This document describes an experiment to optimize the required capacity of a re-regulating dam located downstream of a 2000 MW hydropower plant in Iran. The conventional method would require increasing the dam height by 6 meters at a high cost, but a new method takes advantage of variable power plant operations and water storage in the downstream river to need only a 1.5 meter increase. Modeling the river routing showed that variable outflows from the dam even out downstream. This optimized approach was successfully implemented, reducing construction costs while still providing sufficient flow regulation.

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Towards Optimization of the Required Capacity of Re-regulating Dams:
results of an actual experiment
Hooshang Hassani
Project Manager, Mahab Ghodss Consulting Engineering Co. –Iran
(Hooshang.Hassani@gmail.com )
Alireza Zia (Alireza.Zia@gmail.com)
Assistant Professor at Qom University of Technology - Iran
Nasser Kamjou
Head of Dam, Hydropower and Infrastructures Dept. of Mahab Ghodss Consulting Engineering Co.
(N.Kamjou@yahoo.com)
ABSTRACT
Hydropower plants generally run for peak period of power demands and stop operating during off-peak
hours. Re-regulating dams are usually constructed downstream of these plants to guarantee uniform
release during non-operating hours. The required capacity of these dams is calculated based on the
difference between power plant outflow and re-regulating dam release during plant operation. By
modifying this common procedure and applying it to Gotvand re-regulating dam, an optimization was
achieved. Gotvand re-regulating dam is located downstream of a 2000 MW hydropower plant in
southern Iran. This optimization, which was obtained by taking into account the storing capacity of the
river reach at downstream of the dam axis, led to considerable savings in project costs. In this method,
the geometry of downstream plays a critical role in determining the required capacity.
This paper describes this modified procedure and presents results of its application to a real case.
Keywords: re-regulating dam, reservoir capacity, routing, peak hours
1. INTRUDUCTION
In most cases,any change to an already constructed hydraulic structure such as heightening of a dam
might be more difficult than the construction of the same dam from the beginning. The reason is the
constraints resulting from the operation of the foregoing structure and the work conditions around that
hydraulic structure. Gotvand Re-regulating/ Diversion Dam is one of those previously constructed
hydraulic structures that in view of the change in its upstream conditions, i.e. the construction of the
large Storage Dam of Upper Gotvand with the power generation capacity of 2000 MW, is now in need
of heightening. Gotvand Re-regulating/ Diversion Dam has been operational for 40 years and its aim
has been to provide the communities living downstream of the River Karun with the agricultural,
drinking, and industrial water. So as to avoid any interruption in the operation of the aforementioned
re-regulating dam and to provide adequate volume forregulation, a newapproach wasusedto determine
the required capacity.
2. PROBLEM STATEMENT
The operation of the 2000 MW powerhouse of UpperGotvand Damwithin the peak powerconsumption
hours gives raise to considerable fluctuation in the river flow. The conventional solution for this
problem is to heighten the existing downstream re-regulating dam to obtain the volume required for re-
regulating the downstream flow completely. Based on the existing situation, an additional required
volume of 20 MCM, corresponding to about 6 meter increase in the height of the re-regulating dam is
needed. However, there are structural and operational limitations as follow:

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 The tailrace of the powerhouse of the main dam is located in the reservoir of the re-regulating
dam. Therefore any rise in the re-regulating dam height leads to a decrease in the generation of
power. As per the estimations made, 1 m of increase in the re-regulating dam height is equal to
30 GWh decrease in the annual generated power.
 From the stability point of view, the foundation of the existing re-regulating dam is able to bear
maximum of 2 m of dam heightening. So, any further increase in the dam height entails
reinforcing the dam foundation and proceeding with huge and costly structural activities.
 It is necessary to incessantly have the irrigation canals located in both sides of the regulation
dam operational. So, it makes problems for any structural activities on the dam body.
 In addition, with regard to the topographic conditions of the region downstream of the existing
diversion dam, the construction of a new re-regulating dam with enough capacity entails having
a long dam crest which entails a significant cost apart from having a vast area of farmlands
inundated.
Seeing the problem encountered in the conventional method, several assessments were made to find
better solutions. The most appropriate method was to make use of the present structure and efficiently
control/manage the reservoir to attain flow regulation.
Layout of Upper Gotvand Storage Dam and Gotvand Re-regulating/ Diversion Dam is shown in figure
1.
Figure 1-Layout of Gotvand Dam and river in downstream
3. OPTIMIZED SOLUTION
In order for having an efficient management, the parameters that were studied in more detail include
the real operation of the powerhouse, the current function of the existing reservoir, and the
topography of the River Karun downstream".

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3.1 The Real Operation of the Powerhouse
The investigations made in this regard indicated that the powerhouse outflow is affected by the
following factors:
 As prioritized, the total volume of the diurnal water release is determined by the downstream
agricultural demand.
 The operation hours of the powerhouse units are determined by the national dispatching center
in accordance with the demands of the national grid network.
 Normally, power units start/stop to operate in a given intervals, so it is not practical to start and
stop all units simultaneously.
 In some days, the national grid network demand reaches its maximum amount in two peak
hours of midday and evening.
 The national grid network demands in different days of the week are different. They reach their
minimum amount during the holidays.
 In order to increase the scope for the maneuver of hydroelectric power plants of the national
grid network, in some hours, one of the powerhouse units starts giving service to the network
with minimum power generation capacity.
 There are different peak hours in different seasons of the year.
A more precise study on the above data indicates that the real operation conditions are different from
the initial presumption pertaining to the 6 hours of uniform and non-stop operation. So as to obtain the
optimum volume of the re-regulating dam reservoir, it would be necessary to study different operation
scenarios for different seasons of the year. Figure 2 depicts a sample of different water release patterns
required for power dispatching.
Figure 2- Some of the patterns used as powerhouse daily power generation

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3.2 Reservoir Conditions of Existing Re-regulating Dam
Taking into consideration the problems articulated in Section 2, the computations showed that in case
of 1.5 m increase in the height gates of the existing-regulating dam, by adding a simple steel beam on
top of gates as shown in the figure 3, the corresponding reservoir volume would be equal to 8.0 MCM.
Figure 4 shows the Elevation –Volume Curve of the reservoir under new the condition.
Figure 3-Adding fix steel beam on top of gates
Figure 4-Elevation – Volume curve of the re-regulating reservoir

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So as to investigate the effects of the re-regulating dam reservoir on the powerhouse outflow, 100 cross
sections of the river were extracted from the area under consideration making use of the re-regulating
dam reservoir hydrographic results. Having entered the above sections data into HEC-RAS software,
the routing of the outflow of the Upper Gotvand Dam Powerhouse was computed for each power
generation patterns of the powerhouse starting in the powerhouse and ending in the re-regulating dam
reservoir.
The results are shown in Figure 5.
Figure 5– Result of intermediate model
The following results were obtained in accordance with the above computation:
 The powerhouse outflow goes 10 km inside the re-regulating dam reservoir with delay and a
portion of it is routed (the green graph of Curve No. 1)
 By foreseeing the total volume of the diurnal powerhouse outflow, it would be possible to
release water downstream in two steps in 24 hours (the blue graph of the Curve No. 1 depicts
the steps 250 and 600 m3
/ sec).
3.3 - Study of the River Downstream of Dam
At the second step of flow modeling, surveying of Karun cross sections was carried out up to 300 km
downstream of the re-regulating dam. River location and general layout is shown in figure 6.

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Figure6: Downstream river plan
Subsequently, a mathematical model of the River Karun was developed making use of HEC-RAS
software (750 sections were resorted to in creating the foregoing model). Besides, the boundary
conditions have also been applied for different water release scenarios. For implementing upstream
boundary condition of this model, the results presented in figure 5 are used. By running the model for
different operation conditions of the powerhouse,the velocity, watersurface level and discharge at each
point of the river were calculated. The results thus obtained are presented in Figures 7-9.

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Figure7: Flow in 30 km downstream
Figure No. 8: Flow conditions in 60 km downstream (middle of Shotayt River)
Figure 9: Flow in 120 km downstream (Ahwaz city)
Figures 7, 8, and 9 illustrate that the non-uniform discharge released from the re-regulating dam
gradually becomes uniform at downstream due to the temporary storage capacity of the river reach.
This makes it possible to divide the required capacity behind the re-regulating dam into two parts. The
first part is normally provided by the reservoir of the re-regulating dam and the second part is provided
by the river reach at downstream of the re-regulating dam. In case of no objections regarding the

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morphological, environmental and social conditions at the downstream reach of the re-regulating dam
for the non-uniform release of the water, the project would yield a considerable reduction in the re-
regulating dam costs.
4. CONCLUSION
The results of this experiment show that the conventional approach requires 20 million cubic meter
extra regulating capacity, which can be provided by heightening the existing dam by 6 meter. There
would be some time years interruption in the irrigation water supply canals, an annual 100 GWh
reduction of electrical energy due to the corresponding tail water rise of the power plant, and significant
construction costs.
Our new optimized approach, however, can fulfill the regulation requirement, employing only a simple
steel structure for 1.5 meter rise in water level which provides only 4 million cubic meter additional
volume, making use of routing capacity of the river downstream of the dam. There would be a limited
construction cost, negligible annual energy loss, and no interruption in water supply system.
It should be noted that the conventional approach described in this paper was originally used for detail
design of this project. However, in 2012 the subject was revisited, and based on the results of the
investigation presented here, the new optimized approach was adopted. The dam was therefore
modified accordingly and has successfully been operating since then.
In general, for all hydropower development projects with peak power generation, this method of
discharge regulation can be compared with that of the classical approach. Of course, specific
environmental and social aspects of each project must also be considered in order to achieve an
optimized solution.
5. REFFERNCES:
 Mahab Ghodss consulting Engineer, 1999, Gotvand 2000 MW Power Plant Design report.
 Mahab Ghodss consulting Engineer, 2007, Gotvand re-regulating dam Design report.
 Hydrologic Engineering Centers River Analysis System (HEC-RAS),
http://www.hec.usace.army.mil/software/hec-ras