【正文】 nd seventeenth centuries. In the late eighteenth and throughout the nieenth centuries there was a marked increase in air pollution, because of the greater use of coal by developing industry. From 1750, the chemical industry began to develop, and this caused the discharge of acid fumes into the smoky air of some manufacturing towns. A Royal Commission was set up in 1862 to consider air pollution and this resulted in the first Alkali Act in 1863, which set limits to the concentration of acid in discharged waste gases. However, the increasing domestic and industrial bustion of coal, and the production of piped coal gas from 1815, caused air pollution to steadily get worse. Large cities were particularly affected, and the well known 5 day smog incident in London in 1952 directly contributed to the deaths of 4000 people. As a result, the Beaver Committee on Air Pollution was established in 1953, and the Clean Air Act was passed in 1956. This was the first effective statute to provide the means of controlling atmospheric pollution. Noise pollution probably started when man first developed machines. The increase in industrial plants in the nieenth century produce indoor noise pollution of the working environment for many factory and mill workers over a 6 day week. Outdoors, the development of private and public transport bright environmental noise, as the railway services came into use during the 1830s, motor transport from 1900, and regular aero plane services from 1922. during the first half of the twentieth century environmental noise considerably increased, but it was not recognized as pollution. Industrial and outdoor noise was designated as ‘nuisance’ when the Noise Abatement Act was passed in 1960. Whereas the earlier increase in noise occurred in work places and in connection with transport, during the past thirty years noise has spread into the home and places of leisure and entertainment. Certainly the most rapid increase in environmental pollution has taken place during the last 150 years, and it has been attributed to a number of interrelating factors. THE CHARACTERISTICS OF FLUIDS A fluid is a substance which may flow, that is, its constituent particles may continuously change their positions relative to one another. Moreover, it offers no lasting resistance to the displacement, however great, of one layer over another. This means that, if the fluid is at rest, no shear force (that is a force tangential to the surface on which it acts) can exist in it. A solid, on 。 it may bee difficult to carry out their operation due to devastating floods. A storage or a conservation reservoir can retain such excess supplies during periods of peak flows and can release them gradually during low flows as and when the need arise. Incidentally, in addition to conserving water for later use, the storage of flood water may also reduce flood damage below the reservoir. Hence, a reservoir can be used for controlling floods either solely or in addition to other purposes. In the former case, it is known as ‘Flood Control Reservoir’ or ‘Single Purpose Flood Control Reservoir’, and in the later case, it is called a ‘Multipurpose Reservoir’. (2) Flood Control Reservoirs A flood control reservoir or generally called floodmitigation reservoir, stores a portion of the flood flows in such a way as to minimize the flood peaks at the areas to be protected downstream. To acplish this, the entire inflow entering the reservoir is discharge till the outflow reaches the safe capacity of the channel downstream. The inflow in excess of this rate is stored in stored in the reservoir, which is then gradually released so as to recover the storage capacity for next flood. The flood peaks at the points just downstream of the reservoir are thus reduced by an amount AB. A flood control reservoir differs from a conservation reservoir only in its need for a large sluiceway capacity to permit rapid drawdown before or after a flood. Types of flood control reservoirs. There are tow basic types of floodmitigation reservoir. Storage Reservoir or Detention basins. Retarding basins or retarding reservoirs. A reservoir with gates and valves installation at the spillway and at the sluice outlets is known as a storagereservoir, while on the other hand, a reservoir with ungated outlet is known as a retarding basin. Functioning and advantages of a retarding basin: A retarding basin is usually provided with an uncontrolled spillway and an uncontrolled orifice type sluiceway. The automatic regulation of outflow depending upon the availability of water takes place from such a reservoir. The maximum discharging capacity of such a reservoir should be equal to the maximum safe carrying capacity of the channel downstream. As flood occurs, the reservoir gets filled and discharges through sluiceways. As the reservoir elevation increases, outflow discharge increases. The water level goes on rising until the flood has subsided and the inflow bees equal to or less than the outflow. After this, water gets automatically withdrawn from the reservoir until the stored water is pletely discharged. The advantages of a retarding basin over a gate controlled detention basin are: ① Cost of gate installations is save. 6 ② There are no fates and hence, the possibility of human error and negligence in their operation is eliminated. Since such a reservoir is not always filled, much of land below the maximum reservoir level will be submerged only temporarily and occasionally and can be successfully used for agriculture, although no permanent habitation can be allowed on this land. Functioning and advantages of a storage reservoir: A storage reservoir with gated spillway and gated sluiceway, provides more flexibility of operation