Field Reconnaissance Survey of the Cuckmere River

Methods

A field reconnaissance survey of the Cuckmere was undertaken, to contextualise the information and knowledge obtained from the desk based study of the site, and the wider principles of the Fluvial Audit. Two surveys of site were conducted (June and August 2021). In the June survey 10.6km of the lower course of the river was surveyed (from Arlington reservoir, to the mouth at Cuckmere Haven). The second survey investigated approximately 9.2km of the main channel, orginiating in Hellingly (where the main stem of the channel forms, from its headwaters and tributaries), and terminating downstream at Arlington Reservoir. Where available, public footpaths, bridleways and access roads were used to access and observe the channel.

The channel’s characteristics were logged within a field reconnaissance form, with a new form completed for each discernibly different reach. Maps were also annotated describing the locations of the breaks and lengths of each reach. Notes were made on observable geomorphic and hydrological properties (i.e. sediment supply and transport, channel morphology, sediment sorting, flow types, floodplain characteristics, bedgraident and riparian vegetation). The morphological pressures for each reach are also described. A total of 13 geomorphically distinctive reaches were distinguished. These reaches were then digitised using Google Earth, as line shapefiles (Figure 1).

Individual reaches, differentiated by their geomorphic characteristics — Figure 1. The 13 differentiated reaches of the surveyed River Cuckmere.

Findings

Morphology

In the upstream reaches, the sediment supply is typically from the surrounding hillslopes and steep, incised banks (Figure 2). The single thread channel is more sinuous in the the upstream reaches, with some natural sinuosity present. However, some reaches have been artificially straightened, as characterised by long, straight sections of incised channel, held in place, in some areas, by bank revetment structures.

Figure 2. Straightened and incised section of channel, located in upper course of the river

Conversely, in the downstream course, meanders have been artificially implemented, providing sinuosity to the channel. In the upstream areas, the channels are incised across the majority of the surveyed area, with steep banks (Figure 3).

Figure 3. Artificial meander bends implemented in the lower course of the channel.

In the upstream and middle reaches, the channel bifurcates, diverging into different drainage systems. This creates some smaller in-channel islands, as the channels meet again at confluences. There is also evidence of some artificial channel bifurcation as the channel is split to create islands for private land use.

With increasing distance downstream into the latter most reaches, the channel widens and the banks shallow, allowing for floodplain engagement during higher flows. In these downstream reaches, disengaged meanders are present and the channel becomes, again, more sinuous. Evidence of artificial straightening is also present, as the channel terminates at the sea (Figure 4).

Figure 4. Channel straightening and disconnected meander bends

Across the the study area, evidence of bank failure and slumping is present, providing a possible explanation for the introduction of sediment downstream. Evidence of undercut banks were also observed, primarily in the middle course of the river (Figure 5)

Figure 5. Evidence of bank previous bank slumping and undercutting

Sediment

Despite the morphology of the channel differing between individual reaches, the sediment calibre remains consistent, with clays and silts forming the bed substrate for the majority of the channel, with the occasional, cobble sized sediment. However, where larger sediment sizes are present, they seem to have been artificially placed, or winnowed from bank protection structures, and have since remained in situ (Figure 6). Resultantly, the sediment sorting of the bed is largely indistinguishable, given how fine the bed sediment is.

Flow and Gradient

Similarly, the flow types across the majority of the surveyed river is typically laminar and steady, flowing in the downstream direction. There is little evidence of eddying or turbulent flows. Moreover, in some reaches the flow seems almost stagnant. However, as the river nears its mouth, intertidal flow dominates. The bed gradient across the surveyed reaches is shallow, with no observed breaks in slope.

Vegetation and Floodplain

In the upstream area, the bank vegetation comprises mostly large, established trees, with root networks binding the bank sediments together. The tree branches and roots can be engaged within the flow, trapping wood debris. Between the trees, the banks are lined with brambles and bushes. Resultantly, toppled trees and large woody debris are present in some reaches of the channel and engaged within the flow (Figure 7).

Figure 7. Channel banks covered in riparian vegetation, trees, bushes and reeds.

In the middle and lower reaches, dense reeds cover the riparian zone and in channel bars. However, as the channel progress downstream into the saline and brackish waters, the vegetation composition includes halophytic species. There are fewer reeds in the lowest channel reaches, with sparse shrubs dominating. Vegetation and sediment poaching is also visible here, as seen by the patchy vegetation (Figure 8). This is likely from cattle and walkers, as a public footpath follows the channel banks in the lower course.

Figure 8. a reach of the lower course, characterised by low vegetation and a wide channel.

The floodplains are typically managed grasslands or crop plantations. Cattle fields also line the majority of the floodplains, with evidence of cattle grazing and watering from the degradation of the banks (poaching) close to the main channel. In the northernmost reaches, the floodplains are sloping, in some cases creating tall channel banks (Figure 9).

Pressures

A variety of morphological pressures were identified from the survey, some being ubiquitous (such as channel straightening and farming pressure from cattle grazing leading to bank trampling), whereas others were highly localised (degradation of bank protection structures). Where bank protection measures and culverts have been installed, a variety of methods and materials have been used to mitigate bank collapse and ensure the entrances and exits of these structures remain unimpeded. Derelict and unmaintained wiers also trap debris, creating the potential to impact the transportation of sediment and flow (Figure 10).

Figure 10. Disused and disengaged channel bank protection

Where pipelines traverse the channel, reinforced bank protection has been implemented, using concrete walls and sandbags. These hard structures are both susceptible to degradation but also act as morphological pressures as they force the channel to maintain a specific morphology and flow structure (Figure 11).

Figure 11. Pipeline crossing the channel, with evidence of bank protection.

Culverts and weirs are also present in the upstream reaches, directing flow and sediment into specific areas of the floodplain. The pressures imposed by these structures include: impediment during high flow events, knickpoints within the channel (inducing channel movement), obstruction to the upstream movement of tidal waters and fish species, spatial squeezing of riparian habitats, alterations to sediment supply through slope, and vegetation management. Ultimately, these pressures influence the sources, transport pathways, and deposition of sediment across the channel and its wider catchment.

Predominantly in the downstream section, channel straightening presents a morphological pressure. As the channel is kept in place by soil and hard embankments, it is unable to adjust spatially during periods of high flow. Further, embankments limit the ability of the channel to engage with its floodplain, subsequently reducing the volume of sediment that can be accrued into the channel. Evidence of channel dredging is also apparent in the lower reaches, encouraging the incision of channel, through time. Resultantly, these measures impact the ability of the channel to source, transport and deposits sediment, ultimately, impacting the channels form.

Bridges with footings and piers also present potential pressures, as they can induce changes in channel slope and redirect the passage of sediment and water (Figure 12). This can result in changes to channel form as the velocity of the flows can adjust as the channel passes through the bridge structures.

Figure 12. A bridge situated in the lower course of the river, with concrete piers.

The following section utilises the MIMAS tool to aid in the understanding of the spatiality of these observed pressures and their cumulative impact, in terms of the channels capacity and sensitivity to such pressures.

MIMAS Calculation

The Morphological Impact Assessment System (MIMAS), presented by the Scottish Environmental Protection Agency, to characterise and quantify the likely morphological impact of different channel management methods, across a given length of channel. Impacts of the different management techniques are assessed cumulatively, in terms of how much of the ‘Systems Capacity’ they used (across the channel area and the bank (riparian) zones) (SEPA., 2012) . Further detail on the MIMAS tool can be found here.

MIMAS calculations were performed for 5 different reaches of the river, each presenting a unique combination of pressures and management methods. For each reach the MIMAS methodology was followed , beginning with the identification of key channel attributes. These attributes were identified within the field reconnaissance survey and during the desk based study. The channels typologies were then determined (SEPA, 2012; Appendix A), as described in Table 1.

Table 1. Channel typologies, as defined by SEPA (2012).

Table_Typology

As many reaches of the channel fall between a wandering plane/riffle (typology C) and an actively meandering course (D), the average score values, between these two typologies, were used to provide a hazard score for both the channel and the bank. The pressure length (or activity footprint) of each activity was calculated by considering factors as deflector length, number of pier bridges, the width of the channel (around bridges), etc (Table 2) (SEPA, 2012; Table 2). SEPA (2012) provides guidance on applying the MIMAS tool to reaches of around 500m in length. However, given the lack of dynamism of the channel, larger reach lengths were identified and used.

Table 2. Preliminary calculations for Reach 3

reach_3_activity_length

A matrix was then used to score each activity as a hazard to both the channel and the banks/ riparian zone (Table 3). An average of the hazard scores for both C and D typologies were used for some activities as the channels form aligns with both typologies, for some reaches.

Table 3. Hazard scoring matrix (edited from Jacobs (2018)).

Hazard_Scoring_matrix

The morphological capacity used (%) was then calculated using the following equation:

Capacity_calc

Summation of the scores provide an overall score for each reach, describing the total percentage capacity used, by the pressures that characterise each reach. The summary statistics can be found here.

Table 4. Overall MIMAS calculations for Reach 3 (as identified from the field reconnaissance survey).

Reach_3_MIMAS

The cumulative impact of the activities can be totalled and compared to morphological condition limits (Table 5). These limits allow for the classification of the watercourse, as the greater the morphological capacity used is, the less likely the watercourse will be able to achieve a high morphological condition.

Table 5. Morphological condition limits (SEPA, 2012).

Results and Considerations

Figure 13 describes the pressures and activities observed across the 5 sample reaches of which the MIMAS calculation were applied to. High impact channel realignment features across all surveyed reaches, as the majority of channel has been subjected to striahginenit, in some capacity, for many tens of years (as described in the Desk Based Study).

Much of the floodplain of the Cuckmere is privately owned and managed. Numerous bridges were observed across the channel and resultantly, present the second greatest pressure on the channels ability to store, transport and deposit sediments. Localised areas of low impact realignment, through the introduction of meanders bends, were observed in the lower course of the channel.

Common_pressures — Figure 13. Pressures identified within the sample reaches of the Cuckmere River, following the MIMAS analysis

The confinement of the channel through decades of straightening, diverting flow and occupation of the channel floodplain, has resulted in the Cuckmere’s low value of morphological capacity to accomodate geomorphic pressures. The typology of the reaches also suggests that the channel has a heightened sensitivity to geomorphic perturbations and pressures. Resultantly, many reaches are considered to have low values of morphological classification, for both the channel and the banks/ riparian zone (Figure 14).

Classifications — Figure 14. Channel and Bank Classification totals, following MIMAS analysis

The reaches that displayed the lowest grade of morphological classification (i.e. ‘bad’) were located across the entire surveyed area (Figure 15). However, the classification status of the banks/ riparian zones suggests the capacity of these regions, to accomodate geomorphic change, is greater than in those same reaches, for the main channel. The bank classification status is greater, generally, than the classification of the channel. The areas of the channel that have the greatest capacity to accomodate morphological change arise in the sections of the channel that have not be subjected to as many differing management activities than the other surveyed reaches. Hence, the ability of the channel to absorb change in these areas may be greater, as there is minimal morphological pressure, and greater spatial area for the channel to adjust to such pressures.

Channel_mimas_score

MIMAS_Bank_Classification — Figure 15. Channel (top) and Bank/ riparian (bottom) morphological capacities (see table 5 for key), as calculated from MIMAS.

As the catchment processes are extensively modified with limitations on the development of natural forms and operation of natural recovery processes (as observed through the straightening, dredging and, bank protective measures), the Cuckmere River is likely to be continually adjusting to the range of Potentially Destabilising Phenomena, identified from the desk based and field survey. However, some stresses such as bank poaching and dredging (figure 16) are not parameterised for use within MIMAS . These stresses are prevalent across the study reaches and may further influence the sensitivity and capacity of the channel to absorb morphological change. As these factors are not included within the MIMAS calculation, the accuracy of the results may be limited, resulting in misinformed channel management decisions.

Figure 16. Evidence of Bank Poaching by cattle

As the MIMAS tool was developed and calibrated for its application within Scottish rivers, the results presented within may not accurately capture the sensitivity nor capacity of the Cuckmere to accommodate geomorphic change. however, the MIMAS framework offers a heuristic and semi-quantitative means to rapidly assess the conditions of both the bed and banks of a river, as separate entities. This allows for the identification of localised, and interacting stresses within the surveyed reaches, to guide land managers to provide the most suitable and sustainable means of management and restoration.

References

Jacobs, (2018). Using the MIMAS tool to prioritise restoration and NFM in the River Peffery, Scotland. [ebook] Available at: <https://www.therrc.co.uk/sites/default/files/files/Conference/2018/Presentations/2.1.3._lewin_emma.pdf> [Accessed 12 September 2021].

SEPA, (2012). Supporting Guidance (WAT-SG-21) Environmental Standards for River Morphology. [online] sepa.org.uk. Available at: <https://www.sepa.org.uk/media/152194/wat_sg_21.pdf> [Accessed 28 May 2021].