Design Flood Estimation - 1 | Engineering Hydrology - Civil Engineering (CE) PDF Download

Introduction 

A flood is commonly considered to be an unusually high stage of a river.  For a river in its natural state, occurrence of a flood usually fills up the stream up to its banks and often spills over to the adjoining flood plains.  For a hydraulic structure planned within the river (like a dam or a barrage) or on an adjoining area (like flood control embankments), due consideration should be given to the design of the structure so as to prevent it from collapsing and causing further damage by the force of water released from behind the structure.  Hence an estimate of extreme flood flow is required for the design of hydraulic structures, though the magnitude of such flood may be estimated in accordance with the importance of the structure.  For example, the design flood for a large dam like the Bhakra (Figure 1) or the Hirakud (Figure 2) would be estimated to be more than a medium sized dam like Chamera (Figure 3). It must be remembered that proper selection of design flood value is of great importance.  While a higher value would result in an increase in the cost of hydraulic structures, an under-estimated value is likely to place the structure and population involved at some risk.

Bhakra Dam

First prestigious multipurpose project of Punjab, constructed in independent India across river Sutlej, irrigates 13.35 lac ha. annually. Its hydropower generation (installed capacity) is 1204 MW. Height of the dam is 226 m.

FIGURE 1. Upstream view of Bhakra dam showing spillway gates and chute

Hirakud Dam

The project across river Mahanadi in the state of Orissa, completed in 1957, provides annual irrigation to 2.51 lac ha. Length of the dam (including Dykes) is 25.5 Km.

The project is located across river Ravi in the state of Himachal Pradesh. Its hydel power generation (installed capacity) is 540 MW.

Defining the design flood 

The Design Flood for a hydraulic structure may also be defined in a number of ways, like: 

• The maximum flood that any structure can safely pass.
• The flood considered for the design of a structure corresponding to a maximum tolerable risk.
• The flood which a project (involving a hydraulic structure) can sustain without any substantial damage, either to the objects which it protects or to its own structures.
• The largest flood that may be selected for design as safety evaluation of a structure. 

Design Flood is also known as the Inflow Design Flood (IDF).  It is the flood adopted for design purpose, and could be:

• The entire flood hydrograph, that is, the possible values of discharge as a function of time.
• The peak discharge of the flood hydrograph.

Choice of design flood

The Bureau of Indian standard guidelines IS: 5477 (Part IV) recommends that the Inflow Design Flood (IDF) of a structure, depending on its importance or risk involved, may be chosen from either one of the following:

• Probable Maximum Flood (PMF): This is the flood resulting from the most severe combination of critical meteorological and hydrological conditions that rare reasonably possible in the region.  The PMF is computed by using the Probable Maximum Storm (PMS) which is an estimate of the physical upper limit to storm rainfall over the catchment.  This is obtained from the studies of all the storms that have occurred over the region and maximizing them for the most critical atmospheric conditions.
• Standard Project Flood (SPF): This is the flood resulting from the most sever combination of meteorological and hydrological conditions considered reasonably characteristic of the region.  The SPF is computed from the Standard Project Storm (SPS) over the watershed considered and may be taken as the largest storm observed in the region of the watershed.  It is not maximized for the most critical atmospheric conditions but it may be transposed from an adjacent region to the watershed under consideration. 
• Flood of a specific return period: This flood is estimated by frequency analysis of the annual flood values of adequate length.  Sometimes when the flood data is inadequate, frequency analysis recorded storm data is made and the storm of a particular frequency applied to the unit hydrograph to derive the design flood.  This flood usually has a return period greater than the storm.

The IDF’s for different types of structures constructed across rivers are different.  Some of the structures which are of importance to water resources engineering are: 

• Storage D ams.
• Barrages and Weirs
• Diversion Works and Coffer dams
• Cross drainage works

A brief description of the structures and their corresponding IDF’s are discussed subsequently. 

Design flood for storage dams

Dams are important hydraulic structures which are constructed to serve a variety of purpose, more of which shall be discussed in detail in lesson 3.2.  Most dams have a capacity to store substantial amount of water in the reservoir, and a portion of the inflow flood gets stored and the excess overflows through the spillways.  According to Bureau of Indian Standard guidelines IS: 11223-1985, “Guidelines for fixing spillway capacity”, the IDF to be considered for different requirements

IDF for the safety of the dam It is the flood for which, when used with standard specifications, the performance of the dam should be safe against overtopping, structural failures, and the spillway and its energy dissipation arrangement, if provided for a lower flood, should function reasonable well.

• For large dams (defined as those with gross storage greater than 60 million m³or hydraulic head greater than 30 m), IDF should be based on PMF.
• For intermediate dams (gross storage between 10 and 60 million m³ or hydraulic head between 12 m and 30 m), IDF should be based on SPF.
• For small dams (gross storage between 0.5 to 10 million m³ or hydraulic head between 7.5 m to 12 m), IDF may be taken as 100 years return period flood. 

Floods of larger or smaller magnitude may be used if the hazard involved in the eventuality of a failure is particularly high or low.  The relevant parameters to be considered in judging the hazard in addition to the size would be:

• Distance to and location of the human habitations on the downstream after considering the likely future developments; and  
• Maximum hydraulic capacity of the downstream channel at a level which catastrophic damage is not expected.

IDF for efficient operation of energy dissipation system

It is a flood which may be lower than the IDF for the safety of the dam.  When this flood is used with standard specifications or other factors affecting the performance, the energy dissipation arrangements are expected to work most efficiently.

IDF for checking extent of upstream submergence 

This depends upon local conditions, type of property and effects of the submergence for very important structures upstream like power house, mines, etc.  Levels corresponding to SPF or PMF may be used to determine submergence effects. For other structures consideration of smaller design floods and corresponding levels attained may suffice.  In general, a 25 – year flood for land acquisition and 50 – year flood for built up property acquisition may be adopted.

IDF for checking extent of downstream damage in the valley

This depends on local conditions, the type of property and effects of its submergence.  For very important facilities like powerhouse, outflows corresponding to the inflow design flood for safety of the dam, with all spillway gates operative or of that order may be relevant.  Normally damage due to physical flooding may not be allowed under this condition, but disruption of operation may be allowed. 

Design flood for barrages and weirs 

Weirs and barrages, which are diversion structures, have usually small storage capacities, and the risk of loss of life and property would rarely be enhanced by failure of the structure.  Apart from damage/loss of structure the failure would cause disruption of irrigation and communications that are dependent on the barrage.  According to the bureau of Indian Standard guidelines IS: 6966(Part-I) -1989, “Hydraulic design of barrages and weirs-guidelines for alluvial reaches”, the following are recommended 

• SPF or 500 year return period flood for designing free board
• 50 year return period flood for designing of items other than free board.