1. The Evaluation of Land and sea level in attempts to control drainage system of tidal flood area in Semarang S. IMAM WAHYUDI 1, HENNY PRATIWI ADI 1, GILBERT LEBRAS 2 1 Department of Civil Engineering, Faculty of Engineering, Universitas Islam Sultan Agung (UNISSULA), Semarang, Indonesia 2 Institute de Recherche en Génie Civil et Mécanique (GEM), IUT de Saint Nazaire, Université de Nantes, Nantes, France International Commision Irrigation and Drainage (ICID) 63 rd International EC Meeting & 7th Asian Regional Conference Droughts, Floods, Environment, Managing Consumptive Water Needs Sustainably 24 -29 June 2012 in Adelaide, Australia

2. PLAN OF PRESENTATION 1. INTRODUCTION 2. LITERATURE 3. METHODS AND MATERIALS 4. RESULTS AND DISCUSSION 5. PILOT PROJECT OF POLDER SYSTEM IN SEMARANG, INDONESIA 6. CONCLUSION

3. The phenomenon of land subsidence and sea level rise occurred in the area of the north coast of Central Java, Indonesia. These conditions can be identified when there is no rain in some coastal areas inundated by sea water during high tide Sea water rise affect submerge some of the land flat coastal area through the estuary, river and drainage system. The objectives of this study are; to clarify the land subsidence, the rise of sea level phenomenon and get sustainable drainage system to overcome tidal flood. 1. INTRODUCTION

4. 2. LITERATURE STUDY Sea level rise Changes in average height of sea water in the world since the Year 1889 to 2000 measured from the region with a stable geological environment can be seen in Figure 1a. Arnold Molenaar (2008) and Johan Helmer (2010) who predicted an increase of sea water due to global warming as in the following Figure. (a)Changes in sea surface height (b)Graph of predicted sea water rise in Rotterdam (a)(b) In nord Java is about 1,5 cm per year according to Subandono, 2010

5. Land subsidence Land subsidence in Semarang can be discovered from several data sources. From field observation, example of land subsidence in the Tanjung Mas coastal area is between 5.5 to 7.23 cm per year (Wahyudi, 2001) Drainage system to overcome tidal flood  Polder System Polder system is a way to handle the flood with the infrastructure, which includes drainage systems, retention basin, dike surrounding the area, moveable gate as well as pumps as one integral water managements.

6. 3. Methods and Materials The land level observations were applied by using a stable palm. The height of the palm is tightened to a Bench Mark (BM}. The BM representing the stable land level is within about 5 miles. The measurements of land subsidence area consists of 5 (five) spots in the north of Semarang city. The first level measurements have been carried out in the end of 2008, then it is controlled in the end of 2009 and 2010. To evaluate seawater level, the researcher conducts a periodic observation. The first observation was conducted respectively in November 2008, December 2009, and 2010. The standard method of tide of seawater observation is conducted for a 15 days observation with 60 minutes of interval. Based on measurement and field investigations in tidal flood areas, a mathematical model is created to simulate the water elevation in the drainage system area. The method will be applied to the Semarang Polder System.

7. 4. RESULTS AND DISCUSSION The results can be divided in three points: - land level observation in coastal drainage area, -sea water level fluctuation -drainage system controlling

8. 4.1 Land level observation Based on measurements and analysis, Table below gives a difference of elevation for 1 fixed BM and 5 spots in polder drainage area. The measurement and analysis of land subsidence in the area around of North Semarang shows an average of 6.5 cm per year. BM / spot Name November 2008 (m) December 2009 (m) Land subsidence / year (cm) BM-fixed+ 37.295 0 Spot-1+ 1.854+ 1.789+ 1.7266.4 Spot-2+ 2.654+ 2.591+ 2.5246.5 Spot-3+ 1.956+ 1.890+ 1.8226.7 Spot-4+ 2.252+ 2.193+ 2.1326.0 Spot-5+ 1.442+ 1.375+ 1.3066.8

9. 4.2 Mean Sea level fluctuation analysis Tide of seawater is analysed by using Admiralty method. Base on the result of observation and analysis on mean sea level in November 2008, December 2009 and December 2010, it can be concluded that the mean sea level relatively increasing about 0.775 cm/year.

10. 4.3 Drainage system controlling After analysing land subsidence and sea water level, a simulation of flood discharge is conducted to figure out water level in the drainage area. Results shows that drainage area is inundated during Sea high water level. Another result shows that, dike and road are overtopping during rain intensity of 5 years period (case in midle of February 2012). Rainfall intensity 175 mm/day.

11. 5. PILOT PROJECT POLDER SYSTEM IN SEMARANG Description of the location Semarang city has about 2 million inhabitants and is Capital of Centrak Java province The land elevation in Semarang city is lower that the sea water elevation, and it cause tidal flood in the city. The water from upstream flows through two floodways. The drainage system between the two floodways is using a polder system Drainages system: 1.S ub Sistem Bulu & Tanah Mas 2.S ub Sistem Kali Kali Semarang: Asin, Bandarharjo Barat, Bandarharjo Timur, Kota Lama, Sub Sistem Tugu Muda 3.S ub Sistem Banger: Utara Selatan

12.  Polder Kali Banger has a catchment area of 675 ha in east Semarang district.  Pilot project shared with HHSK Rotterdam water board.  Project is underconstruction in Semarang river drainage polder system. Kali Banger Polder system consists of infrastructures i.e. - Northern Dike protecting the Kali Banger Polder area of sea - Eastern Dike protecting the polder area from East Floodway -- Kali Banger Dam closing the river between polder area and sea water - The function of pump station is to control the water elevation of polder area with a maximum capacity of 6 m 3 /s - Retention basin used to control water elevation before pumping. - The water elevation of polder area is controlled -2 m MSL. Pilot Project of Polder system in Semarang

13. - The institution that is responsible for managing the polder area is named Water- board of Banger Polder or SIMA (Schieland Semarang) established by mayor Decree of Semarang City. - The 1 st Indonesia Polder Institution basing stakeholder organization. The water-board members consist of inhabitants representative, Businessmen, University experts and Municipality officers. Institution management - The main duties of the water-board is occur its operational and maintenance of both technical and non-technical consisting on 5 aspects: Technic al Polder Management Legale Financial Institution Soscial

14. 6. CONCLUSIONS - The measurement and analysis of land subsidence in the area around of North Semarang shows an average of 6.5 cm per year. - Polder systems is chosen to overcome this problem. How to isolate the sea water flow and water elevation controlling. The polder infrastructure consists of pumps, channels, retention basin, dike, dam and movable gate. Polder system of Semarang Banger and Semarang riover area is actually under construction. - The mean sea level relatively increasing about 0.775 cm/year.