Water structure quality

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The structural quality of a water body, even water structure quality is a measure with which the natural reach of the carrying riverbed , including the surrounding flood area ( Aue ) is evaluated. For this purpose, parameters are collected that describe the existing water structure. Structures include the nature of the bank (e.g. vegetation, barriers), the shape of the river bed (e.g. banks, deep / shallow water zones), current and substrate differences or the course of the river bed (e.g. meandering , winding, straightened course).

The assessment of the structural quality is only defined for flowing waters . The term structural quality assessment comes from the 1980s / 1990s. Today we often only speak of structural evaluation.

Origin background

The background to the structural quality assessment was the observation that when the water quality of flowing waters is improved, in particular the "saprobial" water quality , with which the pollution with organic, oxygen-consuming substances is measured on the basis of the community , the abundant original community does not regenerate in all cases how this had been assumed. This directs attention to the morphological changes in the waters. Rivers in the cultural landscape have usually been morphologically strongly changed, in which they z. B. have been straightened. Most of the time, the course was determined, the floor was lowered, and the floor and bank were often fixed by means of technical sheeting. For decades, the effects of this morphological degradation had been overshadowed by the effects of saprobial pollution and thus hidden. The desire arose to have an evaluation method available for measuring these morphological changes, which is analogous to the long-established saprobic system for measuring organic pollution. Structural quality classes should therefore be defined which correspond to the seven levels of stress in the saprobic system (the quality classes).

Structural quality assessment or structural assessment

In a defined stretch of water (mapping / evaluation section), parameters in and on the water are collected, which describe the existing structures and the dynamic processes. These survey data are compared to the unaffected natural state. The deviation from the unaffected natural state is a measure of the naturalness of the considered section of water. Based on the saprobic system, the water structure is also usually rated on a seven-point scale. The following applies: The state of nature without human influence represents the best possible state of water, a completely straightened and fortified body of water the worst possible. The seven levels of structural assessment are shown in maps using the color code introduced in the saprobic system:

Hellbach : completely straightened course with remote, fortified banks = quality class VII
  • Quality class I (dark blue): unchanged stretches of water (near-natural),
  • Quality class II (light blue): Slightly changed sections of water (partially natural),
  • Quality class III (dark green): moderately modified waterway sections (moderately impaired),
  • Quality class IV (light green): clearly changed sections of water (clearly impaired),
  • Quality class V (yellow): heavily modified water sections (noticeably impaired),
  • Quality class VI (orange): very heavily modified water sections (heavily damaged),
  • Quality class VII (red): completely modified water sections (excessively damaged).

In some cases, for example in the current river structure quality mapping in Mecklenburg-Western Pomerania, a rating scale with only five levels is used.

Evaluation procedure in Germany

The survey and assessment of the water body structure takes place in individual mapping / assessment sections, the length of which is based on the width of the water body. In the Federal Republic of Germany, three evaluation methods are used:

  1. The overview procedure ( LAWA, state working group on water )
  2. The procedure for medium-sized to large rivers over 10 m wide ( LUA NRW )
  3. The procedure for small to medium-sized rivers 1 to 10 m wide (LAWA, Länderarbeitsgemeinschaft Wasser), z. Sometimes referred to as LAWA fine process.

The latter has been modified in some federal states (e.g. in Baden-Württemberg), i. In other words, the scope of the survey has been adjusted, but also leads to a seven-stage assessment as in the original LAWA detailed procedure. In addition to the three methods mentioned, there are other mapping methods that take into account the peculiarities of shipping routes (mapping method for federal waterways of the BfG ), or rely on a more detailed assessment of the floodplains (assessment of the condition of the floodplains by the BfN ).

The following application differences must be observed in the above three methods: The overview method is a pure desk method that relies on aerial photo evaluation and evaluates existing data and information. It was v. a. created to give the federal states a quick overview of the state of the rivers. Due to the procedure, not all factors can be reliably assessed in the survey. So is z. B. a tree-lined body of water with continuous riparian trees can hardly be assessed without further data. Intact lines (e.g. meandering) and the tree population can hide the fact that the river bed has largely changed (e.g. due to the intermittent drainage from a retention basin). The status survey and assessment is carried out in the overview procedure for the subject areas of river bed dynamics and floodplain dynamics:

The following indicator parameters are used for the river bed dynamics:

To assess the floodplain dynamics, the following are collected:

  • Outdoor use,
  • Characterization of the banks,
  • Flood protection structures,
  • Expansiveness.

The other two methods mentioned above (cf. 2. and 3.), on the other hand, rely on an on-site inspection in order to obtain the most comprehensive possible picture of the state of the water. They differ in the application v. a. regarding the size of the water. The procedure for large flowing waters (LUA NRW) takes into account the particularities of rivers in the parameter set, while the procedure for small to medium-sized bodies of water (LAWA) is tailored to the particularities of the order of magnitude of 1 to 10 m width. Both have in common that the condition survey and assessment is carried out for six sub-areas, the so-called main parameters (HP):

  • HP 1: run development
  • HP 2: longitudinal profile
  • HP 3: cross profile
  • HP 4: sole structure
  • HP 5: bank structure
  • HP 6: Water environment

Within these main parameters, a different number of so-called individual parameters is recorded and assessed, depending on the size of the water body. A description of some of the main parameters as well as the valley shape from the LAWA fine process can be found below. Detailed information on the individual procedures can be found in the procedure descriptions (see web links).

Description of some survey sizes

The LAWA method for small to medium-sized rivers defines various main parameters, each of which is subdivided by several individual parameters.

Valley shape

Elbe: Meander valley waters

Depending on the geological environment ( solid rock , loose sediments) and the ecoregion (lowlands, mountainous regions, Alps), different valley shapes develop. These in turn have an impact on the mobility of the water and the available sediments (sediment transport of the water). A distinction is made depending on the width of the valley, the presence of a valley floor and the ecoregion Kerbtal, Sohlenkerbtal, Sohlental, Meander, Muldental, Auental and flatland waters.

Main parameters run development

This parameter contains the four individual parameters of running curvature , curvature erosion , longitudinal banks and special running structures . There are inherently unique physical quantities that cause and influence the curvature of most rivers, which is associated with a certain amount of erosion. The curvature of the course depends largely on the slope of the valley. Above all, flowing waters in the flatlands have a meandering course. Since humans have significantly intervened in the running behavior of the water bodies in order to use the floodplains, many bodies of water were straightened. The curvature of the course and curvature erosion are therefore important properties for the description of near-natural waters, as they result in a lengthening of the course and a lower risk of flooding. Typical features of curvature erosion are steep to vertical impact banks and flat slip banks.

Longitudinal banks are a sign of a balanced bed load balance in the water. They arise from the grain size dependent sedimentation in the water. The longitudinal banks include bank, curvature, island and estuary banks. Driftwood blockages, fall trees, island formations, run forks, widening and narrowing of the run are assigned to the individual parameter special run structures . These show that the water body has a high morphological development capacity and that the water body is not hindered by anthropogenic expansion.

Main parameter cross profile

This will u. a. described by the individual parameters of profile depth , width erosion and width variance . By nature, many bodies of water have a relatively flat and wide river bed, which results in a low flood capacity or rapid overflow into the surrounding area. Anthropogenic changes in the water body lead to a higher profile depth, which i. d. Usually causes increased deep erosion. A natural antagonist of deep erosion is latitude erosion, which does not necessarily occur in parallel. If this is present, it counteracts deep erosion. Width variance is the term used to describe the recurring change between wide, shallow and narrow, deep sections of water. A high variance in width favors the damping of flood waves.

Main parameters of the sole structure

This parameter includes a. the individual parameters substrate diversity and special sole structures . The sole substrate is dependent on different currents in a body of water. A selection is made according to grain size, with coarse material being deposited at high flow velocities and fine material being deposited in calmer sections of water. If there are many different zones with different grain sizes due to flow and structure in a stretch of water, then one speaks of a high substrate diversity. However, this is fundamentally dependent on which materials (sediments) are actually available in the catchment area of ​​the water body. The individual parameters special bed structures include pools , deep troughs, still water pools, root areas, shallow water, rapids and cascades. These are mostly caused by the accumulation or erosion of bed material. Flow differences are maintained or reinforced by these shaped elements.

See also

Web links

Individual evidence

  1. ↑ River structural quality mapping in Mecklenburg-Western Pomerania, published by the State Office for the Environment, Nature Conservation and Geology Mecklenburg-Western Pomerania, Güstrow 2011, p. 60 , PDF document.
  2. ↑ State Working Group on Water, Water Structure Quality Mapping in the Federal Republic of Germany - Procedures for small and medium-sized rivers, Schwerin, 2000.