Intensive post-event survey of a flash flood in Western Slovenia:

observation strategy and lessons learned

Lorenzo Marchi1, Marco Borga2, Emanuele Preciso1, Marco Sangati2, Eric Gaume3, Valerie Bain3,

Guy Delrieu4, Laurent Bonnifait4, Nejc Pogančik5

1CNR IRPI, Corso Stati Uniti 4, I-35127 Padova, Italy

2Department of Land and Agroforest Environment, University of Padova, Legnaro, Italy 3Ecole Nationale des Ponts et Chaussées (ENPC), Marne la Vallée, France

4Laboratoire d’étude des Transferts en Hydrologie et Environnement (LTHE), Grenoble, France 5 Environmental Agency of the Republic of Slovenia (EARS), Ljubljana, Slovenia

This is the author’s version of a work published by Wiley. Changes resulting from the publishing process, including peer review, editing, corrections, structural formatting and other quality control mechanisms, may not be reflected in this document. Changes have been made to this work since it was submitted for publication. A definitive version was subsequently published in Hydrological Processes, vol. 23, issue 26, pp. 3761-3770, 2009, DOI: 10.1002/hyp.7542, under the title: “Comprehensive post-event survey of a flash flood in Western Slovenia: observation strategy and lessons learned”.

Corresponding author: Lorenzo Marchi

Abstract:

The limited extent of the areas affected by flash floods and strong spatial and temporal gradients of

rainfall cause conventional measurement networks of rain and discharges to be inadequate for an

effective observation of these events. The documentation of flash floods urges post-event survey

strategies encompassing accurate radar rainfall estimation, field observations of the geomorphic

processes associated to the flood, indirect reconstruction of peak discharges and interviews of

eyewitnesses. This paper describes the methods applied and the results achieved in the survey of a

flash flood that occurred on 18th September 2007 in the Selška Sora watershed (Western Slovenia).

The documentation of this flash flood reveals high peak flood discharges and unit peak discharges

and a complex flood response. Observations on geomorphic activity show widespread erosion and

debris flows, although generally involving relatively small debris volumes. Relevant amounts of

large wood were mobilised, with a large variability of the intensity of supply processes. The field

study of the Selška Sora flash flood outlines the importance of geomorphological surveys as a

prerequisite for flood discharge reconstruction in mountainous watersheds with active sediment

dynamics, the basic role of the accounts of eyewitnesses of the flood, and the need of quality-

controlled weather radar, which permit coupling field observations with rainfall-runoff modelling.

INTRODUCTION

The effects of a flash flood are potentially dramatic and can be measured both in terms of lost

human lives and property damages totalling millions or even billions of Euro. In its 2000 policy

statement, the American Meteorological Society (AMS, 2000) acknowledges flash floods as one of

nature’s worst killers. Many recent examples in Europe and in the Mediterranean basin underscore

this claim. In a flash flood in Algiers on November 9-10, 2001, 740 people were killed and 0.5

billion Euro in property was destroyed in less than one day (Tripoli et al., 2005). Also in central

Europe, the flood of July 20, 1998 in Mala Svinka (Slovakia), which lasted less than one hour,

killed 47 people.

The rapid hydrologic response is a characterising feature of flash floods, with water levels reaching

a peak within less than one hour to a few hours after the onset of the generating rain event (Collier,

2007; Borga et al., 2008; Gaume et al., 2009). The time dimension of the flash flood response is

linked, on the one hand, to the size of the concerned catchments, which is generally less than a few

hundred square kilometres and, on the other hand, to the activation of rapid runoff processes,

generally surface runoff, that becomes the prevailing transfer processes.

Space and time scales of occurrence of flash floods, combined with the space and time scales of

conventional measurement networks of rain and discharges, make these events particularly difficult

to observe. In an investigation concerning 23 major flash flood events occurred in Europe in the

last twenty years, only about one third of the cases were properly documented by means of

conventional stage measurements (Marchi et al., in preparation). In most of the cases, the rivers

impacted by the flash floods were either ungauged or the streamgauge structures were damaged by

the event. Furthermore, even when reliable stage observations are available, the estimation of the

corresponding discharges is made highly uncertain by the often large extrapolation of the rating

curve, by errors induced by the presence of unsteady flow conditions and by changes in the cross

section geometry (Di Baldassarre and Montanari, 2009). Similar considerations apply to the rainfall

estimations, with space and time scale requirements that are generally beyond the sampling

potential offered by even dense raingauge networks.

Flash floods therefore place the ungauged basin problem under rather extreme conditions. Process

understanding is required for flash flood forecasting, since the dominant processes of runoff

generation may change with the increase of storm severity, and therefore the understanding based

on analysis of moderate flood events may be questioned when applied to forecast the response to

extreme storms. However, process understanding and learning from past events is made difficult by

the observational difficulties characterising flash floods.

Post-event estimations of peak discharges have been extensively conducted in various countries and

provided the ground for developing databases of maximum floods, establishing regional peak

discharges envelope curves and enhanced understanding on regional behaviour of extreme events

(Pardé, 1961; Perry, 2000; Costa, 1987; O’Connor and Costa, 2003; Costa and Jarrett, 2008; Gaume

et al., 2009).

The availability of good quality weather radar observations, providing estimates of rainfall

distribution at the flash flood space and time scales (Borga et al., 2007), may greatly enhance the

information content of post-event surveys. A methodology for post-event survey of flash floods

capable to capitalise on the availability of weather radar observations has been proposed and tested

by Gaume and collaborators (Gaume et al., 2004; Gaume, 2006; Gaume and Borga, 2008; Borga et

al., 2008). The methodology is based on three concepts: i) use of radar rainfall estimates for

analysis of the rainfall forcing; ii) post event survey of geomorphic effects and peak discharges; iii)

eye-witnesses interviews to establish the event dynamics. The methodology combines collecting

observations and using rainfall-runoff models to check the internal consistency of the information

gathered by means of the survey. This approach provides an integral cycle of observations and

modelling which enables the determination of the major elements of uncertainty in either the peak

flood and rainfall observations and the rainfall-runoff model.

The methodology has been applied to the survey of an extreme flash flood that occurred in Slovenia

on 18th September 2007. This event motivated the organisation and execution of a survey by an

international team of experts in the context of the European Project HYDRATE

(http://www.hydrate.tesaf.unipd.it), funded by the EU Commission, Sixth Framework Programme.

Twenty-one researchers from various countries (UK, Italy, France, Greece, Romania, Spain and

Slovakia) with different skills (meteorologists, hydrologists, hydraulic engineers,

geomorphologists) and experience (from postgraduate students to senior scientists), and local

specialists from the Environmental Agency of Slovenia (EARS; http://www.arso.gov.si/en/) were

involved in the organisation and execution of the survey.

This paper provides a discussion of the lessons learned from the survey, with the objective to

document problems and issues that were revealed, and to provide recommendations to guide future

investigations when surveying extraordinary flash floods in mountainous terrain.

THE FLASH FLOOD EVENT AND THE SURVEY METHODOLOGY

On 18 September 2007, a Mesoscale Convective System (MCS) affected North-western Slovenia,

starting at 5:00 CET (Central European Time) and lasting for approximately 12 hours. The MCS

triggered extreme rainfall causing several severe flash floods with six casualties. Out of 210 local

communities in Slovenia, 60 reported flood damages, and the total flood-related damage was later

estimated at 210 million Euro (Mikoš et al., 2009). One of the most severely affected watersheds

was that of the Selška Sora River (Western Slovenia, 147 km2) (Figure 1), with approximately 260

mm of basin-averaged storm rainfall accumulation in 6 hours. Hourly rainfall estimates were up to

150 mm/hr, with return time exceeding 200 years (Mikoš et al., 2009).

The watershed is characterised by steep slopes and narrow valleys and ranges in elevation from 450

to 1680 m a.s.l.. The river network is characterised by high-gradient channels (slopes greater than

0.01). Average annual precipitation ranges between 1700 and 2300 mm across the basin, and it is

strongly influenced by the orography. The geology consists of a variety of highly fractured and

fissured rock types, including limestone, schist and shale, with karst geology in the Northern

portion of the basin (Češnijca sub-basin, Fig. 5). The land use is dominated by forests, with

grassland and grazing in the floodplain. In the last century, at least twelve remarkable flood events

are recorded (Komac, 2008); the 2007 flood was by far the most important event in the series.

The Selška Sora River survey was executed in the period November 11-16, 2007, eight weeks after

the flood. Early field visits were organised, with identification of the impacted areas, collection of

rainfall data (including radar observations), GIS data (including DEM, land use and geology), and

compilation of data from previous events. In general, post-flood investigations should start

immediately after the event, before possible obliteration of field evidences due to restoration works.

However, some activities are better carried out after some time has elapsed from flood occurrence.

As an example, it is often not practicable to interview eyewitnesses just after the event: people may

be fully occupied by recovery actions and unwilling to provide accounts to outsiders. Moreover, the

preparation of post-flood surveys requires a sound organisation of logistics, which may require a

few weeks. Gaume and Borga (2008) describe in detail the steps of post-flash flood surveys.

Regarding the spatial scale of post-flood analysis, peak discharge estimation from field surveys and

interviews to eyewitnesses should obviously concentrate on the most impacted areas. However,

radar rainfall estimation and consistency check of rainfall and discharge data should cover a broader

region. In particular, discharge data, which include complete flood hydrographs, are often available

at downstream streamgauge stations and spatial scales such that the peak discharge can be reliably

estimated by streamgauge data. Although these data may not be representative of flood response in

the areas impacted by the most intense rainfall, they are very useful for checking the overall amount

of runoff volumes.

The various steps of the survey are described in the following sections. The KLEM hydrological

model platform (Borga et al., 2007) was used to model the runoff response as a check of the

consistency among the various observations.

Radar rainfall estimation

The study area is covered by a modern volume-scanning Doppler C-band radar located in Lisca at

about 100 km from the impacted watershed (Figure 1). The rainfall measurement system includes a

network of 47 raingauges (with 14 devices operating at the hourly time step and the remaining ones

at daily tine step). As such, this example is quite representative of the post-event analysis context

with radar data coming from a rather remote system, a relatively dense network of daily raingauges

and few sub-daily raingauge time series. The overall strategy consists therefore in using the

raingauge data to control/assess the radar data processing prior to using the radar rainfall estimates

as input in rainfall-runoff models, rather than in ‘merging’ the two data source (Borga et al., 2002).

In the context of the post-event survey, processed radar rainfall estimates are useful not only to

identify the watershed portions most impacted by the event, but also to drive simulations of the

flash flood response by means of rainfall-runoff models. This may help to scrutinise results from

the field survey, enabling identification of erroneous peak and/or time-to-peak estimates which

cannot be sustained by storm rainfall.

The case of the western Slovenian flash flood showed clearly the impact of attenuation due to

extreme rainfall rates. Bouilloud et al. (2009) reported on the use of the Mountain Reference

Technique to correct for the effects of attenuation. The storm total precipitation (Figure 2) exhibits a

gradient of rainfall accumulations from 150 mm in the southern portion of the basin to more than

350 mm in the upstream portions of the Selška Sora river, reflecting the impact of topography and

the shape of the convective bands on the rainfall distribution.

Hydrogeomorphic and large wood survey

The survey of hydrogeomorphic response, in addition to document the hazardous geomorphic

processes often associated to flash floods, includes the recognition of the type of flow processes in

minor streams, enabling distinction between water floods and debris flows. This is an important

prerequisite for the correct evaluation of peak flows and should be carried out before conducting

indirect discharge estimates. Debris flows are non-Newtonian (Costa, 1988; Iverson, 2003), and

using Newtonian-based relations for debris flows leads to gross overestimates of discharges (Jarrett,

1994). However, the identification of flow processes in mountain streams is usually straightforward

because debris flows leave deposits distinctively different from those of water floods (Costa, 1988;

Pierson, 2005).

The September 18, 2007 flash flood event caused widespread geomorphological consequences.

Several tributaries were affected by debris flows, and massive wood accumulations were found at

the basin outlet. Slope failures (Figure 3a) and erosion rills were evident on many of the valley

sides, evidencing that the soils in places were saturated and throughflow through the soil layers was

significant. Due to the combination of small drainage area, stepped shallow gradient, and large

roughness elements, headwater streams typically transport little sediment or coarse wood debris by

fluvial processes. Consequently, headwaters act as sediment reservoirs even for rather long periods

(decades). The accumulated sediment and wood may be episodically evacuated by debris flows and

transported to larger channels. The debris flows generally stopped at the confluence with the

receiving streams, feeding alluvial debris fans in the main river valley. This led to high erosion

throughout upstream reaches in the headwters and significant deposition in the downstream reaches

at the confluence with the main river. The flood caused remarkable changes in the morphology of

the main channel and in the principal tributaries, leading to channel-bed widening (Figure 3b), local

avulsions and overbank deposition.

One of the largest debris flows, which occurred in the Črni Patok, located near the hamlet of

Podrost (Figure 5) was surveyed in detail (Bateman et al., 2007). The size of drainage basin affected

by the debris flow is around 0.9 km2; the estimation of the total debris flow volume leads to

7000 m3, with a yield rate of 7800 m3/km2. The ratio of debris flow volume to channel length leads

to a value of 7.5 m3/m. Both these values indicate an event of moderate magnitude in the context of

debris-flows in the eastern Alps (Marchi and D’Agostino, 2004; Marchi et al., 2009, Mikoš et al.,

2006). In general, the severity of debris flows appears to be limited if compared to the high intensity

of the rainfall forcing. This can probably be referred to the limited debris availability in debris-flow

prone channels, and to the absence of major point source areas of sediment, such as large deep-

seated landslides.

Observations were conducted on large wood transported during the flood, aiming to identify

sources and recruitment processes. Debris flows in small streams, fall of trees into the channel as a

consequence of bank erosion and channel avulsion in forested valley floors were documented and

mapped. This shows that transfer of large wood elements from the headwaters to the main channel

was mostly associated to debris flows and not to the liquid discharges, which were too small to

enable transport of long, newly recruited logs. Other important wood sources were represented by

forested floodplains in reaches where aggradation led to channel avulsion.

An aspect of geomorphic observations of particular importance when dealing with flash floods in

mountainous watersheds is the survey of the remnants of temporary dams formed during the event,

whose breaching and collapse release flood surge which can attain extremely high flow discharges,

unrelated to the rainfall-runoff dynamics. Landslides, debris-flow deposits from tributaries and

accumulation of large wood may be responsible, together with morphological settings of the

channel (e.g. narrow cross sections) and man-made structures (typically under-sized bridges) for the

formation of these temporary dams. In the Davča valley, evidences of temporary blockage of the

channel in correspondence with narrow channel cross-sections or under-sized bridges were

observed. In one surveyed cross-section of the Davča River, channel clogging was caused by a

combination of high water stage, intense coarse sediment transport, and floating large wood.

Backwater effects due to channel obstruction were confirmed by witnesses’ reports and the analysis

of the high water marks. Breaching of the temporarily formed dam has resulted in a flow surge,

which may have significantly increased the peak discharge immediately downstream. The water

volumes stored upstream temporary natural blockages are generally limited. The surges induced by

the breaching of the blockages are rapidly attenuated during their propagation downstream and have

only a local significant effect on the discharges.

Flood response survey

River cross-section surveys were carried out in order to compute the flood peak discharge at

multiple locations along the river network. The surveys were located in reaches where the flow

process was identified as water flood according to the results of the hydrogeomorphic survey.

Several papers (Jarret, 1990; Williams and Costa, 1988; Gaume, 2006; Costa and Jarrett, 2008)

describe and critically analyse indirect discharge estimation methods in flood studies. In this study,

the slope-conveyance method was used for discharge estimation: this requires the survey of one

cross sections, identification of high-water marks, estimation of flow roughness and computation of

the discharge by means of the one-dimensional Manning-Strickler equation. The survey of multiple

cross sections enables to assess representativity of a single cross section for the reach. The

estimation of roughness coefficient was based on particle size, channel geometry and slope,

vegetation, using a proceure that results in reproducible Manning’s n-values, even though with quite

large uncertainties. Typically, the selected Manning roughness values ranged from 0.1 for narrow,

highly vegetated headwater tributaries, to 0.04 for the less-vegetated and large main streams,

leading to average flow velocity values varying generally between 2 and 4 m/s. Identification of

high-water marks is a learned skill that requires experience and thought. The major assumptions

with the slope-conveyance method are that: (1) the cross section is representative of the reach and

(2) the channel slope, the water-surface slope, and the energy slope are all parallel. In spite of the

limitations of these assumptions, the slope-conveyance method is used when multiple flood peak

surveys are required in a limited period of time. The method is particularly useful in high-gradient

channels where a major uncertainty in indirect discharge estimation is represented by unstable

channel behaviour caused by scour and fill, which strongly limits the availability of adequate field

sites. Uncertainty from the various sources of errors was taken into account (Gaume, 2006), and

upper and lower bounds were given for each estimate.

Additional documentation on flood characteristics was gathered from photographs and movies

recorded during the flood. This material was analysed to assess flood water velocity, thus

permitting comparison with computations of flow velocity obtained through the application of

hydraulic models. In several cases, the velocity of floating objects was estimated approximately by

timing the passage of the object, which was visible on the video, between landmarks of know

distance apart (which were surveyed later in the field). Estimates of water velocity can be gathered

by analysing photographs and movies and measuring the super-elevation of water upstream of

obstacles (Figure 4). Application of the Bernoulli equation enables calculation of velocity from

head difference.

Peak discharges were estimated for 22 cross-sections (Figure 5, Table 1). The estimated peak

discharge just downstream of Železniki (cross-section no. 22) is in the range 260-320 m3/s. The

estimate for the Davča (cross-section no. 15), which is the largest tributary of Selška Sora upstream

of Železniki, is in the range 80-120 m3/s. To enable the comparison of flood magnitudes on the

different sub-catchments, the unit peak discharges (ratio of peak discharge to watershed area) have

been mapped in Figure 5. The figure shows that there is a general agreement between the

distribution of the total rainfall and the peak discharge. The highest unit peak discharges are located

in the area of largest total rainfall, which also correspond to the areas affected by the highest

instantaneous rainfall intensities, whereas the lowest unit peak discharges correspond to areas with

lower accumulated rainfall. However, some differences can be noted between sub-basins with

similar rainfall amounts, which may point out to heterogeneity in the catchment properties or errors

in the observations. This is the case of the Češnjica sub-basin (cross-section no. 21), with a

relativley low runoff response with respect to high rainfall amount. This may be explained by the

karst geology of this portion of the catchment, which may have had an impact on the runoff

response. The impact of karst geology on response to short and intense storm events have been

reported by Delrieu et al. (2005), among others. Some tributaries (Danjarska grapa, cross-section

no. 7, Štajnpoh, cross-section no. 8) experienced relatively high unit peak discharge, compared to

other tributaries with similar or even higher rainfall amount (Zadnja Sora, cross-sections no. 2 and

3). The survey of the Danjarska grapa and the Štajnpoh sections showed high bed load transport

(Danjarska) and almost hyperconcentrated flow (Štajnpoh). This may have led to overestimation of

flood discharge, especially for Štajnpoh.

Application of the rainfall-runoff model showed that all the unit peak discharges are consistent

with the rainfall amount, but one (unnamed stream, cross section no. 6). Further observations

revealed that a debris flows dominated the response of the catchment, even though the debris flow

did not flowed through the surveyed cross-section. The debris flow depositional area is located

immediately upstream the surveyed cross-section. It is speculated here that the stopping of the

debris flow may have generated the sudden release of a sharp wave of muddy water, which could

have produced the high water marks used in the survey. Such a phenomenon has been documented,

by means of video recordings and hydrograph measurements, in a small catchment instrumented for

debris-flow monitoring in the Italian Alps (Marchi et al., 2004).

Witnesses interviews

Witnesses were interviewed in order to identify the timing of onset and end of rainfall, the presence

of hail, the time of rise, peak and fall of the flood, and the nature of the flow process (either water

flood or debris flow). This helped to reconstruct the dynamics of the event: start of the rising, time

of the peak, duration of the recession. Witnesses provided also useful information on the rainfall-

runoff processes (observation of surface runoff, origin of the runoff, extent of soil saturation, etc.,

and the local flow characteristics (presence of woody debris, approximated surface water flow

velocities, blockages formed during the flood, timing and influence of the collapse of bridges or

dykes, etc.).

Concerning the dynamics of the event, of great interest were the accounts provided by coach

drivers, which were able to provide accurate timing information for different places in the

catchment, before stopping due to road closure. The comparison of the accounts showed a

remarkable agreement in the timing of the flood event (particularly about the start of the major

storm burst and of the peak flood). According to this information, the shape of the flood hydrograph

was very peaky. A sharp rise of the water levels was noted, with the duration of the most intense

phase of the flood being around 1 hour, and the flood peak occurring around 11.30 CET in the

upstream tributaries of Selšcica Sora, and around 12.30 CET in Železniki. The accounts confirmed

that the peak flows from the major upstream tributaries occurred almost at the same time at their

confluence upstream Železniki. This is consistent with outcomes from the application of a rainfall-

runoff model to the study basin.

The consistency of the accounts suggests that the information collected by means of interviews may

be considered as reliable, with errors in the order of 15-20 minutes. A similar accuracy of the timing

given by some carefully selected eyewitnesses has been reported in past studies (Gaume et al.,

2004, Gaume, 2006). The assistance from the EARS personnel was extremely helpful, both to

translate the questions and the answers and to make the people understanding the aims of the

interviews and of the general survey.

SUMMARY AND LESSONS LEARNED

The documentation of the flash flood that impacted the Selška Sora watershed at Železniki on 18th

September 2007 outlines a complex flood response and provides information for process

interpretation. The event was triggered by a precipitation amount of around 260 mm mean areal

accumulation over the 147 km2catchment area, with a peak discharge at the outlet of approximately

300 m3s-1. Although the highest values of unit peak discharges (> 10 m3/s/km2) are influenced by

intense sediment transport, values in the range of 5 – 7 m3/s/km2 have been observed in several sub-

catchments with area up to approximately 25 km2. These observations confirm the severity of the

studied event, which is placed among the most extreme floods observed in Europe (Gaume et al.,

2009) at the considered spatial scales.

Post flood field estimation of peak discharges revealed strong contrasts in flood response between

different parts of the catchment, owing to spatial variability in both rainfall amount and intensity

and in catchment characteristics. Field observations on geomorphic activity on slopes and in minor

tributaries shown widespread erosion and debris flows although generally involving rather small

sediment volumes. Large amounts of large wood were mobilised within the basin, with a large

variability of the intensity of supply processes.

Concerning the organisation and execution of the post-event survey, a number of lessons can be

recognised.

1. The need to execute a geomorphological survey as a pre-requisite for the peak discharge

estimation, particularly in mountainous areas with abundant sediment supply. The

geomorphological survey is essential to elucidate the nature of the flow and to uncover the

potential for flood surges related to the collapse of temporary dams formed during the event.

Even in difficult settings, the accurate field survey may provide clues on the impact that

these flood surges may have on indirect peak discharge in downstream river reaches. The

survey of the large wood entrained and transported during flash floods provides elements for

refining the interpretation of peak discharge values reconstructed from flood marks.

2. The importance of the information provided with eyewitnesses interview. The systematic

collection of these accounts is capable to generate a reliable picture of the even dynamics

and may provide essential information to check the quality of the observations.

3. The availability of quality-controlled weather radar observations enable the coupling of field

collection of observations with the rainfall-runoff modelling of the same experimental

setting. This triggers an integral cycle of observation-and-modelling, which may be iterated

to provide more accurate understanding of the discrepancies between observations and

model outcomes.

Moreover, the post-event documentation campaign demonstrated the high educational potential of

the post-flood survey concept. The survey gives to graduate and post-graduate students in

hydrology and geosciences the opportunity to improve their field skills in collecting the

interdisciplinary observations necessary to characterize flash flood processes and responses. The

integral cycle of observations and modelling enables the participants in the surveys to grasp the

uncertainty sources in the model input (the rainfall forcing), in the model structure, and in the flood

response observations. This provides insights on both limitations of data and model outcomes and

on the gain generated by interpreting data and models in the field settings where hydrologic

problems arise and where decisions must be made. This is an essential ingredient in the education

and training of next-generation hydrologists.

ACKNOWLEDGEMENTS

This work was supported by the European Community’s Sixth Framework Programme through the

grant to the STREP Project HYDRATE, Contract GOCE 037024. We thank the Environmental

Agency of Slovenia (EARS) and the Municipality of Železniki for the technical and logistic support

to the organisation, execution and discussion of the survey.

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Table I. Peak discharge estimates from field survey. Section reference numbers are shown on map in Figure 5

Ref. Section Watershed

area

Peak discharge

(m3/s)

Unit peak discharge (m3/s/km²)

1 Selška Sora upstream Zadnja Sora inflow 1.9 6 - 8 3.2 – 4.2

2 Zadnja Sora upstream Rovtarjev grapa inflow 2.3 7 - 12 3.0 – 5.2

3 Rovtarjev grapa 2.6 10 - 15 3.8 – 5.8

4 Selška Sora downstream Zadnja Sora inflow 9.0 50 - 70 5.6 – 7.8

5 Selška Sora downstream Globoka inflow 24.7 85 - 125 3.4 – 5.1

6 Anonymous tributary between Globoka and Danjarska grapa inflows * 0.2 5 25.0

7 Danjarska grapa ** 9.2 80 - 120 8.7 – 13.0

8 Štajnpoh ** 3.9 30 - 50 7.7 – 12.8

9 Selška Sora downstream Danjarska grapa inflow 40.7 125 - 155 3.1 – 3.8

10 Selška Sora upstream Zali Log 44.8 140 - 200 3.1 – 4.5

11 Selška Sora between Zali Log and Davca inflow 46.8 170 - 230 3.6 – 4.9

12 Davča upstream reach 9.8 50 - 60 5.1 – 6.1

13 Davča upstream Mustrova grapa inflow 21.4 140 - 170 6.5 – 7.9

14 Muštrova grapa 4.2 6 - 9 1.4 – 2.2

15 Davča upstream the confluence with Selška Sora 31.8 80 - 120 2.5 – 3.8

16 Selška Sora upstream Zadnja Smoleva inflow 80.4 290 - 350 3.6 – 4.4

17 Zadnja Smoleva 7.5 14-18 1.9 – 2.4

18 Selška Sora upstream Prednja Smoleva inflow 95.5 330 - 430 3.5 – 4.5

19 Prednja Smoleva 5.7 7 - 10 1.2 - 1.7

20 Dašnica in Železniki 10.8 25 - 40 2.3 – 3.7

21 Češnjica in Železniki 25.8 35 - 50 1.4 – 1.9

22 Selška Sora downstream Železniki 147.2 260 - 320 1.8 – 2.2 * Influenced by debris flow: see text ** Possible overestimates due to high sediment concentration

Figure 1. a) Location of the Selška Sora watershed and the Lisca weather radar; b) digital terrain model of the Selška Sora watershed

Figure 2. Storm total rainfall (mm) for the September 18, 2007 event. Spatial resolution: 1 km

Figure 3. Geomorphic effects of the flood: a) shallow landslide on a soil-mantled slope; b) channel-

bed widening and damage to road in the Davča valley (from http://www.davca.si)

Figure 4. Pictures extracted form a film taken during the flood at the entrance of the town of

Selezniki. Distance between x1 and x2 = 21 meters. Super-elevation gives V=3m/s and film gives

V=3m/s

Figure 5. Selška Sora watershed with location of discharge estimates, interviews to eyewitnesses

and central values of unit peak discharge