Study of cavernous underground conduits in Nam La (Northwest Vietnam) by an integrative approach   总被引：1，自引：0，他引：1  

V. T. Tam  F. De Smedt  O. Batelaan  L. Q. Hung  A. Dassargues 《Hydrogeology Journal》2005,13(5-6):675-689

This paper presents the result of an investigation of underground conduits, which connect the swallow holes and the resurgence of a blind river in the tropical, highly karstified limestone Nam La catchment in the NW of Vietnam. The Nam La River disappears underground in several swallow holes near the outlet of the catchment. In the rainy season this results in flooding upstream of the sinkholes. A hypothesis is that the Nam La River resurges at a large cavern spring 4.5 km east of the catchment outlet. A multi-thematic study of the possible connections between the swallow holes and the resurgence was carried out to investigate the geological structure, tectonics, cave structure analysis and discharge time series. The existence of the underground conduits was also tested and proven by tracer experiments. On the basis of a lineament analysis the location of the underground conduits were predicted. A remote sensing derived lineament-length density map was used to track routes from the swallow holes to the resurgence, having the shortest length but highest lineament density. This resulted in a plan-view prediction of underground conduits that matches with the cave and fault development. The functioning of the conduits was further explained by analysing flooding records of a nearby doline, which turns out to act as a temporary storage reservoir mitigating flooding of the catchment outlet area.

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Resumen Este artículo presenta el resultado de una investigación de conductos subterráneos, los cuales conectan dolinas y la resurgencia de un río ciego en la cuenca tropical Nam La, compuesta por calizas altamente karstificadas, en el noroeste de Vietnam. El Río Nam La desaparece en el subsuelo por medio de varias dolinas cerca de la salida de la cuenca. Durante la estación lluviosa esto resulta en inundaciones aguas arriba de las dolinas. Una hipótesis es que el Río Nam La resurge en un manantial de caverna grande a 4.5 km al oriente de la salida de la cuenca. Se llevó a cabo un estudio multi-temático de las conexiones posibles entre las dolinas y la resurgencia para investigar la estructura geológica, tectónica, análisis estructural de cavernas, y series de tiempo de descarga. Se evalúa y se demuestra también la existencia de conductos subterráneos por medio de experimentos con trazadores. En base a análisis de lineamientos se predice la localización de los conductos subterráneos. Se utiliza un mapa de densidad de longitudes de lineamientos construido a partir de sensores remotos para seguir las rutas de las dolinas a la resurgencia, teniendo longitudes más cortas pero densidad de lineamientos más altas. Esto da por resultado una predicción en vista de planta de conductos subterráneos que se ajustan con el desarrollo de cavernas y fallas. El funcionamiento de los conductos se explica posteriormente mediante el análisis de registro de inundaciones de una dolina cercana la cual actúa como un reservorio de almacenamiento temporal que mitiga inundaciones en el área de salida de la cuenca.

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Résumé Cet article présente les résultats de linvestigation de conduits souterrains, qui mettent en connexion les pertes et les résurgences dune rivière souterraine dans le bassin versant de Nam La, dans une région tropicale fortement karstique du NW du Vietnam. La rivière Nam La disparaît dans le sous-sol via plusieurs pertes à proximité de lexutoire du bassin. Durant la saison des pluies, les pertes se mettent en charge et des inondations apparaissent. Une hypothèse serait que la rivière ressort 4.5 km à lEst de lexutoire du bassin, dans une grande caverne. Une étude multi-thématique visant les connexions possibles entre les pertes et les résurgences a été réalisée. Elle comprend létude de la structure géologique, la tectonique, lanalyse de la structure karstique et des chroniques de débits. Lexistence des conduits souterrains est également testée et prouvée par des essais de traçages. Sur base de lanalyse des linéaments, la localisation des conduits souterrains est prédite. Grâce à la télédétection et une cartographie de la densité et de la longueur des linéaments, on peut deviner le cheminement de leau entre les pertes et la résurgence (via le chemin le plus court et la densité de fracturation la plus importante). Il en résulte une vue en plan prédisant la localisation des conduits, qui correspond par ailleurs avec le développement des cavités et des failles. Le fonctionnement des conduits est ensuite expliqué sur base de lanalyse des chroniques de débits enregistrés à proximité dune doline, qui joue le rôle dun réservoir temporaire, mitigeant les inondations à la zone de lexutoire du bassin-versant.

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