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Analysis and modeling of flooding in city drainage systems

Abstract

Urban flooding is a worldwide drawback and can have significant economic and social penalties. The major goal of this paper is the event of an built-in planning and management software to permit price effective management for urban drainage systems and prevention of city flooding . This paper follows European Standard EN 752 defining flood frequen cy because the one hydraulic performance criterion. Dual drainage modeling is used here to research urban flooding caused by surcharged sewer systems in city areas . A twin drainage simulation mannequin is described right here in particulars primarily based upon hydraulic move routing procedures for surface flow and pipe flow.

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Special consideration is given to the interplay between surface and sewer move in order to most accurately compute water levels above floor as a basis for further evaluation of possible injury costs. The model utility is introduced for small case research in phrases of knowledge wants, model verification and first simulation outcomes .

Introduction

Climate change is altering precipitation patterns across the globe.

Every year, rainstorms and flooding occasions are rising in both frequency and severity. With municipal water utilities already strained by decades of underinvestment and getting older infrastructure, they now face a complete new spectrum of c hallenges because of climate change and growing urb an populations . So prev ention of flooding in city areas has turn out to be an essential problem. However, drainage methods designed to cope with probably the most excessive storm circumstances can be too expensive to build and function. In establishing tolerable flood frequencies, the safety of the r esidents and the safety of their valuables have to be in steadiness with the technical and economic restrictions.

According to European Standard EN 752, city drainage systems must be designed to face up to intervals of flooding in the range of 10 -50 years, d epending on the sort of urban area and visitors infrastructure served . In the following, the major problems with this standard will be briefly mentioned at the aspect of an analysis of city flooding. A simulation mannequin to assess the hydraulic efficiency of sewer systems and the chance of flooding caused by system surcharge will be described afterwards. Its app lication and information want is demon strated in a case examine.

Analysis of flooding phenomena Flooding in city drainage techniques as defined above might occur at completely different phases of hydraulic surcharge depending on the drainage system (separate or combined sewers), basic drainage design charac teristics as well as specific local constraints.

When non-public sewage drains are instantly linked to the common public sewer system without backwater valves, the possible results of hydraulic surcharge rely upon the degrees of the lowest sewage inlet inside the home (basement), the sewer line and the water stage during surcharge, respectively. Whenever the water stage within the pub lic sewer exceeds the extent of gravity inlets in the house beneath street degree, flooding inside the home will occur as a end result of backwater results. In such a case flooding is possible without experiencing floor flooding. In the identical method, hydraulic surcharge within the sewer system may produce flooding on private properties through storm drains, when their inlet degree is beneath the water stage of the surc harged storm or combined sewer.

In each cases, the prevalence of flooding, being linked on to the extent of inl ets versus water level (pressure height) within the sewer can be ‘easily’ predicted by hydrodynamic sewer flow simulations, assuming the provision of bodily knowledge of the private drai ns and the public sewer system.

Distinct from the situations described ab ove, the occurrence and possible effects of floor flooding rely far more on local constraints and floor characteristics, e.g. avenue gradient, sidewalks and curb peak. These traits, nevertheless, are far more difficult described bodily, a nd these information are usually not out there in practice. In addition, today’s simulation models usually are not fully enough to simulate the related hydraulic phenomena associated with floor flooding and surface move along distinct flow paths.

Consequences of flooding

Flooding in urban areas as a result of failure of drainage systems causes massive harm at buildings and other public and private infrastructure. Besides, avenue flooding can restrict or utterly hinder the functioning of visitors methods and has indi rect penalties such as loss of enterprise and opportu nity. The expected complete injury direct and indirect monetary damage costs in addition to attainable social penalties is related to the bodily properties of the flood, i.e. the water degree above ground le vel, the lengthen of flooding in phrases of water quantity escaping from or not being coming into the drainage system, and the duration of flooding.

With sloped surfaces even the move velocity on the floor might have an effect on potential flood injury .

Methodology

Rainfall -Runoff Simulation :

The RisUrSim model first transforms rainfall into effective runoff utilizing normal strategies for interception, melancholy storage and soil infiltration (previous areas only) as described in literature (e.g. Akan, 1993; Ashley et al., 1999). Surface runoff would then be dealt with in distinct element depending on the particular scenario of a single runoff space. For areas not considered for detailed surface circulate simulation, e.g. roof areas, RisoReff uses a unit -hydr ograph technique to compute floor runoff as input to the sub -surface sewer system (‘uni -directional flow’ ).

Hydraulic surface flow modeling :

The RisoSurf approach contains detailed hydraulic concerns for areas the place floor circulate occurs. Hydraulic (sur face) circulate modeling is generally based mostly upon conservation legal guidelines of fluid move expressed within the Navier -Stokes equations. The proven truth that in floor flow the vertical dimension is far smaller than typical horizontal scale permits a simplified two -dimensional rep resentation, the so -called ‘shallow water circulate equations’ (Hilden, 2003). The software of this detailed hydraulic method could be restricted to small areas only.

Therefore, it solely served as a benchmark for a further simplified two -dimensional approach.

  • Rainfaon of Surface and Sewer Flow

Dynamic sewer flow modeling:

Sewer circulate is simulated making use of fully dynamic flow routing of unsteady, steadily varied circulate and fixing Saint -Venant – Equations numerically in an express difference scheme. The explicit difference scheme is utilized in variable time steps which are permanently adjusted to the COURANT -criterion, guaranteeing numerical stability (Schmitt, 1986).At every time step, the proced ure of dynamic move routing starts by computing circulate values for each conduit (sewer section between nodes) primarily based upon momentum equation and instantaneous water levels on the nodes on the finish of the last time step. In the following step of the dynamic flow routi ng procedure the move quantity is balanced at each node, bearing in mind inlets from home drains and all surface inlets connected, as nicely as inflows and outflows from sewers related on the nodes. The ensuing change of quantity is drawn to free water floor ‘available’ at the node, thus producing a change of water degree at the node. In order to enhance numerical stability, the two phases are utilized in a half -step -full -step process during each time step as described in Roesner et al. (1988) and Schm itt (1986). The underground sewer system is represented by a network of nodes and conduits (sewer section between nodes). In contrast to conventional modeling, not solely manholes but also street inlets and house drains are thought of as additional nodes to totally obtain the connection of floor and underground drainage system in any respect areas the place interaction between floor and sewer flow and doubtlessly flooding might occur. This might be additional mentioned in context with the case research under.

Modeling intera ction of surface and sewer flow :

The simulation of the interplay between floor and sewer flow is predicated upon the definition of exchange areas. Each runoff space is allocated to a minimum of one specified change l ocation as illustrated in Fig. 1 . Here, all re levant data for floor and sewer flow simulation (instantaneous runoff, water level, exchange volume) is available at the beginning of every time step for all simulation modules and is renewed on the end of the time step within the following means:

  • The hydrologic runoff model RisoReff only helps uni -directional flow and is utilized to all areas not considered for surface circulate.

Computed runoff from those ‘hydrologic areas’ is handed to the single change location to which the realm is related. The exchange volume would be the runoff volume within the in accordance time step.

  • Areas simulated with the hydrologic mannequin approach may be linked to the underground drainage system in two other ways :
  • [newline]

‘hydrologic areas’ immediately discharging to the sewer system vi a surface inlets or private drains (se rvice pipes);

‘hydrologic areas’ discharging to surface areas where surface circulate is taken into account by hydraulic model RisoSurf .

  • The hydraulic floor flow module RisoSurf permits bi -directionalexchange of runoff quantity:

from the surface area to the sewer system, if there could be enough sewer capacity;

from the sewer to the hydraulic surface in case of sewer surcharge when the water stage in the sewer system rises above ground level.

  • In case of surcharge, water stage above ground as provided by surface flow simulation module RisoSurf at exchange nodes would be utilized by dynamic sewer flow module HamokaRis in the momentum equation within the following time step. If the balance of flow volume on the nodes in HamokaRis leads to a w ater degree above ground, the associate d surplus quantity can be switch ed to the surface flow simulation by ‘storing’ this volume within the exchange location

Coupling of modules RisoSurf and HamokaRis :

The implementation of coupled hydraulic move routing for surface and sewer move modules RisoSurf and HamokaRis requires specific consideration of numeric stability and remark of continuity as nicely. Numeric stability has been secured by a synchro nized  administration of dynamic time step choice.

Model application case examine :

One of the check areas to show the concept of the RisUrSim technique is a sub -catchment in th e city of Kaiserslautern (Fig. 4 ). Some of the homes in the southern avenue of the take a look at -area have been subject to basement flooding during heavy rainfall prior to now. To put together the mannequin utility, detailed surveying has been carried out to accurately describe circulate -relevant floor a reas. Besides, a flow monitoring system has been installed to gather data throughout rainfall events for model calibration under surcharge circumstances. This, nevertheless, has not been successful as in the course of the interval of monitoring not a single surcharge or even floo ding occasion occurred .

Simulation results :

Due to the fact that no surcharge or flooding occasion might be monitored, the RisUrSim Software has been applied to quite a lot of test scenarios utilizing synthetic design storms.

These purposes have been done to verify probably the most crucial model options of hydr aulic surface flow simulation and the interaction between surface flow and sewer flow beneath surcharge and flooding situations. The simulation outcomes of the real -case system Erzhuetten are proven the Fig. 10 in terms of water degree distribution alongside the st reet floor at 15 and 25 min simulation time, respectively. In this system the surface elevation decreases from north -east (right) to south -west (left) while the sewer – system circulate course is oriented in the different way. This has led to problems w ith flooding in this area up to now. The illustration of simulated water levels within the manholes and on the street floor illustrates the surface circulate pattern from surcharged, flooded manholes to street areas with decrease surface ranges (on the left aspect of the graph). This proves that with the RisUrSim Software the floor flooding might be reproduced realistically.

Conclusions :

It has been proven that European Standard EN 752 triggers more intense consideration of the flooding phenomenon in city drain flow modeling. In the RisUrSim approach specific recognition is given to deta iled floor move simulation and the interplay between surface and sewer circulate during occasions of surcharged sewers. This method is ‘dual – drainage’ idea .

References

  1. Akan, A.O., 1993. Urban stormwater hydrology a information to engineering calculations. Technomic Publishing Company, Lancaster, PA (USA).
  2. Ashley, R.M., Hvitved -Jacobsen, T., Bertrand -Krajewski, J. -L., 1999. Quo vadis sewer course of modelling. Water Science Technology 39 (9), 9 -22. ATV, 1999. Hydraulische Berechnung und Nachweis von Entwa?s -serungssystemen (Hydraulic calculation and verification of drainage techniques.), Arbeitsblatt A 118, ATV -Regelwerk, Hennef, Germany 1999. CEN, 1996. Drain and sewer systems exterior buildings Part 2: Performance Requirements, European Standard,
  3. European Comm ittee for Standardization CEN, Brussels, Belgium 1996. CEN, 1997. Drain and sewer techniques exterior buildings Part 4: Hydraulic design and environmental
  4. considerations, European Standard, European Committee for Standardization CEN, Brussels, Belgium 1997. Djordjevic, S., Prodanovic, D., Maksimovic, C., 1999. An strategy to simulation of twin drainage. Water Science
  5. and Technology 39 (9), ninety five -103. Ettrich, N., Steiner, K., Schmitt, T.G., Thomas, M., Rothe, R., 2004. Surface models for coupled modeling of
  6. run off and sewer circulate in city areas, Conference paper submitted for Urban Drainage Modeling 2004, Dresden,
  7. Germany 2004. Hilden, M., (2003): Extensions of Shallow Water Equations. PhD Thesis, Department of Mathematics,
  8. Kaiserslautern University. Roesner, L .A., Aldrich, J.A., Dickinson, R.E., 1988. Storm Water Management Model User’s ManualManual,
  9. Version 4: Adden -dum I, EXTRAN, EPA/600/3 -88/001b (NTIS PB88236658/AS). Environmental Protection
  10. Agency, Athens, GA, p. 203. Schmitt, T.G., 1986. An environment friendly met hod for dynamic flow routing in storm sewers, Proceedings Of the
  11. International Symposium on Urban Drainage Modeling, Dubrovnik, Yugoslavia 1986 pp. 159 -169. Schmitt, T.G., 2001. Evaluating hydraulic efficiency of sewer systems based on European Stand ard EN 752,
  12. Water21, Magazine of the International Water Association, London 2001, pp. 29 -32. Schmitt, T.G., Thomas, M., 2000. Untersuchung zum rechnerischen
  13. Uberstaunachweis auf der Basis von Modellregen und Regen -serien (Study of simulation verification of
  14. surcharge frequencies on the idea of artificial rainfall profiles and time series), KA Wasserwirtschaft,
  15. Abwasser, Abfall, 1 2000 pp. 63 -69.

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