The experimental basin

The experimental basin location and its stream channel network is presented in Fig. 1.

Soil erosion has cut a dense stream channel network (52 km/km2) that reaches into the bedrock and develops rill channels on the slope divides.

The soil is 1 m think on average, coarse-grained genetically produced by weathering of loose ‘eluvium’ and ‘colluvium’ materials from volcanic Tertiary rocks, which forms the uniform impermeable bedrock underlying the catchment. The bedrock outcrops over about 1 % of the basin area and frequently forms the channel beds (Bellino and Maraga, 1995). Vegetation consists of shrubs and chestnut trees covering 77 % of the basin area, except on the slope divides.

Mean annual temperature is 11.2’C, ranging between 21.0°C (July) and 1.2”C (Jan- uary) from summer to winter (Bellino and Di Nunzio, 1996).

Figure 1: Location map of Valle della Gallina experimental basin (1.08 km2) and their catchment with stream channel network drawn from aerial photographs. The elevation maximum and minimum are indicated as well as the position of the hydrometric and sedimentologic station (arrow).

Rainfall distribution is in accordance with the continental Piedmont pattern in Mediter- ranean conditions (Caroni, 1979). Mean annual rainfall is 1266 mm, with a first maximum in the spring (April-May) and a second maximum in the autumn. In the summer (July) rainstorms are frequent.

Runoff showed a large range of peak flows from 0.005 m3se1 (18 July 1989) to 6.440 m3s-l (19 September 1995), the mean discharge being 0.02 m3sm1.

The mean annual discharge of 0.02 m3ss1 corresponds to the limit exceeding 20 % of frequencies in runoff patterns investigated by annual and monthly discharge duration analysis, computed at intervals of 5 minutes, to detect also short-duration floods. April and October show the highest discharges rates, with a frequency of 10 % in discharge values higher than 0.15 rn’s-l, whereas in the months of June and September the discharge rates higher than 0.15 m3sv1 are less than 2.5 % frequent.

Main channel has a mean slope of 0.06 and the difference in height is 192 m between the higher basin elevation (522 m) and the measuring station at the basin outlet.

The debris delivery is mainly driven by the bed load processes of sand-gravel sized material created by disintegration of the bedrock and introduced into the channel network from the channel heads and directly from their banks. Suspended load is irrelevant in comparison with the bed load sediment transport.

The grain-size distribution of the bed load normally ranges from 0.006 to 256 mm, with a modal size in the 16-32 mm class. Some debris in the 256-512 mm class is transported by occasional bigger flows. The specific weight of the transported loose material varies between 1.4 to 1.6, depending on the grain-size distribution.

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Physiographic and hydrologic characteristics of the Valle della Gallina experimental basin are summarised in Table 1.

The results presented here were obtained from 1982 to 1997 by measuring the annual sediment supply, water runoff and by simultaneous monitoring of water discharge and bed load transport.

Table 1: Physiographic and hydrologic characteristics of the Valle della Gallina experi- mental basin (Alps, North-western Italy). efficiency in collecting material deposited there.

The sediment flowing through the terminal segment is defined for each transport event by the volume of sediments caught in the trap and the grain sizes of the material.

3.1 Topographic survey

The terminal reach is equipped with bench marks nailed in the bedrock of the stream banks, in the crest of the wall and in the edges of the debris trap, which are regularly used to survey the height of the gravel bed and the material caught in the trap. The surveys were taken with the aid of instrumentation enabling the measurements to be automatically recorded on memory modules. The investigations involving detailed topographic surveys of the bed surface start from 1984 (Fig. 2).

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Figure 2: Main channel instrumented reach at the outlet of the basin. Talweg profiles and bed forms ( elevation isolines) are presented before and after bed load transport (modified from Anselmo and Maraga, 1985).

Channel bed surfaces (300 m2) are periodically surveyed at an average of 300 points for each survey, the points being more closely spaced where the bed forms are present, so as to obtain a very accurate picture of local geometrical characteristics.

The bench marks are always surveyed from two station bases; in this manner it is possible to obtain a precision of 0.15 m at the horizontal position and of 0.2 m at the vertical position.

Field data are numerically and graphically processed with software specifically devel- oped to define the topographic features of the channel bed surface by plotting the contour lines or generating the regular terrain grid. Any variations in the channel bed morphology are quantified by their horizontal and vertical changes.

The morphological data on the bed load carried away from the terminal main channel reach were taken by the comparison of the detailed topographic surveys made before and after the following sediment transport events, which occurred after 1984: between November 1986 and July 1987, with 33 m3 of sediment delivery and a maximum peak flow of 5.25 m3se1; between July 1987 and September 1987, with 35 m3 and a maximum peak of 2.93 m3s-‘; between September 1987 and June 1991, with 139 m3 and a maximum peak of 5.79 m3s- ‘; between June 1991 and October 1991, with 28 m3 and a maximum peak of 1.83 m3sm1.

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1000 800 600 400 200

hours

Figure 3: Debris bed load pulses enhanced by geophone related to selected peak flows of 13 October (A 0.53 m3sWf) and 25 November 1990 (B, 0.39 m3s-‘) (volumes trapped in A unknown, in B 0.29 m3). Above: the hydrographs. Below: the sediment transfer drawn by the microseismic impulses number per minute exceeding predetermined threshold values of ground oscillation velocity at the same occasions of above hydrographs.

140 130 4

40 30 20 10 3

00

Figure 4: Annual water and sediment runoff relationships. Except one point the debris bed load is produced above a line starting at 560 m3x104 of water volume, with sediment runoff volumes from 0 to 12 m3.

3.2 Geophones experience hydrologic station. Another seismometer was placed in the stream-bed debris, buried to a depth of 0.3 m immediately upstream of the concrete weir of the sedimentation basin.

Individual sensors were connected to the recorder. Seismometer position in the gravel bed just upstream the concrete wall of sedimentation basin obtained the better response on the sediment runoff (Govi et al., 1993).

During seismic monitoring, seven floods were recorded at peak flows from 0.14 to 0.86 m3s-l and five of them occurred with sediment delivery.

Data were analysed to determine the average velocity of ground oscillations induced by water and sediment runoff and to count the number of impulses per minute exceeding threshold values of velocity starting from the step of average oscillation velocity at peak flow of 0.14 m3s-l.

Data processed from the sensor embedded at the concrete weir proved able to distin- guish water discharges with and without bed load transport.

The velocities approximate the synchronous hydrographs, while the number of impulses per minute display singularities in comparison with the synchronous hydrographs of the floods with sediment transport.

In fact, microseismic anomalies related to sediment delivering floods strengthen the view that the bed load process occurs by pulses during a flood flow. Two or three mi- discharge limits in water volumes calculation.

Sediment and water volumes therefore indicate similarity on the linear evolutionary trend, indicating a divergent pattern over the years (Fig. 5).

Over sixteen years of observation the cumulative sediment-water volume pattern indi- cate a deficiency of sediment supply from the catchment. This indication suggest at long time physical conditions not able to promote soil production nor hydrographic evolution, then exhaustion of sediment delivery.

Topographic data collected from 1984 to 1991 revealed vertical changes in the movable bed elevation, which are produced by alternating processes of cutting and filling. The surveys demonstrate a mechanism of sediment delivery characterised by a wave form

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propagation in the channel, about ten times longer than the width of the channel bed.

Cut and fill at the bed results in a temporary bed load process involving a thickness of 0.1-0.3 m of the gravel bed.

Finally, seismic monitoring data recorded in 1990 reveal that bed load transport changes in intensity during the same flood flow event. Particularly, two or three minor delivery pulses are detected after the peak flow.

14001

Figure 5: Annual water and sediment runoff trends shown by the cumulative curves. The debris bed load curve shows some steps of increase independently of water volumes.