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Application of a geophysical monitoring system on the tidal and salt exposed embankments in the Humber Estuary, UK

Z. Boukalová1, V. Beneš2 & L. Veselý3 1VODNÍ ZDROJE, a.s., Czech Republic 2G-IMPULS Praha spol. s r.o., Czech Republic 3MEASPROG s.r.o. Czech Republic

Abstract

A Geophysical Monitoring System (GMS) has been designed as a new instrument for monitoring of fluvial flood control dikes in the Czech Republic. Having used the GMS to define the hot spots of an existing embankment system, detailed investigation, maintenance and renewal efforts can be concentrated in a cost-effective way on the critical parts of the embankments. But there then raised the question: “Is giving the GMS satisfactory results as well for tidal and salty terrestrial embankments and levees?” Thus, the Humber estuary region, UK, has been chosen as a pilot site to realize quick geophysical survey methods by GMS in June 2010 at 3 localities:

Tidal embankment Humber Estuary Fluvial embankment of Ancholme River Coastal Embankment Immingham.

At each of the pilot sites, first measurements using the DEMP (Dipole Electromagnetic Profiling) method was performed. After having analyzed the data measured by the DEMP method, with the use of the software DIKINS, segments for the measurement by the method of resistivity tomography were selected (the segments with anomalous resistivity structure). The demonstration of the GMS at the dikes/embankments in Hull resulted in valuable findings. First, it is evident that the level of homogeneity of measured dikes in UK is much higher compared to the Czech Republic. Furthermore, all embankments show very low resistivity values. In dikes in coastal areas, saltwater seepage locations and their extent can be quickly mapped by means of the geoelectric methods.

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River Basin Management VI 71

www.witpress.com, ISSN 1743-3541 (on-line) WIT Transactions on Ecology and the Environment, Vol 146, © 2011 WIT Press

doi:10.2495/RM1100 17

Keywords: monitoring, maintenance of dikes, geophysical methods, dipole electromagnetic profiling, flooding prevention, fluvial, tidal and coastal embankments, integrated water resources management, river basin management.

1 Introduction

The Humber estuary region has been chosen as a pilot site to realize geophysical survey methods on the tidal and salt exposed embankments and as well on the fluvial embankments of the river Ancholme. Geophysical Monitoring System (GMS), which was applied, is composed of 3 basic elements.

1. Quick Testing Measurement (QTM): Dipole electromagnetic profiling (DEMP) using multi-frequency apparatus (i.e. GEM 2) is used for this phase. It can measure several frequencies of electromagnetic signal at one moment – 4 working frequencies are used for GMS, but the apparatus can measure up to 15 frequencies. Approximately 10 entries per meter are taken when measuring at the speed of walking person. Output data are continuously linked to spatial coordinates measured with GPS. Selection of suitable frequencies differs, depending on local electromagnetic interference. Easy obtaining of in-situ data and its automated processing using special software which has been developed by consortium comprising of companies VODNÍ ZDROJE, a.s. and G-IMPULS Praha spol.s.r.o.; is the main advantage of DEMP method. The main output of this testing should be the definition of inhomogenous or other potentially unstable segments of dikes. These segments can also be found by comparing the measured data with data sets from previous measurements of the same object.

2. Diagnostic measurements (DM) are used to detect hidden dike defects in disturbed (inhomogeneous) segments. The methodology is based on the application of a complex of geoelectric methods, in particular, the method of resistivity tomography, which is appropriately complemented by another method, in dependence on the searched defect type (microgravimetry, seismic tomography, etc.).

3. The measurement of geomechanical properties (MGM) allows, in combination with the laboratory tests, to observe the geomechanical condition of disturbed dike segments. In analyzing the geomechanical properties of the dikes, particularly the seismic methods and microgravimetry are applied.

The GMS methodology serves to solve, at low cost, the basic tasks and needs in performing maintenance and surveillance of the dike condition. The main benefit and innovation of the GMS methodology is a broad utilization of DEMP (dipole electromagnetic profiling) method for a quick description of the dike condition and monitoring. For the measurement, advanced multi-frequency apparatuses with satisfactory repeatability of the measured data are assumed to be employed.

72 River Basin Management VI


The embankments in Hull estuary have been first analyzed by the dipole electromagnetic profiling (GEM2) to obtain general overview of the embankment status and in the second phase on the determined places the detailed survey by the multi-electrode resistance tomography (RT), by the tool ARES has been carried out. The GMS testing on the fluvial embankment of Ancholme River was completed by drilling of 4 boreholes.

2 Monitoring practices on the selected pilot sites

The Humber Estuary (see Figure 1) is very dynamic with a tidal range of up to six metres near the mouth at Spurn Head. High water levels vary along the estuary, being up to one metre higher (and one hour later) at Goole than at Spurn. Severe storms can raise water levels by up to 1.5 metres above normal and result in waves up to four metres high near the mouth, although upstream of the Humber Bridge waves are rarely more than one metre high.

Figure 1: Map of the pilot site: Humber Estuary, UK.

The monitoring of condition of flooding dikes in Hull Estuary, United Kingdom was conducted in June 2010. For the practical survey three sites with different conditions have been chosen:

Site A - Tidal Embankment Humber Estuary GPS: +53° 40' 45.83", -0° 32' 54.69" Analyzed length (GEM2): 3 km It is a dike reaching a height of between 2 and 4 m, which serves as flood control and also as a regulation for tidal wave near Humber Estuary. The soil tidal embankment has been analyzed by the GEM2 on the embankment crest and partly on both the riverside and the landside. During the



measurements the locations of potential breach or seepage have been located, on these places the detailed ARES survey has been carried out. The main idea was to locate the seepage of the salt water through the embankment body by registering a marked fall of the soil resistivity on the landside of the embankment. Some parts of the measurement have been carried out at both the high and low tide to get the opportunity to analyze the underground water movements influenced by the tidal cycle.

Figure 2: Tidal Embankment Humber Estuary.

Site B - Fluvial Embankment New River Ancholme GPS: +53° 34' 57.43", -0° 30' 48.39" Analyzed length (GEM2): 1.2 km This pilot site represents a typical minor flood control dike reaching a height of between 1 and 2 m on a regulated river. The soil river embankment has been measured by the GEM2 on the embankment crest. After on-site result analysis four locations have been chosen for the borehole analysis, in the surrounding area of two boreholes the ARES measurement has been carried out. The soil embankment hardened by the concrete on the seaside has been measured by the GEM2 on the embankment crest (covered by the concrete pathway) and on the landside of the embankment to potentially locate the seepage of the salt water through the embankment body. In the future sea levels around the UK are predicted to rise more rapidly and severe storms to become more frequent, increasing the risk of tidal flooding on the coast and near estuaries. In the Humber, the rate of rise is expected to average about six mm per year over the next 50 years, so that sea levels will be about 300 mm higher than they are now. As a result there will be a dramatic



reduction in the standard of protection provided by the estuary’s defences. In addition, model studies of the estuary indicate that seaward of Trent Falls the inter-tidal area in front of the defences (the area between high and low water) will decrease by up to 600 ha over the same period due to the predicted rise in sea level, a phenomenon known as ‘coastal squeeze’. The Immingham coastal embankment is a robust dike of a complicated structure (fortified upstream slope, a wave break, fortified top of the dike, earth fill dike body). The dike reaches a height of approx. 4 to 5 m above the ground level, and a width of around 40 m at the footing bottom. At the locality C, the application of geoelectric methods was to a great extent limited by the occurrence of reinforcing components in the dike structure. Tentative measurement using DEMP method was performed for the dike segment in a length of 1100 m. The profiles were laid out in parallel with the longitudinal dike axis; the measurement was conducted for upstream slope, the top and downstream toe of the dike.

Figure 3: Fluvial embankment new river ancholme.

Site C – Coastal Embankment Immingham GPS: +53° 36' 30.73", -0° 8' 35.71" Analyzed length (GEM2): 1.6 km

3 Results of the performed measurements by GMS

3.1 Locality A – Tidal embankment Humber Estuary

According to DEMP method, the dike in the measured segment is built up by materials showing resistivities most often in the interval of between 20 and 25 ohmm; thus the materials concerned are probably of clay – silt type (common local materials). The dike is very homogeneous, without distinctive local



anomalies (for comparison with common resistivity values and corresponding materials used for dikes in the Central Europe – Czech Republic). More distinctive deviations from the mentioned resistivity interval were measured only in locations of cut-off walls (the data were distorted by the effects of iron – accompanying anomalies of magnetic susceptibility) and in the location of a new dike segment (bypass in the location of advancing lateral bank erosion with the original dike). The new dike is built up by materials showing resistivities of up to 35 ohmm: sandy or gravelly clays. Deeper towards the subsoil, the resistivities markedly decline, with the resistivity average for the frequency of 6525 Hz around 15 ohmm. We assume this to be given by the character of the found sediments (again clays and silts) and particularly by the effect of sediment salination caused by penetration of transitional waters into the dike subsoil at high tide. This may be evidenced by the fact that resistivities systematically rise in the direction from the river. It is best apparent in resistivity cross sections according to resistivity tomography (Fig. 4, 5, 6), where the layer of extremely low interpreted resistivities (often even below 1 ohmm) is well observable. Such resistivity values are not quite frequent in the nature, it is a showing of intensive mineralization of the medium, caused by transitional waters. The area influenced by transitional waters from high tides seems to apparently thin out in the direction from the river, nevertheless, it reaches quite far after the dike axis.

Figure 4: Coastal embankment Immingham.

At a profile measured at downstream toe of the dike were well observable areas where resistivities for “shallow” frequency of 47025 Hz and for “deep” frequency of 6525 Hz are adjacent. We interpret these areas as locations of repeated seepages (at high tide or flood). It is probably a showing of “salination” of the subsurface layer of underlying sediments. There was also detected an



anomalous area where, on the contrary, the resistivities for a “deeper” frequency of 6525 Hz rise over the resistivities measured close to the ground surface. We assume this to be a local area with the occurrence of sediments of sand – gravel type, i.e. sediments showing increased permeability. Markedly shallow maximum of resistivities close to the surface of cross section running via the dike connection (Fig. 5) indicates possible development of shallow fissures that in the event of flood might deteriorate the dike stability.

Figure 5: Field A – Tidal embankment Humber Estuary – overview of resistivity cross sections of RT method – RT3 layout (GPS point 122).



Figure 6: Field A – Tidal embankment Humber Estuary – overview of resistivity cross sections of RT method – RT4 layout (GPS points 124, 125, 126).

3.2 Locality B – Fluvial embankment of Ancholme river

According to DEMP method, the embankment in the locality B is built up by materials showing resistivities largely in the interval of between 15 and 22 ohmm. These materials are of clay – silt type which was also confirmed by test pits drilled. The embankment is very homogeneous, showing no distinctive local anomalies.

keying of a new dam



Figure 7: Field A – Tidal embankment Humber Estuary – overview of resistivity cross sections of RT method – RT5 layout (GPS points 127, 128).

3.3 Locality C – Coastal embankment Immingham

With regard to frequent occurrences of armouring in the dike structure, the geoelectric measurements in the locality C were only experimental. The profiles were measured using DEMP method. The method of resistivity tomography was not applied due to hard surface covering most of the dike area, which disallowed to earth the electrodes. The profile in the embankment axis documented well the (negative) effect of armouring on the data measured using DEMP method. Such measurement, nevertheless, may serve to check existence of armouring, or the

area of permeable soils



density of the major armouring elements. Gradual corrosion of armouring elements might probably be documented by repeated measurements. Generally, it can be stated that for the basic dike material description, DEMP method can be applied in places without armouring elements. The method can be used with no limitations in performing the measurement at downstream face of the dike for the investigation of subsoil geology. Seismic methods can be recommended for describing the geomechanical condition of the dike of such conditions. Frequently demanded is the detection of caverns below upstream dike slope reinforcement. For such cases we use the geological radar and microgravimetry.

4 Conclusions

All geophysical measurements require always a qualified interpretation of measured data for which it is necessary to consider a series of factors. The demonstration of GMS in Hull resulted in following valuable findings. First, it is evident that the level of homogeneity of measured dikes in Great Britain is much higher compared to the Czech Republic. Furthermore, all dikes show very low resistivity values. This is given by the material composition of the embankments (which was confirmed by test pits in the locality B) and also by the effect of sediment “salination” in coastal areas. In the interpretation of such measurements then of significance are even small changes in resistivity. Highly beneficial would be, for example, the application of quick measurement using DEMP method for the delimitation of zones with frequent occurrences of fissures. Nevertheless, to verify such possibilities it is necessary to conduct further testing measurements. In embankments in coastal areas (as was for example the Locality C) there may occur problems with saltwater or transitional water seepages through the dike subsoil. Seepage locations and their extent can be quickly mapped by means of the geoelectric methods. The principle of the measurement consisted (similarly to locality A) in the application of DEMP method in several profiles parallel to the longitudinal dike axis, that were situated at upstream and especially at downstream face of the dike. The anomalous areas were documented by cross profiles that were measured using the method of resistivity tomography. Resistivity cross sections could document the thickness of the zone saturated with saltwater and also its gradual thinning out. It has to be admitted that some larger dikes that contain more complicated protective elements (especially coastal dikes with wave-breaks) are not appropriate for the application of the majority of the geoelectric methods (as was experienced in locality C, where it is advisable to apply seismic methods). A very frequent task in the monitoring of most of the coastal embankments is the detection of cavities below upstream slope reinforcement. In tackling this problem, especially geological radar and microgravimetry should be applied.

Acknowledgements

This paper was developed thanks to the E!4584 EUREKA project eGMS, financed by the Ministry of Education, Young and Sports, Czech Republic.



References

[1] Boukalova Z., Beneš V. (2004): Case studies and geophysical methods. Association of State Dam Safety Officials: Dam Safety 2004. Phoenix, Arizona, USA, September 2004.

[2] Boukalova Z., Beneš V. (2005): Long-term monitoring of geotechnical state of flood protection dikes using non-destructive geophysical methods, report. Ministry of Agriculture, Czech Republic, Prague, 2005.

[3] Boukalova Z. and Beneš V. (2007): Dike breaks prevention as the process of flooding protection. IAHR Congress, Venice, Italy, 1-6.7. 2007.

[4] Boukalova, Z., V. Beneš (2008): Application of GMS System in the Czech Republic – Practical Use of IMPACT, FLOODSite and GEMSTONE Projects Outcomes. In the proceedings, FLOODrisk 2008 conference; Oxford (29.9. – 3.10.2008); UK.

[5] Boukalová, Z., Beneš, V., Kořán, P., Veselý, L. (2009): Application of geophysical monitoring system and GIH 01 tool at the river basin scale as a part of integrated water resources management in Czech Republic. In River Basin Management V, WIT Press, ISBN: 978-1-84564-198-6; pages 361 – 372; UK.



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