Department of Civil Engineering CE2021 -HYDROLOGY
Part – A (Questions and Answers)
UNIT – I PRECIPITATION
1. Define Hydrology.
Hydrology means science of water. It is the science that deals with occurrence, distribution and movement of water is on, above and beneath the earth.
2 Define Hydrologic cycle.
Hydrologic cycle is the water transfer cycle which occurs continuously in nature. The three important phases of the hydrologic cycle are
Evaporation and evapotranspiration
Precipitation and
Runoff
3. What is the objective of the hydrological study?
* Estimation of water resources
* Study the processes such as precipitation, runoff, evaporation, transpiration, infiltration and their interactions.
* Study of floods, droughts and strategies to combat them * Hydro power
4. What is the importance of hydrology?
The importance of hydrology is the assessment, development, utilization and management of water resources of any region.
5. Enlist the various phases of a hydrological cycle?
i) Precipitation ii) Infiltration iii) Evaporation iv)Transpiration v) Runoff
6. Define Precipitation?
Any form of moisture reaching the earth surface is called precipitation. The usual forms of precipitation are rainfall, snow, hail, sleet, frost, dew etc.
7. Define infiltration?
8. Define evaporation?
It is the process by which water from liquid state passes into vapour state under the action of sunrays.
9. Define transpiration?
The process by which water passes from liquid to vapour through plant metabolism is termed as transpiration.
10. Define runoff?
It is the precipitation excess after meeting the demands of evaporation, transpiration and infiltration
11. Enlist the various forms of precipitation?
The usual forms of precipitation are
i) Rainfall ii) Drizzle iii) Hail iv) Dew
v) Glaze vi) Snow vii) Frost
12. What are all the types of precipitation?
i) Cyclonic precipitation ii) Convective precipitation iii) Oragraphic precipitation iv) Frontal precipitation
13. How the precipitation can be measured?
It can be measured by rain gauge. The rain gauge may be
i) Recording type rain gauge ii) Non- recording type rain gauge
14. What are all the demerits of Non- recording type rain gauge?
It does not give information regarding Beginning of the rain End of the rain Intensity of rainfall
15. Enlist the three types of recording type rain gauge?
* Tipping bucket * Weighing bucket * Floating bucket
16. Write short notes on rain gauge density?
It is the no. of rain gauges is to erected in an given area Rain gauge density = No. of rain gauges / Area
17. What are all the methods available to find the average depth of precipitation over an area?
18. Define isohyet?
An isohyet is the line joining the points of equal rainfall.
19. What is the use of Double mass curve?
It is used to check the consistency of the rainfall record. In double mass curve a graph is drawn between the cumulative value of average rainfall of base stations as the abscissa against the corresponding cumulative value of rainfall of the station under test as ordinate.
20. Define Hyetograph?
It is a plot of rainfall intensity against time interval. It is derived from mass curve and is usually represented as bar chart. It is used to predict the extreme floods.
21. Write short notes on intensity-duration curves?
Short duration – High intensity Long duration – Less intensity
22. What is the use of frequency analysis?
It is used to find the probability of occurrence of extreme rainfall. The probability of occurrence of rainfall whose magnitude is equal to or greater than specified magnitude is given by
T = N+1/m where T = Return period m= Rank
N= No. of years of rainfall record Define
23. What is a Catchments area?
It is an area of land where stream or a water course at a given location drain in it is also known as drainage area or drainage basin or water shed.
24.Write short note on Divide.
It is the ridge which separates a catchments area from its surrounding areas.
25.Define Front
26.Write short note on cyclones.
A cyclone is a large low pressure region with circular wind motion. Two types of cyclones are recognized, tropical cyclones and extra tropical cyclones.
27.Define Anticyclones.
These are regions of high pressure, usually of large areal extent. The weather is usually calm at the centre, anticyclones cause clockwise wind circulations in the northern hemisphere, winds are of moderate speed and at the outer edges cloudy land precipitation conditions exist.
28.How does the average intensity of rainfall obtained from mass curve.
If any tow points on the rainfall mass curve are jointed by a straight line the slope of that line gives the average intensity of rainfall for the time period between those points denoted by ‘I’
I =
∆
p/∆
t29.State Hydrologic equation?
The Hydrologic equation is simply the statement of the law of conservation of mass and is given by
I = 0 + S
Where I = Infolw O = Outflow
S = Change in storage
30. Define air front?
The surface of contact between two air masses or between an air mass and the surrounding atmosphere is called air front.
UNIT – II
ABSTRACTION FROM PRECIPITATION
1. Define rainfall intensity.
2. Enlist the factors affecting evaporation?
i) Temperature ii) Wind
iii) Atmospheric pressure iv) Soluble salts
3. Enumerate the methods used to estimate the amount of evaporation from a water surface?
i) Evaporimeters ii) Analytical methods iii) Empirical formulae
3. Write short notes on evaporimeters?
It is a device used to measure evaporation. These are water containing chambers which are exposed to atmosphere and the loss of water by evaporation is measured at regular intervals.
4. Enlist the types of evaporimeters?
i) Class A evaporation pan ii) ISI standard pan
iii) Colarodo sunken pan iv) US geological survey floating pan
5. Define pan coefficient?
Pan coefficient = Lake evaporation / Pan evaporation
6. State the Daltons law of evaporation?
It states that the rate of evaporation is proportional to the difference between the saturation vapour pressure to the actual vapour pressure of air.
7. Name the analytical methods of determining the lake evaporation?
* Water budget method * Energy balance method
8. How will you reduce the evaporation from a water surface?
i) Reduction of surface ii) Mechanical covers iii) Chemical films
9. What are all the factors affecting transpiration?
i) Atmospheric pressure ii) Wind iii) Sunshine iv) Temperature v) Characteristics of plants
10. Enlist the instruments used tomeasure transpiration?
* Lysimeter * Field plots
11. Define infiltration capacity?
12. Enumerate the factors affecting infiltration capacity?
i) Compaction ii) Surface cover iii) Temperature iv) Characteristics of soil v) Nature of water
13. Define infiltrometer and mention its types?
Infiltrometres are the devices used to measure infiltration. There are two kinds of infiltrometers
i) Flooding type infiltrometer ii) Rainfall simulator
14. Write short notes on Hortons equation?
Horton developed the mathematical expression defining the infiltration capacity was given by
15. Define o index?
It is the average rainfall above which rainfall volume equals the runoff volume.
16. Write short note on rainfall hyetograph.
The instantaneous rainfall intensity is usually denoted by (i) and expressed in mm/ hrs. A graph showing the variation of rainfall intensity with time is called a rainfall hyetograph.
17. Define rainfall mass curve.
A graph showing the cumulative depth of rainfall against time is known as the rainfall mass curve.
18.Define point rainfall or station rainfall.
The rainfall obtained from a single rain gauge station is known as the point rainfall or station rainfall
19.Define Normal rainfall
The normal rainfall is the average value of rainfall at a particular date, month or year over a specified 30 year period. It is recomputed every decade.
20.What is a DAD curve ?
The average depth of rainfall are plotted against the areas up to the
21.Define PMP.
It has been defined as that depth of precipitation which for a given area and duration can be reached but not exceeded under known meteorological conditions.
22.List out the methods available for estimating missing data.
i. Interpolation from isohyetal maps. ii. Arithmetic average method. iii. Normal ratio method. iv. Inverse distance method.
v. Regression method.
23.Write down the approaches used for estimating the PMP.
Basically two approaches are used for estimating the PMP. They are i. Meteorological Methods
ii. The statistical study of rainfall data.
24.Explain W index
This is refined version of o index. It includes depression storage and
interception from total losses. It is the average infiltrating rate during the time rainfall intensity exceeds the capacity rate.
W = P – Q – S / tf where W = Average rate of infiltration P = Total storm rainfall Q = Total storm runoff
S = Volume of depression storage
Tf = Total time during which rainfall intensity is greater than W
25.What is an infiltrating capacity curve?
The infiltrating rates are plotted against their time intervals as a bar diagram.By joining the mid points of the bars we get the approximate infiltration capacity curv
26. Define Transpiration ratio?
Transpiration ratio = Weight of water transpired / Weight of dry matter produced The average value of transpiration ratio being 700
27. Write short notes on water balance method?
According to Horton’s water balance equation
E = I- Q – S where E = Evaporation I = Inflow
O = Out flow and
UNIT – III HYDROGRAPHS
1. Define runoff?
The portion of the precipitation which appears in the surface either perennial or intermittent in nature is called runoff. The unit of runoff is cumecs.
2. Enlist the components of runoff?
i) Channel precipitation ii) Direct runoff iii) Overland flow iv) Base flow v) Inter flow
3. What is direct runoff?
That part of the runoff which enters the stream immediately after precipitation
4. Explain base flow
The flow reaches a stream essentially as ground water flow is called base flow.
5. What is perennial stream.
A perennial stream is one which always carries some flow. There is considerably some amount of groundwater throughout the layer.
6. What is an ephemeral stream?
A stream which becomes dry soon after the end of the precipitation. There will not be any base flow contribution.
7. What is meant by precipitation excess?
That part of precipitation which contributes directly to the surface runoff.
8. Define a flow mass curve?
It is the plot of cumulative discharge volume against time plotted in chronological order.
9. Define basin lag(Time lag)
Basin lag is defined as the time from the center of rainfall to the hydrograph peak.
10. What are the basic assumptions of unit hydrograph theory?
(i) the time invariance (ii) The linear response
11. Write short notes on drainage density?
12. Define hydrograph?
It is a graph drawn between discharge against time. It consists of direct runoff and base flow.
13. Define unit hydrograph?
It is defined as the hydrograph if direct runoff resulting from an isolated rainfall of unit duration occurring uniformly over the basin and produces unit runoff.
14. What are the methods to determine the unit hydrograph?
Unit hydrograph developed by the following two methods
i) Method of Super position ii) S-curve technique
15. Write short notes on method of super position?
If a D-h unit hydrograph is available and it is easy to develop hydrographs of nD-h wnD-here n is an integer. It is easily accomplisnD-hed by superimposing n-unit nD-hydrograpnD-h with each separated from the previous on D-h.
16. Define S-curve technique?
It is a curve obtained from the summation of infinite series of D-h unit
hydrograph spaced by D-h apart. A smooth curve is obtained resulting to S shape curve called S-curve hydrograph.
17. Define Synthetic unit hydrograph?
Unit hydrographs derived from empirical equations are called synthetic unit hydrograph. The following three parameters are used for development of synthetic unit hydrograph.
i) Base width ii) Peak discharge iii) Lag time
18. Define instantaneous unit hydrograph?
If the duration of effective rainfall approaches zero, the unit hydrograph may be called as instantaneous unit hydrograph.
19. What are all the applications of unit hydrograph?
i) Once unit hydrograph is developed for a basin and it can be used to obtain direct runoff hydrograph for any storm event on the basin.
ii) If the unit hydrograph is applied to max. probable precipitation on the basin, then the max. probable precipitation is obtained.
20. What are all the advantages of instantaneous unit hydrograph over direct unit hydrograph?
i) The advantage of IUH over DUH is that it eliminates the problem of unit duration and the restriction of uniform distribution of rainfall in time.
21. Define stream density?
The stream density of a basin is expressed as the no.of streams per sq.km.
22. Define drainage density?
It is expressed as the total length of all stream channels per unit area of the basin.
23. Define concentration time?
The time required for the falling rain at the most distant point in a drainage area to reach stream outlet is called concentration time.
24. Define overland flow?
When a storm occurs, a portion of rainfall infiltrates in to the ground and some portion may evaporate. The rest flows as a thin sheet of water over the land surface which is termed as overland flow.
25. Define effective precipitation?
The part of the precipitation that contributes entirely to the direct runoff may be called effective precipitation or effective raindfall
26. Define isochrones?
The line joining the points of equal travel time of rainfall is called isochrones. It is helpful in deriving hydrographs.
27. State Virgin flow?
It is a stream flow which is unaffected by the artificial diversions, storage or other works of man in or on the stream channels.
28. Define surface runoff?
It is the part of runoff which travels over the ground surface through channel to reach the basin outlet.
29. Define subsurface runoff?
It is the runoff due to the part of the precipitation which infiltrates the surface soil and moves laterally downward towards the streams.
UNIT – IV
FLOODS AND FLOOD ROUTING
1. Define design flood?
2. Define MPF?
It is defined as the flood that may be expected from the most severe combination of critical meteorological and hydrological conditions that are reasonably possible in the region. This is very large flood and it is very rarely used in design except for reservoir spillways.
3. Define flood?
Any flow which is relatively high and which overtops natural or artificial banka in any reach of the river is called flood. Floods are produced when the capacity of the river channel is inadequate to carry off the abnormal quantity of water arising from heavy rainfall.
4. State any two formulae to calculate flood discharge?
i) Dickens formula, Q = CA ii) Ryves formual, Q = CA
where Q = Max.flood discharge A = Catchment area
C = Dickens constant varies from 6 to 30 C = Ryves constant
5. Define attenuation?
Owing to the storage effect the peak flow of the outflow hydrograph will be smaller than that of outflow hydrograph. This reduction in peak value is called attenuation.
6. Define lag value?
The peak of the outflow occurs after the peak of the inflow. The time difference between the two peaks is termed as lag?
7. Define flood routing?
It is the procedure where by shape of the flood hydrograph at a particular location on the stream is determined from the known or assumed flood hydrograph at some other location.
8. Enlist the types of flood routing?
i) Channel routing ii) Reservoir routing
9. What are all the methods available to determine the floodpeak?
10. Define return period?
It represents the average no. of years within which a given event will be equaled or exceeded.
11. Define prism storage and wedge storage?
Prism storage is proportional to the outflow where as wedge storage is the difference between inflow and outflow.
12.What is reservoir routing?
In reservoir routing the effect of a flood routing wave entering a reservoir is studied to predict the variation of reservoir ,elevation and outflow discharge with time.
13.What is channel routing?
It is a method which uses math medical relations to calculate outflow from a stream channel once inflow ,lateral contributions and channel characteristics are known
14.What are the uses of reservoir routing?
The uses are:
(i) In the design of capacity of spillway and other reservoir outlet structure (ii) In the location and sizing of the capacity of reservoir to meet specific
requirements.
15. What are the uses of flood routing?
(i) Establishing the height of flood peak at a downstream location in short term flood forecasting.
(ii) Estimating the protection that would result from construction of a reservoir. (iii) Determining the required levee height for flood protection.
(iv) Determining the adequacy of spillway.
16.What is the basis of hydrologic routing method?
The basis is continuity equation.
17.What are the forms of storage in a channel reach?
(i) prism storage (ii) wedge storage
18.What is gumbel’s distribution?
It is one of the most widely used probability-distribution functions for extreme values in hydrologic and metrological studies for prediction of flood peaks,maximum rainfall etc,.
19.What is design flood?
20.What is probable maximum flood?
The extreme flood that is physically possible in a region as a result of a severe most combination including rare combination of metrological and hydrological factors.
21. Define standard project flood?
This is the estimate of the flood likely to occur from the most severe combination of the hydrological and meteorological conditions
22. Write short notes on flood control measure?
* By confining the flow between high banks by constructing levees, dykes or flood walls.
* By channel improvement by cutting, straightening or deepening and Following River training works.
* By diversion of a portion of a flood through bypasses or flood ways
* By providing a temporary storage of the flood peaks by constructing upstream reservoirs and retarding basins.
23. Write short notes on flood forecast?
The flood forecasts are issued on the basis of the analysis of weather charts and indicate the likelihood of heavy rainfall over the specified areas with the next 24 to 48 hours. All India forecasts are prepared every day at Poona.
24. Write short notes on flood warning system?
The food warning system basically consists of working out the flood hydrograph at a given point sufficiently in advance, given the flood hydrograph of some gauge or gauges sufficiently upstream, taking into consideration. The flood warning system is prepared by i) Forecasting crest height and time ii) Flood routing techniques
With a properly developed system of scientific flood forecasting and warning, human toll and destruction to movable properties could be greatly mitigated.
UNIT – V
GROUND WATER HYDROLOGY
1. Define aquifer?
An aquifer is a saturated formation of earth materials which not only stores water but yields sufficient quantity of water.
2. Define aquitard?
It is a formation through which only seepage is possible and thus the yield is insignificant compared to an aquifer.
3. Define aquiclude?
It is a geological formation which is neither porous nor permeable. There are no interconnected openings and hence it cannot transmit water.
4. Define confined aquifer?
It is an aquifer which is confined between two impervious beds such as
aquicludes or aquifuges. Recharge of this aquifer takes place only in the area where it is exposed to the ground surface.
5. Define unconfined aquifer?
It is an aquifer in which there exists a free water surface. Recharge of this aquifer takes place through infiltration.
6. Define porosity?
The amount of pore space per unit volume of the aquifer is called porosity.
6. Define specific yield?
The actual volume of water that can be extracted by the force of gravity from a unit volume of aquifer is called Specific yield.
6. State Darcy’s law?
It relating the velocity of flow in a porous medium which was given by V = Ki where V = velocity of seepage
K = Coefficient of permeability I = Hydraulic gradient
7. Define specific capacity?
8.What is meant by cone of depression?
When a well is pumped ,there will be a decline in the water level and gradually it assumes the shape of an inverted cone, which is called cone of depression.
9. Define Transmissivity?
It is the flow capacity of an aquifer per unit width under unit hydraulic gradient and is equal to the product of permeability times the saturated thickness of the aquifer.
10. What are all the test available to find the yield of a well?
i) Pumping test ii) Recuperation test
11. Enlist the formations that serve as good aquifers?
* Unconsolidated gravels, sands, alluvium * Lake sediments and glacial deposits * Lime stones with cavities
* Granites and marbles with fissures and cracks * slates
12. What are all the assumptions made in derivation of Dupuits equations?
* Stabilised draw down
* The aquifer is homogeneous, isotropic, of infinite areal extent and of constant thickness
* Complete penetration of the well with 100% efficiency * Flow lines are radial and horizontal and the flow is laminar
13. Write short notes on spacing of wells?
Pumping wells should be spaced far apart so that their cones of depression will not overlap over each other resulting in the reduction of their yields and/or increased draw downs to avoid well interference. The well should be spaced beyond their radii of influence.
14.What is Steady state flow?
During pumping of a well, a stage occurs when there is no change in water level with time. this condition is known as steady state flow
15.What is effluent stream?
Streams which receive ground water flow are called effluent stream. 16. What is influent stream?
A Storm which contributes to the ground water is known as influent stream.
17.What is meant by radius of influence of a well?
18.What is safe yield?
The safe yield of groundwater basin may be defined as the amount of water which can be drawn from it without producing any undesirable effects. Any withdrawal in excess of safe yield is called over draft.
19.What is pumping test?
In a pumping test,a well is pumped at a constant rate for a considerably period of time.the water level in the pumping as well as in one or more observations well are noted during the period. observation wells are located at varying distance from the pumping well. As the aquifer is pumped, the draw downs are determined as function of time.We can determine the aquifer parameters (T,K and S)based on the data collected from a pumping test.
20.Explain a recuperation test.
