13.4 Results and Discussion
217
This is because the runoff could be generated from the terrace lands in heavy storms
and it took quite a long time to enter the tunnel networks for Tunnel 3 and 4, whereas
the tunnel flow duration for the remaining ones was still quite similar to that in light
and medium storms, owing either to relatively shorter distances between terrace land
and nearest inlets or to higher slope gradients in the tunnel catchments.
(2) Impacts of instability within tunnel systems on tunnel flow hydrology
Deep-seated tunnel systems in this area are characterized by great instability. Collapses within tunnel systems are very common. Small-scale collapses may only cause
oscillation of sediment concentrations in the tunnel flow and no effects on tunnel flow
hydrology. However, large collapses could exert profound impacts on tunnel flow
hydrology. If the collapses are extremely large or associated with surface depression
or sediment deposition, tunnel systems could be totally blocked. Rapid tunnel flow
could, in turn, reopen the blocked tunnel systems. Such a temporal shift of tunnel
systems can be detected using smoke bombs before and after storm events. In the
1989 rainy season, the outlet of Tunnel 1 was connected to two series of inlets, but
the major branch was blocked in the 1990 rainy season. This led to a great disparity
in tunnel flow discharge between 1989 and 1990. In Tunnel 3, the southern branch,
consisting of four tunnel inlets, was blocked throughout the 1989 and 1990 rainy
seasons, but it was reopened in 1992. The middle branch was also blocked during
the storm of August 11, 1990 and reopened by the storm of August 28, 1990, which
caused the tunnel flow discharge of the former storm to be disproportionally low. In
Tunnel 4, three connecting tunnel inlets, about 30 m south of the tunnel outlet, were
abruptly joined into Tunnel 4 during the storm of August 13, 1990. Those newly
joined tunnel inlets subsequently added a large amount of runoff to the tunnel system generated from slope land and terrace land. The most significant event which the
authors observed during the two consecutive rainy seasons was the abrupt initiation
of one tunnel inlet on August 13, 1990. The inlet, with a diameter of 1.5 m and
depth of 1.9 m, was developed in the middle of a road, located on the upper drainage
boundary of Tunnel 6. Smoke bomb tests indicated that it was connected to an inlet
of Tunnel 6 about 40 m away. The runoff from the village and the neighboring subbasin, which used to flow into another basin via the excavated road, was redirected
into Tunnel 6 through the inlet and conduit of the newly developed tunnel. This led to
the discharge of Tunnel 6 being unusually high during the storm of August 13, 1990.
After that storm, the inlet was filled in by the villagers, since it hindered traffic. As a
result, discharge at Tunnel 6 returned to normal in the subsequent storm of August
28, 1990.
Totally blocked tunnel branches can be readily detected with smoke bombs and
thereby their impacts on tunnel flow hydrology can be explicitly evaluated. However,
in most cases, the tunnel systems may not be totally blocked but partially dammed
or blocked first and reopened later during the same storm. In these situations, smoke
bombs are useless and it is extremely dangerous to investigate by crawling into the
tunnel systems after storms. Thus normally no direct evidence is available. However,
217
This is because the runoff could be generated from the terrace lands in heavy storms
and it took quite a long time to enter the tunnel networks for Tunnel 3 and 4, whereas
the tunnel flow duration for the remaining ones was still quite similar to that in light
and medium storms, owing either to relatively shorter distances between terrace land
and nearest inlets or to higher slope gradients in the tunnel catchments.
(2) Impacts of instability within tunnel systems on tunnel flow hydrology
Deep-seated tunnel systems in this area are characterized by great instability. Collapses within tunnel systems are very common. Small-scale collapses may only cause
oscillation of sediment concentrations in the tunnel flow and no effects on tunnel flow
hydrology. However, large collapses could exert profound impacts on tunnel flow
hydrology. If the collapses are extremely large or associated with surface depression
or sediment deposition, tunnel systems could be totally blocked. Rapid tunnel flow
could, in turn, reopen the blocked tunnel systems. Such a temporal shift of tunnel
systems can be detected using smoke bombs before and after storm events. In the
1989 rainy season, the outlet of Tunnel 1 was connected to two series of inlets, but
the major branch was blocked in the 1990 rainy season. This led to a great disparity
in tunnel flow discharge between 1989 and 1990. In Tunnel 3, the southern branch,
consisting of four tunnel inlets, was blocked throughout the 1989 and 1990 rainy
seasons, but it was reopened in 1992. The middle branch was also blocked during
the storm of August 11, 1990 and reopened by the storm of August 28, 1990, which
caused the tunnel flow discharge of the former storm to be disproportionally low. In
Tunnel 4, three connecting tunnel inlets, about 30 m south of the tunnel outlet, were
abruptly joined into Tunnel 4 during the storm of August 13, 1990. Those newly
joined tunnel inlets subsequently added a large amount of runoff to the tunnel system generated from slope land and terrace land. The most significant event which the
authors observed during the two consecutive rainy seasons was the abrupt initiation
of one tunnel inlet on August 13, 1990. The inlet, with a diameter of 1.5 m and
depth of 1.9 m, was developed in the middle of a road, located on the upper drainage
boundary of Tunnel 6. Smoke bomb tests indicated that it was connected to an inlet
of Tunnel 6 about 40 m away. The runoff from the village and the neighboring subbasin, which used to flow into another basin via the excavated road, was redirected
into Tunnel 6 through the inlet and conduit of the newly developed tunnel. This led to
the discharge of Tunnel 6 being unusually high during the storm of August 13, 1990.
After that storm, the inlet was filled in by the villagers, since it hindered traffic. As a
result, discharge at Tunnel 6 returned to normal in the subsequent storm of August
28, 1990.
Totally blocked tunnel branches can be readily detected with smoke bombs and
thereby their impacts on tunnel flow hydrology can be explicitly evaluated. However,
in most cases, the tunnel systems may not be totally blocked but partially dammed
or blocked first and reopened later during the same storm. In these situations, smoke
bombs are useless and it is extremely dangerous to investigate by crawling into the
tunnel systems after storms. Thus normally no direct evidence is available. However,