Monitoring, Simulation, Prevention and Remediation of Dense Debris Flows II
173
Large wood and flash floods: evidence from the
2007 event in the Davča basin (Slovenia)
F. Comiti1, L. Mao1,3, E. Preciso2, L. Picco1,
L. Marchi2 & M. Borga1
1
Department of Land and Agroforest Environment,
University of Padova, Italy
2
CNR – IRPI, Padova, Italy
3
Department of Geography, University of Hull, UK
Abstract
This paper presents the evidence gathered during a post-flood survey in a
Slovenian mountain basin (Davča basin, drainage area of 32 km2) following the
catastrophic flood of September 2007. Channel avulsion, debris flows and
landslides delivered large volumes of wood into the channel, and massive wood
accumulations were found at the basin outlet. Wood-induced dam-break flows
were therefore hypothesised to be responsible for most of the damages. Field
observations and approximate discharge estimations indicate that the damages
suffered in the Davča basin can be attributed to excessive wood load only to a
limited extent, and that the critical factors were narrow road crossings which
acted as traps for sediment and wood.
Keywords: woody debris, flash floods, mountain rivers, Slovenia.
1
Flood hazards due to natural dams and large wood
Mountain valleys are subject to several natural hazards, which include mass
wasting, debris flow and fluvial processes. However, some of the most
hazardous phenomena occur when the three phenomena interact, as in the case of
temporary damming of stream channels by landslides [6, 7, 12] or debris-flow
deposits which often include large wood (LW) [1, 8, 10]. In fact, the subsequent
breaching and collapse of these dams release destructive dam-break waves which
can attain extremely high flow discharges, unpredictable from a mere
hydrological perspective [9]. As a consequence, such events are thought to
WIT Transactions on Engineering Sciences, Vol 60, © 2008 WIT Press
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174 Monitoring, Simulation, Prevention and Remediation of Dense Debris Flows II
represent the most effective and dramatic geomorphic phenomena in mountain
channels [5]. Indeed, natural dam-break processes – transporting huge amounts
of wood (Fig. 1) – have caused some of the largest damages in several valleys of
the Dolomites (Italian Alps) during the 1966 catastrophic flooding [3, 4]. The
extreme magnitude of floods caused by the failure of natural dams is matched by
a typically low frequency (>50–100 yr), such that these events may be easily
forgotten, thus allowing unwise human occupation of flood-prone corridors. This
paper presents a qualitative analysis of the geomorphic processes triggered by an
intense flash flood occurred in a mountain basin of the Slovenian Alps, which
featured considerable LW transport and deposition. The underlying hypothesis is
that the extreme water stages observed and the damages could be ascribed – at
least partially – to the collapse of temporary dams formed by sediments and LW.
Figure 1:
Huge volumes of wood were transported along the Fiorentina river
and invaded the small town of Caprile (Italian Alps) during the
1966 flood (R.I. >100 yr).
2 The flash flood of September 2007 in Slovenia
On 18th September 2007, there was widespread flash flooding in Slovenia. The
flooding caused seven casualties and about €300 million of damages. The
precipitation featured a convective bands structure and extremely large rainfall
gradients. The consequence of such precipitation pattern was that nearby small
basins responded in sharply different ways. The storm lasted for 10-14 hours,
depending on locations. The cumulated precipitation ranged in most basins
between 250 to 300 mm. The storm led to a distributed flash flooding along the
Sava and Selščica Sora rivers [2, 11].
3
Study area and methods
The study area is the Davča basin (32 km2, Fig. 2), one of the tributaries of the
Selščica Sora river most severely hit by the flash flood (230 mm of cumulated
WIT Transactions on Engineering Sciences, Vol 60, © 2008 WIT Press
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Monitoring, Simulation, Prevention and Remediation of Dense Debris Flows II
Figure 2:
175
The Davča catchment with channel subdivision into 13 reaches
and the cross-sections where peak discharge was estimated. The
location map in the upper left corner shows the area affected by
the flood of September 2007 in the Sora River basin.
1100
1
2
3
4
5 6
7
8
9
10
11 12
13
Elevation (m a.s.l.)
1000
900
800
AGGRADATION
DS30
700
DS50
600
DS70
500
0
2000
4000
6000
8000
10000
12000
14000
Distance (m)
Figure 3:
Longitudinal profile of the Davča Torrent. The solid row indicates
the location of a major debris flow tributary, and the dotted line
arrow the confluence with the Muštrova Grapa. The void symbols
(circle, square, triangle) locate cross-sections where discharge was
estimated (see also Fig. 2).
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176 Monitoring, Simulation, Prevention and Remediation of Dense Debris Flows II
rainfall, estimated recurrence interval about 250 yr [2]). The Davča main channel
was separated into different reaches in order to describe the processes related to
sediment and to large wood (LW) source, transport and deposition which took
place during the flood event. The reaches are marked on the longitudinal profile
(Fig. 3), which was derived from the DEM of the basin (12.5 m grid size), and
therefore can be used for describing the channel bed at a large scale (i.e. > 100
m) only. Peak discharges were estimated from the flood marks, surveying with a
total station 3 cross sections (2 in the Davča Torrent and 1 in its tributary
Mustrova Grapa). For each cross section, the longitudinal bed slope and the
highest flood marks were surveyed, and roughness coefficient was estimated
based on grain size and channel morphology. The peak discharge was then
calculated using the Manning-Strickler equation.
4
Field observations
Evidence of intense sediment transport were found at the headwaters (reach 1)
below a 15-m high waterfall, and on a right tributary between the reaches 1 and 2
(hyper-concentrated flow with abundant LW recruited from the hillslopes).
Overall, deposition of sediment and LW characterized the entire reach 2. Wood
from upstream was trapped by the valley floor forest and by trees fallen into the
channel as a consequence of bank erosion. The wide reach 2 ends at a road
crossing where an eyewitness reported that the small bridge did not experience
clogging during the flood, indicating how sediment and wood transport rates
were not “excessive” here, despite a very high discharge (the flood marks are at
Figure 4:
Steep tributaries incised to bedrock and recruited many LW
elements which were too long to be transferred to the main
channel.
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Monitoring, Simulation, Prevention and Remediation of Dense Debris Flows II
177
an elevation 2.5–3 times the bankfull stage) was already present in this upper
part of the basin. Also in the next downstream reaches (3 and 4), the event did
not cause major instabilities in the channel. As a consequence, these two reaches
lack significant wood and sediment sources adjacent to channel. Furthermore,
several steep right-hand tributaries surely experienced high flow rates, but
sediments and LW delivered to the main channel were of minor influence. Most
of them underwent incision (up to 1 m) reaching the bedrock, with widespread
bank erosion and associated recruitment of large trees, which could not be
transported to the Davča (Fig. 4).
However, a road crossing (probably under-sized) between the reaches 4 and 5
likely caused backwater effects which led the channel to shift towards a forested
floodplain just upstream of the bridge. Here many trees were recruited, further
obstructing the bridge. In fact, there is evidence (lateral depositions, wood jams,
Fig. 5(a)) that the main channel here spilled over the bridge. Downstream of it
(reach 5), several huge LW jams (mostly composed of “fresh” trees) were
trapped by standing trees on the floodplain and channel bends (Fig. 5(b)).
(b)
(a)
Figure 5:
The effect of the bridge between reaches 4-5: (a) deposition and
floodplain erosion upstream of the bridge; (b) high wood
accumulations downstream of the bridge.
These jams are up to 2m high, thus testifying a very high stage for a wide
(15–20 m) cross-section. A possible flow surge issued from the bridge may be
envisaged, but its effect must have been limited in length due to its small height
and stored volume. Downstream of a check-dam (which separates reaches 5 and
6), the channel becomes steeper and then flows within a gorge over large
boulders. Several large trees (beeches and spruces) fallen from the adjacent
slopes are also present, but their transport was prevented by the narrow channel
width. Between the reaches 6 and 7, the slope is lower, and the confinement of
the channel on the right bank is less strict. Here the peak discharge was estimated
to about 55 m3s-1 (DS30 in Figs. 2–3, drainage area 9.8 km2, 5.6 m3s-1km-2).
Reach 7 is morphologically comparable to reach 6 and features a small debris
flow on a right-hand tributary. In the reach 8, the main channel presents a milder
slope and the valley floor widens considerably, with the presence of a grassy
floodplain. A probable hyper-concentrated flow on a left-hand tributary delivered
some sediment to Davča. In contrast, the next channel reach (9), flowing in a
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178 Monitoring, Simulation, Prevention and Remediation of Dense Debris Flows II
gorge, features relatively abundant sediment sources from the right bank (debris
flows, landslides).
The reach ends at a bridge which was clogged by a probable combination of
high water stage, intense coarse sediment transport, and floating LW supplied by
nearby debris flows. Bridge clogging is indicated by upstream deposition of
wood and sediment (Fig. 6). The backwater effect led to channel aggradation,
forcing the stream to flow on the road.
(a)
(b)
Figure 6:
The bridge between reaches 9-10: (a) from downstream right after
the flood; (b) from upstream two months later. Source for a:
http://www.davca.si. The arrow indicates the flood marks.
Figure 7:
The narrow reach 11, featuring bank and bed erosion. Source:
http://www.davca.si.
The road embankment and the bridge obstruction finally breached, probably
causing a flow surge downstream where high-elevation (>2 m) marks were
observed at channel banks. Reach 10 displays deposition of coarse sediment (up
to 0.8-1 m in diameter) and further sediment inputs from the left slope (a small
debris flow channel and several small landslides). Intense channel aggradation
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Monitoring, Simulation, Prevention and Remediation of Dense Debris Flows II
179
occurred where the valley widens, upstream of a bridge which collapsed and a
huge LW jam was trapped upstream.
The steeper, narrower reach downstream (11, Fig. 7) was characterized by
diffuse bank erosion and local bed incision. Peak discharge here (drainage area
21.4 km2) was estimated around 150 m3s-1 (i.e., 7 m3s-1km-2, DS50 in Figs. 2–3).
Downstream, the Davča receives two important right-hand tributaries (Farji
Potok and Muštrova Grapa), which did not show evidence of major channel
changes and sediment transport, their bed still showing stable step-pool
sequences. The peak discharge on the Muštrova Grapa just upstream of the
confluence with the Davča (drainage area 4.2 km2) was evaluated to be 13 m3s-1
(3.1 m3s-1 km-2). Although lower than those observed in the upper Davča basin,
these values indicate a high-magnitude flood. The bed stability of Farji Potok
and Muštrova Grapa may be ascribed to a coarser surface layer and to a more
developed bed structuring compared to the upper Davča.
Just upstream of the confluence with the Muštrova Grapa (reach 12), in
correspondence of a widening of the valley floor, the Davča aggraded
considerably; bridges were buried with sediments and wood and the flow
invaded the road at left side (Fig. 8).
The bridge at the Muštrova-Davča confluence trapped some LW, but did not
collapse because most of the floodwater was flowing on the road (Fig. 9). Part of
the floating LW was then trapped by some trees at the road junction, while some
accumulated against a building which was heavily damaged.
Figure 8:
Channel aggradation and channel avulsion along reach 12, with a
bridge clogged by LW. Source: www.davca.si.
Very coarse clasts (up to 0.8–1 m in diameter) were deposited upstream of the
trees, and LW jams were up to 2 m high, both indicating very high stream power
values. The total LW volume was estimated around 80 m3, but much had been
already carried away during restoration works. The longest wood elements (up to
8 m, roughly corresponding to channel width at the near upstream reaches)
appeared to derive from riparian trees eroded by the flood, and the integrity of
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180 Monitoring, Simulation, Prevention and Remediation of Dense Debris Flows II
(a)
(b)
Figure 9:
The Davča at the confluence with the Muštrova Grapa: (a) the
bridge – not collapsed – from downstream right after the flood;
(b) the LW jams trapped at the junction (November 2007). The
arrow indicates the bridge location. Source for a:
http://www.davca.si.
the bark cover hints to a brief transport distance, possibly just a reach length. In
contrast, many shorter pieces showed clear signs of long residence time within
the channel.
Only a minor volume of LW elements likely reached the channel downstream
of the confluence with the Muštrova Grapa) and could be transported
downstream (reach 13) towards the Selščica Sora. It is interesting to note that the
peak discharge estimated for the Davča just upstream of the confluence with the
Sora is less (120 m3s-1, 3.8 m3s-1 km-2, DS70) than at the upstream cross-section
DS50. Bearing in mind the large degree of uncertainty intrinsic in the flow peak
estimates, their interpretation suggests either flood attenuation in the lower part
of the stream (e.g., due to valley widening downstream of section DS50), a dambreak flow surge related to the obstructed bridge between reaches 9-10, or their
combination.
5
Conclusions
Field observations have shown that large amounts of sediment and LW were
mobilised within the basin, with a large variability of the intensity of supply
processes. However, although a quantitative assessment of sediment and LW
discharged by the Davča Torrent to the Sora River was not possible, the amount
of sediment deposited on the floodplain at the confluence between these two
streams appears to be small in comparison to sediment transport and LW
mobilisation observed within the drainage basin. The short duration of the
highest discharges, reported by local witnesses, probably contributed to limit the
distances of transport, thus reducing sediment delivery to the outlet of the
catchment. The post-flood survey gave evidence that transfer of large wood
elements from the headwaters to the main channel was mostly associated to
debris flows. In contrast, fluvial processes in small, low-order streams are not
able to transport long, newly-recruited logs which remain stored in such
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Monitoring, Simulation, Prevention and Remediation of Dense Debris Flows II
181
channels. Other important wood sources were represented by forested
floodplains in reaches where aggradation led to channel avulsion.
In conclusion, with regard to the considerable damages suffered in the Davča
basin, they can be attributed to the combination of: i) very intense, infrequent
precipitation; ii) locally large sediment and LW inputs, mostly from debris
flows; iii) presence of critical narrow sections (road crossing); and, most
importantly, iv) human occupation of valley areas naturally prone to flooding
and channel aggradation.
Acknowledgements
This study was funded by the European Union’s Sixth Framework Programme
through the STREP Project HYDRATE, Contract GOCE 037024. The authors are
grateful to the Environmental Agency of the Republic of Slovenia (ARSO) for
providing the DEM of the studied basin and for the collaboration in field
observations and in the interviews to eyewitnesses of the flood.
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