 60 s 3 50
40 Flow (m
30
20
10
0 01/10/01 09/01/02 19/04/02 28/07/02 05/11/02 13/02/03 24/05/03 01/09/03 Date
The mean monthly discharge is highly variable with peak flows tending to occur during January and February while low flows occur during July and August. The average mean monthly discharge has shown a slight rise over the period 1964-2003 which appears to be mainly the result of higher monthly flows during the winter months (Figure 4.2). Similarly the average annual maximum flow has shown a slight rise during this period (Figure 4.3). However, the highest annual maxima on record were recorded in 1979 and 1969.
Figure 4.2 Mean daily discharge recorded on the River Axe at Whitford Bridge over the period 1964 to 2002. The line illustrates the slight rise in mean monthly discharge over the period.
25 ). -1 y = 0.0036x + 1.6752 s 3 20 R2 = 0.015
15
10
5 Mean Monthly Dicharge (m
0 Dec-64 Dec-66 Dec-68 Dec-70 Dec-72 Dec-74 Dec-76 Dec-78 Dec-80 Dec-82 Dec-84 Dec-86 Dec-88 Dec-90 Dec-92 Dec-94 Dec-96 Dec-98 Dec-00 Dec-02 Date
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Figure 4.3 Annual maxima recorded on the River Axe at Whitford Bridge over the period 1964 to 2002. The line illustrates the slight rise in annual maxima over the period.
300
y = 0.5239x + 103 250 R2 = 0.0152 )
-1 200 s 3
150
100 Dishcarge (m
50
0 1965 1970 1975 1980 1985 1990 1995 2000 Year
The average annual rainfall experienced by the catchment ranges from over 1000mm on the Blackdown Hills to less that 800mm along the coast (Environment Agency, 2003b). While high monthly rainfalls generally correspond with high monthly river flows, their relationship is complex (Figure 4.4). The relationship between rainfall and river discharge for the 1980s, 1990s and 2000s illustrates this complexity but also indicates that there appears to have been an increase in discharge relative to rainfall over the past 30 years (Figure4.5).
Figure 4.4 Comparison of monthly rainfall and mean monthly flow for the Axe catchment between 1979 and 2003.
250 25
200 20
150 15
100 10 Rainfall (mm) Discharge (m3sec-1) 50 5
0 0 Augu Augu Augu Augu Augu Augu Augu Augu Augu Augu Augu Augu Augu st-79 st-81 st-83 st-85 st-87 st-89 st-91 st-93 st-95 st-97 st-99 st-01 st-03 Date
Rainfall Discharge
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Figure 4.5 The relationship between monthly rainfall and monthly river discharge for the Axe catchment divided according to decades.
25
20 R2 = 0.7058
2 ) R = 0.6255 -1 s
3 15 R2 = 0.5375
10 Dicharge (m
5
0 0 50 100 150 200 250 Rainfall (mm)
2000s 1990s 1980s Linear (2000s) Linear (1990s) Linear (1980s)
The increase in discharge relative to rainfall events, particularly at higher rainfall intensities, indicated by Figure 4.5, may result form the increase in the extent of arable farming in the catchment. Arable farming results in periods during which the fields within the catchment are free from vegetation. This reduces both rainfall interception and evapo-transpiration. This will lead to an increase in both overland flow and the volume and rate of ground water movement towards the river channel, which as a result increased river discharge. However, this data must be treated with some caution as less data was collected during the 1980s as compared to the 1990s and the data for the 2000s only covers five years, this may have introduced some bias into the data.
Future climatic change may also influence river discharge. High summer temperatures are likely to become more frequent while cold winters become increasingly rare. Winters are expected to become wetter in contrast to summers which will become drier (www3). This is likely to have significant impact in catchments where arable land use is significant as the ground is likely to be bare during the winter when rainfall and storminess is expected to increase.
4.2.2 Land Use Change Changing farming practices, particularly the increase in arable farming, have been linked an increase in fine sediment (silt) delivery to watercourses (Section 3). Changes in catchment land use over the last century can be examined using agricultural census statistics compiled by Defra and its predecessors. This data provide an indication in the farming trends which may reflect government policy and farming benefit availability. Changes in agricultural land use in Devon (Figure 4.6a) and Somerset (Figure 4.6b) reveal that a reduction in rough grazing began in the 1940s initially in response to increasing arable land. However, arable land use in Devon declined in extent between 1970 and 1980, a trend which has since been reversed in the Axe catchment where an increase in the extent of arable land, accompanied by a decrease in the
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extent of improved grassland of a comparable magnitude, has occurred over the past decade (Figure 4.7).
Figure 4.6 Charts illustrating the change in agricultural land use in Devon (a) and Somerset (b) (www4).
(a)
700,000
600,000
500,000
400,000
300,000
Area Acres 200,000
100,000
0 1900 1910 1920 1930 1940 1950 1960 1970 1980 Year Permanent Grassland Arable land Rough Grazing
(b)
700,000
600,000
500,000
400,000
300,000 Area Acres 200,000
100,000
0 1900 1910 1920 1930 1940 1950 1960 1970 1980
Year Arable Permanent grassland Rough Grazing
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Figure 4.7 Change in broad habitat type in the Axe catchment over the period 1990 -1998, based on data provided by the Environment Agency.
1500
1140
1000
500 263 159 70 79 -9 -7 -6 3 18 0 -60 -53 -52 -29
-500
Change in area 1990-1998 (Ha) -1000
-1500
-1553
-2000
Bracken Inland rock Shrub heath Build up area Acidic grassland Boundary linear Rivers and stream Fen/march/swampNeutral grasslandArable/horticulture Improved grassland Confirous Woodland Cacareous grassland Standing open water Broadleaf/Mixed woodland Broad habitat
Livestock statistics for Devon and Somerset (Figure 4.8) reveal a significant increase in sheep numbers since the 1940s. A smaller increase in cattle numbers occurred in both counties after the 1940s but since the 1970s cattle numbers have begun to decline. Pig numbers in the two counties have increased but remain low in comparison with cattle and sheep. Maize growing and open-air pig rearing leave land bare during the winter resulting in large quantities of silt being washed into adjoining water courses. In addition, tramping (poaching) of channel margins by livestock at crossing at watering points damages riparian vegetation and destabilises the banks and increases sediment delivery to the river channel through erosion. Despite encouragement and grants schemes to encourage farmers to fence riverbanks to reduce poaching have had limited success (Environment Agency, 1999).
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Figure 4.8 Livestock numbers in Devon (a) and Somerset (b) between 1900 and 1999 (www4).
(a)
2,500,000
2,000,000
1,500,000
1,000,000
Livestock numbers 500,000
0 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 Year Total Sheep Total cattle Total pigs
(b)
700,000
600,000
500,000
400,000
300,000
200,000 Livestock numbers
100,000
0 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 Year Total Sheep Total cattle Total pigs
In addition to variations in farming practices changes, in the management of riparian vegetation can also have an impact on river channels. A decline in riverbank management, involving a reduction in coppicing or pollarding, has meant that bank-side trees have developed full crowns making them sensitive to destabilisation during high winds or flood events. Falling trees result in mechanical erosion of the riverbank and expose bank sediments making them vulnerable to water erosion and livestock poaching (Environment Agency, 1999).
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4.3 Channel Changes
4.3.1 Natural Geomorphological Change
Natural geomorphological changes in the catchment have predominantly taken the form of changes in channel planform along the River Axe, primarily though the growth and cut-off of meanders. Channel changes in the catchment between 1891 and the present are summarised in Figure 4.9. Channel changes are concentrated on the River Axe primarily downstream from Chard Junction. While these channel changes have generally involved the cut-off of individual bends, at the confluence of the River Axe and the River Yarty. A series of bends have been cut- off. At Whitford however, approximately 500 metres of channel has moved from the left (east) side of the valley floor to the right (west). This change appears to have been complex and may have occurred in stages, involving a number of bend cut-offs. However, the historical map evidence suggests that the river’s current position on the right bank partially occupies a former ‘fishing lane’ which appears to have been deliberately excavated at some point prior to 1891. This suggests that the channel changes recorded in this location were in part a result of anthropogenic intervention.
There is little documentation relating to the specific geomorphological impacts of hydrological events. While many of the bend changes recorded are likely to have been encouraged by flood events, they may also have occurred as a result of the gradual but progressive growth of bends which is typical of meandering rivers.
A number of additional channel changes along the Axe were initiated by human activity. The river channel at Chard Junction was completely realigned some time after 1891, possibly to facilitate industrial development in this location. At Westford Mill, downstream from Chard Junction a 400m section of river channel was excavated prior to 1891 in conjunction with the development of the mill. Field evidence indicates that this channel now conveys a greater quantity of flow that the original channel. In addition a number of bends were also removed during construction of the railway in the mid-nineteenth century.
4.3.2 Flood Defence
The majority of the Axe catchment is largely unprotected from flooding; although major flood defence schemes have been implemented at Axminster, Stafford Brook and Seaton. Flooding is an issue in number of other locations in the catchment, in these locations flood warnings are provided (Table 4.2).
Table 4.2 Flood Warning Reaches in the Axe Catchment (Environment Agency, 2003b). River Length (km) First Risk Settlement Gauging Station from which flood warnings are provided Axe 14 Mosterton Winsham, Chard Station Axe 8 Chard Junction Winsham, Chard Station Axe 6 Weycroft Winsham, Chard Station Yarty 14 Longbridge Court Place Farm
In each of the sites susceptible to inundation insufficient channel capacity has been identified as the principle cause of the flood hazard, although at Weycroft development on the floodplain has exacerbated the hazard.
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Figure 4.9 Map indicating the location and forms of channel changes observed in the Axe catchment since 1891, based on comparison of the 1891 Ordnance Survey Map and the contemporary OS map of the area. The scale bar refers to the main map.
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
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4.3.3 Channel Maintenance
There is little regular maintenance on the main rivers of the Axe catchment. All maintenance operations are conducted where it is deemed necessary. The majority of the maintenance operations undertaken are designed to improve the catchment fishery, and have included:
· Removal of obstructions · Gravel rehabilitation · Installation of fish passes at large weirs · Riverbankside fencing projects · Riverbankside coppicing/planting schemes (Environment Agency, 2003a)
A further management option under consideration is the removal or replacement of Irish Bridges to allow the passage of migrating fish (Environment Agency, 2001).
In general the Environment Agency encourages natural river processes; however there are locations where bank erosion has been deemed unacceptable. In these situations soft engineering techniques are encouraged such as willow spilling and hazel bundles. Locally however, land owners have excavated sections of river channel flowing through their land. Although many of these works do not require permission they may be damaging to river channel morphology and fish habitats (Environment Agency, 2001).
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2000s Construction of fish passes at high weirs
in Maize
1990s Dec 1992 Sympathetic approach to bank protection promoted by EA. Increase cultivation. Decline in improved grassland. 4 Active quarries in catchment. Sand and gravel extraction at Chard Junction and Kilmington.
1980s Dec 1985
1970s Land drainage improvements
y.
1960s Flood in Dec 1965 with 2.5% probabilit Flood Jul 1968. Nutrient application. Increase in stocking densities.
1950s Ad hoc bank protection and channel works by landowners
off.
- Feb th Dec 1909 th 1940s Jul 1926. - th 1900s Floods on 6 1900, 19 and 17
881 and 1894. Pre 1900 Large floods in 1875, 1 Localised realignment of the River Axe during construction of the railway along Axe valley. Meander growth and occasional cut me chart for the infleunces onsediment and morphology in Axe Catchment
Table 4.3 Ti
Time Floods Capital Works River Maintenance Channel Change (Historic Mapping) Land Use Change (Agriculture) Land Use (Minerals)
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5.0 Characterisation of Watercourses
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5.1 Introduction
The trunk channel and tributaries of the Axe are summarised individually in terms of:
· Physical Character · Historical Change · Geomorphological Behaviour · Significant Issues
Significant issues apparent within the watercourse stem from observation or interpretation during field survey or/and data analyses. The River Yarty is divided into two sections and the River Axe is divided into four as determined by a change in physical characteristics, including valley form, channel gradient and substrate. The “watercourse” boundaries for the Yarty and Axe are described in Table 4.1.
Table 5.1 Watercourse boundaries for the River Yarty and River Axe
Upstream Limit Downstream Limit Reach Codes Bowditch Farm Higher Westwater YAR001-YAR015 Higher Westwater Confluence with the River Axe YAR016-YAR019 Clapton Bridge Chard Junction – Broad Bridge AXE001-AXE014 Chard Junction – Broad Bridge Confluence of River Kit AXE015-AXE021 Confluence of River Kit 100m u/s of A35(T) Road Bridge AXE022-AXE031 100m u/s of A35(T) Road Bridge A035 Road Bridge AXE032-AXE042
5.2 Watercourse Summary Sheets
The river channels of the catchment have been divided into the watercourses, outlined in Table 5.1, where a series of consecutive reaches show similar geomorphological behaviour. The watercourses consist of a series of surveyed reaches with similar characteristics. The Watercourse Summary Sheets detail the physical character, historical change and geomorphological behaviour and also identify geomorphological issues along of each watercourse. This information was compiled by displaying the survey results, stored within a database, in a GIS system. The watercourse summary sheets therefore provide baseline geomorphological information for individual sections of the catchment.
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Watercourse Name: TEMPLE BROOK Catchment Location Map:
Summary Details: Upstream survey limit: ST404030 Broadwinsor Road Downstream survey limit: ST406061 Confluence with River Axe Length of river surveyed: 3.7km Geomorphological reaches: TEM001-TEM011
Physical Character: · Medium gradient channel located within a shallow vee. Planform is sinuous where constrained by valley side slopes and irregularly meandering where confinement is absent. · Bed material generally ranges from silt to fine gravel with coarse gravel and cobbles being concentrated in the upper reaches. · Bank materials are varied throughout and range from silt to cobbles, locally the banks have a composite structure. · Cliffed banks occur extensively along while graded banks are also common. The channel has a U-shaped cross-section the dimension of which varies with planform. · Flow is either varied or highly varied throughout. Riffles, runs and pools being common morphological features, while glides also occur extensively in the lower reaches. · The majority of channel deposits are side bars, point bars and toe accumulations, with side bars being particularly dominant. These deposits consist of both fine and coarse material. · The majority of channel deposits, both coarse and fine, are temporary features, with permanent deposits restricted to fine material in the upper reaches. · The channel is extensively tree lined, with only isolated sections of the upper reaches free from tree lining. · Channel vegetation is extensive throughout consisting mainly of exposed tree roots with emergent reeds where tree cover is less dense. · Channel modification is limited to a short section at Greenham (TEM007) which has been re-sectioned and walled, and two small weirs at the join of reaches TEM001 and TEM002.
Historical Change: There is no evidence of historic channel planform change due primarily to locally high lateral confinement and the presence of dense tree lining. The channel in the lower reaches appears to be relatively incised suggesting change is likely to have been vertical rather than lateral.
Geomorphological Behaviour: The confined nature of the channel, cohesive banks and extensive tree lining mean that bank erosion does not contribute large volumes of sediment to the river channel. There are numerous examples of cliffed riverbanks but erosion rates appear to be low. Sediment delivery is dominated by point sources including field ditches and livestock poaching. Tributary streams are also important sources of both fine and coarse sediment.
The majority of the sediment delivered to the river channel is deposited on the channel bed, as the confined and locally incised nature of the channel prevents out of channel deposition. Many of the channel deposits are temporary indicating that the watercourse acts as a sediment transfer reach.
Significant Issues: · Field ditches common, increase runoff and fine sediment delivery. · Cattle poaching leads to locally high fine and occasionally coarse, sediment delivery. · Invasive species (Himalayan Balsam) is extensive, particularly along field boundaries. · Occasional diseased alders.
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Watercourse Name: BLACKWATER RIVER Catchment Location Map:
Summary Details: Upstream survey limit: ST371009 Sadborrow Mill Downstream survey limit: ST324023 Confluence with River Axe Length of river surveyed: 5.6km Geomorphological reaches: BLA001-008
Physical Character: · The upper section (BLA001) is of high gradient while the majority of the river is of a medium gradient. · As the degree of lateral confinement diminishes from a tight vee to a concave bowl in a downstream direction the channel becomes increasingly sinuous. In the lower reaches the channel is surrounded by a floodplain varying in width from 5-10 river widths. · The channel bed sediments are unsorted and vary in size from clay to cobbles. The coarser material is generally angular to sub-angular throughout. · Bank materials also vary considerably ranging from clays to coarse gravel although fines, particularly silt are more extensive. · The banks are generally steep or cliffed and the channel has a U-shaped cross-section the dimensions of which varies according to planform. · Flow is varied or highly varied reflecting good morphological diversity. Riffles, pools and glides and runs are common throughout. In general morphological diversity is greatest in the upper reaches. · Channel deposits consist of accumulations of both fine and coarse sediment, with side and point bars being the most dominant morphological forms. · The majority of the fine channel deposits are temporary, with some semi-permanent accumulations. Coarse deposits include both temporary and semi-permanent deposits. · Although channel modifications are generally limited to local walling or occasional placed boulders (BLA008), reach BLA006 has been subject to recent channel dredging, tipping and riparian and bank face vegetation clearance. Conducted by a local farmer this has degraded both the ecological and morphological condition of the river channel and threatened the stability of the channel. · In conjunction with riparian tree clearance along BLA006 the land on the right bank has been tilled right to the channel margin.
Historical Change: There is no evidence of historic channel planform change along the watercourse. This appears to reflect local confinement (upper reaches), cohesive banks and tree lining. Channel interference along reach BWR006 has been initiated in response to a perceived increase in lateral channel movement (personal communication with local farmer) however the channel instability in this location appears to have been stable over the past century.
Geomorphological Behaviour: Point sources represent the main sources of sediment along the watercourse. These range from tributaries (coarse and fine) field ditches (mainly fine) and cattle poaching (fine and coarse). Diffuse sources are limited to occasional cliffs, but are of less importance as rates of lateral change are low due to tree lining and bank cohesion. Sediment delivered to the river channel is stored in temporary channel deposits mainly bars, the majority of which are temporary features. This indicates that over decadal timescale the Blackwater River transfers sediment downstream to the River Axe.
Significant Issues: · Tipping, dredging and vegetation clearance along BLA006 have resulted in increased fine and coarse sediment delivery to the river channel and threaten the future stability of the channel in this location. · Poaching is common throughout the watercourse, but is particularly significant in the upper reaches. · Himalayan Balsam occurs in reaches BLA004-BLA008.
BBR No: BWA 0013499 41 Rev R01 October 2004 ENVIRONMENT AGENCY
Watercourse Name: KIT BROOK Catchment Location Map:
Summary Details: Upstream survey limit: ST296063 Narfords Downstream survey limit: ST322015 Confluence with River Axe Length of river surveyed: 6.2km Geomorphological reaches: KIT001-KIT021
Physical Character: · Medium gradient channel with a generally sinuous planform. Locally the channel is straight (KIT008 and KIT011) and anastomosed (KIT003-004). · The valley form varies from a deep vee in the upper reaches and in reaches (KIT010 and KIT013), with a narrow discontinuous floodplain, to a shallow vee or concave bowl with a continuous floodplain of 5-10 river widths. · Bed materials are variable and range from silt to cobbles. Boulders are present in reach KIT013. In general the bed sediments are sorted. Natural debris is extensive and in reach KIT018 artificial material is also present. · Bank sediments generally vary from silt to sand with occasional pockets of fine gravel. Artificial bank materials are found in reaches KIT006, KIT010 AND KIT014. · The channel has a U-shaped cross-section, with the exception of reach KIT004 which is rectangular, and variable with planform. Reaches KIT003-005 and KIT010 are exceptions as cross-section varies without planform as are KIT008 and KIT011 which are uniform. · Flow is highly varied in the upper reaches and varied elsewhere reflecting a medium to high morphological diversity. Runs, riffles, pools and ponds are found throughout while dead water is also common in the lower reaches. Reach KIT008 however is an exception having a uniform bed structure. · Coarse channel deposits occur extensively throughout, while fine deposits are concentrated in the upper reaches. These deposits include side bars, point bars and mid-channel bars. · The majority of the coarse deposits are temporary with some semi-permanent deposits also present, particularly in the upper and lower reaches. Temporary deposits, clustered into the upper reaches are temporary in nature. · Exposed tree roots occur extensively but channel vegetation is most concentrated in the lower reaches where moss and algae occur. · Channel modification is most pronounced in reaches KIT008 and KIT014 which have been resectioned and re-aligned. Occasional bank protection such as walling and concrete is present in all reaches.
Historical Change: No channel changes were detected during an analysis of historic maps. In reach KIT008 man-made levees are present along both banks.
Geomorphological Behaviour: Locally bank erosion is occurring mainly in the form of toe scour and slumping but this has not lead to lateral channel changes and contributes a limited amount of sediment. This lateral inactivity reflects a combination of cohesive banks and tree lining is continuous or fragmentary in nature. Point sources such as tributaries, drains and livestock poaching provide sources of both coarse and fine sediment. To the channel. Much of the sediment introduced to the river channel is deposited on the river bed before being remobilised at high flows and transferred downstream. In the upper reaches some sediment is deposited on the floodplain in the form of splays rather than being transferred downstream.
Significant Issues: · Localised channel modifications limit natural channel behaviour. · Livestock poaching provides a localised source of sediment. · Himalayan Balsam is present in the lower reaches of the watercourse, inhibiting native riparian vegetation.
42 BBR No: BWA 0013499 Rev R01 October 2004 ENVIRONMENT AGENCY
Watercourse Name: RIVER YARTY Catchment Location Map:
Summary Details: Upstream survey limit: ST255055 Bowditch Farm Downstream survey limit: SY275999 Higher Westwater Length of river surveyed: Geomorphological reaches: YAR001-YAR015
Physical Character: · Medium gradient in upper reaches but decline to low gradient in reaches YAR014 and YAR015. · The channel is locally confined within a shallow vee, although elsewhere the valley has the form of a concave bowl. · In its upper reaches the channel has developed irregular meanders. In reach YAR010 the river has a anastomosed planform and in YAR011 the channel is straight. Downstream from YAR012 the channel is sinuous. · The bed materials are highly variable and range from silt to cobbles. Silts, sands and coarse gravels are more common in the upper reaches. Coarse gravels and cobbles are common throughout the watercourse with boulders being restricted to YAR013. · Bank sediment varies considerably from clay to coarse gravel. Silts and sands are most extensive in the upper reaches, with clay occurring in isolated locations. Artificial bank material is present in reaches YAR005- 006, YAR008 and YAR011-14. · Riverbanks are frequently cliffed although graded banks are also common. In cross-section the channel is mainly U-shaped the dimensions of which are variable with planform. · Flow is varied or highly varied reflecting good morphological diversity. Riffles, runs, pools, ponded reaches and dead water are found throughout. Reach YAR013 provides a notable exception with low morphological diversity. · Coarse channel deposits are common throughout and include side bars, point bars, mid-channel bars and islands with occasional berms. Fine channel deposits are only present in reach YAR004 and are temporary features. The majority of the coarse deposits are permanent or semi-permanent although some temporary bars are present in the upper reaches. · Channel vegetation occurs throughout the watercourse but varies considerably in extent. Submerged vegetation, moss, exposed roots and emergent reeds are the most common types of vegetation. · The channel has been culverted in reach YAR011 and reaches YAR014-015 have been realigned. Elsewhere the channel has not been modified.
Historical Change: Although only one cut-off has occurred since 1891 (see section 3.3), historic map evidence suggests that in reach YAR010 where the channel is anastomosed, some small scale channel changes have occurred over the last century in response to sediment deposition. Bank protection has been implemented in numerous places along reaches YAR005-008 and YAR011-014 and included walling and toe protection.
Geomorphological Behaviour: Riverbank erosion is relatively common and includes toe scour, cliff erosion and slumping although its significance as a source of sediment appears to be limited to fine material. Point sources including drains, poaching and tipping provide sources of fine sediment while the only significant sources of coarse sediment are tributaries. Once delivered to the river channel sediment is generally stored in channel deposits, although in reach YAR002 some losses to over-bank deposition occur. Significant Issues: · Poaching, drains and tripping make agriculture an important source of fine sediment. · Tilled land to channel edge locally (YAR006, YAR008 and YAR013). · Himalayan Balsam occurs throughout. · Diseased alders (YAR005-006 and YAR009-011). · Artificial banks impede natural channel behaviour.
BBR No: BWA 0013499 43 Rev R01 October 2004 ENVIRONMENT AGENCY
Watercourse Name: RIVER YARTY Catchment Location Map:
Summary Details: Upstream survey limit: SY 275999 Higher Westwater Downstream survey limit: SY283973 Confluence with River Axe Length of river surveyed: Geomorphological reaches: YAR016-YAR019
Physical Character: · Low gradient channel set within a terraced valley floor with a floodplain generally greater than 10 channel widths wide. · Channel planform principally consists of irregular meanders except reach YAR018 which is straight and YAR019 which has regular meanders. · Bed materials range from silt to cobbles with fines being particularly extensive in the lower reaches. · Bank materials are variable throughout and range from silt to cobbles. Artificial banks are common throughout, particularly in reach YAR018. · The banks are extensively cliffed but also locally graded. In cross-section the channel is mainly U-shaped and variable with planform except reach YAR018 which is rectangular and uniform. · Flow is highly varied reflecting high morphological diversity comprising riffles, ponds, runs and glides. Ponding and dead water also occur in the upper reaches. · Channel deposits include frequent accumulations of both fine and coarse sediments. With the exception of reach YAR018 side bars, point bars and mid-channel bars are extensive throughout. · Fine sediment are predominantly temporary in nature while coarse sediments are mainly permanent, although some temporary and semi-permanent deposits are present. · A range of channel vegetation types are found along the watercourse including exposed tree roots, emergent reeds, submerged vegetation and moss. Vegetation is most extensive in reach YAR018. · Reach YAR016 has been resectioned and locally culverted, while YAR018 has been realigned and resectioned to facilitate road construction.
Historical Change: Numerous channel changes have occurred along the watercourse since 1891. While bend cut-off and enlargement can occur naturally they may also have been encouraged by artificial channel changes, such as the straightening of the channel along YAR018. Under natural conditions meandering rivers in confluence settings tend to experience relatively high rates of channel change. In response to this lateral mobility bank protection has been provided along reaches YAR016-YAR018 varying from rip-rap and concrete (YAR018) to toe protection (YAR016-017).
Geomorphological Behaviour: A lack of bank cohesion and limited riparian vegetation cover has contributed to the lateral changes experienced along this watercourse. These conditions include have resulted in high bank erosion rates, primarily through cliff erosion and geo-technical failure (slumps and slips), which is introducing both fine and coarse sediment to the river channel. In addition to these diffuse sources poaching, field ditches and vehicle access also provide sources of sediment. Sediment delivered to the river channel tends to be deposited locally and contributes to further bank erosion by diverting flow against the riverbanks. Deposition in this location is encouraged both by the decline in channel gradient as the Yarty enters the Axe Valley and by back water effects within the channel. The presence of a river terrace above the present floodplain indicates that this site has a long history of channel activity.
Significant Issues: · Interruption and complication of natural geomorphological processes by anthropogenic interference, such as bank protection and local channel straightening e.g. YAR018. · Diseased alders occur in reaches YAR016-017. · Himalayan Balsam is present throughout the water course.
44 BBR No: BWA 0013499 Rev R01 October 2004 ENVIRONMENT AGENCY
Watercourse Name: RIVER AXE Catchment Location Map:
Summary Details: Upstream survey limit: ST412063 Clapton Bridge Downstream survey limit: ST342047 Chard Junc – Broad Bridge Length of river surveyed: 10.15km Geomorphological reaches: AXE001-AXE014
Physical Character: · Medium gradient channel with irregularly meandering planform located within a valley with a concave bowl cross-section. · The bed materials range from silts and sands to fine and coarse gravel. Clay occurs locally on the river bed in the lower reaches. Fines are particularly extensive in the middle reaches (AXE006-AXE010). Bed armour is present in reaches AXE006-007 and AXE010. · The banks are composed primarily of fine sediments with silt being particularly extensive. Fine gravel also occurs in the banks of the upper reaches, with coarse gravel and cobbles are only found in the banks of the lower reaches. · Flow velocity is highly varied with upper and lower reaches showing the greatest morphological diversity. Riffles, runs and pools occur throughout and in the lower reaches ponded reaches and dead water are also present. · Both coarse and fine channel deposits are found throughout the watercourse; berms and side bars are present throughout while mid-channel bars are mainly located in the upper reaches and point bars in the lower. Bank toe accumulations are found in reaches AXE004-005 and AXE011. · Fine sediment deposits range from permanent to temporary with temporary accumulations most frequent in the middle reaches. Coarse channel deposits are either temporary or semi-permanent throughout. · Channel vegetation is of low-medium density and is high only in reach AXE012. Floating vegetation and algae occur occasionally while moss, exposed tree roots and emergent reeds occur throughout. · Bank protection has been occurs locally and varies from boulder placement and rip-rap (AXE005) to walling (AXE006, 010, 011) and gabions (AXE009).
Historical Change: There has been limited historic channel planform change with one cut-off since 1891 (NGR 335921 105458), similarly lateral channel movement has also been limited during the period 1891 to present.
Geomorphological Behaviour: In general the upper Axe is relatively stable due to extensive tree lining and cohesive riverbanks. However, in reach AXE001 where tree lining is limited, bank erosion is relatively extensive and the meanders appear to be increasing in size. The channel in reach AXE001 has over-widened which is promoting sediment deposition within the channel, which is promoting further bank erosion. This localised channel planform activity appears to reflect natural river channel behaviour. The relatively high bank erosion rate in reach AXE001 is delivering fine sediment to the river channel. In reaches AXE0010-0012 discontinuous river terraces are present and provide evidence of earlier episodes (pre-nineteenth century) of channel change.
Both the coarse and fine sediments stored within the channel of the watercourse have been delivered both from the tributary channels joining the river in the reach, such as the Temple Brook, and as a result of localised bank erosion (particularly in reach AXE001). Poaching by livestock occurs throughout the watercourse and represents and important additional source of fine sediment.
Significant Issues: · Localised livestock poaching. · Diseased alders · Himalayan Balsam is present.
BBR No: BWA 0013499 45 Rev R01 October 2004 ENVIRONMENT AGENCY
Watercourse Name: RIVER AXE Catchment Location Map:
Summary Details: Upstream survey limit: ST342047 Chard Junc– Broad Bridge Downstream survey limit: ST322015 Confluence of River Kit Length of river surveyed: 5.55km Geomorphological reaches: AXE015-AXE021
Physical Character: · Medium gradient channel located within a concave bowl surrounded by a floodplain of 5-10 channel widths on each side. · The planform comprises irregular meanders with the exception of AXE015-016 which are sinuous. · Bed material generally ranges from silt to coarse gravel although clay is also present at the confluence of the Backwater River. The bed is uniform and unsorted and occasionally armoured. · Bank sediments ranges from silt to sand throughout the reach with fine gravel also present downstream from AXE017. In reach AXE015 the bank is frequently artificial. · The cliffed and graded riverbanks are frequent with stepped sections also present along both banks. In cross- section the channel is U-shaped and variable with planform with the exception of AXE015 which is uniform and AXE016 which varies without planform. · With the exception of AXE015 flow is varied or highly varied and reflects the high morphological diversity of the watercourse. Runs, riffles, glides, dead water and pools are all present. Reach AXE015 is an exception here morphological diversity is low and the flow dominated by a long glide. · Both coarse and fine channel deposits are present throughout and are highly frequent in the middle reaches. Channel deposits include berms, islands, point bars and side bars. Toe accumulations occur mainly in the lower reaches. Fine deposits are both temporary and permanent features while coarse deposits are either temporary or semi-permanent. · Channel vegetation occurs with a medium to high frequency, with the exception of AXE015 where it is of low frequency. In-channel vegetation includes floating vegetation, moss, emergent reeds and tree roots. · Reach AXE015 has been realigned and resectioned and the banks have been walled. Elsewhere the banks of reach AXE016 and AXE018 have some bank protection and AXE020 is protected throughout.
Historical Change: Bend cut-offs have occurred since 1891 in reaches AXE017 and AXE019. Examples of bend enlargement are found in reaches AXE018 and AXE019. Reach AXE015 was re-aligned and resectioned at some point after 1891.
Geomorphological Behaviour: Sediment is supplied to the channel from a range of sources. Bank erosion occurs extensively throughout the watercourse and includes toe scour, cliff erosion, slumping and slips. It is mainly fine sediment that is delivered by bank erosion with course sediment delivery occurring occasionally from eroding cliffs. Where bank protection has been provided erosion is absent. Point sources also represent important sources of sediment (mostly fine) and include tributaries, drains and frequent livestock poaching. It appears that much of the coarse sediment in the channel has been transferred from upstream as few sources of coarse sediment are located in this watercourse. The dominance of fine sediment delivery in this water course is a reflection of the composition of floodplain materials. The frequent occurrence of temporary channel deposits reflects relatively active sediment (coarse and fine) delivery and transfer. The deposition of sediment within the channel encourages bank erosion which locally has resulted in bend growth and in two instances bend cut-off.
Significant Issues: · Fine sediment delivery through frequent livestock poaching. · Diseased alders are found throughout the watercourse. · Himalayan Balsam occurs extensively. · Loss of morphological diversity and interruption to natural geomorphological process due to bank protection (mainly in reaches AXE015 and AXE020).
46 BBR No: BWA 0013499 Rev R01 October 2004 ENVIRONMENT AGENCY
Watercourse Name: RIVER AXE Catchment Location Map:
Summary Details: Upstream survey limit: ST322015 Confluence of River Kit Downstream survey limit: SY287977 100m u/s of A35 Road Bridge Length of river surveyed: 7.29km Geomorphological reaches: AXE022-AXE031
Physical Character: · Low gradient channel located within a valley with the form of concave bowl (upper reaches) or terraced valley floor (lower reaches). · The channel planform is varied and includes regular meanders (AXE022, 027, 031); irregular meanders (AXE023, 028-030); straight sections (AXE025, 032) and one anastomosed reach (AXE024). · The floodplain, which is present on both banks, varies in width from 1-5 channel widths in the upper reaches to >10 channel widths in the lower reaches. · The bed materials range in size from silt to coarse gravel. Fine gravel is extensive in the middle and lower reaches of the watercourse. · Bank materials vary from silts to cobbles with fines being particularly extensive in the middle reaches. Artificial bank materials are present in reaches AXE025-031. · Riverbanks are generally steep frequently cliffed, but also extensively graded. The channel has a U-shaped cross-section which is variable with planform. Exemptions occur however such as AXE025-027 which is rectangular and uniform. · Flow is varied and reflects the generally high to medium morphological diversity which includes runs, riffles, pools, glides and dead water. Reaches AXE026-027 are exceptions and have low flow diversity. · Both coarse and fine channel deposits occur although coarse are generally more frequent. Channel deposits include mid-channel bars, point bars, side bars and berms. In reach AXE030 boulders are also found. Reach AXE026 has few channel deposits. · Both coarse and fine deposits are typically permanent or semi-permanent. · Channel vegetation varies from low frequency in the upper reaches to moderate frequency in the lower reaches and includes submerged vegetation, moss, algae roots and reeds, the latter two being common throughout. · Riparian vegetation and tree lining declines in width downstream and become discontinuous in the lower reaches. · Reaches AXE025-027 and AXE030 have been re-aligned with AXE025-026 also resectioned. · Bank protection consists of walling (AXE026-0029) but also riprap (AXE030). Historical Change: Historic bend cut-offs and bend growth have occurred in reaches AXE023-024, elsewhere change is restricted to localised bend growth (see previous section).
Geomorphological Behaviour: Features indicative of bank erosion occur extensively throughout the watercourse and include toe scour, eroding cliffs, slumps, and geo-technical failures. Generally these supply fine sediments with coarse sediment supply restricted to reaches AXE025 and AXE029. Point sinks are locally important, particularly in the lower reaches, and include field ditches and cattle poaching. The sediment delivered to the river channel is stored in frequent fine channel deposits which have a high degree of permanency suggesting limited transportation downstream. Relatively few channel changes have occurred, particularly in the lower reaches, which is at least in part due to the presence of bank protection. No evidence of loss to over bank sedimentation was found. Significant Issues: · Low morphological diversity and interruption to natural geomorphological processes caused by channel modifications. · Himalayan Balsam is present along both banks throughout the reach. · Farming practices (drains and livestock poaching) supply fine sediment to the river channel. · Diseased Alders are present in reaches AXE022 and AXE030.
BBR No: BWA 0013499 47 Rev R01 October 2004 ENVIRONMENT AGENCY
Watercourse Name: RIVER AXE Catchment Location Map:
Summary Details: Upstream survey limit: SY287977 100m u/s of A35 Road Bridge Downstream survey limit: SY259927 A3052 Road Bridge Length of river surveyed: 9.76km Geomorphological reaches: AXE032-AXE042
Physical Character: · Low gradient channel located in a terraced valley floor setting, with a floodplain greater than 10 channel widths present on both banks. · The channel planform is variable ranging from regular meanders (AXE033, 035, 040, 041), irregular meanders (Axe034, 036, 037) to sinuous (AXE039, 042) and straight sections (AXE 032, 038). · Bed sediments are typically non-uniform and ranges in size from silt to cobbles. · Bank materials are similarly variable ranging from clay to coarse gravel. Fine sediment tends to be more widespread than coarse which occurs more locally. Silt and sand is particularly extensive at the confluence of the River Yarty. Clay however is only found in the lower reaches. · The channel generally has U-shaped cross-section with steep banks which are extensively cliffed in the upper reaches. The channel dimensions tend to vary with planform with the exception of AXE037-038. Only reach AXE032 has a rectangular cross-section. · Flow is varied reflecting generally good morphological diversity which includes runs, riffles, pools, dead water, and glides. Reach AXE032 has low morphological diversity. · Channel deposits vary in frequency; both coarse and fine channel deposits have a high frequency in reaches AXE036, AXE038 and AXE040. Elsewhere fine deposits occur with moderate frequency while coarse deposits have a low frequency. Channel deposits are predominantly berms, mid-channel bars, point bars and side bars. Boulders are only found in reach AXE038. · Fine deposits where present are both temporary and permanent, while coarse channel deposits are mainly permanent with occasional semi-permanent and temporary bars. · Channel vegetation occurs more frequently in the upper reaches and than the lower reaches. Submerged vegetation and emergent reeds are the dominant types, but floating vegetation and tree roots also occur. · Artificial bank materials are present locally in each of the reaches. · Reach AXE032 is the only reach that has been resectioned. Historical Change: Numerous historical channel changes have occurred along this watercourse and are mainly clustered around the confluence of the River Yarty (AXE034-036) and Whitford (AXE037) (see previous section). These represent the most geomorphologically active parts of the Axe catchment. Artificial levees (flood embankments) have been installed along reach AXE040. Geomorphological Behaviour: The lack of lateral confinement, limited riparian vegetation and low bank cohesion in the upper reaches mean lateral channel adjustment is unrestricted. High rates of lateral activity are indicated by the frequency of bank erosion which includes toe scour, cliffs, slumping and slips. Both fine and coarse material is being supplied to the channel through these diffuse sources. Point sources are also important, particularly for fine sediment, and include poaching, drains and vehicle access. Much of the sediment delivered to the channel is deposited within the channel often for long periods, mainly in bar features. The deposition of sediment in bars, particularly point bars, promotes bend enlargement. In response to these relatively high rates of channel activity bank protection has been provided throughout the watercourse. This varies from localised protection such as rip rap (AXE036, 037, 040-042) to extensive protection involving walling (AXE035, 042). Significant Issues: · Lack of morphological diversity in reach AXE032. · Reductions in morphological diversity and interruption to natural geomorphological processes caused by channel modifications. · Drains, poaching and vehicle access points provide sources of fine and occasionally coarse sediment to the channel, and attest to the impact of farming practices · Himalayan Balsam is present along both banks throughout the reach. · Diseased alders are present in reaches AXE034, 036, 039, 040.
48 BBR No: BWA 0013499 Rev R01 October 2004 ENVIRONMENT AGENCY Catchment Fluvial Geomorphological Audit of the Axe Catchment
BBR No: BWA 0013499 49 Rev R01 October 2004 ENVIRONMENT AGENCY Catchment Fluvial Geomorphological Audit of the Axe Catchment
6.0 Catchment-Scale Geomorphology
50 BBR No: BWA 0013499 Rev R01 October 2004 ENVIRONMENT AGENCY Catchment Fluvial Geomorphological Audit of the Axe Catchment
6.1 Introduction
While it is valuable to examine the geomorphological character and issues associated with individual watercourses, it is essential to set this in the context of catchment geomorphological behaviour as a whole. This section aims to:
· Describe the sediment regime operating within the Axe catchment. · Examine the internal and external factors governing the sediment regime.
The sediment system in a river, or sediment regime, is a continuum of sediment supply, transport and storage operating at a range of temporal and spatial scales. The nature of the sediment regime is controlled by both internal and external factors that govern the balance between water and sediment inputs to a watercourse. External controls may include catchment geology, topography, soil type, climatic trends, land management practices or downstream base level changes. Internal controls may include the grain size and structure of bed and bank materials, vegetation characteristics of the channel and riparian corridor, gradient and cross-sectional morphology and flow conditions (Sear, Newson & Thorne, 2004).
6.2 The Sediment Regime
This section considers the three main elements of the sediment regime within the Axe catchment: sediment supply, sediment storage and sediment transport. These three elements are interdependent and interact with the flow regime to control channel morphology over a range of temporal and spatial scales (Figure 6.1).
Figure 6.1 Interdependence of the sediment and flow regime
Sediment Storage & Channel Morphology
Sediment Transport Fluid Flow
Sediment Supply Discharge
The purpose of this section is to provide a broadscale overview of the sediment regime and how it functions at a catchment scale. Information regarding the geomorphological behaviour of specific watercourses can be found in the summary sheets within Section 5.0. For reach specific detail regarding sediment supply, storage and transport, the user is referred to the Reach-by-Reach Geomorphological Database and the Geomorphological GIS datasets that accompany this report.
BBR No: BWA 0013499 51 Rev R01 October 2004 ENVIRONMENT AGENCY Catchment Fluvial Geomorphological Audit of the Axe Catchment
6.2.1 Sediment Supply
Sediment is supplied to a watercourse via various pathways, which may be broadly divided into point and diffuse sediment sources.
· Point Sources are those that can be attributed to a specific physical location, and include:
· Tributaries · Scour at structures · Field drain/mill leat · Footpaths · Tipped material · Burrowing · Vehicle access points · Poaching · Outfalls · Fishing platforms · Tree fall
Figure 6.2 summarises the spatial distribution of point sediment supply across the Axe catchment.
The point sources within the Axe catchment contribute both coarse and fine sediment to the river channel. Although there are a greater number of point sources along Kit Brook and River Yarty, point sediment supply is of a greater magnitude along the Temple Brook and Blackwater River. Point sources provide significant along the Axe with the exception of the channel in the vicinity of Chard Junction in the upper catchment and immediately downstream form Whitford in the lower catchment.
Figure 6.2 Point sediment supply in the Axe Catchment
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
52 BBR No: BWA 0013499 Rev R01 October 2004 ENVIRONMENT AGENCY Catchment Fluvial Geomorphological Audit of the Axe Catchment
· Diffuse sources are those that cannot be attributed to a clearly identifiable physical location or a defined discharge channel, and include process such as:
· Fluvial erosion · Hillslope supply · Geotechnical failure · Input from the catchment
Figure 6.3 provides an overview of diffuse sediment supply across the Axe catchment (NB. this does not take into account input from the floodplain).
Sediment is supplied as a result of fluvial erosion and geo-technical failure with fine sediment delivery more widespread than coarse. Along the tributaries diffuse sediment supply is high to very high along both the Temple Brook and Blackwater river together with the lower reaches of the River Yarty. Diffuse sources are also high to very high along the Axe with very high sediment inputs occurring in the upper reaches between Clapton Bridge and Chard Junction between Westford Mills and Wadbrook, around Cloakham and between the Yarty confluence and Whitford.
Hillslope supply, which is typically direct to the channel, is generally absent. There are only two locations in the catchment where hillslope supply was recorded. Along AXE012, on the Axe 500 m upstream from Chard Junction, the channel abuts the slopes of the left valley side and coarse and fine sediment delivery from the slopes is high. The channel in the upper reaches of the Blackwater River, along BLA003, is also in contact with the valley side slopes, however sediment delivery is limited.
Figure 6.3 Diffuse sediment supply in the Axe Catchment
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
BBR No: BWA 0013499 53 Rev R01 October 2004 ENVIRONMENT AGENCY Catchment Fluvial Geomorphological Audit of the Axe Catchment
It is more difficult to evaluate the input of sediment to the channel from the land surrounding the river channel. Land within 50 m of the river is predominantly grazed throughout the catchment with occasional woodland and tilled land, particularly along the River Yarty. The watercourses of the upper catchment are generally bordered by a continuous or near continuous wooded riparian zone. This will act to inhibit the movement of sediment from the surface of the land surrounding the river channel. However, in the lower reaches of the catchment (along the River Axe downstream from Weycroft and along the lower River Yarty) riparian woodland is often indefinite or absent, this may facilitate the transfer of sediment from the land surface to the channel.
6.2.2 Sediment Storage
Sediment is stored in a number of ways within the fluvial system, which may be broadly divided into point and diffuse sediment sinks.
· Point sinks are those that can be attributed to a specific physical location, and may be associated with: · Weirs · Bridges · Dams · Large woody debris · Fords
Figure 6.4 provides an overview of point sediment storage across the Axe catchment.
Figure 6.4 Point sediment storage in the Axe Catchment
Kit Brook
Temple Brook
Blackwater River
River Yarty River Axe
54 BBR No: BWA 0013499 Rev R01 October 2004 ENVIRONMENT AGENCY Catchment Fluvial Geomorphological Audit of the Axe Catchment
Point sediment storage occurs throughout the catchment and is particularly significant along the Kit Brook and Temple Brook. In the Axe catchment point sediment sources are primarily a combination of weirs, bridges and woody debris.
· Diffuse Sinks are those that cannot be attributed to a clearly identifiable physical location. The main diffuse sediment stores are: · Floodplain deposition · In-channel deposition
Figure 6.5 provides an overview of diffuse sediment storage on the floodplain within the Axe catchment. Overall, very limited floodplain sediment storage was found within the Axe catchment. Floodplain deposits recorded were dominated by fine sediment. Floodplain deposits were clustered in the upper reaches of the Kit Brook and the upper and lower reaches of the River Yarty.
Figure 6.5 Diffuse floodplain sediment storage in the Axe Catchment
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
Boulder in-channel deposits are also limited in the catchment, with boulders found in reaches TEM009, BLA006, YAR012, AXE030 and AXE038. All these deposits are small (<10m2) and are permanent in nature.
Storage within the channel in the form of cobble/gravel deposition is more widespread across the catchment (Figure 6.6). Coarse channel deposits are of relatively limited extent along the Temple Brook and the Upper Axe with several reaches showing low maximum gravel extents.
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There are a number of isolated reaches in the central sections the River Axe where there are no cobble/gravel deposits. There is a marked contrast in the degree of permanency of these channel deposits between the upper and lower catchment. Cobble/gravel deposits along the Axe upstream of the Kit Brook confluence and the River Kit, Blackwater River and Temple are predominantly temporary or semi-permanent in nature, while along the River Yarty they tend to be semi-permanent or permanent. Along the lower Axe downstream from the Yarty confluence coarse channel deposits are predominantly of a permanent nature.
Figure 6.6 Diffuse cobble/gravel channel deposition in the Axe Catchment
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
With the exception of the upper reaches of the River Axe and the Temple Brook fine channel deposits are less extensive than cobble/gravel channel deposits (Figure 6.7). Fine channel deposits are frequently absent from the channel of the River Yarty and the Kit Brook. The only significant area of fine sediment deposition in the lower catchment is located along the River Axe downstream from the confluence of the River Yarty. Fine sediment deposition in the tributaries tends to be temporary in nature with permanent deposits only found along the Temple Brook and the lower most reaches of the Blackwater River. Where fine sediment deposits occur along the River Axe they tend to be in the form of both permanent and temporary deposits. Semi-permanent fine deposits are only present in significant numbers in the upper reaches of the River Axe (AXE003-AXE009).
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Figure 6.7 Diffuse fine channel deposition in the Axe Catchment
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
6.2.3 Sediment Transport
The processes of sediment supply and sediment storage are linked by sediment transport. It is therefore possible to deduce general trends in sediment transport through comparison of the distribution of sediment sources and sediment stores. Figures 6.2 and 6.3 show that sediment sources are widespread in the catchment and are greatest along the Temple Brook, Blackwater River and the River Axe.
Under low flow limited floodplain sedimentation was recorded in the catchment the majority of the sediment delivered to the river channel is deposited in the channel and or transferred downstream. Coarse channel deposits are of a relatively low extent in the upper reaches of the River Axe and along the Temple Brook, but are high elsewhere in the catchment. This suggests that the sediment supplied in this location is transferred downstream. This is also indicated by the temporary nature of the coarse channel deposits in this part of the catchment. Fine channel deposits are concentrated along the River Axe and are of low extent in the tributaries. This suggests that fine sediment supplied to the tributaries is transferred to the River Axe. Point storage is widespread throughout the catchment but is greatest on the Temple Brook and Kit Brook and upper River Yarty. Along the Temple Brook, where sediment storage is relatively limited, stored sediment is concentrated into areas where debris or structures interrupt the transfer of sediment downstream. Weirs and bridges provide the most important structure related sediment sinks in the catchment and are located throughout the catchment, the distribution of which is summarised in Table 6.1.
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Table 6.1 The numbers of weirs and bridges where sediment storage occurs in the Axe catchment. Sediment calibre is indicated as fine or coarse sediment.
Watercourse Weir - fine Weir -coarse Bridge - fine Bridge - coarse River Axe 3 4 12 7 River Yarty 0 7 1 6 Kit Brook 2 5 1 6 Temple Brook 0 3 2 1 Blackwater River 1 4 3 3
More specific trends in sediment transport can be deduced through examination of the supply and storage of fine and coarse sediment. Figure 6.8 identifies the main areas of supply and storage of fine and coarse sediment within the Axe catchment.
Figure 6.8 Supply and Storage of Fine and Coarse Sediment in the Axe Catchment
FINE Supply Storage
Temple Brook Temple Brook
Blackwater River Sediment Transport Sediment Transport Lower Kit Brook River Yarty River Axe River Axe
COARSE Supply Storage
Middle Temple Brook Lower Blackwater River Sediment Transport Blackwater River Sediment Transport Lower Kit Brook Kit Brook Lower River Yarty River Yarty Upper Axe River Axe Lower Axe
Figure 6.8 highlights the transfer of fine sediment from the tributaries to the River Axe described above. In terms of coarse sediment Figure 6.8 indicates that much of the coarse sediment delivered to the river channel is deposited locally, particularly along the River Axe. The storage of coarse sediment within the tributaries upstream of the main sediment sources suggests that coarse sediment is also being delivered to the tributary channels from watercourses both upstream and confluent with the tributary channels examined.
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6.3 External Controls
External controls are controlling factors outside of the geomorphological system. External controls influencing the sediment regime within the Axe Catchment include:
· Catchment Form · Surface Water Drainage Network · Natural Flora and Fauna · Landuse and Management Practices
It is recognised that other external factors, such as long term climatic change and downstream base level change, are also of importance, but will not be considered within this report.
6.3.1 Catchment Form
The overall form of a catchment is controlled by the underlying geology and topography (see Section 2.2). These physical characteristics have evolved over geological timescales and act as major controls over the internal fluvial system over shorter geomorphological timescales. Many geomorphological characteristics of the watercourses within the catchment, such as gradient and planform, are directly influenced by geology and topography – these internal controls are discussed further in Section 6.4.
6.3.2 Surface Water Drainage Network
The pathways via which surface water drains from the catchment greatly influence the supply, transport and storage of sediment. Tributaries, field ditches and outfalls act as point sources supplying sediment to the channel. Coarse sediment input from tributaries is high along the Blackwater River but is generally of medium to low importance elsewhere in the catchment. The delivery of fine sediment from tributaries occurs predominantly in the upper reaches of the River Axe and with the exception of single tributaries on the Blackwater River and lower River Axe, is generally low or absent elsewhere. Sediment delivered from field ditches is significant and dominated by fine material and is particularly significant along the middle and lower Axe, the Temple Brook and lower Kit Brook (Figure 6.9). Coarse sediment delivery from drains is restricted to small quantities along the Yarty and upper Kit Brook.
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Figure 6.9 Fine sediment supply form drains in the Axe catchment
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
6.3.3 Natural Flora and Fauna
Natural flora and fauna can have a significant impact on the supply and storage of sediment within the fluvial system. An important cause of increased sediment delivery to river channels is the presence of the invasive species such as Himalayan Balsam (Impatiens glandulifera) which is widespread in the Axe catchment. Himalayan Balsam grows vigorously and spreads rapidly along water courses. Tall, dense clumps, which grow rapidly during the summer months, out- compete native riparian plant species leaving the ground beneath free from vegetation. During the winter months when the Himalayan Balsam dies back the riverbanks are left bare and highly vulnerable to erosion. Himalayan Balsam is by far the most prevalent invasive species in the Axe catchment and is extremely widespread (Figure 6.10).
Alder Root Disease caused by the fungus Pytophthora sp. is a significant cause of riparian Alder damage and fatality in the Axe catchment. Diseased alders are particularly prevalent in the upper and lower reaches of the River Axe and along the River Yarty (Figure 6.11). This may in part explain the occurrence of sediment storage behind woody debris in these locations.
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Figure 6.10 The distribution of invasive species (Himalayan Balsam) in the Axe catchment
Kit Brook
Temple Brook
River Yarty Blackwater River
River Axe
Figure 6.11 The distribution of diseased alders in the Axe catchment
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
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Tree fall can also supply sediment to the channel due to associated bank failure. Sediment supply as a result of tree fall was recorded along the Blackwater River (BLA003) and the River Axe (AXE004 and AXE012). Although tree fall did not appear to be a significant cause of sediment supply elsewhere at the time of survey, this may increase in the future as a result of widespread alder root disease. Large woody debris, such as branches from dead or diseased trees, entering the channel can cause point storage through the creation of debris dams. Sedimentation as a result of woody debris jams is generally greatest in the tributaries and the upper Axe where riparian tree lining is most continuous; this is most significant along the Temple Brook and Blackwater River.
6.3.4 Land Use and Land Management Practices
Land use and the way in which the land is managed have an important role to play in controlling the supply of the sediment to the channel. Poaching (trampling) by cattle, footpath erosion and vehicles accessing the channel all act to create point sources of sediment. Poaching is one of the main causes of fine point sediment transfer to watercourses within the Axe catchment, and is widespread along the River Axe and the each of the tributaries (Figure 6.12). Poaching also contributes some coarse sediment but this is restricted to two sites along the Blackwater River and a single site along the upper Axe.
Figure 6.12 Fine sediment supply form livestock poaching in the Axe catchment
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
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Vehicle access represents minor sources of fine sediment at a catchment scale, but may prove significant on a more local scale. Vehicle access provides localised sources of coarse sediment along the River Kit (KIT015) and River Yarty (YAR016) and fine sediment supply along the Axe in reaches AXE028 and AXE036 and at the confluence of the River Yarty.
The management of bank top trees within the catchment also influences point sediment supply and storage within the Axe catchment (see Section 6.3.3). Locally bank top tree and riparian vegetation removal can be important. For example the riverbanks along reach BLA006 of the Blackwater River have been cleared of vegetation and this appears to be contributing to sediment delivery in this location. The delivery of sediment from the exposed riverbanks in this location is also being augmented by tipped material designed to alleviate the bank instability problems.
Land use and land management practices also greatly influence diffuse supply of sediment to the channel. Land within 50 m of the river is predominantly grazed throughout the catchment with occasional woodland and tilled land, particularly along the River Yarty. This is limited in the upper catchment where watercourses are bordered by a continuous or near continuous wooded riparian zone, which limits the movement of sediment to the river channel. In the lower catchment however riparian vegetation is often indefinite or absent. As such the transfer of sediment from grazed and tilled land is likely to be greatest in the lower catchment.
Tree lining also contributes to bank stability as the tree roots bind the bank material together. The reduced extent of tree lining in the lower catchment may contribute to the occurrence of diffuse sediment supply in the lower catchment although in the upper catchment tree lining does not appear to significantly reduce the occurrence of diffuse sources.
6.3.5 Channel modification
River channel modification ranging from piecemeal bank protection to extended sections of channel re-sectioning or realignment can alter both morphological diversity and the behaviour of the river channels. Channel re-alignment and re-sectioning has been conducted in several locations in the catchment (Figure 6.13). In these locations the channel morphology is often, but not always, simplified. The impacts of channel modification detected in the Axe catchment include reduced bed diversity such as along the Axe at Chard Junction, loss of natural bank materials and such as along the Axe at Wadbrook and a reduction in gravel bar extents also observed at Wadbrook.
Bank protection reduces the input of sediment to the river channel and limits the ability of the channel to adjust laterally. This can lead to unpredictable channel behaviour such as bed erosion or downstream adjustments, although this does not appear to be a significant problem in the catchment. Bed incision following bank protection may reduce the lifespan of the protection measures. The extent of bank protection measures in the catchment is summarised according to individual watercourses in section 5.2.
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Figure 6.13 Channel modifications in the Axe catchment
Kit Brook
Temple Brook
River Yarty
Blackwater River
River Axe
6.4 Internal Controls
Internal controls are factors controlling geomorphological behaviour from within the fluvial system. These controls are interdependent therefore changes in one factor may have knock-on effects on other controlling factors. Diffuse sediment supply and storage are strongly influenced by internal controls.
6.4.1 Gradient
The watercourses of the Axe catchment are of medium gradient in the upper reaches (Temple Brook; Blackwater River, Kit Brook, Upper Yarty and Axe upstream of the Kit Brook confluence) and low gradient in the lower catchment. Features typical of high gradient channels such as rapids waterfalls and boils are absent, the channel bed being dominated by riffles, runs, glides and pools. Channel gradient influences the transfer of sediment within the catchment, particularly the transfer of fine sediment from the tributaries to the River Axe, this is discussed in the following section. The gradient also influences channel width which and is discussed further in section 6.4.3.
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6.4.2 Mode of Adjustment
Rivers are naturally dynamic systems that may adjust to changes in discharge regime and sediment supply in the lateral or vertical plane. The natural mode of adjustment is largely dependant on underlying geology, topography and the gradient of the channel. In areas of relatively high to moderate gradient such as the upper catchment, channels tend to adjust vertically through incision or aggradation while low gradient channels tend to adjust laterally. The lateral stability of the channel planform over historic timescales in the upper reaches of the catchment reflects the dominance of vertical changes. The most significant changes in channel planform (lateral adjustment) in the catchment were recorded in the low gradient reaches of the catchment, both in the vicinity of the Axe/Yarty confluence and downstream. Lateral channel changes are promoted by the sediment deposition within the channel, which tends to be concentrated in relatively low gradient sections.
In meandering rivers such as the lower Axe the channel planform is also governed by complex feedback relationships which influence channel behaviour. For example, the cut-off of a large bend leads to a locally higher channel gradient (as it covers a shorter distance across the same change in elevation). This increases flow velocity which encourages erosion of the channel downstream which can cause bend growth downstream. Channel planform reflects the dynamic equilibrium of fluvial systems which are constantly adjusting to changes in sediment supply, discharge regimes and internal feedback mechanisms.
In general steeper reaches tend to act as sediment transfer reaches, while the lower reaches act as depositional zones. However, transfer reaches may contain relatively high concentrations of coarse sediment, as observed along the Axe, as flows competent to transport coarse sediment only occur episodically and coarse sediment tends to be transferred short distances from channel store to channel store (bars) in a downstream direction. As sediment also delivered from upstream the channels contain a sequence of gravel bars which are continually exchanging sediment during high flow. Depending on long-term flow variability it may take decades or centuries for coarse sediment to be transferred down the length of a tributary.
The contrast in the permanency of channel deposits, particularly coarse sediments, between the dominance of temporary storage in the upper and more permanent storage in the lower reflects the importance of gradient in controlling sediment transfer, through its control on stream power. Temporary channel deposits are concentrated into reaches with moderate gradient, while permanent deposits are more numerous in the lower reaches where gradient is low.
6.4.3 Cross-Sectional Morphology
Downstream trends in cross-sectional morphology act to influence channel sediment storage. Over-widening and/or over-deepening of the channel often leads to slow flow and deposition of sediment. Figure 6.14 illustrates the downstream trend in width:depth ratio along the River Axe. This Figure highlights three distinctive peaks in channel width, which due to limited change in channel depth, is also reflected in the width depth ratio.
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Figure 6.14 Downstream trend in width:depth ratio along the River Axe
25
Axe-028 20 Axe-021
15 Axe-015
10
Dimension (m)
5
0
0 5 10 15 20 25 30 35
Chainage (km)
Width Depth Width:Depth Ratio
At approximately 11 km (Reach AXE015) the channel width shows sharp increase in channel width. This reach is located at Chard Junction, and is known to have been re-aligned and re- resectioned. This represents an artificial change in the channel width and possibly reflects an attempt to reduce the incidence of flooding in this reach, which is surrounded by industrial land use (Dairy).
A second peak in channel width occurs at approximately 16 km (Reach AXE021), this reach is located immediately upstream of the confluence of the River Yarty. This peak represents the culmination of a dramatic rise in channel width downstream from the Blackwater River. This increase in channel width likely reflects the contribution of the Blackwater River to the flow of the Axe and also a change in channel gradient in this area as the valley floor slope changes from a medium gradient to a low gradient channel. As discussed in section 6.4.2, low channel gradients limit incision and encourage lateral change. The decline in channel gradient also increases sediment deposition which then enhances lateral erosion and limits vertical erosion.
A further peak in channel width occurs at approximately 20 km (Reach AXE028) near Weycroft this coincides with a section of channel with non-cohesive banks. This lack of bank cohesion has increased their susceptibility to erosion and has encouraged channel widening. This section also shows historic channel planform change, which is also likely to have been promoted by the decline in bank cohesion.
6.4.4 Bed and Bank Conditions
The nature of bed and bank materials along a channel influences its erodability and stability. Details of the bed and bank materials associated with specific watercourses are found in the Watercourse Summary Sheets (Section 5.2). The composition of the riverbanks varies
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throughout the catchment, with many banks consisting of both fine and coarse sediments. Fine sediments are dominant however. The majority of banks along the surveyed watercourses frequently have vertical banks which reflect the widespread bank cohesion in the catchment (Figure 6.15).
Figure 6.15 The distribution of cohesive and non-cohesive riverbanks in the Axe catchment.
Kit Brook
Temple Brook
Blackwater River
River Yarty River Axe
While some of the most significant channel planform changes recorded in catchment (Figure 3.9) occur in areas of low bank cohesion, this is not always the case, for example at Whitford. This suggests that lateral channel changes are influenced by other factors. The presence of coarse horizons, in otherwise cohesive banks, and the occurrence of weathering processes mean that sediment supply and lateral channel activity does not simply reflect the degree of bank cohesion. In addition anthropogenic activities can also influence channel changes. The channel changes recorded at Whitford (Figure 4.9), along the lower Axe, for example, appear to have been influenced by anthropogenic activity (Section 4.3).
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6.4.5 Floodplain Connectivity
The degree to which the channel is connected to the floodplain influences the storage of sediment on the floodplain. The upper reaches of the Axe catchment have smaller and less continuous floodplains than the lower catchment reducing the capacity for out-of-channel sediment deposition. The channels of the upper catchment are also frequently incised below the floodplain further reducing the loss of sediment to floodplain deposition. In the lower Axe however, floodplain connectivity is greater although discontinuous flood embankments reduce the frequency of floodplain inundation. As a result much of the sediment, particularly cobble/gravels, are transported downstream and are often deposited, at least temporarily, within the channel.
6.4.6 River Continuity
Sediment transport is strongly influenced by river continuity. In-channel structures, such as dams, weirs, culverts, fords, and to a lesser extent bridges, act to disrupt the longitudinal continuity of a river. These barriers impede the downstream transport of sediment and in the case of weirs and Irish Bridges the migration of fish. A total of 25 weirs, 43 bridges and 6 fords were observed acting as points sinks in the catchment. These are distributed widely throughout the catchment. As a result sediment transport is segmented to some extent along all the watercourses in the catchment. Weirs and Irish bridges in the catchment are frequently associated with sediment deposition. This can impede the passage of migratory fish.
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7.0 Summary
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7.1 Catchment Influences
The most significant influences on channel behaviour and morphology in the catchment are:
· Sediment supply · Lateral confinement · Gradient · Bank cohesion
In general the geomorphological quality of the Axe catchment is very good. The morphological diversity of the channel is either medium or high. Anthropogenic modifications tend to be localised and urban influences are minimal. This means that the main influences on channel morphology and behaviour tend to be natural. However, there are a number of additional factors which lead to localised deviation from the natural behaviour of the river, these are:
· Livestock poaching · Channel modifications including bank protection, realignment and weirs. · Invasive species primarily Himalayan Balsam. · Diseased alders · The increase in maize farming · The absence of riparian vegetation, mainly in the lower catchment.
7.2 Geomorphological Processes
In broad terms, the channel processes are split between the tributaries and the River Axe, which is partially a reflections channel gradient and the degree of lateral confinement (Map 7.1). The tributaries are dominated by sediment transfer and planform stability. Coarse sediment transfer is episodic and involves a continual exchange between temporary storage in the channel. Fine sediment transfer in contrast is more frequent and occurs over greater distances. In the lower catchment, where the gradient is lower, lateral channel adjustment (planform change) occurs. Sediment deposition within the channel is widespread along the River Axe; the presence of coarse sediment has promoted good morphological diversity within the channel. There are however factors which complicate this general picture including anthropogenic channel modification, bank properties and riparian landuse.
· Point sediment sources are mainly fine and located throughout the catchment. Livestock poaching and drains are the most extensive point sources of sediment in the catchment. Tributaries and tipping also provide sources of sediment but these are less extensive.
· Diffuse sediment sources such as fluvial erosion and geotechnical failure are also wide-spread in the catchment. Diffuse sources are dominated by fine sediment, particularly in areas where tilled land occurs. Coarse sediment sources are also widespread, especially along the lower Blackwater River, lower Yarty and locally along the River Axe. The occurrence of diffuse sediment sources is increased by the absence or degradation of natural riparian vegetation due to a combination of farming practices and invasive species.
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· In addition to bank erosion, coarse sediment is delivered to the watercourses from tributaries. Located in the upper, steeper reaches of the catchment stream powers these minor tributaries experience higher stream powers capable of transporting coarse sediments. In addition these tributaries are also confined by valley side slopes leading to relatively efficient transfer of sediment from hill slopes to the river channel.
· Coarse sediment sinks are located throughout the catchment, although these are generally less extensive along the Temple Brook and the River Axe upstream of Chard Junction. Coarse sinks consist of channel bars and include point bars, side bars, mid- channel bars and vegetated islands. The permanency of coarse sediment deposits strongly reflects the gradient of the catchment which influences stream power and as such the capacity of the flow to transfer sediment.
· Fine sediment sinks are located primarily along the River Axe and are of low extent in the tributaries. The most extensive fine deposits are located along the Axe upstream of the River Kit confluence and along a 3 km stretch downstream from the River Yarty confluence. Fine sinks also consist of channel bars and include point bars, side bars, mid-channel bars and vegetated islands. The permanence of fine channel deposits is more variable than coarse. The presence of fine sediments is likely to be seasonal with rapid summer vegetation colonisation stabilising deposits, while vegetation die-off in the winter increases the vulnerability of fine material to entrainment.
· Although much of the sediment stored in the river channel is in the form of diffuse bar deposits there are numerous point sinks in the catchment. These include weirs, bridges, fords and large woody debris. Point sinks are important along the Temple Brook, the River Kit and the River Yarty.
· Despite the widespread occurrence of diseased alders in the catchment, sediment supply as a result of tree collapse is relatively limited as is sedimentation behind woody debris. This suggests that despite that at present woody debris supply to the channel is not sufficient to cause serious disruption to fluvial processes in the catchment.
· Channel modification ranging from local bank protection work, to re-sectioning and culverting results in localised alterations to the channel morphology and fluvial processes. These modifications both interrupt the natural behaviour, by reducing bank erosion, and limit the morphological diversity of the river channel reducing the ecological status of the channel. However, this reduction in channel quality is typically localised and the morphological diversity of the river channel in general is medium to high in the catchment.
· Weirs are located throughout the catchment and are widespread along the River Yarty and Kit Brook in particular. These interrupt sediment transport by trapping sediment immediately upstream. Similarly there a number of “Irish Bridges” in the catchment which are vulnerable to sedimentation, this can prevent fish migration.
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7.3 Key Issues
· High volume of fine sediment supplied through a combination of bank erosion caused by channel weathering; invasive species and livestock poaching together
· High inputs of sediment from the wider catchment, through field ditches and surface runoff from arable land.
· Channel modifications such as bank protection; channel re-alignment; re-sectioning; culverting and weir construction lead to reductions in channel morphological diversity, inhibit natural geomorphological processes and in some instances, such as weirs, inhibit the passage of migratory fish species.
· Channel planform changes and associated bank erosion along the River Axe are in part a reflection of the natural behaviour of meandering rivers. However, localised channel modifications such as bend removal, channel re-alignments and bank protection, conducted throughout the last 200 years, have complicated the natural behaviour of the river channel. It is therefore extremely difficult to determine the extent to contemporary river behaviour is natural or a result of the anthropogenic interference. It is likely that channel activity represents a combination of natural and artificial influences. Further anthropogenic interference may lead to an unpredictable and undesirable channel response.
· Himalayan Balsam is extremely widespread in the catchment. The detrimental effect of this plant on riparian vegetation and riverbank stability is contributing to the transfer of sediment, particularly fine material, to the river channel.
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8.0 References
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Countryside Agency, 1999. Countryside Character. Volume 8: Southwest.
Environment Agency, 1999. Local Environment Agency Plan. Axe and Lim Consultation Draft.
Environment Agency, 2001. Local Environment Agency Plan. Axe and Lim Action Plan.
Environment Agency, 2003a. River Axe Salmon Action Plan.
Environment Agency, 2003b. Eat Devon Catchment Flood Management Plan, Draft Inception Report.
Environment Agency, 2004. The Otter, Sid, Axe and Lim Catchment Abstraction Management Strategy.
Grieve and Clarke, 2003. Macrophyte surveys of the River Axe SAC. Report to English Heritage.
Holmes, N.T.H., 2000. Macrophyte survey of the River Axe – Summer 1999. Report to the Environment Agency.
Sear, D.A., Newson, M.D., Thorne, C.R., 2004. Guidebook of Applied Fluvial Geomorphology. DEFRA/Environment Agency R&D Technical Report FD1914, 233pp
Sweet, R.J. 2004. Computer Modelling of flood flows and floodplain sedimentation. Unpublished PhD thesis, University of Exeter, 295pp.
www1 English Nature http://www.english-nature.org.uk/pubs/gis/tech_na.htm
www2 British Hydrological Society Hydrochronology. www.dundee.ac.uk/geography/cbhe/
www3 DEFRA Climate Change. www.defra.gov.uk/environment/statistics/globatmos/gagccuk.htm
www4 DEFRA Historic Land Use Change. www.farmstats.defra.gov.uk/cs/farmstats_data/DATA/historical_data/hist_pub_search.asp
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