FLOOD FORECASTING AND WARNING
8.9 FLOOD FORECASTING AND WARNING
The flood forecasts are issued on the basis of the analysis of weather charts and indicate the likelihood of heavy rainfall over the specified areas with the next 24 to 48 hours. All India forecasts are prepared every day at Poona.
Radar is very effective in the detection and tracking of severe storms. Meteorological satellites give an excellent idea of the cloud cover over the whole of India and neighbouring countries.
After the formulation of forecast, it must be dessiminated amongst persons concerned at the fastest speed. This can be done by utilising all available media of communications like telegraph, teleprinter, telephone and wireless and organising a hierarchy for onward trans- mission, if necessary.
One of the methods of forecasts in The Bureau of Meteorology, Australia comprises the following procedural steps:
(i) Collection of previous 24-hour rainfall in respect of each catchment (ii) Updating the Anticident Moisture Index (iii) Working out the average depth of rainfall (iv) Assessment of likely extent of the rain from weather charts
(v) Collection of river stages (vi) Issue of preliminary warnings with all factual data collected at every 6-hour interval;
the final advice is issued using the crest relationship.
A schematic diagram of the flood forecasting and warning system contemplated for the Tapti basin (central India) is given in Chart 8.1.
C-9\N-HYDRO\HYD8-3.PM5
Table 8.9 Economic analysis of flood control—by constructing levees (Example 8.8)
Annual project Ratio af (1000 cumec)
Flood peak
Total damage Increment of Recurrence
Increment
Annual benefit
Total annual
for the peak
damage
interval of in recurrence
from protec-
benefits from
cost for the benefit
(Rs. crores)
(Rs. crores) flood peak
interval
tion of incre-
protection for
flood peak to cost
(Yr)
(Yr)
mental damage
flood peak
(Rs. crores) (7) ÷ (8)
Rs. crores
(Rs. crores)
2.67 2.4 1.10 HYDROLOGY
FLOODS—ESTIMATION AND CONTROL
Stream gauging &
Raingauge stns.
Radio wind &
wireless stns. u/s
in catchment
radio sonde stns.
Talegaon Chalisgaon
Ukai Kakrapur
Burhanpur
Kathor
Bhusaval Nanded Sindkheda Taloda Ukai Kakrapur Kathor
Flood forecasting cell surat
Focal officer
D.S.P.
Collecter
Municipal Municipal authorities authorities
Wireless sets in vulnerable areas- 10 nos.
Chart 8.1 Flood forecasting system in Tapti basin (after B.S. Rao, 1969)
The flood warning system basically means the working out of the flood hydrograph at a given point sufficiently in advance, given the flood hydrograph of some gauge or gauges suffi- ciently upstream, taking into consideration.
(i) the effect of the incidence of rainfall forecast for or that has fallen in the intervening catchment and the consequent contribution of floods enroute, and (ii) the moderation affected by the valley storage from the upstream gauge to the point under consideration.
The flood warning system is prepared by (i) forecasting crest height and time. (ii) flood routing techniques. With a properly developed system of scientific flood forecasting and warning, human
toll and destruction to movable properties could be greatly mitigated.
Summary —The various steps involved in the design of a flood control project are:
(i) Determination of the project design flood and flood characteristics of the basin. (ii) Assessment of flood damages for different flood stages and socio-economic problems involved. (iii) Field survey of flood-prone areas and defining areas to be protected.
HYDROLOGY
(iv) Determination of the possible methods of flood protection. (v) If flood control is feasible by construction of reservoirs and levees, selection of suit-
able sites and their physical characteristics. (vi) Design details of the flood control structures proposed and preparation of cost esti-
mates. (vii) For a combination of flood control measures, selection of a flood peak that offers the
desired protection at minimum cost.
(viii) Making the cost-benefit analysis of the project and its economic justification.
(ix) Development of a scientific flood forecasting and warning system. (x) Preparation of a detailed project report, indicating the alternative flood control meas-
ures explored, the combination finally selected with the economic justification for the same, and the degree of flood protection offered.
(xi) Construction of the project proposed after approval and sanction of the budget from the authority concerned.
QUIZ VIII
I Match the items in ‘A’ with the items in ‘B’ (more than one item may fit in ‘B’)
(i) Floods
(a) South India
(ii) SPF
(b) Central India
(iii) Design storm
(c) Maharashtra State
(iv) Maximum flood discharge
(d) U.S.A.
(v) Ryves formula (e) Flood frequency as a factor in flood-estimation (vi) Dicken’s formula
(f) Flood control
(vii) Inglis formula (g) Construction of a cofferdam (viii) Fuller’s formula
(h) ≈ twice coefficient of variation (2C v ) (ix) Creager’s formula
(i) Increase HMD by deepening (x) Annual flood series
(j) Rational formula
(xi) Partial duration series
(k) Moisture adjustment
(xii) Coefficient of skew, C s
(l) Spillway design flood
(xiii) Retarding basins (m) High intensity of rainfall (xiv) Channel improvement
(n) 80% of MPF
II Say ‘True’ or ‘False’; if false, give the correct statement: (i) Floods are caused by succession of high intensity storms on a basin. (ii) The design flood is taken as the maximum probable flood that may occur on a basin. (iii) The maximum probable flood includes the extremely rare and catastrophic floods and is
usually considered for the spillway design of very high dams. (iv) The standard project flood is a fare estimate of the flood resulting from the most severe
combination of meteorological and hydrological conditions of the basin. (v) The PMP is derived from the most critical combination of storms and moisture adjustment which, after minimising losses, when applied on the design unit hydrograph for the basin,
will produce the MPF.
FLOODS—ESTIMATION AND CONTROL
(vi) The MPF for a basin is usually around 80% of the SPF. (vii) In flood control project, 100% flood protection is provided and hence there is absolutely no
flood risk. (viii) In a partial duration series, there may be 2–3 high floods of the same year and does not give
a true distribution series. (ix) Annual series are useful during the construction period of a large dam project, while the partial duration series are preferred for the spillway design flood. (x) The critical (design) intensity of rainfall occurs when the duration of storm is more than the concentration time. (xi) When enough storm data for a basin is not available, the PMP is estimated by storm transpo-
sition from a neighbouring catchment which is meteorologically homogeneous. (xii) The Gumbel distribution provides a satisfactory fit for partial duration foods or rainfall data. (xiii) While the annual floods are plotted against their probability of their occurrence in 100 years,
the partial duration floods are plotted against percentage of time. (xiv) In channel improvement (for flood control), widening is preferred to deepening. (xv) While Radar gives an excellent idea of cloud cover, the meteorological satellites are very
effective in the detection and tracking of severe storms. (xvi) A flood forecasting and warning system basically means working out of the flood hydrograph
at a given point sufficiently in advance, given the flood hydrographs at sufficiently upstream point/s.
(xvii) The design flood is usually selected after making a cost-benefit analysis. (xviii) Land management is ineffective in flood control. (xix) If a certain flood occurs once in 50 years, then its chance of occurring in any one year is 2%. (xx) Generally, the flood stage (flood peak) for which the ratio of annual benefits to cost is a
maximum is adopted for the design of flood protection works. (xxi) The 100-pr flood will be equalled or exceeded 10 times in a 1000 year period. (xxii) Protection against rare floods is uneconomical and hence there is always a certain amount of
flood risk involved. (xxiii) The first to suggest that frequency should be considered as a factor in estimating flood was: (a) Allen Hazen
(b) Gumbel
(c) Fuller
(d) Weibull
(false: ii, vi, vii, ix, x, xii, xiii, xiv, xv, xviii) III Classify the following into tangible (direct) and intangible (indirect) benefits due to flood control. (i) Prevention of damage to structures (ii) Loss of life and property (iii) Money saved under insurance and Workmen’s Compensation Laws (iv) Higher yields from intensive cultivation of protected lands (v) Elimination of losses arising from interruption of business (due to disruption of communica-
tion) (vi) Reduction in flood-prone epidemic diseases (vii) Damage to crops. IV Choose the correct statement/s in the following: 1 The methods of estimating high flood discharge are (i) by applying PMP on the design unit hydrograph for the basin.
HYDROLOGY (ii) By empirical formulae developed for the regions.
(iii) By applying rational formula. (iv) By flood frequency studies. (v) By stochastic methods. (vi) From the percent flood risk that can be allowed in the project. (vii) All the above methods. 2 The methods of mitigating floods are (i) by construction of reservoirs across tributaries (ii) by construction of a large reservoir across the main stream (iii) by construction of a retarding basin across a large river with a tributary joining the river
downstream (iv) by construction of levees, flood banks and dykes (v) by channel improvement (vi) by land management (vii) by temporary evacuation of low lying areas and flood warnings (viii) all the above methods. 3 The ‘design flood’ is taken as (i) flood peak derived by applying PMP on the design unit hydrograph (ii) 80% (approx.) of MPF (iii) SPF (iv) the flood of the recurrence interval corresponding to the percent risk that can be allowed in
the project (v) the flood stage for which the ratio of benefit to cost is maximum (vi) the 25-year flood for a land drainage project (vii) the 160-year flood for a spillway design or bridge (viii) all the above.
(1–except vi, 2–i, iv, v, vi, vii; 3 vii).