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Sewer Simulation

Drainage network computation is used as a basis for design of new systems and also to assess the situation of existing networks. Following surface runoff and the dry weather flow data, the resulting runoff is incorporated within the channel.

The requirements for calculation of sewer networks are shown in DWA-A 118th

Background

The workflow for computation includes the following steps:

  • Provision of appropriate data for precipitation in the form of time series
  • Calculation of surface runoff
  • Determination of the dry-weather runoff
  • Calculation of runoff in the sewer network

For the calculation of runoff in the sewer network it is important to distinguish two methods; hydrological and hydrodynamic processes.

Hydrological methods take into account the transport (Translation) and attenuation (retention) of drainage runoff by means of a transfer functions. Following this, a water level can be calculated. Backflow will not be considered. The hydrological models are not suited to represent overload conditions in the sewer network.

The hydrodynamic methods are based on the flow process according to the Saint-Venant equations. Water levels and discharges are computed by iteratively time steps.

Applications

The applications for a drainage system hydrodynamic simulation are limitless, below are some examples :

  • Re-assessment
  • Recalculation of existing systems
  • Calculation of remediation alternatives
  • Calculation of backflow Frequency

Depending on the application, an appropriate calculation method should be selected.

Design procedure

For the assessment of a planned drainage system a sewer network model flow is calculated. The dimensioning is adapted to ensure that the maximum load is at 90%  (Qges/Qvoll).

The precipitation data per second per area  rD,n is given by the relevant authority or can be taken from the database KOSTRA; data of DWD. The frequency of the data n depends on the type of drainage area;

  • Rural areas: 1 time of 1 Year (n=1a-1)
  • Residential areas: 1 time in 2 years (n=0,5 a-1)
  • City centers, industrial and commercial areas: 1 time in 5 years (n=0,2 a-1)
  • Underground transport systems: 1 times in 10 years (n=0,1 a-1)

The re-assessment may be carried out with a rain model Euler type II, by means of hydrodynamic sewer simulation. Following the dimensioning a validation should be done especially for large channels.

Verification procedure

In order to investigate congestions and backwater within the system a detection method should be used within the hydrodynamic simulation. The target of the calculated network should be proofed as per DWA-A 118. It is important to identify threshold values of backwater.

It can be calculated with Euler type II rain model or with a long-term series simulation. The DWA-A 118 calls for the following frequencies that are depending on the type of drainage area:

  • Rural areas: 1 time of 1 Year (n=1a-1)
  • Residential areas: 1 time in 2 years (n=0,5 a-1)
  • City centers, industrial and commercial areas: 1 time in 5 years (n=0,2 a-1)
  • Underground transport systems: 1 times in 10 years (n=0,1 a-1)

When using long-term series simulation (heavy rain Series) the frequency nÜ is calculated from the number of events :

nÜ = x / M 

with
x:number of impounding events per shaft
M:number of years

Software

Common software solutions for drainage system simulation include:

  • InfoSWMM from InnoAqua / Innovyze,
  • InfoWorks ICM from InnoAqua / Innovyze,
  • the open source software SWMM from the United States Environmental Protection Agency (EPA),
  • HYSTEM EXTRAN from itwh GmbH,
  • MIKE URBAN from DHI

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Author
Dipl.-Ing. Frauke Jakobs
+49 3342 3595-22
f.jakobs[at]sieker.de