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Hydraulic effectiveness of flood-control measures

The hydraulic effectiveness compares the situation with a flood control measure in place to the situation without such measure and, for a range of historical floods, calculates the extent to which the flooding would have been prevented by the flood control measure concerned. For that purpose one of the flood parameters discussed above has to be used which can be considered to characterise the flooding in the area concerned.

Design of flood-control measures

In the foregoing sections the analysis of floods and flooding was discussed. Such an analysis is made by collecting first of all data on topography, bathymetry, geology, hydrology, agronomy, agro-economy, regional and urban development plans, and last but not least, development plans for water resources of the river basin and its delta as a whole. Floods, flooding and damage caused by flooding are then studied through modelling or other means. As soon as the flooding phenomenon is thoroughly understood the designer is in a position to formulate a number of physical measures for arriving at a certain degree of flood control. Obviously, some of these measures would have been already provisionally formulated, at an early stage, in order to provide a reference point for the study of the various disciplines. However, it is now possible not only to formulate more definite flood control works, but also to evaluate these on their technical and economic merits. The five types of flood control measures as mentioned earlier will now be looked at in some detail.

Temporary storage of floods in reservoirs or in parts of the floodplain

As such, it does not matter whether the reservoirs are located in the upper part of the river (in the mountains), or in its middle or lower reaches. In practice, however, reservoirs located in the middle or lower reaches are not very effective, as the land is generally too flat and the reservoir must be very large to provide substantial storage. Obviously, a large reservoir reserved solely for the storage of flood waves and, in many cases, occupying fertile lands is not always looked upon favourably. In addition, as the lifetime of such a shallow reservoir is generally limited, because of rapid silting-up, this solution is mostly disregarded. This leaves the reservoir located in the mountains as a possible solution for flood control. This would be feasible provided the community, aiming at flood protection in the middle and lower reaches of a river, has the power to carry out works in the upper reaches. However, this is not always the case, as large rivers are, in many cases, also international rivers. Examples of countries facing this problem are: Bangladesh, which could hope to achieve a (limited) degree of flood control if large reservoirs were built in Nepal to check the floods of the rivers Ganges and Brahmaputra, and Argentina and Paraguay, who do not completely know what the effects of Brazil's works on the river Parana and its tributaries will be.
For the Rharb plain in Morocco , all possible reservoir sites, identified in the upper reaches of the Ouerrha-Sebou river system were scrutinised for flood control purposes. For each reservoir the size ( i.e. the height of the dam) was varied and its hydraulic effectiveness in preventing flooding in the Rharb plain calculated. In Table 1 (Source: Van Duivendijk, 1981-83) various particulars of this study are given. An interesting feature is the interdependence of the reservoirs as far as flood control is concerned. The hydraulic effectiveness of Rhafsaii (260) is 36 %, and of Bab Ouender (350) 43 %, but if operating together, they have an effectiveness of 55 to 60 % and not 79 % as would be expected. As already stated, the idea of utilising reservoirs for flood-control in the upper reaches, was disregarded in the case of the Parana/Paraguay studies. However, it is possible to create a very large reservoir at the site of a natural depression, the lbera marshes. Floods, coming from the Upper Parana and entering the Yacyreta reservoir, would be diverted to the lbera reservoir. The lbera reservoir in turn, would discharge through the river Mirinay into the river Uruguay and through the river Corrientes into the Middle Parana. Though, from an ecological point of view, the siting of a large reservoir in the lbera marshes now a days would probably not be accepted, it is, technically speaking, a sound solution

Improvement of river channels

The increase of conveyance capacity in a river or estuary to achieve a certain degree of flood control, is mentioned here for the sake of completeness. As previously mentioned, the enlargement of the wet profile of the river, widening of flood plains, increase of the slope of the river bed and decrease of the roughness on the flood plains, will individually and collectively contribute to an increase in conveyance capacity. Relatively cheap improvements like removing local bends, and clearing the flood plain of obstacles and bushes, will no doubt be worth considering when contemplating flood control measures. Other measures like lowering of the flood plain or deepening and/or widening of the river channel are either costly or have to be repeated periodically (dredging), and the result (degree of flood control) is often marginal. Moreover, in the tidal zone the increased conveyance capacity will increase the inflow of tidal waves and wind set-up caused by storm surges. Thus, its effect on flood control in the coastal zone would be negative!

Creation of additional flood ways

By creating flood ways an attempt is made to enlarge the discharge capacity of the conveyance system. There are situations where the flood water can be diverted through an old river course (bypass) to a point downstream where it re-enters the river, or to a second outlet into the sea. Such flood ways are also called flood diversion schemes.

In Argentina, it was proposed to divert the flood water to another river basin while in Morocco, the combination of a movable weir (already required for facilitating the intake of water for irrigation anyhow), and a flood way to a newly constructed outlet into the sea seemed to be attractive, as well as a diversion channel (bypass) on the left bank. Again, generally speaking, it can be said that flood ways are only economically justified when formed by natural processes. In this case extensive excavation works are not required and, in most cases, agriculture is not seriously affected by periodic flooding of the land in the flood way. Nowadays, it may, however, be difficult to introduce a flood way where none already exists.

Advanced agricultural production will not allow such flooding, or alternatively, the landowners will exaggerate the annual damage to crops. All the same, from a technical point of view flood ways are an efficient means of flood control.

Flood embankments along rivers and around flood prone areas

Flood embankments are also called levees, sea walls, sea defences or dikes. In this Paper the words embankment and levee will be used to illustrate the flood protection function along rivers. Only when the protection against wave attack and high tides is part of the function of embankments would the term sea defence be used. These sea defences will not be discussed in this Paper. Embankments along rivers or around specific flood prone areas like towns are part of what is called impoldering, and have been the standard solution for local protection against flooding through the centuries, in many river valleys and deltas throughout the world. There is nothing wrong with this solution provided that the river retains sufficient space (flood way, floodplain) for the discharge and storage of flood waves, the embankments are well maintained and flood levels are monitored. However, these points also indicate the weakness of the system. In flat low-lying areas, the river may require its storage and large floodplains at the time of floods (as is the case in Bangladesh), in which case only limited impoldering is possible. In addition, the continuous periodic inspection and maintenance of embankments, together with a foolproof flood warning system (see Section 4), require a mentality of both peasants and local authorities, which can only develop in time. Assuming the embankment is structurally sound (slopes not too steep, no seepage underneath, no danger of slips, no settlement, no lapse in maintenance), it is mainly the height of the crest which determines its risk of overtopping. One of the more difficult decisions the designer has to make when fixing the horizontal alignment of an embankment is the rate of meandering of the river. Obviously, the peasant would like to have the levees as near as possible to the river, while the authorities responsible for maintenance do not like the idea of frequent rebuilding of embankment sections. The designer also has to bear in mind the need for (a) floodplain(s) along the river channel to enable flood waves to pass safely. In Bangladesh, the 200km long embankment, along the right bank of the Brahmaputra river was constructed at a distance of 0.5 to 1.5 kilometres from the bank. However, this mighty (braided) river is continuously shifting to the left or right resulting in no less than 1 Okm of embankment being rebuilt in a period of 14 years. Clearly, the cost of such periodic reconstruction's has to be included in the economic calculations. It must be added that river training works (ie works preventing the river from meandering) can only seldom be economically justified for rural areas, so the meandering of the river and the realignment of the levees have to be accepted

The consequences of flood control

In the natural situation, ie without any flood control or flood protection whatsoever, the inhabitants of a certain flood prone area will have learnt "to live with the floods". This is for instance the case in Bangladesh where, as a minimum, each year 30 % of the country is flooded. as a consequence of:

  1. high water stages in the main rivers entering this deltarc country
  2. heavy local rainfall
  3. impeded drainage.

The situation can change dramatically when flood management is introduced. In the classic approach the degree of flood-control is expressed by the return period of flooding (N years). Flood-control schemes are designed in such manner that theoretically and statistically all flood waves ( or ,alternatively, all peak discharges lower than a given value) having a lower return period than the said N years do not result in flooding. Now it will depend on the local situation what value is given to N. For economic reasons overtopping of embankments, resulting in flooding is generally not allowed in urban areas, but is considered to be quite acceptable in rural areas in developing countries. Embankments surrounding urban areas are therefore designed for overtopping once every 1 000- 3 000 years (the Netherlands and Argentina), while the farmers, living in the rural areas, may expect to see the embankments overtopped once every 20 (Bangladesh) to 1 250 (the Netherlands) years. There are designers who maintain that a high frequency of flooding is to be preferred, as the alternative ( once in every 50-100 years) would create a false sense of safety, and consequently, when flooding occurs, it would incur a great loss of life, both human and animals.

In fact this was demonstrated recently (October 1999) in Nigeria when the authorities decided to open the spillways of two upstream reservoirs (Kainji and Shiroro). While in the original natural situation (ie prior to 1969) the river Niger flooded its banks every year and the peasants were used to this situation, during the past decades, because of the size of the reservoirs as well as their operation, aiming at producing hydropower throughout the year; flooding had been practically annulled. Accordingly, the peasants not being aware of the dangers started to live in the floodplains. It is not known whether or not warnings were given and if they actually reached the occupants of the flood plains in time. The newspapers reported that more than 1 000 persons lost their life. Apparently, the inhabitants of the flood plain of the river Niger had expected 100 % protection from floods and, as every hydrologist knows, this is impossible. It is also not realised that the said number of years N is a figure based on statistics, to which a probability is attached which by no means is 100 %. Moreover, the hydrological situation in the catchment may change in time for various reasons.

Failure of dams and flood embankments

Dams as well as flood embankments may fail and because of the use of the "protected" floodplains by mankind for housing and work such failures can result in huge losses in life and significant material damages. (For failure of dams, refer to !COLD Draft Bulletin)

Failure of Flood embankments

If one ignores breaching of sea defences due to storm surges (low lying countries around the North Sea) and the disastrous effect of cyclones on the countries around ·the Bay of Bengal, it can be stated that breaching of flood embankments (due to overtopping or other structural reasons) in general mainly results in material damage. Loss of life in most cases is very limited or does not occur because the inhabitants of the area behind the embankments live near the embankment under threat, the water level rises gradually and everybody is aware of what may happen. Still, material damage can be considerable because of economic development inside the protected area but will always be much less, or in any case be more bearable, than in the situation without flood control when, without economic development, the flooding is more frequent but the damages in each event are less. Here, the significance of the flood prone area for agriculture, human settlement and industry also starts to play a role. In other words: Is there an alternative for the fertile lands (agriculture), the ideal (flat) topography for human settlements, the transport opportunities ( road and rail along the river valley, river transport) in these flood prone areas? In most cases such an alternative does not exist and population pressure during the last decades has only encouraged human occupation of flood prone areas.

Return flows to rivers impeded by dikes

It has been suggested that the return flow of bank overspill towards the river could be hampered by dikes (ie flood embankments). This must for the greater part be based on a misunderstanding. First of all it is quite feasible in most circumstances to drain the flood water towards the river (after the river stage has gone down) provided there is an efficient drainage system (including drainage sluices through the flood embankment) in place. Secondly, one should realise that a sediment loaded river, at time of bank overspill, tends to deposit coarse sediment on the banks near the place of overspill. Consequently, prior to any embankments being built the river in the natural situation will have formed its own "embankments". It follows that flooding in alluvial plains built up by sediment deposited in the course of time by the river will always be deeper in locations remote from the river than on and near the banks (iii).Another reason for difficulties with drainage of flood waters towards the river can be the sediment transport of the river in the situation with flood embankments. A river, which has been embanked for a long time, and which transports sediment, will gradually heighten its bed. If also the embankments (as a consequence thereof) are gradually increased in height, the end result will be a river bed which is higher than the level of the flood plain. It may lead to channel shifting and it definitely hampers drainage after flooding. This situation can be found in China as well as in Italy (river Po)

Current approaches to planning for flood management

The extreme floods of the two last decades have led to a, what one could call, more sophisticated approach to planning for flood management. There are a number of reasons for this approach. First of all increase of population in flood prone areas together with a higher standard of living results in high material damages. These damages in turn have been aggravated by the complete lack of flood awareness of the general public. Here, the (false ) perception of safety created by the long period without any significant flooding becomes apparent. Secondly, the desire to introduce or restore, but in any case not to destroy, ecological values in flood management has completely changed the nature of the evaluation of flood control schemes. Thirdly, intensive land use and population pressure require careful masterplanning and urbanisation. Finally, the knowledge that extreme floods will cause flooding and that this is unavoidable have encouraged the introduction of a wide range of, what are called, " non-structural measures". This modern approach to flood management comprises the following principles:

  1. study the floods and flooding on a basin-wide scale, and, accordingly, further trans-basin(transnational) co-operation;
  2. rather than to work with design discharges or design river stages coupled to a return period try to reduce flood- related risks;
  3. risk assessment provides a tool for a better comprehension of these risks;
  4. as extreme flooding may only be influenced to a limited extent and no flood management will ever be able to pre-empt all future flooding, introduction of non-structural measures is vital to decrease damages in case flooding occurs;
  5. ecological values and improvement thereof shall be looked at when designating areas as flood plain while simultaneous integration of ecological values and flood control measures is recommended.
  6. To achieve the aforementioned goals it is necessary:
  7. to take actions on a local, regional, national and transboundary scale;
  8. to establish contributions from the side of water management, master planning and urban development, nature protection, agriculture and forestry, public and private enterprise.

Role of Dams in Flood-control

Flood-control as add-on

In the case of multi-purpose operation of reservoirs flood control can be one of the aims. This in fact implies that the operation· of the reservoir needs to serve various sectors like energy generation, agriculture, flood management. The easiest way would appear to reserve space to store water in the reservoir for each sector separately. Apart from the fact that this is costly, it is not always possible because of topographical constraints which limit the maximum size of the reservoir. It is in this context that integrated reservoir operation may prove its value. How such operation is carried out will depend strongly on the hydrology of the basin concerned, the characteristics of dam(s) and reservoir(s), and, last but not least, the requirements of the sectors using the water of the reservoir.

For the flood protection of the Rharb plain (Morocco) multi-purpose use of the M'Jara reservoir was studied in combination with a (limited ) increase in discharge capacity of the river Lower Sebou. This multi-purpose use of the reservoir was based on a system of pools and rule curves valid for the whole of the Sebou- lnaouene river system comprising a number of reservoirs and diversion tunnels in the basin (Sbihi & Suning, 1976). The rule curves were established by simulating historical time series for the period 1933- 1973. Much later, when the M' Jara dam (now called Al Wahda) came under construction the calculations were repeated in greater detail by simulating 43 historical floods (period 1933/34-1984/85) in a more sophisticated mathematical model. The results are presented in Table 3 ( Source: NEDECO, 1997). The multi purpose use of the reservoir would reduce the number of floods causing bank overspill from 43 to 4, while, depending on the increase in discharge capacity of the Lower Sebou, the hydraulic effectiveness would be 90% to 95%.

Also here, though storage and increase of conveyance capacity are combined and the reservoir is very large in relation to the inflow, a 100% effectiveness is not possible and, given the said flood control measures, flooding would have occurred four times in a period of 52 years.

Flood parameters and frequency curves

To compare various flood control measures with one another it is necessary to check their effectiveness on a common basis. In order to do this, one should first of all define flood parameters whose values can represent the magnitude of a flood, of flooding and of the damage caused by it. Secondly, frequency curves (or better: exceedance curves) must be drawn up for values of the selected flood parameter. Selection of a flood parameter implies that sufficient data are available about floods and flooding which occurred in the past. In fact the whole phenomenon of floods and flooding must be thoroughly understood before a flood parameter can be selected and exceedance curves drawn. A flood parameter should represent a flood in such a manner that the different values of the parameter valid for the different floods, reflect the magnitude of these floods in relation to each other. Flood parameters can be:

  1. maximum discharge during flood at a certain location;
  2. maximum water level during flood at a certain location;
  3. maximum extent of flooding in a certain area;
  4. duration of flooding in a certain area;
  5. bank overspill in a certain area;
  6. duration of the actual flood wave travelling down the river.

The results of hydrological and hydraulic studies should indicate which flood parameter best represents the flooding phenomenon. It is known from experience that the hydrological and topographical situation as well as the size of the area, determine to a major extent the flood parameter to be used. During studies in Morocco (NEDECO, 1973), it turned out that flooding of the Rharb plain by the Sebou and Ouerrha rivers was best represented by the overall bank overspill into the plain. In the case of the Parana-Paraguay studies in Argentina-Paraguay (Motor Columbus, 1979),which covered a much larger area, no less than seven locations were earmarked at which the maximum annual water level could have been considered as the representative flood parameter for a certain area (such an area was called a sub-area). Exceedance curves can be drawn for the values of various flood parameters attained during recorded historical floods. In addition, the relationship between various flood parameters can be established such as:

  1. total discharge over and above a certain flood level vs. the bank overspill;
  2. maximum discharge during a flood vs. the duration of flooding;
  3. maximum discharge during a flood vs. the bank overspill;
  4. maximum water level during a flood vs. the extent of flooding.

As will be demonstrated later on, exceedance curves for the selected flood parameter play an important role in the economic calculations. In Figure 1 the exceedance curves for the total volume and the overspill volume into the Rharb plain are presented. The data for the overspill curve were found by comparing the overspill volumes (calculated for a number of floods by means of a mathematical model), with the volume of the same floods above 2, 100 m3 Is as obtained from the hydrographs at the Sebou-Ouerrha confluence. The selected parameter for the Parana-Paraguay studies for each sub-area was the water level.

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