Tender Ref. 12709
Geomorphological Assessment of the River Otter, Devon
Report UC0672
March 2004
GeoData Institute
Tender Ref. 12709
Geomorphological Assessment of the River Otter, Devon
Emery, J.C., Hill, C.T. and Sear, D.A.
Report UC0672
March 2004
GeoData Institute University of Southampton Southampton SO17 1BJ
Tel: 023 8059 2719 Fax: 023 8059 2849 e-mail: [email protected]
CONTENTS
1 INTRODUCTION...... 3
2 AIM AND OBJECTIVES ...... 3
3 CATCHMENT CHARACTERISTICS ...... 4
4 BANK EROSION THEORY ...... 9
5 METHODOLOGY ...... 13 5.1 DATA COLLECTION ...... 14 5.1.1 Contemporary Field Survey ...... 14 5.1.2 Historical (archive) data ...... 15 6 HISTORICAL GEOMORPHOLOGY ...... 17 6.1 RAINFALL, FLOOD MAGNITUDE AND FREQUENCY ...... 17 6.2 CHANNEL MODIFICATION AND MANAGEMENT ...... 24 6.3 HISTORICAL PLANFORM CHANGE ...... 27 7 CONTEMPORARY GEOMORPHOLOGY ...... 49
8 GEOMORPHOLOGICAL INTERPRETATION ...... 69
9 MANAGEMENT IMPLICATIONS ...... 74 9.1 MANAGEMENT OPTIONS ...... 74 9.2 CASE STUDY: TIPTON ST JOHN ...... 78 10 USE OF THE REPORT ...... 81
11 REFERENCES ...... 83
12 APPENDICES A - C
River Otter Geomorphological Audit
Executive Summary
A geomorphological assessment that utilised historical information, contemporary field survey and scientific literature resources has been undertaken for the River Otter between the tidal limit and the end of ‘main’ river upstream of Honiton.
The findings of this study can be summarised as follows:
1) Bank erosion is system wide and occurs over 23% of the bank length. Of this some 6.4% is considered to be severe. 2) Bank erosion is intrinsically linked to channel adjustment, particularly (but not exclusively) at river bends. 3) The adjustment (and erosion activity) increased system-wide at some point between 1930 and 1950. During this period channel planform became more sinuous and bends started to migrate more rapidly. The precise cause is unknown at present. 4) Channel migration and subsequent bend curvature has increased over the intervening periods, with increases in activity associated with periods of increased flood frequency. 5) The primary drivers of bank erosion (in no priority) on the Otter are considered to be: a) Periods of increased flood frequency, in particular long duration floods and rainfall. b) Increased bend curvature leading to flow impingement and higher outer- bank shear stress at many bends. c) Widespread dredging and shoal removal in the period 1960 – 1990 that reduced sediment supply, dropped bed elevations and increased bank heights along the study reach. d) The presence of composite banks with the exposure (through incision) of weaker gravel layers at the toe of most banks.
The Implications of these findings are summarised as follows:
1) The system-wide scale and causes of channel adjustment and associated bank erosion requires a system-wide plan for channel management. Piecemeal bend protection such as been practiced in the past, does not treat the root causes. Indeed evidence suggests that the channel adjusts elsewhere as a result of the protection. 2) Conditions at most banks are not conducive to the use of bio or biotechnical engineering approaches to bank protection. 3) Local bank protection measures where necessary, are likely to require heavy blockstone in combination with channel realignment to reduce flow impingement.
GeoData Institute Page 1 River Otter Geomorphological Audit The current lack of an overall strategy for managing bank erosion and channel adjustment does not work for the Environment Agency, Stakeholders or the river environment. This report is seen as a step towards developing a more comprehensive management plan. This process might involve the following:
1) Education of stakeholders as to the system-wide nature and cause of the adjustments through a series of meetings in combination with a scaled down information brochure based on this report. 2) Take an adaptive management approach to channel adjustment whereby stakeholders are informed and involved in developing a Channel Migration Zone (CMZ) plan for the Otter, based on procedures developed in the US, and the analysis undertaken in this study. This CMZ would aim to develop floodplain zones to allow adjustment of the river where possible, and to highlight where adjustment would not be permitted. In the latter case a plan for bank protection would be required. A monitoring procedure to track the adjustment of the channel bank line would be implemented and used as the basis for action where this is necessary on the basis of economy, efficacy and environment. 3) At specific sites (e.g. Tipton St John) this report can be used to highlight the underlying causes of erosion and the costly nature of the protection and re- alignment works. Alternative solutions including set-back (removal of the threatened infrastructure) should be considered. 4) Recognise that the baseline data contained in this report and accompanying GIS provides a valuable live resource against which to base subsequent monitoring (e.g. 5 year review of channel planform and erosion at severe sites).
GeoData Institute Page 2 River Otter Geomorphological Audit River Otter Geomorphological Audit
1 INTRODUCTION
The River Otter is a dynamic gravel-bed river in East Devon. Evidence of active channel migration is widespread throughout its catchment and consequently severe bank erosion is, and has been, a persistent problem. In response a Geomorphological Audit of the River Otter has been undertaken to identify and understand the geomorphological processes which influence and control this channel activity.
A 30km length of the River Otter between the main river limit, 1.5km north of Honiton (ST173023), and the Normal Tide Limit (NTL) at the Budleigh Brook Confluence has been investigated. The audit was two fold; i) a desk study to collate historical data sources which record changes in the catchment, both natural and anthropogenic, which may have disturbed the fluvial system (discharge or sediment supply), and had direct implications on the channel morphology (change in channel planform, long profile and cross sectional geometry); ii) a contemporary field survey which integrated the key components of both a Detailed Catchment Baseline Survey (DCBS) and Fluvial Audit (FA), and emphasised assessment of the controls and extent of erosion and deposition along the channel.
By classifying the stability of channel morphology, in particular the susceptibility of the banks to erode, on a reach-by-reach basis, an assessment was made as to whether the current channel form and features were either stable and adjusted to the prevailing flow processes and sediment dynamics, or unstable and actively adjusting (eroding) to attain a new dynamic equilibrium. By differentiating between reaches classified as stable or unstable it was possible to isolate the potential causes or exacerbating factors of bank erosion in operation on the River Otter.
2 AIM AND OBJECTIVES
The aim of this project is to develop an understanding of the dynamic geomorphology of the River Otter and to elucidate those variables and geomorphological processes which are, or have been, influential in controlling or modifying channel adjustment, in particular those which are associated with bank erosion.
The specific objectives of the study are six-fold:
• To examine changes to the channel planform over time using historical maps and aerial photographs.
GeoData Institute Page 3 River Otter Geomorphological Audit • To investigate channel and catchment changes over the past 150 years using historical records and consultation with EA staff, and to develop a time chart which identifies Potentially Destabilising Phenomena (PDP’s).
• To identify the major geomorphological features and processes occurring in the River Otter, between the main river limit and the mouth of the estuary, by undertaking a field survey of present erosion, deposition, bank protection, flow types and sediment sources.
• To identify the key areas of channel instability (geomorphological activity).
• To determine the possible factors affecting this instability, in particular those contributing to the severe bank erosion evident along the Otter.
• To provide guidance on appropriate methods of erosion control.
3 CATCHMENT CHARACTERISTICS
Figures 3.1 and 3.2 illustrate the location of the River Otter within East Devon, and the drainage network of its principle tributaries, respectively, while Table 3.1 summarises the physical characteristics of the catchment. The River Otter rises in the Blackdown Hills in the north east of the catchment and drains in a south-westerly direction to its estuary at Budleigh Salterton on the south coast of Devon, England. The upstream limit of the 30km section of channel investigated coincides with where the river becomes designated as ‘Main River’, at the confluence with the River Love (near Langford Bridge). At this point the floodplain has widened to approximately 200 to 300m, although the valley remains steep-sided.
The River Otter then follows the line of the A30 as it skirts the suburbs of Honiton. The A373 crosses the Otter immediately upstream of the confluence with The Gissage, a heavily culverted stream which is the main drainage channel for Honiton. Between the A373 bridge and Weston (Trafalgar Bridge) the course of the Otter is very sinuous, although further meander development is constrained by the A30 and a natural embankment at Cottarson. From Weston to Fenny Bridges the planform is predominantly straight and the main channel is heavily reinforced as it passes under the A30 and South- Western Railway viaduct.
Downstream of Fenny Bridges the underlying geology switches from Mercia Mudstone to Otter Sandstone (Figure 3.1), the floodplain widens further and the valley sides decrease in gradient. Just upstream of Gosford Bridge a disused railway embankment, which runs from Honiton to Budleigh Salterton, constrains the planform migration of the Otter to the west, while natural Otter Sandstone Cliffs which outcrop repeatedly impinge upon the rivers development to the east.
GeoData Institute Page 4 River Otter Geomorphological Audit Figure 3.1 Geology map of Otter (hardcopy)
GeoData Institute Page 5 River Otter Geomorphological Audit Figure 3.2 Location map of Otter (hardcopy)
GeoData Institute Page 6 River Otter Geomorphological Audit After its confluence with the River Tale, the River Otter passes through the western side of Ottery St Mary. For approximately 1km downstream of Ottery St Mary the channel is extremely active and bank erosion is particularly intense. Thereafter the channel is confined again between the disused railway to the west and bedrock bluffs to the east, and follows a straight course to Tipton St John.
At Tipton St John the Otter crosses the disused railway embankment and meanders across a wide, flat floodplain, constrained only by the bridge which fixes the location of the apex of the central meander bend. Downstream the channel crosses back under the disused railway and follows a straight course along the embankment of the main road between Tipton St John and Harpford.
At Harpford the channel actively meanders to Newton Poppleford Bridge, thereafter a large, stable meander bend takes the river west, under the disused railway, until it impinges against a series of sandstone cliffs. Here, the river passes through the Dotton EA gauging station and is narrowly constrained between the cliff line and railway embankment until the river passes east under the railway just north of Colaton Raleigh. Between Colaton Raleigh and Otterton the Otter follows a fairly straight course along the base of a series of sandstone cliffs to the east of the river. The disused railway embankment is some 350m to the west, however this narrows to approximately 100km at Otterton Bridge.
Downstream of Otterton the valley sides steepen and clearly define the floodplain. The Otter flows between the sandstone cliffs on its eastern bank and a wide, flat floodplain to the west, until its outfall to the sea. A flood protection embankment lines the west bank of the Otter from the normal tide limit at the confluence with Budleigh Brook (downstream limit of the Otter in this study) to its outfall at Budleigh Salterton.
It is clear that the course of the River Otter has been, and remains today, significantly affected by anthropogenic interventions. Some 17 road and railway bridges (many disused) cross the Otter, while 7 major weirs and numerous smaller weirs are used to control river flow, check bed erosion and supply the mill leats which run through Ottery St Mary, Tipton St John, Otterton and Tracey Mill Farm north of Honiton.
GeoData Institute Page 7 River Otter Geomorphological Audit Table 3.1 Summary of River Otter catchment characteristics (sourced from EA 2000; Mott MacDonald 1997; CEH 2001, 2003)
CHARACTERISTIC DESCRIPTION
Catchment area 230km2 River Otter length 44km Study reach length 30km Principle tributaries River Love, River Wolf, The Gissage and River Tale Rises at 300m in the Blackdown Hills and falls to sea level (0m) at its Relief/slope estuary in Budleigh Salterton. Average channel gradient is 6.4m/km. From its source in the Blackdown Hills to Fenny Bridges (confluence with Vine Water) the geology is dominated by Permo-Triassic Mercia Mudstone (calcareous clays and mudstones) with the northern and eastern border of the catchment capped with deposits of Cretaceous Solid geology Upper Greensand and Quaternary Clay with Flints. Downstream of Fenny Bridges the geology is dominated by Permo-Triassic Otter Sandstone, with outcrops of Budleigh Salterton Pebble Beds and Litteham and Aylesbeare Mudstone along the western border of the catchment (see Figure 3.1). Although the River Otter has a fairly shallow gradient (6.4m/km) the steep valley sides and low permeability of the upper catchment produces a very rapid rise and fall of water levels, with high flood peaks in response to rainfall. Downstream of Fenny Bridges, groundwater Hydrology flow from the Otter Sandstone deposits makes a significant contribution to river flow. The River Otter is gauged at two locations; Fenny Bridges (SY114985) and Dotton (SY086884), at which the mean discharge is 2.16m3/s and 3.14m3/s, respectively. Otter Sandstone and Pebble Beds provide regionally important aquifers which are extensively exploited for both public and private water supplies. The Upper Greensand deposit has limited value as an aquifer Hydrogeology due to its restricted coverage within the catchment, although it may be significant in maintaining a base flow in the headwater tributaries of the River Otter in dry summers Otter Sandstones produce well-drained fertile soils (suited to arable and grass production) while the Mercia mudstone produces a heavier soil Soils (suited to permanent pasture). The Upper Greensand and Clay with Flints produce very little soil (associated with heath and rough grass). Honiton, Ottery St Mary and Budleigh Salterton, with numerous smaller Main Urban areas settlements including Weston, Fenny Bridges, Tipton St John, Newton Poppleford, Harpford, Colaton Raleigh and Otterton (see Figure 3.2).
GeoData Institute Page 8 River Otter Geomorphological Audit Predominantly rural (pasture and arable agriculture with some areas of forestry). The Pebble Beds which support the East Devon Pebble Heaths (wet and dry heath and mire communities) are designated as a SPA for breeding birds. The Blackdown Hills (including Hense Moor SSSI) support mires, neutral and calcareous grasslands, wet and dry Landuse and woodlands and lowland heath and a traditional farming landscape (ESA Conservation status scheme). The Otter Estuary is also an SSSI (saltmarsh). The catchment is covered by two Areas of Outstanding Natural Beauty (AONB); the East Devon AONB and the Blackdown Hills AONB, and two of English Nature’s Natural Areas; the Devon Redlands and the Blackdowns. The estuary is part of the proposed Dorset and East Devon Coast World Heritage Site (UNESCO).
4 BANK EROSION THEORY
Given the emphasis of bank erosion and channel migration in this study of the River Otter it is relevant to consider here the different factors which can be influential in bank erosion systems (described in detail in Appendix A; Table 12.1). These factors have been integrated into the standard list of geomorphological attributes collected during the field survey and desk study within the fluvial audit process.
Bank erosion is a natural process inherent to an active fluvial system. Although generally seen in a negative light, bank erosion and lateral channel migration are key components of floodplain renewal; reworking the floodplain sediments and maintaining a high biological diversity. However, in the ‘wrong’ place, bank erosion can have serious consequences; undermining infrastructure (both in-channel and riparian), loss of land for farmers and increased downstream sedimentation.
As illustrated in Figure 4.1, the size and shape of a river channel will depend primarily on the interaction between the flow and sediment discharge supplied to it, the strength of the bed and bank materials, and their vertical arrangement or “bank structure”. For example, the presence of a gravel toe affords a weaker layer that can be undercut by fluvial scour. Through processes of erosion and deposition, the channel attempts to adjust to an ‘equilibrium’ form which allows the most effective conveyance of flow and sediment. However, variability in flow, sediment supply and bank and bed materials induces a ‘dynamic equilibrium’ condition in which channel form is continually adjusting and fluctuates about the equilibrium (mean condition) form. Bank erosion therefore represents a process by which a river naturally adjusts to changes in upstream sediment supply and flow regime. For example, channel widening in response to a phase of increase runoff brought about through increased storminess or urbanisation of the catchment.
Figure 4.1 highlights the importance of bank material strength in controlling erosion, but also notes the significance of continuity of sediment supply in controlling channel adjustment. Thus it is apparent that bank modification at one location can have significant implications on the channel condition at another. Bank protection upsets the ‘equilibrium’
GeoData Institute Page 9 River Otter Geomorphological Audit of the fluvial system and can instigate or exacerbate bank erosion and channel change in reaches further downstream by reducing the supply of sediment (the role of the protection). In addition, changes to the morphology of the channel through anthropogenic activities can affect the balance of fluvial system drivers. For example; i) direct modification to the channel geometry (dredging, realignment and resectioning), channel flow (weirs, embankments and abstraction) and bank materials (protection and poaching), or ii) catchment-scale changes in land-use (urbanisation and agricultural practices).
Bank and bed erosion occurs when the shear stress exerted by the flow on the bed and banks of the channel exceeds the strength of the material. Shear stress is a function of the flow depth and gradient of the channel (though with important local controls such as bend geometry and channel roughness). Hence channel deepening can locally increase shear stress, while straightening can locally increase channel gradient. Lawler et al. (1997 p148) differentiate between bank erosion and bank failure, defining the former as the ‘detachment, entrainment and removal of bank material as individual grains or aggregates by fluvial and subaerial processes’, and the latter as the ‘collapse of all or part of the bank en masse, in response to a geotechnical instability process’; in practice the two processes are inter-linked. Bank erosion occurs in all channels to some extent, and is commonly associated with high flows and zones in the channel where shear stress is consistently high (i.e. the outer bank of bends). The distinctive flow circulations which occur in meander bends (Appendix A Figure 12.1, Dietrich and Smith 1984; Hey and Thorne 1975) result in the highest velocities to be recorded at the foot of the bank, increasing the likelihood of basal scour which undercuts the bank material and induces failure.
However, excess boundary shear stress is not the sole factor controlling bank erosion, especially in the case of cohesive banks which are highly resistant to fluvial entrainment, but have low shear strength and are more susceptible to mass failure due to excess porewater pressures within the bank materials. Bank erosion does not occur in a singular, consistent manner, which is ubiquitous to every channel. There is a myriad of causes, mechanisms and processes associated with different site (i.e. bank material/structure, flow regime) characteristics, and which often operate in concert to produce bank retreat (Lawler et al. 1997).
Establishing the dominant mechanisms from the interplay of bank weakening, direct fluvial entrainment and mass failure processes which can induce serious bank instability is very difficult, but ‘a detailed consideration of the geomorphology and hydrology of the bank, the river reach and catchment context, as well as the geotechnical and hydraulic characteristics of the site usually provides the basis for significant insights which can then be fed into engineering aspects of the problem’ (Lawler et al. 1997 p148). Figure 4.2 illustrates the most common mechanisms (and processes responsible) of river bank failure (Thorne et al. 1996).
GeoData Institute Page 10 River Otter Geomorphological Audit Figure 4.1 Flow diagram illustrating the predominant controls and feedbacks within a bank erosion system
Relief (i.e. slope, Catchment Climate Landuse Geology Scale Controls catchment area) on Water and
Sediment Discharge
Sediment Water Drivers of Bank material fluvial system Discharge discharge
Reach scale System Incision controls on bank Bank structure or aggradation erosion systems: •Channel properties bank •Flow properties Width-depth ratio material drainage •Bank properties
Reach slope Stream power Bank material strength
Planform / pore water sinuosity pressure
Bank Bend Strength of geometry tightness outer bank fluvial (height and shear stress scour angle)
Bed material (channel roughness) Basal end-point bank BANK control (toe status) failure STABILITY
Vegetation Bank moisture cover and content rooting
GeoData Institute Page 11 River Otter Geomorphological Audit
Figure 4.2 Mechanisms of bank failure
1. Shallow slide Line of previous Movement of a block of soil along a slide plane bank profile just below and parallel to the bank surface. Slide plane Occurs in bank materials of low cohesion and where bank angle exceeds the angle of internal Debris at friction. Direction of bank toe Caused by fluvial scour (or boatwash) at the toe movement undercutting the bank. Toe Supplies a large volume of loose debris to the scour toe where it is rapidly removed by scour.
2. Rotational Slip Line of previous bank profile Deep-seated movement of material both downwards and outwards along a curved slip Slide plane plane. Results in a bare upper bank face and the upper slope of the failed block is tilted inwards towards the bank. Pop-out failures at toe Occurs in cohesive bank materials with bank angles < 60°. Caused by fluvial scour at the toe and high pore water pressures (rapid drawdown). Failed material is removed by scour and weathering.
3. Slab Failure Line of previous Severe failure where a block of bank material bank profile falls into the channel. Occurs in cohesive bank materials with bank angles > 60°. Tension Slide plane cracks Caused by toe scour undercutting the bank, high porewater pressures and development of Debris at toe tension cracks inland from the banktop. Results in rapid bank retreat and a steep Toe scour exposed bank face. Debris forms a talus slope at the toe, but it is often quickly removed by scour.
4. Cantilever Failure Line of previous bank profile Severe failure where an overhanging block of bank collapses (topples outwards) into the channel. Can be shear (vertical plane), tensile (horizontal) or beam (axis of rotation) failure. Tension Slide plane Occurs in composite banks where cohesive cracks Fluvial material overlies non-cohesive material which is undercutting easily eroded by scour (or boatwash). Results in rapid bank retreat and a steep Fallen debris exposed bank face. Debris may protect the toe temporarily, but it is often quickly removed by scour.
GeoData Institute Page 12 River Otter Geomorphological Audit Figure 4.2 cont.
5. Wet Earth Flow Scar on upper bank Soil moves out of the bank in a low angle, lobate mass (mudflow). Plane of movement Occurs even on very low angled banks where a high pore water pressure (poor drainage) causes the soil to become liquefied. Seepage Lobe of from bank soil at toe Failed material forms very weak lobes on the bank toe which are rapidly scoured.
5 METHODOLOGY
The Fluvial Audit (FA) was conducted following the R+D 661 approach (Universities of Nottingham, Newcastle and Southampton 1998), which uses contemporary (field survey) and historic (archive desk study) data collection methods to gain a comprehensive understanding of the river system. The data requirements for this methodology are diagrammatically shown in Figure 5.1.
Figure 5.1 Primary inputs and outputs of the fluvial auditing process
DATA Field Survey EA Records Historic Maps COLLECTION • Forms • Flow and Reports • Maps • Rainfall • LEAPS
• Landuse • FAS
GIS / ACCESS DATABASE PROCESSING AND PRESENTATION
GIS Maps: Data Tables Time chart • Current status and graphs • Historical change
ANALYSIS Zones, periods and type of Potentially Destabilising channel change Phenomenon (PDPs)
IMPLICATIONS IDENTIFY CAUSE OF CHANNEL INSTABILITY
AND SOLUTIONS IMPACTS
GEOMORPHOLOGICAL INPUT TO ENGINEERING SOLUTION
GeoData Institute Page 13 River Otter Geomorphological Audit 5.1 Data Collection
5.1.1 Contemporary Field Survey
In the field, the methodology included:
1. Field mapping at 1:1525, which divided the 30km study reach into a series of smaller homogeneous geomorphological reaches (between 0.025 to 0.5km in length) and indicated the specific location of the following attributes: • geomorphological reach breaks • bank height • bank erosion type and severity • bank protection type • poaching • inchannel barforms • inchannel modifications (i.e. weirs, fords) • flow type
2. Field forms, which collated reach-aggregated information on the following: • bank properties (bank height, material type, structure, vegetation cover, erosion process, and toe condition) • channel properties (wetted width, in-channel sediment storage (bar deposition), flow types, anthropogenic controls on hydraulics and bank erosion, and evidence of reach instability (incision or aggradation)) • catchment influences (landuse and sediment sources)
3. Photographic record which summarised the overall geomorphological character of each reach, and provided detailed visual information on specific attributes of the river considered to be of importance in understanding the controls on bank erosion operating on the River Otter.
The standard methodology for FA has been augmented for the River Otter by the addition of field data parameters specific to bank erosion systems (discussed in Appendix A). The amendments made to the field forms are:
• bank property attributes are separated into left and right banks (while the standard FA methodology just summed the erosion within a reach) • description of the condition of the bank toe has been expanded to include its status, material calibre and coverage and age of vegetation. • addition of dominant erosion mechanism ( slide, rotational slip etc)
The methods of data collection in the field, a copy of the field form used and an example of the information collected in mapped format are included in Appendix B. The mapped field data is converted into GIS format, while the form-based information is entered into
GeoData Institute Page 14 River Otter Geomorphological Audit an ACCESS database which is linked to the digitised reaches within the GIS as attribute data.
5.1.2 Historical (archive) data
Changes within a catchment can have profound affect on the geomorphology and stability of a river channel through influencing the sediment and water regime (Figure 4.1). These changes are referred to as Potentially Destabilising Phenomena (PDP) (University of Newcastle 1998) and those influencing the Otter catchment are indicated in Table 5.1.
Table 5.1 Potentially Destabilising Phenomena (PDP’s) within the River Otter catchment (both natural and anthropogenic)
SCALE PDP INFLUENCE Catchment Land-use change Affect sediment supply and run-off to channel Agricultural drainage Increase sediment supply and run-off to channel Agricultural practices Affect sediment supply and run-off to channel Soil erosion Increase sediment supply to channel Sediment management Decrease sediment supply to channel Change in rainfall – affect channel discharge and Climate change bank moisture content Natural vegetation succession Decrease sediment supply and stabilise banks Reach Channel realignment Affect sinuosity, gradient and bed shear stress Channel regrading Affect gradient and bed shear stress Channel widening Affect width-depth ratio and bed shear stress Gravel extraction Affect width-depth ratio and bed shear stress Dredging Affect width-depth ratio and bed shear stress Embankments Affect width-depth ratio and bed shear stress Bank protection works Decrease sediment supply Affect flow regime – induce scour downstream and Weirs and groynes ponding upstream Agricultural run-off Increase sediment supply and run-off to channel Vegetation of banks Decrease sediment supply and stabilise banks Poaching Increase sediment supply and destabilise banks Tributary supply Increase sediment supply and run-off to channel Planform change Affect sinuosity, gradient and bed shear stress Affect width-depth ratio, sediment supply and bed Erosion and deposition shear stress
The primary aim of the archive data is to provide evidence to support whether any of the PDP’s listed in Table 5.1 have, or are affecting, the geomorphology of the River Otter, and if so to identify the precise location, timing and impact of the PDP within the river system. The methodology includes:
1. Analysis of rainfall and discharge records in the Otter catchment. 2. Review the Agencies archives (reports and management plans) regarding the River Otter. 3. Review academic literature discussing bank erosion, particularly on rivers within East Devon.
GeoData Institute Page 15 River Otter Geomorphological Audit 4. Discussion with Environment Agency Officers, landowners and interested parties on previous channel and catchment management practices. 5. Overlaying historic maps of the River Otter to explore historical planform change.
Information from these sources is entered into the time-chart (Table 6.6) to facilitate the exploration of links between both the natural and anthropogenic forces (PDP) and channel change (i.e. planform change, channel aggradation, increased bank erosion).
The fundamental approach of the geomorphological assessment process is based on identifying the behaviour of the channel adjustment with the aim of determining whether;
1. The channel responds at similar times and over its entire course. This provides evidence of system wide adjustment to large scale driving processes such as increases in runoff or large scale channel modification. Treatment of such large scale factors is generally unsuccessful when applied locally. 2. The channel response is local and persistent (or not). This provides evidence of local controls on adjustment processes that may be treated locally
The use of historical information is necessary because river systems (in common with other natural systems) often experience a lag time between cause and response. Furthermore, the time a river takes to adjust to a perturbation is often measured in decades or centuries. Generally large-scale system-wide changes take longer to adjust to, and in the case of geological events (glaciations, etc.) may take millennia to adjust. The second rationale for utilising historical data is that it provides information on the type and even rate of channel adjustment. This is particularly useful when interpreting the evolution of meander bends.
GeoData Institute Page 16 River Otter Geomorphological Audit 6 HISTORICAL GEOMORPHOLOGY
This section discusses and explores the archive data for evidence of Potentially Destabilising Phenomena (PDPs) within the River Otter catchment to attempt to establish the extent to which past events have affected the river system. Firstly, rainfall and river discharge records are examined to determine periods of flood and drought. Secondly, the location and timing of capital works (flood alleviation schemes), river maintenance (dredging, bank vegetation clearance), and catchment land use changes are described. Finally, by overlaying historic maps, the location, timing and type of active channel change (bank erosion) can be identified and potentially linked to the occurrence of PDPs within the Otter system via the time-chart (Table 6.6). The time-chart is a key element of the fluvial audit as it seeks to identify the temporal coincidence of factors (PDPs) influencing the catchment or channel to help explain the observed sequence of river response.
A distinction must be made between that evidence which derives from objective, ‘hard’ sources (i.e. local hydrometric data records and historical maps) and that which derives from subjective information (i.e. memoirs, generalised south-west hydrometric patterns and events). The latter produce more tentative explanations than the former with respect channel change.
6.1 Rainfall, flood magnitude and frequency
Daily discharge (m3s-1) on the River Otter has been gauged at Dotton (SY087885) and Fenny Bridges (SY115986) since 1962 and 1974, respectively. Daily rainfall (mm) has been monitored in the Otter catchment near Fenny Bridges (Feniton Court gauge) and Ottery St Mary (Ottery St Mary Greatwell gauge) since 1960, and in the catchment source area (Blackdown Hills) at Upottery (ST206089) and Churchingford (ST213125) since 1954 and 1974 respectively. Rainfall at the source is the primary input to river flow and therefore is used in preference to those gauges located further downstream in the catchment to analyse the relationship between rainfall and discharge. Figure 6.1 shows graphically the daily rainfall measurements in the source area at Upottery 1954-2000 and Churchingford 2000-2003, while Figure 6.2 plots the daily maximum discharge at Dotton and Fenny Bridges.
To determine whether the discharge regime of the Otter is rainfall driven, the Pearson’s correlation coefficient (r2) was used to measure the strength of relationship between the two variables. Figure 6.3 illustrates the relationship graphically between discharge at Fenny Bridges and a) daily rainfall in the source area and b) API(5), while Table 6.1 gives the r2 values for the discharge at the two gauge stations with daily rainfall and antecedent precipitation indices of varying durations. The dates, daily rainfall and Antecedent Precipitation Index over the 5 days previous (API(5)) in which discharge at either Dotton or Fenny Bridges exceeded 75% bankfull flow (66 m3s-1 at Dotton and 54.75 m3s-1 at Fenny Bridges) are listed in Table 6.2.
It is apparent that the discharge regime of the Otter is predominantly rainfall driven. Although all of the r2 values in Table 6.1 are statistically significant (an artefact of the
GeoData Institute Page 17 River Otter Geomorphological Audit very large sample size (n)) there is a distinct trend of increasing relationship strength with a reduction in the duration of the API, resulting in the strongest relationship between daily rainfall and daily discharge. As expected for a small, steep-sided catchment, the results suggest there is a very short lag-time (limited storage) in the response of the river to rainfall, discharge being most closely related to the most recent rainfall. The large scatter in the temporal coincidence of high rainfall and high flow events in Figure 6.3 is attributed to differences in the intensity of rainfall rather than total daily amount, and variability in the extent of catchment saturation (API). For example, a high API(5) correlating with a low flow is likely to be attributed to low intensity, prolonged frontal rain, whereas conversely a disproportionately high flow for rainfall may relate to an intense convectional storm.
Variability in the antecedent precipitation conditions may explain why two high magnitude rainfall events produced very different flow responses on the Otter. The major rainfall event (111mm at Upottery) on the 10 July 1968 coincides with the highest flow event recorded at the Dotton gauge (347m3s-1), whereas the rainfall event (88.5mm) on the 28 July 1969 only produced a lower flow peak (45m3s-1). Table 6.3 gives the API values for 5 to 30 days prior to each major rainfall event. It is apparent that the catchment saturation status in July 1968 and 1969 were very different; in 1968 the catchment was heavily saturated, with 151mm of rainfall in the month previous, while in 1969 just 19mm of rain had fallen. The extent of antecedent soil saturation (soil pore water pressure) in the catchment may explain why some rainfall events do produce flood flows in the River Otter, while other, possibly larger, storms have relatively little impact on the flow.
Figure 6.1 Gauged records of daily rainfall (mm) at Upottery (1954-2000) and Churchingford (2000-2003) (Environment Agency and BADC) 120 Upottery Churchingford
100
80
60
40 Daily Rainfall in(mm) Blackdown Hills 20
0 1954 1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002
Year (JDD)
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A 400 Daily Max Flows 350 Bankfull Flow 75% Bankfull Flow 300
250
200
150
100 R. Otter Discharge at Dotton (cumecs) 50
0
1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 Time (Julian Decimal Day)
200 B Daily Max Flows 180 Bankfull Flow 75% Bankfull Flow 160
140
120
100
80
60
40
20 R. Otter Discharge at Fenny Bridges (cumecs)
0
1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 Time (Julian Decimal Day)
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200
180 )
-1 160
s 3
140
120
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80 y = 0.7754x + 1.162 R2 = 0.3827 60
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0 0 10 20 30 40 50 60 70 80 90 B Source Area Daily Rainfall (mm)
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100 y = 0.2401x - 0.1329 R2 = 0.3187
10 FB Daily FlowFB Q (m3/s)
1
0.1 0 25 50 75 100 125 150 API(5) Source Area (mm)
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Table 6.1 Pearson’s correlation coefficients (r2) and sample size (n) values for relationships between daily river discharge and rainfall measurements
Discharge Gauge site Rainfall Variable Pearson’s r2 n Dotton Daily Rainfall 0.3375 13808 Dotton API(5) 0.3409 13808 Dotton API(10) 0.2431 13808 Dotton API(20) 0.1691 13808 Dotton API(30) 0.1499 13808 Fenny Bridges Daily Rainfall 0.3827 10176 Fenny Bridges API(5) 0.3187 10176 Fenny Bridges API(10) 0.2280 10176 Fenny Bridges API(20) 0.1555 10176 Fenny Bridges API(30) 0.1397 10176
Table 6.2 Temporal coincidence of flow events >75% Bankfull recorded at A) Fenny Bridges and B) Dotton, with daily rainfall and API(5)
A. Julian Decimal Date Rainfall at Source (mm) API(5) (mm) Daily Discharge (m3 s-1) 2000.934 50.0 100.7 184.325 2000.847 48.0 60.8 172.892 1997.600 8.9 137.7 141.245 2002.868 48.2 76.6 140.796 1981.411 35.5 45.4 131.734 1989.970 37.2 101.4 113.759 1978.074 37.6 54.9 100.257 2001.000 38.0 38.7 89.831 1979.989 57.4 81.6 87.355 2000.828 65.7 98.5 82.278 1981.997 25.2 84.5 81.669 1975.052 21.3 56.1 80.198 1974.740 19.2 70.4 77.381 1977.940 25.1 64.4 73.488 1989.151 12.8 49.2 72.151 1997.907 26.4 58.8 66.614 1985.984 62.2 106.4 65.594 1994.090 24.9 37.4 65.594 1994.093 3.0 37.3 65.594 1990.074 38.6 59.9 62.427 2002.789 52.3 144.5 61.666 1992.964 8.4 39.9 60.458 1994.855 44.1 58.6 57.192 1975.055 3.0 51.4 56.759 1974.868 17.5 51.3 56.547 2000.831 8.0 105.9 55.954 1986.019 35.5 57.1 55.159
GeoData Institute Page 21 River Otter Geomorphological Audit B. Julian Decimal Date Rainfall at Source (mm) API(5) (mm) Daily Discharge (m3 s-1) 1968.525 111 130.6 347.013 2000.934 50 100.7 144.633 2000.847 48 60.8 132.848 2002.868 48.2 76.6 131.754 1997.600 8.9 137.7 108.624 1979.989 57.4 81.6 104.244 2001.000 38 38.7 102.511 1989.970 37.2 101.4 100.919 1978.074 37.6 54.9 100.801 2000.831 8 105.9 99.777 1981.997 25.2 84.5 97.77 1978.077 1.5 47 97.607 2002.871 4.8 91.6 91.606 2000.828 65.7 98.5 89.842 1975.055 3 51.4 82.236 1997.910 2.4 41.9 81.895 1997.907 26.4 58.8 81.494 1965.912 5.3 61.2 80.379 1981.411 35.5 45.4 80.377 1992.964 8.4 39.9 80.297 1981.414 0 41 79.9 1968.527 0 130.6 79.205 1986.019 35.5 57.1 79.187 1989.151 12.8 49.2 78.556 2002.789 52.3 144.5 78.006 1994.855 44.1 58.6 77.222 1994.093 3 37.3 76.676 1985.984 62.2 106.4 76.365 1977.940 25.1 64.4 76.186 1963.123 26.4 43 75.524 1993.970 10.2 70.2 74.202 1974.123 20.6 53.7 73.081 1984.068 22.6 62.2 72.674 1965.910 32.8 60.7 72.661 1978.148 6.1 45.9 72.006 1997.882 10.7 53.8 70.934 1969.145 4.1 63 70.897 1990.074 38.6 59.9 70.709 1963.301 6.9 58 70.117 1999.715 84 115.7 69.959 1983.970 17 73.6 68.545 1974.011 37 43.7 68.104 1999.310 1.7 77.7 67.88 1975.052 21.3 56.1 67.603 1974.740 19.2 70.4 66.907 1987.252 30.2 37.5 66.704 1990.093 15.6 69 66.704 1999.718 0.1 115.8 66.704
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Table 6.3 API values and discharge (Dotton gauge) for two major rainfall events
Daily Rainfall at Daily River Julian Decimal Date API(5) API(10) API(20) API(30) Source (mm) Discharge (m3 s-1) 1968.525 111 130.6 140 235.3 262.2 347.013 1969.573 88.5 90.1 90.1 90.2 107.7 45.283
With the absence of pre-1960 flow records the close relationship between rainfall and discharge allows long-term rainfall records to be used as a surrogate for indicating periods when the Otter is likely to have been in flood. Rockbeare (SY031940) is the nearest gauging station (5km) to the River Otter with a daily rainfall record pre-1950. Although giving an insight into the gross wetter and drier periods throughout the past century in East Devon, this data is largely unrepresentative of the rainfall patterns in the wetter, source areas of the Otter (the correlation between daily rainfall data at Upottery, in the Blackdown Hills, and Rockbeare is very weak (r2 = 0.054 when n = 13114)). However, some of the high rainfall events do coincide with significant floods reported in the EA archives (as indicated in the time-chart (Table 6.2)).
Figure 6.4 Gauged records of daily rainfall (mm) from Rockbeare gauge station between 1922 and 1980 (BADC)
1100
1000
900
800
700
600
500 Rainfall (mm) 400
300
200
100
0 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 Time (Year)
In other gravel-bed river systems, such as the Tyne and Severn, decadal episodes of increased flooding have been linked to periods of increased channel adjustment and incision at the catchment scale. Although only justified at the gross scale, the rainfall
GeoData Institute Page 23 River Otter Geomorphological Audit record in Figure 6.4 for the Otter catchment does show periods of increased flood frequency prior to 1930 and post 1950, that may coincide with those reported from other UK and European river systems (1910 – 1930; 1950 – 1970, and 1998 – present, Rumsby & Macklin, 1994). However, the second period of reduced flood frequency (1970-1998) is not supported in either the recent daily flow or rainfall records for the Otter (Figures 6.1 and 6.2). The potential link between flood events and increased channel adjustment (bank erosion) is not supported by the earliest phase of increased flood frequency (1910-1930) which does not show marked lateral planform adjustment (except around Newton Poppleford). The second phase (1950-70) is however coincident with the evidence of a phase of increased sinuosity and possible channel incision and recent high flows, in particular the prolonged duration of the millennium floods, has been associated with a phase of bank erosion activity. The strength of this causal link and the extent to which other destabilising factors were involved is not known. Periods of extensive channel modification in association with high flood frequency within the study reach are likely to have reinforced any response.
The lack of a lateral channel response to the earlier period of increased flood frequency may reflect the dominance of incision within the existing planform, with subsequent re- working of the incised channel during the subsequent period up to the 1950’s. This might be a mechanism for explaining the apparent increase in lateral channel activity that appears to coincide with a phase of relatively low flood frequency and has precedence in other UK systems (Macklin & Lewin, 1989). Additional analysis of the floodplain sediments and the use of additional historic data (e.g. Tithe maps) to map channel planform activity in the mid 19th Century might help to confirm this hypothesis.
Alternatively, (and in the absence of a reliable long-term rainfall or flow record evidence to the contrary), the River Otter may not have experienced the same generalised wet and dry periods reported for other UK rivers (Rumsby and Macklin, 1994), and as such, periods of increased flooding might coincide with the increased channel adjustment evident from the post-1930 datasets.
6.2 Channel modification and management
In-channel structures and anthropogenic activities have influenced the channel along much of the 30km study reach of the River Otter. It was evident from the field visit and analysis of historic maps and reports that many of these features are of considerable age. Engineered features include: weirs, mills and associated structures (i.e. mill leats), embankments, bank protection works (e.g. blockstone rip-rap, willow spiling), flood alleviation schemes and channel realignment. Information on the timing and location of these changes have been extracted from reports and depicted on the time-chart (Table 6.6).
The earliest capital works on the Otter were associated with the development of mills and agricultural improvement. With each mill requiring a head of water for power, flow in the Otter is repeatedly interrupted by large head weirs (i.e. Otterton, Dotton, Tipton Mill (Plate 6.1), Cadhay bridge and Clapper Lane Bridge), mill leat channels and realignment to control the channel dimensions. Although the mills on the River Otter have long been
GeoData Institute Page 24 River Otter Geomorphological Audit redundant, the relic channel modifications still exert significant control over the local flow regime. By locally controlling the gradient of the channel, the weirs significantly affect the depositional and erosional environment both upstream (ponding reduces the flow velocity and promotes the accumulation of fines) and downstream (increased local bed slope increases flow velocity and induces bed scour) of the structure. In addition, sources within the Environment Agency suggest that agricultural driven realignment has occurred widely in the catchment and probably pre-1800, although no documentary evidence is available to support this belief.
Although the catchment is predominantly rural, several small towns and villages border the banks of the Otter in association with several road and railway bridge crossings which locally fix the position of the river channel. Along much of the study reach the Otter is repeatedly constrained by the embankment of the disused railway which ran from Feniton to Budleigh Salterton (Figure 3.2), and which may also have required the channel planform to be realigned in some locations.
Unfortunately a paucity of available reports and persons to interview relating to channel management activities between 1900 and 1960 has resulted in an ‘information gap’ in the time-chart (Table 6.6) at the time when significant channel change in the system was initiated (section 6.3). However it has been suggested that in this period semi-commercial gravel extraction from the channel was occurring, which observations from the field survey and historical planform activity appear to support.
From the 1960s to present day the archive data is more substantial and provides evidence for significant and widespread channel realignment, bank protection, embankment construction and general maintenance works between the late 1960s to the mid 1970s. It is likely that this system-wide ‘channel improvement’ strategy for flood defence was in response to the unprecedented 1968 flood, which caused severe damage to several inchannel and floodplain structures (bridge collapse), and initiated significant morphological change (Hooke 1977; Mott MacDonald 1997). In particular, numerous blockstone weirs (grade control structures) were constructed to reduce flow velocity and check bed erosion initiated during the 1968 flood (pers. comm. Mike Williams), although some may have been related to the creation of more favourable fishing habitat.
Bank protection is found both in conjunction with other channel management works (embankments or weirs) and independently of any other works where bank erosion is a persistent problem, or has been in the past. 15.5% (9.03km) of the total 58.36km of bank surveyed is protected, almost entirely by blockstone rip-rap (750 to 1500kg blocks laid or tipped against the bank toe and/or face). Rip-rap can cover long lengths of river bank, for example a stretch of bank over 350m in length at Tipton Mill is continuously protected. However, bank protection along the Otter is generally piecemeal (i.e. located on the outer bank of tight bends or to protect inchannel and floodplain structures) and has largely lacked regular maintenance since the 1980s.
A sequence of major, but localised, capital works (Flood Alleviation Schemes (FAS)) have been undertaken since the 1970s to protect Honiton, Tipton St John and Ottery St
GeoData Institute Page 25 River Otter Geomorphological Audit Mary from spate flows of the Otter and several of it’s tributaries. Brief details of these schemes and two currently proposed schemes are given on the time-chart (Table 6.6).
Interviews with agency staff and the lack of reports available have highlighted an absence of local management, although a number of piecemeal schemes have been undertaken in recent years by landowners (Plate 6.2). This reflects a shift away from the heavy maintenance of channels and the control of bank erosion that occurred in recognition that it was in most cases unjustifiable on economic and environmental grounds.
Plate 6.1 Tipton Mill head Weir
GeoData Institute Page 26 River Otter Geomorphological Audit Plate 6.2 New blockstone rip-rap bank protection downstream of Clapper Lane Bridge (north of Honiton)
6.3 Historical planform change
Planform change is the natural response in many river systems to variations in input of water discharge and sediment load. Although adjustments in channel position are inherent, the cause of change is predominantly external to the fluvial system, and can be either natural (i.e. climate change or extreme flood/drought events) or a response to human interventions. The latter can be either indirect forces, which alter the catchment characteristics (i.e. land use change), or direct, through modification of the channel itself (i.e. channelisation). “The nature of the channel response depends on the inherent instability, the freedom to adjust [competence of flow to rework it’s banks] and the sensitivity of different environments and channel reaches” (Hooke 1997).
Morphological change in a river channel is not exclusive to the planform dimension, affecting both the cross-sectional and long profile form. However, the planform generally incurs the greatest adjustments and together with the availability of documentation (in the form of historical maps), it is the most commonly used dimension to monitor channel change.
Historical maps from 1888, 1903, 1933 and 1956 were overlayed with contemporary maps (2000 Landline data) to identify the location, timing and magnitude of planform change along the River Otter study reach. Transects were drawn perpendicular to the banklines at the apex of each bend in which significant planform change (movement which exceeded more than one channel width between 1888 and 2000) had occurred to measure both the movement between maps of successive dates and the cumulative movement of the bankline between 1888 and 2000. The channel change described by each transect
GeoData Institute Page 27 River Otter Geomorphological Audit throughout the study reach is plotted in a downstream series in Figure 6.5. Each bar in Figure 6.5 represents a transect and illustrates the cumulative (and non-directional) distance (m) the channel has shifted over time.
It is apparent that different locations in the Otter system have undergone planform change at different periods of time. For example, at Newton Poppleford the river exhibits significant change between 1903 and 1933, whereas near Alfington the channel was predominantly active between 1933 and 1956. Throughout the system, the magnitude of channel change between 1888 and 1903 is consistently very low, and with the exception of a 4km stretch centred on Newton Poppleford, it is also generally low between 1903 and 1933. However ambiguity in the validity of a number of the maps dated 1933 has rendered the channel change measurements between 1903 and 1933 uncertain. Overall though, the results suggest system-wide planform change initiated between 1933 and 1956, and has remained highly active between 1956 and 2000.
Figure 6.5 highlighted 15 reaches (labelled M1 to M15) which have experienced a high magnitude of planform change, the locations of which along the River Otter are shown in Figure 6.6. The planform (and transect locations) of these reaches for each of the map dates are plotted in Figure 6.7 (Historical change maps 1-15), while the magnitude of planform shift along each transect is plotted for each map in Appendix C (Map 1-15).
GeoData Institute Page 28 River Otter Geomorphological Audit Figure 6.5 Magnitude of planform channel change (m) downstream of Langford Bridge (anomalous changes at Nod Lodge and artificial realignment excluded)
120 M10 1956-2000 M6 M9 M13 1933-1956 M2&3 1903-1933 100 M11&12 1888-1903
M8 M7 M15 80 M14 M5 M4
60
M1 BankErosion (m) 40
20
0
0 1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5
10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5 18.5 19.5 20.5 21.5 22.5 23.5 24.5 25.5 26.5 27.5 Clapper Lane Distance Downstream (km) Otterton Bridge Harpford Tracey Mill Farm Colhayes Gosford Burnthouse Farm Fenny Bridge Little Burcombe Hams Cottage Newton Pop'ford Bridges Colaton Raleigh Cottarson (oxbow Cadhay Pixies Alfington Wrinkley Cliff complex) Bridge Parlour Weston (N OSM) (S OSM) Tipton St John Dotton Farm Nod Lodge Northmostown Cottages
GeoData Institute Page 29 River Otter Geomorphological Audit Figure 6.6 Map Locations (supplied as PDF file)
GeoData Institute Page 30 River Otter Geomorphological Audit Figure 6.7 Historical Maps 1-15 (supplied as PDF file)
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GeoData Institute Page 45 River Otter Geomorphological Audit Although the exact form and rate of planform change is heavily affected by the composition and resistance of the banks (and bank vegetation) and by both natural (i.e. bedrock bluffs) and anthropogenic (i.e. railway embankments, bank protection works and bridges) constraints on the floodplain, systematic tendencies in meander development are still evident above random forces (Hooke 1995). Figure 6.8 provides a conceptual model of the sequence of changes in meander form typical of active gravel-bed rivers and a classification of the types of meander growth and shift, while Table 6.5 attempts to classify the predominant planform change type for each of the selected reaches in Figure 6.7 (Map 1-15).
Figure 6.8a Model of sequence of meander change (Hooke 1991)
1. MIGRATION
2. EXTENSION 4. INTERSECTION
3. DOUBLE 5. CUTOFF HEADING
Figure 6.8b Types of meander growth and shift (Knighton 1998)
GeoData Institute Page 46 River Otter Geomorphological Audit Table 6.5 Planform change description for each of the 15 selected reaches in Figure 6.7
Map number Planform change description (in Figure 6.7) Meander initiation 1933-56 1 Channel realignment between 1956-2000 (1965 scheme) 2 Significant meander growth and downstream migration of bend apex 1933-2000 Significant meander growth (towards double-heading) and downstream migration of bend 3 apex 1933-1956, but straightened 1965. Increased sinuosity 1966-2000 Significant meander initiation 1933-1956, realigned 1965. 4 Nod Lodge cut-off complex. 5 Significant meander growth 1933-1956, realigned 1965, meander migration 1966-2000 Meander initiation, significant meander growth and downstream migration 1933-2000 6 (attempts to reduce sinuosity in 1965 not evident in 2000) 7 Lateral movement of low sinuosity bends, complicated by confluence with the River Tale 8 Significant downstream migration and meander initiation 1956-2000 9 Significant meander growth (increased sinuosity) 1956-2000 10 Significant lateral movement of channel and meander initiation 1933-2000 11 Significant meander growth and downstream migration 1933-2000 12 Significant meander growth (double-heading), but realigned 1965 to reduce sinuosity Meander initiation 1903-1933, growth 1933-1956, downstream migration 1956-2000 13 (attempts to reduce sinuosity in 1965 not evident in 2000) 14 Meander initiation 1903-1933, growth 1933-2000 15 Meander growth and downstream migration 1933-2000
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Table 6.6 Timechart (supplied as separate file)
GeoData Institute Page 48 River Otter Geomorphological Audit 7 CONTEMPORARY GEOMORPHOLOGY
Data collected in the field survey (both map- and form-based) provided information describing the contemporary channel form and bank stability throughout the study reach. Of most importance to this study is the data concerning the reach bank characteristics (described in Section 2 of Appendix B) and the maps which illustrate the precise location, mechanism and severity of bank erosion.
In total 23.6% of the banks along the River Otter are currently eroding which amounts to 13.80km (both left and right bank) of the 58.36km of bank surveyed. Of this 23.6%, 6.4% (3.74km) are considered to be severely eroding, while 8.7% (5.07km) and 8.6% (5.03km) are of intermediate and low severity.
As expected, the locations where contemporary erosion was found to be most severe (Figure 7.1 Maps 1-15) coincide with those reaches which have experienced significant historical planform change (Figure 6.7 Maps 1-15). The maps in Figures 6.7 and 7.1 show identical stretches of the river and allow the spatial comparison of historical channel change and present bank erosion. It is apparent that the bank sections which were identified as severely eroding (erosion score high (3)) in the field survey are located where channel change historically has been most active and where continued channel migration has not been attenuated by bank protection.
The effect of bank reinforcement in curtailing and modifying planform adjustment is seen in the tight meander loop of Map 2 (Figures 6.7 and 7.1) where bank protection at the bend apex has prevented further meander extension and potentially initiated meander expansion with severe pockets of erosion located on the right bank of each flank of the meander loop. In reaches where the channel has been straightened in the mid-1960s (i.e. upstream of Weston in Map 2, and downstream of Tipton St John in Map 12) the pattern of bank erosion suggests a tendency for the reach to return to the planform prior to the realignment (1956 planform geometry), unless the banks have been heavily reinforced.
Although the majority of contemporary bank erosion is associated with the locations of active planform change since the 1930s, four additional sites along the River Otter have been identified which despite having maintained a relatively stable planform geometry in the past, are currently experiencing severe bank erosion and active planform adjustment. Figure 7.1 (Maps 16-19) illustrates the erosion severity and historical planform change (labelled ‘a’ and ‘b’, respectively) of these four sites, which are ordered in a downstream sequence. The locations of these extra sites (Maps 16-19) in relation to sites 1 to 15 are shown in Figure 6.6. The erosion at these additional sites, with the exclusion of Alfington (Map 17) is important because of the proximity of the banks to infrastructure, in particular the pylon in Map 16 (Plate 7.1), the flood embankment in Map 18 and a caravan site and farm buildings in Map 19 (Plate 7.2).
The predominant natural bank structure along the Otter study reach is composite and non- cohesive, where sand overlies a layer of gravel at the bank toe. Consequently the predominant bank erosion mechanism is fluvial scour of the gravel toe which undercuts the upper bank material causing it to fail by cantilever failure (Figure 4.2).
GeoData Institute Page 49 River Otter Geomorphological Audit Plate 7.1 Foundations of a pylon are under threat upstream of Tracey Bridge (A373)
Plate 7.2 Severe bank erosion upstream of Harpford Bridge
GeoData Institute Page 50 River Otter Geomorphological Audit Dependent on the severity of erosion and the capacity of the flow to remove the failed material, a toe of the failure blocks may accumulate and vegetate over time, thus reducing further erosion. Where erosion is most severe the failed material is rapidly entrained and scour of the bank continues unimpeded. This mechanism of bank failure is exemplified at Tipton St John (where channel migration has recently caused a footpath and several trees to collapse into the channel and threatens the stability of two tennis courts in the playing field bordering the channel, Plate 7.3) and north of Ottery St Mary (Plate 7.4).
Plate 7.3 Severe outer bank erosion at Tipton St John
Plate 7.4 Severe bank erosion 0.6km upstream of Ottery St Mary
GeoData Institute Page 51 River Otter Geomorphological Audit Figure 7.1 Erosion Maps 1-19 (supplied as PDF file)
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GeoData Institute Page 68 River Otter Geomorphological Audit 8 GEOMORPHOLOGICAL INTERPRETATION
Although the results from the field survey explain how (failure mechanism and erosion process) the banks along the River Otter are currently eroding, the historical data sources must be utilised to ascertain a cause for the bank instability. It is apparent that the bank erosion is not isolated to a discrete location on the Otter, but is, and has been since 1933, a system-wide problem. From Figure 4.1 four key factors have been isolated as potential causes of bank instability in the study reach; climate (rainfall), planform (sinuosity), channel incision and channel works.
Climate: The effect of climate, in particular rainfall events which induce high flows and fluvial scour, is system-wide spatially and episodic temporally. The sensitivity of the river to flood driven erosion has been exemplified in the comments of several riparian landowners who observed a distinct increase in the magnitude of bank erosion during the recent winter floods of 2000 and 2002. However, the lack of a reliable long-term rainfall or flow record for, and prior to, the period (1933-1956) when major channel planform adjustment appears to have been initiated prevents the definitive association of climate and erosion as the primary initiator of system-wide bank instability in the 1930s-50s.
Planform: The planform of the River Otter remained relatively unchanged between 1888 and 1933. Since 1933 planform sinuosity has increased throughout the system and continues to increase to the present day (with the exception of those locations where the channel was realigned (straightened) in 1965/66). The rate of migration is controlled largely by the initial bend geometry and in particular the channel curvature (rc/w, where rc is the radius of curvature of the meander bend and w is the channel width) (Knighton 1998). Meander change studies have demonstrated that the rate of migration reaches a peak when 2 < rc/w < 3 and confirm the general form of the curvature-migration rate relationship illustrated in Figure 8.1 (Hickin and Nanson 1984; Hooke 1987). The meanders of the 2000 planform of the River Otter have predominantly an rc/w between 4 and 5, which suggests that the current planform, although approaching the peak of channel migration, is likely to continue increasing in sinuosity for some time to come.
Figure 8.1 Relationship of channel migration rate to bend curvature (Nanson and Hickin 1983)
GeoData Institute Page 69 River Otter Geomorphological Audit
The development of a progressively more sinuous planform has caused the incidence of bank erosion to rise (due to an increase in the impingement of flow onto the banks), in particular at the outer banks of bend apices. Although the presence of terraces in the floodplain indicates the Otter is inherently a dynamic system, the dramatic increase in sinuosity between 1933 and 1956 is likely to have been triggered by an external stimulus.
Channel Incision: Throughout the study reach, evidence for channel incision (bed level lowering) is widespread, including undermined bank protection structures, the presence of bed check weirs (Plate 8.1), bank erosion on both banks exposing gravels at the bank toe etc. An over deepened channel, by increasing the bank height and angle (Appendix A), and reducing the width-depth ratio (increasing flow velocity and fluvial scour) may cause the banks to become unstable. It is likely that some form of channel works in the 1940’s artificially incised the channel which destabilised the banks, increased stream power and consequently initiated meander development. Although the evidence for such direct management activity is lacking, it has been suggested that the timing coincides with the development in 1940 of the ‘Taunton Stop Line’ WW2 defences, which ran between Highbridge and Seaton following the line of waterways and railways (pers. comm. Mike Williams). However subsequent investigation with specialists in this area has revealed that it is unlikely that the River Otter was modified at any location as a WW2 defence measure, with only Honiton being utilised as a possible ‘anti-tank island’ which involved the construction of road blocks rather than channel realignment, scarping and dams structures as used on the River Axe (pers. comm. David Hunt).
Plate 8.1 Rip-rap bed check weir located downstream of Nod Lodge
An alternative, but related, stimulus for channel manipulation throughout this period may have been the intensification of agriculture since the 1930’s and in particular throughout WW2. Channel incision and realignment for agricultural purposes (i.e. to increase
GeoData Institute Page 70 River Otter Geomorphological Audit drainage and manipulate flood waters), and land-use and production intensity changes would have significant implications directly on the channel and flow geometry, and indirectly on the run-off and flow regime. An objective assessment on the magnitude and timing of keys changes in agriculture requires the analysis of land utilisation survey results (1936 Dudley Stamp; 1965-8 Alice Coleman) and parish returns.
Channel Works: Despite a lack of archive information regarding channel management on the River Otter pre-1960, numerous types of channel works have been implemented post-1960. The location and dates of these activities are included in the time-chart (Table 6.2).
Although somewhat speculative, it is likely that artificial channel incision throughout the River Otter in the 1940s is the root cause of the bank erosion problems on the Otter today. It is possible that these changes to the geometry of the channel caused the system to cross an intrinsic threshold (Schumm 1973), which initiated increasing planform sinuosity (meander development) and bank instability. It is likely that this was compounded by the exceptional flood in 1968, which is speculated to have resulted in large shoals of gravel being deposited on the floodplain and not returned to the channel (pers. comm. Mike Williams). Although these observations are contradicted by the experiences of Steve Moore (pers. comm.), several of the annual reports of the Devon River Authority (1968-73) refer to the ‘usual gravel shoal removal’ in response to floods with respect the maintenance operations on the River Otter. Any significant removal of gravel from the system would have reinforced the downcutting instigated in the 1940s, and evidence of channel incision at this time is supported by the construction of grade control structures (weirs) to reduce velocity and keep the bed erosion in check.
It is acknowledged that without the availability of ‘hard’ documentary evidence in this period, speculative judgments based on various, and potentially contradictory, subjective observations have been utilised.
The factors revealed by this study present a picture of increasing trends towards susceptibility to bank erosion throughout the River Otter. These are in part due to human modification, but also result from phases of increased flood frequency and reinforcement of channel sensitivity resulting from increasing bend curvature and planform sinuosity. The relationship between these pressures on the Otter channel and the response of the channel through bank erosion and channel adjustment are mapped out in Figures 8.2 and 8.3. The implications of these interrelationships are that simply increasing bank strength by protection is not a complete (and therefore sustainable) solution to bank erosion and channel adjustment.
Figure 8.3 illustrates the impacts of human activities (grey boxes), in association with flooding, on the factors which control bank erosion and failure (slumping) by overlaying their effect on the original flow chart depicting the natural bank erosion system in Figure 4.1. The factors in Figure 4.1 affected by these activities are outlined in red boxes, and the direction of their affect can either be an increase or decrease (i.e. dredging causes an increase in bank height and angle, but a decrease in the width-depth ratio), or a non- specific change (i.e. system incision will cause a change in the bank structure).
GeoData Institute Page 71 River Otter Geomorphological Audit Figure 8.2 Schematic model describing the planform and cross-sectional evolution of the River Otter between 1888 and 2003
Pre 1933: Stable cross section and low sinuosity planform.
1933 – 1956: Channel incision and widening initiated causing bank erosion and planform migration.
1956 – 2003: Continued incision and planform migration where channel allowed to adjust. Where channel reinforced, realigned and bed check weirs installed – 1956 cross section maintained.
GeoData Institute Page 72 River Otter Geomorphological Audit Figure 8.3 Bank Management implications flow chart (supplied at separate file)
GeoData Institute Page 73 River Otter Geomorphological Audit 9 MANAGEMENT IMPLICATIONS
This study has found that the problem of bank erosion is not in isolated reaches, but is occurring system-wide and as such cannot be successfully treated through the current piecemeal bank protection works. It is speculated that the cause of this instability was channel works implemented some 60 years ago, the implications of which have produced a channel sensitive (bank erosion and channel migration) to high flow (intense rainfall) events, particularly after long periods of rainfall which create antecedent saturated conditions (i.e. 2000 flood series).
Unfortunately, the primary driving variable of contemporary bank erosion is the climate (rainfall) and as such very little can be done to control or modify this. However there are a number of management options which could potentially treat the bank erosion either by a) reducing the extent of flow impingement (fluvial scour) against the bank by decreasing the planform sinuosity, or b) increasing the bank strength through reinforcement (soft or hard engineering works), and these are outlined in Section 9.1.
9.1 Management options
The choice of management options that the Environment Agency can adopt to address the bank erosion issue on the River Otter, which range from ‘do nothing’ to the complete realignment and revetment of the study reach, are as follows:
1. Allowed natural adjustment This option accepts that the River Otter is still actively adjusting to the events in the 1940s, which massively lowered the bed levels of the channel, and as suggested in Figure 8.1 is likely to continue to adjust for some time in the future. It therefore may be considered cost effective to retire form the river margins and allow the river to migrate freely across the floodplain. Although this option appears to be ‘do nothing’, it is actually adaptive management and requires an intensive programme of monitoring, both site-specific and system-wide, to ensure the situation does not worsen and the expected benefits arise (EA 1999).
Benefits: Stock fencing will produce a riparian buffer zone and reduce poaching which currently exacerbates the bank erosion on the River Otter. Natural, unconstrained adjustment (erosion and deposition) will allow the river to rebuild the bed level of the channel (aggradation) and potentially stabilise the banks and planform to some extent. Creation of ecological habitats in-channel, on the banks and in the floodplain through erosion and deposition process (floodplain renewal).
Constraints: Requires a sufficiently wide corridor to accommodate future erosion and channel migration. Evidence of the magnitude and time-scale of future channel migration to define the limits of the corridor to set-aside, and to determine whether the channel will ultimately stabilise is essential.
GeoData Institute Page 74 River Otter Geomorphological Audit This option would not be acceptable or achievable in urban areas or where infrastructure is under threat (i.e. the pylon at Tracey Mill Farm, Figure 7.1 map 16). Compensation to landowners for the loss of land to erosion. Cost of a monitoring programme.
2. Complete channel realignment and bank revetment (structural engineering) The bank erosion issue on the River Otter is extensive and system-wide, therefore the solution to prevent it must also be systematic. The current piecemeal pattern of bank reinforcement in zones of intense erosion may delay bank erosion in that specific location, but the cause (flow impingement onto the bank) has not been treated and eventually the bank protection may be undermined. Alternatively the zone of bank erosion will be shifted upstream or downstream beyond the limits of the bank defence and the problem will still persist and propagate throughout the system.
The basis of this option is to treat the cause and effect of the bank erosion by both reducing the channel sinuosity (and therefore angle of flow impingement) and reinforcing the banks. This option firstly the complete realignment of the River Otter system to produce a very low sinuosity planform, and secondly to maintain and fix this artificial planform with heavy bank revetment works.
Benefits: Bank erosion issue entirely removed. Potential to incorporate flood protection into the scheme by increasing the flow capacity of the channel. Remove issue of land loss and threats to infrastructure.
Constraints: Massive and unviable financial costs to straighten and reinforce every meander bend. Aesthetic losses as the river is converted into a virtual canal. Environmental losses – huge habitat destruction both inchannel and on the banks.
3. Interim solution (bioengineering/biotechnical engineering) Options one and two are at the extremes of what could be done to resolve the bank erosion issue on the River Otter, and neither are acceptable in their entirety for the reasons outlined above. Option three therefore suggests an intermediate response, using softer and shorter term techniques (i.e. willow spiling and planting, bank vegetation encouragement and root wad deflectors) which will not stop bank erosion, rather significantly delay it in some locations.
In addition it is suggested that all gravel removal from the channel should cease, with the exception of that to enable flood conveyance. For the latter, gravels should only be removed to the level of the riffle crests, and any excess should be returned to the system by filling the downstream pools.
Benefits: Environmentally sound bank management techniques Aesthetically pleasing
GeoData Institute Page 75 River Otter Geomorphological Audit Redistribution, rather than removal of gravels may reduce further channel incision.
Constraints: Only slows bank erosion, does not prevent it entirely. In many locations on the River Otter the conditions are too extreme (fluvial scour, very high banks and steep angles) for soft engineering to be effective, thus it could be a potential waste of time and resources.
4. Hybrid solution It is apparent that aspects of each of the options could be applied at different locations in the system according to the specific geomorphological and flow conditions. Numerous types of bank reinforcement have been implemented along the River Otter in the past, the vast majority being hard engineering (blockstone), the success of which has been variable spatially and temporally. Soft engineering (willow spiling) has been utilised on a 180m length of bank between Otterton and Colaton (Plate 9.1). Although the bank heights on this stretch are relatively high (1.8 to 2.0m) the planform is very straight which may explain the success of soft engineering in this specific reach.
Plate 9.1 Bioengineering bank protection technique (willow spiling) used between Otterton and Colaton
Table 9.1 attempts to define the conditions (i.e. bank height and angle, bankfull velocity and land-use significance) under which different types of management should be applied.
GeoData Institute Page 76 River Otter Geomorphological Audit Table 9.1 Management options for different site condition scenarios (adapted from the Waterway Bank Protection Manual, EA 1999)
Bank geometry and flow conditions
Bankfull Management Land-use Bankfull Bank angle velocity Appropriate material height (m) (◦) Option criteria (m/s)
Availability of a Allow natural wide corridor Bank geometry and flow conditions analysed to determine the adjustment (1) (CMZ) in spatial and temporal extent of channel migration and whether the which land can be lost to banks are likely to stabilise within acceptable limits. erosion
Low bank <3 <2.5 Willow spiling Bioengineering loading and <3 <1 Reeds / willows / trees (3a) fairly low <1.5 <4 <34 Grass revetment priority land- <1.5 1-2.5 >50 Willow spiling use <1.5 1-2.5 <50 Grass and reeds
Medium Biotechnical loading (i.e. engineering non-metalled Toe rolls / open cell <3 <4 (3b) road, revetment (OCR) individual building)
<3 <4 Toe rolls / OCR 1.5-3 <1 <34 Geotextiles Low loading <1.5 2.5-4 <34 Geotextiles <1.5 1-2.5 <34 Geotextiles <1.5 <1 <60 Pocket fabric
All bank geometries can be protected by structural engineering Structural therefore used predominantly for banks >3m with steep bank face solution (2) High priority and in high velocity flow conditions. Actual material and design of land-use (high structure used dependent on precise bank and flow criteria loading) i.e. particularly the bank failure mechanism i.e: metalled road, • cantilever failure - the bank toe must be protected buildings. • slab failure - regrade to a lower slope unless insufficient space - vertical walling is then employed For all structural solutions, good bank drainage must be ensured. Where channel incision has increased bank height causing bank Flow control failure, deflectors, such as groynes, can be used to slow the flow structures velocity and shift the scour zone away from the bank toe. (deflectors) However flow control structures can create as well as control scour, therefore need careful design of deflector spacing, dimensions and orientation and associated bank revetment measures. Only for localised erosion issues – too costly too implement over long reaches, which may be necessary to prevent the transfer of the problem downstream.
GeoData Institute Page 77 River Otter Geomorphological Audit In the 19 locations (Figure 7.1 maps 1-19) where bank erosion has been identified as most severe, the channel conditions indicate channel realignment and structural engineering is likely to be the only option which could attain long term bank stability (Table 9.1). The flowing section (9.2) discusses the management options for Tipton St John (Figure 7.1, map 11).
9.2 Case Study: Tipton St John
The erosion appears to have started in the upstream bend sometime before 1956 with the downstream bend (Plate 7.3, 9.2, 9.3) only starting in the early 1990’s becoming severe in recent years (during and after the 2000 and 2002 winter floods).
The cause of the erosion is not attributable to just one factor but the lower bend erosion is strongly affected by the development of the upstream bend. Flood flows, though passing over the channel bars, are now focussed as impinging flow at the downstream bend (Figure 9.1). The increasing bend curvature associated with the bank erosion and sediment accumulation is now developing a much more sinuous channel with erosion now starting on the right bank (west bank) opposite the blockstone revetment by the playground. In short the geometry of the bends is changing and with it the locations and
severity of the erosion.
The fact that the bank material is composed of sands overlying a gravel toe containing material of similar calibre as that currently transported by the river in flood contributes to the erosivity of the bank. Furthermore the scale of the banks (2.5 – 3.5m) mitigates against bank stabilisation by existing (sparse) vegetation/tree growth.
A further factor that might be helping exacerbate the erosion processes is that the channel has widened through the erosion, with the result that more of the flood flow is contained within the channel banks rather than dissipating over the floodplain. Whilst this offers flood protection it increases the power available for bank and bed erosion.
In summary the banks adjacent to the playing fields are severely eroding and this is due principally to: 1. the upstream bend geometry “aiming” flood flows directly at the bank 2. the weak nature of the bank material and scale of the banks in relation to vegetative protection 3. widening of the channel leading to confinement of flood flows and increased stream power within the reach.
The current geometry of the river bends and associated gravel deposits strongly suggest that the river is entering a phase of bank erosion on the right bank. Should this continue then the channel will ultimately focus flows on to the existing blockstone towards the road bridge. This might lead to failure of this wall under impinging flows and high flood magnitude. If erosion continues then there is the possibility over time (uncertain how
long since this is driven by flooding) that the bank erosion will also reach the railway (flood) embankment.
GeoData Institute Page 78 River Otter Geomorphological Audit Figure 9.1 Schematic diagram of the channel configuration at Tipton St John
Road bridge Key
Rip-rap
Bank erosion
Flow path
Gravel shoal
Plate 9.2 Bank erosion into the playing fields (and tennis courts) at Tipton St John
Plate 9.3 Conservation value of sandy banks for sand martin nests (Tipton St J.)
GeoData Institute Page 79 River Otter Geomorphological Audit Management options for Tipton St John:
• In accordance with the Waterway Bank Protection Manual (EA 1999) to stop further channel migration under the conditions exhibited at Tipton St John, structural engineering is the only viable solution. However, as with much of the land bordering the River Otter, the minimal bank loading and limited value of the land under threat cannot justify the massive expense of a structural solution (unless the bridge and railway embankment (flood protection) can be shown to be threatened).
• Bioengineering and biotechnical engineering solutions offer an interim solution (such as the emplacement of root wads to act as a buttress at the bank toe, Figure 9.1) which may delay fluvial scour for sufficient time to promote vegetative colonisation and further slow the bank erosion. However, under the extreme conditions observed at Tipton St John it is likely that such measures will be very short term, particularly as current climate change scenarios tend to suggest generally wetter winters and high flood frequency thus increasing the occurrence of the main driver of bank erosion – flooding.
Figure 9.1 Schematic diagram illustrating the implementation of root wads as a bank protection measure in A) planform and B) cross section
A
B
• The final option is to retire from the riparian zone (movement of facilities to adjacent land within the playing fields) and allow the channel to adjust and potentially self- stabilise. This principal of allowing the river a corridor of land to freely migrate within has been utilised in the USA and is termed a ‘Channel Migration Zone (CMZ)’. A combination of historical information and field data to interpret past and current channel conditions is used to predict future trends in channel migration (bank erosion) and delineate an area in which channel processes will occur over a specified period of time (Rapp and Abbe 2003, Figure 9.2).
GeoData Institute Page 80 River Otter Geomorphological Audit The CMZ concept in association with semi-continuous monitoring of the planform, long profile and cross-sectional geometry of the channel, may provide a sustainable, system- wide solution to the bank erosion problem which persists extensively throughout the River Otter.
Figure 9.2 An example of a Channel Migration Zone as the cumulative product of the HMZ, AHZ, EHA and DMA based on historical and field analysis and interpretation (Rapp and Abbe 2003)
10 USE OF THE REPORT
The value of this report lies in the future use of the information collected, both as a reconnaissance survey and as a basis for monitoring. It is a ‘live’ document, and its incorporation within a GIS / ACCESS database environment will allow it to be updated with additional information and integrated with a range of data sources.
• Continued monitoring, both system-wide (aerial photo records) and site-specific (i.e. monitor the cross-section, planform (GPS actual bank configuration) and long-profile of the 19 locations highlighted in Figure 7.1), for channel widening, planform and bed-level change respectively, is a necessary part of ‘adaptive management’.
For example, at Harpford Bridge (Figure 12.3, Appendix B) the channel configuration has significantly altered in response to the recent winter floods (2002). Shoals of gravel deposited in-channel have initiated a sinuous planform (and therefore impinging flow) when the river stage is low and the shoals are exposed. The management appropriate for
GeoData Institute Page 81 River Otter Geomorphological Audit this specific reach is contrary to that advised generally for the Otter system, in that shoal redistribution may be advised to remove the sinuosity in the flow. However, gravel redistribution in such a sensitive river must employ caution as the removal of the riffle just downstream of the bridge could trigger instability in both the bed and banks for the whole reach and propagate to those upstream and downstream. Stock fencing may also be utilised to reduce poaching in this reach.
Such site-specific, adaptive management is dependent on intensive long-term monitoring and may prevent the need for costly structural responses, and allow for uncertainty in system response.
• It is possible the report could be utilised as a decision support system in association with the Waterway Bank Protection Manual (EA 1999) such that it produces a series of management options specific to the geomorphological circumstances of the channel (Figure 10.3).
Figure 10.3 Potential strategy for utilising geomorphological audit data within a decision support system for channel management
GIS AND ACCESS DATABASE (BASE LINE INFORMATION)
PROCESS UNDERSTANDING SITE SPECIFIC CONDITIONS (KEY CONTROLS / FACTORS IN • bank height, angle and material SYSTEM) • planform geometry (bend curvature and evolution) • landuse in river corridor • flow conditions
EROSION CONTROL TOOLS (WATERWAY BANK PROTECTION MANUAL)
MANAGEMENT OPTIONS
GeoData Institute Page 82 River Otter Geomorphological Audit 11 REFERENCES
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GeoData Institute Page 85 River Otter Geomorphological Audit