Mangrove forest against dyke-break-induced tsunami on rapidly subsiding coasts

Abstract

Abstract. Thin coastal dykes typically found in developing countries may suddenly collapse due to rapid land subsidence, material ageing, sea-level rise, high wave attack, earthquakes, landslides, or a collision with vessels. Such a failure could trigger dam-break tsunami-type flooding, or "dyke-break-induced tsunami", a possibility which has so far been overlooked in the field of coastal disaster science and management. To analyse the potential consequences of one such flooding event caused by a dyke failure, a hydrodynamic model was constructed based on the authors' field surveys of a vulnerable coastal location in Jakarta, Indonesia. In a 2 m land subsidence scenario -which is expected to take place in the study area after only about 10-20 years -the model results show that the floodwaters rapidly rise to a height of nearly 3 m, resembling the flooding pattern of earthquakeinduced tsunamis. The depth-velocity product criterion suggests that many of the narrow pedestrian paths behind the dyke could experience strong flows, which are far greater than the safe limits that would allow pedestrian evacuation. A couple of alternative scenarios were also considered to investigate how such flood impacts could be mitigated by creating a mangrove belt in front of the dyke as an additional safety measure. The dyke-break-induced tsunamis, which in many areas are far more likely than regular earthquake tsunamis, cannot be overlooked and thus should be considered in disaster management and urban planning along the coasts of many developing countries.

Effectiveness and Limitation of Coastal Dykes in Jakarta: The Need for Prioritizing Actions against Land Subsidence

“…This is not surprising considering that the last major floods took place in 2007, as reported in Takagi et al [12,28]. Essentially, this was an important benchmark event that could be used to calibrate the responses of those interviewed, giving the authors confidence in the results of the questionnaire.…”

“…Indeed, the results for a 5-m dyke demonstrate that it would be effective to almost completely protect coastal areas until 2050 (Figure 7). However, as Figure 10a illustrates, SLR and land subsidence will both increase the relative difference between the sea surface and the ground, which would create the risk of a dyke-break induced tsunami taking place [28]. Although analyzing the structural stability of the dykes is out of the scope of this study, the thin coastal dykes found in many communities of Jakarta could suddenly collapse due to material aging, the occurrence of high waves, SLR, land subsidence, landslides, a collision with ships, earthquakes, or other adverse mechanisms.…”

“…After these surveys, the authors identified this small coastal village as one of the most vulnerable communities in Jakarta, as it is only protected by a thin coastal dyke ( Figure 5). The rate of land subsidence near this coastal area was considered particularly fast [2,28].…”

“…The rate of land subsidence near this coastal area was considered particularly fast [2,28]. A total of 200 responses were collected, all of which were valid (n = 200).…”

“…Substantial leakage of seawater through a concrete joint made the road wet, as can be observed in the photo. The details on the dyke and community were described by Takagi et al [28].…”