Study of Interaction between Broad Oak Reservoir and Richborough Connection Project South East Water & National Grid
Geomorphology Fluvial Audit
B14000AG/BORStudy/1016 | 0 July 16
Geomorphology Fluvial Audit South East Water & National Grid
Geomorphology Fluvial Audit
Study of Interaction between Broad Oak Reservoir and Richborough Connection Project Project no: B14000AG Document title: Geomorphology Fluvial Audit Document No.: B14000AG/BORStudy/1016 Revision: 0 Date: July 16 Client name: South East Water & National Grid Client no: Project manager: Alaistair Smith Author: Sera Roberts and Katy Kemble
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Document history and status
Revision Date Description By Review Approved
0 2nd Dec Draft for client comment Sera Roberts Andrew Brookes Ros Vincent 2015 Katy Kemble
1 4th July 2016 Final for Stage 1b report rev 1 Sera Roberts Andrew Brookes Ros Vincent Katy Kemble
Distribution of copies
Revision Issue Date issued Issued to Comments
approved
As appendix to report 0 A J Smith 15-12-15 Report issued under control of main report B14000AG-BORStudy-901
As appendix to report 1 A J Smith 04-07-15 Report issued under control of main report B14000AG-BORStudy-901
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Contents Executive Summary ...... 34 1. Introduction ...... 45 1.1 Background ...... 45 1.2 Report Aim and Objectives ...... 45 2. Fluvial Geomorphology ...... 67 2.1 What is Fluvial Geomorphology? ...... 67 3. Methodology ...... 78 3.1 Geomorphological Reconnaissance Survey and Spot Checks ...... 78 3.2 Data Collection ...... 78 3.3 Analysis and Reporting ...... 78 4. Desk Study ...... 910 4.1 Catchment Overview ...... 910 4.2 Historical Channel Change ...... 910 4.3 Geology and Soils ...... 1011 4.4 Hydrology ...... 1011 5. Contemporary River Characteristics ...... 1112 5.1 Geomorphological Characteristics ...... 1112 5.1.1 Channel Planform ...... 1112 5.1.2 Floodplain Connectivity ...... 1213 5.1.3 Flow Variation ...... 1314 5.1.4 Bed Substrate ...... 1415 5.1.5 Sediment Sources ...... 1516 5.1.6 Erosion ...... 1617 5.1.7 Deposition ...... 1718 5.1.8 Land Use and Riparian Zones ...... 1920 5.1.9 Channel Modifications and Structures ...... 1920 5.2 Analysis ...... 2122 5.2.1 Channel Planform Changes ...... 2122 5.2.2 Channel Substrate and Bank Erosion ...... 2122 5.2.3 Potential Sediment Sources ...... 2223 6. Summary and Recommendations ...... 2324 6.1 Summary of the Sarre Penn Characterisation ...... 2324 6.2 Recommendations for Diversion Design ...... 2324 References ...... 2627
Appendix A. Point Sediment Sources Appendix B. Structure Types
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Executive Summary
South East Water’s Water Resources Management Plan (2014) identified a new reservoir at Broad Oak as a ‘preferred plan water resource option’ for development within the 25 year plan period. A concept stage study proposed a top water level of 36.0mOD. The reservoir would be filled principally from an abstraction from the River Stour at Plucks Gutter (in an adjacent catchment) and pumped to the reservoir through a pipeline. The proposed scheme includes a water treatment works located downstream of the dam from where potable water would be pumped into the supply.
Approximately 2km of the Sarre Penn runs through the footprint of the proposed reservoir. Construction of the reservoir would lead to the loss of the river channel and associated riparian habitats. The Sarre Penn is classified under the Water Framework Directive and a preliminary assessment suggests that without mitigation the reservoir development could result in a deterioration of the status or potential of the waterbody. At this stage the option discussed with the Environment Agency to address this WFD impact, is to create a river diversion channel to maintain the continuity of the stream and its habitat function. This will involve a diversion channel perched on the southern edge of the reservoir within SEW’s land holding.
A geomorphological reconnaissance survey of the river has been undertaken, particularly focusing on the upstream lengths, to establish the character of the river and the parameters for an acceptable design of a river diversion. This assessment aims to establish the baseline conditions based on a walkover survey and also analysis of data/information. Where possible these findings are supported by literature and professional judgement.
The Sarre Penn is not an active alluvial gravel bed river. Whilst secondary currents are present in the channel, they are insufficiently powerful to erode the bed and banks. It has been assessed that the Sarre Penn has historically been heavily modified, leading to a modified channel that is slowly attempting to adjust within the bank limits to the channel changes. The clay banks provide a definitive barrier to lateral migration, with the course of the river not having adjusted in recent history. There is little evidence of a sustainable sediment source within the upstream reaches of the catchment, with minimal gravel observed in the banks and in tributaries/drains. The channel bed (either natural or post-dredging) is generally segregated and consolidated and is not thought to be mobile as a result. The finer sediments (sand and silt) have primarily been winnowed out and transported downstream. The small localised areas of mobile deposits that are present are anticipated to be a response to historic channel modifications and dredging, exposing and/or dislodging lenses of gravels in some sections of the river bank. These mobile deposits were observed to be small, intermittent and unsustainable.
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1. Introduction
1.1 Background
South East Water’s Water Resources Management Plan (WRMP) (2014) identified a new reservoir at Broad Oak as a ‘preferred plan water resource option’ for development within the 25 year plan period. A concept stage study proposed a top water level of 36.0mOD. The reservoir would be filled principally from an abstraction from the River Stour at Plucks Gutter (in an adjacent catchment) and pumped to the reservoir through a pipeline. The proposed scheme includes a water treatment works located downstream of the dam from whence potable water would be pumped into the supply.
Approximately 2km of the Sarre Penn runs through the proposed footprint of the reservoir (Figure 1.1). Construction of the reservoir would lead to the loss of the river channel and associated riparian habitats in the area. The Sarre Penn is classified under the WFD and a preliminary assessment suggests that without mitigation the reservoir development could result in a deterioration of the status or potential of the waterbody. At this stage the option discussed with the Environment Agency to address this WFD impact, is to create a river diversion channel to maintain the continuity of the stream and its habitat function. This will involve a diversion channel perched on the southern edge of the reservoir within SEW’s land holding.
Jacobs’ geomorphology team were commissioned to undertake a geomorphological reconnaissance survey of the river, particularly focusing on the upstream extents, to establish the character of the river and parameters for design of a potential river diversion. This work aims to establish the baseline conditions following a walkover survey and to undertake analysis based on professional judgement informed by the literature.
1.2 Report Aim and Objectives
The aim of the geomorphological reconnaissance survey is to establish the baseline character of the Sarre Penn to inform the design of the potential watercourse diversion. To achieve this, a number of objectives have been defined to focus the study: 1) Define key reaches along the Sarre Penn in relation to the geomorphological features and processes and changes in land use. 2) Characterise forms and processes (in general), using literature to support deductions. 3) Assess what functions and processes would be relevant to a river diversion design.
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Figure 1.1 : Broad Oak reservoir site and Sarre Penn
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2. Fluvial Geomorphology
2.1 What is Fluvial Geomorphology? This study concerns fluvial geomorphology which is the study of landforms associated with river channels and the processes that form them. It considers the process of sediment transfer (erosion, transport and deposition) in river systems and also the relationship between channel forms and processes.
The geomorphological form of a river channel and valley floor is complex and influenced by many different factors and inter-related processes. Controls influencing the river system include external controls and internal controls. External controls include catchment geology, topography, soil type, climatic trends and land management practices. Internal controls may include bed and bank materials, vegetation characteristics, channel gradient, cross-sectional morphology and flow conditions. These controls interact to determine fluvial processes, such as flow and sediment transport, which in turn, influence channel form.
As a natural system, a river evolves in response to natural influences. However, rivers are often significantly affected by human activities. Artificial structures in the river, alterations to the channel dimensions and land management around it can have major implications for river forms and processes. Changes in one part of the river catchment either through natural or human activity can result not only in geomorphological adjustment over time at that point, but also in changes upstream and downstream. An understanding of the controls on channel morphology is required before an action to enhance conditions or reduce and mitigate the impacts of current or future activities is decided upon.
Hydromorphology is a specific term coined by the Water Framework Directive (Directive 2000/60/EC). It refers to the relationship between hydrological processes and the morphological effects and encompasses key factors such as river width and depth, riparian zone, longitudinal profile, lateral profile, groundwater connectivity and bed substrate. Within this report hydromorphology and fluvial geomorphology are taken to be synonymous.
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3. Methodology
3.1 Geomorphological Reconnaissance Survey and Spot Checks
A targeted survey of the Sarre Penn from its source to its confluence with the River Stour was undertaken by two experienced geomorphologists from 5th October 2015 to the 7th October 2015. A detailed geomorphological reconnaissance survey was undertaken of the section from the source at Dunkirk to Calcott (approximately 9.7km) and from Upstreet to Sarre (approximately 3.7km).
Following this, two spot checks were undertaken at TR 19095 62457 (near Westbere) and TR 21001 63445 (near Hoath) to provide context of the Sarre Penn in the reaches without full access to private land. Figure 3.1 provides an overview of the study area.
3.2 Data Collection
The survey assessed the baseline condition of the main channel within the Sarre Penn catchment potentially affected by the proposed scheme. This also provided an understanding of existing geomorphological conditions of the water body. The detailed geomorphological reconnaissance survey logged all features within the channel throughout the length walked including: depositional features, pools/riffles, bank erosion, point sediment sources (e.g. poaching and outfalls), diffuse sediment sources (e.g. tilled arable land), general geomorphological function and process, substrate, riparian corridor characteristics and flow types. A photographic record of the general character of the watercourse was collated.
Spot checks mapped the river in a similar manner to the continuous reconnaissance survey, logging all the features observed from the location. A general overview of the area around the spot check and the watercourse along with a photographic record were also noted.
3.3 Analysis and Reporting
Following write up of the field survey an analysis of each of the reaches has been undertaken to establish the fundamental geomorphological processes and the nature/character of the watercourse. The analysis has been informed by literature where possible, to support the assessments made. Following this, a general summary on the Sarre Penn has been made to establish the type and character of the channel where it would be flooded by the reservoir.
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Copyright OS Open Data (2015)
Figure 3.1 : Geomorphology study area of the Sarre Penn (NB: because of land access a continuous detailed walkover was not always possible so there are gaps in information/data)
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4. Desk Study
4.1 Catchment Overview
The main river within the study area is the Sarre Penn, which rises upstream of the proposed reservoir site to the west near Dunkirk. The river then flows east through the area of the proposed scheme footprint and continues to a confluence with the Great Stour near Sarre. The river is fed by an artificial network of small drainage ditches (typically man-made) in the upstream extents and one main tributary near Vale Farm within the proposed scheme footprint.
A desk based assessment of the entire Sarre Penn catchment has been undertaken using aerial photography. Upstream of the study area towards the source of the river, the Sarre Penn becomes narrower and exhibits both depositional and erosional features. The riparian corridor is semi-continuous. Downstream of the study area, the river appears to be sinuous in the section west of Hersden, beyond which it appears to have been historically modified with an artificially straight planform and uniform channel continuing as far as the confluence with the Great Stour. In the section downstream of Hersden, the river appears to be embanked for the majority of its length with a number of structures present, including weirs. The riparian corridor in the upper reaches is semi-continuous, but then non-existent in the lower reaches with only a few isolated trees present. The upper section shows some evidence of erosion and deposition of the channel; however, the lower section is uniform with no features evident.
The principal tributary of the Sarre Penn within the study area (Figure 1.1) joins the main channel at Vale Farm, upstream of Calcott. The tributary, referred to in this Report as Trib01, has a semi-continuous buffer zone varying in width from 5-10m.
The surrounding land use is predominantly agricultural. Within the vicinity of the study area, there are a number of ancient woodland areas classified as Sites of Special Scientific Interest (SSSI), including the West Blean and Thornden Woods, which in total are approximately 35ha in size. These woodlands comprise broadleaved, mixed and Yew woodland, and the condition is currently considered to be unfavourable – recovering (assessed in 2009). The Stour catchment is also a priority catchment of the Catchment Sensitive Farming Delivery Initiative (2011-2016).
4.2 Historical Channel Change
Historically, there has been little significant change to the planform of the Sarre Penn and its tributaries within the study area. The most significant change identified is the realignment of the Sarre Penn around Mayton Lane Bridge. A reservoir was also constructed adjacent to the river, first appearing on maps in 1972, consisting of a small outfall structure within the channel.
Outwith the study area, the planform of the Sarre Penn has also had little significant change since 1869, with a straightened planform in the lower section.
However, there are historical records held in the British Library showing for the Kent River Board Area (documents in their Annual Reports and Accounts) that the Sarre Penn was subject to a significant arterial drainage scheme in the early 1960’s (over a period of 4 years). Arterial drainage schemes usually involve comprehensive channel improvement, particularly by deepening a channel to facilitate the discharge of field drains (arterial drainage). The schemes were often carried out with little or no change of planform. There is some field evidence of the spoil from deepening of the Sarre Penn being deposited locally along the river banks. Additional works were carried out downstream at Chislet Marshes in 1974, also involving channel resectioning.
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4.3 Geology and Soils
The majority of the Sarre Penn within the study area is underlain by London Clay, from the Palaeogene Period (34-56 million years ago), compromising silt and clay. This is typical of a geological environment previously dominated by deep seas. Superficial deposits consist of head clay and silt deposits, formed up to 3 million years ago in the Quaternary Period (BGS, 2015).
The soils surrounding the northern section (Figure 2.1) of the catchment are classed as ‘slowly permeable, seasonally wet, slightly acid but base-rich loamy and clayey soil’, draining into the watercourse networks. Habitats associated with this soil type are seasonally wet pastures and forested areas, as the drainage is typically impeded (Cranfield Soil and Agrifood Institute, 2015). The southern section of the catchment is classed as ‘freely draining, slightly acid loamy soils’, draining into local groundwater and rivers. Habitats associated with this soil type include neutral and acid pastures and deciduous woodlands (Cranfield Soil and Agrifood Institute, 2015). The soil type surrounding the Sarre Penn and tributaries is classed as ‘loamy soils with naturally high groundwater’, draining into local shallow groundwater (Cranfield Soil and Agrifood Institute, 2015).
4.4 Hydrology
The nearest gauging station along the Sarre Penn is located in Calcott (grid reference TR173824) (Centre of Ecology and Hydrology, 2015). This 19.4km2 catchment is classified as being mostly rural. Table 4.1 outlines the average flow data taken from the gauging station (1999-2011). A more detailed hydrological assessment is being undertaken in parallel with this assessment.
Table 4.1 : Flow data for the Sarre Penn (source: Centre for Ecology and Hydrology, 2015)
Measure Flow (m3/s)
Mean Flow 0.094
Q95 0.001
Q70 0.009
Q50 0.024
Q10 0.242
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5. Contemporary River Characteristics
A geomorphological reconnaissance survey was undertaken along the Sarre Penn from Dunkirk to Sarre to gain an understanding of the baseline conditions and the geomorphological character of the watercourse. During the fieldwork, the river was sub-divided into three reaches based on professional judgement, taking into consideration changes in land-use, river corridor characteristics, river cross-section and level of modification. The reaches are shown in Figure 5.1. These are:
Reach 1: Dunkirk to Tyler Hill;
Reach 2: Tyler Hill to Upstreet; and,
Reach 3: Upstreet to confluence with River Stour (Sarre).
Copyright OS Open Data (2015)
Figure 5.1 : Reach location plan
5.1 Geomorphological Characteristics
5.1.1 Channel Planform
The Sarre Penn from Dunkirk to Tyler Hill (Reach 1) has an irregular sinuous planform predominantly flowing through established deciduous woodland. The watercourse was stable with no evidence of an actively meandering channel across its contemporary floodplain. A 125m bypass channel which appears to have been artificially created cuts off a tightly meandering section of the river south of Blean Woods National Nature Reserve. Water did however, appear to be flowing within both channels at this section. Some sections of the channel appear to have been historically artificially straightened, particularly at the following locations:
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Along an access track to Cosendene House at Dunkirk;
Around the periphery of the airstrip at Dunkirk;
For approximately 350m within Blean Wood;
Around agricultural fields at Luckett’s Farm, south west of Blean; and,
For approximately 1km south west of Tyler Hill (forming agricultural field boundaries).
The channel planform from Tyler Hill (Reach 2) typically consists of a stable irregularly sinuous channel, becoming increasingly straight towards Upstreet at the downstream extent. A number of reaches were observed where it appeared that the channel had been artificially straightened and subject to dredging. These works are likely to have led to short-term channel instability particularly upstream of the works as the Sarre Penn adjusted to reach a new equilibrium. The channel appeared to still be adjusting in locations to these works, with gravel deposits that are likely to be derived from the disruption and possibly dislodged (by machine) from the exposed gravel lenses deposited locally in the banks. These are now working through the system, often temporarily depositing on the hard segregated bed. The areas where works were observed included:
South east of Calcott;
North of Hersden at the Ash Plantation; and,
From Chislet Business Park (north west of Upstreet) to Chislet South Level (north of Upstreet).
The channel from Upstreet to the confluence with the River Stour (Reach 3) has an artificially straightened planform throughout. Several artificial field drains extend the drainage network and feed into the river forming a detailed drainage network, particularly at Chislet (Chislet Marshes).
5.1.2 Floodplain Connectivity
The majority of the Sarre Penn from Dunkirk to Tyler Hill (Reach 1) is connected to its floodplain with banks approximately 0.5m high (Figure 5.2). There are a few exceptions where areas appeared to be artificially overdeep with high banks (approximately 1m). The channel within this reach is set within a shallow valley; however, communication with a local farmer confirmed that flooding was not frequent within the headwaters of the river. This could be an indication that the channel now has a greater capacity (as a result of resectioning and modification) to contain a normal range of flood flows.
The reach from Tyler Hill to Upstreet (Reach 2) is predominantly disconnected from its floodplain due to vertical, high banks (approximately 1.5m high), particularly north of Chislet Business Park where the channel appears to be significantly artificially incised (Figure 5.3).
In the lower reach of the Sarre Penn (Reach 3), from Upstreet to its confluence with the River Stour, the banks are typically vertical and approximately 1-2m high. Artificial deepening appears to have disconnected the channel from its floodplain, particularly emphasising the issue during low flow conditions. The channel is also embanked (approximately 1.5m high) throughout the majority of this section, further reducing lateral floodplain connectivity (Figure 5.4).
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Figure 5.2 : Channel connected with its floodplain in Reach 1 Figure 5.3 : Incised, overdeep channel within Reach 2 (note: natural bank full height circa 0.5m) limiting connectivity with the floodplain (artificial bank height in excess of 1m)
Figure 5.4 : Embanked and overdeep channel within Reach 3
5.1.3 Flow Variation
Flow variation within Reach 1 was observed to be relatively diverse with runs, glides and reaches of no perceptible flow. A riffle-pool sequence was observed throughout, enhancing the varied flow types (Figure 5.5). Exposed tree roots and large woody debris were also observed, creating local variations in flow throughout the reach. A knickpoint (1m high) created a vertical free-falling drop south of the Blean Woods National Nature Reserve. Some areas of slacker flow (no perceivable/smooth flow) were also observed, particularly where the channel became wider within Blean Wood.
Reach 2 was observed to consist mainly of a riffle-pool sequence, with run and glide flow (Figure 5.6). Some woody debris dams caused large ponded areas and cascading water. Structures, such as the large culvert under the dismantled railway at Tyler Hill, also created large ponded areas altering the local flow regimes (Figure 5.7).
A uniform flow type of smooth/no perceivable flow was observed throughout Reach 3 (Figure 5.8).
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Figure 5.5 : Riffle-pool sequence in Reach 1 Figure 5.6 : Riffle-pool sequence in Reach 2
Figure 5.7 : Ponded area downstream of large culvert Figure 5.8 : Smooth flow throughout Reach 3 beneath dismantled railway
5.1.4 Bed Substrate
The bed substrate in the upper section of Reach 1 at Dunkirk was noted to consist predominantly of sand and fine gravels (Figure 5.9). The bed appeared to be consolidated and formed a predominantly segregated bed. The substrate throughout the lower part of Reach 1 and most of Reach 2 consisted of fine and coarse gravels with some cobble bars (Figure 5.10 and 5.11). The substrate in these sections appeared to be highly consolidated forming a segregated bed with little evidence of ‘mobile’ features. Finer substrate including sand and silt were evident, primarily filling interstices between the gravels and deposited at the river margins, particularly in the lower reaches where diffuse agricultural sediment sources were evident.
The bed substrate between Upstreet and the confluence with the River Stour (Reach 3) was not visible due to deep, turbid water and dense algae in the lower section.
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Figure 5.9 : Sand substrate in upper part of Reach 1 Figure 5.10 : Consolidated gravel and sand substrate in middle section of Reach 1
Figure 5.11 : Consolidated segregated coarse gravel bed typical of Reach 2
5.1.5 Sediment Sources
Limited sources of sediment were identified throughout the length of the Sarre Penn and those present typically consisted of fine sands and silt (Appendix A). A series of minor forest drains were noted (Figure 5.12), particularly in Reaches 1 and 2, with the sediment delivery from these channels consisting mainly of silt and sand. Some small tributaries (less than 0.4m wide) were also observed just upstream of Tyler Hill (Figure 5.13), which appeared to be delivering some fine and coarse gravels to the river channel.
There appeared to be some evidence of gravels present in the banks of the channel in Reach 1; however, evidence of erosion was limited in this section and it is assumed that minimal sediment is being input to the system from this potential source (Figure 5.14).
Within Reach 2 the Sarre Penn was observed to be predominantly bordered by tilled agricultural riparian land; this combined with a very limited vegetated riparian buffer means that the potential for fine sediment to be input to the watercourse was high, acting as a diffuse sediment source (see Section 5.1.8). A similar land use was noted in Reach 3 from Upstreet to Sarre; however, the embankments on either bank are likely to locally minimise the amount of fine sediment reaching the watercourse.
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Figure 5.12 : Small forest drain with coarse gravel substrate Figure 5.13 : Small tributary with silt and gravel substrate
Figure 5.14 : Evidence of gravel within the banks of the Sarre Penn in Reach 2
5.1.6 Erosion
There was evidence of erosion within Reach 1, particularly on the outside of meander bends. Erosion predominantly appeared historic (Figure 5.15) and only a few sections appeared to be actively supplying material (consisting of clay) to the channel. There were some areas of undercutting, particularly where tree roots bound material in the upper layers of the bank. A small ford facilitates cycle and footpath access across the watercourse. This feature had locally eroded a small portion of the bed and banks at Blean Wood (Figure 5.16). There were also some areas of enhanced bank erosion as a result of poaching by livestock, particularly at Blean.
In locations, the banks within Reach 2 were undercut and vertically eroded, with some evidence of mass failure in the form of slumping, particularly on the outside of meander bends. This is likely to be a result of the overdeep channel and the vertical nature of the banks. As in Reach 1, debris dams and structures have amplified rates of scour upstream of the obstructions.
Reach 3 had several areas of mass failure where banks had slumped in, potentially as a result of poaching pressures and by overdeep and overstep channel banks (Figure 5.18). The bank slumping could also be attributed to changes in the strength of the bank material as a result of reduced soil moisture conditions during lower flow conditions; i.e. the banks are artificially steep and the channel is overwide, causing the banks to be unsupported and subject to slumping during lower flow periods (Knighton, 1998; Thorne, 1982).
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Figure 5.15 : Historic vertical erosion (presence of moss and Figure 5.16 : Poaching in Reach 1 – impromptu cycle and vegetation confirms this is not an actively eroding bank) in footpath crossing Sarre Penn Reach 1
Figure 5.17 : Eroding bank in Reach 2 on the outside of a Figure 5.18 : Mass failure and slumping within embanked meander bend section with reprofiled banks in Reach 3
5.1.7 Deposition
A number of depositional features were noted from the source of the Sarre Penn in Dunkirk to Upstreet, including riffles, side bars, mid-channel bars, point bars and berms. The majority of the depositional features within this section were not vegetated.
Within Reach 1, the majority of the fine and coarse gravel bars were observed to be consolidated and did not appear to be mobile (Figure 5.19). In some instances a top layer of loosely consolidated sediment was observed (consisting of fine gravels and sand) that is likely to be mobile in higher flows (Figure 5.20). However, these are not thought to be indicative of an active meandering river, but rather localised depositions moving down the system as a result of historical deposits disrupted by man’s activities upstream.
Similarly, between Tyler Hill and Upstreet, several coarse gravel bars were observed located mainly on the inside of meander bends. The majority of these deposits appeared consolidated, with some observed to be more mobile (Figure 5.21 and 5.22).
The only evidence suggesting potential areas of deposition are present in Reach 3. Clumps of vegetation are found growing at the channel margins, which are likely to be trapping fine sediment such as sand and silt (Figure 5.23).
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Figure 5.19 : Consolidated gravel side bar in Reach 1 Figure 5.20 : Large cobble mid-channel bar in Reach 1 appeared stable
Figure 5.21 : Temporary gravel bar deposit on the segregated Figure 5.22 : Gravel side and point pars in Reach 2 consolidated bed in Reach 2. These features are unlikely to be sustainable due to lack of sediment sources
Figure 5.23 : Narrowing channel in Reach 3, likely to be trapping fine sediment such as silt and sand
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5.1.8 Land Use and Riparian Zones
The main land use along the Sarre Penn from its source to Tyler Hill (Reach 1) was observed to be dense deciduous woodland, including other woodland vegetation types such as scrub and low lying vegetation (Figure 5.24). This provided a substantial vegetated buffer between the Sarre Penn and the agricultural land on the periphery of the woodland. Some organic arable farming (i.e. using no fertilisers) was noted (pers. comm local farmer) around Dunkirk, further reducing the likelihood of chemicals entering the river in Reach 1.
The predominant land use in Reach 2 was found to be arable, tilled land (Figure 5.25). The majority of the Sarre Penn in this location had no substantial vegetated riparian corridor between the agricultural land and the watercourse, with only very few areas having a significant buffer strip (i.e. 1-2m of complex vegetation). There are some small built up urban areas within Reaches 1 and 2, including Dunkirk, Blean, Tyler Hill and Upstreet.
The land use from Upstreet to the confluence of the Sarre Penn with the River Stour at Sarre (Reach 3) consisted primarily of tilled arable land. A 1.5m high embankment and overgrown field drainage ditches acted as potential buffers/filters for transfer of fine sediment to the channel (Figure 5.26).
Figure 5.24 : Deciduous woodland – predominant land-use Figure 5.25 : Arable tilled land – predominant land use type type for Reach 1 for Reach 2
Figure 5.26 : Arable land-use, embankment and field drain (which does not connect to the main channel) acting as a buffer/filter
5.1.9 Channel Modifications and Structures
Longitudinal connectivity was noted to be impacted throughout the course of the Sarre Penn in all reaches. The watercourse was culverted under several roads, including the A2 (Figure 5.27), A290, A291 and A28. The
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Sarre Penn was also culverted under an operational railway at Sarre, the dismantled railway at Tyler Hill (Figure 5.28) and several smaller tracks and footpaths (Figure 5.29). Several small footbridges were also present along the course of the Sarre Penn, particularly within the wooded areas between Dunkirk and Blean (Figure 5.30). Some weirs were identified along the course of the Sarre Penn, altering the channel cross-section and gradient, with the two major ones noted at Tyler Hill and upstream of Sarre.
Several additional structures were noted to be present between Upstreet and Sarre (Reach 31), namely a pumping station, an adjustable weir and a sluice gate (Figure 5.32). All the structures in Reach 3 typically consisted of a concrete channel bed and banks.
Appendix B provides an indication of the location of the structures along the Sarre Penn.
Figure 5.27 : Twin culvert under A2 at Dunkirk Figure 5.28 : Culvert (1.3m diameter) under dismantled railway at Tyler Hill
Figure 5.29 : Twin culvert under forest track in Church Wood Figure 5.30 : Footbridge over the Sarre Penn at Church Wood
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Figure 5.31 : Weir at Calcott Figure 5.32 : Sluice at Sarre
5.2 Analysis
The site walkover and assessment of baseline characteristics (described in Section 5.1), has enabled the Sarre Penn to be characterised as a clay lined channel. Whilst there is evidence of secondary currents there is limited or no energy for channel migration (as would occur in an active gravel bedded river in a higher energy location). The watercourse is not regarded to be active but instead adjusting within its banks to historic man-induced channel modifications. There is documented evidence of extensive channel works in the early 1960’s. The following provides an understanding of the key channel characteristics.
5.2.1 Channel Planform Changes
Having studied historical maps, the channel planform appears to have remained stable over at least the past hundred years. Following the site walkover it was apparent that the channel was not actively eroding or laterally migrating and minimal extents of bank erosion were observed. Where bank erosion was present it was determined to be primarily minor scour and no evidence of meander migration was found, nor presence of knickpoints created by channel straightening which might otherwise cause destabilisation.
Research by Coleman (1952) supports this, stating that the floodplain “upon which … [the Sarre Penn’s] reconstruction is based … [lies] close to the river, indicating that it has not … [migrated] far from its present line during the whole period of its development”. This supports that the Sarre Penn has not altered its existing course probably since the last glaciation.
5.2.2 Channel Substrate and Bank Erosion
The presence of the gravel/cobble bed is highly likely to be linked to historic dredging and capital works, deepening the channel and exposing gravel lenses within the clay and forming a consolidated segregated bed. The dredging of the channel has probably locally extended the channel depth to more resistant gravel layers, forming the river bed that is observed today. The nature of the gravel lenses has also probably prevented any subsequent/further adjustment of the channel bed (through downcutting), creating a form of ‘natural’ bed protection.
The few mobile deposits on top of this consolidated segregated bed are not deemed to be a sustainable source of gravels and are interpreted to be a result of adjustment to large-scale modifications during the last century (probably since the early 1960s). Watercourses of this nature can take anything from 60 to 150 years (or more) to respond and adjust significantly to channel modifications such as dredging or re-profiling (Brookes, 1988). This is because of their relatively low energy and lack of competence to erode the bed and banks.
Scour of the banks in the reach close to the Broad Oak reservoir site has also been assessed to be a response to the documented historic modifications. No major erosion was observed in the reaches upstream or
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downstream from this point. The adjacent watercourse appears to have been straightened, altered in cross- section and potentially maintained by dredging. As a response to this modification some form of channel adjustment has occurred. As no defined knickpoint was noted along the reach of watercourse from Tyler Hill to Calcott it is thought that the mobile deposits are most likely a result of localised erosion of the gravel lenses exposed in the banks. Loose deposits have subsequently formed in the channel directing the flow towards the banks, encouraging scour of the banks during higher flows. This leads to a cycle of localised sediment provision and localised narrowing. This provides the illusion of a natural active gravel bed river. When on site the banks of the Sarre Penn at this location were noted to predominantly consist of clay, suggesting this small source is no longer feasibly providing loose sediment into the channel.
5.2.3 Potential Sediment Sources
The clay lined channel was observed to have resisted change within the watercourse and to be a limited source of gravel to the channel. After walking the upstream catchment of the Sarre Penn and assessing the characteristics, it has become evident that the majority of the fine sediment delivered to the channel is sourced from periglacial deposits located within the clay banks (possibly fossil wind blown deposits) following disruption by human activity (such as dredging and realigning/straightening). Of all of the tributaries, field drains, outfalls and woodland drains only three were observed to provide a source of fine/coarse gravels, with the rest typically consisting of sand and silt. This implies that the channel is not a self-supplying system, i.e. providing a downstream supply of sediment through erosion and tributary input. It is not an active gravel bed river.
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6. Summary and Recommendations
6.1 Summary of the Sarre Penn Characterisation
Overall, it has been assessed that the Sarre Penn has been heavily modified historically, leading to an unstable channel that is slowly attempting to adjust within its banks. The clay banks provide a definitive barrier to lateral migration, with the course of the river having not adjusted in recent history. This is not an active gravel bed river. There is little evidence of a sustainable sediment source within the upstream reaches of the catchment, with minimal gravel observed in the banks and in tributaries/drains. The channel bed is segregated and consolidated and is not determined to be mobile as a result, the finer sediments (sand and silt) have primarily been winnowed out and transported downstream (and eventually to the sea). The small areas of mobile deposits present are anticipated to be a response to historic channel modifications and dredging, exposing lenses of gravels in some sections of the river bank. These mobile deposits were observed to be very localised, intermittent and small in extent and not judged to be sustainable.
6.2 Recommendations for Diversion Design
The following are some guiding principles for the design of a potential river diversion. It should be noted that these have not been agreed in principle with the Environment Agency but are expected to be discussed, refined and agreed as part of taking the project design forward.
With respect to the design of a potential Sarre Penn diversion, it is considered that a watercourse with a consolidated gravel bed and clay banks would mimic closely that of the existing channel. It is not considered necessary for the channel to be ‘active’ (i.e. able to adjust laterally) and that the transfer of larger gravels, pebbles and cobbles is also not believed to be required as there is no sustainable source of sediment upstream to maintain this through the system. The channel would need to be designed to ensure that any fine sediment, including silt and sands, is prevented from depositing and smothering the gravel bed, with the ability to move some finer gravels in higher flows also considered to be essential.
The creation of a two stage channel is likely to mimic the existing channel characteristics, with berms or deposits used to create a narrow sinuous low flow channel. The low flow channel would be approximately one metre wide, as observed on site, with a pool-riffle sequence and include in sections tick channel design as expressed as a preference by the EA. If possible, creating slightly steeper banks for the low flow channel would enable some bank slumping and adjustment within the defined cross-section.
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Appendix A. Point Sediment Sources
Copyright OS Open Data (2015)
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Appendix B. Structure Types
Copyright OS Open Data (2015)
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References
Brookes, A (1988) River channelization – John Wiley and Sons
Butler, A., Headley, M. and Litten, V. (1996) The Sarre Penn…the story of a stream, Canterbury
Coleman, A. (1952) The Relief and Drainage Evolution of the Blean, received 11 September 1952
Knighton, D. (1998) Fluvial Forms and Processes: A New Perspective, London
Thorne, C.R. (1982) Processes and Mechanisms of River Bank Erosion. In: Hay, R.D. Bathurst, J.C. and Thorne, C.R. eds. Gravel Bed Rivers. Chichester; Wiley, pg. 227-259
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