Slope models

The goal is to develop models combining surface, lateral and groundwater flow as well as geomorphological slope dynamics (Kirkby 1996). It is clear that this task is especially important for mountains. The general problem is tackled only partially by soil erosion experts (e.g. Hurni 1988) or by geologists or soil scientists dealing with landscape denudation at different scales (Montgomery etal. 1995, Schreier 2002).

During the last decade, more and more GIS models have been applied to describe changes in land use and/or vegetation cover in mountain regions and to speculate about the future impact of climatic or hydrological changes (Leavesley 1994). The most important parameter linked to vegetation change is ET. Only major changes have an impact on slope water and geomorphological stability. For example, the hydrological impact of reforestation of former alpine meadows is often less important than the construction of a network of forest roads on steep trough slopes (Megahan 1981). In the Soelktal, there are very serious examples of the impact of new forest roads on slope stability, but related impact studies do not exist yet.

In the Dischma and Solk valleys, the sequences of soils from the rocky ridges to the valley bottom are comparable. The typical catena (Krause & Peyer 1986) begins with shallow silicate soils on the rocky ridges (5-15 cm), followed by humus-rock soils (10-30 cm) on alpine meadows down to the tree line. Regosoils are very typical for trough slopes. Here, no distinct horizon is detectable, and because of ongoing erosion, weathering products are rare. The thickness of these soils range between 30 and 50 cm. Brown soils and podsols are dominant in the valley bottom and the neighbouring terraces. The distribution of''moderhumus'' is of high interest since this surface cover forms a very good buffer and storage for rainwater. Tensiometer studies at the Stillberg site, Dischma, showed that even during the summer the dominant shrub and meadow vegetation suffer no water shortage (Bednorz etal. 2000). Plants will indicate water stress only when rainfall fails for more than 10 days.

The subsurface sediment and regolith must be considered in models of local slope hydrology. Here again, geomorphological zoning of slopes with different properties is very important. On the lower slopes of schist or gneiss rocks, several meters of weathered material often lie below covers of periglacial scree and deposits of morainic material. In the Tauern, schists often trigger mass movements. In the Dischma, recent and pre-Wuermian mass movements can be found even in harder gneiss rocks (Figure 17.3a and b). As shown in this example, mass movements can be clearly distinguished from high-resolution remotely sensed images. They decisively influence lateral water transport in slopes and for this reason geomorphological maps are important tools for flow routing in hydrological modelling. At the foot of mass movements, a spring horizon is common.

Both the gneiss of the Dischma as well as the schist of the Tauern are prone to mechanical weathering. Talus slopes of weathered material preferentially develop below rock faces in the corrie zone above the present tree line. As indicated for the Braeualm valley (Figure 17.4), long steep trough slopes are often covered by scree. The talus slopes in the corries are only partially covered by short grass and shrubs. These typically develop according to the Richter model (Selby 1993). During phases of intensive precipitation, the surplus of sudden infiltration water causes groundwater outbursts at the lower end of the cones and may even trigger small debris flows. Since debris flows cannot travel far because of breaks in slope on the corrie floor, these cannot feed into the main valley. For modelling purposes, talus slopes are very important since these combine impermeable rock faces with the highly permeable screes below.

The older scree and talus slopes of the trough valley south of St. Nikolai develop differently because of creep processes (e.g. east-facing slope below Mt. Scheiben, Figure 17.5). This slope is covered by large and steep debris lobes moving slowly towards the valley bottom. The lobes are delineated at their upper end by a zone of detachment with a steep rim. Creep slopes are common in the Solk valleys and are a function of the depth of scree material and its hydrological properties. The movement of such debris creeps can continue across thousands of years but in certain zones they can suddenly accelerate to form debris flows or mass movements. Such an event occurred in the summer 1994 in the neighbouring valley of Klein Solk (Hermann & Becker 1998). Signs for this transition can be observed from three small incised channels feeding larger debris fans below the Scheiben. Infiltration of creep slopes is reduced to moderate rates, thereby also reducing surface runoff (Figure 17.5). Zones with more intensive runoff are located at upper rim above scree slopes. Such aspects need consideration in spatially diversified hydrological models and can be integrated accordingly.

Hence, in terms of modelling sediment sources, creeping slopes are ''silent sources'' that can be activated randomly, whereas other sources are more continuously mobilised through avalanche and debris flow activity. An important geomorphological boundary exists between trough slopes and corrie areas (Figure 17.4). This boundary splits the watershed into two parts. The higher part of the basin is hydrologically active, creating high discharge rates on rock faces down to the talus. Discharge is transmitted through the corrie lakes and depressions, but coarse sediments cannot pass. Under present conditions, coarse sediments can only be transferred from the steep trough slopes into the main valley via avalanches and debris flow channels.

In principle, the corrie zone in the Dischma resembles that of the Solk valley. Since the weathering rates of gneiss are lower than those of schist, less scree material is produced on the trough slopes and debris creep is reduced accordingly. On the other hand, the Dischma is still presently glaciated with a more intensively eroded upper valley (de Jong et al. 2002). This has caused a mass movement near Duerrboden mobilised on the west-facing slope (Figure 17.3). However, this material stagnated at the edges of the valley. Intensive erosion only occurs along the southern edge of the mass movement. Here, scree material is carried towards the river by debris flows. Locally, the mass movement is extremely porous. Corresponding to high rates of infiltration over this zone, a distinct spring horizon has formed at the lower end.

The importance of frozen ground for infiltration and groundwater recharge is significant in zones with rock glaciers. These occur in the Dischma at altitudes above 2400 m and in the Solk valleys above 2250 m. Permafrost is another key factor combining local hydrology, meteorology and geomorphology in high alpine valleys and slopes (Schrott 1998). This is well documented for the Dischma (Haeberli & Beniston 1998). At the end of the winter, the distribution of permafrost is as important as temporarily frozen ground. The latter is dominant during cold winters with shallow snow cover and wind drift. Under such conditions, water from snowmelt and first rain will be transported close to the surface as long as subsurface sediments and soils are frozen.

Other geomorphological zones important for hydro-logical models include terminal and ground moraines in corries and at the head of the trough valleys. Morainic

/// Avalanche Area with storage (with or without vegetation) /Mil anc| of coarse sedimen

| I Forest ^ debris cone

* Watershed I I Scree slope

Ridge and corrie

Rock fan

Figure 17.4 Corries and zones of sediment sources, River Braeualmbach south of St. Nikolai (Grosssoelk/Tauern/Austria)

Alluvial

Area of coarse sediment sources for the river

Area of coarse sediment sources for the river

Geomorphological zoning:

^^ J Stadial moraine

Avalanche and debris cone

Hydrological setting:

• Hydrological Response Units (HRU)

Immediate surface runoff

Fast infiltration and subsurface storage + some surface runoff

Fast surface runoff

Moderate surface runoff + moderate infiltration Dominant infiltration and subsurface storage

Figure 17.5 Hydrogeomorphological zoning of the trough slopes in Braeualm valley south of St. Nikolai (Grosssoelk/ Tauern/Austria)

material is normally rich in silt and can even contain some clay. This causes significant differences in infiltration over short distances. Bogs are quite common amongst the Holocene terminal moraines and demonstrate a low potential both for infiltration and for through-flow. According to Parriaux & Nicoud (1990), surface runoff and/or mass movement are dominant in water lain tills and glaciolacustrine beds, whereas deep infiltration only dominates in glaciofluvial deposits, kames terraces and terminal moraines. Shallow infiltration is typical for ground moraines and many lateral moraines.

These examples cover the broad spectrum of options that can be obtained from geomorphological interpretations of slopes for regional hydrology and even more so their potential in complex ecological models, including sediment dynamics.

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