Levees

One of the major reasons of levee failure is erosion due to overtopping. The overtopping of levees is occasionally the consequence of a flood or Hurricane. In these events, the surface layers of levees become saturated in advance of any overtopping due to rain and storm events. In this investigation, the effect of the saturation ratio and almost vertical impingement of overtopped water from floodwall on scour development are studied. For this purpose, laboratory-physical models of a typical levee on the banks of Mississippi river with a scale of 1:20 were constructed. A nearly 3 mm thick wooden plate, which was embedded in the crest of the levee represented the floodwall. Silty soil materials with various saturation ratios were used to observe scour potential of the soil. In all the tests, the scour development and the stability of the wall were monitored and analyzed for each test during overtopping. Hydraulic and geometric parameters including Densimetric Froude number, compaction level, water content and levee dimensions were recorded to have a representative comparison between the laboratory models and constructed levees in practice. In addition, the erodibility of the levee materials was determined using Erosion Function Apparatus (EFA). The results of EFA tests were compared to physical model test results to explore and discuss the vulnerability of levee systems to erosion when the surface soils are saturated and floodwall overtopping is experienced.

In Flood Control 2015, technology is used as a means to enable people to better assess and respond to risky situations. From manager to citizen, people make decisions depending upon the circumstances at hand. If there is more certainty about the current situation, and if pertaining information is purposefully provided, the adverse consequences of flooding can be limited. By making smart use of the most advanced information and technology available, Flood Control 2015 makes it possible to take informed action. As illustration of the applicability and need of the Flood Control 2015 advances, ample use was made of these innovations not only within the Netherlands, but also in Indonesia and the US. Improved flood forecasting systems or real-time monitoring of flood defences are not objectives in themselves. A characteristic feature of the Flood Consortium 2015 is that it develops knowledge and tools that are suitable for use in day-to-day practice of several professional disciplines, not only for those responsible for operational water management but also for decision-makers on different levels, crisis managers, communication advisors, news agencies and the emergency services. All of the aforementioned benefit from the yields of this program. Let us not forget about the communities within the areas at risk who will now be better equipped to help themselves and their neighbours by using this improved information and means of communication.

The floodplain embankments of the Serchio River (Tuscany, Italy) have experienced several failures during their life. After the event of December 2009 a detailed geotechnical investigation has been carried out. This paper discusses the necessary criteria for a cost-effective investigation taking into account the total length of the embankments (30 km), the requested level of detail and analyses and the capability of indirect methods (CPTu, 2D geo-electric tomography) to infer the soil stratigraphy. In this respect, it has been confirmed that CPTu test is an expeditious, economical and effective indirect method for soil profiling, if the results are correctly calibrated against borehole-logs. On the other hand, geo-electric investigations can be very useful to highlight anomalies and hetero-geneities in the embankment cross section. Following the geotechnical characterization of the Serchio River a number of analyses have been carried out for various purposes: to clarify the causes of the December 2009 failures, to design appropriate repair of the failures and retrofit of the embankments in proximity of the failures and to identify the risk areas considering the whole extension of the embankments. The following Ultimate Limit States have been considered: Slope failure and hydraulic heave (under steady state flow conditions) by the Limit Equilibrium Method. The stability analyses have been carried out using the Bishop simplified method with circular sliding surfaces and using three different commercial codes. Safety factor against hydraulic heave has been computed according to NTC 2008. Slope failure under transient flow conditions by means of FEM analysis (PLAXIS Flow), in order to assess the time to approach the steady state flow conditions. FEM analysis also considered the stability in the case of partial saturation since the water table is usually few meters below the embankment base. Susceptibility of the embankment and foundation soil to internal erosion phenomena. The paper also tries to clarify the redundant terminology that is used to describe the same phenomenon and, at the same time, points out the need to use appropriate terminology to describe different phenomena related to internal erosion. Appropriate design of both repair of the failures and retrofitting of the surrounding areas has been achieved on the basis of the above analyses. Assessment of the riskiest areas, considering the whole extension of the embankments, has been carried out by means of expeditious criteria, which were based on the embankment geometry and the mechanical soil characteristics. The analysis results show that, for the selected cross sections, the safety factors are rather small and approaching to unit if the seepage forces are not considered. In the case of steady state flow, safety factor drastically reduces to values lower than one. The results of the FEM analyses suggest that the partial saturation of the embankment-material mainly contributes to its stability in the absence of filtration, leading to acceptable safety margins. A good characterization of the material under conditions of partial saturation would allow applying the most appropriate hydraulic conditions.

Nell'ultima decade del dicembre 2009 il fiume Serchio ha visto aumentare a dismisura la propria portata, a causa dello scioglimento delle nevi provocato da un innalzamento delle temperature e di un periodo ininterrotto di pioggia. La notte del 25 dicembre l'eccessivo flusso idrico ha provocato il cedimento di un tratto di argine sia nella frazione di Santa Maria a Colle (comune di Lucca) che tra le frazioni di Nodica e Migliarino (comune di Vecchiano) causando una grave alluvione nei territori circostanti, allagati da acqua mista a fango, alta fino a 2 metri, che ha provocato ingenti danni sia alle infrastrutture presenti sul territorio che alle abitazioni. Il vicino tratto dell'autostrada A12, all'altezza dell'uscita Pisa Nord e dello svincolo con l'autostrada A11, è stato chiuso in quanto sormontato dall'ondata di piena e per pericolo di smottamenti, e le acque hanno raggiunto il vicino lago di Massaciuccoli ingrossandolo notevolmente oltre il livello di guardia. A seguito di tale evento è stato condotto uno studio con l'obiettivo di definire lo stato dell'arginatura, individuare le possibili cause di rottura e definire delle soluzioni progettuali di massima della nuova sezione arginale. È stata programmata una campagna di indagini, per pervenire ad una caratterizzazione geotecnica dei materiali costituenti il corpo arginale ed il terreno di fondazione e, allo scopo di giungere alla comprensione del funzionamento del tratto arginale in questione in occasione delle piene ed alla possibile spiegazione dei fenomeni che hanno portato alle rotte, sono state effettuate delle analisi numeriche con i codici di calcolo PLAXIS V9 e PLAXFLOW. L'esame della risposta strutturale calcolata ha consentito di individuare i possibili meccanismi di collasso degli argini e successivamente di individuare le possibili misure da adottare per incrementare i margini di sicurezza dell'opera. Questa attività si colloca nell'ambito di un dottorato di ricerca che ha come obiettivo finale la definizione di linee guida per il progetto, la verifica in corso d'opera, il controllo nel tempo ed il monitoraggio degli argini fluviali.

This paper presents the possibilities for using an unmanned aerial system for evaluation of the condition of levees. The unmanned aerial system is equipped with two types of sensor. One is an ultra-light laser scanner, integrated with a GNSS receiver and an INS system; the other sensor is a digital camera that acquires data with stereoscopic coverage. Sensors have been mounted on the multirotor, unmanned platform the Hawk Moth, constructed by MSP company. LiDAR data and images of levees the length of several hundred metres were acquired during testing of the platform. Flights were performed in several variants. Control points measured with the use of the GNSS technique were considered as reference data. The obtained results are presented in this paper; the methodology of processing the acquired LiDAR data, which increase in accuracy when low accuracy of the navigation systems occurs as a result of systematic errors, is also discussed. The Iterative Closest Point (ICP) algorithm, as well as measurements of control points, were used to georeference the LiDAR data. Final accuracy in the order of centimetres was obtained for generation of the digital terrain model. The final products of the proposed UAV data processing are digital elevation models, an orthophotomap and colour point clouds. The authors conclude that such a platform offers wide possibilities for low-budget flights to deliver the data, which may compete with typical direct surveying measurements performed during monitoring of such objects. However, the biggest advantage is the density and continuity of data, which allows for detection of changes in objects being monitored.

Usually, dikes that have to be improved because of the phenomenon ‘uplifting’ are improved by constructing a berm. When the space a berm requires is not available, expensive heavy and anchored sheet pile walls are installed within the dikes. This paper presents the results of research to a cheaper solution: shorter, deeply installed non-anchored sheet pile walls.
Two centrifuge tests have been carried out, one with and one without a short sheet pile wall. The centrifuge tests showed that the short sheet pile wall improved the stability of the dike. A design method for short sheet piles has been developed using the finite element program Plaxis.
The paper also considers the installation procedure and costs. The paper concludes that short sheet pile walls sufficiently improve the dike stability. In the province South Holland in the Netherlands, use of short instead of the long sheet pile walls saves some millions of euros (25-30 % of the average costs). Safety philosophy is still a major subject for improvement.

The two most significant parameters which affect the erodibility of soils are soil plasticity and soil compaction during the levee construction. The erodibility potential of levee material has an influence on scour generation due to overtopping from floodwalls. In this investigation, 1/20 scaled physical models with different soil plasticity indexes and compaction levels are considered to evaluate levee performance against scouring. Typical levee construction methods for Mississippi river levees were followed for construction of physical models. In each test, the soil mass was compacted in several thin layers to reach the targeted field compaction. During the lab tests, different geometric and hydraulic parameters of levee and flow were monitored to identify the scour development. The Froude number of the flow was measured in each test to ensure a reasonable comparison between the flow in laboratory-scaled test and typical overtopping in the field. The results of physical modeling tests indicate how the scour potential for levees vary with different soil plasticity and compaction levels during the construction.

The last decade increasing evidence of soil erosion by sediment runoff predating agriculture has been found in different areas of west and central Europe. A central discussion is whether pre-agricultural erosion was triggered by vegetational disturbances caused by hunter-gatherer activities (trampling, controlled forest fires) or natural processes (e.g. climatic anomalies, wildfires, wind-throws). This paper contributes to this discussion using data recently gathered during archaeological excavations of a levee, within the floodplain of the River Scheldt in NW Belgium, occupied by hunter-gatherers during the Early Holocene. These excavations revealed the presence of a ca. 40 cm thick slope deposit, which was radiocarbon dated to the late Preboreal and 1st half of the Boreal. A high-resolution, multi-proxy analysis of this deposit demonstrates a close correlation between the type of vegetation , forest fires and erosion intensity. It is concluded that repeated burning of pine-dominated forests was most likely the main trigger. Furthermore, this study provides strong evidence against an anthropogenic origin of these Early Holocene fires as burning and slope erosion already started long before prehistoric hunter-gatherers occupied the levee top and the Scheldt basin. Furthermore, this study demonstrates that pre-agricultural slope erosion was not limited to hilly regions with pronounced topography but also occurred in low-land regions with subtle topographical gradients.

Climatic changes are expected to increase both the frequency and severity of flooding across the world. It is not possible to plan a continuous increase of levee height and its geometry. Therefore levees should resist to overtopping. This Technical Note gives an updated review of the surface erosion phenomena that can occur during floods and mainly deals with a case history on the effect of overtopping on a trial embankment in stabilized soil. The possible use of recycled soil is also discussed.

The paper illustrates an innovative method to evaluate the degree of compaction of both existing and new river embankments after their completion. A tip resistance target - profile is inferred from laboratory tests in a mini calibration chamber (CC) using a mini CPT. The “laboratory” tip resistance (qcLAB) is expressed as a function of the expected density and of the vertical – horizontal stress components. Such a dependence of qcLAB is obtained carrying out a number of repeated tests in the CC at given density and different consolidation stresses. In situ stresses are inferred by combining DMT results and an estimate of the vertical stress component. A comparison between the qcLAB profile, from CC testing, and the qc, as inferred from in situ CPT, gives the possibility of assessing the density of existing embankments,
while, for new embankments, the method defines the expected in situ qc for a given target density. This method has been successfully used in some case histories (already published data). The paper, after the description of the method, focuses on the effects of stresses on the tip resistance in the case of dry sand and partially saturated fine soil. Eventually, a correlation is attempted between the tip resistance and the dynamic modulus (Ed) as inferred from LFWD. Indeed LFWD could be a useful and expeditious tool for controlling the degree of compaction during the construction stage.

L'obiettivo dello studio è la messa a punto di un metodo per il controllo del grado di compattazione e/o delle caratteristiche prestazionali di rilevati (manufatti) in terreni a grana fine. La metodologia proposta è da tempo oggetto di studio presso l'Unità di Ricerca di Pisa ed il presente lavoro illustra lo stato di avanzamento del progetto. Il metodo prevede l'utilizzo di prove in sito e prove di laboratorio. In particolare le prove CPT E in laboratorio, in scala ridotta, in una mini-camera di calibrazione vengono utilizzate per definire un profilo penetrometrico di riferimento; le prove CPT E e DMT in sito servono rispettivamente a verificare che il manufatto, oggetto del controllo, esibisca un profilo penetrometrico non peggiore di quello di riferimento e a determinare lo stato tensionale in sito. L'aspetto di maggior difficoltà, riguardante la metodologia proposta, consiste nella valutazione dello stato tensionale (in sito e in laboratorio) e dei fattori che influenzano la resistenza penetrometrica (in sito e in laboratorio) nel caso di un manufatto di terre a grana fine parzialmente sature e compattate meccanicamente. Le prove penetrometriche in laboratorio, in scala ridotta, sono state eseguite in una camera di calibrazione (diametro 320 mm) sviluppata e modificata presso il Laboratorio Geotecnico dell'Università di Pisa con il supporto della Pagani Geotechnical Equipment, utilizzando un mini-penetrometro avente caratteristiche simili a quello in scala reale ed un diametro di 8 mm. La camera di calibrazione ha contorni flessibili lateralmente e inferiormente e contorno rigido all'estremità superiore. Le prove in camera di calibrazione sono state eseguite su campioni di una sabbia di frantoio in condizioni asciutte e su un limo sabbioso – argilloso addensato con un contenuto d'acqua pari a circa il 13%. Tutte le prove sono state eseguite con tensioni al contorno costanti (BC1). La presente nota si concentra sull'influenza delle tensioni al contorno sulla resistenza penetrometrica. Si propone inoltre, in via preliminare, una possibile correlazione tra la resistenza alla punta ed il modulo dinamico misurato con prove LWD.

This paper illustrates a case history, based on full-scale tests (trial embankment). More specifically, the settlements of a full-scale trial embankment on a very compressible soil (organic clay) for a period of 5 years are reported. The observed settlements are compared with those obtained from an A type prediction, as well as a B type prediction. The analysis results suggest the importance of an appropriate modelling of secondary settlements of organic soils. The difficulties in obtaining good quality undisturbed samples of organic clay and the peculiar creep behaviour point out the importance of full-scale tests.

Dykes are increasingly becoming structures of major importance in view of sea level changes. Dyke inspectors monitor dykes visually whilst walking the dyke. It has been proposed that remote sensing could be used to make dyke inspection faster and cheaper. This paper presents the results
of a study that investigates remote sensing for evaluating the soil moisture as a means to investigate the quality of a peat dyke. Correlations were found between soil moisture and thermal remote sensing data. Remote sensing provides useful information for soil moisture evaluation for dyke inspection and may be useful for dyke quality inspection.