Lecture 2 Storage Structures Reservoir

Storage structures: reservoir, dam….

CE405IU Hydraulic Structures

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Dr. Pham Ngoc

• A reservoir is a natural or artificial lake, storage pond, or impoundment from a Dam which is used to store water.

• Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.

• Types of a reservoir based on purposes: Irrigation, Flood control, Water supply, Hydropower, Navigation, Recreation, or multi-puposes

• Pertinent structures – dam, spillway, intake, outlet, powerhouse…..04/18/23 2

I. Reservoir engineering:

Son La Hydropower plantDam – downstream face

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http://www.vncold.vn/Web/Content.aspx?distid=128

Son La Hydropower plant: Reservoir

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Area of free water surface: 224 km2 Storage capacity: 9,26 Bil. m3 nước,Electricity: 10,2 tỉ kwh

Spillway Crest

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Phuoc Hoa Reservoir, Binh Phuoc province

Hoover Dam – Spillway

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Hoover dam – Outflow Channel

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Hoover Dam – Outlet Tunnel

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How many reservoirs are there in Vietnam?

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Based on  Directorate of Water Resources, Vietnam:

Totally, there are 6.648 reservoirs till 2012, in which:

-Storage capacity (SC) greater than 10 Mill. m3: 103 ones;

-SC in the range of 3-10 Mill. m3 : 152 ones;

-SC smaller than 3 Mill. m3: 6.393 ones.

Reasonable water resources development and utilization in DNRB:

S.

Beù

S.

Beù

S. L

a N

gaø

S. L

a N

gaø

S. Ñoàng Nai

S. Ñoàng Nai

S. Ñ

oàng

Nai

S.

Ñoà

ng N

ai

Trò AnTrò An

Daàu Daàu TieángTieáng

Thaùc MôThaùc Mô

ÑaÑaNhimNhim

Ñaïi Ñaïi NinhNinh

ÑN3ÑN3

ÑN4ÑN4

Caàn ÑônCaàn Ñôn

SRFu SRFu MieângMieâng

PhöôùcPhöôùc HoaøHoaø

Haøm Haøm ThuaänThuaän

ÑaÑa MiMi

Taø Taø PaoPao

Voõ ÑaêtVoõ Ñaêt

S. Saøi Goøn

S. Saøi Goøn

S. V

aøm C

oû Ñoâng

S. V

aøm C

oû Ñoâng

S. Vaøm Coû Taây

S. Vaøm Coû Taây

ÑN5ÑN5

ÑN6ÑN6

ÑN8ÑN8

ÑN2ÑN2

SRFu SRFu MieângMieâng

ÑN3ÑN3

ÑN8ÑN8

Existing

Under construction

Planned

Soâng Caùi Phan Rang

Soâng Luõy Bình Thuaän

Haï löu Ñoàng Nai

TP. Hoà Chí Minh

La Ngaø 3

Soâng Dinh/Ray BThuaän/ÑNai

ÑN6AÑN6A

Taø LaøiPhuù Taân 1 Phuù Taân 2

Thanh SônNgoïc Ñònh

Raïch Chanh

ProposedÑN8ÑN8

Procedure to design a reservoir

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A Reservoir

Field Investigation

Determine technical

parameters

Design optional structures

Analyze and select real

option

c

TopographyGeologyHydrology

c

Models, software,Other Cal.

Tech

cDrawings

cEconomic and environmental criteria and tools

Criteria for Site Selection of a Reservoir

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1) Large storage capacity2) Suitable site for the dam: downstream, good foundation, shorter

length3) Watertightness of the reservoir: pervious foundation are preferable4) Good hydrological conditions: adequate runoff available for storage; catchment area of the river should give high yield; Should not be heavy water losses (evapo-transpiration..)5) Deep reservoir: reduce water losses, cost of land acquisition, weed

growth6) Small submerged area: mitigate negative impacts (ecology, social

economic)7) Low silt inflow: extend life span of a reservoir8) No objectionable minerals: prevent water contaminated9) Low cost of real estate: dam, dwellings…

Basic (technical) terms and definitions of a Reservoir

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FRL, NPL

How to calculate Available Storage Capacity (ASC) of a reservoir?

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The ASC of a reservoir depends upon thetopography of the site and the height of dam. To determine the ASC of areservoir up to a certain level of water, a topographic survey of the reservoir areais usually conducted, and a contour map of the area is prepared.

scale of 1 cm = 100 m or 150 m with a contour interval of 1 to 3 m, dependingupon the size of the reservoir

Steps to calculate ASC of a reservoir:

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(1) Derive Elevation-Area curve from topographic map. (2) Derive Elevation-Capacity Curve:-Trapezoidal formula: ΔV1 = h(A1+A2)/2

V = Σ Vi; i = 1-n-Cone formula:

- Prismoidal formula:

Combined diagram Do an example?

Example 1

A reservoir has the following areas enclosed by contours at various elevation. Determine the capacity of the reservoir between elevation of 200.0 to 300.0 by various formula above.

Elevation 200.0 220.0 240.0 260.0 280.0 300.0

Area of contour

(km2)

150.0 175.0 210.0 270.0 320.0 400.0

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How to determine Required Capacity (RC)?

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RC depends on the inflow available and demand. - If available flow > demand: no storage required- If inflow in the river is small but the demand is high, a large reservoir capacity is required The RC for a reservoir can be determined by rge following methods:

(1) Graphical method, using mass curves.(2) Analytical method(3) Flow-duration curves method

(1) Graphical methods to determine the RC

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(a) Storage required for uniform demand. - used when the mass demand

curve is a straight line. - Procedures:1. Prepare a mass inflow curve

from the flow hydrograph of the site for a number of consecutive years including the most critical years (or the driest years)

2. Prepare the mass demand curve (MDC) corresponding to the given rate of demand.

3. Draw the lines AB, FG, etc..(parallel to the MDC and tangential to the crests of MIC)

4. Determine vertical intercepts CD, HJ…

5. Determine the largest vertical interception = RSC

The same scale

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(b) Storage required for Non-uniform demand.

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(c) Storage required when the demand is equal to the average discharge of the river.

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Determination of Yield of a Reservoir

Yield is the volume of water which can be withdrawn from a reservoir in a specified period of time. The time period for the estimation of yield is selected according to the size of the reservoir. It may be a day for a small reservoir and a month or a year for a large reservoir. The yield is usually expressed as Mha-m/year or Mm3/year for largeReservoirs

Safe yield (Firm yield) Safe yield is the maximum quantity of water which can be supplied from a reservoir in a specified period of time during a critical dry year. Generally, thelowest recorded natural flow of the river for a number of years is taken as the critical dry period for determining the safe yield.

The yield is equal to the slope of flattest tangent

Home work 1

The average annual discharge of a river for 11 years is as follows:

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Determine the storage capacity required to meet a demand of 2000 cumecs throughout the year.

Deadline: 15 Oct. 2014 by paper

2. Analytical Method for Determination of Storage Capacity

• The storage capacity should be adequate to supply the water equal to the demand during the critical period.

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• a long period, the total outflow volume from the reservoir must be equal to the total inflow volume minus the volume of water lost and wasted during the period.

• The capacity of the reservoir is determined from the net inflow and demand. The storage is required when the demand exceeds the net inflow.

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The procedure is used for the determination of storage capacity:1. Collect the stream flow data at the reservoir site during the critical dry period. Generally, the monthly inflow rates are required. However, for very large reservoirs, the annual inflow rates may be used.2. Ascertain the discharge to be released downstream to satisfy water rights or to honour the agreement between the states or the cities.3. Determine the direct precipitation volume falling on the reservoir during the month.4. Estimate the evaporation losses which would occur from the reservoir. ‘The pan evaporation data are normally used for the estimation of evaporation losses during the month.5. Ascertain the demand during various months.6. Determine the adjusted inflow during different months as follows:Adjusted inflow = Stream inflow + Precipitation - Evaporation - DownstreamDischarge7. Compute the storage capacity for each months.Storage required = Adjusted inflow - DemandThe storage would be required only in those months in which the demand is greater than the adjusted inflow.8. Determine the total storage capacity of the reservoir by adding the storages required found in Step 7.

Home work 2The monthly inflow and monthly pan-evaporation during a critical dry year at the

site of a proposed reservoir are given below.

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The net increase in pool area is 500 ha and the prior rights require the release of the full stream flow or 10 ha-m, whichever is less. Assume that 40% of the precipitation that has fallen on the submerged area reached the stream earlier and 60% of that directly falls on the reservoir. Determine the storage capacity. Take pan coefficient as 0.80.

Deadline: 15 Oct. 2014 by paper

Economic Height of Dam

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Theoretically, economic height of dam is the height of dam corresponding to which the cost of dam per million cubic metre of storage is the minimum.

Reservoir Losses

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(1) Evaporation Losses: They depend on reservoir area and are expressed in cm of water depth.The other factors influencing evaporation are temperature, wind velocity,

relative humidity, proximity of other structures etc. Evaporation losses can be measured on standard pans and after applying

suitable coefficients, reservoir evaporation losses can be evaluated.

(2) Absorption Losses:They depend on the type of soil forming the reservoir basin. They may be quite

large in the beginning, but gradually reduce as the pores get saturated.

(3) Percolation or Seepage Losses:They are usually small but may be quite significant where there may be

continuous seam of porous strata or cavernous or fissured rock.

Sedimentation in Reservoirs

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Mechanism of SedimentationIn many respects deposits in a reservoir resemble those in a delta, made by a stream where it discharges into a lake or sea. These deposits are: (i) bottom set beds, consisting of the fine sediments brought in by the stream; (ii) the fore set beds formed of the coarser sandy sediments; (iii) top set beds consisting of coarser particles; and (iv) density current deposits

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Factors Affecting Sedimentation( i ) Extent of catchment area and the Unable nature of its different zones.( ii ) Amount of sediment load in the rivers.(iii) Type of rainfall and snowfall in each zone.(iv ) Mean monthly and annual temperature in each zone.( v ) Monthly and annual run-off from catchment or sub-catchment.(vi) Slope of each zone of catchment.(vii) Vegetation in each zone of catchment.(viii) Geological formations of each zone and estimated relative weathering and erosion with due regard to climatic conditions.(ix) Presence of upstream reservoir and extent of trapping of sediment therein.(x) Amount of sediment flushed out through sluices.(xi) Degree of consolidation of the accumulated sediment depending upon the extent of exposure to air, sun and wind.(xii) Volume of water in the reservoir and its proportion to the mean annual flow in the river i.e. capacity inflow ratio.(xiii) Operation schedule of the reservoir.

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Control of Silting of Reservoirs1. Adequate consideration of the silt factor for the selecting of site and design of the dam.(a) Selection of reservoir site—If reservoirs of equal size could be constructed in eitherof the two watersheds at approximately the same cost, the watershed having less erosion should be preferred.

(b) Ratio of reservoir capacity and size of drainage area. It also affects the annual rate of storage depletion. According to Carl. B. Brown, of U.S. Deptt, of Agriculture, a reservoir should hold at least 3.6 hectare metre of water per square km of drainage area in order to have a safe life of 100 years.

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Control of Silting of Reservoirs (con’t)2. A plan of water release designed to eject the maximum possible sediment load before deposition.a) Reservoir design —A lower darn in the first instance with provision in its structural design for its being raised in stages, as its capacity is encroached, would be a better proposition.(b) In some of the reservoirs, sluices may be provided to take advantage of formation of density currents and thus eject a significant share of Sediment load.(c) The reservoir may be filled, only after passing the peak flood-design of Aswan clamin Egypt provided for this.

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Control of Silting of Reservoirs (con’t)

3. Control of soil erosion and sediment movement in the catchments area.(a) Control of sediment inflow —Small check dams may be constructed on all tributaries of the main river. Vegetation screen on the catchment would go a longway in reducing erosion.

(b) Control of sediment deposit—The outlets may be opened at the time when there is maximum inflow of sediment in the reservoir i.e. during monsoon periods; also ejection of reservoir water at lower levels would help in reducing silt in the basin.

(c) Removal of sediment deposit— Scouring, excavation, dredging etc. may beresorted to. But these methods are expensive. Loosening the sediment and orpushing it towards the sluices by mechanical means simultaneously with scouring would increase the effectiveness of the scouring action to some extent.

(d) Erosion control in the catchments area Soil conservation methods, likeafforestation, control of grazing, terrace cultivation, provision of contour bunds,gully formation by providing small embankments, where necessary, debris barriers, weed growth etc. all help to control soil erosion and thus reduce sediment entry in the reservoir.

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Life of a Reservoir

The dead storage provided in reservoir capacity is allowed for sedimentation. Actually all the sediment load does not go in dead storage. It encroaches upon live storage also. The encroachment and its distribution depend upon many factors such as reservoir operation, valley characteristics, capacity inflow ratio, sediment content in the inflow etc. The useful life of a reservoir is taken till its capacity is reduced to about 20% of’ the designed capacity.The rate of sedimentation is higher in the initial stages and it decreases with years. This is due to fall in the trap efficiency of the reservoir, consolidation and shrinkage of deposits and formation of delta.