USING GIS TECHNIQUES AND RAINFALL WSR-98D RADAR ESTIMATIONS FROM FAST PRECIPITATIONS MONITORING IN SMALL CATCHMENTS.

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1 USING GIS TECHNIQUES AND RAINFALL WSR-98D RADAR ESTIMATIONS FROM FAST PRECIPITATIONS MONITORING IN SMALL CATCHMENTS Petru Ioan ROMAN 1, Octavian ŞTRENG 2 1 Crisuri Rivers Authority, Oradea, 35 Ion Bogdan str., , Romania petru.roman@dac.rowater.ro 2 Crisuri Rivers Authority, Oradea, 35 Ion Bogdan str., , Romania octavian.streng@dac.rowater.ro Abstract: The demand for rainfall diagnosis / prognosis with spatial and temporal high resolution is increasing, concerning flash floods in small catchments. The main objective of this paper is represented by precipitations amount fast monitoring and small catchments impact, with no available precipitations measurements data. Using rainfall WSR-98D radar products and Crisuri watershed layers, a diagnosis rainfall GIS project was done. The precipitations case study causing flash floods in middle course Bistra s small catchments and the calculus of medium precipitations amounts was realized in Barcau basin. The precipitations falling between August 23 rd 24 th 2005, were produced by severe convective storms across western Romania, presented there. The question of the diagnose utility and the limitations of the operational Weather Surveillance Radar 1998 Doppler (WSR-98D) was posed. Built-in strategies (GIS, hydrology and radar) were also presented. The application results could be useful for rainfall monitoring and floods decision making in small catchments with no available precipitations measurements data. Keywords: GIS, georeference, vectorization, flash floods, rainfall radar estimations. 1. Introduction Natural causes of recent floods are generally associated (as some specialists say) to climate changes. The damages that are generating by hydro meteorological phenomena have aroused the interest of studying a special kind of floods (flash floods). This type of floods is produced as a result of short duration rainfall (short duration rainfalls with a relatively high peak discharge) especially in watersheds less than 60 km 2. The fast convective systems development and rapidly rainfall (depth) accumulations increase the river discharge and the flood probability in their small watershed. The magnitude of this rainfall that conduct to depth accumulation of water mainly depends of climatic conditions and watershed features: area, vegetation and shape of watersheds, slope and slope gradient, river bad, soil permeability and moisture, and land use (human modifications). One of the main contributions of GIS is to build and maintained in the spatial database all the changes that appear in these river basins. The meteorological radars are used in spatial diagnose of precipitation accumulations. Generally, the anticipation time of the phenomena that are coming with convective systems, causing flash floods is between minutes and up to 2 3 hours. Often, the convective systems are developing and can modify their trajectory. This reduces the forecast precipitation chances up to a few minutes before the start of the rain. Page 1 of 9

2 The fast localization and signaling of the rainfalls in the small watershed represents precious information that is used by the authorities in rapid actions that are meant to reduce the negative effects after the event. The radar rain rate charts can be quickly georectificate and easily use to crisis situation management - monitories floods, especially flash floods. 2. Study area - Bistra s watershed A small area between Crisul Repede and Barcau basin was chosen. The study area (Figure 1, Table 1) is represented by the Bistra hydrographic basin affected by flooding. Drainage divide is between Bistra s watershed (tributary of Barcau) and three small catchments: Izvor, Loranta and Valea Morii. We chosed this subject due to the amount of precipitation collected between August 2005, with big discharge and material damages. Figure 1. Localization of study area (rectangle distinct) Bistra watershed (tributary of Barcău) in Crisul Repede Basin. River Table 1. Bistra and Izvor morphometric watershed data Confluence position Length Altitude (m) (km) upstream downstream Medium slope ( ) Tortuosity coef. Area (km 2 ) Medium altitude (m) Content forest area (ha) Bistra S Up confl. Cuzap Cuzap D Up. confl. Varvizel Varvizel D Up. confl. Rovine Rovine S Izvor D Up. confl. Valea Morii Page 2 of 9

3 3. Working methods The information quality and processing is most important in flash flood efficient management. This includes: acquisition, validations, fast processing, diagnose / prognoses elaboration and the measures taken for the diminution of the effects and/or post management event. Generally, rainfall data that is included in the hydro meteorological system (measured with rain gauge) is available only at a certain time interval that is not enough for a crisis flood management. For the small catchments, generally less than 20 km 2, these interpolations and extrapolations can lead to errors that are amplified, when related to convective systems with strong cores that mostly effect small areas. Digital radars estimate rainfall rate over a large area with a very good spatial and temporal resolution. The observations are made continuous and the data are immediately obtainable. The objective of this paper is the processing of these products in order to monitorize and to use operatively the information, by using GIS techniques. Radar data products were obtained from Oradea Radars Observatory. Radar chart (Figure 2) presents water quantities accumulated in approximately 20 hour (since on 23 August until on 24 August 2005). Figure 2. Storm Total Rainfall radar chart estimation since on 23 August until on 24 August 2005 (radar product STP80 (Storm Total Rainfall) Weather Surveillance Radar 98Doppler S-band.) Page 3 of 9

4 First stage is to improve the radar information by superposition of hydrological themes over the radar chart that has to be georectificated. This was made by GIS application. The radar chart was added general layers (relief, subbasins, river streams etc.) to show the relative positions of the areas with large amount of precipitations. The results of the first stage can provide to Dispecerat / Defence departments a map of the subbasins with large amounts of precipitations estimated within a certain period of time (between one hour up to 24 hours) that can lead to the rapid increase of rivers discharge. This GIS project may be useful for monitoring nearly real time floods (Figure 3). Figure 3. Stage 1 - superposition of hydrological themes over the radar chart with the amounts of precipitations (the estimated amounts of precipitation since on 23 August until on 24 August 2005) 4. The obtaining of the medium quantities of precipitations in small watershed from radar data Second stage consists in using GIS application to calculate the amounts of precipitations in flash floods susceptible watershed. Few preliminary radar chart treatment methods (limitation intermediate colors, reducing shade) were used. Page 4 of 9

5 Few automatically vectorisation softs contain this preliminary methods. There was also necessary to make manually adjustments to eliminate or reduce redundancy of radar chart elements: standard distance rings, axes, name of localities, various administrative contours etc. The magnitude of radar bin (pixel) was taking into consideration. 5. Case Study: the flood between August 2005, in the Bistra watershed For the interval between 23 th 24 th August 2005, E.S.S.L. (European Severe Storm Laboratory) estimated the possibility of severe convective storms in Romania. In the opinion of E.S.S.L., the meteorological models have forecast a wind shear of m/s in layer 0 6 km, enough to provoke convective storms with long duration ascending currents and possible rotations (mesocyclons). Besides, in some areas they estimated small movement speeds of the convective cells that can lead to local accumulation of large amounts of precipitations and possibly to flash floods. In the 22 nd of August 2005 convective cores produced large amounts of precipitations: in Popesti (Bihor) the amount of precipitations have reached 57.0 mm, between , that led to the increase of soil moisture and of the runoff coefficient of the following days (Table no. 2).Daily amounts of precipitations (mm) at hydrometrical station in the studied and surrounding area Station / Date 23 August August August 2005 Marghita Marca Chiribiş Balc Sărsig Pădurea Neagră Although the precipitations in the Bistra watershed have totalized a medium calculated water depth (Figure 4., Table no. 3) of 32.8 mm (since on 23 August until on 24 August 2005), due to the short period of the rainfall they caused (over the moistured soil since the 22 nd of August) the important runoffs from slopes and the integration in the permanent and impermanent hydrographical network of the Bistra s tributaries. Medium values of the precipitation classes from the radar chart were used in calculations. The calculation of the medium precipitations from the hydrographical watershed was made similar to the method of the Thiessen polygons (Drobot şi Şerban, 1999), by using the (formula 1): h W m 1 = hi f i, (1) F i=1 where: : the area (partial) with a certain amount of precipitations estimated by radar; f i h i : medium value of the precipitation classes from the radar chart, in mm; m : the number of areas from the same class of precipitations; F : the area of the watershed, in km 2. Page 5 of 9

6 In the second stage, the medium amounts of precipitations from the watersheds were established. The areas with different precipitations amounts (estimated by the radar) were measured (Table 3). Table 3. Medium amounts of watersheds precipitations calculated by radar data, since on 23 August until on 24 August 2005 Watershed Area(km 2 Medium calculated amounts ) of precipitations (mm) Valea Hraptinului* Valea Vestică* Valea Ţiganului* Cuzap Varvizel Rovine Bistra Izvor Secătura Valea Morii * uncadastred rivers. Figure 4. Medium values of the precipitations amounts from the watersheds. Page 6 of 9

7 For the calculations of the medium amounts of water in the flooded watershed were used: areas of the hydrographic watershed, vectorized areas of the precipitation amounts estimated by radar. The amounts of precipitations accumulated between August 2005 lead to the formation of the flood wave that affected the localities from Bistra s lower and medium basin (Table 4). On the southern slope of the Meseş Mountains the effects of the flood lead to the blocking of the national road DN 1H Aleşd - Şinteu at 6 km, due to the overflow of the Izvor River. Table 4. The effects of the hydro meteorological phenomena between August 2005 in the localities on the Bistra River The damaged objectives in the localities on the: Bistra river upstream tributary Cuzap Cuzap Varvizel Bistra River downstream tributary Varvizel Physical criterion / Localities Budoi Voivozi Popeşti Cuzap Vărzari Varviz Bistra Affected houses 4 No. of flooded houses evacuated 2 2 Yards & gardens Agriculture land (hectares) 80 Fountains 185 Bridges 4 roads km 0,55 National road DN 108H Pădurea Restricted road area Neagră -Voivozi Ciutelec inpermanent accumulation did not function during this flood *Authorities Report National road DN 108H Vărzari Varviz 6. Discussion and perspectives Flash floods are extreme hydrological event with the societal impact. A good cooperation between hydrologists and meteorologist was realized. This case study refers a flash flood with a 5% assurance at Chiribis (hydrological station located near to Bistra s confluence with Barcau River). The maximum discharge in 2005 year at Chiribis 54 m 3 /s, were produced in 24 th August, during this flash flood. At distances > 100km, due to the opening angle of the antenna and polar system of coordinates, the radar bin (pixel) increases (WSR 98Doppler S-band) (Ferree, J., 2002). Fig. 5. The increase of the radial distance of the radar polar bin. Page 7 of 9

8 The increase of the radial distance of the radar polar bin, with an opening of 1 degree and 2 km length can induces in the case of small catchments located at distances > 100km some limitations of the areal precipitations estimations. The precision degree of GIS processing does not induce large errors in the evaluations of the areas with different amounts of precipitations in small catchments. The automatization of these methods in currently in an experimental stage, mostly in obtaining medium estimated quantities of precipitations in hydrographical basins with the help of an estimation method of the precipitation field (the transformation of the radar chart information in digital grid format). An increase of the efficiency in flash floods monitorization might be realized through common methods and strategies (GIS, hydrology and radar). the delimitation of the basins on the map scale of 1: or by using DTM with fine resolution; knowing the main characteristics of each basin and the definition of the flash flods (larger than) >3 km 2 ; the establishment an specifically medium rain / basin from witch the risk of the flooding increases and of a threshold of flash flooding by using the characteristics of the hydrographically basin between 3 60 km 2 ; the comparison of the accumulated rain amounts with the value of the flooding threshold; the decrease of the positioning uncertainties of every radar bin towards each basin. 7. Conclusions The processing of radar charts with the help of GIS techniques improves the information regarding the precipitations amounts in the hydrographical basins system that can be rapidly flooded and offers useful indices in hydrological precipitations diagnosis and in the after event interventions. The calculation of medium rainfalls on subbasins from radar data represents an alternative to the diagnosed estimations of precipitations for the subbasins without available data. The calculation results of the precipitation amounts estimated with meteorological radar in hydrographical basin can be used as input data in later processing and in rainfall runoff models. 8. Acknowledgement Authors acknowledge gratefully to Mr. Radu Barbus from Oradea Radars Observatory for radars products support. 9. References Moore, D., Imbroane, M. (1999): Initiatory in GIS and Remote Sensing. 150p, Editura Presa Universitară Clujeană, Cluj-Napoca, Romania (in Romanian). Drobot, R., Şerban, P. (1999): Water management and hydrology aplications. 376 p, Editura H*G*A*, Bucureşti, 16-20, Romania (in Romanian). Şerban, P., Stănescu, V. AL., Roman, P. (1989). Dynamical Hydrology. p 38-43, Editura Tehnică, Bucureşti, Romania (in Romanian). Ferree, J., (2002). Flash Flood Monitoring and Prediction. (FFMP). Warning Decision Training Branch. Norman, OK. Severe Weather/Flash Flood WDM. July 9-12, 2002, Page 8 of 9

9 Musy, A. (1998) Hydrologie Appliquée, 365 p, Ed. HGA, Bucharest, Romania. - Convective Forecast - valid Tue. 23 Aug 06:00 Wed. 24 Aug 06: (UTC), issued: 22 Aug 23:52 (UTC), forecaster: Van der Velde); Page 9 of 9