Flood Damage Mitigation Reservoirs

RESERVOIR

Types of Reservoirsa. Storage/Conservation Reservoirb. Multi-purpose Reservoirc. Distribution Reservoird. Flood Control/Mitigation Reservoir

Storage/Conservation 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.

Multi-purpose ReservoirA 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

Distribution Reservoir

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 emergency.

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 downstream.

Types of Flood Mitigation Reservoirs

a. Storage Reservoirb. Retarding Basins

Storage ReservoirThe 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 flood.

Retarding Basin 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.

Next…Purpose of Flood Mitigation Reservoirs

References• http://www.fao.org/docrep/005/ac675

e/ac675e04.htm• Slideshare• Google• Wikipedia

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:

individual properties

small communities

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 the building.

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

Location of Reservoir

Rim stability Water-holding capability Loss of reservoir water Seismicity Sedimentation Bank storage Topography Geology Local condition

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 effectiveness,

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. 

THANK YOU

http://www.aboutcivil.org/How-Plan-Reservoirs-Factors-Affecting-Location.html

https://en.wikipedia.org/wiki/Flood_mitigation

http://google.com (images)

References

Flood Damage mitigation Reservoirs

Topics:Size of Reservoirs

Operation ProblemsReported by: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.

Dams

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

-Single Purpose -Multi-Purpose

2. Classification Based on Size-Major Reservoir-Medium Reservoir-Minor Reservoir

Classification of Reservoirs

3. Classification Based on Storage

- Seasonal storage reservoir- One-year reservoir

Okay Let’s Begin!!

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.

Different Dams found in the Philippines

Location: Brgy. San Lorenzo, Norzagaray Bulacan Phils.Dam and spillways:Impound – Angat RiverHeight: 131m (430 ft.)Length: 568m (1864 ft.)Width(base): 550m (1800ft)Reservoir:Creates- Angat ReservoirTotal Capacity- 850 Million cu.m.

Angat Dam

Location: Bokod, BenguetType of Dam – Central Core Rock-fill EnbankmentImpounds – Agno RiverHeight – 129mLength – 452mWidth(base) – 8.5mSpillways – 8

ReservoirCreates – Ambuklao reservoirTotal capacity – 327, 170, 000 cu.mActive capacity – 258 , 000, 000 cu.mCatchment area – 690 sq.kmSurface area – 7.7 sq.km

Ambuklao Dam

Location : Pantabangan, Nueva EcijaType of Dam: Enbankment, EarthfillImpounds: Pampanga RiverHeight –107 m Length –1615mElevation at Crest –232mWidth(crest) – 12mWidth(base) – 535mDam Volume – 12 million cu.yd(9, 174, 658 cu.m.)

Pantabangan Dam

Reservoir Creates: Pantabangan LakeTotal Capacity: 2, 996, 000, 000 cu.mActive Capacity: 2, 083, 000, 000 cu.mCatchment Area: 853 sq.kmSurface Area: 69. 62 sq. kmNormal Elevation: 230 m (755ft. Max)

Location: Greater Lagro, Quezon CityImpounds – Tullahan RiverTotal Capacity – 50.5 Million cu.m

La Mesa Dam and Reservoir

Metro Manila and its sorrounding areas are divided into two water concessionaries : Maynilad water (red) and Manila water (blue)

San Roque Dam

Location: San Manuel and San Nicholas Pangasinan and Itagon, Benguet Phils.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 mitigation.

2. A flood mitigation reservoir has its maximum potential for flood reduction when it is empty.

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 tributary.

Operational Problems

References

Water Resources Systems Planning and Management by S.K. Jain and V.P. Singh

Wikipedia : Category, Dams in the Philippines

Google for Images

E.N.D