Figure 13.2 is used for the simulation of flow operations of a reservoir.
Before running the program, V SPILL and N j are set to zero and the values of
V MAX , V 1 , Q OUT and V MIN must first be defined. Notwithstanding what the geologists, surveyors, engineers are proposing, the engineering hydrologist must address
the questions above.
Q OUT : The maximum flow Q OUT that can be obtained from the river through an
impounding reservoir is an average of all the flows constituting the record of N years
(N ¼ 10, 20, 30 or more). No reservoir can provide more than the average. This
already puts an upper limit to the maximum drawoff flow Q OUT , unless this is for a
short period only. However, Q OUT will, in practice, be decided by the water
infrastructure supplier.
V 1 : Before starting operation of the reservoir, it is judicious that the reservoir
contains sufficient water, particularly if a dry period is approaching, so that water
supply need not be stopped entirely or rationed. A minimum value to start for V 1 is
around 0.25V MAX , although a higher value (even V MAX ) is better.
V MIN : During operation over many years, sediments, etc. will tend to accumulate
at the bottom of the reservoir, thereby rendering it impossible to draw water beyond a
minimum volume, without conveying muddy water to the filters. A good precaution
is therefore to consider a minimum of 1% of V MAX as V MIN during this type of
simulation.
V MAX : The maximum capacity is guided by many factors (geological, hydrology,
etc). Despite this value being available, the higher the capacity of the reservoir, the
higher is its cost. At some time, the analyst will find an optimum reservoir size which
is probably much less than the real maximum capacity possible (dictated by parameters mentioned above).
What is significant however, if that apart from the actual river flow series Q IN
which are given and which cannot be modified (see climate change simulation),
later, however, the values of V MAX , V MIN , V 1 and Q OUT may be varied in a series of
“what-if” scenarios.
To come back to Fig. 13.2, following the flow process, the volume V 2 resulting
from the day’s (in theory) operation ((Q IN -Q OUT ). ΔT + V 1 ) is first tested against the
minimum volume, V MIN decided upon, whether V 2 is less than V MIN . (V 2 < V MIN ).
Case (1): If this is the case (V 2 < V MIN ), then water cannot be supplied. Thus, N J
(which was initially assigned a value of 0) will equal to: (0 + 1 ¼ 1). Thus, the total
volume of water in the reservoir is the sum of water already present and the inflow
volume, V 1 + Q IN ΔT ¼ V 2 . The next day, however, the same value of V 2 is assigned
to the variable V 1 , the volume already in the reservoir at the beginning of the day’s
operation.
Case (2): In the opposite case (V 2 ! V MIN ), water is successfully supplied.
However, if the total volume V 2 is higher than that of the capacity of the reservoir,
then the excess water, V SPILL must be discharged over the spillway (and is accumulated over an initial amount set to 0). The volume of water, V 1 , used for the next day
is then V MAX .
13.6 Simulation of Operating a Water Reservoir
377
Before running the program, V SPILL and N j are set to zero and the values of
V MAX , V 1 , Q OUT and V MIN must first be defined. Notwithstanding what the geologists, surveyors, engineers are proposing, the engineering hydrologist must address
the questions above.
Q OUT : The maximum flow Q OUT that can be obtained from the river through an
impounding reservoir is an average of all the flows constituting the record of N years
(N ¼ 10, 20, 30 or more). No reservoir can provide more than the average. This
already puts an upper limit to the maximum drawoff flow Q OUT , unless this is for a
short period only. However, Q OUT will, in practice, be decided by the water
infrastructure supplier.
V 1 : Before starting operation of the reservoir, it is judicious that the reservoir
contains sufficient water, particularly if a dry period is approaching, so that water
supply need not be stopped entirely or rationed. A minimum value to start for V 1 is
around 0.25V MAX , although a higher value (even V MAX ) is better.
V MIN : During operation over many years, sediments, etc. will tend to accumulate
at the bottom of the reservoir, thereby rendering it impossible to draw water beyond a
minimum volume, without conveying muddy water to the filters. A good precaution
is therefore to consider a minimum of 1% of V MAX as V MIN during this type of
simulation.
V MAX : The maximum capacity is guided by many factors (geological, hydrology,
etc). Despite this value being available, the higher the capacity of the reservoir, the
higher is its cost. At some time, the analyst will find an optimum reservoir size which
is probably much less than the real maximum capacity possible (dictated by parameters mentioned above).
What is significant however, if that apart from the actual river flow series Q IN
which are given and which cannot be modified (see climate change simulation),
later, however, the values of V MAX , V MIN , V 1 and Q OUT may be varied in a series of
“what-if” scenarios.
To come back to Fig. 13.2, following the flow process, the volume V 2 resulting
from the day’s (in theory) operation ((Q IN -Q OUT ). ΔT + V 1 ) is first tested against the
minimum volume, V MIN decided upon, whether V 2 is less than V MIN . (V 2 < V MIN ).
Case (1): If this is the case (V 2 < V MIN ), then water cannot be supplied. Thus, N J
(which was initially assigned a value of 0) will equal to: (0 + 1 ¼ 1). Thus, the total
volume of water in the reservoir is the sum of water already present and the inflow
volume, V 1 + Q IN ΔT ¼ V 2 . The next day, however, the same value of V 2 is assigned
to the variable V 1 , the volume already in the reservoir at the beginning of the day’s
operation.
Case (2): In the opposite case (V 2 ! V MIN ), water is successfully supplied.
However, if the total volume V 2 is higher than that of the capacity of the reservoir,
then the excess water, V SPILL must be discharged over the spillway (and is accumulated over an initial amount set to 0). The volume of water, V 1 , used for the next day
is then V MAX .
13.6 Simulation of Operating a Water Reservoir
377