218
13 Tunnel Flow and Erosion Processes …
in July of 1989, the authors did manage to pass through the last section of Tunnel 6,
a 76 m long tunnel conduit, and found a bridge-like constriction inside. Apparently,
it used to be a dam caused by a collapse and sometime later tunnel flow penetrated
the dam and formed the opening under the ‘bridge’. Here, in contrast to the intended
objective of this section, we use the monitored tunnel flow hydrologic processes to
identify the possible partial damming during the event. Owing to the lack of direct
evidence, the results presented here must be considered tentative and to be examined
further in the future. In the storm of July 26, 1990, tunnel flow processes in all tunnels
except Tunnel 4 were characterized by an early discharge peak, which was caused
by an immediate intensity peak after rainfall onset. However, discharge at Tunnel 4
was very low in the first hour and peak discharge did not occur until 77 min after
rainfall onset. After the peak, the discharge sharply dropped to a very low level and
lasted for another 40 min or so. The total discharge appeared to be normal and the
absence of the first peak probably resulted from partial damming, which was flushed
away later by accumulated water inside the tunnel. In contrast, in Tunnel 6, after the
first discharge peak, tunnel flow simply stopped. It was unlikely that no runoff had
been generated by the second rainfall peak from the tunnel catchment, the largest
one in the basin. The total discharge from Tunnel 6 during this storm was also quite
low. Twelve hours later, another storm occurred, with a rainfall of 19.8 mm. Flow
in Tunnel 6 started 18 min after rainfall onset and the discharge was so high that
the trapezoidal weir shifted. Accordingly, no water discharge data were collected,
although sediment samples were still taken throughout the event. Actually, the mean
rainfall intensity for the first 20 min (0.09 mm/min) was very low in this storm.
Though the antecedent soil moisture was very high, it is unlikely to have produced
such a high flow if no water had been trapped by damming during the previous
storm. Two rainfall peaks produced four discharge peaks in Tunnel 3, may have
been caused by runoff generation zonation as well. To further evaluate the impact
of instability on tunnel flow discharge, the authors compared it with overland flow
discharge. Owing to the limited number of events that were monitored on the surface
plots during our monitoring periods, the authors have used the data collected by the
Shanxi Institute of Soil and Water Conservation during the period of 1963–1968
from the Yangdaogou sub-basin (SISWC 1982). Three surface plots with areas of
400,1855 and 4167 m
2 were selected in the sub-basin comprising upper slope, lower
slope, and combined slope, respectively. It was found that a good correlation exists
between runoff discharge and rainfall with an intensity of more than 0.2 mm/min
for all three surface plots. For tunnel systems, such a good correlation could only
be found for Tunnel 3. However, if we disregard those events affected by tunnel
instability, identified above, correlation coefficients are improved for all tunnels,
especially for Tunnel 1 and 4. The still poor correlation for Tunnel 6 may be ascribed
to unidentified tunnel instabilities within this large and complex system.
13 Tunnel Flow and Erosion Processes …
in July of 1989, the authors did manage to pass through the last section of Tunnel 6,
a 76 m long tunnel conduit, and found a bridge-like constriction inside. Apparently,
it used to be a dam caused by a collapse and sometime later tunnel flow penetrated
the dam and formed the opening under the ‘bridge’. Here, in contrast to the intended
objective of this section, we use the monitored tunnel flow hydrologic processes to
identify the possible partial damming during the event. Owing to the lack of direct
evidence, the results presented here must be considered tentative and to be examined
further in the future. In the storm of July 26, 1990, tunnel flow processes in all tunnels
except Tunnel 4 were characterized by an early discharge peak, which was caused
by an immediate intensity peak after rainfall onset. However, discharge at Tunnel 4
was very low in the first hour and peak discharge did not occur until 77 min after
rainfall onset. After the peak, the discharge sharply dropped to a very low level and
lasted for another 40 min or so. The total discharge appeared to be normal and the
absence of the first peak probably resulted from partial damming, which was flushed
away later by accumulated water inside the tunnel. In contrast, in Tunnel 6, after the
first discharge peak, tunnel flow simply stopped. It was unlikely that no runoff had
been generated by the second rainfall peak from the tunnel catchment, the largest
one in the basin. The total discharge from Tunnel 6 during this storm was also quite
low. Twelve hours later, another storm occurred, with a rainfall of 19.8 mm. Flow
in Tunnel 6 started 18 min after rainfall onset and the discharge was so high that
the trapezoidal weir shifted. Accordingly, no water discharge data were collected,
although sediment samples were still taken throughout the event. Actually, the mean
rainfall intensity for the first 20 min (0.09 mm/min) was very low in this storm.
Though the antecedent soil moisture was very high, it is unlikely to have produced
such a high flow if no water had been trapped by damming during the previous
storm. Two rainfall peaks produced four discharge peaks in Tunnel 3, may have
been caused by runoff generation zonation as well. To further evaluate the impact
of instability on tunnel flow discharge, the authors compared it with overland flow
discharge. Owing to the limited number of events that were monitored on the surface
plots during our monitoring periods, the authors have used the data collected by the
Shanxi Institute of Soil and Water Conservation during the period of 1963–1968
from the Yangdaogou sub-basin (SISWC 1982). Three surface plots with areas of
400,1855 and 4167 m
2 were selected in the sub-basin comprising upper slope, lower
slope, and combined slope, respectively. It was found that a good correlation exists
between runoff discharge and rainfall with an intensity of more than 0.2 mm/min
for all three surface plots. For tunnel systems, such a good correlation could only
be found for Tunnel 3. However, if we disregard those events affected by tunnel
instability, identified above, correlation coefficients are improved for all tunnels,
especially for Tunnel 1 and 4. The still poor correlation for Tunnel 6 may be ascribed
to unidentified tunnel instabilities within this large and complex system.