Impervious Soil Coverage (Sealing of Soil Surface) 2021

Introduction

The impervious coverage of natural soils by construction and non-porous materials has a multitude of negative effects on the ecosystem, the microclimate in the city and on the human habitat. The effects of impervious coverage are primarily noticeable in cities and metropolitan areas, where a high proportion of the total area is impervious.

Definition

Impervious coverage is defined as the paving (or covering) of the soil with solid materials. Impervious areas may be divided into built-up impervious areas, i.e. buildings of all kinds; and non-built-up impervious areas, i.e. roads, parking lots, paved walkways, etc.

In addition to building complexes and surfaces completely, imperviously paved with asphalt or concrete, more porous paving types are also considered impervious, although these often have very different ecological qualities. Such coverings as grass trellis stones or paving stones with wide seams still permit reduced plant growth, are partially permeable to water, and provide for a considerably more favourable microclimate.

The occurring types of pavement of non-built-up impervious areas have been grouped into four pavement classes, including their different effects on the ecosystem (cf. Table 1).

Tab. 1: Overview of the pavement classes of non-built-up impervious areas

• Pavement class 1:
  Asphalt, concrete, paving stones with joint sealer or concrete substructure, synthetic surface materials

  extreme effects
  on ecosystem

• Pavement class 2:
  Artificial stone and plates (edge length > 8 cm), concrete-stone composites, clinker, medium and large-sized paving stones

  high effects
  on ecosystem

• Pavement class 3:
  Small-stone and mosaic paving (edge length < 8 cm)

  medium effects
  on ecosystem

• Pavement class 4:
  Grass trellis stones, water-bound pavement (e.g. ash, gravel or tamped ground), gravel lawn

  low effects
  on ecosystem

The Effects of Impervious Coverage on the Ecosystem and the Urban Climate

The complete impervious coverage of the soil causes the irreversible loss of natural soil functions.

Impervious coverage and densification moreover strongly impair the water storage capacity of the soil that is available to plants, as well as its buffering and filtration capacity. Obstructing the water and oxygen supply causes most organisms in the soil to die. Since no more water can seep down, the pollutants entering the soil via air and precipitation are no longer retained in the soil and are partially washed into surface waters. The formation of new groundwater is prevented or reduced.

Impervious coverage of the soil also results in changes in the water balance and the water composition, due to the loss of evaporation and seepage surfaces for precipitation. The rainwater runoff from impervious areas, heavily polluted by tire abrasion, dust, dog excrement, etc., is passed by via the sewage system either directly into the tributaries or into sewage-treatment plants (cf. Environmental Atlas Map “Management of Rain and Waste Water” (02.09)). The drainage of polluted rainwater after heavy rainfalls often causes the eutrophication of water bodies.

The complete impervious coverage of the soil consequently causes the complete loss of all flora and fauna, but even partial impervious coverage always means habitat loss. Biotopes are fragmented or isolated, while sensitive species are crowded out in favour of more adaptable species.

Pervious soils strongly impact the urban climate due to their capacity to store and supply water to plants. On the one hand, evaporation caused by plants and by the (pervious) soil surface leads to cooling of the air. On the other hand, the air is heated by the high heat-storage capacity of buildings, impervious areas and asphalt streets, contributing to the development of a specific urban climate. Especially in summertime, night-time cooling is reduced (cf. Fig. 1 and the Environmental Atlas Map “Nocturnal Cooling Rate between 10:00 p.m. and 04:00 a.m.” (04.10.4)).

Fig. 1: Temperature Range above Various Surfaces

At the same time, the atmospheric humidity is also reduced, due to the lack of vegetation areas and the evaporation that emanates from them. This may lead to the occurrence of extreme values that may impair human well-being considerably. In this context, pervious areas such as parks play a major role. Positive climatic effects on human well-being may already be observed for parks as small as 1 ha in size. Vegetation areas also have an effect on dust and pollutant levels in the air, as their large leaf surfaces enable them to bind dust particles and other air pollutants.

The effects of impervious coverage on the Berlin urban climate are described in detail in various maps in the Climate section.

In addition to the effects on the ecosystem described above, the degree of impervious coverage of an urban area also has an immediate effect on the human habitat. A high degree of impervious coverage is usually associated with a disparity of open space per capita. Long rows of buildings, frequently interrupted only by asphalt or concrete surfaces, may have a depressing, monotonous effect on residents. It becomes impossible to truly experience nature, such as the change of the seasons, in the immediate residential environment. Turning to the outskirts for recreation, however, generates traffic with equally negative impacts on the environment.

Data on impervious soil coverage is regularly used in important contexts for environmental protection, as well as urban and landscape planning. For example, this data is used and processed particularly often in various models (urban climate, water balance), or evaluation methods, e.g. in soil protection. The documentation regarding the damage to nature and landscape due to impervious soil coverage is also of great importance. Policy-makers, too, are increasingly requesting frequent, regular updates on impervious coverage data, in order to monitor and measure the success of environmental or urban-planning strategies (cf. Reusswig et al. 2016, SenStadtUm 2016a, SenUVK 2019, AfS 2021).

Methods to Reduce Impervious Coverage and New Land Consumption

As part of the National Sustainability Strategy, the indicator “new land consumption” was developed for empirical studies and risk assessments on the impact of land consumption. New land consumption is calculated from the daily increase in built-up and traffic areas. This is not equal to the impervious area, since these also include areas which are only slightly impervious (gardens in residential areas, allotments, park facilities or green median strips on roads, etc.). The Federal Government’s goal is to limit the average new land consumption to less than 30 ha per day by 2030. By 2050, the aim is to establish a closed-loop land-use regime, in which the total land consumption is reduced to zero, in net terms, by means of land recycling and a reduction in new land consumption (cf. Federal Statistical Office 2021). In the years from 2004 to 2019, the daily new land consumption decreased continuously from 131 ha to 45 ha. In the year 2020, however, it rose again to 58 ha per day. The 30-ha target originally set by the Federal government for 2020 was thus not met, despite a deceleration in new land consumption (Federal Environment Agency 2020).

At the UN Summit in New York in September 2015, the 2030 Agenda for Sustainable Development was adopted. The German Sustainability Strategy 2021 (The Federal Government 2021), based on the same, identifies the special need for sustainable soil protection as a resource against the background of increasing urbanisation and climate change (Sustainable Development Goal – SDG 15). Working towards the 2030 Agenda goal of a land and soil degradation neutral world, emphasises the crucial role that soil plays in biodiversity, climate protection and as carbon storage (The Federal Government 2021).

In a conurbation area such as Berlin, however, the increase in built-up and traffic areas as described above does not represent land consumption accurately (cf. Environmental Atlas Map “Open-Space Development”). For this reason, “soil imperviousness”, which is indicator No 15.1 of seventeen core indicators for monitoring sustainability, was chosen to document the economical use of soil as a resource from a sustainability perspective. Environmental Atlas data is also used to illustrate the development of the degree of imperviousness over time (Statistical Office for Berlin-Brandenburg 2021).

In 2005, the federal and regional-state soil conservation board (LABO) appointed a group of federal and state experts to develop a suitable assessment procedure for ascertaining the impervious soil coverage at the state level, in order to expand the sustainability indicator “new land consumption for residential and traffic areas” to include the component of impervious coverage.

The results of the experts’ group were incorporated into the Environmental-Economic Accounting of the German States (EEAL), and documented in the report “Indikator Versiegelung”, (the indicator “Impervious Coverage”) (Frie & Hensel 2007).

LABO reports to the Environmental Minister’s Conferences (UMK) document the development of new land consumption and the measures taken by the federal German states to reduce new land consumption (LABO 2020). According to the Environmental-Economic Accounting of the German States (EEAL), impervious areas in Germany accounted for 6.3 % of the total area in 2019. This corresponds to an impervious area of 2.2 million ha. In Berlin, impervious areas covered 34.7 % (approx. 30,894 ha) of the total area in 2019 (Statistical Offices of the German States 2021).

See also the Excursus Impervious Coverage Data 2005, 2011, 2016 and 2021, which compares the present results with those of the “Indikator Versiegelung” (“Impervious Coverage” indicator), by the Environmental-Economic Accounting of the German States (EEAL, Statistical Offices of the German States 2021).

The reduction of land consumption, which is a goal of the National Sustainability Strategy, shall be achieved through land-saving and compact construction, densification of inner-city areas, concentration of infrastructure, provision of compensation areas, and the rehabilitation of areas that are no longer used (space recycling). By improving the quality of the living environment in residential areas, dense housing in the city shall be reestablished as an alternative to the “home in the country” (The Federal Government 2021). Germany’s states and municipalities shall realise these targets in the context of their spatial and construction planning. In March 2017, the amendment of the Building Code was adopted in the course of adapting urban planning laws to the Strategic Environmental Assessment (SEA) Amendment Directive. A key focus is the introduction of a new area category “urban area”, which is to enable a higher level of density of mixed uses while reducing land consumption (German Bundestag 2017). With the federal soil protection legislation coming into effect, soil including all its functions was protected by uniform nationwide regulations for the very first time. The Soil Protection Law, however, does not provide any direct legal claim, in regard to use changes or land consumption by construction. Generally, the requirement of impervious-coverage removal under Section 5 of the Federal Soil Protection Law (BBodSchG 2021) represents an instrument for areas that are permanently no longer in use to be rendered pervious once again, and thus regain their natural soil functions, as per Section 2 Para. 2 BBodSchG (Oerder 1999). However, since costs and reasonableness are taken into account as additional criteria, this regulation has not proven its worth in practice.

Furthermore, the Federal Building Code (BauGB 2022) and, in some cases, nature conservation legislation comprise pertinent regulations relating to soil as a subject to be protected. These include the “soil protection clause” according to Section 1a Para. 2 BauGB and the requirement of deconstruction and removal of impervious coverage according to Section 179 BauGB. With the introduction of the Strategic Environmental Assessment in 2004, recording and describing soil functions became mandatory. As a result, measures to avoid, reduce and compensate for adverse effects must be described and assessed, and planning alternatives must be identified. According to Section 1 Para. 3 No. 2 of the Federal Nature Conservation Act (BNatSchG 2022), soils must be preserved in such a way that they can fulfil their functions within the ecosystem. According to Section 15 Para. 1 and Para. 2 BNatSchG, inevitable interventions in nature and landscape are to be balanced out or offset.

With an increase in imperviously covered areas in the State of Berlin, there should be a qualitative assessment as to which soils are in use or need of extra protection. The Environmental Atlas Map “Planning Advice for Soil Protection” (01.13) and the summarised “Leitbild und Maßnahmenkatalog für den vorsorgenden Bodenschutz” (Catalogue of models and measures for precautionary soil protection in Berlin, SenStadt UVK 2021, only in German) serve this purpose.

In its Environmental Report of 2020, the German Advisory Council on the Environment demands, among other things, that a duty of review be introduced to determine whether the impervious cover of an existing area may be removed, whenever a new area is rendered impervious (SRU 2020). In this context, a project is highlighted that systematically records areas that could have their impervious cover potentially removed developed by the State of Berlin. As part of nature conservation compensation measures, these areas could then be made available permanently to the ecosystem after the removal of their impervious cover and the restoration of their soil functions (Environmental Atlas Map “Potential for the Removal of Impervious Soil Coverage (Soil De-sealing)” (01.16), SenSW 2021b).

Financial incentives for private individuals may also contribute to the reduction of impervious coverage. On January 1, 2000, Berlin introduced separate billing for rainwater and waste water charges. Splitting of the charges was introduced based on a ruling by the Federal Administrative Court (resolution of June 12, 1972) and the Higher Administrative Court of Lüneburg (rulings of June 14, 1968 and of April 10, 1980). It requires municipalities, in which the costs for the discharge of rainwater exceeds 15 % of the total costs of waste water disposal, to account for the charges separately. Thus, the rainwater charge is no longer proportionally linked to the waste water charge. It is calculated according to the proportion of the impervious area of the property from which rainwater is discharged into the sewage system (BWB 1998). Since 2000, owners have therefore been eager to keep impervious areas on their property as small as possible to save on sewage costs. Since the NWFreiV (Precipitation Water Exemption Ordinance, Ordinance on the exemption from permits for the safe percolation of precipitation water – NWFreiV of 24 August 2001) came into force, it has been possible to obtain a partial or complete exemption from the rainwater charge without a permit by allowing rainwater to percolate on one’s own property (SenStadt 2001). Since 2018, construction projects have been required to take precautionary action to ensure rainwater management on the property in accordance with Section 29 (1) BauGB. If it is impossible to prevent the discharge of rainwater into the sewage system or directly into a body of water, the volume shall be reduced (BReWa-BE, SenUVK 2021).