The influence of green roofs on reducing storm runoff

. The present research studies the way to reduce storm runoff in urban conditions by means of installing green roofs when projecting, constructing, re-constructing buildings, as well as performing major repairs. Sewer systems of rapidly growing modern cities are often unable to cope with the load. This is why today environmentally friendly and efficient reduction of storm runoff is one of the major challenges in the field of urban development. Despite the fact that foreign research on the topic and the experience of applying the green roof technologies abroad proved considerable reduction of runoff, the question of feasibility of this method in the Russian reality remains open. It is important to take into account not only water-retaining capacity of green roofs per se, but also meteorological features of the area of the planned construction, production technology methods and used building materials. Until now, little attention has been paid to this subject in the Russian Federation, which has consequently delayed the introduction of modern green roof technology in our country, as well as the development and approval of the necessary regulatory framework. The article provides theoretical proof of the efficiency of green roofs in reducing storm and snowmelt runoff in the climate conditions of Central Russia based on statistical and empirical data. The results obtained may prove valuable for further development of green technologies in construction and for ensuring environmental safety in our country.


Introduction
Each year big cities across the country face localised temporary flooding of streets and areas around buildings caused, among other things, by clogging or malfunctioning of local storm sewer systems [1].For a certain time (until being eliminated by utility service providers) they disrupt normal functioning of transport and recreation infrastructure, which, in turn, hinders comfortable life of citizens.Furthermore, water flows from localised flooding capture street garbage, dust, de-icing agents and other urban wastes, which not only damages city's storm sewer system, but also negatively affects urban environmental safety [2].One of the reasons for temporary flooding of urban areas is connected with the fact that municipal authorities in our country do not frequently make decisions to rebuild or modernise local surface drainage systems.The sealing of streets with such impermeable pavements as asphalt, paving stones and rubber pavements as well as the general increase in building area and density change the amount of hydraulic volume collected from such watertight pavements [3].At the same time, outdated storm sewer systems of the cities do not cope with the significantly increased hydraulic load, which leads to temporary overflow of manholes of the diverter system [4].
The negative consequences of flooding include the pollution of the ecosystems of the water bodies surrounding cities and the erosion of their shorelines.The reason for this is that rain and melt water containing harmful suspended or dissolved substances are often discharged directly into nearby water bodies bypassing the sewage disposal plant.According to the 2018 Izhevsk study, the average concentration of harmful substances in storm runoff in Russia significantly exceeds the maximum permissible values.For instance, COD (chemical oxygen demand) and BOD (biological oxygen demand) indicators associated with the concentration of organic pollution of rain and melt water by microorganisms are on average 3-4 times above the allowed level in Russia [5].Such an excessive biological pollution of storm runoff negatively affects the ecological condition of water bodies.Invasive microorganisms rearrange the ecosystems of rivers, lakes and reservoirs, displacing the local established flora and fauna, which subsequently leads to the degradation and destruction of aqua ecosystems [6].
The above-stated facts indicate that nowadays it is extremely important to solve the problem of frequent flooding of urban residential areas in our country.Therefore, the search for effective method of dealing with excessive runoff, as well as preventing clogging and malfunctioning of storm sewer systems in urbanised areas, is more relevant and urgent than ever.One of the efficient ways to prevent temporary flooding can be roof greening as part of major repairs, construction and reconstruction.The introduction of federal or municipal programmes for installation of green roofs or state incentives and support for voluntary roof greening can probably reduce localised flooding and provide positive change in the environmental safety of cities and adjacent areas [7][8][9].

Methods
To prove the efficiency of green roofs in reducing storm runoff and the load on the sewer system in the cities with the climate conditions typical of Central Russia, the study analyses foreign statistical research and processes information on their successful applications.Furthermore, a system of retaining rain and melt water in the structural layers of the green roof is simulated within the framework of the study.The following stage of the research establishes maximum retention of storm runoff in litres per square meter of the surface.Then, the reduction of storm runoff by means of green roofs is calculated for a number of major cities in Central Russia based on collected archival meteorological data on the absolute maximum daily precipitation for these cities.As a result, conclusions are made on the efficiency of using green roofs to reduce storm runoff in the climate conditions of Central Russia.

Results
In many countries around the world, green roofs have established themselves as a workable way of dealing with excessive storm runoff and preventing street flooding.In Western and Northern European countries, particularly Germany, the UK and Sweden, mandatory greening of roofs of residential and office buildings is regulated and incentivised by municipal authorities [10,11].Such municipal programmes have been used in foreign countries, among other things, to reduce storm runoff and the load on urban sewers.In the German city of Bonn, for example, the local authorities have been levying an additional tax on excessive storm runoff since 1996.However, citizens can significantly reduce the tax by installing a green roof on their house.The reduction of the sum depends directly on the green roof's capability to retain rain and melt water runoff.It is worth mentioning that the funds received from the tax are invested by the authorities of Bonn in the maintenance of the municipal sewage disposal plant, which also contributes to the stable and smooth operation of the storm sewer system.
Extensive green roofs have been the most commonly constructed green roofs in Europe and North America.The schematic arrangement of the structural layers of an extensive green roof is provided in figure 1.A statistical analysis of studies by foreign scientists has led to the conclusion that, in a large city, even the use of light weight (extensive) green roofs can help to achieve up to 50% retention of storm runoff on average [12].Such roofs have substrate layer thickness of 50+ mm (less in some cases, providing sufficient project justification) and are usually planted with low-growing, small plants, such as Sedum succulents.During use, a number of invasive plant species are added, which in about 5 years completely renew the composition of the green roof vegetation, competing with and even displacing the original ones.Such local "natural selection" of plant species on the roof allows the greenery cover to adapt to urban flora and facilitates the creation of a sustainable and self-replicating greenery.Usually, extensive green roofs are not used and are accessed only for repairs or to observe the vegetative layer.This type of roof greening has gained popularity due to the fact that it is the most economical type to build or renovate and is easy to maintain [13].In order to simulate probable retention of precipitation by green roofs, it is important to analyse how the green roof structure absorbs and retains water.For instance, the retention of a considerable amount of water by a green roof is conditioned by the structural layers of its covering.Significant quantities of precipitation in such structures for a variety of reasons accumulate in the vegetation cover, substrate and drainage layers [14].Some water is absorbed by the plants, primarily for their nutrition and as a result of the evapotranspiration process [15].The amount of the water absorbed by vegetation increases over time, since sedum covering is replaced after a few years of use by invasive plant species with higher water consumption.
The water flows then accumulate and are retained in the substrate layer, which consists of a nutrient substrate mixture.Substrate mixtures mostly have spongy, non-cohesive texture capable of high water absorption.On average, one cubic metre of substrate for extensive landscaping can hold up to 200 litres of water, which is 10 litres per square metre of 50 mm layer.Lightweight aggregate should comprise about 80% of the total volume of the substrate mixture.On average, one cubic metre of substrate for extensive greening can retain up to 200 litres of water, which is 10 litres per square metre of 50 mm layer.The remaining volume of rain and melt water then enters the drainage layer, often consisting of drainage and retention elements.Drainage elements enable accumulation of the amount of water required for the nutrition of selected plants, which is regulated by the thickness of the layer.At the same time, drainage elements can effectively divert water flows to the receiving waters in case of excessive water saturation.According to the manufacturer, the 25 mm thick drainage and retention element can absorb and retain up to 16 litres of water per square metre.In addition, when such drainage and retention elements are used, a protective water retention mat should be provided.Such fibrous mat is necessary to redistribute the point load from the cells of the drainage and retention element.The mat is generally made of felt or its synthetic counterpart.The water absorption of such mats can be up to 3 litres per square metre.A graph of the retained water distribution exclusive of vegetative layer evapotranspiration and surface evaporation is provided in figure 2.

Fig. 2. Graph of the retained water distribution in the structural layers of an extensive green roof (substrate layer thickness if 50 mm).
Based on the above data, it can be concluded that water absorption of extensive roof greening can be up to 29 litres (1), in other words, the simulated greening system can retain up to 29 mm of atmospheric precipitation.Where V ev -the amount of water retained due to evapotranspiration and evaporation; V sub -the amount of water absorption by the substrate layer; V dr -the amount of water absorption by the drainage layer, V mat -water retention mat.
In order to assess the efficiency of a simulated extensive green roof, it is essential to estimate its water retention at the calculated daily precipitation depth (see formula 2), the average daily table value, the average maximum and at the absolute maximum precipitation depth.Where H av -average daily precipitation depth; F -normed average deviation of precipitation; С v -coefficient of variation.These input data are presented in table 1.The use of average maximum data only for the summer period is due to the fact that the predominant precipitation depth in absolute daily maximums in the observed cities is recorded between April and October.Based on the data obtained (see table 1) table 2 was compiled that shows water retention efficiency of an extensive green roof for the cities of Central Russia (3)(4)(5)(6).Where V rettotal water absorption by all green roof layers; H cal -calculated daily precipitation depth; H av -average daily precipitation depth; H av.max -precipitation depth at average daily maximum; Н max -precipitation depth at absolute daily maximum.Thus, with the calculated value of precipitation depth, the water absorption of the green covering was over 100%.The calculated value is based on 63% probability, i.e. at one excess per year.With the average daily precipitation depth in accordance with the the table data, the water retention by a square metre of a green roof averages 86.3 %, which shows the effectiveness of green roofs in reducing storm runoff in Central Russia.However, the calculation of the average daily value does not take into account sudden abnormal effects of rainfall or increased precipitation.At the absolute maximum of recorded meteorological observations, the water retention rate of extensive roof greening was 34.7 %.At the average daily precipitation maximum, the reduction in storm runoff could be 51.5 % on average under the conditions adopted in the study.

Discussion
Based on the above, green roofs are able to store and retain a significant amount of rain and melt water due to their design features, as well as the natural circulation of water as part of vegetation lifecycle.This subsequently leads to a reduction in overall storm runoff and hence a reduction in the load on the city's sewer system.An important factor is that this method of dealing with excessive storm runoff has so far proven successful in a number of foreign countries with climate conditions similar to those of Central Russia.

Fig. 1 .
Fig. 1.Main structural layers of an extensive green roof.

Table 1 .
Input data to assess the efficiency of storm runoff containment with extensive green roofs in the major cities of Central Russia.

Table 2 .
Possible storm runoff retention efficiency of extensive green roofs in the major cities of Central Russia at average daily precipitation depth, at average daily maximum and absolute daily maximum.