Designs to reach new heights15 August 2006
As a follow up to their paper published in the February 2006 issue of IWP&DC which focused on building two auxiliary dams and reservoirs to increase storage capacity at Marathon dam, E C Kalkani and P Foteinopoulos present research which investigates the alternative of increasing the height of the dam to improve storage
At the beginning of the 20th century the Greek city of Athens experienced a rapid population expansion, which brought with it an increase in demand for water supply. The existing Hadrian Aqueduct was not sufficient for the supply of water to the Athenians, and as a result US-based ULEN Co was contacted by the country’s government to plan the design and construction of a new water supply system to serve the consumers of Athens and Piraeus.
In 1925 a consortium was formed between the Greek Government, the Bank of Athens and ULEN and a contract was signed to raise finance for the new water system. With that contract a new company was established named the Greek Water Company (EEY), with the goal to design, construct and operate the water supply system.
The first storage project was the design and construction of the Marathon dam (1926-1929), a masonry dam that had a total height of 54m and a length at the crest of 285m. Water is conveyed from Marathon reservoir to Athens via the Boyati Tunnel, with the water brought to a new water treatment plant at the outskirts of Athens, called the Galatsi Water Treatment Plant. A particularly interesting feature of the project is that the stepped downstream face of the dam is lined with white marble from the Pentelicon Mountain in Attica, Greece. Despite the construction of this project, water supply needs continued to grow and new resources for water supply were investigated. In 1950 the water of the Yliki Lake in the neighbouring perfecture of Viotia was considered adequate for the population’s demand. Since the Yliki Lake is close to sea level, a pumping station was needed to transport the water to Athens. This was constructed and began operating in 1959 – the nominal supply from Yliki is almost 750,000m3/day.
In 1974, however, ULEN withdrew from the consortium of the Greek Water Company (EEY), leaving the operation of the water supply system to the other partners. In 1980, the Athens Sewerage Organization (OAP) merged with EEY to form a new water and sewerage utility – the Water and Sewerage Company (EYDAP). The new company was responsible for the water distribution network, as well the sewage and drainage networks.
Due to the continuing increase in water demand of Athens and Piraeus, a larger storage project was built on the Mornos river, 200km west of Athens. The Mornos dam construction started in 1969 and finished along with the aqueduct in 1981. The Mornos dam is an earth dam with height of 126m, and the reservoir created behind it has a storage capacity of 780Mm3. The length of the Mornos aqueduct is 192km and consists of pressure tunnels, open conveyance channels, and siphons, and is gravity operating from the Mornos reservoir to Athens.
The last major project of providing water to Athens is the Evinos dam and reservoir, from which a conveyance tunnel transports water to the Mornos reservoir. The construction of the Evinos dam and diversion tunnel started in 1992. The diversion tunnel with a length of 29.4km carries 100Mm3/year. In the operation of the water supply system, the main aqueducts of Mornos, Yliki and Marathon are combined with additional aqueducts that provide security in the operation of the system, in case of maintenance and repairs on certain portions of the aqueducts. The network of interconnected aqueducts, with a total length of 500km, is operated electronically and provides a safe management of the water supply system.
From the various sources the water is transported to Athens, where four drinking water treatment plants are operational: Galatsi, Polydendri, Acharnes, and Aspropirgos. The combined capacity of the four treatment plants is 1.9Mm3 of water per day.
From the treatment plants the water is transported to 50 storage reservoirs and tanks within Athens and Piraeus. An extensive distribution network of 7.55Mm, which is constantly expanded and refurbished, brings the water to the consumers in Athens, Piraeus and the suburbs. The drinking water is of excellent quality and is one of the best waters in Europe. The following paper considers the existing situation of the Marathon dam and reservoir and researches the possibility of increasing the dam height by 10m to increase water storage for the Greek cities.
The Marathon reservoir is associated with the Marathon dam, which is located at the NW edge of the Marathon valley and west of the town of Marathon. The Marathon area is located in the prefecture of Attica, in the northeast of Athens, Greece.
The Marathon valley is open to the sea towards the east, where a crescent shaped bay is present, closed to the west and to the south from the hills of Vrilissos, and to the North from the hills of the Diakria mountains. A peninsula is present in the NE of the valley that is called Kynosoura (tail of dog). The Marathon valley, named after the ancient town of Marathon, has the shape of a crescent 9.7km long and 4.8km wide, the concave side is directed towards the sea and the convex to the mountainous perimeter. The Marathon dam is located west of the town of Marathon and NNE from Athens.
As mentioned earlier, the dam construction started in October 1926 and lasted three years. The water transport from the reservoir was designed through a tunnel to the area of Helidonou, where the water treatment plant was located. The conveyance structure was finished in 1931. Within that year the water from the reservoir was transferred to the water purification plant of Helidonou and the water distribution system of Athens.
The reservoir would be the first part of the new aqueduct, which later might be extended to the north slopes of the Parness Mountain. The watershed area is 190km2, from which 124km2 feeds the reservoir. The rock formations at the dam site consist of muscovite schist, which is considered good rock for the foundation of the dam. At the reservoir the geology consists of watertight formations. The mean annual rainfall is 772mm of rain, which corresponds annually to 95.7Mm3 of precipitation. The minimum outflow of the Charadros springs is 61 litres/sec. The coefficient of inflow to the reservoir is 0.43 of the precipitation (rain and snow). Possible evaporation is 12.17mm annually per square meter of the reservoir. Water loss through seepage from the dam is 2.00 litres/sec per 1Mm3 of stored water or 880,000m3 of water annually, assuming an annual average stored volume of 14Mm3.
The Marathon dam has a height at the maximum cross section of 54m. The elevation at the toe of the dam is 173m. The dam crest is at el. 227m, while the spillway crest is at el. 223m. The crest has a length of 285m. The width of the dam at the foundation is 48m and at the crest 5.25m (clear width 4.5m). The upstream slope of the dam is 6.35:1 and the downstream slope is 1.54:1. The volume of the dam is 180,000m3. The drainage area is 120km2 and the inflow equal to 10-17Mm3 per year. The total capacity of the reservoir to the crest of the spillway according to the design is 41Mm3. The useful storage of the reservoir according to the design is 33Mm3. The area of the reservoir at the spillway crest level is 2.4km2. The conveyance tunnel is 13.4km long, with dimensions 2.6m x 2.1m (width x height).
About 10km2 of forested land was flooded to form the Marathon reservoir. The Dionysos-Nea Makri road, part of highway GR-83 passes through a traffic-light-controlled one-lane driveway on the crest of the dam. The Marathon valley lies to the southeast of the Marathon dam. The beach of Schinias is located southeast of the town of Marathon, a small town with a population of 13,000. The beach is a popular windsurfing spot and the location of the Olympic Rowing Center for the 2004 Summer Olympics.
The research presented in this paper refers to the figures obtained when increasing the storage of the Marathon reservoir by increasing the height of the dam. Using digital topographic data, available from the Military Geographic Service (GYS) of Greece, one can reproduce the plan of the dam and the reservoir in its present condition within the watershed. The reservoir volume is 22Mm3 (different from what was calculated in the original design) and the area covered is 2.4km2 (the same as calculated in the original design), for reservoir elevation at the crest of the spillway, which is at el. 223m.
Since the dam is a masonry dam with the downstream face lined with marble, any refurbishment of the dam should be directed towards the reservoir, so that the existing downstream face of the dam remains untouched and unspoiled as much as possible. This condition is interpreted as moving the crest of the dam upstream while increasing its height, so that the downstream face remains at the same position. Since the research considers increasing the height of the dam at every 2m, the crest is advancing upstream at every 1.3m, as estimated from calculations shown hereafter.
The upstream advancement of the dam while raising the crest is calculated following the logic that considering the dam crest 4m above the spillway crest, the dam width at the crest being 4.5m, and with slopes 1.54:1 downstream and 6.35:1 upstream, the width of the dam is calculated as equal to 4/6.35+4.5+4/1.54 = 0.63 + 4.50 + 2.60 = 7.73m at the spillway crest level. Moving upwards at every 2m height, the dam crest moves upstream at a distance of 2/1.54 = 1.3m. This upstream translation of the dam is obtained by considering a guideline along the river valley, and moving the center of the crest upstream, figuring the outline of the curved crest at 400m radius of curvature and finding the cross section of the crest and the dam contours on the upstream and downstream faces of the dam with the same level contours of the topography of the site. The guideline and the positions of the cross sections of the dam at the spillway crest levels of el. 222m, 224m, 226m, 228m, 230m, and 232m are shown in Figure 2.
The radius of curvature of the existing dam is 400m, which should remain the same as the dam gains elevation. The increase of the elevation was considered from el. 223 to el. 233m, a total range of 10m, mainly every two meters on the even elevations: 224m, 226m, 228m, 230m, 232m. For each of those elevations the reservoir and the dam areas were computed, and next the volumes of the reservoir and the dam from bottom to top were calculated. The computer software used is the one developed at the School of Civil Engineering, National Technical University of Athens (NTUA) by Panayiotis Foteinopoulos (Kalkani and Foteinopoulos, 2005). The use of the software has the advantage to produce shapes of dams and give the areas and volumes of the dam and the reservoir from the lower to the upper elevations and plot the graphs of elevations versus volumes. The outline of the topography and the location of the reservoir and the dam for spillway crest elevation at 22m, as produced by the software used, is shown in Figure 3. The plan of Figure 3 is the existing condition on which the research of this paper is based and from which the trials to increase the elevation of the dam crest will start.
The original designer of the Marathon dam considered a curved crest of the dam with radius of curvature of 400m. The positioning of the dam is such, as the contours of the topography at the right and the left abutments of the dam coincide with the extension of the curved crest of the dam. Hence, there is an exact fit of the curved crest to the topography, bridging the two abutments with a continuous curved line, which helps to build the alignment of the highway over the dam without large excavations for the access roads. This can be seen in Figure 2 at the spillway crest cross section of the dam at el. 222, which is close to the existing spillway level at el. 223m.
Keeping the same dam curvature and moving to higher elevations and upstream, when drawing electronically the higher parts of the dam using the computer software, the dam contours on the upstream and downstream surfaces do not cross the topographic contours, and the foundation of the dam is left above ground at the right abutment. To solve this problem in this research, the following directions were decided: the direction of the crest is rotated towards the guideline clockwise and the radius of curvature is increasing with the increase of the height of the dam.
The rotation of the crest of the dam clockwise to the guideline and the increase of the radius of curvature is shown in the table, along with the start and end point coordinates of the guideline and the respective crest elevations.
The plans of the dams developed are shown in Figure 4, Figure 5 and Figure 6, for respective spillway crest elevations at el. 224, el. 228 and el. 232. Values for the volumes of the dams and the reservoirs were calculated every 2m spillway crest elevation increase. The crest of the dam shown on Figure 4, Figure 5 and Figure 6 is at the level of the spillway crest with width 7.73m, while the crest of the dam is 4m higher and has a width of 4.5m. In the calculations of the volume of the reservoir, the areas at consecutive levels were considered and the volume calculated from bottom to top. In the case of calculating the volume of the dam, the areas were found and the volume was calculated from bottom to top up to the spillway crest level and then the volume of the dam 4m above the spillway crest was calculated and added separately.
The plots of the areas and volumes for the reservoir can be produced from the computer program for each dam calculated, and presented on the same figure by using different scales for the area and the volume on the horizontal axis, while the reservoir elevations are shown in the vertical axis. As the dam moves upstream along the guideline there is an increase of dam volumes as the dam crest increases. Comparing the dams with spillway crest at el. 222m and at el. 232m one can see that that there is an increase of the volume starting from the very bottom of the dam at el. 180m up to the top of the dam. This increase of volumes is shown in Figure 7, with the total volume of the dam moving from 150,000m3 to 260,000m3 as the crest increases from el. 227m to el. 236m.
As the computer program gives the outline of the reservoir in plan, one can calculate the additional area needed for the additional storage of the reservoir and the extent of the reservoir within the agricultural communities of the area. The reservoir elevation versus the area of the reservoir is shown in Figure 8. From checking the 1:50,000 maps of the Military Geographic Service (GYS) of Greece, no villages are within the area defined by the spillway crest elevation of 232m, or the reservoir flood level at el. 237m.
The variation of the reservoir volume as the dam rises from spillway crest el. 222m to 232m, is shown in Figure 9. From this figure the variation of the reservoir volume for the different dams entering towards the reservoir as the crest elevation increases, cannot be differentiated on the figure due to the large scale of reservoir volumes, since dam volumes are 100 times less than the reservoir volumes. From Figure 9, one can see that the volume of the reservoir at el. 222m is of the order of 22Mm3 and increases to 53Mm3 for the spillway crest at el. 232m. This is a considerable increase indicating that the reservoir storage is more than doubled for dam height increase of approximately 10m.
For comparison purposes and to focus on the design aspects, Figure 10 was plotted that shows the variations of the volumes of the dam and the reservoir for el. 222m to el. 232m. From Figure 10 one can select the height of the dam needed in case one considers the volume of the dam within economic possibilities for financing the development and the volume of the reservoir not exceeding the requirement in extra water storage or extending of the reservoir area beyond certain limits.
From Figure 10 one can determine the rate of increase of the dam volume that is 20x103m3 /2m height increase, and the rate of increase of the reservoir volume that is 60x105m3 /2m height increase. Beyond spillway crest level at el. 232m there is a saddle to the topography that should require a saddle dam with foundation at el. 234m. The condition with the saddle dam to support the reservoir was not examined in this research, while even for reservoir crest levels at el. 232m a small saddle dam is needed for the crest of the dam to be 4m higher at el. 236m. Extrapolating the graphs on Figure 10, one can forecast the change of volumes for elevations above el. 232m. However, the study shows that the increase of the spillway crest from el. 222m to el. 232m produces increase of the dam volume from 140x103 m3 to 250x103 m3, and increase of the reservoir volume from 22x106 m3 to 53x106 m3, which is an approximate doubling of the volume of the dam and more than doubling for the volume of the reservoir.
The research presented in this paper has the advantage of developing with accuracy alternative solutions for dams at different elevations, and giving charts to the planning engineer who will take the decision to select the dam and reservoir with volumes within the limits allowed by the finances of the development and by other factors regarding the volume and the area of the stored water in the reservoir.
Since the dam and the reservoir are in a historical and known region, the importance of the history is outlined in this paper. In the planning process, the plans of the dam and the reservoir at different elevations have to be examined considering the extended area upstream and downstream of the dam, and the charts of elevations versus reservoir area and versus volumes of the dam and the reservoir to be used for accurate estimates in complementary engineering designs.
Prof. Efrossini Kalkani and Panayiotis Foteinopoulos, School of Civil Engineering, National Technical University of Athens, Patission 42, Athens 10682, Greece, [email protected], [email protected], [email protected]TablesTable 1