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Flooding prevention using the example of the City and metropolitan area of Manila

Manila, capital of the Philippines, has around 1,8 Mio inhabitants and is situated in the agglomeration “Metro Manila” with a total of 12,9 Mio citizens. The immense population pressure since the middle of the 20th century caused an uncontrolled expansion of the city Manila and the surrounding conurbation.

The planning and construction of the installations of sewage discharge and treatment systems as well as storm water distribution could not keep up with this growth. Thus, drainage systems yes do exist, but do not meet in any way to the objectives of integral urban drainage systems. Especially the inadequate hydraulic dimensioning of the distribution pipelines, which are mostly done in the diameter of DN/ID 900 mm, regularly lead to overload flooding in a lot of districts of Manila.

People trying to escape the enormous amounts of sewage watersPicture 1: People trying to escape the enormous amounts of sewage waters

The hydraulic failure of the systems is even intensified by the need of relining and repair, wrongly constructed manholes, false installation of the pipelines as well as root infiltration and blockings. On basis of the applied regulations of the German association for water management, waste water and waste (Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e. V. (DWA)) and DIN norms possibilities for a safe discharge of waste water as well as the protection against flooding shall be presented.The current situation in Manila is difficult: Due to the demographic development of the last decades in the agglomeration of „Metro Manila“ and the city of Manila, the populated areas have multiplied. This expansion of populated areas happened without a sustainable planning of a secure waste water discharge and storm water distribution system. Thus, consequently, the protection of humans and goods against flooding in the residential areas and a proper discharge of waste water were not ensured. The sewage system based on a sewage and storm mixed system was extended with the growing settlement but without adapting the existing system to the hydraulic additional load. Natural discharge ways and retention areas were overbuilt without creating sufficient countervailing measures. The still existing open water channels were often limited and restricted by dense constructions. Bridges without sufficient freeboard, encroaching buildings of all kinds as well as deposit of rubbish is existent, which reduces the drainage system width considerably.

Storm sewage systemScheme 1: Storm sewage system

On the basis of the population of 1,8 Mio for Manila and an urban area of around 39 km², there is a population density of around 46.000 inhabitants/km². Thus, the area is very densily crowded and built and discharge areas are very high. Green and other permeable areas are not really existent. Manila`s climate is semi-humid tropical. Annually it rains approx. 2.070 mm, ¾ of it during the months of the monsoon from June to September. Heavy rain falls during the monsoon lead to continuous flooding in the city area, which also result from an insufficient drainage system. It is expected that in the future the rainfalls will increase in intensity and frequency due to the climate change. During the flooding when the drainage systems that transport the waste water fail, inacceptable hygienic conditions and large damages are the results.

Possible approaches and solutions on the basis of the German guidlines have been evaluated in the past time: A future-oriented and strategic draining system plan for the agglomeration “Metro Manila” respectively the city of Manila is not directly derivable from the European norms and regulations due to the different climate zones. Especially the protection against overflow and flooding has to be respected because of the strong and heavy rainfalls in the subtropical areas. Generally the procedure of planning and construction of drainage systems in Germany is given among others by the DIN norms and regulations of the DWA. The study area Manila is a grown mega city. Existing drainage systems are based on combined storm and sewage systems. This means that domestic and operational waste water are discharged together with storm water in one line. Sewage treatment plants to clean the waste water do not exist for all areas of Manila yet. Partly the sewage is led without any treatment directly into surface water.

A combined sewage and storm system in Germany always leads to a sewage treatment plant that cleans the waste water and part of the storm water.

Sewage treatment plants are not designed to treat the complete incoming storm water. For this reason surcharge storages and storm water overflows should be planned and situated under streets and places in highly populated areas. To create cost-effective, underground storage volumes large diameter pipe systems should be considered. With the help of the storm water overflows the heavily diluted waste water is being transported to receiving waters after being mechanically treated. Before starting to create sewage treatment plants the drainage system of the city areas should be prepared and secured. Therefore the hydraulically powerful pipe systems should be constructed starting at the site of a potential sewage treatment plant to the living areas. To determine the water amount for the hydraulic dimensioning of the necessary line system in Germany the regulation DWA-A 118 “Hydraulic dimensioning and proof of drainage systems” ( „Hydraulische Bemessung und Nachweis von Entwässerungssystemen“) is used. Exemplary is the determination of the water amount according to the rain classification applied in the Philippines for a rain of the classification “orange”.

Rainfall advisories, classification and measurementPicture 2: Rainfall advisories, classification and measurement

On the basis of scheme 2 the corresponding occurring sewage and storm water quantity for a classified rainfall of the colour “orange” will be determined for the city areas of 20 ha, 50 ha and 100 ha. Due to the climate change an additional 20% of intensity of rain fall will be added in order to guarantee a future-proof drainage system. Simplified exemplary calculation for the determination of the amount of precipitation:

Drainage area: 20 ha; 50 ha; 100 ha
Covered areas: 90 %
Duration of rain Orange: 60 min
Intensity in 60 minutes: 30 mm = 30 l/m² h = ~ 83 l/ha s
Addition for climate change: 20 %

The quantity of sewage water will be calculated with assumed values of 4 l/(s 1000E) => Qs = 4 l/(s *1000 E) * 46.000 E/km² / 1000 = 184 l/(s * km²)

Quantity of sewage and storm water EZG 20 ha:
QM = QR + QS = 1.800 l/s + 37 l/s = 1.837 l/s

Quantity of sewage and storm water EZG 50 ha:
QM = QR + QS = 4.480 l/s + 92 l/s = 4.572 l/s

Quantity of sewage and storm water EZG 100 ha:
QM = QR + QS = 8.960 l/s + 184 l/s = 9.144 l/s

Sewage lines are dimensioned in Germany according to worksheet DWA-A 110 „Hydraulic dimenisiong and performance record of sewage lines and cannels“ („Hydraulische Dimensionierung und Leistungsnachweise von Abwasserleitung und –kanälen“.) The considered city area is topographically gently sloped. Thus, also the waste water lines can only be installed with a low longitudinal slope. On the basis of the water amount calculated above the following necessary dimensioning derives for lines with an assumed longitudinal slope of I = 0,25 % and kb (operative roughness of the pipe system) = 0,75 mm:

table drainage area

The large dimensioned pipelines should consist of a smooth inside surface in order to avoid deposits during dry seasons. Especially large Krah-Pipes out of Polyethylene High Density are very suitable. Another big advantage of these kinds of pipes are the self cleaning behavior, due to a homogenous system with a low system roughness.

This example shows that in the city area efficient and high performance large dimensioned pipelines are required. Because of the dense population only few waters and flood channels still exist. Thus the rainwater has to be collected, drained and locally teed in qualified and hydraulically efficient waters through pipeline systems laid under the streets.

Due to the dense population only few water bodies or flood channels are existent. The occurring storm water thus has to be collected and transported necessarily by underground installed pipelines and should be lead locally if possible into hydraulic capable waters. It is highly recommended to design local storages as well as surface water seepage areas. The storage of the storm water is done with the help of storm water overflows or storm water retention tanks. Storm retention tanks have an additional storage area for polluting loads. As an example please refer to the following shown storm water retention tank, designed with an overhead load removal. The highly diluted waste water and storm water coming from the pipe system will be deducted mostly from the retention tank to surficial waters.

storm water retention tankScheme 2: Storm water retention tank

The outlet is, under normal conditions, as big as the double amount of the normal waste water. A sewage treatment plant should be situated close to powerful waters and at a geographically lowest point of the drainage area. When developing city areas the guidelines of an integral urban drainage system (working sheet DWA-A 100) should be respected. Here, all objects of protection should be considered like waste water management services for residential areas, water protection, safety in use as well as other issues as nature protection, soil protection etc. For new developments sewage and storm water systems (mixed systems) as well as separate sewage systems are possible. For the conduction of storm water for both systems however, big diameter pipes are absolutely necessary. Additionally a sewage pipeline is necessary beside the storm drain pipeline. In order to avoid flooding in waters and city areas situated below, ground basins as storm water retention basins should be planned for new developments.

Those basins will store the surplus storm water during heavy rains and will then deduct it throttledly to the receiving waters. As final word you can say that the development of an integrated sewage and drainage system for the city and agglomeration of Manila, considering the protection objects “hygiene”, “flood prevention” and “water protection” is only realizable with high technical and economical efforts, due to the insufficiently available infrastructure as well as the dense and still growing population. In the living areas, a comprehensive and big-volume pipe system with retention tanks for collecting and deducting sewage and storm water should be developed and expanded. The expansion of the drainage systems should be started at existing or potential sites for sewage treatment plants and/or powerful receiving waters with direction to the living areas. The existing waters should be carefully checked for possibilities to improve the hydraulic comprehensive system in order to use them by conducting storm water out of the living areas and to support and relief the pipelines.

For future extensions and developments of living as well as commercial and industrial areas also sufficiently large volumes of storage and retention possibilities should be created. Beside the necessary measurements for the handling of storm water it is also crucial to consider and create flood protections for the surrounding areas and drainage areas in order to avoid flooding of populated areas coming from outer areas. Due to the seismic activities a flexible pipe system should be used, which is capable of adapting to the earth movements. Furthermore the tightness should be of high value as well as to avoid infiltration of roots in the pipe joints. A welded pipe system made of Polyethylene corresponds to the today´s requirements of an integral pipe system. Suppliers as Krah Pipes can offer the complete pipe system – a pipe system out of Polyethylene (kb= 0,25 mm) with integrated electro-fusion jointing in large diameters with all manholes and fittings necessary.

Author:
Dipl.-Ing. Ulf Heinemann
Ingenieurbüro Heinemann
Frankfurter Str. 23
57610 Altenkirchen, Germany