The following considerations refer to road drainage in the form of road gully systems consisting of road gullies and connection sewers. They serve both to absorb surface water draining off paved areas (road gully) and to drain it off into the sewer (see Figure 1). In addition, road gullies fulfil the function of restraining gross solids in the road drain, and by this means, to a certain extent, to perform a preliminary purification of the road drain before it is drained off into the sewer network.
At this stage, there are no specific strategies as well as requirements and standard procedures for road gully system maintenance yet. In the relevant laws/regulations as well as rules and standards for drain and sewer systems, road gully systems have not been considered explicitly so far. Not until serious damages have occurred, which result or resulted in breakdown or malfunction of the road gully system including visible effects on the surface, e.g. on the sewer, on the environment, or on traffic, or which lead or led to flooding of cellars close-by, costly and normally little lasting immediate measures are initiated according to firefighting strategies.
This approach and the disregard of road gully systems respectively contradict the requirement of an integral maintenance of our drain and sewer systems stated in DIN EN 752 [1]. For this reason, given by the restoration of both sewer systems and drain systems, there is the risk of a shift of technical, ecological, and economical consequences of damages to the defective road gully systems. As a result, all current and future efforts that are made at high expense to improve the state of drain and sewer systems would be taken ad absurdum.
On this account, a nationwide survey on the state of road gully systems was realised by the authors. The results of this survey will be introduced in two parts. In the first part, the approach to the survey, the data pool, as well as project-specific key figures will be presented. The second part includes the results of the batteries of questions regarding inspection, leak tightness testing, requirement for restoration, and costs of modernisation.
The nationwide survey to determine the status quo on the actual state, construction, maintenance expense, operation, as well as rehabilitation of road gully systems was carried out on the basis of surveys on the state of public sewers and manholes plus private drain systems [2] that were realised regularly since 1984/85, and most recently in 2004, by the German Association for Water, Wastewater and Waste (DWA). With its surveys, the DWA pursues the aim to obtain data and information on the state of the sewer system in Germany and to be able to make statements about the investment needs in this particularly cost-intensive area of sewage disposal. At the same time, technical developments, as for example construction methods and rehabilitation procedures, shall be presented and documented. So far, in these surveys, road gully systems have not been recorded and taken into consideration, so that there is no information available in this regard.
For the survey, local authorities and network operators that were chosen according to statistical aspects nationwide were contacted by mail. In choosing them, a well-balanced distribution regarding both the federal states, and the size classes, relating to the number of inhabitants, were taken into account.
The survey was composed of the following five paragraphs:
- General Information on the Drain and Sewer Network
- General Information on Road Gullies
- Cleaning of Road Gullies
- Inspection of Road Gullies
- Rehabilitation of Road Gullies
The questions of the first and second paragraph referred to drain and sewer networks and road gully systems in general. The questions in paragraph three targeted on cleaning in the broadest sense. The fourth paragraph dealt with the area of inspection, whereto the occurring kinds of damages in the road gully systems were also asked in addition to the methods of inspection. In the fifth paragraph, it was challenged to what extent the rehabilitation of road gully systems is carried out, which experiences, if so, were gained so far, and which financial and other edge conditions have to be considered from the point of view of the operator.
The survey took place in 2006, over a period of time from June to December. With these kinds of surveys, from experience, the participation strongly depends on the expenses the local authorities have in processing them. Partly, especially for small or rural local authorities, it is impossible to give the required particulars, because data are often not available in digital form.
In this survey, a total of 180 local authorities from the whole federal territory of Germany were contacted - of these, a total of 50 sewer system operators answered the questions asked. This equates to a return rate of 27.8%.
In Figure 2, the distribution of the participating local authorities in relation to the individual federal states of Germany is shown. The predominant part, with 66%, comes from North Rhine-Westphalia, followed by Lower Saxony and Bavaria with 8% each. The size class GK 4 (100,000 – 500,000 inhabitants) is represented strongest with 38% (see Figure 3). The Hanseatic town Hamburg forms the biggest local authority with a network length of 5,400 km and a catchment area of 75,600 ha, the town called Billerbeck is smallest with a network length of 72 km and a catchment area of 800 ha.
If you add the number of inhabitants of all returns, it is reflected that the survey covers a total of 19.86 million inhabitants. These represent about 24.1% of the German population (about 82.5 million inhabitants).
Overall, a sewer network length of 41,922 km was covered by the survey (see Table 1). According to specifications of the Federal Statistical Office (SB), this corresponds to 8.6 % of the total sewer network in Germany. Compared to the sewer system recorded by means of the DWA survey in 2004 [2, 3], the network length that was covered here amounts to approx. 50%.
In Figure 4, the distribution of drain and sewer systems of the participating local authorities is shown. Thus, 53% of the sewer networks consist of combined sewers, 24% of wastewater sewers, and 23% of surface water sewers. These figures correspond approximately to those of the DWA survey in 2004 [2].
Figure 5 shows the percental distribution of the lengths of drain and sewer networks in relation to the participating local authorities. From this follows that approx. 44% of the local authorities operate a sewer network of more than 600 km in length. The smallest proportion, with 4% each, is constituted by both local authorities with network lengths of < 100km, and those with network lengths between 400km and 500 km.
2.1 General Information on Road Gullies The questions regarding the amount, type distribution, material distribution, and age distribution of road gully systems were answered by 47 of the 50 local authorities taking part in the survey. This amounts to a proportion of approximately 94%.
2.1.1 Total Number of Road Gullies Depending on a large number of influencing factors, as for example the length of network, drainage method, or land utilization, the number of road gullies in the registered drain and sewer networks varies between 2,020 and 160,000 (see Figure 6). Here we can establish that about 72% of the questioned local authorities operate more than 10,000 road gullies in their sewer network.
Overall, a total number of 1,465,804 road gullies was registered as a result of the survey in 50 local authorities. On average, two road gullies (one on each side of the road) are installed at intervals of about 44 m sewer length in the cross section of a road. Relating to the total length of the public combined sewers and surface water sewers of 331,531 km according to the data given by the Federal Statistical Office [3], and based on the average road gully interval of 44 m mentioned above, the total number of road gullies in the public traffic areas of the Federal Republic of Germany can be estimated at 15.2 million.
For comparison of these data, Table 2 contrasts the number of manholes and shafts, which was registered within the scope of a survey of Falk et. al. [4]. The number of shafts of about 11.5 million is based on a projection of the network length of combined sewers, wastewater sewers, and surface water sewers, whereas, according to the construction, only the length of combined sewers and surface water sewers could be considered in the determination of the number of road gullies.
2.1.2 Length of Connection Sewers The length of connection sewers between road gullies that serve for discharging the road drain and the next manhole varies between 2.5 and more than 10.0 m (see Figure 7).
In more than 86% of the local authorities, connection sewers show a length of 2.50 m to 5.0 m. Thus, the average length of a connection sewer amounts to approx. 4.90 m.
Given the total number of 1,465,804 road gullies registered by means of the survey, and an average length of a connection sewer of 4.90 m, the theoretical overall length of connection sewers in the sewer systems of the participating local authorities adds up to about 7,200 km. Relating to the recorded network length of 41,922 km, the proportion amounts to 17.7%. Applied to the whole of Germany, the length of connection sewers therefore covers about 1/5 of the length of the public sewer system (see Table 3).
2.1.3 Types of Road Gullies Installed Road gullies are standardised in DIN 4052 [5]. Accordingly, depending on their mode of operation, two types of road gullies are distinguished (see Figure 8):
- Road gullies with floor drain, also called dry systems or road gullies for dry sludge (Straßenabläufe mit Bodenauslauf, auch Trockensystem oder Straßenablauf für Trockenschlamm) [6], subsequently called SB in short (see Figure 8 a and b).
- Road gullies with sludge space, also called wet systems or road gullies for wet sludge (Straßenabläufe mit Schlammraum, auch Nasssystem oder Straßenablauf für Nassschlamm) [6], subsequently called SS in short (see Figure 8 c and d).
Figure 9 shows the distribution of the types of road gullies installed so far. Thus, the road gully with floor drain (SB) (see Figures 7a and 7b) represents almost 2/3 of the road gullies with 61%. The percentage of road gullies with sludge space (SS) (see Figures 7c and 7d) amounts to 39%.
2.1.4 Material and Age Distribution of Road Gullies According to the results of the survey, the majority of road gullies in sewer networks of the participating local authorities consists of concrete (85.8%), followed by road gullies made of vitrified clay (9.2%). Road gullies made of other material (4.7%), plastics in particular (0.3%), play a secondary role (see Figure 10).
Figure 11 shows the age distribution of road gullies. Concerning the age, the surveyed local authorities could not give any particulars for 45% of the recorded road gullies. For the remaining road gullies, the bigger part (45.4%) is between 0 and 50 years old. Around 7.6% of the road gullies are aged from 51 to 75 years, and only 0.3% are more than 100 years old. From the data concerning the age distribution, it can be assessed that the average age of road gullies in sewer systems of the participating local authorities amounts to about 34 years.
For reasons of comparison, the age distribution of road gullies and sewer systems according to the DWA survey in 2004 is contrasted in Figure 12. About 1/3 of the existing sewers were newly built in the last 25 years, whereas the proportion of the newly built road gullies in this period of time amounts to only 20%. The number of road gullies with an unknown year of construction adds up to 45%. Compared to the degree of anonymousness of the year of construction of sewers (9%), this percentage is very high.
[1]
DIN EN 752-1 (1996): Entwässerungssysteme außerhalb von Gebäuden: Allgemeines und Definitionen (Drain and sewer systems outside buildings: Generalities and definitions), Publishing House: Beuth Verlag, Berlin, Germany.
[2]
Berger, C.; Lohaus, J.: Zustand der Kanalisation in Deutschland – Ergebnisse der DWA-Umfrage 2004, Hennef (2004).
[3]
Federal Statistical Office: Öffentliche Wasserversorgung und Abwasserbeseitigung 1998, UmweltTechnical Series 19, Volume 2.1, Publishing house: Metzler und Pöschel, Stuttgart, 2001.
[4]
Falk, Ch., Weidemann, S.: Zustand, Sanierungsbedarf und Sanierungsmöglichkeiten der Einsteigschächte in kommunalen Entwässerungsnetzen; Ergebnisse einer im Auftrag der Elastogran GmbH, Lemförde durchgeführten Umfrage, September, 2004.
[5]
DIN 4052 (2006): Betonteile und Eimer für Straßenabläufe; Teil 1: Bauart und Einbau (Concrete elements and buckets for road gullies; Part 1: General requirements and installation); Teil 2: Zusammenstellungen und Bezeichnungen (Part 2: Summary and designations); Teil 3: Betonteile (Part 3: Concrete elements); Teil 4: Eimer (Part 4: Buckets), Publishing House: Beuth Verlag, Berlin.
[6]
RAS-Ew (2005): Richtlinien für die Anlage von Straßen (RAS), Teil: Entwässerung (RAS-Ew), Forschungsgesellschaft für Straßen- und Verkehrswesen (FGSV), Köln (Cologne).
[7]
Stein, R. (2008): Auswirkungen optimierter Straßenabläufe auf Feststoffeinträge in Kanalisationen (Effects of optimised road gullies on the entry of solids into the sewer system), Dissertation, RWTH-Aachen.
