Engineering Hydrology

👉 Do You Know

• Tensiometer is used to measure capillary potential
• Phytometer measures Transpiration
• Lysimeter measures Evaporation
• Infiltrometer measures Infiltration
• Speed of wind is measured using Anemometer.
• Average annual rainfall over whole of India is 119 cm.

👉Measurement of Precipitation

• Rain Gauges
• Pluviometer
• Ombrometer
• Hyetometer

👉 Types of Gauges

• Non-Recording Gauges

(a) The Non-Recording gauge used in India as SYMONS Raingauge

• Recording Gauges

(a) Tipping Bucket Type

(b) Weighing Bucket Type

(c) Natural Syphon Type

⟶ The record from Tipping bucket gives data on the intensity of rainfall

⟶Weighing bucket gives the idea of mass curve of rainfall (Plot of the accumulated rainfall against the elapsed time)

⟶In India Natural Syphon Type is adopted as Recording Rain Gauge.(This type of gauge gives a plot of the mass curve of rainfall) 

👉Inconsistency of Records

Some of the common cause of inconsistency of the records are:

• Shifting of raingauge station to a new location
• Neighborhood of the station undergoing a marked change
• Replacement of old instrument by new one
• Method of observation 

👉Inconsistency of record is corrected by using DOUBLE MASS CURVE technique. 

👉To convert the point rainfall values at various stations into an average value over catchment, several methods are available

(a) Arithmetical mean method

(b) Theissen- polygon method

(c) Isohyetal method

👉Accuracy: Isohyetal method > Theissen-polygon method > Arithmetical mean method

👉Frequency of Point Rainfall

The purpose of the frequency analysis of an annual series is to obtain a relation between the magnitude of the event and its probability of exceedence (P)

If the annual extreme series is arranged in descending order of magnitude and each position given a number 1 to N, 1 being given to Ist i.e largest value and N given to last value or least value. Then probability P of a rainfall at position m being equaled or exceeded is given by :

                             Method                                            Probability (P) 

                       

                             Weibull                                                 m/(N+1)

                             

                             California                                                m/N

                              Hazen                                                     (m - 0.5)/N

                              Blom                                                    (m-0.44)/(N+0.12)

👉 Probable Maximum Precipitation(PMP)

The probable maximum precipitation (PMP) is defined as the greatest or extreme rainfall of a given duration that is physically possible over a basin.

                    PMP =  Mean annual rainfall + k. σ

                        σ = standard deviation of series

                        k = Frequency factor

👉Isopleth

A line drawn on the map along which the value of a particular property is uniform .

             Name                              Isopleth of (i.e line joining places of equal) 

             ISOBAR                                              Pressure

            ISOBATH                                         Depth in sea

         

            ISOCHRONE                     Travel time from a common centre

            ISOHALINE                                      Salinity

            ISOHELS                                           Sunshine

 

           ISOHYETS                                         Rainfall

            ISONIF                                          Snow fall amount

            ISOTHERM                                     Temperature

Engineering Hydrology

 👉 A flow duration curve is a plot of discharge against the percentage of time the flow was equaled or 

exceeded. This curve is also known as discharge frequency curve.  

👉Line joining points of equal rainfall depth is called isohyet.

👉 Isopleths are line on a map through points having equal depth of evapotranspiration.

👉                    Name                       Isopleth of (i.e line joining places of equal)

                        Isotherm                                           Temperature

                         Isobar                                               Pressure

                 

                         Isobath                                             Depth in sea

 

                        Isochrone                                        Travel time from a common centre

                       Isohaline                                              Salinity

                        Isohyets                                                 Rainfall

                        Isonif                                                Snowfall amount

                       Isoryme                                                Frost

                        Isohels                                                Sunshine

👉  Bowen's ratio is defined as the ratio of Sensible heat transfer from water surface to air (Ha) to Heat energy used up in evaporation (He)

👉               Property                                  Measuring Instrument

                  Evapotranspiration                          Lysimeter

                Capillary Potential                            Tensiometer

                Transpiration                                    Phytometer

                 Evaporation                                     Atmometer

                Hydraulic Conductivity                     Permeameter

                Infiltration Capacity                          Infiltrometer

                                                                           Rainfall Simulator

              Humidity                                             Hygrometer

              Relative Humidity                              Psychrometer

              Wind Speed                                        Anemometer

 👉 Ratio of total channel length to the discharge area is called drainage density. 

👉         Hyetograph:  Rainfall intensity versus time 

   

               Mass Curve: Cumulative rainfall versus time

               Hydrograph: A hydrograph is a graph showing the rate of flow versus time past a specific point in a river, channel, or conduit carrying flow

Engineering Hydrology

 👉The hydrological budget equation is:

                          P-R-E-T-G = △S

Where, P = total precipitation

             R = Net runoff = R2-R1 = surface runoff flow- surface runoff inflow

            E = Total evaporation

            T = total Transpiration

          G = Net ground water flow

         △S = Total storage increase

 👉 Hydrological cycle is the cycle in which water is transported from the oceans to the atmosphere as vapours, from the atmosphere to the land as precipitation and back from land to oceans as runoff.   

👉Drizzle:  A fine sprinkle of numerous water droplets of size less than 0.5 mm and intensity less than 1 mm/h

Rain: The precipitation of liquid water with drops of sizes larger than 0.5 mm 

Hail: It is showery precipitation in the form of irregular pellets or lumps of ice of size more than 5 mm.

👉 Average rainfall over a catchment area can be estimated as

1. Arithmetic mean method: 

• every rain gauge station is given equal weightage regardless of its location.
• Rainfall is almost uniformly distributed over the whole catchment area which rarely occurs in nature.
• method is fast but does not give accurate results.

2. Theissen polygon method: 

• In this method weightage to the various rainfall datas based on area close to the rain gauge station 
• Method is fast when once the weights are known
• It does not take care of the variability in rainfall due to elevation difference(topographical influence are not take care of)
• New polygon is required to be drawn when due to addition or deletion of raingauges to the network, weight of each station changes.

3. Isohyetal method :

• This is most accurate method
• It utilizes all relevant data's and properly interpret them.
• method is slow and laborious

4. Normal ratio method: 

• It is used to estimate missing rainfall data.