Chapter 6 Dam and Spillways2

Chapter 6: Dam and Spillways

Identify the types and functions of dam and spillways

Evaluate the dam and its storage

Consider the design concept of dam and spillway structures

Lesson goals

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

Introduction

Dam is any artificial barrier and its appurtenant works constructed for the purpose of holding water or any other fluid.

Detention/Retention Basin is any structure that functions as a dam.

Typically, it serve multiple purposes, including water supply, flood control, hydroelectric power generation, navigation, and water based recreation activities.

Introduction

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

Types of reservoir

Valley dam reservoir, dam constructed in a valley relies on the natural topography to provide most of the basin of the reservoir. Dams are typically located at a narrow part of a valley downstream of a natural basin. The valley sides act as natural walls with the dam located at the narrowest practical point to provide strength and the lowest practical cost of construction

Bank-side reservoirs may be constructed to store the water pumped from the river. The use of bank-side reservoirs also allows a water abstraction to be closed down for extended period at times when the river is unacceptably polluted or when flow conditions are very low due to drought.

Service reservoirs store fully treated potable water close to the point of distribution. It perform several functions including ensuring sufficient head of water in the water distribution system and providing hydraulic capacitance in the system to even out peak demand from consumers enabling the treatment plant to run at optimum efficiency.

Type of dams

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

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

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

Earthen embankment dam, more than 50% of the total volume of an earthern dam consists of compacted earth materials. Normally have an impervious core of clay or other material of low permeability that prevent water from rapidly seeping through or beneath the foundation of structure. Have drain installed along the downstream toe of the dam.

Type of reservoir and dams

Coffer dam, a temporary structure enclosing all or part of the construction area so that construction can proceed in the dry. A diversion cofferdam diverts a stream into a pipe, channel, tunnel, or other watercourse.

Diversion dam , a dam built to divert water from a waterway or stream into a different watercourse.

Hydropower Dam, a dam that uses the difference in water level between the reservoir pool elevation and the tailwater elevation to turn a turbine to generate electricity.

Type of reservoir and dams

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.Melaka – Jus dam, Durian Tunggal damPerlis – Timah Tasoh damPerak – Bukit Merah dam, Perting dam, Pontian dam, Kemeriang dam, Temenggor dam, Piah dam.Pahang – Repas Baru dam, Perting dam, Pontoan dam, Chematu dam, Chereh dam, Chini dam, Kelau dam.Kedah – Beris dam.Kelantan – Bukit Kwong dam, Pergau dam.Langkawi – Padang Saga dam.Penang – Itam dam, Mengkuang dam, Teluk Bahang dam.N.Sembilan – Gemencheh damSelangor – Batu dam, Langat dam, Selangor dam, Semenyih dam, Subang dam, Tinggi dam.Terengganu – Kenyir dam.KL – Klang gates dam.

A typical dam is a wall of solid material built across a river to block the flow of the river thus storing water in the lake that will form upstream of the dam as water continues to flow from the river upstream of the dam.

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

The dam must be watertight (ie impermeable or impervious to water) so that water does not leak out of the dam and escape to downstream.

How dam works

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

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

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

Dams 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; 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

Uses of dams

Dam failures are of particular concern because the failure of a large dam has the potential to cause more death and destruction than the failure of any other man-made structure. This is because of the destructive power of the flood wave that would be released by the sudden collapse of a large dam.

Dam failures

The most common causes of dam failures are:

1) 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 dams

Dam failures

A gravity dam shall be: Safe against overturning at any horizontal plane within the 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 Existing 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 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.

Concept of Dam Design

The rock that constitutes the foundation is strong enough 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 without rock movements of detrimental magnitude.The rock formations are homogeneous and uniformly elastic 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 a satisfactory bond between the concrete and rock materials

Basic assumptions in the design of masonry (rock/ concrete) dams.

The concrete is uniformly elastic in all parts of the structure, Contraction joints are properly grouted under adequate pressures, 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 deformations Action at locations along the sloping

Basic assumptions in the design of masonry (rock/ concrete) dams.

Stability Analysis

The 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 is repeated until an acceptable cross section is attained.

Dam Analysis – Gravity Dam

Stability analysis and failure sequence

Flow Net Analysis

Flow 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 to show ground water flow.

The 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

Dam Analysis – Gravity Dam

Stress Analysis

i. 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 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. 

Static and Dynamic Stress Analyses

Finite element analysis.

Finite element models are used for linear elastic static and dynamic analyses and for nonlinear analyses that account for interaction of the dam and foundation.

Provides the capability of modeling complex geometries and wide variations in material properties. (ie; stresses at corners, around openings, tension zones, thermal behavior and couple thermal stresses and etc

Dynamic AnalysisThe structural analysis for earthquake loadings consists of two parts: an approximate resultant location and sliding stability 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 maximum design flood;

A spillway is shaped as a rectangular concrete channel that connects the upstream and downstream regions of a weir;

A spillway is located at the top of the reservoir pool. Dams may also have bottom outlets with valves or gates which may be operated to release flood flow, and a few dams lack overflow spillways and rely entirely on bottom outlets..

There are two main types of spillways: controlled and uncontrolled.

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

An uncontrolled spillway, in contrast, does not have gates; when the water rises above the lip or crest of the spillway it begins to be released from the reservoir. The rate of discharge is controlled only by the depth of water within the reservoir. All of the storage volume in the reservoir above the spillway crest can be used only for the temporary storage of floodwater, and cannot be used as water supply storage because it is normally empty.

Free overall spillways, ogee spillways, drop inlet or morning glory spillways, and chute spillways are common types.

The main decisions that have to be taken during the design of a spillway for a dam are:How large a flood should the dam be capable of withstanding, ie what is the Probable Maximum Flood (PMF);

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

To what extent is it necessary to line the spillway with concrete;

To what extent is it necessary to provide energy dissipation structures at the downstream end of the spillway.

Main decision in designing spillway

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

Functions of a Spillway

Spillways have been classified according to various criteria such as;-

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

a) According to the most prominent featurei. Ogee spillwayii. Chute spillway iii. Side channel spillwayiv. Shaft spillwayv. Siphon spillwayvi. Straight drop or overfall spillwayvii. Tunnel spillway/Culvert spillwayviii. Labyrinth spillwayix. Stepped spillway

Classification of spillway

Classification of spillway

b) According to Functioni. Service spillwayii. Auxiliary spillwayiii. Fuse plug or emergency spillway

c) According to Control Structureiv. Gated spillwayv. Ungated spillwayvi. 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/dam

The storage zone as shown in figure below present a simplified view of reservoir capacity, since sedimentation storage capacity must generally be provided in all storage zones.

Maximum pool elevation

Normal pool elevation

Minimum pool elevation

Flood storage capacity

Active storage

Dead storage

DAM

Firm & secondary yield

Storage and Capacity in the reservoir/dam

Typically, sediment reserve storage capacity is provided to accommodate sediment deposition expected to occur over a 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.

100dim

dim(%) x

enteringamountentSe

depositedamountentSeefficiencyTrap

Example 1

A reservoir covers an area of 850 km2 and has an average depth of 18.7m. The inflow to the reservoir is from a river with an average flowrate of 2500 m3/s and a suspended sediment concentration of 250 mg/L. Estimate the rate at which the depth of the reservoir is decreasing due to sediment accumulation and the time it will take for the reservoir storage to decrease by 10%. Assume that the accumulated sediment has a bulk density of 1600 kg/m3.

Given;Inflow rate =

Sus.sediment =

Area of reservoir =

Sediment load = inflow rate x suspended sediment concentration

Storage capacity = area of reservoir x average depth

3 10 32500 / 7.88 10 /m s x m yr

3250 / 0.25 /mg L kg m

2 8 2850 8.5 10km x m

yrkgx

mkgxyr

mx 103

210 1097.125.01088.7

31028 1059.17.18105.8 mxmxmx

chartseex

x

lowannual

capacitystroragestorageofRatio 2.0

1088.7

1059.1

inf 10

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

yrmx

mkg

yrkgx

36

3

10

1045.111600

1083.1

Sediment volume accumulation rate

Rate of sediment accumulation

yrcm

yrm

x

x35.10135.0

105.8

1045.118

6

This rate, it will take approximately 1385 yrs for the reservoir capacity to decrease by 10% due to sediment accumulation.Since 93% sediment is trapped, 1.83x1010kg/yr sediment load delivered by river,

Sediment release from the reservoir =

Sediment cont. =

yrkgxyrkgx 910 1028.1)1083.1)(93.01(

Lmgmkg

yrkgx

yrkgx65065.0

1097.1

1028.1 310

9

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

Planning Guidelines

Recommended 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 surveysv. Hydrologic Investigationsvi. Hydrologic analysis