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, and 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 but a few of the parameters 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 the surface is impermeable.

Rainfall that infiltrates the soil and penetrates to the underlying strata is called groundwater. The quantity of water that can be accommodated under the surface depends on the porosity of the sub-surface strata. The water-bearing strata, called aquifers, can 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 that make the rock, en masse similar to a porous rock in its capacity to hold water and act as an aquifer.

The various contributing components of a natural hydrograph are shown in figure 7.1. To begin with there is baseflow only; that is, the groundwater contribution from the aquifers bordering the river, which go on discharging more and more slowly with time. The hydrograph of baseflow is near to an exponential curve and the quantity at any time is represented very nearly by Q_t = Q₀e^-αt where Q_o = discharge at start of period Q_t = discharge at end of time t a = coefficient of aquifer e = base of natural logarithms.

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 can 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 can be determined by consideration of similar data for a point upstream. In other words it is a process that shows how a flood wave can 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 that can be used to provide quantitative answers to many questions.

In the estimation of runoff from natural catchments, the determination of T_p, time to peak, and SPR, standard percentage runoff, contain an URBAN term. Similarly in chapter 9, the equation for , the mean annual maximum flood for Region 6, departs from the general equation and includes an URBAN term. URBAN, it may be recalled, is the fraction of catchment in urban development. The reason for this special term’s inclusion is the need to take account of the impermeable areas of buildings’ roofs, roads, pavements, car parks, etc. in a built-up environment where there is negligible infiltration and runoff is accelerated by drainage systems of gulleys, pipes and sewers. Obviously the time to peak of a hydrograph and the percentage runoff will be decreased and increased respectively compared with a natural catchment of permeable soils and vegetation.

Since the Flood Studies Report was published, many of the techniques devised for it have been subsequently developed and their range of application extended. One of the investigations arising from this development was the World Flood Study [1] which, in the words of its authors, “was conceived with the aim of examining and classifying the characteristics of floods in as many countries and from as wide a range of climates as possible.” Much of this work has been subsequently reported in the literature [2] .

All hydraulic engineering design carries with it an implied structural life during which the structure is expected to meet its design specification. Some structures have specified design lives (for example, a cofferdam or a diversion tunnel) but often such a life is not made explicit. How long, for example, are the design lives of a canal, a dam, or a breakwater?