Multifunctional Floodprotection, Indonesia

Introduction

Rotterdam University of Applied Sciences (RUAS) and the Unissula University in Semarang, Indonesia, are cooperating to develop solutions for the water-related problems in the Banger polder in Semarang and surrounding areas. The Banger polder is a densely populated low lying area with an out-dated polder system established in the colonial era. The area is subsiding due to groundwater extractions. Currently about half of the area is situated below mean sea level. Heavy rain showers cannot be drained anymore under free flow leading to frequent pluvial and fluvial flooding. In addition the probability (and risk) of coastal flooding is increasing due to the relative see level rise. A full description of the problems in the Banger polder and potential solution strategies can be found.

This project focuses on multifunctional usage of flood protection. The Dutch experience in the field of flood protection is very important in this project. For the Indonesian colleagues in Semarang a tutorial will be made about maintaining a water retaining structure.

Background

Semarang is the fifth biggest city in Indonesia with almost 1.8 million inhabitants. Another 4.2 million people are living in the surrounding areas of the city. The economy in the city is booming, in the past years a lot has been changed and in the future there will be more changes. The urge of trading and the need of industry are causing an increasing economy, which increases the business climate. These developments cause an increase in the purchasing power of the population. It can be concluded that the city is growing, but unfortunately there is also a growing problem: the city faces floods which are frequently increasing. These floods are mainly caused by the subsidence of the inner land which is decreasing by extracting groundwater in large quantities. These withdrawals cause a subsidence of about 10 centimetres per year. (Rochim, 2017) The consequences are big: the local infrastructure is damaged which results in more accidents and traffic congestions. In addition, more and more people leave their homes as a result of the increasing floods. The locals are trying to deal with the problems, but it is more a solution to live with the problems. The solutions are abandoning the low laying homes or raising the current infrastructure. These solutions are short term solutions and will not be very effective.

Objective

The objective of this paper is to look into the possibilities of protecting the city of Semarang against flooding. The main problem is the sinking soil in the city, this will increase the number of floods in the future. First of all the multifunctional flood barrier will protect the inhabitants of Semarang. The most important part of this objective is to tackle the societal and professional problems. The societal problem is, of course, the flooding in the Semarang area. The professional problem is the lack of knowledge about defence against water, the subsidence of the soil layers is part of this lack of knowledge. These two problems are the fundament of this research. In addition to the main problem, it is an objective to teach the inhabitants of Semarang how to maintain a (multifunctional) flood barrier.

More information about the information about the delta project in Semarang can be found in the following article;

https://hrnl-my.sharepoint.com/:b:/g/personal/0914548_hr_nl/EairiYi8w95Ghhiv7psd3IsBrpImAprHg3g7XgYcNQlA8g?e=REsaek

The first step is to find the right location for a water storage area. For our case this location is off the coast of Semarang. This location was first used as a fishpond, but is now no longer in use There are a two rivers in this area. By making a water storage here, the discharge of these rivers can be stored in the water storage area. In addition to the function as water storage, the dike also acts as a sea defense. So this makes it the perfect location to use this location as water storage area.

To build a dike, an investigation into the soil structure is important. The construction of a dike must be done on solide ground(sand). If the dike is built on a soft ground, the dike will settle and no longer meet safety requirments.

If the soil consists of a soft clay layer, a soil improvement will be applied. This soil improvement consists of a sand layer. When it is not possible to adjust this soil improvement, than it will be necessary to think about adapting other floodprotecion constructions. The following points offer a few examples for a flood protection;

• beach wall
• sand supplementation
• dune's
• sheet piling

the third step is to analyze the information for determining the height of the dike. The dike will be designed for a number of years and therefore, a number of data will be examined to determine the height of the dike. in the Netherlands there are five subjects that are being investigated to determine the height;

• Reference level (Mean Sea Level)
• Level rise due to climate changes
• Tide difference
• Wave run-up
• Soil subsidence

By determining the dike trajectory, the dike lengths can be determined and what the surface of the water storage area will be.

For our case the polder needs 2 types of dikes. One dike that meets the requirements of a flood defense (red line) and one that functions as a dike for the water storage area (yellow line).

The length of flood defense dike(red line) is about 2 km and the length of the dike for the storage area (the yellow line) is about 6.4 km. The surface of the water storage is 2.9 km².

In order to determine the height of the dike(yellow line), a water balance will be required. A water balance shows the amount of water that flows into and out of an area with a significant precipitation. From this follows the water that has to be stored in the area to prevent flooding. On this basis, the height of the dike can be determined. If the height of the dike is unrealistically high, another adjustment will have to be made to prevent flooding such as; higher pomp capacity, dredging or larger surface area of the water storage.

the information to be analyzed to determine the water that has to be stored is as follows;

• Significant precipitation
• Surface water catchment
• evaporation
• pump capacity
• water storage area

Waterbalance

For the water balance of our case, a normative preciptation of 140 mm(Data Hidrology) a day has been used. The drainage area that runs off on the our water storage covers 43 km². The water that flows out of the area is the average evaporation of 100 mm a month and the pump capacity of 10 m³ per second. These data have all been brought to m3 per day. The outcome of the inflow data in and outflow data gives the number of m³ of water that needs to be recovered. By spreading this over the storage area, the level rise of the water storage area, can be determined.

Dike 2

Water level rise

The height of the dike is partly determined by the water storage area level rise.

Design life

The dike is designed for a lifespan up to 2050, this is a period from 30 years from the design date.

Local soil subsidence

The local subsidence is one of the main factors in this dike design because of the subsidence of 5 – 10 centimetres a year due to groundwater extraction. The maximum is assumed, this gives a result of 10 cm * 30 years = 300 cm equals 3.00 metres.

Volume balance construction dike

Length of the dike is about 6.4 kilometres.

Area clay = 16 081.64 m²

Volume clay = 16 081.64 m² * 6400 m = 102 922 470.40 m3 ≈ 103.0*10^6 m3

Area sand = 80 644.07 m²

Volume sand = 80 644.07 m² * 6400 m = 516 122 060.80 m3 ≈ 516.2*10^6 m3

The following points were used to determine the height of the dike for the sea dike

Dike 1

Design life

The dike is designed for a lifespan up to 2050, this is a period from 30 years from the design date.

Reference level

The reference level is the base of the design height of the dike. This level is equal to the Mean Sea Level (MSL).

Sea level rise

Surcharge for high water rise for the coming 30 years within a warm climate with a low or high value change of airflow pattern. Due to lack of information and location specific knowledge the maximum of 40 centimetres is assumed.

High tide

The maximum flood in januari that occurs for our case is 125 centimeters(Data Tide 01-2017) on top of the reference level..

Overtopping/wave run-up

This factor defines the value that occurs during wave run-up at maximum waves. Assumed is a wave height of 2 meters(J.Lekkerkerk), wavelength of 100 m and a slope of 1:3. The calculation for the overtopping is als volgt;

R = H * L0 * tan(a)

H = 2 m

L0 = 100 m

a = 1:3

R = 2 * 100 * tan(1:3) = 1.16 m

Local soil subsidence

The local subsidence is one of the main factors in this dike design because of the subsidence of 5 – 10 centimetres a year due to groundwater extraction. The maximum is assumed, this gives a result of 10 cm * 30 years = 300 cm equals 3.00 metres.

Volume balance construction dike

Length of the dike is about 2 kilometres

Area clay = 25 563.16 m2
Volume clay = 25 563.16 m2 * 2000 m = 51 126 326 m3 ≈ 51.2*10^6 m3

Area sand = 158 099.41 m2
Volume sand = 158 099.41 m2 * 2000 m = 316 198 822 m3 ≈ 316.2*10^6 m3

Dike management is the maintenance of the dike; this will mean that the outside part of the dike must be maintained. Next to spraying and mowing, there will be a check on the strength and the stability of the dike. It is important that the conditions of the dike are agree with the safety requirements.

The Dikemanagmener is responsible for supervision and controlling at critical moments. This will mean that the dike must be inspected in case of a high predicted waterlevel, prolonged drought, high rainfall runoff river floats of floating containers. This work is carried out by trained personnel who know how to handle in critical situations.

Necessary materials

• Report pick
• Measuring pick
• Map
• Note

The "capacity building material" gives further information about the importancy of dike management and the use of the needed materials.

failure mechanism

There are various possible threats for a dike to collapse. An threat can be caused by high water, drought and other influences that can make the dike unstable. These threats can grow to the aforementioned failure mechanisms.

The following bullet-points shows all of the failure machanism;

• Micro instability
• Macro instability
• Piping
• Overflow

Piping can occur when groundwater flows through a layer of sand. If the water level is too high, the pressure will rise, which increases the critical flow velocity. The critical flow of the water will exit the dike in a ditch or seepage. As time goes by, the pipe will be wide by the flow of water and sand. During the widening of the pipe, sand can be carried along, which can cause that the dike will collapse by its own weight.

fase 1

Water pressures in the water-bearing sand package under the dike can become so high during high water that the inner covering of clay or peat will bulge. At an eruption, water exits take place in the form of wells.

fase 2

After the eruption and flooding of water, sand can be entrained if the water flow is too high. An outflow of quicksand is created

fase 3

In case of a too large discharge flow of sand, an excavation tunnel will arise by size. If the pipe becomes too wide, the dike will collapse.

measure againt dike failure

In order to make the dike stable, counterpressure must be provided, which can be done by placing sandbags around the source.

For more information and examples of failure mechanics, view the following powerpoint;

https://hrnl-my.sharepoint.com/:p:/r/personal/0914...