The Zagreb Sewer System serves cca one million inhabitants and large industry. It consists of various types of catchments with both very steep and mild slopes. Creeks from the neighbouring mountain Medvednica are introduced into the Sewer System. It has cca 30 km from the most western to the most eastern point, very big conduits, almost no CSO, etc. The main recipient is the Sava river which divides city and the sewer system in two separate systems, planned to be joint at the location of the future WWTP. The very beginnings of the systematic development were in 1880. Up to date, Zagreb Sewerage System has more than 1200 km of public and cca 1000 km of industrial sewer pipe. Total catchment area is cca 22500 ha. The evident problems are: backflow effects, floodings, bottle necks, pollution, etc. The effects of the Sewer System malfunctioning are multiple: drainage low quality, high communal risk of damages, temporarily/permanent inadequate living conditions in some areas and last, but not least, high maintenance costs.
The City of Zagreb has undertaken a PHASE I of the ZSSO aiming to create the Hydraulic and Hydrological existing sewer model and renew the knowledge about the system. That project, initiated in 1994, has established new standards to urban drainage modelling within the area. The “PRONING DHI” company was proposed to undertake the necessary actions and provide results determined for the ZSSO Project PHASE I. A number of Croatian and European companies and experts were included in its realisation. The PHASE I was finalised at the beginnings of 1996. The methodology applied was adjusted to the latest European Standards and the adopted approach was based on the MOUSE concept (Danish Hydraulic Institute). The existing Zagreb sewer system model consists only of the main trunks and channels. Total length of the trunks modelled was 150 km, what represents cca 10% of the total sewer network length, but 80% of the total sewer network volume.
The sequence of activities of ZSSO Project PHASE I includes the following:
1. Data Collection (GIS/ MOUSE)
2. Hydrologic analysis
3. Hydraulic analysis
4. Short Term (Flow and rainfall) Monitoring
5. Verification of the Sewerage Planning Tool
6. Description of the hydrodynamic processes in the Main Outflowing Channel
7. Long-Term Statistics for the Existing System
8. Training and Technology Transfer
The SW used were: MOUSE, MIKE 11, AutoCAD and ARC Cad.
The initiation of the ZSSO Project represents the choice of the city of Zagreb and its inhabitants towards the modern and ecological urban drainage solution and living quality improvement. This project brings out numerous new dilemmas and problems. Necessary financial investments will provide both safety and quality of the City Sewerage System, and in a long term prospect it will bring savings in comparison to the investments based on the traditional engineering approaches and technologies.
