Flood early warning system on the Đetinja river Basin in Serbia

The Đetinja River is a true example of an ungauged torrential watercourse where, during the first decade of the 21st century, numerous floods have caused financial loss and sometimes even human casualties. The Republic Hydro-meteorological Service of Serbia's existing observation system cannot be used to forecast floods in the basin, due to lack of measuring points or stations capable of transmitting observation data in real time, so that the competent authorities are alerted and can react to minimize the consequences.

Systems for observing rainfall intensity and runoff that are capable of recording observations and sending them automatically to a given address have already been developed in similar basins (Gavrilović et al. 2004). Today, they are computerized, and offer a whole new dimension to the monitoring and announcement of floods in real time. The development of communication systems has significantly improved the functionality of measuring equipment as well as the transfer of data to users. Unlike previous systems that, with the inevitable delay, transmitted data to functionally separate centers (meteorological, hydrological, water management, etc.), modern systems can transmit data directly to all centers and their competent staff.

The system design was based on the flood early warning systems (FEWS) on the Topčiderska River in Belgrade and on the River Crnica in Paraćin (Milovanović et al. 2010). No hydraulic measurements were made on the River Đetinja for this study. Because of that, the methodology used was the same as that applied to ungauged basins. Maximum attention is focused on the analysis of data collected to determine early warning alarm criteria.

Hydrologic analysis is used to define the flow characteristics of the desired profile, to determine criteria for FEWS. For this study, the maximum flow discharges were calculated for flooding probabilities of 1%, 2%, 5% and 10%, correlating, respectively, with 100-, 50-, 20- and 10-year return periods. All relevant meteorological, climatological and hydrologic data for the study area were processed and analyzed.

Hydrologic analysis was carried out for a profile on the Đetinja where it was proposed that the measuring device be installed. Maximum water levels were estimated for the profile using a rainfall–runoff model. This was based on the synthetic unit hydrograph theory for determining peak runoff, as well as the SCS method for determining effective rainfall USDA (1986).

Data from the main meteorological station at Zlatibor, which is nearby, were used as input for the analysis (Institute for the Development of Water Resources ‘Jaroslav Černi’, 2009). Flow curves and water levels for individual profiles were used where there were appropriate measurements covering sufficiently long periods.

The calculations yielded the probable maximal flows – P = 1, 2, 5 and 10% – for the return periods noted above. The maximum flows calculated were used to define the FEWS criteria, as well as water levels and flow curves available.

The software was developed at the Institute for the Development of Water Resources ‘Jaroslav Černi’, in the Department of Torrent and Erosion Control (Gavrilović et al. 2004). It can be adapted to user needs, and has a clear interface with options to connect to other databases. The equipment can be programmed to give an emergency alarm under specific conditions.

The hardware must be of good quality and reliable, with fast and efficient service in the event of malfunction. It must also be of simple and flexible structure, resistant to atmospheric effects and simple to operate. The rain sensors must have a measurement accuracy of +/− 0.1 mm of rainfall and the water level sensors must be accurate to +/− 1 cm.

The study yielded criteria for FEWS alarms. The definitions were based on the analyses of data on rainfall and flow rates, as well as the flow curves and water levels available from the target area. These criteria are crucial to the timely announcement of a flood surge, so it is very important to define them precisely.

The alarm system criteria for rapid flood warning were determined from the causal analysis of data from previous events. There are three criteria, as noted above.

• Level I – a warning is given when it rains at an average intensity exceeding 20 mm/30 min;

• Level II – an alarm is given when it rains at an average intensity exceeding 30 mm/30 min;

• Level III – an alarm is given when the average intensity of the rain exceeds 40 mm/30 min and it continues raining at this rate for than an hour.

The alarm time interval of 30 minutes is used rather than 1 hour, due to the size of the basin, to enable an efficient and timely response.

High intensity rainfall causes surface runoff, which can become a substantial flow. Because of this and as noted, three alarm level criteria – I, II and III – have been defined for two stations in the basin. The criteria relating to the Đetinja profile were defined on the basis of detailed hydrologic analysis and the potential hazards arising from the water level for the downstream areas of Užice.

FEWS was designed on the model of existing systems on the Crnica River, near Paraćin, and the Topčiderska River in Belgrade. These systems have proved to be very reliable and effective, especially that on the Topčiderska, which has been in operation for many years.

The main results of the study are the criteria on which almost the entire system depends, so, three criteria levels are defined:

• a warning to be ready (level I),

• engagement of regular flood defense (level II),

• emergency flood protection (level III), i.e., the need to react with all available manpower and equipment to mitigate potential flood consequences downstream of the monitoring point.

It is important that the water level monitoring profile of the River Đetinja is checked and maintained regularly. Every serious flood leaves large amounts of sediment on the river bed. The alarm criteria were defined on the basis of water levels relating to a specific river channel profile. It is necessary, after every flood, to clean the channel bottom so that the criteria specified remain relevant.

The Đetinja River is characterized by torrential flow, requiring fast warning flood defense systems that differ from the classic form. Long-term and detailed analysis of the data available indicated that torrential floods on the Đetinja are formed and reach their peak approximately two hours after heavy rain.

The specifics of the Đetinja basin mean that detailed hydrologic analytical data are needed to determine FEWS criteria. Experience gained on the Topčiderska River indicates that it is necessary to have at least two rain gauges and one river stage/water level monitoring device.

When the alarm for emergency flood defense is given, the competent services have 2 to 3 hours lead time before the critical water level is achieved at the flood zone in Užice city center. That is long enough for citizens to be warned to remove or protect property susceptible to flooding, organize additional flood defense and, if necessary, organize and execute the evacuation of vulnerable zones.

The system plays a key role in active flood control and prevention, by providing the lead time to secure and strengthen the passive flood defense system, thereby minimizing the adverse impacts of torrential floods.

The research presented in this paper is funded by Serbia's Ministry of Education and Science as part of a project ‘Assessment of Climate Change Impact on Water Resources in Serbia’ (TR-37005) for the period 2011–2015.