this is my presentation of hydraulic and water resources engineering. I have discussed in this ppt about network density for given rain gauge and calculations and index of witness.
1. GUIDED BY:-
PROF . Ravi patel
PREPARED BY:-
Divyen devani 140050106018
Devanshi desai 140050106017
BITS EDU CAMPUS
2. Rainfall data is the most important and
fundamental data required for all hydrological
investigations.
They are required for,
Analysing storms
Fixing design flood
Forecasting flood in a river
Reservoir regulation
3. The rain gauges density or network density is
define as the ratio of total area of the catchment
to the total number of rain gauges in the
catchment.
To obtain reliable results , the various rain
gauges should be evenly and uniformly
distributed within a given catchment.
The total number of rain gauges installed
within a given catchment area should neither
be too many as to be costly nor should be to
less as to give unreliable results.
4. The world meterological organisation (WMO)
has laid down the following norms for
minimum network density.
Ragio
n
Description Network density
Minimum tolerable
1 Flat region of temperate,
mediterranean and tropical
zones.
1 gauge for 600 to
900𝑘𝑚2
1 gauge for 900 to
3000𝑘𝑚2
2 Mountainous area of
temperate, mediterranean
and trpical zones.
1 gauge for 100 to
250𝑘𝑚2
1 gauge fo 250 to
1000𝑘𝑚2
3 Arid and polar zones 1 gauge for 1500
to 10000𝑘𝑚2
-
5. IS recommendations on raingauge density
Is : 4987-1968 recommendends the following
raingauge densities
i. In plains – 1 station for every 520sq.km.
ii. In regions with average elevation 1000m – 1
station per 260-390 sq.km.
iii. In hilly areas with heavy rainfall – 1 station for
every 130 sq.km.
6. Optimum number of raingauges:
– The optimum number of raingauge stations that
should exist in order that the mean rainfall can be
estimated with an assigned percentage of error is
given by
Where,
N = optimum number of raingauges
𝐶𝑣= coefficient of variation of rainfall
E = allowable percentage error in estimate
of mean rainfall
𝐶𝑣 can be computed as follows:
7. 1. Calculate mean average annual rainfall
𝑃 =
𝑃
𝑛
where,
𝑃= total rainfall
= 𝑃1 + 𝑃2+….. 𝑃𝑛
n = number of raingauge
existing
2. calculate 𝑃 − 𝑃 2
8. 3. Calculate sample standard derivation
𝜎 =
𝑃− 𝑃 2
𝑛−1
4.Calculate the coefficient of variation
𝐶𝑣=
100×𝜎
𝑃
9. Index of Wetness
• Index of Wetness =
𝑎𝑐𝑡𝑢𝑎𝑙 𝑟𝑎𝑖𝑛𝑓𝑎𝑙𝑙 𝑖𝑛 𝑔𝑖𝑣𝑒𝑛 𝑦𝑒𝑎𝑟 𝑎𝑡 𝑎 𝑝𝑙𝑎𝑐𝑒
𝑛𝑜𝑟𝑚𝑎𝑙 𝑎𝑛𝑛𝑢𝑎𝑙 𝑟𝑎𝑖𝑛𝑓𝑎𝑙𝑙 𝑎𝑙𝑙 𝑎𝑡 𝑡ℎ𝑎𝑡 𝑝𝑙𝑎𝑐𝑒
It gives an idea of the wetness of that year and hence
is a measure of the deficiency of rainfall. A 60% index
of wetness means a deficiency of 40%.
• Deficiency ~ 30-45% – Large
• Deficiency ~ 45-60% – Serious
• Deficiency ~ >60% – Disastrous
Annual rainfall < Average Annual Rainfall – Bad
(Subnormal) Year
Annual rainfall ~ Average Annual Rainfall – Normal
Year
Annual rainfall > Average Annual Rainfall – Good
Year