Source: Molyvann, Vann. Modern Khmer Cities. Phnom Penh, Cambodia : Reyum ; [Chicago, IL] : Sales and distribution, USA, Art Media Resources, c 2003.
“…lessons learned from the 1960s, actions which have proved their efficacy and durability over the last few decades in which the city has been large left unmanaged. The methods for managing the hydraulic system of the “Four Faces” adopted during the 1960s, coupled with a coherent set of urban development plans, could create a synergy provoking the dynamic growth of the Greater Phnom Penh Region.
Some measures which should be urgently reinstated include:
_Reintroduction of dredging in the navigations channel of the Mekong River. Dredging must also be extended to the Basaac and Tonle Sap Rivers in order to work against the natural phenomena of silting found throughout the area of the “Four Faces” and the larger hydraulic system tied to the Great Lake
_Soil gathered through this dredging should be used to raise and expand the elevated riverbanks
_The mouth of the Basaac River should be unblocked in order to increase the current of the river as it flows south, thus enabling it to carry silt farther away from its mouth
_No new obstacles to flood waters should be created. Instead , the unobstructed flows of the major rivers should be aided and encouraged
The Tonle Sap and Bassac Rivers have almost identical types of banks. The Natinonal Routes lie close to these banks, general at altitudes around 10-11 meters above sea level, which in some places is lower than the highest flood levels. Between the National Routes and the rivers lies a band of planes and beaches occupied by large scale developments (factories, hotels, schools, embassies etc) as well as domestic housing and public spaces. Most large scale developments are situated on land which has an altitude higher than the highest flood waters, while most of the domestic housing in this band of riverside land is constructed at a lower level, more susceptible to flooding. South of the city, on the way to Takhmau, the riverbank road remains largely unflooded. The riverbank road turns into National Route 2 after Takhmau and provides excellent protection against floods of the Bassac River to the southern region.
Japanese Master Plan
Two alternative solutions for protecting areas surrounding the city from flooding.
_The first propositions suggests construction dikes on the riverbanks in order to protect not only the area behind the National Routes but also the houses in the narrow band between the National Routes and the rivers which flood each season
_An alternative proposition suggests building protective flood structures at low sections of the National Routes in order to completely protect all land lying behind these Routes. The narrow plans along the river before the National Routes would then be left as is and would be allowed to flood.
(Van Molyvann) prefer the first proposition since it will allow for better management of the areas between the river and the National Routes which, to date, have been primarily settled by a variety of anarchic squatter settlements.
The city of Paris project (Aditem) conducted an analysis of the water drainage system of Phnom Penh during 1994-5 and submitted a final report on December 1995.
The report notes that the drainage system of Phnom Penh remains vulnerable in many ways. The volume of water coursing through the Mekong and Tonle Sap Rivers during their flood stages (40,000 cubic meters per second and 8,000 cubic meters per second respectively) indicates the size of these rivers and the force of their waters.
The early city of Phnom Penh, which developed on the naturally elevated banks of the river, was spared the force of these floodwaters. As the city expanded into low-lying areas behind these elevated riverbanks however, urban areas came to lie below normal flood levels. It was necessary therefore to protect these areas by creating successive dikes and filling in the spaces between these dikes. Today, the primary flood risk to the city is the inadequate drainage capacity of these artificially raised areas.
Originally, the extensions of the city were accompanies by well-planned water management systems: a network of carefully maintained prek and beng located throughout the city, both collected and drained rainwater. Today, however, the carefully planned and minutely calibrated system which once kept a fragile equilibrium during flooding has been abandoned. What remains of the drainage system is obstructed with garbage and household waste, or has been filled in with new construction, which disregards or does not understand the existing system.
Almost all of the prek and beng have been filed in, dramatically reducing the places in the city where water can ruff off or be stored. No new alternative spaces for such water storage have been built. Water drainage is treated on an adhoc basis without logic and without a comprehensive vision of the city as a whole. Individual citizens think of their own needs and desires without considering water as a communal resource, which must be collectively managed.
The essential role of water in the city has been forgotten and once central water drainage canals and ponds have been filled in or completely neglected. There are only three large beng (Beng Kak, Beng Salang, and Beng Trabaek) remaining in the city, two of which sever as drainage areas for used water as well as acting as reservoirs for rain water. These two reservoirs – Bengs Saland and Trabaek – are gradually filling n with sand and garbage while being ringed by squatters. These two critical beng can be said to be threatened and must be given urgent attention.
P176 Aditem Plan – French
The Aditem study concluded that the system of water drainage in this district can be rehabilitated. This would require creating a separate drainage system connected the already existing system. The existing system of water drainage would have be completely cleaned and rehabilitated and then would be exclusively used to drawing used household water. The present number of drawings would be increased and used water would drain towards the basin of Beng Trabaek. A new system of rain water drains would be installed, running perpendicular to the Tonle Sap River, and draining rain water from this district directly into the river.
A drainage system for this district has to be developed. The existing system of drainage must be extened to comprehensively cover the area. All rainwater runs into canals and then drains which are controlled by pumping stations. The capacity of the canals must be calculated, and all canals must be cleaned and possibly re-sized in order to be able to fulfill their required drainage functions. A second system of PVC pipes must be laid in order to collect used household water. The main drain of this network will be located to the southwest of the Tuol Kok district. Crossing the dike at the site of the pumping station, this new network of used household water could join the main drainage system sketched in the master drainage plan for the city. Used water thus will enter the main drain for used city water located to the south of the cty.
The basin near Beng Salang
The existing drainage network must be rehabilitated, reinforced and then maintained in order to collect used household water and rain water. The single system used for such drainage leads all water towards Beng Salang which, if well managed can contain up to about 150,000 cubic meters of water. A system using storm gutters to separate rain water from household waste is installed at the entrance to the beng, while used household water is led to the pumping station with the help of a new system of channels constructed along the beng.
The basin towards the Khmer-Soviet Hospital
In the same manner as the Tuol Kok district, this district will come to rely on a new single system of drainage leading all water leads towards a cut in the dike while is already part of the comprehensive system for the cty of Phnom Penh.
The basic of Beng Trabaek
The existing drainage network must be rehabilitated, reinforced and then maintained in order to collect both used household water and rain water. This single system discharges upstream even in times of flooding thanks to the installation of storm gutters at flood stations along the Tonle Sap River. Wesaater not discharged is directed to an organized set of canals. These canals are regulated by flood gates and they conduct excess water to the Beng Trabaek station where it passes out under the dike. If Beng Trabaek is cleaned and properly maintained it can hold up to 500,000 cubic meters of water.
The new Basaac Front
For the areas within the district, which are to urbanized, a drainage system must be established. I propose the construction of a system of drainage following the north-south axis prposed for extension of the city as a whole. This drainage system will pass under the Monivong Bridge and flow into the southern drains of the city-wide system. Rain water will drain into the Bassac River through a new system of drawings running perpendicular to the river.
Japanese team of CTI Engineering and Nippon Koei
Area of study: delimited by the Tonle Sap and Bassac Rivers to the east, the floodplain of prek Phnou to the north, the floodplain of Prek Thnot to the south and the rising hills to the west of the city.
The Japanese study delineated three distinct zones within this area. The first zone is that of the central city with the natural embankment of the riverbank running along the Tonle Sap River. Elevations within this area vary from 8 to 11.5 meters above sea level. A second zone consistes of a lower terrace, hollowed out at its center, where the Prek Phnom probably ran from north to south n ancient times linking a network of pengs which still exist. Levels in the second zone are generally lowever than 8 meters above sea level. The third zone lies to the west where Pochentong Airport is located. This zone consists of a higher terraces sloping down from west to east and ranging in elevation from 14 to 8 meters.
The Japanese team proposed a master plan for these three zones which aims to improve drainage and allow for greater flood control by the year 2010. The degree of flood protected offered by this plan is calculated on flood data gathered over the last thirty years, while measure proposed to improve local drainage are based on statistics from the last two years. The master plan is comprised of eight components:
_Reinforcement of the banks of the Tonle Sap River
_Reinforcement of the dikes of Kop Srov and Tompun
_Improvement of drainage in the basin of Tompun by increasing pumping capacities, repairing and cleaning drains, and installing some flood gates and other new regulatory measures
_Improvement drainage in the basin of Beng Trabaek by increasing pumping capacities, repairing and cleaning drains and installing some flood gates and other new regulatory measures
_Improvement of drainage in the central area of the city by increasing pumping capacities, repairing and cleaning drains, and installing some flood gates and other new regulatory measures
_Improvement of the drainage east of Pochentong Airport by establishing pumping stations, regulating ponds, flood gates and a system of drains
_Improvement of the areas of drainage to the northeast and northwest of the city through repairs to existing drains and floodgates as well as the installation of new drains and floodgates
_Improvement of overall drainage environment through general cleaning of all canal and beng areas
The project to dam the Prek Thnot was approved in 1966. The project had multiple goals, which included providing irrigation, producing electricity and controlling flooding. At the time, plans consisted of a main dam with a principle reservoir, a central hydroelectric station, a secondary diverting dam on the Prek Thnot 12 kilometers up stream from the main dam, and an irrigation canal system with a regulating gauge. Construction of the main dam and principal reservoir began in 1969 but suspended in 1970 due to the war.
In 1992, an Australian Engineering firm called Snowy Mountain Engineering Corporation prepared a report for the National Mekong Commission evaluating the Prek Thnot region. The report summarized the state of affairs in 1992. The firm found that the diverting dam at Roleng Chrey and the regulatory gauge for the left bank irrigation system had been completed by the time work was stopped in 1973. The embankment of the project was 13% completed while all electrical and mechanical equipment had been abandoned and was unusable, or had disappeared. Four kilometers of irrigation canal had been complete and there were two completed water outlet gates. In additional tertiary canals were irrigating about 300 hectares of land. The Australian firm estimated that diverting dam could be repaired and reused, while construction of the main dam could go ahead with some modifications to the initial 1960s plans. The firm noted that in the interceding years, a number of ‘non-coordinated irrigation developments’ had occurred, particularly downstream along Prek Thnot.
The Australian study found that there had been considerable environmental degradation in the area and that some villages had been relocated into the area planned for the reservoir. Several poorly planned irrigation and drainage systems had been established through Khmer Rouge era labor. Along with subsequent equally ill-planned and hastily executed irrigations and drainage projects, these systems had damaged the aquatic eco-system of Prek Thnot. What cultivation there was in the area was simple single harvest rice cultivation depending on rain water. The Australian firm emphasized that any return to the Prek Thnot Development Project as planning in the 1960s must be coupled with strict environmental management policies which should include restocking of fish populations, protections of forests, and control of water resources. In addition, the delicate problem of moving villages out of the area to be covered by the reservoir must be addressed through community development projects and the active participation of those affected.
Option 1: No dam and or reservoir. Instead use high quality rice seed and fertilized to increase rice production. Over a nine year projected period, infrastructure could be developed that would allow for two rice harvest annually in areas with sufficiently good soil. Fish populations could be protected and restocked in order to develop aquaculture business.
Option 2: Dam that irrigates but no electricity. Dam could produce an irrigation system, which covers 34,000 hectares of rice fields producing at least two harvests annually.
Option 3: Dam that irrigates and produces electricity. 27,000 hectares of rice fields could be sufficiently irrigated while maximum electricity production would occur during the rainy season when water was in surplus