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Urban flooding is a global problem and can have significant economic and social consequences. The main objective of this paper is the development of an integrated planning and management tool to allow cost effective management for urban drainage systems and prevention of urban flooding . This paper follows European Standard EN 752 defining flood frequen cy as the one hydraulic performance criterion. Dual drainage modeling is used here to analyze urban flooding caused by surcharged sewer systems in urban areas . A dual drainage simulation model is described here in details based upon hydraulic flow routing procedures for surface flow and pipe flow.
Special consideration is given to the interaction between surface and sewer flow in order to most accurately compute water levels above ground as a basis for further assessment of possible damage costs. The model application is presented for small case study in terms of data needs, model verification and first simulation results .
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Every year, rainstorms and flooding events are increasing in both frequency and severity. With municipal water utilities already strained by decades of underinvestment and aging infrastructure, they now face a whole new spectrum of c hallenges due to climate change and growing urb an populations . So prev ention of flooding in urban areas has become an important issue. However, drainage systems designed to cope with the most extreme storm conditions would be too expensive to build and operate. In establishing tolerable flood frequencies, the safety of the r esidents and the protection of their valuables must be in balance with the technical and economic restrictions.
According to European Standard EN 752, urban drainage systems should be designed to withstand periods of flooding in the range of 10 -50 years, d epending on the type of urban area and traffic infrastructure served .
In the following, the major issues of this standard will be briefly discussed in conjunction with an analysis of urban flooding. A simulation model to assess the hydraulic performance of sewer systems and the risk of flooding caused by system surcharge will be described afterwards. Its app lication and data need is demon strated in a case study.
Analysis of flooding phenomena Flooding in urban drainage systems as defined above may occur at different stages of hydraulic surcharge depending on the drainage system (separate or combined sewers), general drainage design charac teristics as well as specific local constraints.
When private sewage drains are directly connected to the public sewer system without backwater valves, the possible effects of hydraulic surcharge depend on the levels of the lowest sewage inlet inside the house (basement), the sewer line and the water level during surcharge, respectively. Whenever the water level in the pub lic sewer exceeds the level of gravity inlets in the house below street level, flooding inside the house will occur due to backwater effects. In such a case flooding is possible without experiencing surface flooding. In the same way, hydraulic surcharge in the sewer system might produce flooding on private properties via storm drains, when their inlet level is below the water level of the surc harged storm or combined sewer.
In both cases, the occurrence of flooding, being linked directly to the level of inl ets versus water level (pressure height) in the sewer can be 'easily' predicted by hydrodynamic sewer flow simulations, assuming the availability of physical data of the private drai ns and the public sewer system.
Distinct from the situations described ab ove, the occurrence and possible effects of surface flooding depend much more on local constraints and surface characteristics, e.g. street gradient, sidewalks and curb height. These characteristics, however, are much more difficult described physically, a nd these data are usually not available in practice. In addition, today's simulation models are not fully adequate to simulate the relevant hydraulic phenomena associated with surface flooding and surface flow along distinct flow paths.
Flooding in urban areas due to the failure of drainage systems causes large damage at buildings and other public and private infrastructure. Besides, street flooding can limit or completely hinder the functioning of traffic systems and has indi rect consequences such as loss of business and opportu nity. The expected total damage direct and indirect monetary damage costs as well as possible social consequences is related to the physical properties of the flood, i.e. the water level above ground le vel, the extend of flooding in terms of water volume escaping from or not being entering the drainage system, and the duration of flooding.
With sloped surfaces even the flow velocity on the surface might have an impact on potential flood damage .
The RisUrSim model first transforms rainfall into effective runoff using standard methods for interception, depression storage and soil infiltration (previous areas only) as described in literature (e.g. Akan, 1993; Ashley et al., 1999). Surface runoff would then be handled in distinct detail depending on the specific situation of a single runoff area. For areas not considered for detailed surface flow simulation, e.g. roof areas, RisoReff uses a unit -hydr ograph method to compute surface runoff as input to the sub -surface sewer system ('uni -directional flow' ).
The RisoSurf approach includes detailed hydraulic considerations for areas where surface flow occurs. Hydraulic (sur face) flow modeling is generally based upon conservation laws of fluid flow expressed in the Navier -Stokes equations. The fact that in surface flow the vertical dimension is much smaller than typical horizontal scale allows a simplified two -dimensional rep resentation, the so -called 'shallow water flow equations' (Hilden, 2003). The application of this detailed hydraulic method would be restricted to small areas only.
Therefore, it only served as a benchmark for a further simplified two -dimensional approach.
Sewer flow is simulated applying fully dynamic flow routing of unsteady, gradually varied flow and solving Saint -Venant - Equations numerically in an explicit difference scheme. The explicit difference scheme is applied in variable time steps that are permanently adjusted to the COURANT -criterion, guaranteeing numerical stability (Schmitt, 1986).At each time step, the proced ure of dynamic flow routing starts by computing flow values for each conduit (sewer segment between nodes) based upon momentum equation and instantaneous water levels at the nodes at the end of the last time step. In the next step of the dynamic flow routi ng procedure the flow volume is balanced at each node, taking into account inlets from house drains and all surface inlets connected, as well as inflows and outflows from sewers connected at the nodes. The resulting change of volume is drawn to free water surface 'available' at the node, thus producing a change of water level at the node. In order to improve numerical stability, the two phases are applied in a half -step -full -step procedure 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 segment between nodes). In contrast to conventional modeling, not only manholes but also street inlets and house drains are considered as extra nodes to fully achieve the connection of surface and underground drainage system at all locations where interaction between surface and sewer flow and potentially flooding might occur. This will be further discussed in context with the case study below.
The simulation of the interaction between surface and sewer flow is based upon the definition of exchange locations. Each runoff area is allocated to one specified exchange l ocation as illustrated in Fig. 1 . Here, all re levant information for surface and sewer flow simulation (instantaneous runoff, water level, exchange volume) is available at the beginning of each time step for all simulation modules and is renewed at the end of the time step in the following way:
Computed runoff from those 'hydrologic areas' is passed to the single exchange location to which the area is connected. The exchange volume would be the runoff volume in the according time step.
'hydrologic areas' directly discharging to the sewer system vi a surface inlets or private drains (se rvice pipes);
'hydrologic areas' discharging to surface areas where surface flow is considered by hydraulic model RisoSurf .
from the surface area to the sewer system, if there is sufficient sewer capacity;
from the sewer to the hydraulic surface in case of sewer surcharge when the water level in the sewer system rises above ground level.
The implementation of coupled hydraulic flow routing for surface and sewer flow modules RisoSurf and HamokaRis requires particular consideration of numeric stability and observation of continuity as well. Numeric stability has been secured by a synchro nized administration of dynamic time step selection.
One of the test areas to prove the concept of the RisUrSim method is a sub -catchment in th e city of Kaiserslautern (Fig. 4 ). Some of the houses in the southern street of the test -area have been subject to basement flooding during heavy rainfall in the past. To prepare the model application, detailed surveying has been carried out to accurately describe flow -relevant surface a reas. Besides, a flow monitoring device has been installed to gather data during rainfall events for model calibration under surcharge conditions. This, however, has not been successful as during the period of monitoring not a single surcharge or even floo ding event occurred .
Due to the fact that no surcharge or flooding event could be monitored, the RisUrSim Software has been applied to a variety of test scenarios using synthetic design storms.
These applications have been done to verify the most crucial model features of hydr aulic surface flow simulation and the interaction between surface flow and sewer flow under surcharge and flooding conditions. The simulation results of the real -case system Erzhuetten are shown the Fig. 10 in terms of water level distribution along the st reet surface 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 flow direction is oriented in the opposite direction. This has led to problems w ith flooding in this area in the past. The representation of simulated water levels in the manholes and on the street surface illustrates the surface flow pattern from surcharged, flooded manholes to street areas with lower surface levels (on the left side of the graph). This proves that with the RisUrSim Software the surface flooding could be reproduced realistically.
It has been shown that European Standard EN 752 triggers more intense consideration of the flooding phenomenon in urban drain flow modeling. In the RisUrSim approach particular recognition is given to deta iled surface flow simulation and the interaction between surface and sewer flow during times of surcharged sewers. This approach is 'dual - drainage' concept .
Analysis and modeling of flooding in urban drainage systems. (2019, Nov 20). Retrieved from https://studymoose.com/analysis-and-modeling-of-flooding-in-urban-drainage-systems-essay
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