Homework Sourse1 Answer the following a What is the rating curve How is it
Solution
a) RATING CURVE: A rating curve is a graph showing relationship between stage (water surface elevation) and discharge corresponding to that stage.
Usefulness of Rating Curve: Rating curve is immensely used to estimate the discharge in natural and artificial open channels. Rating curve plays an important role in planning, design, and management of water resources projects. For ex: The hydrological rainfall-runoff models are usually parameterized on the basis of observations of rainfall and discharge from streams at a particular point of interest.
For streams and rivers, which doesn\'t display any change in its flow regime, a relationship between stage and discharge is established to estimate the discharge corresponding to the water elevation. This relationship is called as Rating Equation of the stream.
b) PAN EVAPORATION is the evaporation of water observed from a Class-A evaporation pan. This is multiplied with Pan coefficient (Cp) to estimate the evaporation of water (in depth) from lakes under similar climatic and exposure conditions.. This is measured for proper planning and operation of reservoirs and irrigation systems.
REFERENCE EVAPOTRANSPIRATION is the maximum quantity of water capable of being evaporated from the soil and transpired from a hypothetical reference grass with an assumed crop height of 0.12 m, a defined fixed surface resistance of 70 s m?1, and an albedo of 0.23.
CROP EVAPOTRANSPIRATION under standard conditions (ETc) refers to the evaporating demand from crops that are grown in large fields under optimum soil water, excellent management and environmental conditions, and achieve full production under the given climatic conditions.
(c) RATIONAL METHOD: ASSUMPTIONS & LIMITATIONS: 1) Rainfall is distributed uniformly over the entire catchment both spatially and temporally. 2) The catchment area must be very small. 3) Predominance of overland flow 4) The duration of the rainfall is equal to the catchment’s time of concentration. 5) The Return period of peak discharge is assumed to be equal to that of the rainfall event. 6) Rainfall intensity is constant throughout the rainfall duration. 7) The rational method does not account for storage in the catchment area. 8) The runoff frequency curve is parallel to the rainfall frequency curve. This is a consequence of constant C for all recurrence intervals.
(d) UH method Assumptions:
1) Rainfall is distributed uniformly over the entire catchment and at a uniform intensity. 2) The unit hydrograph is a linear response function of the watershed. It assumes that the time base of the hydrograph remains constant regardless of the amount of runoff resulting from different storms with the same duration. 3) the ordinates of all DRH\'s of a common base time are directly proportional to the total amount of direct runoff represented by each unit hydrograph. 4) The hydrograph of direct runoff resulting from a given pattern of effective rainfall will remain invariable irrespective of its time of occurrence. This assumption is called principle of time invariance.
(e) Tc is the time required for the water to travel from the hydraulically most remote portion of the watershed to its outlet or design point. It represents the maximum time of translation of the surface runoff of the catchment. The hydraulically most distant point is the point with the longest travel time to the watershed outlet, and not necessarily the point with the longest flow distance to the outlet. Time of concentration is generally applied only to surface runoff and may be computed using many different methods.
Time of Concentration is also explained as the time taken by the total catchment or drainage area to contribute runoff at the desired point on the stream. When total catchment area contributes at Tc, then maximum discharge is obtained which is also called as peak discharge.
Another definition of Tc is based on a rainfall hyetograph and the resulting runoff hydrograph. In hydrograph analysis, time of concentration is the time from the end of excess (effective) rainfall to the point on the falling limb of the direct runoff hydrograph (point of inflection) where the recession curve begins.
(g) The response of total drainage area to runoff for a given rainfall occurs only at the Time of concentration. This can be explained by Clarke\'s Time-Area method or by using isochrones in a catchment. If the design is considered for a time less than Tc, then intensity of rainfall will be more. This can be explained with an example:
Assume rainfall of 100mm for Tc of 1 hr. As per Rational method Q=KCIA, assuming C & A as constant, Q is directly proportional to I (=RF/Tc)
Then I = 100/1 = 100mm/hr.
If duration of rainfall (=50min) is assumed to be lesser than Tc (1 hr), then I=100/(50/60)= 120mm/hr. That means, for this intensity the Peak Discharge comes out to be more than what it is expected to come at Tc (Q1>Q2). That means for less Tc, we are over-estimating the discharge, which leads to higher sizes of openings of culverts, bridges, and drains. Due to these bigger sizes, cost of construction will be more, which is not entertained.
As the peak discharge when total catchment is contributing to runoff is lesser (and maximum) at ACTUAL Tc than at lesser Tc, there is no need to consider less Tc for designs of hydraulic structures.
