Japanese Experience

Within the Tokyo metropolitan area urbanisation increased rapidly. At the same time rainwater runoff, due to heavy rainfall, increased as well. The basin of the Motoara-river suffered large damages caused by floods. Within Koshigaya Lake Town (Fig. 4.16), a new development, a regulation pond was implemented in order to reduce flood damages (Koshigaya Land Development Office, 2005). The purpose is to create an environment, which is safe for the residents and comfortable (Fig. 4.17).

Tokyo Spatial Plan
Fig. 4.16 Location of the regulation pond (Source: Koshigaya Land Development Office, 2005)
Tokyo Spatial Plan
Fig. 4.17 A safe and comfortable environment (Source: Koshigaya Land Development Office, 2005)

The regulating pond is developed as an integrated part of the urban development of Koshigaya Lake Town. The regulating pond captures part of the excessive water during heavy rain from the Motoara-river preventing the river from flooding. In order to function well the development contains a flood control function, which includes an overflow-type of dike, a control gate, a diversion channel a drainage channel and a drainage gate (Fig. 4.18). Furthermore, the water quality and water level is controlled by a sluiceway for purification the channels and gates and a drainage pump station.

Fig. 4.18 The overflow dike, control gate, diversion and drainage channels and drainage gate (Source: Koshigaya Land Development Office, 2005)

The different elements of the system are visualised in Fig. 4.19. The regulation pond stores temporally water, which flows in the river at times of heavy rain. The drainage pump station pumps water from the regulation pond into the Naka-river. The diversion and drainage channels are connecting the pond both with the Motoara and Naka-river. The overflow-type of dike, which is realised lower than the neighbouring dikes, allows water to flow over the dike into the regulation pond in times of floods. The sluiceway for purification regulates the amount of water that runs in the regulation pond from the Motoara-river and cleans it at the same time.

Fig. 4.19 Functioning of the system with facilities (Source: Koshigaya Land Development Office, 2005)

The mechanism of the regulation pond works as follows (Fig. 4.20). Before a flood occurs the water level is kept low. During a flood the water flows over the overflow dike and the control gate is open. The drainage gate is closed to prevent that the water flows immediately in the Naka-river. After the flood happened the control gate is closed and the drainage gate is opened in order to let the water flow in the Naka-river and he water level is brought down to the original level.

Another mechanism is also used. In order to keep the water quality high, the regulation pond needs to be flushed through once in a while. The tidal differences are used to operate the system. In times the tidal level is raised the drainage pond is closed in order to raise the water level in the regulation pond. If the tide is falling, the gate is kept open to flow the water through the system.

4.4.3 Types of Houses

The houses typology is based on possible adjustments in building techniques and adjustments in the installations. Water deserves a structural function starting in the initiative phase and ending with the usage phase, and maintenance. Water is allowed in the house, beneath or under it. Two categories types of houses are distinguished (Fig. 4.21): the independent and dependant house. The dependant waterproof house requires human support to become waterproof during inundation (for

Rame Produzione
Fig. 4.20 Functioning of the pond in times of flood (left) and cleaning the system using tides (right) (Source: Koshigaya Land Development Office, 2005)

instance: placing of waterproof partitions in front of façade openings), while the independent waterproof house is waterproof by itself.

4.4.3.1 Wet Proof House

During a flood, water is allowed to enter the house until a certain height, 1,5 metres above the floor. Inside the house the damage is minimised by using waterproof floor, walls and other amenities inside and outside the house. The furniture is by preference removable and waterproof. The house needs to have enough openings in order to give the water the space to flow through (Fig. 4.22).

4.4.3.2 Dry Proof House

This house has a waterproof ground floor and outside walls and these houses are waterproof until 90 cm height. Recent research (Bowker, 2007) advises to use waterproof measures, meant to protect the building skin, up to 60 cm, because the chance at tears in the wall, shrinkage and leakages increase above that height. Furthermore, this research states that above a flood level of 90 cm higher risks are apparent at floating rubbish, trees and branches. If other materials than bricks are used it is possible to make the construction waterproof. The decisions and choices depend on the costs and benefits of different solutions. If a flood takes place open parts of the façade and the ground floor are closed by hand (Fig. 4.23). If this is done the house is prevented - up to 90 cm - from entering water. This house does not have free space under the ground floor. This prevents water to enter the house from beneath during a flood.

Dependant waterproof house

Wetproof house

Dry proof house

Indépendant waterproof house

Shore house

A

Column house

Floating house

¿Q

Amphibic house

ùk

Slight drainage house

Ù.

Fig. 4.21 Typology of houses (Source: www.waterbestendigbouwen.nl)

Dry Proofing Housewww.waterbestendigbouwen.nl)"/>
Fig. 4.22 Wet-proof house, example of measures inside the house: waterproof floor, waterproof walls and electricity above maximum water level (Source: www.waterbestendigbouwen.nl)
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Fig. 4.23 Dry proof house measures outside the house keep the house dry (Source: www.waterbestendigbouwen.nl)

4.4.3.3 Shore House

One side of this house - the waterside - is waterproof. The entrance is on the opposite (land) side. The ground floor is made waterproof and the walls are waterproof up to 1.5 m above ground level at the waterside. Above this height space can be used for windows in order to provide enough daylight to enter the house (Fig. 4.24).

4.4.3.4 Column House

This house is built on columns, anchored on the fundament poles. The columns and ground floor are above ground level and water level. Despite the fact that the water level fluctuates, the house will be dry any time, because the columns provide dry circumstances - the ground floor is high enough (Fig. 4.25).

Tokyo Spatial Plan

4.4.3.5 Floating House

The underground of this house is water. Fundament is not necessary. The house is fixed horizontally, by means of mooring poles, which allow the house to move vertically along with the water level. The floating house is put on a floating body, which can be a hollow concrete reservoir (www.goudenkust.nl) or a plate, made from EPS (exponant polystyrene) as heart. FlexBase (www.flexbase.eu) is an example of this (Fig. 4.26).

Fig. 4.26 Floating houses (Source: Dura Vermeer Buiness Development)

4.4.3.6 Amphibic House

The amphibic house consists of a traditional fundament as well as a floating body. In dry circumstances the house rests on the fundament, but in case of the rise of the water level the floating body enhances the loosening of the fundament and the house is capable of floating. This house is connected horizontally with mooring poles as well and is able to move along with water level fluctuations (Fig. 4.27).

Fig. 4.27 Amphibic houses (Source: Dura Vermeer Business Development)

Fig. 4.27 Amphibic houses (Source: Dura Vermeer Business Development)

4.4.3.7 The Slight Drainage House

This house has to deal with high groundwater levels and the ground floor and a part of the outside walls and facades, which are below water level, needs to be waterproof. This house does not have any free space under the ground floor, preventing the groundwater from entering the house (Fig. 4.28).

Fig. 4.28 Slight drainage house (Source:

www.waterbestendigbouwen.nl)

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