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