HYDROLOGY & DAMS - QUICK REVISION SERIES 2
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1.Precipitation denotes all forms of water that reach the earth surface from the
atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew. Precipitation is the natural stating point for the hydrologic cycle and main input to the hydrologic systems. For formation of precipitation, atmosphere must have sufficient moisture, nuclei aid.
2.Forms of precipitation
1.) Rain – 0.5 mm < d < 6.0 mm; light if intensity < 2.5 mm/h & heavy > 7.5 mm/h 2.) Snow – ice crystals into snowflakes
3.) Drizzle – 0.1mm < d < 0.5mm; intensity < 1mm/h; appear to float in air
4.) Hail – balls of ice 5mm<d<125mm; thunderstorms with strong vertical currents 5.) Sleet – frozen raindrops when fall through subfreezing zone
6.) Glaze – ice coating on exposed surface by freezing rain or drizzle.
3. Importance of Hydrological Data:
1) Estimation of water resources becomes easy.
2) For adequate study of processes such as run off, evapotranspiration, precipitation and their interaction.
3) To take problems like floods and drought in proper strategical manner.
4) Much helpful for design and operation of water resources project as the design also based on the data.
4.Mechanics of Precipitation occurs when air saturated; this occurs when air rises; which in turn usually occurs in one of three ways.
Types 1. Convective precipitation occurs when air rises vertically through the (temporarily) self-sustaining mechanism of convection. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent.
2. Stratiform precipitation occurs when large masses over air rise slant-wise as larger-scale atmospheric dynamics force them to move over each other. Stratiform rainfall is also caused by frontal systems surrounding extratropical cyclones or lows, which form when warm and often tropical air meets cooler air. Stratiform precipitation falls out of nimbostratus clouds.
3. Orographic precipitation is similar, except the upwards motion is forced when a moving air mass encounters a rising slope. Orographic or relief rainfall is caused when masses of air pushed by wind are forced up the side of elevated land formations, such as large mountains.
5.Hyetograph is a plot of intensity of rainfall against the time interval in mm/hr. The hyetograph is derived from the mass curve and is usually represented as bar chart. It is very convenient way of representing the characteristics of a storm and is practically particularly important in the development of designs storms to predict extreme floods. The area under a hyetograph represents the total precipitation received in the period. The time interval used depends on the purpose in urban drainage problems small durations are used while in flood flow computations in larger catchments. The intervals are of about 6 hours. It is usedfor hydrological analysis of catchment for estimation of design storms for prediction of flood, estimation of run off and derivation of unit hydrograph.
6Types of rain gauge: Symon’s (non recording) gauge. Weighing Bucket type, Tipping bucket type, Siphon rain gauge or floating gauge, storage rain gauge, telemetering rain gauge, radar measurement of rain fall and automatic radio reporting rain gauge. Rain guage network (a) plains - 1 in 520 km2 (b) elevation upto 1000m - 1 in 260 km2 (c) hills - 1 in 130 km2.
7.Mass curve of rainfall: The mass curve of rainfall is a plot of the accumulated precipitation against time plotted in chronological order. Mass curve of rainfall is very useful in extracting the information on the duration and magnitude of a storm. Also, intensities at various time intervals in a storm can be obtained by the slope of the curve.
8.Average Mean precipitation over an area Raingauges represent only point sampling of the areal distribution of a storm. In practice hydrological analysis requires a knowledge of the rainfall over an area. To convert the point rainfall values at various stations into an average value over a catchment the following three methods are used (a) Arithmetic mean (b) Thiessen polygon and (c) Isohyetal method.
9.Depth- Area duration Curve In designing structures for water resources, one has to know the areal spread of rainfall within watershed. However, it is often required to know the amount of high rainfall that may be expected over the catchment. It may be observed that usually a storm event would start with a heavy downpour and may gradually reduce as time passes. Hence, the rainfall depth is not proportional to the time duration of rainfall observation. Similarly, rainfall over a small area may be more or less uniform. But if the area is large, then due to the variation of rain falling in different parts, the average rainfall would be less than that recorded over a small portion below the high rain fall occurring within the area. Due to these facts, a Depth-Area-Duration (DAD) analysis is carried out based on records of several storms on an area and, the maximum areal precipitation for different durations corresponding to different areal extents.
10.Intensity duration frequency Curve: The analysis of continuous rainfall events, usually lasting for periods of less than a day, requires the evaluation of rainfall intensities. The assessment of such values may be made from records of several part storms over the area and presented in a graphical form. An Intensity-Duration-Frequency curve (IDF Curve) is a graphical representation of the probability that a given average rainfall intensity will occur. Rainfall Intensity (mm/hr), Rainfall Duration and Rainfall Frequency are the parameters that make up the axes of the graph of IDF curve. An IDF curve is created with long term rainfall records collected at a rainfall monitoring station. i= kTx/(D+a)n. here k and a are constant, and x. n are exponent for a given catchment. Variation of intensity = i with duration D and return period T.
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