Small-scale mild rainwater management systems: optimal design and the impact of climate

This Diploma thesis aims to shed light on the performance of a hypothetical system of mild rainwater management, under different climatic conditions. It is located on the Hydraulics building in the National Technical University of Athens, Zografou. The rainwater collection and storage system, contains two tanks, one underground and one on the roof, and receives runoff from the roofs of the hydraulics building, the hydraulics laboratory and the coastal engineering building, as well as from the impermeable surfaces that surround the above buildings and are bounded by the hydrocriter.

To serve the purpose above, a daily water balance model has been developed, which receives daily rainfall, potential evapotranspiration and water demand data calculates on a daily basis, the stored water, the quantities of water offered for domestic and irrigation uses and the failures to meet demand. The meteorological data used, were taken from stations in two basins of Great Britain, located near Edinburgh, Scotland (time series of 45 years) and Brighton, England (time series of 45 years), also in a basin of France (time series of 6 years), in the Livadi of Parnassos (time series of 45 years), in Hellinikon, Attica (time series of 65 years) and in the island of Astypalaia (time series of 6 years). In these are areas prevail different climatic conditions and it is exactly the fact on which the creation of six (6) different case studies was based.

The term of reliability was used to evaluate the hypothetical rainwater harvesting system. Reliability expresses the lack of demand coverage failures in relation to the total length of the examined period. The other criterion that has been used το asses the operation of the RWH system in this study haw a financial basis. The annual cost has been estimated, by taking into account the annual depreciation for the installation and replacement of parts of the rainwater storage and pumping devices, as well as the annual cost for the purchase and supply of water from the conventional network, that is being supervised by water supply company of Athens (EYDAP), each time the system showed inability to serve the water needs of the study area.

At first, the hydrometeorological data on which each case study was based, were analyzed by constructing diagrams of daily, monthly and annual rainfall and potential evapotranspiration. At the same time, cumulative drought curves of at least 5 days, were calculated in a Matlab environment. Through this thorough analysis we extracted important information about the climatic conditions of each region.

In the second phase, we constructed reliability and energy demand curves, in the context of investigating the "behavior" of the model, for each climate area, for 60 combinations of underground and aboveground tank volumes. Energy requirement was calculated only for the underground tank, by determining the daily amount of water that has to be pumped and the power of the pumping unit. Finally, all the resulting diagrams were examined in detail and the respective results were analyzed. In the third and final phase, we proceeded on sizing the tanks that are included in the hypothetical mild rainwater management system, by solving an optimization problem. The optimal sizes for the tanks were calculated, aiming to minimize the annual cost. Finally, for each case study, the reliability values and the annual final cost after optimization were compared, for given tank volumes. This comparison confirms that the practice of rainwater harvesting, for uses that do not require drinking water, could constitute an extremely important secondary source of water intake, under some conditions. These conditions include a sufficient supply of precipitation. In particular, rainwater harvesting is suitable for areas with frequent rainfall episodes and without long periods of drought, such as those in France and the United Kingdom, which demonstrated high reliability values (> 80%), while having very low annual costs. The above is possible even with not very high annual rainfall. In contrast, in areas such as the Livadi of Parnassos, which is characterized of high annual rainfall that is not evenly distributed and long periods of dryness, the performance of these systems drops dramatically. Finally, the installation of the above systems in fundamentally arid areas such as Elliniko and Astypalaia, which receive relatively low rainfall, mainly concentrated in few large events, is considered uneconomical and unreliable.