FLOOD FORECASTING
16.8 FLOOD FORECASTING
With the operation of flood forecasting centres in India since 1969, heavy loss of life and suffer- ing of people are greatly minimised due to advance warning. In the country, there are 8 fore- casting centres with 25 sub-centres and more than 200 observation sites have been equipped with wireless. In addition to this, rainfall data from 30 ordinary rain gauge stations and 50 self-recording rain gauge stations are also collected to supplement the gauge and discharge data.
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C-9\N-HYDRO\HYD16-2.PM5
MA
Table 16.2 Operation study for a storage reservoir (Example 16.3)
Pan Eva-
Demand
D /s Release
Change in
storage, ∆ s
storage,
capacity
DOWNLOADED FROM http://annacivil.tk
flow, Q
volume
P (mm)
E p (mm)
MODELS IN HYDROLOGY
(5)-(6)-(7)
V 15039
BLOG
V 13600
W ||
Σ Deficit = 13045 ha-m
1. †27 cumec × 30 days = 27(30 × 86400)/10 4 = 6998 ha-m 2. †56 cumec × 30 days = 6 (30 × 86400)/10 4 = 1555 ha-m 135 × 0.75 − 155 × 0.7
× 6000 – 650 – 1555 = +4758 ha-m
1000 4. Reservoir capacity = sum of negative quantities in col. (8).
HYDROLOGY
ANNACIVIL accurate weather forecast is available. In practice, short-term forecasts of weather elements
Factors governing forecasting can be divided into two groups—initial and final. The initial factors govern conditions existing at the time when the forecast is made and can be estimated on the basis of current hydrometeorological observations. The final factors include the future weather conditions and has to be taken into account in hydrological forecasts, if an
are being used in compilation of hydrological forecasts and warnings. The elements of forecasts include forecast of crest stages, discharge and time of occur-
rence, etc. In some cases, the other basic water regime elements to be known are: (i) volume of runoff in respect of various periods of time (ii) flow distribution (iii) MWL in reservoir and the data of occurrence The data required for making an accurate forecast are: (a) stage and discharge of upstream base station (b) stage and discharge of forecasting station (c) change in stage and discharge of these stations (d) stage and discharge of any tributary joining the main stream between the base sta-
tion and forecasting site (e) the intensity, duration and distribution of rainfall in the main, intercepted or sub-
catchment (f) topography, nature of vegetation, soil type, land use, population density, depth of
GWT etc., of the main or intercepted catchment (g) the atmospheric and climatic conditions. The factors (a) to (d) are the basic parameters used in developing correlation curves or
factors (e) and (f) are taken into account for introducing rainfall and antecedent precipitation BLOG
mathematical models; factor (d) can be neglected if its contribution is not appreciable, and
index as additional parameters; however, (g) stands as a future factor. The forecasing methods currently used in the country are: (a) based on laws governing the movement of water in the channel, i.e., using the hydro-
dynamic methods to determine the movement and transformation of flood waves (b) based on the analysis of hydrometerological data of the river basin, i.e., water balance
studies taking into account precipitation, the water equivalent of snow cover, soil moisture, ground water and other factors and estimating runoff, which require the use of a computer.
For small catchments, approximate calculations of flood movement and transformation can be made by:
(i) multiple correlation between stage and discharge observations (ii) streamflow routing on river reaches (iii) mathematical model Multiple correlation has the advantage of using parameters like rainfall or antecedent
precipitation index. Streamflow routing method includes the effect of channel storage on the shape and movement of flood wave; Muskingum method is generally used. For example, the routing equation developed between Sikanderpur and Rossera on Burhi Gandak (Bihar) is
MATHEMATICAL MODELS IN HYDROLOGY
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O 2 = 0.612I 1 + 0.033I 2 + 0.355O 1 ...(16.33) where
K = 36 hr, x = 0.3 and t = 24 hr