Flash flood forecasting in Slovakia Michal Hazlinger Slovak Hydrometeorological Institute Ljubljana

Input informations - precipitation Problems with measurement – discrete raingauge stations Problems with measurement – discrete raingauge stations Problems with data gaps Problems with data gaps Automatic station network since 2004 Automatic station network since 2004 Necessarity of spatial precipitation field – based on combination of radar measurement and station data – INCA precipitation field Necessarity of spatial precipitation field – based on combination of radar measurement and station data – INCA precipitation field

Input data in Myjava watershed and „Orava“ pilot basins

INCA precipitation field

Forecast of runoff Hydrologic models - problems with events with very short duration Hydrologic models - problems with events with very short duration Testing of HBV variation for Slovak condition – HRON Testing of HBV variation for Slovak condition – HRON 1 hour forecasting step 1 hour forecasting step Calibrated for extreme events in 2011 (Gidra basin) and 2010 (Oravica river basin) Calibrated for extreme events in 2011 (Gidra basin) and 2010 (Oravica river basin) Validation in 2010 resp Validation in 2010 resp. 2011

Results of HRON simulation in Gidra river basin Rainfall input – APS and INCA precipitation field analyzes Rainfall input – APS and INCA precipitation field analyzes

Dispersion of simulated and observed datas Calibration data setValidation data set

Hydrologic model Rainfall – runoff model HEC-HMS Rainfall – runoff model HEC-HMS Basin model based on CN method Basin model based on CN method for calibration used input data from raingauge stations (1 minute and 1 hour time step), from watergauge stations and INCA precipitation field (based on shp files, 5 min. time step, 1 x 1 km spatial resolution) for calibration used input data from raingauge stations (1 minute and 1 hour time step), from watergauge stations and INCA precipitation field (based on shp files, 5 min. time step, 1 x 1 km spatial resolution) Calibration in Handlovka and Gidra river basin Calibration in Handlovka and Gidra river basin Calibrated for extreme events Calibrated for extreme events Output - Hydrogram of outflow or possibility / propability of exceeding warning level Output - Hydrogram of outflow or possibility / propability of exceeding warning level

Hydrological models Input from INCA precipitation field analyses (based on *.shp, 5 min. timestep, 1 x 1 spatial resolution) Input from INCA precipitation field analyses (based on *.shp, 5 min. timestep, 1 x 1 spatial resolution) HEC – HMS rainfall – runoff model – able to compute runoff in small watersheds HEC – HMS rainfall – runoff model – able to compute runoff in small watersheds System based on estimation of flood potential by CN method – 3 states of saturation System based on estimation of flood potential by CN method – 3 states of saturation

Calibration results for Handlovka and Gidra river basins Wave time of peak peak flow volume Real16:4544,5410 Minutovky16: INCA17:0545,20315 APS Modra 17:5048,8383 Wave time of peak peak flow volumereal15:008,685,23 Dry15:409,2100,3 Average15:20993,3 Saturated15:0025,9276,3 Gidra Wave time of peak peak flow volumereal14: Dry12:1551,1547 Average12:41179,22315 Saturated12:18298,43958 Handlovka Handlovka

Case Study – Gidra (Píla) 30 km – 600 m a.s.l. Average slope 10-15° granite + limestone region forested almost 100% 1000 –year flood

Calibration outputs

Handlovka basin model

Handlovka – calibration and validation outputs

Other methods available for flash flood forecasting 3 different tools 3 different tools 1. calibration of hydrological models for various types of watershed (based on natural condition) – weak coverage of Slovakia 1. calibration of hydrological models for various types of watershed (based on natural condition) – weak coverage of Slovakia 2. estimation of flood potential - based on CN method 2. estimation of flood potential - based on CN method 3. estimation of flood potential – based on parameters of watershed 3. estimation of flood potential – based on parameters of watershed  All these methods require cooperation with INCA analyzes of precipitation field and nowcasting and knowledge of antecedent precipitation (API)

CN method - estimate peak discharge We can use CN method to determine the peak discharge for relatively homogeneous watersheds ASSUMPTIONS: We shouldn´t use this method for runoff amount < 38 mm CN < 60 -T c time of concentraction < 10 hours

We need folloving inputs: - A - area of watershed (in square km) - determine antecedent moisture conditions (I,II,III) - determine CN (I,II,III) - estimate Tc – watershed time of concentration (in hours) - determine potential max storage A - determine initial abstraction IA IA = 0,2*A IA = 0,2*A if IA >P then rainfall event wouldn´t produce runoff if IA >P then rainfall event wouldn´t produce runoff

- P - total amount of precipitations - determine the accumulate runoff -determine the unit peak discharge coefficients c0,c1,c2 are estimated with regard to IA/P (see in following table) IA/P (see in following table) if IA/P is outside the bounds of table, use more precise method (for example HEC-HMS)

Rainfall Type I a /P C0C0C0C0 C1C1C1C1 C2C2C2C2 II III Ponding Adjustment Factor % Ponded/Swamp Area Factor (F) determine the pond adjustment factor F as follows: compute the peak discharge: coefficients c0,c1,c2 are estimated with regard to IA/P (see in following table) IA/P (see in following table) if IA/P is outside the bounds of table, use more precise method (for example HEC-HMS)

where: Q = peak discharge (m 3 /s) Q = peak discharge (m 3 /s) q u = unit peak discharge (m 3 /s/km 2 /mm) q u = unit peak discharge (m 3 /s/km 2 /mm) A = drainage area (km 2 ) A = drainage area (km 2 ) R = runoff (mm) R = runoff (mm) F = ponding factor F = ponding factor We use this method for basin Handlovka for flash flood The results: Time of concentration CNCNCNCNQsimQobs 6II11,309,854 III52,339,854 5II12,9079,854 III57,1279,854

For basin Gidra: Flashflood (Precipitation amount from INCA analyse 85,9 mm) Time of concentration CNQsimQobs 5II28,244,5 III71,1644,5 6II24,4844,5 III61,9544,5

Operative system of flash flood forecasting Based on analyses of precipitation field and on nowcasting of precipitation Based on analyses of precipitation field and on nowcasting of precipitation Covering of layer of precipitation and layer with CN method Covering of layer of precipitation and layer with CN method Simple program for computing flood peaks Simple program for computing flood peaks 2012 summer storm season – calibration period 2012 summer storm season – calibration period Output in the form of ansambel forecast Output in the form of ansambel forecast Necessarity of trainings for stakeholders Necessarity of trainings for stakeholders

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