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Hydraulic failure of flood protection dykes
Défaillance du circuit hydraulique des levées de protection contre les inondations
Brandl H., Szabo M.
Vienna University of Technology, Vienna, Austria
ABSTRACT: The increase in frequency, magnitude and duration of floods during the past decades has become an outstanding
challenge to geotechnical engineering. Appropriate measures against hydraulic fracture due to underseepage of dykes or levees
require comprehensive knowledge of failure modes. This paper describes various forms of hydraulic failure and its critical values for
different types of soil. Furthermore, measures to prevent hydraulic failure by placing berms or by installing relief elements at the
landside dyke toe are discussed.
RÉSUMÉ : L'augmentation de la fréquence, l'ampleur et la durée des inondations au cours des dernières décennies, est devenu un défi
exceptionnel à la géotechnique. Des mesures appropriées contre la fracturation hydraulique en raison de l´écoulement phréatique de
barrage de rivière ou levées nécessitent une connaissance approfondie des modes de défaillance. Cet article décrit les différentes
formes de défaillance du circuit hydraulique et de ses valeurs critiques pour différents types de sol. En outre, des mesures pour
prévenir une panne hydraulique en plaçant des bermes ou en installant des éléments de relief à l'orteil terrestre digue est discuté.
KEYWORDS: dykes, flood protection, hydraulic failure, uplift, relief drainage.
1 INTRODUCTION
Floods have affected millions of people worldwide in recent
decades. In several regions the magnitude and frequency of
flood waves have increased dramatically since long-term
measurements and historical reports have existed. In Austria, for
instance a 2000 to 10 000-year flood event was back-calculated
from the flood disaster in the year 2002. Such hitherto singular
values cannot be taken as design values for flood protection
dykes, but they underline the need for local overflow crests or
spillway sections. Moreover, they clearly demonstrate that a
residual risk is inevitable – despite most costly protective
measures.
The risk of dykes or levee failure increases not only
with the magnitude of a flood but also with its duration. For
instance, the peak period of flood waves along the Austrian
section of the river Danube usually lasts one to three days,
whereas its tributary, the river March/Morava (Austria/Slovak
border) frequently undergoes flood waves up to three or six
weeks (Fig. 1). Figure 1 also illustrates the increase of
magnitude and frequency of the floods since the 1990s.
Especially long-lasting flood waves exhibit in
combination with a required groundwater communication below
dykes a high risk potential regarding hydraulic failure. But also
periodic short hydraulic loadings of flood protection dams and
their subgrade can produce a failure caused by an inner erosion
processes in a long-term.
2 FAILURE MODES OF DYKES
The knowledge of possible failure modes is an essential
prerequisite for a reliable quality assessment of existing dykes
and levees, and for an optimized design of new ones and for
rehabilitation work. Moreover, it helps to optimize emergency
measures during flood defence.
The dominating failure modes for typical ground
conditions along rivers (near-surface, low-permeability sandy to
clayey silts underlain by high permeability sand or gravel) are:
overtopping or overflowing of the dyke/dam crest,
hydraulic fracture,
surface erosion and failure of the water-side slope due to
wave action,
piping due to animal activities, especially from beavers and
rats,
slope failure due to excessive pore-water pressures, seepage
or inner erosion,
slope failure due to a rapid drop of the flood water level,
unsuitable planting of dykes (especially trees with flat
roots).
Actually, it is often difficult to precisely determine the
causes of a dyke failure. Several types of processes might be
involved in a breach and multiple modes in a dyke failure.
Statistical analyses show that overtopping and internal erosion
are the most common modes of failure. While many of these
failure mechanisms occur relatively fast, the erosion by
underseepage develops more inconspicuously. If a groundwater
communication below the dyke is possible, the aquifer or the
overlaying low permeable layer can be progressively eroded
during hydraulic loading. Hydraulic failure is critical because
there may not be any external evidence, mostly only soil boiling
can be found.
Due to this unpredictable behaviour hydraulic failure
is frequently underestimated in practice and may occur in
different forms (e.g. Eurocode 7; CEN 2004):
By uplift (buoyancy). The pore-water pressure under the
low-permeability soil layer exceeds the overburden
pressure.
By heave. Upward seepage forces act against the weight of
the soil, reducing the vertical effective stress to zero; soil
particles are then lifted away by the vertical water flow.
