September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.
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Keywords: flood-control works, piping, environmental consequences, model boundaries, escalation. This paper aims to show what happens when engineering decisions are taken based on an artificial model that, though perfectly coherent within its boundaries, is constrained into an area which does not match the actual natural limits. The case study considers the escalation of flood-control works, such as levees, levee elevations and cut-off walls, built over the years, to protect the riparian town of Boretto (RE) along the Po river. It illustrates how the exposition to piping, along the Po river (Italy) instead of being solved, has been transferred constantly downstream in the past 70 years.
Extraordinary flood events, in the river Po, occurred in November 1951 and 1994 and in October 2000. This latter flood caused 23 deaths, 11 missing and 40,000 homeless people. Between 1995 and 2000 cut-off walls were driven into the river embankments to reduce seepage forces, working from the crest downwards. Between 1998 and 2001 the same stretch of levee was raised to meet the system’s standard of protecting against the 200 year flood. As shown in Figure 1, three sand boils formed after the events in 1951, 1994 and 2000 (reactivated in 2014), each one appearing downstream from the previous one, on the landside of the main earthen levee without a significant change in distance from the levee toe.
Figure 1. Sand boils appeared in 1951 (nr. 35),in 1994 (nr .36) and a third one in 2000 which reoccurred in 2014. On the right, the freeway fly-over. Photo of 2017.
Every time a levee section was lifted and a cut-off wall was embedded in the embankment, its design and effectiveness was proved by the results of 2-D numerical calculation using Flow Nets and, more recently, SEEP/W (GEO –SLOPE 2004) limited to the area of the intervention. Here we see how engineering solutions that have been chosen and implemented in such an artificially designed area, though perfectly coherent and locally effective, have just transferred the problem to the areas beyond its boundaries, leading to an escalation in man-made interventions that may not be easily nor safely governed.
Until recently, as can be seen in the Po river Levee Atlas in Figure 2, flood control measures have been merely of structural kind, aiming at improving the hydraulic capacity of rivers. The atlas shows existing levees as well as relevant information concerning these levees. It provides a baseline description of the flood protection structures along the channel banks, in addition to locating sand boils and leaks in adjacent areas. Its purpose is to give a basis for decision making after the risk assessment phase.
M.D. Giliberti
1, M. Cirincione
2, L. La Torre
1& G. Zanichelli
11Department of Flood Risk Prevention and Mitigation - Interregional Agency of the Po River, Parma, Italy 2IHO Cat. A Hydrographer
September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.
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Figure 2. Po river Levee Atlas showing sand boil locations: nr. 35 and nr. 36. Blue and red lines indicate cut-off walls and their geometry. The green line indicates the raised levee stretch. The orange line indicates spur levees.
In order to evaluate levee performance and fixes, the approach was collecting: 1. Historical data: past levee performance, investigations, and repairs, supplemented with interviews to personnel from the Agency. 2. Topographic data: LIDAR survey along urban levees provides some significant advantage over the only use of aerial photographic imagery, like the smooth transition to GIS-based digital terrain models (DTM.2 m grid), 3. Subsurface data: intrusive and nonintrusive investigation methods. Sources were: a. Topographic maps; b. Aerial photography, c. Geological maps; d. Soil survey reports; e. Field reconnaissance observations.
Because structural works are extremely site dependent the choice of model boundaries must be given top priority. However levee geometry and fixes, to safeguard flood-prone areas, answer always to economic, social, environmental and political demands, thus influencing an optimal choice of the above mentioned boundaries. As a result of a not always optimal approach, we see an escalation of flood protection works unable to solve the situation. Following this escalation, we can notice a “maximum adding up” constraint that is not evident when defining boundary conditions in seepage and stability analysis of the designed measures. Also, protection from detrimental effects due to flooding and piping in soil foundation cannot lie in mere adding up of structural works, where design standards are based upon probability of event occurrence, or return period. Taking upstream, downstream and environmental consequences into consideration, should be common design practice.
The emerging, though still incomplete, research suggests the key concept of “engineering threshold”. In this case, building a new cut-off wall to avoid the reoccurrence of sand boils, seems to find its threshold in another civil and environmental engineering work, with an assessed higher vulnerability or exposed value, i.e. the freeway fly-over. But the fly-over as well, answers to a new social and economic demand. The new cut-off wall, though fit for purpose, could shift further downstream the sand boil under the fly-over, adding another step to the escalation.
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