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,