Flood Related Studies


Flood : An Introduction

      The flow pattern in a stream constituting a distinct progressive rise culminating in a peak or summit together with the flow is considered an unusually high stage of the river. It is perhaps better described as that stage at which the stream channel gets filled and above which it overflows its banks. The valley then becomes flooded.

Importance of Flood Estimation

       An estimate of probable maximum flood and the corresponding stage are necessary for the design of irrigation, hydropower and flood control projects. Proper selection of the design flood is of utmost importance as it effects both the safety and cost of any structure. Too small a design flood for a major structure involves a high risk, not only of total failure of the structure and the services rendered by it, but also to the safety of the persons and the property located downstream. An excessive design flood, on the other hand, will result in an unnecessarily costly structure which may adversely affect the economic feasibility of the project.

Causes of Floods

All floods are primarily due to surface runoff. Floods may result from-

  • An intense rainfall
  • The melting of accumulated snow
  • Melting of snow combined with rain
  • Glacier obstructing the normal flow
  • Climate change
  • Land slide
  • Storm Surge
  • High Ground Water Level
  • Dam Failures
  • Embankment Failures
  • Floodplain Encroachment

       The Magnitude of flood depends upon the intensity of rainfall and its duration and ground conditions. Major river systems in India, and especially those in the north, rise in spate during the monsoons frequently inundating vast tracts of land causing damage to crops and properties and bringing in train considerable misery to the community.

Factors Affecting Flood Flow:


  • Variation in rainfall intensity
  • Duration of rainfall..
  • Cloud burst..
  • Storm distribution.
  • Direction of movement of storm

Heavy Rain and Snow-Melting

Characteristics of Catchment-

  • Area of catchment
  • Shape of catchment
  • Slope of catchment
  • Land condition
  • Bank storage

Other factors -

  • (a) Occurrence of very intense storm
  • (b) Centering of the storm over the catchment so as to produce flood runoff most       effectively
  • (c) Critical time sequence and movement of storm
  • (d) Occurrence of the storm at a time when the catchment is already more or less saturated      from the preceding rainfall

Classification of Floods:

  • Ordinary Flood
  • Annual Flood
  • Maximum Known Flood
  • Maximum observed Flood
  • Peak Flood
  • Design Flood
  • Flash Flood
  • Mean Annual Maximum Flood
  • Foundation Design Flood
  • Maximum Probable Flood
  • Standard Probable Flood

Flood Frequency-

       Flood frequency (F) denotes the possibility of flood being equaled or exceeded. A 10 per cent frequency means that the flood has 10 out of 100 chances of being equalled or exceeded.

  • Recurrence Interval: Recurrence interval (T) denotes the average number of years within which a given flood can be equalled or exceeded.

T = (1/F) X 100

Flood Management -

Structural Measures :

  • Embankments/Banks, Flood Walls, Flood Levees
  • Dams, Reservoirs and Storages other Water
  • Channel Improvement
  • Desilting/Dredging of Rivers
  • Drainage Improvement
  • Diversion of Flood Water
  • Catchment Area Treatment/Afforestation
  • Anti-erosion Works
  • Sea Walls/Coastal Protection Works
  • Alignment, Location, Design and provision of waterway i.e vents, culverts, bridges and causeways in national highways, state highways, district and other roads and railway embankments.

Non Structural Measures :

  • Inspection
  • Rehabilitation
  • Maintenance

Facilities of different studies related to Flood available at IRI, Roorkee

  • Estimation of design flood for Hydro electric multipurpose Projects, bridges, river, and flood protection works, etc.
  • Forecasting of HFL of flood protection projects, river, etc. by mathematical modeling and physical hydraulic modeling
  • Optimization and performance of flood protective measures
  • Optimization and hydraulic design of river training works ( such as guide bund, marginal guide bund, afflux bund, spur and stud, etc.)
  • Concurrent Evaluation and performance studies flood protective works, etc