On the Verge of a Global Climate Crisis: CENN Analyses the Hazards of Disastrous Flooding in Tbilisi

The global climate crisis is expected to lead to drastic changes in our environment. Extreme weather events in future are likely to increase the number and scale of natural disasters. Urban areas, and big cities in particular, are complex and interdependent systems, extremely vulnerable to threats from natural hazards. Considering this, it is essential to have a comprehensive strategy of urban hazard mitigation aimed at the creation of resilient cities, able to withstand potential threats.

With this in mind, CENN recently published a new edition of the study ‘Hazards of Disastrous Flooding in the City of Tbilisi’. The study was conducted with the support from the Austrian Development Cooperation (ADC) and analyzes the physical-geographical and geo-ecological preconditions for the occurrence of catastrophic flash floods on the tributaries of the River Mtkvari within the limits of the city of Tbilisi. In addition, it offers an assessment of the rehabilitation works undertaken since the 13 June 2015 Vere River flash flood, and gives some general recommendations for the mitigation or prevention of negative effects of such extreme natural events. This comprehensive look at the River Mtkvari urban watershed in Tbilisi is a key storm water management planning tool to prevent future catastrophic flooding and loss of life and property.

The CENN study notes that the tributaries to the River Mtkvari are characterized by narrow channels and steep gradients, which already predisposes them to flash flood events under certain climactic conditions. A long history of watershed development, including deforestation in the upper reaches, urban development in low lying areas, disposal of household waste and illegal dumpsites in the riverbeds leaves the River Mtkvari tributaries without the capacity to contain and convey storm water runoff during peak storm events. Increases in precipitation intensity and duration due to climate change underscore the need to catalog and address these anthropogenic factors. As an example, in photo 1 we can see intensive construction taking place on top of the tunnel laid beneath the Vake-Saburtalo highway. This will create unfavorable conditions for the implementation of preventive measures aimed at improving the water conveyance capacity of the tunnel. At the same time, the surface of the right bank of the river at the intake of the water tunnel is being elevated with extracted soils to use this area for construction, which leads to the narrowing of the riverbed. It is unlikely that the tunnel, with its current water conveyance capacity, will be able to convey large volumes of runoff generated by strong flash floods. Under such conditions, water levels may reach higher marks than those that were observed at the intake of this water tunnel during the June 13, 2015 flash flood.

While reflecting on the situation in the Vere River, the CENN study states that in the aftermath of the devastating 2015 flood, several storm water runoff deficiencies in the Vere River were corrected, including reconstruction of the intake tunnel beneath Heroes Square, reinforcement of the riverbed, and construction of a dam at Mziuri Park. However, the height of the protection dam is under 4 m, and 2015 peak flood levels exceeded 7 m. Two bars were built into the Vere River to trap sediment and other debris. These two bars may be inadequate in preventing damage from large volume flash floods. While these measures are certainly beneficial, a more effective, long-term solution would be to remove the underground water tunnels and restore the natural riverbed. And while the implementation of an early flood warning system may prevent death and injury, populations in the flood zones and relevant agencies will have a maximum of 10-15 minutes in which to respond to the flood threat. This limited time highlights the importance of pursuing mitigation measures now to reduce potential flood risk and threats to life and property.

After analyzing the geo-ecological conditions on the 13 tributaries within Tbilisi‘s urban portion of the River Mtkvari watershed, the recurring issues identified include undersized storm water conveyances, lack of storm water system maintenance and obstruction of channels with debris.

To demonstrate this, in photo 2 we can see the confluence section of one of the River Mtkvari tributaries - Gldanistskali River - being used for the disposal of household and industrial-construction waste, polluting the river and so reducing its water conveyance capacity.

The CENN report concludes with a number of important lessons learned for future development and flood prevention:

• Development should not occur in a floodplain zone;

• If floodplain development is unavoidable, storm water conveyances need to be sized for a hundred-year flood peak discharge event;

• Future urban development needs to take into consideration watershed geomorphology and stream discharge patterns, as well as incorporate storm water management infrastructure;

• Proper maintenance of storm water management systems (culverts/tunnels) is essential to prevent blockages that result in dangerous impoundment of floodwater;

• Controlling and reducing illegal dumping of household, construction, and industrial waste in stream channels is imperative to keep stream channels and tunnel intakes clear;

• Wherever feasible the natural stream channel should be restored.