Types of Reservoirs
a. Storage/Conservation Reservoir
b. Multi-purpose Reservoir
c. Distribution Reservoir
d. Flood Control/Mitigation Reservoir
A storage/conservation reservoir can retain
such excess supplies during periods of peak
flows and can release them gradually during
low flows as and when the need arises.
A multi-purpose reservoir is planned and
constructed to serve not only one purpose
but various purpose together.
Managed to balance:
- Water supply - Navigation
- Flood control - Recreation
- Soil Erosion - Irrigation
- Environmental management
- Hydroelectric power generation
A distribution reservoir connected with the
conduits of a primary water supply; used to
supply water to consumers according to
fluctuations in demand over short time
periods and serves for local storage in case of
Flood Control/Mitigation Reservoir
A flood control reservoir stores a portions of
the flood flows in such a way as to minimize
the flood peaks at the areas to be protected
Types of Flood Mitigation Reservoirs
a. Storage Reservoir
b. Retarding Basins
The discharge from a storage reservoir is
regulated by gates and valves operated on the
basis of the judgment of the project engineer.
It differs from conservation reservoirs as they
need large sluiceway capacity in order to
allow rapid drawdown before and after a
A detention basin or retarding basin is an
excavated area installed on, or adjacent to,
tributaries of rivers, streams, lakes or bays to
protect against flooding and, in some cases,
downstream erosion by storing water for a
limited period of time.
Prepared by: Ivy Diane L. Ramos
Purpose of flood-mitigation reservoirs and
locations of reservoirs
In environmental engineering, flood mitigation involves the management and
control of flood water movement, such as redirecting flood run-off through
the use of floodwalls and flood gates, rather than trying to prevent floods
altogether. It also involves the management of people, through measures
such as evacuation and dry/wet proofing properties.
Purpose of Flood-mitigation reservoir
This is accomplished by discharging all reservoir inflow until the outflow
reaches the safe capacity of the channel downstream. All flow above this rate
is stored until inflow drops below the safe channel capacity, and the stored
water is released to recover storage capacity for the next flood
The reservoir must be operated so as to produce a minimum water level at
the protected area, rather than a minimum at the dam
The prevention and mitigation of flooding can be studied
on three levels:
whole towns or cities.
Property owners may fit their home to stop water entering by blocking doors
and air vents, waterproofing important areas and sandbagging the edges of
Protection of individual properties
When more homes, shops and infrastructure are threatened by the effects of flooding, then the
benefits of greater protection is worth the additional cost. Temporary flood defenses can be
constructed relatively quickly in certain locations and provide protection from rising flood waters.
Rivers running through large urban developments are often controlled and channeled. Water rising
above a canal's full capacity may cause flooding to spread to other waterways and areas of the
community, which causes damage. Defenses (both long-term and short-term) can be constructed to
minimize damage, which involves raising the edge of the water with levees, embankments or walls.
The high population and value of infrastructure at risk often justifies the high cost of mitigation in
larger urban areas.
Protection of communities
The most effective way of reducing the risk to people and property is
through the production of flood risk maps. Most countries have produced
maps which show areas prone to flooding based on flood data.
Flood risk management
Loss of reservoir water
Factors Affecting Location of Reservoirs
Topography a narrow site will minimize the amount of material in the dam
thus reducing its cost, but such site may adaptable
Geology the foundation of the dam should be relatively free of major faults
and shears. If these are present they require expensive foundation treatment.
Local Condition site availability of water supply, sewerage disposal, electric
power for construction purposes, telephone service should be chosen.
Rim stability and water-holding capability are interrelated. Rim failure can be
caused due to either the sliding or the erosion of a segment of the reservoir
rim. Seepage of water is mainly responsible for such failures.
Loss of reservoir water to the atmosphere occurs due to direct evaporation
from the reservoir surface. The evaporation losses are affected by the climate
of the region, shape of the reservoir, wind conditions, humidity, and
temperature. From considerations of evaporation, a reservoir site having a
small surface area to volume ratio will be better than a saucer-shaped
reservoir of equal capacity.
Bank Storage is the water which spreads out from the body of water, filling
interstices of the surrounding earth and rock mass.
The increased seismic activity is attributed to the
changes in the normal effective stresses in the
underlying rock because of the increased pore
pressure. The transmission of the hydrostatic
pressure through discontinuities in the underlying
rock can have a triggering effect where a critical state
of stress already exists.
The most effective flood mitigation is obtained from an adequate reservoir located
immediately upstream from the point (or reach) to be protected. A single reservoir may
not be able to protect a number citied at a different distances downstream.
Economical analysis and other factors often favor the upstream site despite its lesser
No general rules can be set forth because each problem is unique, and several alternatives
must be evaluated.
The use of several small reservoir offers the possibility of developing initially only those
units of the system that yield the highest economic return and constructing the additional
units as the development of the area increases the potential benefits.
The life of a reservoir is predicted on the basis of the amount of sediment
delivered to it, the reservoir size, and its ability to retain the sediment.
Sediment deposition at the initial stage may be beneficial in the sense that it
may have the effect of a natural blanket resulting in reduced seepage loss
Stabilization of the unstable mass can also be achieved by strengthening or
replacing weak material. Grouting is the most common remedy for
strengthening such weak masses. It may be desirable to plan the steps to be
taken to mitigate the effects of potential slide after it has occurred in spite of
all preventive steps.
Flood Damage mitigation
Size of Reservoirs
Edwardo R. Batas
Are constructed in two main function:
1. First is to store water in the lake behind
the dam to even out the fluctuations in river
flow and match the availability in demand.
2. Second is to create a hydraulic head of
water in the reservoir upstream of the dam so
that water can be diverted into a canal and flow
due to gravity.
1. Using a reservoir, the natural streamflow
can be regulated so that the outflow
follows the desired pattern.
2.To control flood.
3. To store water enough to sustain the
demands on water during dry season.
4. Most probably as a source of energy
such as Hydroelectric power.
Why do we need reservoirs?
1. Classification Based on Purpose
2. Classification Based on Size
Classification of Reservoirs
3. Classification Based on Storage
- Seasonal storage reservoir
- One-year reservoir
1. The potential reduction in peak flow by
reservoir operation increases as reservoir
capacity increases, since a greater portion
of flood water can be stored.
2. For this reason a second criterion for
evaluation of a flood-mitigation reservoir
is its storage capacity.
Size of Reservoirs
3. It must no be presumed that the basic
rule of design is “the bigger the
better”, for the economic factors
control the decision.
4. The maximum capacity required is the
difference in volume between the safe
release from the reservoir and the
design and the design flood inflow.
Location: Brgy. San Lorenzo,
Norzagaray Bulacan Phils.
Dam and spillways:
Impound – Angat River
Height: 131m (430 ft.)
Length: 568m (1864 ft.)
Width(base): 550m (1800ft)
Creates- Angat Reservoir
Total Capacity- 850 Million cu.m.
Location: Bokod, Benguet
Type of Dam – Central Core Rock-fill
Impounds – Agno River
Height – 129m
Length – 452m
Width(base) – 8.5m
Spillways – 8
Creates – Ambuklao reservoir
Total capacity – 327, 170, 000 cu.m
Active capacity – 258 , 000, 000
Catchment area – 690 sq.km
Surface area – 7.7 sq.km
Location : Pantabangan, Nueva
Type of Dam: Enbankment,
Impounds: Pampanga River
Height –107 m
Elevation at Crest –232m
Width(crest) – 12m
Width(base) – 535m
Dam Volume – 12 million cu.yd(9,
174, 658 cu.m.)
Reservoir Creates: Pantabangan
Total Capacity: 2, 996, 000, 000
Active Capacity: 2, 083, 000, 000
Catchment Area: 853 sq.km
Surface Area: 69. 62 sq. km
Normal Elevation: 230 m (755ft.
Lagro, Quezon City
Impounds – Tullahan
Total Capacity – 50.5
La Mesa Damand Reservoir
Metro Manila and its
sorrounding areas are
divided into two water
Maynilad water (red)
and Manila water
Location: San Manuel and San
Nicholas Pangasinan and Itagon,
Dam And Spillways
• Impounds- Agno River
• Height – 200 m(660ft.)
• Length – 1, 130 m (3,710ft.)
Total Capacity – 835 million cu.m
1. Streamflow forecast are necessary in
planning reservoir operations for flood
2. A flood mitigation reservoir has its maximum
potential for flood reduction when it is
3. Third operational problem develops when
flows in excess of natural flows are released
from a reservoir and synchronize at some
point downstream with flood flows from a
Water Resources Systems Planning and
Management by S.K. Jain and V.P. Singh
Wikipedia : Category, Dams in the
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