Project: Property protection “Haus Kliffende”, Kampen/Sylt (planning/construction 1990; monitoring until 2000)
Editorial preparation by Naue: 2025
Coastal protection on Sylt: Geotextile sand cushions were installed at Haus Kliffende as a flexible, environmentally friendly second line of defence against storm surges and remained functional even after severe winter storms.
Abstract Coastal protection on Sylt
The “Haus Kliffende Property Protection” project demonstrates the successful use of geotextile sand cushions as a flexible coastal protection measure in front of an exposed building on Sylt. Calculations by the BAW and large-scale tests confirmed the stability under wave and overflow loads. The measure remained functional during several severe storm surges and is an environmentally friendly alternative to rigid protective structures.
- Coastal protection on Sylt: Geotextile sand cushions acted as a robust “second line of defence” at the Kliffende house.
- Verification: BAW calculations + large-scale wave channel tests confirmed stability.
- Design idea: Flexible rather than rigid systems in protected areas; deep integration mobilises friction and prevents washout.
- Critical parameters: embedment depth, permeability/filtering effect, tensile strength, front protection against abrasion/UV.
- Long-term effect: Monitoring + biotechnical measures promote natural dunes and increase durability.
1. Classification: The challenge of coastal protection on Sylt
In early 1990, storm surges caused significant morphological changes to Sylt’s west coast. At times, the historic Kliffende house was only about 5.4 metres from the cliff edge. Traditional, rigid protective structures (e.g. stone revetment, seawall) were not eligible for approval due to the location in nature conservation zone 1. An effective, landscape-compatible solution that could be implemented in the short term was sought.
2. Objective of the measure
To supplement regular beach nourishment, a 165-metre-wide, geotextile-reinforced artificial dune was designed. It serves as a second line of defence in case the offshore sand deposit is eroded again. Objective: to prevent cliff erosion and ensure long-term protection of buildings.
3. Variants & decision (rigid vs. flexible)
Rigid, semi-flexible and flexible construction methods were examined. Due to nature conservation regulations and erosion dynamics, a flexible system consisting of geotextile sand cushions was chosen. Advantages: deformability, repairability, low visibility, approvable in the protected area.
4. Dimensioning & stability verification (BAW)
- Approach: Outgoing waves and the associated pore water pressure effects were considered to be decisive.
- Concept: Coordinate fabric/fleece permeability and drainage effect so that pore water is quickly released; embed depth in unaffected subsoil to mobilise friction; tensile strength of the covering prevents sand loss.
- Tests: Calculations by BAW Karlsruhe; liquefaction under waves considered according to approaches by BAW/Delft Geotechnics.
- Result: Optimised geometry (total height approx. 8 m; embankments approx. 1:2 at the bottom / 1:4 at the top); construction resistant to scouring and overflow.
5. Geotextile sand cushion system (structure and materials)
Purpose of the covering: to hold the sandbags in position, anchor them and protect the fronts against abrasion/UV radiation. A highly stretchable, abrasion-resistant fleece was used on the front to protect the PP fabric. The cushions were arranged in layers in a staircase formation (first layer approx. 30 m in front of the cliff edge; partially integrated below sea level).
Loads: Extreme water levels (≈ +4.5 m above sea level), significant wave heights (Hs ≈ 2.5 m), periods (Ts ≈ 5.5 s).
Material requirements: high tensile strength with low elongation, suitable filter/drainage properties, UV stability, abrasion resistance.
Technical characteristics (composite material — excerpt)
Values according to project documentation (composite of PP tape fabric + PES fleece)
| component | raw material | Mass [g/m²] | Layer thickness [mm] | Maximum longitudinal tensile force [kN/m] | Maximum transverse tensile force [kN/m] | Opening width O90,w [mm] |
|---|---|---|---|---|---|---|
| Non-woven fabric (e.g. Terrafix 601 S) | PES | 620 | 6.6 | ≥ 12 | ≥ 18 | 0.15 |
| fabric | PP | 340 | 0.33 | 80 | 80 | — |
Note: Observe project-specific makes/test conditions; data serves to illustrate the composite concept.
6. Construction work (laying)
- Create an excavation pit at the foot of the cliff, protected by an approx. 5 m high sand embankment.
- Lay marked/numbered geotextile sheets; overlap approx. 1.5 m; fill with sand and compact, fold back sheets without creases.
- Reinforce fronts facing the sea with highly stretchable, abrasion-resistant fleece; temporary concrete formwork for precise front formation.
- Cross-securing (full-surface fabric/fleece bulkheads) to prevent lateral washout in the event of partial damage.
- Result: stabilised beach area, stepped layers of padding, total height ≈ 8 m.
7. Monitoring, Performance & Events 1993–2000
- After completion (December 1990), biotechnical measures: sand trap fences, beach grass planting; approx. 2 m of sand cover formed within ~2 years.
- Tests: Winter storms in 1993/1994/1998 → Exposures without structural damage.
- Large-scale wave channel tests (Universities of Hanover/Braunschweig, 1991): Stability verification under continuous loading, pressure surge, overflow, scour, floating debris.
- Series of storms in 1999/2000 (including hurricanes “Anatol”, “Kerstin” and “Liane”): severe coastal erosion; erosion at the ends of structures with risk of scouring → immediate measures implemented; basic system proved effective.
8. Conclusion & recommendations for practice
Coastal protection using geotextile sand cushions is an effective option, especially in protected areas and where there is limited scope for obtaining permits. Deep integration, suitable filter/drainage characteristics and front protection against abrasion/UV are crucial to success. Monitoring after severe events and biotechnical measures strengthen long-term stability. For exposed locations, a buffer zone (pre-flushing) in front of the geotextile line is recommended to reduce the duration of exposure.
What are geotextile sand cushions?
Sand bodies that are encased/anchored by high-strength geotextiles and serve as flexible, deformable coastal protection elements.
Why are they suitable for coastal protection on Sylt?
They are environmentally friendly, visually unobtrusive, easier to obtain approval for than rigid structures and, when correctly dimensioned, can withstand overflow/flushing.
What factors determine durability?
Embedding depth, filter/drain design (pore water pressure reduction), tensile strength, front protection, maintenance/monitoring and beach replenishment (pre-flushing).
This article was first published in 2019 at the Naue Colloquium.
Geotechnical classification
- Exposed cliff coastline on the west coast of Sylt near Haus Kliffende, characterised by storm-surge driven erosion, cliff retreat and a very small stand-off distance between the building and the cliff edge.
- After severe storm surges in early 1990, the distance from the historic property to the cliff was reduced to about 5.4 m, requiring urgent property protection within a nature conservation area where massive, rigid structures could not be approved.
- Coastal protection concept combining regular beach nourishment with a 165 m wide artificial dune reinforced by geotextile sand cushions, forming a flexible second line of defence against storm surges.
- Hydraulic and geotechnical boundary conditions include extreme water levels of about +4.5 m, significant wave heights of roughly Hs ≈ 2.5 m and periods Ts ≈ 5.5 s, with failure mechanisms such as scour, overflow, liquefaction and pore water pressure build-up in the sand body.
- Design mechanisms: deep embedment of the geotextile sand cushions in relatively stable subsoil, coordinated filter and drainage behaviour of the composite geotextile, abrasion and UV protection at the seaward face and integration with biotechnical dune stabilisation measures.
Typical project questions
- How can coastal properties on eroding cliff coasts be secured when massive, rigid protection structures are not permissible due to environmental regulations or approval constraints?
- Which geometry, embedment depth and material properties are required for geotextile sand cushion systems to remain stable under repeated storm surges, overflow and local scour without loss of sand or loss of function?
- How should monitoring, emergency measures and biotechnical stabilisation be combined with geotextile systems so that protective dunes retain their function over many years despite morphological changes of the coastline?
Suitable Naue products & system solutions
A project-specific system selection enables protective structures to be implemented efficiently, durably, and tailored to local conditions.
Geotextile sand cushions and internal filter system
- Secutex® nonwoven geotextile – provides the internal filter and protection layer in geotextile sand cushion systems, limiting sand loss, controlling pore water pressures and protecting the structural fabric under cyclic wave loading.
Stabilising coastal slopes and transition zones
- Secugrid® Geogrids – used to reinforce steep dune toes, access ramps or landward embankments behind the sand cushion line, increasing overall slope stability and reducing deformation during storm surges.
- Secugrid® m3 systems (reinforced soil) – enable the construction of reinforced soil structures in the backland transition zone, for example to stabilise terraces or berms that tie the protected cliff section into existing topography.
Surface protection and dune development
- Secumat® erosion control systems – protect the surface of artificial dunes and covered sand cushion structures against wind and rain erosion, while supporting vegetation establishment and long-term morphological integration.
Drainage and pore water pressure control
- Secudrain® drainage mats – installed as planar drains adjacent to geotextile sand cushion systems or landward structures, they accelerate dissipation of pore water pressures induced by wave loading and reduce the risk of uplift or sliding.
- Bentofix® GCL – where required, provides a low-permeability barrier layer beneath or behind coastal protection elements, limiting seepage paths and helping to protect sensitive groundwater or hinterland areas in combination with drained geosynthetic systems.
More Naue solutions and geosynthetics
Secutex® nonwoven geotextile
Secumat® erosion control systems
Secudrain® drainage mats
Secugrid® Geogrids
Secugrid® m3 systems (reinforced soil)
Bentofix® GCL
Carbofol® geomembrane