Falsterbonäsets skyddsvalls sårbarhet mot framtida stormhändelser: En analys av vågöverspolning och erosionsbeständighet

Abstract (Swedish)

Falsterbonäset är ett låglänt kustområde med hög sårbarhet för översvämning vid stigande havsnivåer och framtida stormhändelser. Som skydd mot detta anläggs ett 21 kilometer långt översvämningsskydd bestående av skyddsvallar och murar med en krönnivå på +3,0 m i RH2000. Föreliggande studie analyserar skyddsvallens sårbarhet genom att kombinera beräkningar av vågöverspolning med experimentell utvärdering av vallens erosionsbeständighet. Överspolningsflöden beräknades med överspolningsberäkningsmanualen EurOtop (2018) utifrån vågförhållanden från den numeriska vågmodellen SWAN, för olika kombinationer av vattenstånd, vindhastighet och vindriktning. Vallens motståndskraft mot erosion undersöktes genom fältexperiment på en representativ testvall med ytskikt av lermorän, sand och gräs.

Resultaten visar att vallens funktion i hög grad beror på ytskiktets skick. Ett tätt vegetationstäcke ger ett betydande skydd mot erosion, medan sand eroderar redan vid låga överspolningsflöden. Exponerad lermorän, vilket uppstår på vallen vid ytor med bristande vegetation, uppvisar högre beständighet än sand, men kan få kritisk erosion vid flöden omkring 5 l/s/m. Kämpinge och Norra Höllviken identifieras som de mest utsatta delsträckorna, där kritisk överspolning kan uppstå vid det historiskt vanliga vattenståndet +1,5 m (RH2000) om ytskiktet är skadat. Studien visar att den faktiska utformningen med lermorän är mer robust än den sandbaserade minimilösning som tilläts enligt mark-och miljödomstolen, men att historiska och framtida stormscenarier ändå kan överskrida skyddets kapacitet. Skyddsvallen bör därför betraktas som en riskreducerande åtgärd som kräver kontinuerligt underhåll och kompletterande beredskapsplanering.

Popular Abstract

The Falsterbo peninsula is a narrow and low-lying stretch of land in the southwestern corner of Sweden,
surrounded by the sea on three sides. Because of its geography, flooding has always been a part of its
history. In November 1872, the storm surge known as Backafloden inundated large parts of the peninsula
and caused extensive damage to homes and infrastructure. A few decades later, the New Year's Storm
of 1904 brought somewhat less severe, but still comparable, destruction.

More than a century later, the same vulnerability remains. Rising sea levels and changing storm patterns
are expected to increase flood risks in many coastal areas, including the Falsterbo peninsula. To address
this challenge, Vellinge municipality is constructing a 21-kilometer-long flood protection system
consisting of sea dikes and seawalls. The structures are built to a crest level of +3.0 meters in RH2000,
the Swedish national height reference system, and are intended to protect the peninsula from future
storm surges and high water levels.

The purpose of this study is to evaluate how well this flood protection is likely to perform during its
planned lifespan and to identify the conditions under which it may become vulnerable to erosion or
failure.

To answer these questions, the study combines two complementary approaches. The first focuses on
wave overtopping, which occurs when waves wash water over the crest of a dike or seawall. Although
some overtopping can be tolerated, excessive overtopping may erode the landward side of the structure
and eventually threaten its stability. Overtopping was calculated along the entire flood protection using
EurOtop (2018), the current European standard for overtopping assessments. Wave conditions were
obtained from the numerical wave model SWAN, which was used to simulate 120 combinations of
water level, wind speed and wind direction. Together, these simulations provide a detailed picture of the
storm conditions the flood protection may encounter.

The second part of the study examined how resistant the sea dike is to erosion. To do this, a series of
controlled field experiments was carried out on a smaller test dike designed to represent the geometry
and materials of the actual structure. Four different test sections were constructed with varying slopes
and surface materials, including clay till, sand and grass-covered surfaces. Using an overtopping
simulator, the test dikes were exposed to controlled flows ranging from 0.33 to 1.67 liters per second
per meter (l/s/m). This made it possible to observe how different surfaces responded to overtopping and
to determine when erosion began to develop.

The experiments clearly demonstrated that the condition of the dike surface plays a crucial role in its
ability to withstand overtopping. Dense grass cover proved to be by far the most erosion-resistant surface
tested. No growing erosion was observed on the grass-covered sections, even at the highest flow rates
used in the experiments. Sand showed the opposite behavior. Erosion channels began to form already at
0.33 liters l/s/m, a relatively low flow rate that can occur during moderate storms in the most exposed
parts of the flood protection.

The performance of clay till, which is the primary material used in the actual sea dike, fell somewhere
inbetween. It proved considerably more resistant than sand, with growing local erosion first appearing
at 1.67 l/s/m on a slope of 1:3. The experiments alone were not sufficient to determine the ultimate
erosion limit of the material, but when combined with previous studies of clay and mixed till sea dikes,
they provide a useful indication of its performance. Based on this combined evidence, the study
estimates that the risk of structural damage becomes significant at around 5 l/s/m, while the critical
threshold of the dike is likely exceeded somewhere around 10 l/s/m, if the vegetation cover is damaged
or absent.

These erosion thresholds become particularly important when viewed together with the overtopping
calculations. By combining the results, the study identified Kämpinge and northern Höllviken as the
most exposed sections of the flood protection. In these areas, overtopping could become critical at water
levels of approximately +1.5 meters (RH2000) if the dike surface is in poor condition, for example due
to a combination of sparse vegetation, holes, pits or other forms of damage. This finding highlights an
important point of the study: the effectiveness of the flood protection depends not only on its height and
design, but also on how well it is maintained over time.

The importance of maintenance becomes even clearer when considering future water levels. An analysis
of historical water level and wind data shows that levels above +1.5 meters (RH2000) are uncommon
but certainly possible, where water levels around +2.0 meters (RH2000) are rare but could occur during
the lifetime of the sea dike. Under such conditions, critical overtopping could occur in Kämpinge and
northern Höllviken if the vegetation cover is not intact. Field observations show that paths and areas
with sparse vegetation already exist along parts of the dike today, suggesting that some sections may
already be more vulnerable than intended.

But sometimes even a dense vegetation cover is not enough. When examining the overtopping rates of
more extreme storm scenarios involving water levels above +2.5 meters (RH2000), some sections of the
flood protection would experience overtopping flows that may become critical regardless of the
condition of the vegetation cover. In other words, while the flood protection substantially reduces flood
risk, there are still storm events (e.g. Backafloden 1872 or The New Years storm of 1904) for which
overtopping may exceed what the dike can safely withstand.

Beyond evaluating the current design, the study also provides insight into an important design decision
made during the planning process. The sea dike that is currently being built differs significantly from
the minimum design approved by the Swedish Land and Environment Court. The approved design
allowed a sand core and did not require a protective clay layer on the landward side. The results of this
study suggest that such a structure would have been considerably more vulnerable to erosion. Applying
the experimentally observed erosion threshold for sand to the overtopping calculations indicates that
water levels around +1.5 meters (RH2000) could have generated overtopping flows capable of causing
serious erosion and potentially breaches in the most exposed areas, particularly around Kämpinge and
northern Höllviken.

The dike that is actually being constructed is therefore substantially more robust than the legally
approved minimum alternative. By using clay till as the core material, its resistance to erosion is greatly
improved, reducing the likelihood of damage during overtopping events. Combined with a dense and
well-maintained grass cover, this design provides a significantly higher level of protection.
The study therefore leads to two main conclusions. First, under present-day conditions, the flood
protection on the Falsterbo peninsula provides meaningful protection against coastal flooding, provided
that the grass cover is actively maintained and that surface damage is repaired before it develops into
larger weaknesses. Second, there are both historical and plausible future storm scenarios that the current
dike may not be able to withstand, regardless of maintenance. For this reason, the flood protection should
not be viewed as a complete solution on its own. It should be complemented by a contingency plan that
can help prevent or minimise the consequences of overtopping and potential dike failure during extreme
storm events.

• author: Uddgren, Carl and Gyllfors, Adam
• supervisor: Caroline Hallin
• organization: Division of Water Resources Engineering
• course: VVRM05 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Overtopping, Falsterbo Peninsula, Sea dikes, Erosion resistance, Sea level rise, Clay till dike, Embankment, Overtopping simulator
• publication/series: TVVR 5000
• report number: TVVR26/5018
• ISSN: 1101-9824
• language: Swedish
• additional info: Examiner; Björn Almström
• id: 9241474
• date added to LUP: 2026-06-23 10:41:15
• date last changed: 2026-06-24 17:21:25