Application of a Novel Automatic Erosion and Deposition Monitoring System at a Channel Bank Site on the Tidal River Trent, U.K.

Estuarine, Coastal and Shelf Science (2001) 53, 237–247 doi:10.1006/ecss.2001.0779, available online at http://www.idealibrary.com on

Application of a Novel Automatic Erosion and Deposition Monitoring System at a Channel Bank Site on the Tidal River Trent, U.K. D. M. Lawlera,d, J. R. Westb, J. S. Couperthwaitea and S. B. Mitchellc a

School of Geography and Environmental Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. b School of Civil Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. c School of the Environment, University of Brighton, Lewes Road, Brighton BN2 4GJ, U.K. Received 16 November 2000 and accepted in revised form 1 May 2001 There is a well-defined need to improve understanding of the dynamics of sediment erosion and deposition on intertidal channel banks, given their importance to channel stability, sediment budgets, depth maintenance, pollutant and nutrient transport, and ecological processes in estuarine systems. Conventional, manual methods for field monitoring of erosion and deposition, however, normally deliver information of low temporal resolution conditioned by infrequent field resurveys. To address this problem, this paper discusses a recently developed and improved automatic erosion and deposition monitoring technique, the Photo-Electronic Erosion Pin (PEEP) system, and its application to a tidal channel bank site at Burringham on the River Trent, U.K. The PEEP system allows the magnitude, frequency and timing of individual erosion and deposition events to be monitored much more precisely than with conventional manual methods. PEEP sensors also monitor light intensity and sediment temperature, variables which can influence bank stabilizing and destabilizing processes. Example results at both the event and spring-neap timescales are presented from a short PEEP system deployment between March and May 1997 at Burringham. These establish that discrete erosion events of >60 mm and 100 mm can occur in response to individual tidal cycles, events which are readily monitored automatically and quasicontinuously by the PEEP system. The capability of the PEEP approach to enhance temporal resolution of monitoring is demonstrated by the determination of the timing of the 100-mm bank erosion incident to an ‘ event window ’ of 2·75 h: this converts to mean bank erosion rate of 36 mm h
1 over the period of inundation. In addition, the PEEP system defines the magnitude and date of two example deposition events of 47 and 92 mm on the lower bank during a sequence of rising spring tides. These represent mean deposition rates of 4·5 and 8·4 mm h
1 respectively over the periods of inundation. The Burringham site is shown to be highly active, with regular and dynamic erosion and deposition cycles. Upper bank surface elevation oscillations, driven by this sediment cycling, were characterized by a strong 14-day cycle which clearly reflected the spring-neap cycling of tidal range. Sediment was deposited on the bank relatively quickly, but removed by erosion rather slowly, giving an asymmetric sediment cycling profile. Higher bank elevations were strongly correlated with high tidal ranges, and especially to water level peaks 2 days previously. Incorporation of a simple Wind Stress Index further improved the statistical explanation of tidal bank elevation, and suggested that high on-shore wind speeds were associated with increased bank erosion. Such vigorous sediment cycling means that many erosion-deposition sequences on tidal banks can be self-concealing and therefore may not be recorded by infrequent manual resurveys which will inevitably underestimate total activity. This reinforces the need for an automated method such as the PEEP system to determine these typically cyclic sequences of self-cancelling accretion and removal activity if site dynamism is to be correctly quantified. The ability of the PEEP approach to generate such detailed and high-resolution information on the temporal distribution of erosion and deposition events in tidal environments should significantly enhance future process and applied studies, especially of entrainment thresholds, sediment recycling, estuarine sediment supply and sediment fluxes, the operation of biomediation mechanisms, bank erodibility changes, storage and residence times of contaminants and site management options.  2001 Academic Press Keywords: erosion; sediment accretion; tidal rivers; tidal cycles; intertidal zone; PEEP system; instrument design; turbidity maximum; River Trent

Introduction Knowledge of sediment deposition, erosion and transport dynamics within tidal channels is fundamental to d

Corresponding author. E-mail: [email protected]

0272–7714/01/080237+11 $35.00/0

a complete understanding of estuarine system operation. The interaction of tidal, river flow, topographical and buoyancy effects in estuaries leads to complex hydrodynamic fields which have profound effects on sediment fluxes and sediment exchange processes  2001 Academic Press

238 D. M. Lawler et al.

across channel boundaries. The most obvious effect in medium to high tidal range estuaries is the occurrence in their upper reaches of a zone of high suspended solids concentration termed the turbidity maximum (Uncles et al., 1994). This may extend longitudinally for many kilometres and regularly have concentrations of suspended solids between two and four orders of magnitude greater than the connected sea or river (Dyer, 1986). The suspended solids material tends to be dominated by the finer, cohesive fractions which may have been brought into the estuary by advective or dispersive processes from the sea or delivered from the upstream catchment. Intertidal banks generally have lower shear stresses than main channels and thus have the ability to trap sediment through sedimentation over high water. The intertidal zone is also subject to the intense turbulence generated by breaking waves that occur on rising and falling tides in response to wind, tidal action or boat wash, and this facilitates erosion. The degradable organic matter naturally associated with fine sediment particles makes the intertidal zone one of the most productive zones in the estuarine system by providing an ideal environment for primary producers, invertebrates, birds and fish. As many persistent organic and inorganic pollutants also have an affinity for fine sediments, there is the potential for profound ecological effects. However, little knowledge has yet emerged of the detailed dynamics of tidal bank erosion and deposition events at a time resolution comparable to that available for flow and sediment transport rates derived from continuous water-level measurement, currentmetering and turbidity monitoring. Traditional, manual, field methods for monitoring tidal bank erosion and deposition [e.g. erosion pins or repeat profiling (e.g. Lawler, 1993; West & West, 1991)], though useful for spatial detail, simply reveal net change in the position of a bank surface since the previous survey, which may take place at weekly, 14-day or monthly intervals. They do not quantify the precise temporal distribution of change which is often highly episodic within intertidal zones (e.g. Kirby et al., 1993, p.390). This means that the exact bank response to individual flow, tidal, sediment-flux or meteorological events is generally unknown. Clearly, process explanations and model building and testing will be enhanced when (1) the full episodicity of tidal bank change is detected, (2) the magnitude, frequency and timing of individual erosion and deposition events is known with some certainty, and (3) these specific erosion and deposition events can be related to continuous information on the temporal fluctuations in the suspected driving forces.

Therefore, despite considerable research into the mechanisms of cohesive sediment deposition, consolidation and erosion, the complexity of the underlying processes, for example biological effects (Black, 1997; Paterson, 1997), has led to semiempirical expressions being used in mathematical models (e.g. Teisson, 1997). The coefficients have been difficult to quantify or test because of the difficulty of obtaining good quality, high-resolution data under field conditions. The purpose of this paper, then, is to demonstrate that the application of a newly developed and modified automatic erosion and deposition monitoring technique—the PEEP system—can deliver a clearer, more detailed, event-based picture of the dynamics of channel bank changes at an ITZ (intertidal zone) site. Such data can provide a very useful platform for investigations of process efficacy, controls and implications. The Photo-Electronic Erosion Pin (PEEP) system PEEP measurement principle The Photo-Electronic Erosion Pin (PEEP) system has recently been developed to tackle erosion monitoring problems in fluvial systems (e.g. Lawler, 1991; 1992; 1994; Lawler et al., 1997). The PEEP sensor consists of a row of photovoltaic cells connected in series, enclosed within a waterproofed, transparent, acrylic tube of 10 mm I.D. and 16 mm O.D. (Figure 1). The sensor generates an analogue voltage directly proportional to the total length of tube exposed to visible light (designed such that 1 mV of cell series output Y1 mm of tube length). Networks of PEEP sensors are inserted into the eroding/accreting feature, and connected by screened cable to a datalogger housed in a weatherproofed enclosure nearby. Subsequent erosion (retreat of the bank face) exposes more photosensitive material to light which increases PEEP voltage outputs. Conversely, deposition reduces sensor voltage outputs. Data periodically interrogated or downloaded from the logger thus reveal the magnitude, frequency and timing of individual erosion and deposition events much more precisely than has hitherto been possible, as demonstrated for fluvial sites by Lawler (1992, 1994). Logging intervals are user-defined and depend solely on datalogger capabilities, as PEEP sensors output continuously. For most field monitoring purposes our scan frequencies have ranged from 1–30 min, but they can be less than 1 s if desired. PEEP approaches have shown that fluvial banks can be much more dynamic than previously assumed, identified highly complex bank

Automatic monitoring of tidal bank erosion 239 STAR20 PEEP SENSOR

Note: Not to scale

Measurement pcb datum Diode 1 (front reference cell)

Diode 20 (back reference cell) Photodiodes

Cable entry gland

Cable

To datalogger Thermistor 1 Threaded nylon bolt

Thermistor 2

Acrylic tube

Inner acrylic tube

F 1. The Photo-Electronic Erosion Pin (PEEP) sensor for the automatic monitoring of sediment erosion and deposition events. The STAR20 version depicted was redesigned for tidal applications and is 0·66 m long, contains an array of 20 photovoltaic cells, including two reference cells for signal normalization, and incorporates two thermistors for temperature monitoring at the sediment surface and at 68 mm depth (pcb=printed circuit board). Different designs are possible to suit the application. Not to scale.

responses to flow events, defined the complete erosional and depositional ‘ event structure ’ and sediment balance for lower-bank zones, and helped to clarify the operation of key processes (e.g. Lawler, 1992, 1994; Lawler et al., 1997; 2001). Prior calibration establishes relationships between PEEP voltage outputs and known exposed tube lengths. Under such ideal conditions, the position of the sediment surface is generally known with 95% confidence to within 1–2 mm, or 2–3 mm in the field (Lawler, 1992), although very low light levels can affect this. PEEP sensors can be reset periodically to keep pace with especially active erosion and deposition activity. A power supply is not required for PEEP sensors, because they are photovoltaic devices, and generate electrical current as a function of incident light: this greatly simplifies deployment logistics in the field. A reference cell is incorporated in the sensor to allow normalization of voltage outputs for varying light intensity (Figure 1). PEEP outputs drop to zero at night, however, so that the detection of nocturnal events is delayed by a few hours till the following morning (though temporal resolution is still much better than with conventional manual methods: see below). Also, nocturnal event timings can often be refined now using thermal consonance timing (TCT) (Lawler et al., 2001).

Ouse-Trent-Humber system [Figure 2(a)]. To cope with greater site dynamism, sensor tube wall thickness was increased to 3 mm, total sensor length was increased to 660 mm, and ‘ active ’ sensor length was increased to 200 mm by raising the number of photovoltaic diodes in the array from 12 to 20 (Figure 1). An extra reference cell was added, to give two in total. These were located at each end of the array so that the sensors could be installed in either ‘ cable-up ’ or ‘ cable-down ’ mode depending on site conditions. The second, back, reference cell also allows doubleconfirmation of minimum erosion magnitudes in the case of large bank retreat events which expose the entire photocell array. Two thermistors were also incorporated to allow automatic monitoring of temperature at the sediment surface and at 68 mm depth (Figure 1). Temperature has been shown to be an important control on variables which influence sediment entrainment and accretion, especially biological processes such as vegetation growth, biofilm development and the photosynthesis of intertidal microphytobenthos (Blanchard et al., 1996). Moreover, the light intensity data generated by PEEP sensors can also be used to help explain such biological and biomediation effects at tidal and fluvial sites (e.g. vegetation development and photosynthetic processes; Blanchard et al., 1996).

PEEP redesign for tidal applications

Materials and methods: field trials at the Burringham tidal site

Various PEEP designs are possible to suit the application (e.g. bank, mudflat, saltmarsh, beach, dune, gully, channel bar, hillslope). The ‘ fluvial ’ PEEP sensors were specifically redesigned here for firsttime deployment at a tidal channel bank site at Burringham (Couperthwaite et al., 1998; Mitchell et al., 1999), and for two other bank sites in the tidal

Bank erosion activity was intensively and automatically monitored using PEEP systems and erosion pins at three tidal and five fluvial sites within the Yorkshire Ouse and Trent catchments, and at a further 18 sites using erosion pins alone. This paper will concentrate solely on example erosion and deposition events and

240 D. M. Lawler et al. W

ha

rfe

Ou

Selby

se Drax Ai re

Don

Kingston upon Hull

Keadby

Trent Falls Immingham Burringham Grimsby

Trent

Gainsborough

N

Cromwell Weir

Tidal limits North Muskham 0

10

Flood bank

Approximate high water (spring tide)

x

pro

0m

ap

Bottled samples

6

PEEP sensors

3 5

20 kilometres

1 4

2

Position of pumped 1 samples 2 3 Approximate low water (spring tide)

6 Direction of ebb

Pressure transducer (stage)

Turbidity meter

F 2. Study site and instrumentation deployment: (a) the location of the Burringham study site within the Trent–Humber system, U.K.; (b) schematic arrangement of the six PEEP sensors on the intertidal bank at Burringham.

sequences on the west-facing intertidal channel bank at Burringham on the River Trent, northern England [Figure 2(a)]. The aim is simply to demonstrate the potential of using redesigned and vertically-installed PEEP sensors to quantify, at both event and springneap timescales, the timing and magnitude of erosional and depositional activity on fine-grained, cohesive banks subject to tidal flow conditions. The Burringham site is situated 15 km upstream of the Trent–Humber confluence, and experiences a mean spring tidal range of almost 5 m [Figure 2(a)]. Between March and May 1997, six PEEP sensors were installed on the upper, middle and lower zones of

the vegetation-free bank face (Couperthwaite et al., 1998; Mitchell et al., 1999), between the approximate limits of high- and low-water under typical spring tide conditions [Figure 2(b)]. Each PEEP was installed vertically in the ‘ cable-down ’ position, with the photocells facing west towards the open channel to maximize light reception, and each PEEP was supported by a wooden stake positioned behind. Sensors were set to an initial, known exposure length of approximately 80 mm to allow for fluctuations in either direction in the elevation of the bank surface. PEEP sensor cables were carefully led away from below each sensor, beneath the surface material, to a Campbell Scientific CR10X data logger and multiplexer housed on a flood bank within a nearby Environment Agency compound. The logger was programmed to scan outputs from each PEEP at 1-min intervals and store 15-min averages. The six PEEP sensors ran without significant problems for up to 13 months following installation in March–May 1997. This short paper, however, focuses exclusively on events in the 70-day period between 17 March and 27 May 1997 recorded by PEEP sensors 3 (upper bank) and 6 (lower bank) [Figure 2(a)]. All timings here are reported in Greenwich Mean Time (GMT). Direct, manual, field measurements of PEEP tube exposure were made at c.14-day intervals to provide ‘ groundtruth ’ checks of the PEEP sensor readings. Conventional erosion pins were also installed at each site to provide a more complete spatial picture of activity on the bank-face, but these data will be presented elsewhere. Evidence from the site suggests that the installation of the PEEPs and buried cables caused little apparent disturbance to the bank material. Furthermore, accretion of fine particles onto the smooth sensor tubes proved to be minimally important under most flow conditions. Stage data were obtained from the Environment Agency gauging station at Keadby, 2 km downstream of Burringham [Figure 2(a)]. Results Example erosion events Figure 3 shows an erosional sequence in May 1997 detected by PEEP sensor 3 on the upper bank at Burringham [Figure 2(b)] during a run of declining tides (Figure 3, A–G). At the beginning of the sequence, on 9 May, the entire PEEP sensor is covered by sediment, giving zero daytime readings for both the PEEP reference cell and cell series outputs (Figure 3). An erosion event of at least 60 mm then becomes apparent by 11.45 GMT on 10 May 1997, immediately following site inundation by tide C

Automatic monitoring of tidal bank erosion 241

175 150

Event E2: 100 mm erosion

PEEP sensor fully covered by sediment

125 100 mm

PEEP sensor outputs (mV)

200

100 75 50

Event E1: >60 mm erosion

25

5

0

4

2 A

B

C

D

E

F

G

1

Stage (m O.D.)

3

0 9-May 00:00

10-May 00:00

11-May 00:00

12-May 00:00

–1 13-May 00:00

F 3. Two erosion events on the upper bank, automatically recorded by rises in the cell series voltage outputs from PEEP 3 on the upper bank at Burringham, 9–13 May 1997, in relation to individual tidal cycles (lettered A–G for clarity: see text). Stage was recorded at the Environment Agency gauging station at Keadby, 2 km downstream [see Figure 2(a)]. PEEP ref. cell (- - -); PEEP cell series ( ); stage (  ).

(Figure 3; Event E1). Exposure of the PEEP sensor by erosion at this time is indicated both by the sudden rise at 11.45 GMT in cell series voltages and the return of PEEP reference cell signals to normal daytime values (>150 mV). To demonstrate the key ability of the PEEP technique to increase the accuracy of determining the timing of erosional and depositional activity, Figure 3 shows a further example of a large erosion event, Event E2, on 12 May 1997. At 09.45 GMT on 12 May, inundation of the PEEP site by turbid waters during the peak of tide G caused both reference cell and cell series outputs to drop substantially (Figure 3). Once the tidal peak had passed, however, the PEEP sensor fully re-emerged from the water at 12.30 GMT on 12 May to deliver the normal daytime reference cell outputs of approximately 175 mV (Figure 3; Event E2). Moreover, it is clear from the pronounced increase in cell series values to above 125 mV, immediately following peak water level on 12 May (Figure 3), that a significant erosion event (Event E2) had occurred during bank inundation by tide G. Erosion Event E2 is therefore timed to a 2.75-h window between 09.45 and 12.30 GMT on 12 May 1997 (Figure 3). The magnitude of erosion event, calculated from differencing the calibrated pre- and post-tide PEEP cell series outputs (37 and 133 mV respectively), is revealed to be 100mm

(Figure 3; Event E2). This converts to an average bank erosion rate of 36 mm h
1 or 0.01 mm s
1 over the 2.75 h inundation period.

Sediment surface stability PEEP monitoring systems can also confirm the stability of a tidal sediment surface during exposure to hydraulic or meteorological events. This is a very useful advantage because stability confirmation in the face of (variable and transient) potentially destabilizing forces can be as helpful as erosion definition when deciphering process dynamics. This is especially true for the interpretation of complex sediment entrainment thresholds which may shift in response to temporal changes in material erodibility. Note in Figure 3, for example, following the >60 mm erosion of Event E1 after tide C on 10 May (discussed above), the apparent absence of any further, lasting erosion associated with tides D, E and F. Zero net change to the bank elevation is confirmed by relatively similar diurnal cell series values, relative to reference cell signals, for 10 and 11 May and the first part of 12 May. This is despite peak water levels sufficiently high to inundate the sediment surface at the PEEP 3 site on three occasions to a depth of 0·5–1·1 m (Figure 3).

242 D. M. Lawler et al. 200 PEEP fully exposed PEEP sensor outputs (mV)

175 Event D1: 47 mm deposition

150 125

Event D2: 92 mm deposition

100 75 50

4

0

3 2 1 0

Stage (m O.D.)

25

Stage 20-Mar 00:00

21-Mar 00:00

22-Mar 00:00

23-Mar 00:00

24-Mar 00:00

–1 25-Mar 00:00

F 4. Two deposition events on the lower bank, automatically recorded by falls in cell series voltage outputs from PEEP 6 on the lower bank at Burringham, 20–25 March 1997, in relation to individual tidal cycles. Stage was recorded at the Environment Agency gauging station at nearby Keadby, 2 km downstream [see Figure 2(a)]. PEEP ref. cell (- - -); PEEP cell series ( ); PEEP back ref. cell (——); stage (  ).

Example deposition events Figure 4 shows a depositional sequence in March 1997 during a series of rising spring tides, as revealed by PEEP sensor 6 installed in the lower part of the Burringham bank [Figure 2(b)]. The sequence begins on 20 March 1997 with the complete PEEP photocell array exposed to light, giving maximum cell series outputs virtually identical to the signals from both reference cells (Figure 4). By 21 March, however, sedimentation at the site has been sufficient to halfobscure the Back Reference Cell (Figure 4). At least two distinct depositional episodes then follow. The first of these, Event D1 (21–22 March), occurred in an 18-h window ending 10.30 GMT on 22 March. This ‘ event window ’ embraced two periods of tidal inundation of the PEEP site when sedimentation could have occurred, the first lasting 5·5 h and peaking at 18.15 GMT on 21 March, the second lasting 4·9 h and peaking at 06.45 GMT on 22 March (Figure 4). The magnitude of the deposition event, quantified by calibration of the depression in the PEEP cell series data, is 47 mm (Figure 4; Event D1). This converts to a mean deposition rate of 4·5 mm h
1, or 0·001 mm s
1, over the total 10·3-h inundation period. A second, much larger, depositional episode of 92 mm, Event D2, occurred on 22–23 March, within an 18·5-h event window ending 11.30 GMT on

23 March 1997 (Figure 4). Once again, we can refine the timing a little by ignoring the possibility of aeolian deposition or flow-accretion of material between periods of inundation and assuming that deposition was possible only during inundation. Two individual tidal cycles occurred within this event window, each covering the PEEP site for 5·5 h, with stage peaks at 19.00 on 22 March and 07.15 on 23 March (Figure 4). This accretion event of 92 mm gives a much higher time-averaged deposition rate over the total 10·9-h inundation period of 8·4 mm h
1, or 0·002 mm s
1. The magnitude, timing and accretion rate information for these two example depositional events is delivered by the PEEP system at a much finer temporal resolution than could be obtained routinely by manual resurvey methods.

Sediment cycling on channel bank surfaces To develop the consideration of specific short-lived erosion and deposition events above, 15-min PEEP data can be aggregated to the daily level to ease the examination of longer-term changes in bank surface elevations. To illustrate this, we briefly focus here on results for a 70-day period beginning 17 March 1997 for PEEP 3, located in the upper bank at Burringham. By stripping out all nocturnal PEEP data, then averaging the 15-min results for the remaining daylight

4 3

300 250 200 150 100 50 0

300

200 2

periods, a mean daily bank elevation time series was obtained (Couperthwaite et al., 1998; Mitchell et al., 1999). This is plotted in Figure 5 alongside a mean daily maximum water level series for Keadby. Water-level values for most days are derived as the mean of the two semi-diurnal tidal stage maxima. A striking pattern of sediment cycling is clearly evident in Figure 5. Successive deposition–erosion sequences occur on a 14-day cycle which is strongly and positively related to the 14-day spring-neap cycling of tidal range. Rising spring tidal ranges are strongly associated with depositional sequences and higher bank elevations (Figure 5). The two series are a little out of phase, however, with bank elevation appearing to lag behind water level slightly. Lagged correlation analysis for the 60 days of concurrent record revealed that the strongest relationship emerged when bank elevation was correlated to water level two days previously, WLd-2 (r2 =58·9%;

ELEV = 14.872 (WLd-2) – 26.458 (WLd-2) + 5.97 2 R = 0.6032

150 100 50 0

-M 10

F 5. Sediment cycling on the upper bank at Burringham on the tidal Trent shown by PEEP 3 (——), in relation to spring-neap cycling of tidal water levels (——) measured at Keadby, March–May 1997.

ELEV = 70.977 (WLd-2) – 145.7 2 R = 0.5887

250

17 ar -M 24 ar -M 31 ar -M 7- ar A 14 pr -A 21 pr -A 28 pr -A 5- pr M 12 ay -M 19 ay -M 26 ay -M ay

2

Mean daily bank elevation (mm)

Mean daily maximum water level (m O.D.)

5

Bank elevation, ELEV (mm)

Automatic monitoring of tidal bank erosion 243

1 2 3 4 5 6 Maximum water level two days previously, WLd-2 (m)

F 6. Lagged relationship of mean daily upper bank elevation at Burringham, R. Trent, to mean daily maximum water levels at Keadby, March–May 1997, showing least squares fits.

r= +0·767; n=60; F<0·0001; Table 1). While this lagged relationship is highly significant, explaining 58·9% or 60·3% of the variation in bank elevation for linear or second-order polynomial regression fits respectively, the associated bivariate plot shows that considerable scatter remains (Figure 6; Table 1). (Exponential or power-function fits explained less variation in bank elevation than simple linear and quadratic fits.) The simple linear regression equation for mean daily bank elevation (ELEV, in mm) against mean daily maximum water level recorded two days previously (WLd-2, in m) is: ELEV=
145·7+70·977 WLd-2

High wind speeds were thought likely to generate wave-induced stresses on the bank and assist erosional activity, and rationalize some of the scatter evident in Figure 6. However, there was no statistically significant relationship between bank elevation and either mean 6-hourly wind speed or wind direction [data

T 1. Results for regression analyses of mean daily bank elevation at the Burringham upper bank PEEP 3 site (ELEV, in mm) against mean daily maximum water level at Keadby (WL, in m) for n=60 days of observations, March–May 1997

Regression equation ELEV=
88·3+54·004 WL ELEV=
132·4+67·170 WLd-1 ELEV=
145·7+70·977 WLd-2 ELEV=
116·7+62·027 WLd-3 ELEV=
137·6+73·478 WL d-2


0·0310 WSI

(1)

r

r^2

Standard error (mm)

0·5935 0·7324 0·7673 0·6667 0·7907

0·3522 0·5364 0·5887 0·4445 0·6252

55·15 46·66 43·95 51·07 42·32

F significance 5·8 2·9 8·7 6·0 7·1

E-07 E-11 E-13 E-09 E-13

Lagged correlations and regressions are indicated by ad-n suffix to the WL variable, e.g WLd-2 is mean maximum water level two days previously. The final equation listed is a multiple regression expression incorporating a simple daily Wind Stress Index, WSI, derived as WSI=Uw·Dw, where Uw is daily mean 6-hourly wind speed at Gainsborough (m s
1) and Dw is daily mean 6-hourly wind direction ().

244 D. M. Lawler et al.

from the Humber Observatory at Gainsborough, 20 km to the south; Figure 2(a)]. Moreover, the addition of either wind variable, or both, to the simple bivariate regression model of bank elevation in equation (1) to produce a multivariate relationship did not significantly improve the explanation. However, the Burringham bank is west facing, and likely to be especially susceptible to winds from the 180–360 sector. Therefore, a very simple daily Wind Stress Index, WSI, was derived as WSI=Uw. Dw, where Uw is daily mean 6-hourly wind speed at Gainsborough (m s
1) and Dw is daily mean 6-hourly wind direction (). Interestingly, when WSI was added to the bivariate model of equation (1), explanation of bank elevation was significantly improved at the P=0·05 level, r2 rising from 58·9% to 62·5% (Table 1), and yielding the multivariate expression: ELEV=
137·6+73·478 WLd-2
0·03101 WSI (r2 =0·625%; n=60; F<0·0001) (2) The partial correlation coefficient (
0·298) for WSI in equation (2) shows that, when the effects of water level are accounted for, there is a statistically significant negative relationship (P<0·01) between WSI and bank elevation. The sign of this WSI coefficient is in the anticipated direction, in that, for a given water level maximum, increased wind-generated stresses from the western sector would be expected to induce erosion and a decrease in bank elevation. Clearly, the site was highly dynamic over the 70-day trial period: at this daily timescale, few extended periods of sediment surface stability emerge, with the bank almost always undergoing elevation increase or decrease as a result of deposition or erosion (Figure 5). Indeed, the site proved to be so dynamic that the ceiling response of 250 mm for the model of PEEP used here was occasionally exceeded in the latter half of the study (Figure 5), but PEEPs can be periodically reset, like normal erosion pins, to keep pace with bank changes. The amplitude of the 14-day bank elevation cycles varies from 125–>225 mm. Interestingly, sediment tends to be deposited on the bank very quickly, but is then removed by erosion rather slowly, giving an asymmetric sediment cycling profile (Figure 5). This is most evident in the first two cycles ending 21 April (Figure 5), in which sediment is accreted on the bank at a rate of approximately 30–60 mm per day for 3–4 days, but it then takes 9–10 days for this to be removed by erosion (Figure 5). The erosion event E2 of 12 May shown in detail in Figure 3 can be seen to be part of a longerterm, 6-day sequence of erosional activity leading to sustained bank elevation decreases (Figure 5).

Discussion These few example results demonstrate the considerable promise that PEEP system approaches offer in improving the understanding of tidal bank erosion and deposition event dynamics, and for the design of future investigations of tidal bank processes. Furthermore, the PEEP data reveal how dynamic the tidal bank surface at Burringham is, and define the striking 14-day cyclical nature of sediment removal and accretion, which is clearly linked to the spring-neap tidal range cycle. The study also shows that, at the event timescale, there is rarely a simple positive relation between the magnitude of individual erosion or accretion events and maximum tidal water levels. This ‘ complex response ’ suggests that entrainment is not only a function of hydrodynamic stresses applied to the channel boundary, but also reflects, amongst other processes, the impact of changing sediment erodibility (e.g. Paterson, 1997). This suggestion parallels the recognition of the importance of similar transient changes in material erodibility on fluvial banks (e.g. Lawler, 1992; 1994; Lawler et al., 1997; Leopold, 1973; Prosser et al., 2000). These brief results also underline the desirability of adopting multivariate approaches to tidal channel erosion and deposition explanation which embrace not only boundary shear stresses but a whole range on influences which control particle entrainment and settling (Couperthwaite et al., 1998; Mitchell et al., 1999). These include, for example: the possibility of sediment saturation and dewatering time-lags (Anderson, 1983); aggregated impact and the exceedance of threshold sensitivities; wave and wind energy conditions (Anderson, 1983; Kirby et al., 1993); biomediation, vegetation, consolidation and weathering effects on sediment erodibility (e.g. Black, 1997; Paterson, 1997); the chemistry of pore and eroding fluids; and suspended sediment concentrations and floc characteristics in the overlying water column. We would argue, therefore, that a greater focus is desirable in future on the dynamic, and possibly transient and recoverable, changes in those material properties which control sediment entrainment potential. Such complex influences and responses also reinforce the need to deploy automated erosion/ deposition monitoring techniques like the PEEP system to record the full ‘ event structure ’ at an appropriate temporal resolution for explanation (Lawler, 1994, p.172)—especially the magnitude, frequency, timing and duration of individual bank retreat and advance events. Such data, collected on a routine basis, should permit in future a more rigorous building, testing and calibration of high-resolution

Automatic monitoring of tidal bank erosion 245

tidal bank erosion, sediment supply and estuarine sediment transport models (e.g. Le Hir et al., 1989; 1993). Advantages of PEEP system approaches The advantages of automated erosion monitoring approaches are manifold. PEEP systems, because they detect sediment-surface change quasi-continuously, deliver a clearer picture of the temporal distribution of erosional and depositional activity. Information on specific, individual erosion and deposition events can thus be ‘ matched up ’ against particular driving events, such as tidal cycles, fluvial floods, storms, migration of estuarine turbidity maxima, and freezethaw cycles, to improve process inference and explanation. PEEP monitoring approaches also deliver a more complete picture of total activity on a tidal bank surface, because self-cancelling erosion-deposition cycles are less likely to be missed as they can be with infrequent manual resurveys. Automatic monitoring also reduces the risk of disturbance to bank surfaces caused by the repeated manual resurvey or reading of conventional erosion pins. There can be field safety advantages, too, through a reduction in the need to encroach repeatedly onto difficult bank surfaces at dangerous times to conduct manual surveys. PEEP sensors are also useful in defining shifting threshold stresses required for erosion [e.g. in response to erodibility changes brought about by biomediation effects (Black, 1997; Paterson, 1997) or prior weathering activity such as freeze-thaw or desiccation processes (Lawler, 1992; 1994)]. This is because the erosional impact of each individual storm or tidal cycle, for instance, is more likely to be determined. Also, the addition of thermistors to PEEP sensors facilitates the investigation of temperature effects on the rheological properties of sediments (e.g. Anderson, 1983; Bassoullet & Jestin, 1995), freeze-thaw or desiccation activity, or on biologically-related disturbing or stabilizing processes like bioturbation effects or seasonal or longerterm biofilm development and vegetation growth (e.g. Paterson, 1997). Similarly, the light-intensity data generated by PEEP sensors can be used to help explain and model biological processes within ITZs, such as vegetation colonization, growth and decay rates or photosynthetic processes (e.g. Blanchard et al., 1996). PEEP sensors are compatible with virtually any datalogging system, and can be used in a variety of ways according to study aims and site conditions. Furthermore, PEEP systems ensure that erosional and depositional activity is monitored on the same time base as turbidity, and this should permit

relationships between sediment transport in the flow and sediment supply to and from accreting and eroding surfaces to be defined more readily. This should help to define the process linkages which control sediment fluxes and turbidity patterns at various time and space scales (e.g. estuarine turbidity maxima). PEEP sensors can also be linked to data telemetry platforms to generate interrogable real-time data which can aid scheduling of fieldwork, provide early warnings of site stability problems, and assist in erosion management. PEEP sensors are likely also to be applicable to other coastal/tidal environments, such as saltmarshes, beaches, dunes and tidal mudflats; indeed, field trials are already underway in some of these contexts.

Limitations of PEEP systems However, these relatively simple PEEP systems are not without limitations, though many are amenable to solution. For example, until recently, nocturnal events were not detected until the following morning when light re-activated the photodiode array: even so, the achievable temporal resolution was still much better than could be commonly obtained with traditional manual resurvey methods. Furthermore, preliminary experiments indicate that nocturnal data gaps can be plugged with short bursts of artificial light programmed by the datalogger at the instant of logger scanning. It is likely that cells operating at different spectral sensitivities could obviate this problem. Also, Lawler et al. (2001) showed for a fluvial site that the timings of some nocturnal erosion events can now be refined by using PEEP thermistor data to produce a ‘ thermal consonance timing ’ (TCT), and it is likely that such benefits will transfer to the tidal environment. Some PEEP data can be lost or degraded under low-light conditions, and especially when covered by highly turbid water (e.g. Figure 3, tide G). Local scour effects around the PEEP sensors, as with much bank and bed instrumentation, can occur at times, but were found to be of minor importance here. This is because the low viscosity of the surface mud layers at Burringham led to the filling of any small scour holes as the tide receded. Also, like traditional erosion pins, PEEP sensors are invasive (although their small size minimises this), and could potentially disturb a site: little evidence of this was found here, however. The addition of data transmitters to PEEP sensors will preclude the need for cabling, reducing still further installation time and residual site interference. In addition, PEEPs may be difficult to install in gravelly materials and, like most bank instruments, are less

246 D. M. Lawler et al.

suitable for mass failure situations where large blocks of material collapse downslope.

Conclusions The following conclusions may be drawn from this study: (1) Photo-Electronic Erosion Pin (PEEP) automatic erosion and deposition monitoring approaches reveal clear benefits. In particular, such techniques can define the timing and magnitude of individual erosion and accretion events on tidal banks, in relation to specific tides and storms, much more clearly than with traditional, manual methods. Hence, for the first time, a reasonably complete picture of the range and variability of the dynamic response of tidal banks to driving forces can be determined, and this should enhance future process studies; (2) the Burringham tidal bank site is highly dynamic, and experienced erosion or deposition events up to 100 mm in magnitude over a single tide. Examples given for individual tidal cycles revealed mean bank erosion rates of 36 mm h
1 and mean deposition rates of 4·5–8·4 mm h
1 over the periods of inundation. Over a 70-day monitoring period, few extended periods of sediment-surface stability emerged. (3) at the event timescale, there does not appear to be any simple relation between the magnitude of erosion/ deposition events and tidal water level maxima, suggesting that activity rates also reflect the influence of changing sediment erodibility, sediment supply and wind stresses; (4) a striking pattern of sediment cycling is clearly evident, however, over the 70-day monitoring period on the upper bank at Burringham. Bank surface elevation oscillations, driven by erosion and deposition sequences, were characterized by a strong 14day cycle which clearly reflects the spring-neap cycling of tidal range. Sediment tended to be deposited on the bank very quickly, but was removed by erosion rather slowly, to give an asymmetric sediment cycling profile. Higher bank elevations were clearly correlated with high tidal ranges, and especially to water level peaks two days previously and strong winds from the western sector. Such vigorous sediment cycling means that many erosion-deposition sequences on tidal banks can be self-concealing and therefore may not be recorded by infrequent manual resurveys which will inevitably underestimate total activity. This clearly reinforces the need for an automated monitoring technique, which generates at least quasi-continuous time series, to quantify the magnitude, frequency and timing of individual bank advance and retreat events.

Such information is necessary to correctly determine levels of dynamism in such tidal environments, facilitate strong process inference and produce effective management of problem sites. These sample PEEP results from the Burringham tidal bank site suggest that automated monitoring techniques offer considerable promise in the understanding of erosion and deposition event distributions and processes, sediment recycling at the channel boundary, the testing of estuarine sediment-supply and sediment-transport models, the operation of biomediation and other mechanisms which control changing bank sediment erodibility, the residence times of pollutants within intertidal sediment storage zones, and for site management. PEEP sensors can also monitor light incidence and temperature, variables which are useful in explaining the operation of biological effects within ITZs, including mediation of erosion and deposition rates. In particular, sediment erodibility is an important influence on tidal bank response, but erodibility itself is clearly subject to dynamic change at short timescales. Further work is therefore needed on the total bank erosion/deposition ‘ event structure ’, and the processes which control transient shifts in material erodibility and entrainment potential, to balance recent developments in the understanding of estuarine hydrodynamics and sediment fluxes.

Acknowledgements This project was funded under NERC Land-Ocean Interaction Study (LOIS) Special Topic 231 RACS (R) & (C) (NERC grant reference GST/02/184) which is very gratefully acknowledged. We much appreciate the help with fieldwork and data processing from Richard Johnson, Adam Sawyer and James Grove and many others from the School of Geography and Environmental Sciences at Birmingham. We thank also: the Environmental Agency for stage data for Keadby and permission to establish monitoring equipment at Burringham; the Humber Observatory for wind data; and Associated British Ports for permission to install PEEP sensors and erosion pins at the Burringham bank site. We are very grateful to Lynn Buckler and Kevin Burkhill for drawing Figures 1 and 2 respectively.

References Anderson, F. E. 1983 The northern muddy intertidal: seasonal factors controlling erosion and deposition—a review. Canadian Journal of Fisheries and Aquatic Science 40, 143–159.

Automatic monitoring of tidal bank erosion 247 Bassoullet, P. & Jestin, H. 1995 Characterization of intertidal mudflat environments by means of in situ experiments, MAST 2—G8 Overall Workshop, Gdansk, Sept., 4 pp. Black, K. S. 1997 Microbiological factors contributing to the erosion resistance in natural cohesive sediments. In Cohesive Sediments (Burt, T. N., Parker, W. R. & Watts, J., eds). J. Wiley and Sons Ltd., Chichester, pp. 231–244. Blanchard, G. F., Guarini, J.-M., Richard, P., Gros, Ph. & Mornet, F. 1996 Quantifying the short-term temperature effect on lightsaturated photosynthesis of intertidal microphytobenthos. Marine Ecology Progress Series 134, 309–313. Couperthwaite, J. S., Mitchell, S. B., West, J. R. & Lawler, D. M. 1998 Cohesive sediment dynamics on an inter-tidal bank on the tidal Trent, U.K. Marine Pollution Bulletin 37, 144–154. Dyer, K. R. 1986 Coastal and Estuarine Sediment Dynamics. J. Wiley and Sons Ltd., Chichester, 342 pp. Kirby, R., Bleakley, R. J., Weatherup, S. T. C., Raven, P. J. & Donaldson, N. D. 1993 Effect of episodic events on tidal mud flat stability, Ardmillian Bay, Strangford Lough, Northern Ireland. In Nearshore and Estuarine Cohesive Sediment Transport (Mehta, A. J., ed.). Amer. Geophys. Union, Washington D.C., pp. 117–143. Lawler, D. M. 1991 A new technique for the automatic monitoring of erosion and deposition rates, Water Resources Research 27, 2125–2128. Lawler, D. M. 1992 Process dominance in bank erosion systems. In Lowland Floodplain Rivers: Geomorphological Perspectives (Carling, P. A. & Petts, G. E., eds). J. Wiley and Sons Ltd., Chichester, pp. 117–143. Lawler, D. M. 1993 The measurement of river bank erosion and lateral channel change: a review. Earth Surface Processes and Landforms, Technical and Software Bulletin 18, No. 9, 777–821. Lawler, D. M. 1994 Temporal variability in streambank response to individual flow events: the River Arrow, Warwickshire, UK. In Variability in Stream Erosion and Sediment Transport (Olive, L. J., Loughran, R. J. & Kesby, J. S., eds). Internat. Assn Hydrol. Sci. Pubn, 224, pp. 171–180. Lawler, D. M., Harris, N. & Leeks, G. J. L. 1997 Automated monitoring of bank erosion dynamics: applications of the novel Photo-Electronic Erosion Pin (PEEP) system in upland and lowland river basins. In Management of Landscapes Disturbed by Channel Incision (Wang, S. Y., Langendoen, E. J. & Shields, F. D., eds). The University of Mississippi, Oxford, Mississippi, 249–255.

Lawler, D. M., Couperthwaite, J. S. & Leeks, G. J. L. 2001 Using the Photo-Electronic Erosion Pin (PEEP) automatic monitoring system to define bank erosion event timings: the R. Wharfe, UK, In: Proc. 7th Federal Interagency Sedimentation Conference on ‘ Sediment: Monitoring, Modeling and Managing ’ (Bernard, J., ed.). Reno, Nevada, 25–29 March 2001, Vol. 2, V-39– V-46. Le Hir, P., Bassoullet, P. & L’Yavanc, J. 1989 New developments about mud transport models: application to a macrotidal estuary. In Sediment Transport Modelling (Wang, S. S. Y., ed.). Proc. International Symposium, New Orleans, Aug., pp. 94–99. Le Hir, P., Bassoullet, P. & L’Yavanc, J. 1993 Application of a multivariate transport model for understanding cohesive sediment dynamics. In Nearshore and Estuarine Cohesive Sediment Transport (Mehta, A. J., ed.). Amer. Geophys. Union, Washington D.C., pp. 467–485. Leopold, L. B. 1973 River channel change with time: an example. Geological Society American Bulletin 84, 1845–1860. Mitchell, S. B., Couperthwaite, J. S., West, J. R. & Lawler, D. M. 1999 Dynamics of erosion and deposition events on an intertidal mudbank at Burringham, River Trent, U.K. Hydrological Processes 13, 1155–1166. Paterson, D. M. 1997 Biological mediation of sediment erodibility: ecology and physical dynamics. In Cohesive Sediments (Burt, T. N., Parker, W. R. & Watts, J., eds). J. Wiley and Sons Ltd., Chichester, pp. 215–230. Prosser, I. P., Hughes, A. O. & Rutherfurd, I. D. 2000 Bank erosion of an incised upland channel by subaerial processes: Tasmania, Australia. Earth Surface Processes and Landforms 25, 1085–1101. Teisson, C. 1997 A review of cohesive sediment transport models. In Cohesive Sediments (Burt, T. N., Parker, W. R. & Watts, J., eds). J. Wiley and Sons Ltd., Chichester, pp. 367–382. Uncles, R. J., Stephens, J. A. & Barton, M. L. 1994 Seasonal variability of fine-sediment concentration in the turbidity maximum region of the Tamar estuary. Estuarine, Coastal and Shelf Science 38, 19–39. West, M. S. & West, J. R. 1991 Spatial and temporal variations in intertidal zone sediment properties in the Severn estuary. In Estuaries and Coasts (Elliott, M. & Ducrotoy, J. P., eds). Olsen and Olsen, Fredensborg, pp. 25–30.