Engineering Hydrology

The science of hydrology deals with the occurrence and movement of water on and over the surface of the earth. It deals with the various forms of moisture that occur, the transformation between the liquid, solid and gaseous states in the atmosphere and in the surface layers of land masses. It is concerned also with the sea: the source and store of all the water that activates life on this planet.

The hydrology of a region depends primarily on its climate, secondly on its topography and its geology. Climate is largely dependent on the geographical position on the earth’s surface. Climatic factors of importance are precipitation and its mode of occurrence, humidity, temperature and wind, all of which directly affect evaporation and transpiration.

Evaporation is important in all water resource studies. It affects the yield of river basins, the necessary capacity of reservoirs, the size of pumping plant, the consumptive use of water by crops and the yield of underground supplies, to name only a few of the factors affected by it.

When rain falls upon the ground it first of all wets the vegetation or the bare soil. When the surface cover is completely wet, subsequent rain must either penetrate the surface layers, if the surface is permeable, or run off the surface towards a stream channel if it is impermeable.

Rainfall which infiltrates the soil and penetrates to the underlying strata is called groundwater. The quantity of water which can be accommodated under the surface depends on the porosity of the sub-surface strata. The water bearing strata, called aquifers may consist of unconsolidated materials like sands, gravels and glacial drift or consolidated material like sandstones and limestones. Limestone is relatively impervious but is soluble in water and so frequently has wide joints and solution passages which make the rock, en masse similar to a porous rock in its capacity to hold water and act as an aquifer.

Civilisation has always developed along rivers, whose presence guaranteed access to and from the sea coast, irrigation for crops, water supplies for urban communities and latterly power development and industrial water supply. The many advantages have always been counterbalanced by the dangers of floods and in the past levees or flood banks were built along many major rivers to prevent inundation in the flood season. In more recent times storage reservoirs have been built as the principles of dam construction became better understood and other measures like relief channels, storage basins and channel improvements are continually under construction in many parts of the world. It is important for such works that estimates can be made of how the measures proposed will affect the behaviour of flood waves in rivers so that economic solutions may be found in particular cases. Flood routing is the description applied to this process. It is a procedure through which the variation of discharge with time at a point on a stream channel, may be determined by consideration of similar data for a point upstream. In other words it is a process which shows how a flood wave may be reduced in magnitude and lengthened in time (attenuated) by the use of storage in the reach between the two points.

In the previous chapters the various physical processes involved in the hydrologic cycle have been enumerated and examined in detail. Methods of evaluating each process have been suggested and often explained, and techniques discussed which may be used to provide quantitative answers to many questions.