CE 434 – IRRIGATION ENGINEERING

T-Th (1:00-2:30 PM)

CHAPTER VII – RESERVOIR

STUDENT REPORTERS

Miral, Pearly Dawn T.

Oliveros, Mia Andrae B.

Pis-an, Shaness G.

ENGR. IRISMAY T. JUMAWAN, Ph. D.

Instructor

October 2024
TABLE OF CONTENTS
 I. INTRODUCTION

Reservoirs are an integral part of many water supply systems worldwide. A reservoir is a

natural or artificial place where water is collected and stored for various uses. It is an open-air

storage area, usually formed by masonry or earthwork where water is collected and kept in quantity

so that it may be drawn off for use. Reservoirs are typically constructed by damming a river to

form an artificial lake. They can also be formed from a natural lake whose outlet has been dammed

to control the water level. They may also be built by excavation or conventional construction

techniques such as brickwork or cast concrete.

People have been creating reservoirs for thousands of years as changes in weather cause

the natural flow of streams and rivers to vary greatly with time. Periods of excess flows

and valley flooding may alternate with low flows or droughts. During stormy times or when

mountain snow is melting, the water in a river rises and sometimes overflows its banks. By limiting

the amount of water allowed to continue downriver, reservoirs help control flooding. During

droughts or extended dry periods, the water level in a river may be extremely low. Under these

conditions, more water is released from the reservoir so farmers can water their crops, and homes

and businesses can function normally. The role of water-storage reservoirs, therefore, is to

impound water during higher flows, thus preventing flood disasters, and then permit gradual

release of water during lower flows.

Reservoirs also serve other purposes such as recreation, water supply, irrigation, flood

control, navigation, development of fish and wildlife, soil conservation, and hydroelectric power

generation. Overall, reservoirs are an important feature of many water supply systems around the

world.
 II. LEARNING OBJECTIVES

At the end of the discussion, students should be able to:

 Define reservoir, its types, and purposes;

 Identify the zones of the reservoir and its best site location;

 Know the criteria for site collection of storage reservoir;

 Differentiate reservoir yield from reservoir sedimentation

 Determine the reservoir sediment control measures

III. RESERVOIR

A reservoir is an artificial lake where water is stored. Most reservoirs are formed by

constructing dams across rivers. A reservoir can also be formed from a natural lake whose outlet

has been dammed to control the water level.

Source: https://www.nab.usace.army.mil/missions/dams-recreation/raystown-lake/raystown-lake-dam-risk-
communication/

Figure 1: Parts of A Reservoir

 Dams are designed to regulate water for many uses, including managing flood risks up and

downstream. Dams restrict water flow by holding it back or releasing it downstream. Some dams

typically hold water behind them in a reservoir, while others are dry until there is a lot of rain.

Whether the reservoir is typically dry or wet, it can only hold a finite amount of water.

When precipitation occurs upstream of the dam, water fills the reservoir and is slowly

released through the dam. Once the water reaches a certain threshold in the reservoir, a spillway

may be used to release more water. Some dams have an approved document or authorization that

requires water to be spilled through the dam once it reaches a certain level within the reservoir.

Depending on the dam, spillways can either operate on their own or require an operator to

decide when to use them. In either case, dam owners and operators typically know in advance if

the dam will have to spill water and can work with up and downstream communities and

emergency managers to give them advanced notice and support actions to protect people and

property.

Many dams serve multiple purposes. This often requires those who operate dams to

carefully monitor and manage the dam to try to provide all the intended benefits. For example,

water may need to be held back and released in increments to support water supply or power

generation, or water may need to be released to create space in advance of a large rain event.

IV. TYPES OF RESERVOIRS

Reservoirs can be classified into several types based on their purpose, construction, and

location. They may serve functions such as water storage, flood control, and hydropower

generation, with storage and multipurpose reservoirs being common.

 In terms of construction, reservoirs can be impounded by dams or designed as off-stream

storage systems. Their location also plays a role, as they can be built in valleys (valley-dammed)

or away from main watercourses (off-stream). Additionally, reservoirs vary in size from small-

scale systems for irrigation to large ones for regional water supply, with dams made from earth or

concrete depending on the terrain and budget. The three main types of reservoir are as follows:

A. Valley-Dammed Reservoir

Source: https://ohwhataknight.co.uk/elan-valley

Figure 2: Ellan Valley Dams

Valley-dammed reservoirs are formed in valleys in the middle of the mountains. There is

often an existing lake or water, and the sides of the mountain are used as reservoir walls to hold

water. A dam or artificial tank wall is built at the narrowest point to hold water.

Before constructing valley-dammed reservoirs, the water flow or river must be diverted.

The dam construction process involves laying the foundations for the dam, and then the concrete

cladding is laid and the construction of the dam can proceed. The construction of the dam usually

takes many years, but when it is finished, valley ponds and a large water source can be used to

serve the purpose of irrigation, hydropower generation, and domestic and industrial water supply.
B. Flood Control Reservoir

Source: https://www.shutterstock.com/search/flood-control-project

Figure 3: Flood Control Reservoir

This type of reservoir, also known as a flood mitigation reservoir, is built to store the

floodwater from a high-flow water stream to reduce flooding in protected areas or populated areas.

The entire stream entering the water storage is discharged until the outflow reaches the safe

capacity of the lower channel. The excess inflow is stored in the reservoir, and the stored water is

gradually resealed to create a storage capacity for the next flood. There are two types of flood

control reservoirs:

1. Storage Reservoir

Storage reservoirs are also called conservation reservoirs because they are used

to conserve water. Storage reservoirs are constructed to store the water in rainy seasons

and to release it later when the river flow is low. This type of reservoir has gates and

valves installed at its spillways and sluice outlets. It requires a manual operation to

open and close the gate, which gives complete control over the amount of water

discharged.
 Source: https://californiawaterblog.com/2011/09/13/water-storage-in-california-2/

Figure 4: California Water Storage

2. Retarding Reservoir
A reservoir that has ungated outlets and the flow is uncontrolled is known as

retarding reservoir or retarding basin. The retarding reservoir has some advantages over

the storage reservoir, such as no necessity of installing gates at sluiceways and spillway

crests. During maximum floods, the water present in the land is submerged temporarily

and driven out in a few days after the flood is controlled or minimized.

Source: https://www.ptvnews.ph/two-retarding-basins-to-ease-flood-woes-in-cavite1/

Figure 5: Imus, Cavite Retarding Basin

C. Distribution/Service Reservoir

Source: https://www.zenitaka.co.jp/english/works/achievements/21-03.html

Figure 6: Distribution Reservoir

A distribution reservoir is connected to the main water supply channels or pipelines. The

main purpose of this type of reservoir is to serve or supply water to consumers according to

changing demands or requirements of the local population. It also serves as local storage in the

event of an emergency. Here the water is stored in the reservoir by pumping at a specific rate, and

later, this stored water can be used or supplied at a rate higher than the inflow rate during high

demands.

The main advantage of this type of reservoir is it can store water during the demand period

and supply water during the demand period. Distribution reservoirs mainly depend on the

population’s demand for water at a particular period.

D. Multi-Purpose Reservoir
These reservoirs are built to store and supply the water to meet more than one purpose;

hence they are known as Multipurpose Reservoirs. For Example, multi-purpose reservoirs are

designed for irrigation, flood control, power generation, etc. In India, Bhakra Dam and Nagarjun

Sagar Dam are examples of important multi-purpose projects that serve more than two purposes.
 Source: https://www.thekikarlodge.com/bhakra-dam/

Figure 7: Bhakra Dam

V. WATER LEVELS AND STORAGE ZONES

Reservoirs are vital components of water management, playing a crucial role in the storage

and distribution of water for various purposes. These artificial lakes, created by damming rivers or

streams, come in different types based on their primary functions. The classification includes

potable water reservoirs, designed for supplying drinking water to communities, and agricultural

reservoirs, which serve the irrigation needs of farmlands. The storage zone of a reservoir is the

volume of water that can be stored in the reservoir.

A. Reservoir Water Levels

These reservoir water levels help regulate the storage, release, and usage of water based on

demand and environmental factors. The key water levels in a reservoir are:

 Full Reservoir Level (FRL)

The full reservoir level is the highest water level to which the water surface will

rise during normal operating conditions.

 Maximum Water Level (MWL)

The maximum water level is the maximum level to which the water surface will

rise when the design flood passes over the spillway. It is higher than the full reservoir level

so that some surcharge storage is available between the two levels to absorb flood.

 Minimum Pool Level

The minimum pool level is the lowest level up to which the water is withdrawn

from the reservoir under ordinary conditions.it generally corresponds to the elevation of

the lowest outlet (sluiceway) of the dam.

B. Reservoir Storage Zones

These storage zones are used to describe the different operational stages of a reservoir,

each serving a specific function in water management. These are vital in managing the reservoir's

storage, ensuring both water availability and safety in varying conditions.

 Dead Storage

The volume of water held below the minimum pool level is called dead storage.

It is provided to cater for the sediment deposition by the impounding sediment laid in
 the water. Normally it is equivalent to the volume of sediment expected to be deposited

in the reservoir during the design life reservoir.

 Live/Useful Storage

The volume of water stored between the full reservoir level (FRL) and the

minimum pool level is called useful storage. It assures the supply of water for a specific

period to meet the demand.

 Bank Storage

It is developed in the voids of soil cover in the reservoir area and becomes

available as seepage of water when water levels drop down. It increases the reservoir

capacity over and above that given by elevation storage curves.

 Valley Storage

The volume of water held by the natural river channel in its valley up to the top

of its banks before the construction of a reservoir is called valley storage. It is the

storage available in the stream channel even before the construction of the dam and

depends upon the cross-section of the river.

 Flood/Surcharge Storage

It is the storage contained between the maximum reservoir level and the full reservoir

level. It varies with the spillway capacity of the dam for a given design flood.
 Source: http://www.altunkaynak.net/public/upload/Yeni_Water_Resources_Lecture222_08102013_133728.pdf

Figure 8: Reservoir Water Levels and Storage Zones

VI. USES OF RESERVOIR

Reservoir serve a variety of purposes, including:

A. Water Supply

Reservoirs serves as vital water storage facilities, ensuring a steady supply for diverse

needs. From homes and offices to schools, factories and hospitals, reservoirs provide essential

liquid that sustains daily life. Designed to capture and store rainwater during wetter seasons, these

structures act as reservoirs of freshwater, safeguarding communities against water scarcity during

drier periods.
 Before being used, reservoir water undergoes a meticulous purification process. At water

treatment plants, chemicals are employed to eliminate harmful bacteria and naturally occurring

mineral particles. Additionally, filtration systems, consisting of sand and gravel beds, or chemical

additives, capture dirt and other small impurities, ensuring the water cleanliness. Once treated, the

purified water is transferred to service (or storage) reservoirs, ready for distribution to consumers.

Source: http://www.altunkaynak.net/public/upload/Yeni_Water_Resources_Lecture222_08102013_133728.pdf

Figure 9: Hoover Dam, Colorado River at Arizona-Nevada border, USA

B. Hydroelectric Power Generation

Reservoirs serves as crucial components in the generation of hydroelectric power. By

storing water at elevated levels, these structures create a potential energy source that can be

harnessed to produce electricity. The process involves the controlled release of water from the

reservoir to drive turbines, which, in turn, rotate generators.

 Hydroelectric power stations are typically located near reservoirs. In some cases, the

power station is situated directly adjacent to the dam, with pipes channeling water directly from

the reservoir to the turbines. In other instances, the power station is located at the lower elevation,

requiring water to be transported through long pipes or tunnels known as penstocks. These conduits

carry the water from the reservoir to the turbines, where its kinetic energy is converted into

electrical energy.

Source: https://www.energy.gov/eere/water/types-hydropower-plants

Figure 10: Hydroelectric Power Plant Diagram

C. Flood Control

Reservoirs play a pivotal role in flood risk management. By capturing excess water

during rainfall, they help regulate the flow of rivers and streams. This regulation serves several

crucial purposes:

 Reducing peak flows – Reservoir act as temporary storage for rainwater, absorbing a

portion of the runoff. This reduces the peak flow rate, preventing it from overwhelming

downstream areas and causing severe flooding.

 Delaying flood arrival – By holding back water, reservoirs can delay the flood arrival of

flood peaks. This gives communities and authorities more time to prepare and implement

mitigation measures, such as evacuations and flood defenses.

 Extending hydrograph base – The hydrograph is a graph that shows the flow rate of a river

over time. Reservoirs can help extend the base of the hydrograph by releasing water

gradually, reducing the overall impact of the flood event. This helps prevent rapid changes

in water levels and minimizes damage to infrastructure and property.

Source: https://www.watereducation.org/aquapedia/shasta-dam

Figure 11: Shasta Dam, California

D. Recreation

Many reservoirs often allow some recreational uses, such as fishing, boating and other

water-based activities. To ensure the safety of the public and protect the delicate ecosystem specific

rules and regulations are typically in place. These may include restrictions in boat size, speed limits

and fishing practices.

E. Irrigation

In regions with arid climates, irrigation plays a crucial role in sustaining agriculture. By

providing a reliable water source, irrigation systems can help overcome the challenges posed by

dry soil and promote growth of vegetation. Reservoirs serve as vital components of these systems,

storing water during rainy seasons and realizing it during drier periods. This stored water is then

distributed to agricultural lands through a network of canals, often flowing naturally under the

influence of gravity or being pump unto fields.

According to the World Commission on Dams, water stored due to dams will help

irrigate nearly 40% of agriculture around the world and help generate 19% of the world’s

electricity.
VII. RESERVOIR PLANNING AND INVESTIGATIONS

Reservoir planning and investigation is a critical process that involves a series of steps to

determine the feasibility, design, and construction of a reservoir. This process is essential for

ensuring that the reservoir meets its intended purposes, such as flood control, water supply,

hydropower generation, or recreation, while minimizing environmental impacts.

A. Reservoir Planning

Reservoir planning is essential for optimizing water source use, protecting against

natural disasters, promoting sustainable development and ensuring the well-being of communities

and the environment. It involves complex decision-making process to balance competing demands

and address evolving challenges, making it a critical component of water management and

infrastructure development.

 Site Selection

According to Alrammahi & Khassaf (2020), a good site for a reservoir should have the

following characteristics:

a. Large storage capacity – The topography of the site should be such that the reservoir

has a large capacity to store water.

b. Suitable site for the dam – A suitable site for the dam should exist on the

downstream of the proposed reservoir. There should be good foundation for the

dam. The reservoir basin should have narrow opening in the valley so that the length

of the dam is small. The cost of dam is often a controlling factor in the selection of

a site for the reservoir.

c. Watertightness of the reservoir – The geological conditions of the reservoir site

should not be such that the reservoir basin is watertight. The reservoir sites having

pervious rocks are not suitable. The reservoir basins having shales, slates, schists,

gneiss, granite are generally suitable.

d. Good hydrological conditions – The hydrological conditions of the river at the

reservoir site should be such that adequate runoff is available for storage, the

catchment area of the river should give high yield. There should not be heavy losses

in the catchment due to evaporation, transpiration and percolation.

e. Deep reservoir – The site should be such that a deep reservoir is formed after the

construction of the dam. A deep reservoir is preferred to a shallow reservoir because

in the former the evaporation losses are small, the cost of land acquisition is low

and the weed growth is less.

f. Small submerged area – The site should be such that the submerged area is a

minimum. It should not submerge costly land and property. It should not affect the

ecology of the region. Monuments of historical and architectural importance should

not be submerged.

g. Low silt inflow – The life of the reservoir is short of the river water at the site has

a large quantity of sediments. The reservoir site should be selected such that it

avoids or excludes the water from those tributaries which carry a high percentage

of silt.

h. No objectionable minerals – The soil and rock mass at the reservoir site should not

contain any objectionable soluble minerals which may contaminate the water. The

stored water should be suitable for the purpose for which the water is required.
 i. Low cost of real estate – The cost of real estate for the reservoir site, dam,

dwellings, roads, railways, etc. should be low.

According to the Nia General Guidelines and Criteria for Planning, Design,

Construction, Operation and Maintenance of Reservoir Dams, the selection of dam and reservoir

site and the type of dam to be adopted shall be governed by, but not limited to the following general

factors enumerated below:

1. Topography

2. Geology and Foundation Conditions

3. Availability of Materials

4. Vital Appurtenances (Spillways, Outlet Works, Tunnels, Galleries & Adits) Size and

Location

5. Climatic Condition, Legal, Esthetic, Social, Environmental, Economic and Cost

Considerations

The following Site Specific/ Special Conditions and Criteria in addition to the above

general factors (items 1-5) shall be considered for the selection and prioritization of Reservoir Area

Sites.

1. Shall not be covered nor within the Protected Area Management Bureau (PAMB)

jurisdiction of the DENR.

2. Shall not submerge or encroach permanent settlement site in large coverage

(Barangay/Municipality), important heritage sites, major infrastructure of

 provincial/regional/national impact, value, interest and holy grounds, burial sites,

settlement sites of indigenous people.

3. No known presence of existing subterranean channel, volcanic vents, sink holes,

concentrated leak (highly pervious strata/layer) of reservoir rim/banks and other

unfavorable geologic features.

4. The reservoir area shall not be identified/mapped traversed or crossed by an Active or

Potentially Active Seismic Fault Line/s.

5. The reservoir area and adjacent or surrounding vicinity of the site has no identified

potential peace and order issues or not susceptible to human induced (sabotage) hazard and

risk.

B. RESERVOIR INVESTIGATION

The following investigations are usually conducted for reservoir planning; engineering

surveys, geological investigations and hydrological investigations.

 Engineering Surveys

Engineering surveys are conducted for the dam, reservoir and other associated works.

Generally, the topographic survey of the area is carried out and the contour plan is prepared.

The horizontal control is usually provided by triangulation survey and vertical control is

precise leveling.

a. Dam site – For the area in the vicinity of the dam site, a very accurate triangulation

survey is conducted. A contour plan to a scale of 1/250 or 1/500 is usually prepared.

The contour interval is usually 1m or 2m. the contour plan should cover an area at
 least up to 200m upstream and 400m downstream and for adequate width beyond the

two abutments.

b. Reservoir – For the reservoir, the scale of the contour plan is usually 1/15,000 with

a contour interval of 2m to 3m, depending upon the size of the reservoir. The area-

elevation and storage-elevation curves are prepared for different elevations up to an

elevation of 3m to 5m higher than the anticipated maximum water level (MWL).

In addition, the National Irrigation Administration (NIA) mandates that maps and survey

data are essential for all stages of dam and reservoir projects. These materials are crucial for

planning, design, construction, and operation. NIA must collaborate with the National Mapping

and Resource Information Authority (NAMRIA) to obtain aerial and satellite imagery for

infrastructure projects. All maps and surveys used, especially during detailed design, must be

the most recent or in the process of being updated by a licensed geodetic engineer.

 Geological Investigations

Geological investigations for the dam and reservoir are done for the following purposes:

a. Suitability of foundation for the dam

The type and height of the dam mainly depend upon the type of foundation.

Subsurface explorations are carried out to determine the depth of overburden to be

removed for laying the foundation of the dam, the type of rock, the nature and extent

of the fault zones, if any, present in the rock.

 Depending upon the location of the bedrock, the following methods of

subsurface explorations are used:

 Excavation of open pits or trenches of suitable size

 Drifting (or tunneling) into the sides of the valley

 Driving vertical inspection shafts into the rock

 Core drilling to obtain the samples and to determine the configuration of the

strata

If the hard rock lies far below the surface, the dam site may be suitable for

a low gravity dam or an earth dam. The dam in that case may have to be founded on

soil foundations. Subsurface investigations are done to determine the type of soil, the

properties of soil, the soil profile and the location of the water table.

The information obtained from the geological investigations is used for determining

a suitable program of foundation treatment and grouting, if necessary.

b. Water tightness of the reservoir basin

The reservoir basin should be watertight so that the stored water is not

wasted due to seepage through bed and banks; otherwise, the very purpose of

constructing the reservoir would be defeated. Geological investigations are

conducted to detect the presence of cavernous rock formations, which have cavities

and are porous. The stored water may escape through such cavities into adjacent

valleys. If such formations exist in small areas, they may be treated and made

watertight. However, if they are widespread, the site may have to be abandoned.
 c. Location of quarry sites for the construction materials

Large quantities of construction materials such as stones, aggregates, sand,

soil, rockfill, etc. are required for the construction of a dam. Geological

investigations are conducted for location of suitable quarries for stones and burrow

areas for soils. The quantity and quality of the available construction materials are

ascertained.

 Hydrological Investigations

Hydrological investigations are conducted for the following purposes:

a. To study the runoff patter and yield

The most important aspect of reservoir planning is to estimate the quantity

of water likely to be available in the river from year to year and season to season.

For determination of the required storage capacity of a reservoir, the runoff pattern

of the river at the dam site is required. If the stream gauging has been done for a

number of years before the construction of the dam, the runoff pattern will be

available from the record. It is generally assumed that the runoff pattern will be

substantially the same in the future as well. The available record is used for

estimating the storage capacity. The inflow hydrographs of 2 or 3 consecutive dry

years when the discharge is low are frequently use for estimating the required

capacity. However, if the stream gauging records are not available, the runoff and

yield have to be estimated by other methods.

 b. To determine the flood discharge at the site

The spillway capacity of the dam is determined from the inflow hydrograph

for the worst flood when the discharge in the river is at maximum. Flood routing is

done to estimate the maximum outflow and the maximum water level reached

during the worst flood.

VIII. RESERVOIR MANAGEMENT CURVES

Reservoir management curves are graphical tools used to visualize and analyze the

relationship between reservoir storage, inflows and outflows over time. These curves are

essential for effective reservoir operations and decision-making. The following are

common reservoir management curves:

A. Ruling Curve

Ruling curve (or rule curve) is a frequently used management technique for reservoir

operation. On a specific date, a rule curve provides information about the storage or empty space

to be maintained in a reservoir.

The rule curve specifies the ideal storage or empty space to be maintained in a reservoir

during different times of the year. The implicit assumption is that a reservoir can satisfy its

purposes to the maximum possible extent if the storage levels or empty space specified by the rule

curve are maintained in the reservoir at different times. The rule curve, as such, does not give the

amount of water to be released from the reservoir. This amount will depend upon the actual inflows

to the reservoir. However, it provides guidance for daily operation of a reservoir.

Source: https://www.researchgate.net/figure/a-b-Location-of-the-Angat-basin-in-the-Philippines-
c-The-observed-reservoir_fig1_222797835

Figure 12: Angat Reservoir

 As shown in Figure 12, Panel (a) Location of Angat basin in the Philippines, Panel (b)

shows Angat basin and the upstream and downstream of Angat reservoir, Panel (c) The observed

reservoir storages during 1996-2001 in the Angat reservoir on Luzon Island. It shows the currently

adopted water supply rule curves (upper and lower) and flood rule curve (dotted line) along with

the recorded storages during 1996-2001.

B. Mass Inflow Curve

Mass inflow curve is a plot between the cumulative inflow in the reservoir with time.

As indicated below, a mass curve inflow can be prepared from the flow hydrograph of a

stream for a large number of consecutive previous years.

Source: https://www.engineeringenotes.com/water-engineering-2/storage-reservoir/how-to-
determine-capacity-of-a-storage-reservoir-water-engineering/44144#

Figure 13: Hydrograph

 In Figure 13, it shows a typical flow hydrograph of a stream for six consecutive years.

The area under the hydrograph from the starting year up to any time (𝑡1 ) [shown by hatching]

represents the total quantity of water that has flown through the stream from 1953 up to time (𝑡1 )

and hence it is equal to the ordinate of the mass curve at time (𝑡1 ).

Source: https://www.engineeringenotes.com/water-engineering-2/storage-reservoir/how-to-
determine-capacity-of-a-storage-reservoir-water-engineering/44144#

Figure 14: Mass Curve

 In Figure 14, it shows the ordinates of the mass curve corresponding to different times

are thus determined and plotted at the respective times to obtain the mass curve. A mass curve

continuously rises as it shows accumulated flows.

The slope of the curve at any point indicates the rate of inflow at that particular time. If

there is no flow during certain period, the curve will be horizontal. If there is high rate of flow, the

cure rises steeply. Thus, relatively dry periods are indicated as concave depression on the mass

curve.

C. Mass Demand Curve

Mass demand curve (or demand curve) is a plot between accumulated demand with time.

Source: https://www.engineeringenotes.com/water-engineering-2/storage-reservoir/how-to
determine-capacity-of-a-storage-reservoir-water-engineering/44144#

Figure 15: Demand Curve

 As shown in the Figure 15, (a) if the demand is at a uniform (or constant) rate then the

demand curve is a straight line having its slope equal to the demand rate. However, (b) if the

demand is not constant then the demand will be curved indicating a variable rate of demand.

IX. RESERVOIR YIELD AND CAPACITY

Reservoir yield and capacity are two crucial terms in the design and operation of

reservoirs. They play a significant role in determining the reservoir's ability to meet water demands

and manage flood risks.

A. Reservoir Yield

It is used to characterize the capacity of a water resource to serve as a long-term water

supply. It is a fundamental water-supply planning concept, and an understanding of its attributes

is critical for those who participate in water-supply issues. In the context of surface-water

resources, yield is often synonymous with safe yield or firm yield. Safe yield or firm yield in the

context of water reservoirs is defined as the maximum quantity of water which can be guaranteed

during a critical dry period (Linsley and Franzini, 1979).

It means the quantity of water which can be withdrawn from storage in the reservoir.

Reservoir yield is determined by the rate of flow of the stream into the reservoir, losses due to

evaporation from the reservoir surface, and the volume of water impounded in the reservoir. It is

the amount of water that can be supplied by the reservoir in a specified interval of time. The

specified time interval may vary from a day for a small distribution reservoir to a month or year

for large conservation reservoirs.

 If we say that three million cubic meters of water can be supplied from a reservoir in a year

then its yield is 3000000 m3 / year. The yield of the reservoir is dependent upon the inflow and

varies from time to time.

The simplicity of this definition, however, belies two "complicating" factors. First, yield

changes as watershed conditions, such as land use and ground-water-surface-water interactions,

evolve. Second, yield is uncertain because of our inability to know the severity and duration of

future drought periods.

Source: https://civil.colorado.edu/~balajir/CVEN5393/lectures/Lecture2-ReservoirAnalysis-
revised.pdf

Figure 16: Reservoir Yield

Figure 16 shows that the yield depends on the active storage capacity of the reservoir. It is

often expressed as the % of mean annual flow. Example, 70% yield means the reservoir can provide

a regulated release of 70% of the mean annual flow.

Reliability of Yield: probability that a reservoir will be able to meet the demand in any

particular time interval (usually a year)

Reliability = Ns/N

Where: Ns = is number of intervals in which demand was met;

N = is the total number of intervals

Firm Yield: can be meet 100% of time

1. Types of Reservoir Yield

a. Safe Yield

It is also known as “Firm Yield”. It is the maximum quantity of water that

can be supplied from the reservoir with full guarantee during the worst dry period.

b. Secondary Yield

It is the quantity of water which is available during the period of high flow

in the rivers when the yield is more than the safe yield. This yield is available during

a period of high inflows. In addition, secondary yield of the reservoir can be used

either to generate extra hydroelectric power or for irrigation of extra lands.

c. Average Yield

The average yield is the arithmetic average of the firm yield and the

secondary yield over a long period of time.

 d. Design Yield

The critical period for a reservoir is generally considered when natural flow

in the reservoir is minimum. Hence, a lower value than guaranteed yield or safe

yield may be taken for design purposes. This yield whose value is smaller that safe

yield is known as Design yield.

The design yield is the yield adopted in the design of a reservoir. The design

yield is usually fixed after considering the urgency of the water needs and the

amount of risk involved. In the case of reservoir used for irrigation purposes, the

yield may be taken slightly more than the safe yield as crops can tolerate some

deficiency of water during the exceptionally dry season.

2. Determination of Yield: Mass balance Equation of Reservoir

Reservoir Yields are often determined by operation studies. Operation studies

simulation of the physical system based on the principle of conservation of mass. A

generalized mass balance equation for a single-purpose water-supply reservoir is:

St-1 + Qt – Rt – Lt = St ------------------------------------------------------------- Eq. 1

Where:

St-1 = is active storage at end of previous time interval

St = active storage at end of current time interval

 Qt = is inflows at current time interval

Rt = release at current time interval

Lt = loss (evap/seepage) at current time interval

Reservoirs have a fixed storage capacity, K, so St <= K for each interval

B. Reservoir Capacity

Reservoir capacity is the maximum volume of water that the reservoir can hold at its full

pool level. It is typically expressed in units of volume (e.g., cubic meters).

1. Determination of Reservoir Storage Capacity

The storage capacity of a reservoir is conceptually divided into a number of zones

based on the useful purposes that a reservoir is required to serve. Fig. 2 gives a schematic

of various storage zones of a reservoir. Dead storage zone is the bottom-most zone of a

reservoir. Major storage space is occupied by the conservation zone. If the reservoir is

operated to control floods then the flood control storage is provided above the conservation

zone flowed by the surcharge storage.

 Source: https://www.youtube.com/watch?v=MTe7FFeRibQ

Figure 17: Schematic Diagram of a Reservoir Showing the various Storage Zones

A number of techniques are available for computing storage capacity for conservation

purposes like irrigation, municipal and industrial water supply, hydropower generation etc.

Depending upon the type of data and the computational technique used.

a. Mass Inflow Curve

This method is also known as the (Rippl) mass curve method after the developer of

this method. This is a simple method which is commonly used to estimate the required

storage capacity of a reservoir in project planning stage.

b. Sequent Peak Algorithm

Sequent Peak Algorithm overcomes some shortcomings associated with the mass

curve method. This method is particularly suited for the analysis of large data with the

help of a computer. It was proposed as a method which circumvents the need to choose

the correct starting storage which is required in the mass curve procedure. The

computations are quite simple and can be carried out as follows.

Let It be the inflow to the reservoir in the period t, Rt be the release from the

reservoir, and St the storage at the beginning of the period t. The reservoir is assumed

to be empty in the beginning. The mass curve of cumulative net flow volume (Inflow -

Outflow) against chronological time is used. This curve will have peaks (local

maximums) and troughs (local minimums). For any peak Pi the next following peak of

magnitude greater than Pi is called a sequent peak. The computations are performed for

twice the length of the inflow record by assuming that the inflows repeat after the end

of first cycle. This assumption is made to take care of the case when the critical period

falls at the end of the record.

The variable St is calculated by the following equation:

St = | St-1 + Rt – It ----------------------------------------------------------- Eq. 2

|0
Sample Problem:

A reservoir is to be constructed at a location where monthly flow data are available for 10

months. It is required to release 35 MCM of water from the reservoir every month. Find the

minimum size of the required reservoir by the Sequent Peak Algorithm.

Given: Required release Rt = 35 MCM

Solution: The computations are illustrated in the following table where the inflows are as

given in column 3.

Note: We can directly solve and input the data that we acquired based on the given solution since

Sequent peak Algorithm is a numerical method process.

All values are in Million Cubic Meter
(St-1 + Rt – It = St )
Period (t) Storage (St-1) Inflow (It) Release (Rt) Storage (St)
1 0 17.1 35 17.9
2 17.9 47.2 35 5.7
3 5.7 76.7 35 0.0
4 0.0 2.6 35 32.4
5 32.4 0.7 35 66.7
6 66.7 0.0 35 101.7
7 101.7 0.0 35 136.7
8 136.7 40.7 35 131
9 131 184.9 35 0.0
10 0.0 527.2 35 0.0
11 0.0 17.1 35 17.9
12 17.9 47.2 35 5.7
13 5.7 76.7 35 0.0
14 0.0 2.6 35 32.4
15 32.4 0.7 35 66.7
16 66.7 0.0 35 101.7
17 101.7 0.0 35 136.7
18 136.7 40.7 35 131
19 131 184.9 35 0.0
20 0.0 527.2 35 0.0
 Based on the result given, we’ve acquired graphical representation on the minimum storage
capacity required of the reservoir using the Sequent peak Method.

160
140
Storage (MCM)

120
100
80
60
40
20
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Time (Month)

Figure 11: Sequent peak Algorithm

The required storage is given by the maximum of the values in the last column which is

136.7 MCM. Here the calculations have been repeated for the second cycle of the inflows. Fig. 3

shows the graphical presentation of the method and the concept of sequent peak. In the sequent

peak algorithm, it is very easy to consider the variable release from the reservoir. The reliability of

the reservoir can be obtained indirectly. Since the reservoir would be able to meet the worst drought

from the record, the implied probability of failure would be 1/(N+1). Sequent peak algorithm is

very fast and easy to program. A single historical record is used to compute the storage and hence

the method is limited in that sense. It is also not possible to exactly consider the losses, these can

be approximately included in the releases.

 X. RESERVOIR SEDIMENTATION AND SEDIMENT MANAGEMENT

Within the catchment area, soil erosion and rock disintegration occur due to various factors.

This leads to the deposition of sand and silt in the reservoir, a process known as reservoir

sedimentation. To counteract the negative effects of sedimentation, sediment management

strategies are implemented to control and mitigate the accumulation of sediment in the reservoir.

A. Reservoir Sedimentation

Reservoir Sedimentation is a difficult problem for which an economical solution has not

yet been discovered, except by providing a “dead storage” to accommodate the deposits during the

life of the dam. The disintegration, erosion, transportation, and sedimentation, are the different

stages leading to silting of reservoir.

A river entering a water reservoir will lose its capacity to transport sediments. The sediment

will therefore deposit in the reservoir and decreases its volume.

The deposition of sediment in the reservoir is called Reservoir Sedimentation or Reservoir

Silting. Sedimentation is a complex hydro-morphological process which is difficult to predict.

It has been underestimated in the past and perceived as a minor problem which can be

controlled by sacrificing certain volume of the reservoir for accumulation of the sediment (dead

storage).
Source: https://www.linkedin.com/pulse/scientific-paper-sedimentation-challenges-reservoir-its-
sharma
Figure 18: Conceptual Sketch of Density Currents and Sediment Deposits in reservoir

B. Causes of Sedimentation

Generally, soil erosion is the major cause of reservoir sedimentation and subsequent

sedimentation of reservoirs is a complex process dependent upon a number of natural and

anthropogenic factors. The causes are classified into two with respect to the factors, namely:

1. Natural Causes

a. Geomorphology

In geological sense, geomorphology is the configuration of the land surface,

and it includes the location, size and shape of such physical features as hills, ridges,

valleys, streams and lakes. Topographic maps show these features.

b. Hydrology

Hydrology is the science relating to water of the earth, its distribution and

its phenomena. To be successful, a dam and reservoir project must have an adequate
 and continuous supply of water suitable for theory intended uses of the reservoir.

Hydrologic information and investigation will be required in varying degree,

depending upon the size of the project. The annual rainfall, the ratio of watershed

area to reservoir area, and the volume of stream of the year must be known.

c. Hydrogeology

Hydrogeology to determine whether groundwater would contribute to the reservoir

or whether the reservoir would lose water to the groundwater system is also

essential.

d. Geology

It has been said that construction of a dam and reservoir causes more interferences

with natural conditions than does any other civil engineering operation. Knowledge

of the geological situation is essential as a basis for sound engineering, especially

in the investigation of dam and reservoir sites, for an error in geological

interpretation or the failure to discover some relatively minor geologic detail may

be costly and sometimes hazardous.

e. Soil Characteristics

The type of soil and its properties such as porosity and permeability can cause or

lead to erosion within and around the reservoir.

 2. Anthropogenic Causes

a. Tillage Practices – Wrong tillage can cause loose soil thereby leading to washing

away of top soil.

Source:https://encryptedtbn0.gstatic.com/images?q=tbn:ANd9GcR6rJgK3WwKUjK9gMbzyQAt
AuaaYBC5_JKCLg&s

Figure 19: Tillage Practice

b. Overgrazing – Too much grazing of vegetation by animals can lead to exposure of

the soil in an area thereby causing erosion.

Source: https://i.ytimg.com/vi/Nwr_IAZHf1k/hq720.jpg

Figure 19: Overgrazing

 c. Mining and Logging – Mining activities can lead to erosion due to wearing off of

the surface through surveys and excavation as well. Logging is the cutting, on-site

processing, and loading of tree or logs on trucks. It is a process of cutting trees,

processing them, and moving them to a location for transport.

Source: https://rmn.ph/ilang-alkalde-nasasangkot-sa-illegal-mining-at-logging-ayon-sa-
dilg/illegal-mininhlogging/
Figure 20: Mining and Logging

C. Effects of Sedimentation

Loss of reservoir storage reduces flexibility in generation and affects the reliability of water

supply. Without storage, hydropower facilities are entirely dependent on seasonal flows. These

flows might not occur when energy is needed, eliminating one of the key benefits that hydropower

provides over other renewables. Sediments discharged from an upstream dam in a cascade system

can increase tail water levels, reducing power generation (Morris, 1998).
 This would impact the generation potential of all plants in the cascade and increase the

possibility of powerhouse flooding. Other effects due to Sedimentation of Reservoir are the

following:

i. Reduced storage capacity iii. Reduced availability of water irrigation

ii. Retrogressive Progression iv. Shortening the life of Reservoir

D. Sediment Management

Sedimentation of storage reservoirs is a natural process, since large part of the silt eroded

from the catchment and transported by the river, gets deposited on the bed of a reservoir. This

causes reduction in the life as well as dead storage capacities of the reservoir. Progressive loss of

capacity due to sediment accumulation results in reduced benefits and may even cause operational

problems. Below are some sediment management actions that we need to consider:

1. Catchment vegetation

A catchment is an area of land where water collects when it rains, often bounded

by hills. As the water flows over the landscape it finds its way into streams and down into

the soil, eventually feeding the river. Vegetation has the potential to decrease channel

erosion and sediment transfers in dryland environments by increasing channel bed

resistance and roughness. Plants soak up lots of water in their roots, preventing the soil

from becoming over-saturated. Their roots also hold the soil together, preventing it from
 washing away, and the foliage reduces the impact of raindrops and disperses the water

gently over a wider area.

Source: https://ponce.sdsu.edu/catchment_wetting_and_water_balance.html

Figure 21: Catchment Vegetation

2. Construction of Coffer dams

A cofferdam is a temporary barrier designed to keep water out of a work area, creating

dry environment for construction activities. Cofferdams, particularly the Aqua-

Barrier®, provide a less invasive alternative. By containing the work area, cofferdams

prevent sediment from spreading, thus preserving water quality and protecting aquatic

habitats.
 Source: https://eddypump.com/education/cofferdam-construction-using-dredge-pumps/

Figure 22: Construction of Coffer Dam

3. Flushing and Desilting of sediments

Reservoir sediment flushing, one of the most effective strategies for alleviating

reservoir sedimentation, involves discharging sediment-laden flows downstream through

bottom tunnels. Flushing and desilting is a common measure to manage and reduce the

amount of sediment stored in reservoirs. Basically, it is the process of removing sediment

from a water supply dam or reservoir.

Source:https://fullspeedchartering.com/pontoons/desilting-using-pontoons-in-rivers-lakes-and-
seas-by-city-government/
Figure 23: Flushing and Desilting of Sand
4. Low level outlets/Sediment Sluicing

The objective of sediment sluicing is to maintain the sediments in suspension when

they enter the reservoir and convey them out via sluicing gates. Reservoir sluicing necessities

a partial emptying of the reservoir prior to flood periods and the incoming of sediment-laden

flows.

Source:https://www.encyclopedieenvironnement.org/en/zoom/some-examples-of-sediment-

management/

Figure 24: Low Level Outlets/Sediment Sluicing

XI. POST – TEST

REFERENCES
McCartney, M. P., & Smakhtin, V. (2010). Water storage in reservoirs: Impacts and issues.
International Water Management Institute.

Smith, J. (2021, March 15). Reservoir water levels and types. Water Resources Insights.
https://www.waterresourcesinsights.com/reservoir-water-levels

Williams, D. A. (2015). Classification and management of reservoirs: Definition, types, and

operational zones. Journal of Hydrology, 43(2), 123-137.
https://doi.org/10.1016/j.jhydrol.2015.04.012

https://dailycivil.com/what-is-reservoir-purpose-and-types-of-reservoir/

https://www.nab.usace.army.mil/missions/dams-recreation/raystown-lake/raystown-lake-dam-
risk-communication/

https://study.com/academy/lesson/video/what-is-a-reservoir-definition-formation-
characteristics.html

https://www.thekikarlodge.com/bhakra-dam/
https://www.linkedin.com/pulse/scientific-paper-sedimentation-challenges-reservoir-its-sharma
https://www.youtube.com/watch?v=MTe7FFeRibQ
Alrammahi, F. S., & Khassaf, S. I. (2020). Reservoir Planning 1st part. ResearchGate.
https://doi.org/10.13140/RG.2.2.25110.98884
GENERAL GUIDELINES AND CRITERIA FOR PLANNING, DESIGN, CONSTRUCTION,
OPERATION AND MAINTENANCE OF RESERVOIR DAMS (1st ed.). (2019). NIA