Rehabilitation and Maintenance of Drains and Sewers

Hydraulic Examination

The solving of hydraulic problems requires that all the relevant information for determining the situation is collected and evaluated. A calculated determination of the dry and storm weather flows, with the addition of an inventory of bad connections and leaks, is insufficient in many cases for comprehensively describing the hydraulic loads on the sewers. The actual flow in a drain and sewer network, or part network, can deviate substantially from the results of hydraulic calculations and the simulation of flow models and they are greatly influenced by limiting conditions - for example growth or loss due to in- and exfiltration. In critical cases, the estimates must be replaced by measured data. The most important of these are flow measurements that can depict the loading directly in the sewer. This point of view is also represented in EN 752-5 [DINEN752-5b]. There, it is mentioned in point 7.3 "Testing and inspection procedures can be required in order to ensure an adequate evaluation of flows (dry weather and storm) infiltration, exfiltration and wrong connections." These are the subject of the following paragraphs and also "… can include … the identification of wrong connections and groundwater measurements" (Chapter 4.4.3.2.1) (Chapter 4.6.2.11).

"A sewer flow simulation model can be necessary in order to understand the hydraulics of the system. However, a model may not be necessary:

  • Where there are no known hydraulic problems (particularly where the sewer system takes only wastewater flows);
  • And where there are no combined sewer overflows;
  • And where structural problems are to be solved using techniques, which do not reduce the hydraulic capacity of the sewer.

Information on the use of computer based sewer flow simulation programs is given in clause 11 of EN 752-4" [DINEN752-4].

"Verification and/or calibration of the models is necessary. The procedures used depend on the program.

If suitable agreement is not obtained, the model input data should be checked and then the sewer records. Having identified possible causes of error, it will often be necessary to confirm these by site inspection and then adjust the model accordingly. Data shall not be modified without justification based on an inspection of the system" [DINEN752-5b].

The aim of the subsequently described flow measurements within the scope of hydraulic testings of the sewer system is to be able to make a statement about:

  • The flow dynamics (dry weather and runoff, hydraulic capabilities);
  • The exfiltration; as well as
  • Extraneous water inflow due to leakiness and possible wrong connections.

These saved sizes are used again as input parameters in hydraulic models for calculating the sewer system.

Rehabilitation and Maintenance of Drains and Sewers