BFW 40103- Water Resources E i iEngineering

Chapter 6:Dam and Spillways

Prepared by:

Mohd Shalahuddin Adnan PhDMohd Shalahuddin Adnan, PhD

Lesson goalsAt the end of this topic, student should be able to:-

Identify the types and functions of dam and spillwaysof dam and spillways

Evaluate the dam and its storage

Consider the design concept of dam and spillway structures

Introduction

Dam is any artificial barrier and its appurtenantworks constructed for the purpose of holding water

th fl idor any other fluid.

Detention/Retention Basin is any structure thatDetention/Retention Basin is any structure thatfunctions as a dam.

Typically it serve multiple purposes includingTypically, it serve multiple purposes, includingwater supply, flood control, hydroelectric powergeneration navigation and water based recreationgeneration, navigation, and water based recreationactivities.

Introduction

Photo credit; http://thecolumbiaexperience.wordpress.com/dams/

Types of reservoir

Valley dam reservoir, dam constructed in a valley reliesa ey da ese o , da co st ucted a a ey e eson the natural topography to provide most of the basin ofthe reservoir. Dams are typically located at a narrow part ofa valley downstream of a natural basin. The valley sides actas natural walls with the dam located at the narrowestas natural walls with the dam located at the narrowestpractical point to provide strength and the lowest practicalcost of construction

Bank-side reservoirs may be constructed to store thewater pumped from the river. The use of bank-sidereservoirs also allows a water abstraction to be closeddown for extended period at times when the river isdown for extended period at times when the river isunacceptably polluted or when flow conditions are very lowdue to drought.

Service reservoirs store fully treated potable water closeto the point of distribution. It perform several functionsincluding ensuring sufficient head of water in the waterg gdistribution system and providing hydraulic capacitance inthe system to even out peak demand from consumersenabling the treatment plant to run at optimum efficiency.

Type of dams

Gravity concrete dam, is a solid concrete structure that uses its massto hold back water. It requires massive amount of concrete to providethe weight necessary to withstand the hydrostatic force exerted by thewater impounded behind the dam.

Buttress dam have triangular supports called buttresses on thedownstream side to strengthen it and to distribute water pressure to theg pfoundation. Used to support a foundation that too week to stand thepressure of gravity dam.

Concrete arch dam, has a curvature design that arches across acanyon and has abutments embedded into solid rock walls. Requiresless concrete but must have solid rock as anchor for the abutments.

Earthen embankment dam, more than 50% of the total volume of anth d i t f t d th t i l N ll hearthern dam consists of compacted earth materials. Normally have an

impervious core of clay or other material of low permeability thatprevent water from rapidly seeping through or beneath the foundationof structure. Have drain installed along the downstream toe of the dam.

Type of reservoir and dams

Coffer dam a temporaryCoffer dam, a temporarystructure enclosing all or part ofthe construction area so thatconstruction can proceed in thedry. A diversion cofferdamdiverts a stream into a pipe,channel, tunnel, or otherwatercourse.watercourse.

Diversion dam , a dam built todi t t f tdivert water from a waterway orstream into a differentwatercourse.

Hydropower Dam, a dam that uses the difference in water level between

Type of reservoir and dams

the reservoir pool elevation and the tailwater elevation to turn a turbine to generate electricity.

Photo credit; WTC Berhad

Dam in Peninsula Malaysia

List of dam location in Peninsular Malaysia:

Johor – Machap dam, Sembrong dam, Sultan Iskandar Reservoir, Bekok dam.M l k J d D i T l dMelaka – Jus dam, Durian Tunggal damPerlis – Timah Tasoh damPerak – Bukit Merah dam, Perting dam, Pontian dam, Kemeriang dam, Temenggor dam Piah damTemenggor dam, Piah dam.Pahang – Repas Baru dam, Perting dam, Pontoan dam, Chematu dam, Chereh dam, Chini dam, Kelau dam.Kedah – Beris damKedah Beris dam.Kelantan – Bukit Kwong dam, Pergau dam.Langkawi – Padang Saga dam.Penang – Itam dam, Mengkuang dam, Teluk Bahang dam.g , g g , gN.Sembilan – Gemencheh damSelangor – Batu dam, Langat dam, Selangor dam, Semenyih dam, Subang dam, Tinggi dam.Terengganu – Kenyir dam.KL – Klang gates dam.

How dam worksA typical dam is a wall of solid material built across a riverto block the flow of the river thus storing water in the lakethat will form upstream of the dam as water continues tothat will form upstream of the dam as water continues toflow from the river upstream of the dam.

The main purpose of most dams is to create a permanentreservoir of water for use at a later time.

The dam must be watertight (ie impermeable or imperviousto water) so that water does not leak out of the dam andescape to downstreamescape to downstream.

Dam wall must have sufficient strengthi. to stand permanently under its own weightii. resist the water pressure in the lake (greater the depthii. resist the water pressure in the lake (greater the depth

of water stored behind the dam and the greater thewater pressure on the dam wall.

A dam must have some way of releasing water in controlledamounts as it is needed ( ie an outlet valve of sometype).

Uses of damsDams are usually built for one or more of the following reasons:

i. To provide a supply of water for towns, cities and mining sites

ii To contain and store waste (tailings) from mines;ii. To contain and store waste (tailings) from mines; iii. To provide a supply of water for the irrigation;iv. To generate electricity in hydro-electric power

stations; v. To help control or mitigate floods

Dam failures

Dam failures are of particular concern because the failureof a large dam has the potential to cause more death andd t ti th th f il f th ddestruction than the failure of any other man-madestructure. This is because of the destructive power of theflood wave that would be released by the sudden collapseof a large dam.

Dam failures

The most common causes of dam failures are:

1) O t i f b k t d d t i d t1) Overtopping of embankment dams due to inadequate spillway discharge capacity to pass flood waters,

2) Faults in construction methods,3) Geological problems with the dam foundation,4) Landslides which fall into the storage reservoir,5) Earthquakes can certainly cause damage to dams5) Earthquakes can certainly cause damage to dams

Concept of Dam Design

A gravity dam shall be: Safe against overturning at any horizontal plane within th dthe dam. Safe against sliding at any horizontal place within the dam. So proportioned that the allowable stresses in both the concrete and the foundation shall not be exceeded.

Concept of Dam Design

Existing Dams - Design Flood

E i ti d th t b i h bilit t d h ld hExisting dams that are being rehabilitated should have adequate spillway capacity to pass the following floods without overtopping:

Hazard Classification Spillway Design Flood (SDF)A 100 yearA 100 yearB 150% of 100 yearC 50% of PMF

The Service Spillway Design Flood (SSDF) for existing dams is the same as shown for the new dams on Table 1.

Basic assumptions in the design of masonry ( k/ t ) d

The rock that constitutes the foundation is strong enough

(rock/ concrete) dams.

to carry the forces imposed by the damThe bearing power of the geologic structure is great enough to carry the total loads imposed by the dam withoutenough to carry the total loads imposed by the dam without rock movements of detrimental magnitude.The rock formations are homogeneous and uniformly elastic in all directionselastic in all directionsThe flow of the foundation rock under the sustained loads adequately.The base of the dam is thoroughly keyed into the rock formations along the foundations and abutments.Construction operations are conducted so as to secure aConstruction operations are conducted so as to secure a satisfactory bond between the concrete and rock materials

Basic assumptions in the design of masonry ( k/ t ) d

The concrete is uniformly elastic in all parts of the

(rock/ concrete) dams.

The concrete is uniformly elastic in all parts of the structure, Contraction joints are properly grouted under adequate

l t l fill d ith tpressures, or open slots are properly filled with concrete,Sufficient drains are installed in the dam to reduce such uplift pressures as may develop along areas of contact between the concrete and rock materialsAssumptions of maximum earthquake accelerationsEffects of foundation and abutment deformationsEffects of foundation and abutment deformations Action at locations along the sloping

Dam Analysis – Gravity Dam

Stability Analysis

Th b i l di diti i ti t d i th d i dThe basic loading conditions investigated in the design and guidance for the dam profile and layout.

Preliminary layout of the structure followed by a stability and stress analysis. If the structure fails to meet criteria then the layout is modified and reanalyzed This process isthen the layout is modified and reanalyzed. This process is repeated until an acceptable cross section is attained.

Stability analysis and failure sequence

Dam Analysis – Gravity Dam

Flow Net Analysis

Fl t d t ti t di ti d lFlow nets used to estimate seepage direction and volume and pore pressure at points within the embankment (CANMET 1977).

A flow net is a graphical solution of Darcy's law to show steady flow through porous media and is often used tosteady flow through porous media and is often used to show ground water flow.

The ariables incl de flo characteristics (either in terms ofThe variables include flow characteristics (either in terms of flow or head), information on the boundaries of the area to be modeled, and information on the hydraulic conductivity within the area

Static and Dynamic Stress Analyses

Stress Analysis

i T d t i th it d d di t ib ti f ti. To determine the magnitude and distribution of stresses throughout the structure for static and dynamic load conditions

ii. To investigate the structural adequacy of the sub-structance and foundation.

Gravity dam stresses are analyzed by either approximate simplified methods or the finite element method depending on the refinement req ired for the partic lar le el of designon the refinement required for the particular level of design and the type and configuration of the dam.

The finite element method is ordinarily used for the feature and final design stages if a more exact stress investigation is required.

Finite element analysis.

Finite element models are used for linear elastic static anddynamic analyses and for nonlinear analyses that accounty y yfor interaction of the dam and foundation.

Provides the capability of modeling complex geometriesProvides the capability of modeling complex geometriesand wide variations in material properties. (ie; stresses atcorners, around openings, tension zones, thermal behavior

d l th l t d tand couple thermal stresses and etc

Dynamic AnalysisThe structural analysis for earthquake loadings consists oftwo parts: an approximate resultant location and slidingstability analysis using an appropriate seismic coefficientstability analysis using an appropriate seismic coefficientand a dynamic internal stress analysis using site-dependent earthquake ground motions

SPILLWAYS A spillway is built in a reservoir to allow the flow of water

to safely move downstream when the reservoir is full;

The spillway capacity must accommodate the maximumdesign flood;

A spillway is shaped as a rectangular concrete channelthat connects the upstream and downstream regions of athat connects the upstream and downstream regions of aweir;

A ill i l t d t th t f th i l A spillway is located at the top of the reservoir pool.Dams may also have bottom outlets with valves or gateswhich may be operated to release flood flow, and a fewd l k fl ill d l ti l b ttdams lack overflow spillways and rely entirely on bottomoutlets..

There are two main types of spillways: controlled andThere are two main types of spillways: controlled anduncontrolled.

A controlled spillway has mechanical structures or gates toA controlled spillway has mechanical structures or gates toregulate the rate of flow. This design allows nearly the full height ofthe dam to be used for water storage year-round, and flood waterscan be released as required by opening one or more gatescan be released as required by opening one or more gates.

An uncontrolled spillway, in contrast, does not have gates; whenthe water rises above the lip or crest of the spillway it begins to bep p y greleased from the reservoir. The rate of discharge is controlled onlyby the depth of water within the reservoir. All of the storage volumein the reservoir above the spillway crest can be used only for thetemporary storage of floodwater, and cannot be used as watersupply storage because it is normally empty.

F ll ill ill d i l t iFree overall spillways, ogee spillways, drop inlet or morning glory spillways, and chute spillways are common types.

Main decision in designing spillway

The main decisions that have to be taken during the design of a spillway for a dam are:H l fl d h ld th d b bl fHow large a flood should the dam be capable ofwithstanding, ie what is the Probable Maximum Flood(PMF);

How much of the PMF does the spillway have to handle(ie maximum discharge capacity);(ie maximum discharge capacity);

To what extent is it necessary to line the spillway withconcreteconcrete;

To what extent is it necessary to provide energydissipation structures at the downstream end of thespillway.

Functions of a Spillway

The principal function of a spillway is to pass down thesurplus water from the reservoir into the downstream river,th i l f ti th t b i d tthere are precisely seven functions that can be assigned tospillway as follows:1. Maintaining normal river water functions (compensation water supply)2. Discharging water for utilization3 Maintaining initial water level in the flood-control3. Maintaining initial water level in the flood control operation4. Controlling floods5 Controlling additional floods5. Controlling additional floods6. Releasing surplus water (securing dam and reservoir safety)7. Lowering water levels (depleting water levels in an emergency)

Classification of spillwaySpillways have been classified according to various criteria such as;-

a) Most prominent featurea) Most prominent featureb) According to functionc) According to Control Structure

a) According to the most prominent featurei. Ogee spillwayg p yii. Chute spillway iii. Side channel spillwayiv Shaft spillwayiv. Shaft spillwayv. Siphon spillwayvi. Straight drop or overfall spillwayii T l ill /C l t illvii. Tunnel spillway/Culvert spillway

viii. Labyrinth spillwayix. Stepped spillway

Classification of spillwayb) According to Function

i. Service spillwayii Auxiliary spillwayii. Auxiliary spillwayiii. Fuse plug or emergency spillway

c) According to Control Structurei. Gated spillwayii. Ungated spillwayg p yiii. Orifice of sluice spillway

OGEE SPILLWAY OVERFALL SPILLWAY

CHUTE SPILLWAY

LABYRINTH SPILLWAY

SHAFT SPILLWAY

TUNNEL/CULVERT SPILLWAY

SIFPHON SPILLWAY

STEPPED SPILLWAY

Storage and Capacity in the reservoir/damThe storage zone as shown in figure below present a simplified view of reservoir capacity, since sedimentation storage capacity must generally be provided in all storagestorage capacity must generally be provided in all storage zones.

Maximum pool elevation

Normal pool elevationFlood storage capacity DAM

Normal pool elevation

Minimum pool elevation

Active storage

Dead storageFirm & secondary yield

Storage and Capacity in the reservoir/damTypically, sediment reserve storage capacity is provided to accommodate sediment deposition expected to occur over a specified design life which for large projects is typicallya specified design life which for large projects, is typically on the order of 50 - 100 years.

Reservoir sedimentation amounts are predicted as the sediment yield entering the reservoir multiplied by trap efficiency.y

dim depositedamountentSe 100dimdim(%) x

enteringamountentSedepositedamountentSeefficiencyTrap

Example 1A reservoir covers an area of 850 km2 and has an average

depth of 18.7m. The inflow to the reservoir is from ariver with an average flowrate of 2500 m3/s and ariver with an average flowrate of 2500 m3/s and asuspended sediment concentration of 250 mg/L.Estimate the rate at which the depth of the reservoir isdecreasing due to sediment accumulation and the timeit will take for the reservoir storage to decrease by10%. Assume that the accumulated sediment has a bulkdensity of 1600 kg/m3.

Given;Inflow rate = 3 10 32500 / 7.88 10 /m s x m yr

Sus.sediment = 3250 / 0.25 /mg L kg m

2 8 2850 8 5 10kArea of reservoir =

Sediment load = inflow rate x suspended sediment

2 8 2850 8.5 10km x m

pconcentration

yrkgxm

kgxyrmx 10

3210 1097.125.01088.7

Storage capacity = area of reservoir x average depth

yrmyr

31028 1059.17.18105.8 mxmxmx

chartseexcapacitystroragestorageofRatio 201059.1 10

chartseexlowannual

storageofRatio 2.01088.7inf 10

Based on ratio of storage capacity = 0.2, sediment trapped = 93%.

The rate sediment accumulate in the reservoir

yrkgxyr

kgx 101083.197.193.0

kgx 1010831

Sediment volume accumulation rate

yrmx

mkg

yrx36

3

1045.111600

1083.1

m

Rate of sediment accumulation

yrcm

yrm

xx 35.10135.01058

1045.118

6

yryrx105.8

This rate, it will take approximately 138.5 yrs for the reservoir capacity to decrease by 10% due to sediment accumulation.Since 93% sediment is trapped, 1.83x1010kg/yr sediment pp , g yload delivered by river,

Sediment release from the reservoir =

yrkgxyrkgx 910 1028.1)1083.1)(93.01(

Sediment cont. =L

mgmkgyrkgxyrkgx 65065.0

1097.11028.1 3

10

9

Therefore, reservoir reduce the suspended sediment concentration = 250 mg/L to 65 mg/L = (74%)g g ( )Reservoir trap efficiency (93%) ≠ reduction suspended sediment cont. (74%)

Planning GuidelinesRecommended steps in the planning and investigation of dam and reservoir projects are as follows :

i. Identification of project objective approximate magnitudes

ii. Selection of a dam and reservoir siteiii. Preliminary sizing and determination of dam typeiv. Preliminary surveysy yv. Hydrologic Investigationsvi. Hydrologic analysis