Retention potential

Water retention in the catchment area

In order to be able to operate preventive flood protection, it is necessary to depart from local-level consideration and to look at the overall problem on a regional, national and if necessary international scale. The question of flood risk of a particular area is partly determined directly by the way in which the body of water is handled going up the river. As the basic rule on a river is that “everyone downstream is upstream”, responsible handling of every kilometre of river is required and co-operation is essential when designing protective measures.

To lessen a flood, retention of rainwater in the catchment area is of vital importance. The following influencing factors affect the water level by varying degrees and can moreover contribute substantially to environmental protection through targeted support.

 

 

Vegetation

Vegetation cover must be viewed positively in three respects concerning the reduction of rapid surface runoff.

1.    Interception
Organic material on the ground creates a buffer effect, which results in slowing of the flow. Depending on their surface area, plants can absorb up to 50% of the rainwater for a certain period and release it to the ground only with a delay. A small proportion of this intercepted water is returned directly to the atmosphere through evaporation, although the majority is passed to the ground/surface after a short resting period. Sealing of the ground by silting as a result of rainfall that is too heavy is therefore prevented and the storage capacity of the ground is increased.

2.    Root penetration
As well as the stabilising effect of roots, they also have a substantial effect on the infiltration capacity of a soil. Dense vegetation, for example in woodlands, exhibits a significantly higher infiltration rate than open spaces or even vegetation-free arable areas. Surface runoff and therefore rapid introduction of large volumes of water into the outlet channels are therefore reduced.

3.    Roughness
From a purely mechanical perspective, greater surface roughness causes a reduction in runoff speed. Vegetation creates such roughness and can equally contribute effectively to the retention of surface water.

 

 

Soil

Soils are a frequently underestimated resource, which render an exceptional service for humans (drinking water production and purification, habitat etc.) in addition to their use for agricultural production. With respect to relevance for water retention, soils have huge potential. In combination with vegetation, they form one of the largest reservoirs in the hydrological system. However, their infiltration capacity is dependent on a number of variables, which can differ both spatially and temporally. As well as pore size, the condition of the soil is particularly important:

1.    Frost
In the winter half year, the topsoil might be continuously or temporarily frozen and it can absorb no water in this condition. The rainfall that lands on it is entirely converted into surface runoff and results in a rapid rise in the level of the outlet channel.

2.    Saturation
If the soil is saturated, for example due to a previous rainfall event, then it can absorb no additional quantities of water.

3.    Silting
If the rainfall hits vegetation-free soil unchecked then the pores in the topsoil close within moments and no further water can penetrate through them, despite sufficient capacity in the subsoil.

As soon as the water is able to infiltrate into the soil, it percolates downward due to gravity, might be transported towards outlet channels, or eventually reaches the water table then stands in contact with the body of water as the base flow. Extensive infiltration of rainwater can sometimes delay runoff by days and is desirable in respect of preventive flood protection.

 

 

Sealing

Particularly in urban areas, the proportion of sealed surfaces is extremely high. Impeding the natural infiltration capacity of a surface by sealing (transport routes, building development etc.) directly results in an increase in surface runoff. The retention capacity of the soil, which can create a strong time delay in runoff, is completely cut off. The flowing body of water has to accommodate and discharge huge quantities of water within the shortest time, which can result in a rising water level with floods that cannot be ruled out.

Removal of sealed surfaces, for example by choosing accordingly porous paving stones, a larger proportion of green space and renaturation of smaller streams and river courses can reduce surface runoff and restart the intrinsic retention mechanisms of nature.

 

 

Topography / relief

The relief and geological structure of the near-surface subsoil determine the extent of the catchment area. The stronger the relief intensity of an area, the thinner the soil cover and with it the storage capacity of this hydrological parameter. Surface runoff therefore increases, so rainfall takes less time to reach the outlet channels.

Considered locally, small (mountain) streams can therefore grow into disproportionately large currents, which carry many times their usual volume of water into the valley. At the same time, as well as this line-based damage, extensive soil erosion occurs, which further reduces the already limited soil cover of such areas.

Moving from local to a higher level, rainfall events from which the runoff reaches the river within the shortest time therefore result in flash floods and flood waves, which might overlap with other inflows and can add up. Those who are downstream from larger catchment areas are exposed to such a risk.