(New bridge at Hua Ro in approximate location)
Text, photographs & map by Tricky Vandenberg - May 2009
Updated October 2019
(Example of a weir on the Noi River)
"Thamnop" is the Thai language for a weir or water barrage. "Ro" means a breakwater or weir. "Thamnop Ro" is a pleonasm, unless "Ro" refers
to Hua Ro, the north-eastern point of Ayutthaya's city island.
The north-eastern point "Hua Ro" could have been called after the weir, but also after the area were people had to wait to be ferried between the
Noblemen Landing" and the opposite landing near Wat Mae Nang Plum in the Ayutthaya era. The "Noblemen Landing" was the second official
boat landing in connection with the
Front Palace, the seat of the viceroy or Uparat (the first official landing being near the Grand Palace).
Thamnop Ro was situated at the confluence of the old
Lopburi River and a shunt that connected to the Khu Khue Na or Front City moat. The shunt
was dug in front of the present-day defunct
Maha Chai Fortress.
(The shunt dug after the fall of Ayutthaya in 1569)
When was the weir constructed?

The "Geographical description of Ayutthaya: Documents from the palace" states that the Burmese besieged the city of Ayutthaya in 1556 AD (918
CS - a year of the Dragon) during the reign of King Chakkraphat (reign 1548-1569). In an attempt to conquer the city, the Mon drove logs of toddy
palm in the water, to make a weir, filled it in with earth, and made a bamboo bridge across the waterway into the city. After the war, the causeway
was kept as it was convenient to have a bridge to cross the river into the city. When the old Mon causeway rotted and collapsed, the Thai repaired it
and gradually converted it into a large bridge. [1]

The point of discussion here is the year of the occurrence of the event. The earliest appearance of Burmese troops in the area of Ayutthaya is 1549
AD. The Burmese army encamped north of Ayutthaya, but there is no mentioning of the Burmese attacking the city itself, let alone making a bridge
over a stretch of water, which was not existent at that time. After this war, King Chakkraphat increased his defenses by, inter alia replacing the
earthworks surmounted with wooden posts around the city with brick walls plastered over with quicklime.

There was no Burmese attack on Ayutthaya in 1556. The next Burmese attack after 1549 AD, was in 1563-64 AD, also known as the "White
Elephant War". Ayutthaya was invested by the Burmese; a conference was accordingly held between the two monarchs and King Chakkraphat
agreed to the onerous terms imposed by the King of Burma. The City of Ayutthaya was as thus not taken nor destroyed.

The next Burmese invasion was nearly 5 years later in 1568-69 AD. 1568 was a Dragon year so maybe the old documents were referring to this
war. The Chronicles of Ayutthaya show that the eastern city limit of Ayutthaya in 1569 was present
Khlong Makham Riang. Ayutthaya had a natural
defense line because of the Lopburi River on the northern, western and southern sides, while a man-dug moat protected the eastern side. This
defense moat became later known as Khlong Makham Riang ala Khlong Nai Kai. (See
Eastern City Walls in 1569).  

The old Khmer town of Ayodhya (Mueang Doem) was situated in an oxbow of a branch of the
Pa Sak River. As it was completely encircled on
three sides by the river, its remaining western side was likely the defensive moat became later known as the Khu Khue Na. It retained its function as
a secondary defensive moat when Ayutthaya was established in 1351 by King Ramathibodhi I (reign 1351-1369).

On the evening of the Burmese attack in 1569, the eastern city wall ran along Khlong Makham Riang, being the eastern defensive moat of the city at
that time. The northeastern corner of the city wall ran from
Pratu Khao Pluak towards the confluence of Khlong Ho Ratanachai and Khlong Makham
Riang at present. The whole northeastern corner was outside the city walls and a land bridge existed towards Thale Ya area (Sea of Grass). This
land bridge was defended by an advanced stockade bearing already at that time the name Pom Maha Chai (RCA 35). Outside the city wall in the
northeast stood
Song Monastery with a 'phaniat' or elephant kraal in its vicinity. The eastern city wall was the weakest point of defense for
Ayutthaya, hence the use of the Khue Na moat as a forward defensive buffer.

The Burmese built three causeways to cross the Khu Khue Na; one in front of
Wat Khwang, another in front of Bang Ian village (village at that time
outside the city walls) and the last one at the corner of Kaeo Island. The Royal Chronicles of Ayutthaya do not mention an attack on the Maha Chai
Fort and the northeastern land bridge, neither does it speak about the construction of a causeway in that place. The Burmese Chronicles are also
silent. [2]

"When the inventory of all the troop supplies ordered by the King was completed, the King of Hongsawadi had preparations made for
attacking the Capital and ordered causeways laid extending in towards the front ramparts of the city from three points; from the positions
of the Uparat the first was to be laid extending in to Fang Monastery and a second to Ian Village, and from the positions of the King of
Ava a third was to be laid extending in to the corner of Kaeo Island. In laying the earth in towards the city the men of Hongsawadi
constructed shields and set them up running in towards the city to protect themselves from firearms, and they heaved the earth on ahead
over the shields."
Why was the weir constructed?

Ayutthaya finally fell to the Burmese. After the Burmese armies withdrew, the Burmese vassal King Maha Thammaracha (reign 1569-1590) realized
the poor state of Ayutthaya’s defenses. Also, the Cambodians came to that conclusion and took advantage of the situation, invading Siam numerous
times. King Maha Thammaracha assessed that the eastern part of the city needed better protection. Around 1580, he ordered the construction of a
number of defensive structures such as the Chan Palace, the eastern city wall extension towards the Khue Na moat, fortresses and canal widening.
[3] Wat Song became now situated within the city walls and behind the Maha Chai Fortress. A shunt was dug from the northern Lopburi River bend
Wat Sam Wihan to the Khu Khue Na, which in its turn was dredged 6 meters deep and widened up to 20 meters. The elephant kraal near
Wat Song was moved to the Thale Ya area (present Phaniat area). The Maha Chai Fortress received its position at the confluence of the Lopburi
River with the shunt. The Lopburi River was thus split up in a western and a southern direction, surrounding the City of Ayutthaya completely.

Hydraulic play is a risky business, and soon the Lopburi River tended to flow heavier in its new southern direction (the shunt) than in its normal river
bed in western direction. In "Tamnan Krung Kao" Phraya Boran Rachathanin wrote fearing that the river would become shallow, due to insufficient
water flow on the northern side of the city, (where the Grand Palace and the
Royal Boathouse were situated) a weir was built with large wooden
beams at the beginning of the shunt in front of the Maha Chai Fortress. The weir reduced the river current and blocked off its waters so that they
partly continued to flow north of the city through the old river bed. [4]

The version regarding the weir given by Phraya Boran Rachathanin in “Tamnan Krung Kao” in comparison which what is written in the "Geographical
description of Ayutthaya: Documents from the palace" seems the most plausible. A causeway bridge was built on top of the heavy wooden beams of
the weir, functioning as support.

The Tamnop Ro Bridge stood next to the Maha Chai Fort near the storehouse of the Front Palace. The dam, paved with planks on both sides, was 6
meters wide and allowed the passage of boats through a channel in the middle. On both sides, there were steps for descending the slope down to the
bank. The bridge must have been strong as carts, elephants, and horses could pass it. The crossing was only possible with an official order and
strictly controlled. For this purpose, there was a manned guard pavilion beside the bridge. [5]

Depicting the Tamnop Ro Bridge

To my knowledge, the bridge is shown for the first time on the map of Jean de Maguelonne de Courtaulin (1638?-1...) titled "Siam ou Iudia,
Capitalle du Royaume de Siam Dessigné sur le lieu Par Mr Courtaulin missre Apostolique de la Chine" and published in 1686 by Franciscus Jollain,
the elder (1641-1704).
de Maguelonne resided at Ayutthaya from October 1672 until June 1674 as a missionary of the Missions Etrangères de Paris and left thereafter for
Cochinchina. He likely transited through Siam on his return to Paris in 1685.
Simon de La Loubère (1642-1729), who was in Ayutthaya in 1687 for 3 months as a French Ambassador to Siam, also shows the bridge on his
map. De la Loubère refers to the bridge as "une chaussée par laquelle, seule come par un isthme, on peut sortir de la ville sans passer l‘eau" (a
Causey, by which alone, as by an Isthmus, People may go out of the City without crossing the water.) [6]
Engelbert Kaempfer (1651-1716) resided in Ayutthaya in 1690 on his travel to Japan. Kaempfer wandered around in the city and described the dam
in his journal entry for 28 June 1690 (fol 24r), as  "[...] die situation der 3 palaien besehen item den Dam, so doppell von pfahlen u. erde gefult" ([…]
got a look at the situation of the 3 palaces as well as the dam, being a doubly of poles and earth-filled) (Sl 2921 fol. 24.) Kaempfer, when visiting the
Temple of Berklam (Phra Klang's Monastery) passed "over the Dyke made not long ago for shutting up the Southern arm of the great River." One of
Kaempfer's sketches shows the dam as a partial barrage of which the narrow passage was thereafter bridged as given in the "Tamnan Krung Kao" of
Phraya Boran Rachathanin. [7]
Francois Valentyn (1666-1727), a Calvinist preacher on the Spice Island of Amboina, never traveled to Ayutthaya, but nevertheless created a
magnificent map of the Chao Phraya River titled “De Groote Siamse Rievier Me-Nam ofte Moeder der Wateren inharen loop met de in vallende
Spruyten Verbeeld”. The map was published in 1726 as part of his work “Oud en Nieuw Oost-Indien” (1724-1726). Valentyn shows the Thamnop
Ro Bridge, linking the city island with the Phaniat area noted as “61. Duing Pinjet” in the map description.
When did the weir disappear?

The weir collapsed during the Burmese attack in 1767. It stood close to the area were the Burmese breached the city wall near the Maha Chai
Fortress. The Lopburi River flowed now without obstruction into the Khu Khua Na. As the weir was destroyed, the Lopburi River running on the
northern side of the city became shallow. The river banks developed larger and made the river narrower, which makes it look like a canal today,
bearing the name
Khlong Mueang or City Canal. [8]

A map of an unknown mapmaker, dating back to the reign of King Mongkut or Rama IV (reign 1851-1868), does not show the bridge anymore, but
only two boat landings (Tha Wilanda & Tha Chang Wang Na) are linking the city island with the mainland.
Phraya Boram Rachathanin mentioned in his work “Wa duai phaenthi Krung Sri Ayutthaya” that around 1875 people went investigating the area and
still found support posts of the weir on the bottom of the canal. He added that today (that was in 1907) you will have to dig 1 to 1.5-meter-deep in
the river mud to find the logs. [9]

It would take 176 more years before the city island of Ayutthaya was linked again with the mainland. A 60-meter-long-span bridge was constructed
in 1940 and opened officially on 14 July 1943, the anniversary birthday of Field Marshal Plaek Phibunsongkhram, Prime Minister at that time. The
bridge was named after the former Prime Ministers of Thailand, Professor Pridi Phanomyong (1900 – 1983) and Rear Admiral Thawal Thamrong
Navaswadhi (1901-1988). All construction material and methods were developed in Thailand by Thai engineers under the consultation of a German
engineer - Dr. Kruck - employed by the Department of Highways. The bridge was precambered during construction in order to counteract future
deformation, while the main reinforcements in the deck were pre-tensioned and released after the concrete had been cast; a technique later known as
"partially prestressed concrete". The overall Pridi-Thamrong Bridge length is 168.6 m, while the roadway width is 6.5 m. [10]
Pridi-Thamrong Bridge (Photo courtesy of Sean Alcock)

[1] Phanna phumisathan phranakhon si ayutthaya: ekkasan jak ho luang [Geographical description of Ayutthaya: Documents from the palace] -
Edited by Winai Pongsripian. Bangkok: Usakane, n. d. [2007] - page 82.
[2] Cushman, Richard D. (2006) - The Royal Chronicles of Ayutthaya - page 62 / Source: Thonburi fragment (1779) Khurusapha (1963) - The
Hongsawadi Armies Besiege Ayutthaya.
[3] Cushman, Richard D. (2006) - The Royal Chronicles of Ayutthaya - page 82 / Source: Luang Prasoet, Phan Canthanumat & British Museum -
Ayutthaya’s Fortifications Rebuilt, 1580.
[4] Boran Rachathanin, Phraya (1907) - Tamnan Krung Kao - page 93-94.
[5] Boran Ratchathanin, Phraya (1907) - Wa duai phaenthi Krung Sri Ayutthaya [About the map of Krung Sri Ayutthaya.
[6] de La Loubère (London - 1693) - A new Historical Relation of the Kingdom of Siam (2 Tomes) - Edited by John Villiers (1986) - White Lotus,
Bangkok - page 6.
[7] Engelbert Kaempfer, Werke 4. Kritische Ausgabe in Einzelbänden. Herausgegeben von Detlef Haberland, Wolfgang Michel, Elisabeth
Gössmann - Engelbert Kaempfer in Siam - Iudicum Verlag GmbH München 2003 - edited by Barend Jan Terwiel.
[8] Boran Rachathanin, Phraya (1907) - Tamnan Krung Kao - page 93-94.
[9] Boran Ratchathanin, Phraya (1907) - Wa duai phaenthi Krung Sri Ayutthaya [About the map of Krung Sri Ayutthaya] - page 162.
[10] Wai-Fah Chen, Lian Duan (2013) - Handbook of International Bridge Engineering - CRC Press - page 1156.