A flood risk assessment and mapping for Riga city U. Bethers, J.Seņņikovs Laboratory for Mathematical Modelling of Environmental and Technological Processes, Faculty of Physics and Mathematics, University of Latvia Riga (population ca. 800,000) is the capital of Latvia. The city is located at the southern coast of Gulf of Riga along the lower stretch of the River Daugava. 4. Scenario calculation The water bodies adjacent or within the territory of the city are the River 2. Model Daugava itself, its connection (Bullupe) to another major river – Lielupe, The finite-element based The hydrodynamics of the flood several minor tributaries of the River Daugava, as well as an interconnected shallow water model was scenarios were calculated for the W lake system of estuarine origin. The flow in the lower stretch of the River a built for the area, model storm situations. The t Daugava is regulated by Riga hydropower plant which is situated approx. 25 potentially vulnerable for importance of the dynamical e kilometers upstream the river mouth. The lowest 12 kilometers of the river r flooding. Heterogeneous modeling of flooded areas was l channel is dredged to ensure the operation of the Riga Freeport. depth/terrain information shown for the domain with a e The aims of the study were (1) the identification of the flood risk situations, (2) the v from various sources was complex channel system and e quantification of the flooding scenarious of different return periods, (3) the building integrated in the model. typical length of storm event l and calibration of the hydrodynamical mathematical model for the domain potentially , The linear objects below 18 hours. Figure: water m vulnerable for flooding, (4) the calculation of flood events with different return period, (watercourses, dams, level time series for storm with and (5) the detailed (horizontal accuracy around 10 m) mappping of the potentially etc.) of hydraulic return period 10 years. Time flooded areas. importance were included in the model. The typical spatial resolution of 1. Scenarious approx. 50-100 m was reached with total number 5. Flood mapping of finite elements around 250,000. The method for the mapping of the results of hydrodynamical calculations on the digital terrain map of much higher (10 m) spatial resolution was proposed and applied. See 325000 illustration of flooded area correction. 18.33 324800 14.62 324600 10.91 324400 7.204 324200 3.494 324000 -0.216 323800 -3.926 323600 -7.636 323400 -11.35 323200 -15.06 323000 322800 -18.77 322600 -22.48 The combination of storm surges in the southern part of Gulf of Riga with 501000 501500 502000 502500 503000 503500 unfavorable regime of hydropower plant operation was found as the most dangerous flooding situation. The time series of water level at the mouth of the 6. Examples of flooded areas River Daugava was analysed for more than 130 year long time period. The significant trend was found in the annual peak water level. Five significant storm events were found in time period 2001-2007 which roughly correspond to storm surges with return period once in 5, 10, 20, 50 and 100 years. The model 3. Calibration storm events were created by scaling waterlevel and meteorological conditions 3 during these selected events, and superposing them with hydropowerplant Andrejosta - observed operation regime. K īš ez ers - obs erved 2,5 R īgas HE S - obs erved Andrejos ta - calculated Ķ īš ez ers - calculated 2 R īgas HE S - calculated Waterlevel, m 1,5 The hydrodynamical model was calibrated on the basis of water level observations in 5 different locations during 5 selected real storm events. Model 1 Traffic junction (left above), river delta forcing was hourly observations of 2 sea levels (Lielupe, Daugavgrīva), (right above), northern part of Ķīšezers 0,5 Daugava (at HPP) and Jugla river discharges, and wind speed. Calibration lake (below). Return period 10 years results are illlustrated for the storm of Jan-2005 comparing hourly observed and 2005.01.09 1:00 2005.01.09 3:00 2005.01.09 5:00 2005.01.09 7:00 2005.01.09 9:00 2005.01.08 9:00 (green), 100 years (red). 2005.01.08 13:00 2005.01.08 15:00 2005.01.08 17:00 2005.01.08 19:00 2005.01.08 21:00 2005.01.08 23:00 2005.01.09 11:00 2005.01.09 13:00 2005.01.09 15:00 2005.01.09 17:00 2005.01.09 19:00 calculated water levels 2005.01.08 11:00.