VOL.5

Vol.5

The Ara Riverbed: Japan's Largest

The shape of the Ara River has a distorted appearance, its wide middle reaches making it look like a snake that has swallowed a frog.

The middle and lower reaches of the river flow through alluvial lowland. Natural forces have forced the Ara's course to change gradually, but towards the beginning of the Edo Period it joined the Tone River after flowing to the east of the Omiya Highlands.

Immediately after Shogun Tokugawa Ieyasu's triumphal entry into Edo Castle in 1590, he launched a project to divert the Ara River in order to separate it from the Tone River. This project, the new Shogunate's most important, was placed under the supervision of Ina Bizen-no-kami Tadatsugu. The diversion of the Tone River eastward to Choshi (Chiba Prefecture) began in 1594 and took over 60 years to complete.

In 1629, Tadaharu, Tadatsugu's second son, dammed the Ara River flowing eastward near Kumagaya (Saitama Prefecture), directing it southward to the west of the Omiya Highlands and into the Edo region by connecting it to the lower Sumida River. As a result of this diversion, people living along the new course of the Ara Rive suffered from the effects of flooding and a continuous circular embankment was constructed, within which they were protected from such damage. All these flood prevention and irrigation projects were carried out by the Ina family. Being very familiar with the geographical features of the flat, low Saitama plain, they were able to construct such protective circular embankments and to channel water into a huge flood control area located outside them.

The muddy streams rushing into the lower river were diverted into this flood control area. People living along the Sumida River, the lowest tributary of the Ara River flowing through the downtown districts of Edo, were protected from flooding by the construction of the funnel-shaped Sumida Zutsumi and Nihon Zutsumi embankments on either side of the Sumida River. This type of flood control method later became known as the Ina (Kanto) style.

However, as the alluvial land was originally flood land, the construction of continuous embankments to guard against flooding reduced the width of the river. The level of water in the river became higher, resulting in the frequent and serious flooding of villages in the area.

Towards the end of the Edo Period, financial difficulties forced the Shogunate to abandon its flood control works. The rigid encircling embankments collapsed nearly every year, causing serious damage. The risks of frequent flooding preyed on people's minds and led to conflicts and disputes over water.

It was not until the Meiji Period that the Ara River, together with other major rivers in Japan, became the subject of improvement work sponsored by the Government.

In the lower reaches of the Ara River, a project which included the excavation of the Ara River drainage canal was launched in 1911. Improvements to the middle reaches were implemented in 1918, but were not completed until 1954, due to rapid and dramatic political and social changes, as well as to financial difficulties.

Because the Ara River runs through the important Tokyo Metropolitan Area, numerous improvement projects, which made use of modern civil engineering technology, were implemented and have succeeded in reducing the incidence of flood damage attributable to this river.

The modern embankments constructed since 1910 are still modeled on the paths of those constructed early in the Edo Period. The Ara riverbed, Japan's largest, is always associated in our minds with Ina Tadatsugu and his son, Tadaharu.

Ashford Common

On October, 4th, 2004, we enjoyed a successful visit to Thames Water's Ashford Common Water Treatment Works in London. Arrangements for the visit were finalized only a few days before we left Tokyo, thanks to our friend in London, Mr. Robin Cosby, who spent many hours setting up the appointment. We had spent the previous night at the home, near London, of another friend, Ms. Vicky Gardner, who drove us to the town of Ashford in the morning.

Thames Water's Chief Engineer, Mr. Terry Bridgman, showed us round the installation. He explained that Thames Water, which has five water treatment works ? Ashford Common, Kempton Park, Hampton, Walton and Coppermills ? supplies water, taken mainly from the River Thames, to between 3.5 and 4 million households in London, via 11 distribution stations.

Water drawn from the River Thames is stored in reservoirs for between 50 and 100 days, to allow solids time to settle.

The water is then subjected to its principle processing procedure, comprising a primary ozone treatment, dual-layer filtration, a secondary ozone treatment, slow sand filtration and chemical treatment. The treated water is then fed into the Ring Main, Thames Water's main water distribution network, which consists of 86 km of pipes, 2.54 m. in diameter, located 40 meters below ground...

The Ashford Common Treatment Works has 32 slow sand filtration reservoirs. These are drained regularly and the top layers of sand cleaned. This cleaning process is carried out once a month on average.

Mr. Bridgman explained that because this monthly skimming work is done by machine, the utmost care is taken to avoid any leakage of oil. Nevertheless, we saw very few people working there.

An optical fiber network installed inside the Ring Main sends data such as water pressure, flow volumes, reservoir levels and water quality to a centralized control facility in Hampton.

Although such a modern water supply system has been constructed in the UK, the use of slow sand filtration has not been discontinued and Mr. Bridgman explained to us why this was so. "The most important reason is that we have a large area of land available to us to accommodate the filtration processes," he said. "We now know that there is no better treatment method for removing bacteria than slow sand filtration. It doesn't need constant monitoring because no chemicals are used. It is also very resilient. It looks primitive, but it has many advantages."

Mr. Bridgman continued, "Our customers do not like the use of chemicals, and if we were to build a new type of water treatment plant today, it could cost $100 million. We can continue to use our existing facilities, which employ natural filtration methods. I believe that this slow sand filtration system is a good one for developing countries."

The Queen Mary Reservoir, which is within the Ashford Common Treatment Works, is not open to the public for security reasons, but its huge water surface is utilized by sailing clubs on a contract basis.

There are numerous large reservoirs like this in London. Most of the lakes that we could see from the aircraft when we left London from Heathrow Airport are, in fact, reservoirs. These huge stretches of water provide Londoners with valuable shoreline; and the slow sand filtration ponds, which will last for many years if they are properly looked after, are priceless properties. They function as a natural infrastructure, withstanding time and environmental changes.