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Ljubljana Marsh: Modelling The Surface-groundwater Interaction In Floods Events Branka Bracic Zeleznik1 & Lidija Globevnik2 1 Public Water Utility Jp Vodovod-kanalizacija D.o.o., Vodovodna Ces

Congress: 2015
Author(s): Branka Bracic Zeleznik (Ljubljana, Slovenia), Branka Bracic Zeleznik
Public Water Supply JP VODOVOD-KANALIZACIJA d.o.o. Ljubljana1

Keyword(s): Sub-theme 2: Surface water and groundwater,
AbstractLjubljana Marsh: Modelling surface-groundwater interaction in flood events BRANKA BRACIC ZELEZNIK1 & LIDIJA GLOBEVNIK2 1 Public Water Utility JP VODOVOD-KANALIZACIJA d.o.o., Vodovodna cesta 90, 1000 Ljubljana, Slovenia branka.bracic.zeleznik@vo-ka.si 2University of Ljubljana, Faculty of Civil Engineering and Geodesy, Jamova cesta 12, 1000 Ljubljana, Slovenija lidija.globevnik@ INTRODUCTION TO LJUBLJANA MARSH The Ljubljana Marsh lies in the central part of Slovenia. On the North-East it stretches towards Ljubljana, the capital of Slovenia that is located on the Ljubljanica River. The river is the main stream of a large plain area with elevation between 288 and 300 meters that was covered by a lake approximately 4000 years ago. Due to the natural succession it changed into a high bog and marsh, named Ljubljana Moor. The system of vertically stratified aquifers is one of the biggest and important drinking water sources in the central part of Slovenia. Water resources of the area are under significant pressure. Agricultural land managers expect amelioration canals are regularly maintained. Land owners take advantage of high prices of land for urban development. Agricultural land and less commercially productive areas (marshes) are being changed to urban areas. New roads and transportation facilities are built, therefore the Ljubljana Moor landscape and its water regime are continually changing. Environmental problems include water pollution, rising demand for water, flood and drought risk and decline of water retention capacities, decreasing groundwater levels and terrain subsidence. As a consequence, wetland ecosystems are also at risk as are the economic and social stability of the area. To give answers and proposals for sustainable water and soil management measures with a view to protect water resources and wetland ecosystems, we develop an integrated water resources modelling and decision support system. MATERIAL AND METHODS Regular hydrological monitoring of water levels and water temperature is performed by the national hydrological service at Slovenian Environmental Agency. They also performed discharge measurements. The monitoring network of groundwater levels is operated by the Public Water Utility JP VODOVOD -- KANALIZACIJA d.o.o., Ljubljana. Monitoring is performed at 22 piezometers on the Iška gravel fan area and Ljubljana Moor and 12 wells at the Brest drinking water pumping station. For the modelling we obtained daily data of water level at all locations for 2010 and 2011. Ljubljana Moor is a basin with stone bedrock that consists in the southern part of Triassic dolomite and limestone and in the northern part of permocarbonic schists. The basin is filled with lacustrine, marshy and fluvial sediments. The upper layer, a zone up to 20 m deep, of the Ljubljana Moor consists of peat, peat mud and silty loam known as "snail clay". Below this is a thick zone of gravel with silt and sand. The edge of Ljubljanica Moor in the south consists of a very permeable gravel fan.When it leaves the narrow valley at the village Iška, the Iška River recharges the gravel fan. The gravel fan extends towards the flat surface of the Ljubljana Marsh. Modelling Analysis of water regime areas with wetland habitats typical for the Ljubljana Marsh are done by modelling surface and groundwater flow and interactions in flood events and during periods without precipitation. The water regime depends on two main factors. The general hydraulics of the Ljubljana Moor is driven by the Ljubljanica River flow. The river is 30 km-long and has a catchment area of 1700 km2. It drains the majority of the south western part of the Slovenia karstic area . The Ljubljanica River is a tributary of the Sava River and belongs to the Danube River catchment. It has many karstic springs in the western side of the Ljubljana Marsh and flows in a West--East direction. The yearly average discharge of the Ljubljanica River is 24.5 m3/s and in Vrhnika and Ljubljana is 55.4 m3/s. The water level of the Ljubljanica River is controlled by a dam located in the Ljubljana city. By a protocol the water level at the dam should be set constant at 285.6 m a.s.l at discharges lower than120 m3/s to hold water level high along the whole river stretch in the Ljubljana Marsh. When the water level starts rising at larger discharges, the gates at the dam open and water level of the Ljubljanica River drops. By this protocol the channel flow capacity at high water is increased, otherwise the hydraulic head in the Ljubljanica River along the Ljubljana Marsh holds soil water level. In recent years, due to flood control, the gates are released at lower discharges or set to lower level than 285.6 m a.s.l. Consequently, the water level in the Ljubljanica River drops more frequently and for a longer time. Furthermore, farmers demand that channels of tributaries and ditches are regularly cleaned to keep groundwater low. There is permanent drainage and recharge of groundwater from tributaries. The Iska River, one of the main tributaries of Ljubljanica River, flows from the South to the North and drains the southern part of the mountains. The river is 25 km long, of which 5 km lies in the Ljubljana Marsh area. The catchment area at the Iška vas is 74.5 km2. When it leaves the narrow valley at the village Iška vas, the Iška River recharges the gravel aquifer of Iška fan. The low river flow decreases to a minimum and increases after the geo-morphological break between the Iška fan and the flat Ljubljana Marsh surface. Here the river is partly recharged by shallow groundwater and springs. Springs exist along the whole gravel fan -- moor edge, recharging the wetland area in a northerly direction. A three level nested modelling approach was adopted. The first level was rainfall - runoff modelling of the Iška River. The second was a surface water network modelling of the Ljubljanica River and Iška River. We used flood hydrographs formed due to high precipitation in the catchment and runoff characteristics during periods without precipitation to define the dynamics. The third level iwas the surface -- groundwater flow exchange modelling of the Ljubljana Moor aquifer in the Iška River recharge area. The rainfall -- runoff was modelled using the HBV light model. The surface flow was modelled with MIKE 11 and MIKE FLOOD model (DHI) and the surface - groundwater modelling was done with the MIKE SHE module. RESULTS The daily precipitation values for the Iška River catchment are obtained by Thiessen method from stations Pokojišče, Želimlje, Črna vas, Sodražica and Cerknica for the period 2001-2011. All stations are situated around the watershed and are correlated. The efficiency of the calibrated rainfall- runoff model of the Iška using HBV software for the year 2010 is 0.80, an acceptable level for model performance. Nevertheless, the first calibration of the model coefficients with precipitation data from two stations (Pokoji ARSO Agency of Republic of Slovenia for Environment. Archive of hydrological data. http://vode.arso.gov.si/hidarhiv/pov_arhiv_tab.php. ARSO Agency of Republic of Slovenia for Environment (2013). Archive of meteorological data. http://www.meteo.si/met/sl/archive/ Breznik, M. (1975) Podtalnica Iškega Vršaja. Geologija 18, 289–309. Globevnik, L. and Vidmar, A., (2010) Poplave na Ljubljanskem barju September 2010 (Flood on Ljubljana Moor September 2010). Mišičev vodarski dan 2010, 22–26. Maribor. Globevnik, L, & Železnik, B. (2011). Drinking water extraction facilities at risk of flooding from rivers and groundwater – flood impact assessment for water extraction facilities in Ljubljana area. Risk in Water Resources Management. IAHS Publ. 347, 2011. GURS. Geodetic Survey of the Republic of Slovenia. Archive of digital elevation data in grid 5 m* 5 m and 1*1 m. Mencej, Z. (1976) Hidrogeološke razmere na zahodnem obrobju Ljubljanskega barja, GeoZS, Ljubljana, Slovenia. Mencej, Z. (1989) The gravel fill beneath the lacustrine sediments of the Ljubljansko barje. Geologija 31–32, 517–553. (1988/1998). Novak, D. (1977) Meritve pretokov izvirov Iškega vršaja. GeoZS, Ljubljana, Slovenia. Seibert, J.; Vis, M.; Käser, D. HBV light - A user-friendly catchment-runoff-model software. EGU General Assembly 2012, held 22-27 April, 2012 in Vienna, Austria., p.12203. VGI (1984) Vodnogospodarski inštitut. Ljubljansko barje. Hidrologija in analiza poplavnosti. Zvezek 1. C-350. Ljubljana. ZEVS, 2011. Statistika vremenskih podatkov. [http://statistika.zevs.si].
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