Discharged suspended solids from the sewer system are deposited or transported downstream depending on the hydraulic conditions in the receiving water. The domains erosion, sedimentation and transport of TSS in function of particle size and flow velocity are shown in the following figure.
| Figure: | behaviour of particles in function of particle size and flow velocity (from Graf W.H. (1984): Hydraulic of sediment transport. S. 513 ff, Water resources publications, Colorado, USA.) |
For each rainfall event the TSS concentration is computed directly after the discharges from the sewer systems. A full mixing in the receiving water is assumed. The computed concentration is compared with concentration-duration curves in order to determine if a critical event caused by turbidity occured.
The program also computes if the TSS are accumulated or resuspended due to the hydraulic conditions in the receiving water. Depending on the relation between actual and critical shear stress accumulation or erosion of TSS is modelled. The critical shear stress is different from that for bedload motion and must be defined by the user. Critical TSS accumulation on the riverbed with regard to colmation, toxicity or oxygen depletion is assessed by computing the percentage of time where a critical TSS density is exceeded.
Further details of the model can you find in the following publications:
Fankhauser R., Kreikenbaum S., Krejci V., Rossi L., Rauch W. (2004). Rebeka II - a stochastic software tool for assessing impacts of urban drainage on receiving waters. 6th Int. conf. on Urban Drainage Modelling (UDM), pp 175-184, Dresden, Germany.
L. Rossi, R. Fankhauser, and N. Chèvre (2006). Water quality criteria for total suspended solids (TSS) in urban wet-weather discharges. Water Science and Technology, 54(6-7):355-62
In German:
Rossi, L., Gujer,W., Kreikenbaum, S., Fankhauser, R. (2004): Gesamte ungelöste Stoffe (GUS) bei Regenwetter. GWA Gas, Wasser, Abwasser 10, 753-761.
Same paper in French:
Rossi, L., Gujer,W., Kreikenbaum, S., Fankhauser, R. (2004): Modélisation des matières en suspension (MES) dans les rejets urbains en temps de pluie, gwa No 10, p. 753–761.