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Author(s): Sandra Collin
Editorial office: FVA, Germany
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Flooding, erosion and stagnant moisture

Handbook: Water in the Forest
"Forest Crisis Management" Advisory Guide

Heavy rains are weather extremes which are difficult to deal with. There are studies and measures to protect the forest and the population. This article describes the buffering effect of forests as well as the problematic conditions of flood protection.

Content

Heavy rain and torrential streams

Prolonged heavy rains cause time and again flooding of entire regions, mudslides, fluvial sediments transport (flood with debris) and infrastructure damages. The communities and forest owners affected by the floods and torrential streams face immense damages. Good planning such as in emergency situations and preventative measures can save lives and livelihoods. A good starting point is the EU Floods Directive, which should be implemented by 2015. In addition, a public and user-friendly platform following the model of the "Natural Hazard Overview & Risk Assessment Austria" should appear on the internet. On this website, current data on potential danger zones can be found.

Hochwasser in Sankt Lorenzen, Steiermark, im März 2012
Fig. 1: Flood in St. Lorenzen, Steiermark, March, 2012. (Photo: BMLFUW, WLV, Gerhard Baumann)

Forests in torrential stream catchment areas usually act as protection forests against flooding and erosion, but they can also send driftwood such as trees and branches, which can cause damage when they get suck on bridges for example.

In 2010, the Forestry Engineering Service for Torrent and Avalanche Control (in German Wildbach- und Lawinenverbauung (WLV)) undertook the job to document the torrential streams. The results showed that 80% of the observed streams were carrying debris. This shows again that the observation and documentation of torrential streams are essential for the development of appropriate protection measures.

Measures when risk is identified

A differentiated and intensive forest management is necessary along the banks of creeks. For example, young and permanent stands provide the opportunity on the one hand to create high pumping effects and prevent erosion, on the other to minimize the risk of wood debris and potential for blockage. Especially “slope reliefs” should only then be carefully used when there is precise knowledge of the site conditions. In particular, the toe of the slope should not be abruptly released because the weight of the stand can increase the rigidity when the slope angle does not exceed a certain limit.

Land movements

Trees secure with their matted roots the top-soil and significantly stabilize the slopes. If there are, however, too few secondary pores (animal and root holes), which act in way as a valve for excess pressure in unforested areas, then the pore pressure can increase and raise the danger of landslides. The buffer potential of forests is limited. It is especially critical when a lot of slope and surface water from higher lying unforested areas get suppressed during heavy rains. In the forest areas erosion is mainly caused by runoff concentration as a result of improper management measures. The buffer effect of forest soils highly depends on the state of the soils. There are large differences also between the pores of forest soils and other utilization practices such as grazing lands. The soil reaction is particularly sensible to mechanical stress such as compaction or grazing. These factors, which often span generations, reduce the share of large pores through which the water from heavy rains can easily flow to deeper layers. "Jammed soles" arise and the water permeability of upper soils is greatly reduced. The danger of landslides also increases.

Mudslides
Fig. 2: Land movements: mudslides are common especially in the mountain regions.
Measures when risk is identified

Catchment barriers at creeks above houses have proved useful for areas prone to mudslides. They catch the debris and the freed up water can drain. In places where such systems have been built, the extent of damages from extreme weather is significantly lower.

Flooding – and then?

Floods are part of the natural development dynamic for forest communities living along water bodies. With regard to climate change and the probability that the snow will melt earlier in the spring, this may lead to extreme floods. For this reason, for example, as part of the Integrated Rhine Program (IRP) large areas are planned as retention areas for flood protection. Since a large part of these areas are used for forestry, the knowledge of tree species composition, site characteristics and flood tolerance of tree species is existential.

Studies have shown that in areas, which flood naturally, the flood duration is the decisive criterion which builds up the flood tolerance of trees. For forests, which flood rarely or have not been flooded in while and thus particularly for the flood retention areas, the overflow water level can be used as the criterion to predict the damages. This means that the flood tolerance is greater affected by the overflow water level than by the duration. The environmental impacts of episodically occurring retention placements can be characterized by relatively short floods with big flooding levels. These short-term floods buffer floods from rivers and slow down the further outflow. The negative impact of floods on trees happens, however, when the oxygenation between the bark and trunk are interrupted. A risk analysis should identify and assess the impact of retention placements, ecological flooding and groundwater rise on forests and trees. Based on the research results, silvicultural opportunities for the further development of forests have been developed.

Measures when risk is identified

Forestry measures and programs based on scientific data already exist in action plans for preventative flood protection in lowland forests as well as wetlands such as moors. Among other things, there is the concept of the Bavarian State Forest Service for a natural management of forests along rivers or the manual on raised bogs renaturalization. All in all, the following recommendations can be used for preventative flood protection:

  • The preservation of site-specific, stable, functionally compatible forest cover is of crucial importance.
  • Avoiding severe large-scale disturbances (storm, beetles) or minimizing their damaging potential is more effective than additional silvicultural “fine-tuning” of already “satisfyingly” functioning stands.
  • Rehabilitation of disturbed, over-aged, thinned-out or reconstruction of not site-specific stands to stable natural growing stocks.
  • Soil protection through the minimizing of soil compaction when harvesting with technical and organic means.
  • Opening–ups inside forests are to be properly planned and roads are to be properly maintained.
  • Wet sites are to be preserved or restored to their near-natural state. Drainage systems are to be no longer made or dismantled.
  • An assessment of the hydraulic effectiveness of measures at different sites can be useful in terms of setting priorities control measures when resources are limited (especially protected forest)

Stagnant moisture on poorly permeable soils

Wetlands and flooding region often act as transition zones between terrestrial ecosystems and the water as a sheer aquatic living space, where the link is predominately established through water. Examples include the coastal/shore areas of seas and running waters (spring, creeks, and rivers) and stagnant waters (lakes, ponds, moors, puddles and swamps).

Stehendes Wasser im Wald
Fig. 3: Stagnant water in the forest.

Short-term flooding does not pose a major problem in these areas. Prolonged or repeated stagnant moisture causes oxygen deficiency and thereby root asphyxia. This manifests itself in yellowing and reduction of leaf cover as well as in roots and trunk rots. The litter decomposition also suffers from lack of oxygen. The stark accumulation of raw humus can lead to peat formation. Soil saturation, also called in general stagnant water, is chemically not comparable with flooding water. It has a poor nutrient-oxygen ratio. This is one cause for phytopathogenic stress (higher mortality rates) in alder for example. Extreme high pH values in soil saturation hinder the alder growth. In anaerobic decomposition, i.e. without the involvement of oxygen, the toxic hydrogen sulphide develops. If a dry phase occurs, these have particularly severe effects on the vegetation as the previously reduced root mass can no longer assure the adequate water supply of trees.

Beech and fir are particularly sensitive to water-logging. Pine and spruce are less so. Alder, poplar, elm, common oak and soft pine trees are resistant to stagnant moisture and temporary flooding.

Measures when risk is identified

Especially in sites vulnerable to soil saturation, the stocking of inappropriate stock should be avoided. When choosing the tree species, attention should be paid to soil saturation and flooding tolerant species as well as their origin. If necessary, hill or contour planting can be chosen.

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