Lesson 2 Reservoirs.pdf

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LESSON 2 – RESERVOIRS



Why Reservoirs?To stabilize the flow of water, either by regulating a varying supply

in a natural stream or by satisfying a varying demand by the ulti

mate consumers

LESSON 2 – Reservoirs

Storage, or conservation reservoir can retain excess

water from periods of high flow for use during periods

of drought Storage of floodwater may reduce flood damage below

the reservoir

Distribution reservoir to provide the varying rate of

demand for water during the day

Stock tanks or farm ponds may conserve the intermittent

flow from small creeks for useful purposes



Physical Characteristics of Reservoirs

Area – elevation curve

is constructed by

planimetering thearea enclosed within

each contour within

the reservoir site

STORAGE CAPACITY

Elevation – storage and elevation –area

curves for Cherokee Reservoir on the

Holston River, Tennessee













NORMAL POOL LEVELmaximum elevation to which the reservoir surface will r ise during

ordinary operating condi tions

determined by the elevation of the spillway crest or the top of the

spillway gates

MINIMUM POOL LEVELlowest elevation to which the pool is to be drawn under normal

conditionsmay be fixed by the elevation of the lowest outlet in the dam or,

in the case of hydroelectric reservoirs, by conditions of operating

the efficiency of the turbines





USEFUL STORAGEstorage volume between the minimum and normal pool levels

for multipurpose dams, these may be subdivided into conservation

storage and flood- mitigation storage

DEAD STORAGEwater held below the minimum pool level

SURCHARGE STORAGEdischarge over the spillway; normally uncontrolled; it exists only

while a flood is occurring and cannot be retained for later use





BANK STORAGEincreases the capacity of the reservoir above that indicated by the

elevation-storage curve

VALLEY STORAGEa variable volume occupied by water in natural stream channel








Reservoir Yield

is the amount of water that can be supplied from the reservoirduring a specified interval of time; it is dependent on inflow

and wil l vary from year to year

YIELD

SAFE or FIRM YIELD is the maximum quantity of water thatcan be guaranteed during a critical dry period

SECONDARY YIELD water available in excess of safe yieldduring periods of high flow




Selection of Distribution Reservoir

Capacity for a Given YieldExample 1. The water supply for acity is pumped from wells to a dis

tribution reservoir. The estimated

hourly water requirements for the

maximum day are as fo llows. If

the pumps are to operate at auniform rate, what distribution

reservoir capacity is required?

Hour

ending

Demand

(m³/h)

Pumpingrate

(m³/h)

Hour

ending

Demand

(m³/h)

Pumpingrate

(m³/h)

0100 273 529.3 1300 759 529.3

0200 206 529.3 1400 764 529.3

0300 256 529.3 1500 729 529.3

0400 237 529.3 1600 671 529.3

0500 257 529.3 1700 670 529.3

0600 312 529.3 1800 657 529.3

0700 438 529.3 1900 612 529.3

0800 627 529.3 2000 525 529.3

0900 817 529.3 2100 423 529.31000 875 529.3 2200 365 529.3

1100 820 529.3 2300 328 529.3

1200 773 529.3 2400 309 529.3




Hour endingDemand

(m³/h)

Pumping rate

(m³/h)

Required from

reservoir (m³)Hour ending

Demand

(m³/h)

Pumping rate

(m³/h)

Required from

reservoir (m³)

0100 273 529.3 1300 759 529.3

0200 206 529.3 1400 764 529.3

0300 256 529.3 1500 729 529.3

0400 237 529.3 1600 671 529.3

0500 257 529.3 1700 670 529.3

0600 312 529.3 1800 657 529.3

0700 438 529.3 1900 612 529.3

0800 627 529.3 2000 525 529.3

0900 817 529.3 2100 423 529.3

1000 875 529.3 2200 365 529.3

1100 820 529.3 2300 328 529.3

1200 773 529.3 2400 309 529.3

TOTAL TOTAL




GRAPHICAL SOLUTION

∫ −b

a

I O )(

where:

O = Outflow (demand)

I = Inflow pumping rate

RRC =




Selection of Capacity for a River

Reservoir

Sequent – Peak Algori thm - values of cumulative sum of inflow

minus withdrawals





Reservoir

Mass Curve (Rippl Diagram) –

a cumulative plotting of

net reservoir inflow





ReservoirExample 2. What reservoir capacityis required to assure a yield of

75,000 acre-ft/yr for the inflows

shown?





ReservoirExample 3. What yield will be availableif a reservoir of 30,000 acre-ft

capacity is provided at the site for which

the mass curve of Fig. 7.7 applies?




Reservoir Reliability

RELIABILITY of the reservoir is defined as the probabil ity that

will deliver the expected demand throughout its li fetime without

incurring a deficiency




LET LEARNING CONTINUE….

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Lesson 2 Reservoirs.pdf