Dams and Reservoirs

WRS Final

water supply

water needs and supply aren’t well matched

water needs are being met by depleting resources that took an extended period of time to accumulate (water use increases, but water availability stays the same)

water is being depleted by:

• agriculture

• industrilal

• domestic

no growth in ground/surface water

Dams

Defn: solid barrier used to store water

Use: Hydroelectric Power generation, irrigation, flood controls, recreation

Storage Dams

for impounding water for development use

diversion dams

for diverting streamflow into canals or other conveyance systems

detention dams

hold water temporarily to delibrately slow down flood flows

Types of Dams - function(3)

1. storage dams

2. diversion sams

3. detention dams

Overflow Dams

Carry discharge over the crests

Nonoverflow dams

not designed to be overtopped

Dam classification (2)

1. overflow dams

2. non-overflow dams

Earthfill dams

built by compacting successive layers of earth, using the most impervious materials to form a core and placing more permeable substances on the upstream and downstream sides

Rockfill Dams

mainly made from natural materials. They are suited to sites with wide valleys and shallow slopes, creating relatively wide and shallow reservoirs

Concrete Dams (3)

a dam constructed of concrete and or masonry, which relies on its weight and internal strength for stability

• concrete gravity dams

• concrete arch dams

• concrete buttress dams

• hollow gravity dam

Stone Masonry

a dam constructed of concrete and or masonry, which relies on its weight and internal strength for stability

• stone masonry gravity dams

• stone masonry arch dams

• hollow gravity dam

Hollow gravity Damn (concrete/masonry)

a dam constructed of concrete or masonry on the outside, but having a hollow interior relying on its weight for stability

Steel Cofferdams

a cofferdam is a substantially water tight enclosure consisting of steel sheet piling and structural bracing components which will permit substructure construction in waterways, or other areas where construction must be isolated from the surroundings

inflatable cofferdams

can change them to control flow

types of dams (5-7)

1. earthfill dams

2. rockfill dams

3. concrete dams

4. stone massonry dams

5. hollow gravity dam

6. steel cofferdam

7. inflatable cofferdam

Hydropower station

big dams used to generate power

• power dependent on elevation of water, efficiency, and losses

underflow gates

gates are frequently used to control discharge in canals or on spillway crests

vertical slice gate is most common

found at bottom of reservoir

Reservoir Sedimentation

Defn: sedimentation is the process of filling natural lakes and man-made reservoirs by sediment transforming finally into the land again (will shorten lifespan of reservoir)

Cause: sediment yield transported by rivers as suspended or bed load into the reservoirs. The inflow into the reservoir is mixture of water and sediment

finest silt particle of diamm < 0,1mm are carried up to the deepest reservoir zone in front of the dam and normally near the outlet

Issues w Sedimentation

as the amount of sediment rises the storage of the reservoir goes down

• eventually they will have to dredge the reservoir (costly

Sedimentation Solution (general)

calculate the amount of sediment coming in

the water with most the sediment in it is allowed to flow over/past the reservoir (taking the sediment with it) and then the reservoir will open to intake the rest of the water.

issues with general solution:

• causes silt to accumulate downstream instead of in the dam

• reduces the cross section of the stream

• stream depth decreases and the stream width increases, causing more flooding

Check Dams

small structures, ditches, and brooks

use: Build a dam before the big dam to collect the sediment, then let the overflow from that dam go to the big dam

pros:

• easier to dredge

• reduces sediment in the big dams

Flushing

how much rain will impact that area of the soil and how much settlement will come through

letting water go instead of catching it bc you plan for a big storm to come

sediment amount depends on length of dry period before rainfall

cons:

• risky because weather is often unpredictable

Situation 1: Rate at which the water resources are available is always in excess of the requirements

(supply > demand)

Solution: run-of-river project can be formulated in which water can be used directly from the stream as need arises. A run-of-river project primarily incorporates a conveyance system to transport water to the location of its use.

Situation 2: The total quantity of available resources over a period of time is equal to or in excess of the overall requirements, but the rate of requirements at times exceeds the available rate of resource supply

(rate of requirements > availability)

Solution: a storage reservoir. A reservoir project includes a storage structure, a control structure, and a conveyance system

Situation 3: Total available resources are less than the overall requirements

(available resources < required)

Solution: a supplemental source or an alternative site has to be explored (the conveyance structures, desalination…)

Seasonal (within-year) storage

when the demand for water can be satisfied by holding some of the high flow each year for release during a later period of low flow

Carryover (over-year) Storage

when there is not enough high flow every year to raise the flow to the desired level, extra water must be stored during wetter years, to release during dry year

Types of Storage (2)

1. seasonal - Within year

2. Carry over - over year

Active storage

storage to manage peak demand, provide firefighting reserves, ensure supply during dry periods, and support irrigation

includes:

• flood control, hydropower, water supply and irrigation, and a buffer

storage required to meet the water demand for intended uses as well as a buffer amount of water

live storage is part of it

Dead storage

storage provided to collect sediment and maintain a minimum pool level

prevents it from clogging intakes while maintaining habitat for fish and supporting environmental minimum downstream flows during droughts

live storage

the storage required to meed the demand for intended uses

amount of active storage without the buffer section

Reservoir Storage Capacity

active storage: (live storage + buffer)

dead storage

gate useful to control water supply

Demand for Water

use of water: comprises studying availability of and demand for water contemplating a project that can meet the expected needs from available supplies

Goal: varying the quantity and quality, time of supply, and place of use of water resources in accordance w need

Questions (4)

1. How much water is needed?

(how much demand/ supply )

2. How much water is Available?

(are demand and supply in sink? How can we manage resources and risks to see what the demand is?)

3. How are the requirements satisfied by the supplies?

(multiple objectives to play bw them)

4. How is the used-up water disposed of?

How is the used up water disposed of?

disposed of through the drainage system

(important because main intake from river and water waste will be disposed of back into the river and used by communities downstream.)

The volume of water you can dispose of into the river depends on the size of the river itself

reconciliation of demand w available resources in an optimal manner

combines demands for agriculture, industry, etc

optimize this solution to balance water availability as well as demand (not a static problem-both variables vary)

water withdrawal uses

involve diverting surface water or groundwater from tis source of supply (for irrigation…water supply…etc)

(demand that requires withdrawal)

Nonwithdrawal uses

onsite uses such as navigation and recreatoin

(doesnt require withdrawal)

requires a certain amount that remains in the network

(keep certain volume at certain flow to avoid cavities)

consumptive water use

the portion of withdrawn quantity that is no longer available for further use because it has been degraded from use by plants, humans, industry, evaporation…etc

(consumptive water use is small since 80-90% of the total intake is returned)

Demand for Water Supply

withdrawal uses

non withdrawal uses

water requirements usually have the highest priority

consumptive water use is small

most water will be returned to the system degraded which is why it is treated

Water supply visual (magnitude of consumption)

well field: (literally a field of wells) used for backup storage that pump the water from aquifers

(in this example) the municipal is the main water demand source

water is withdrawn and is returned to the river (80-90%) - very little loss from start to finish

water we dispose of in the river will need to be a certain ratio of the water flow because people downstream need the water so it cant all be degraded from use

Municipal Requirements (public water use)

1. domestic uses

2. public uses

3. commercial uses

4. small industrial use

5. losses in distribution system

Parameters:

1. population estimates

2. per-capita water usage

(highly variable-depending on size of city, characteristics of the population, nature, and size of commercial and industrial establishments, climactic conditions, and cost of supply)

plan to anticipate demand

Water requirements: Categories

1. Municipal (public use)

2. large industrial

3. waste dilution

water supply vs water consumption

water supply: the recorded amount of water put into the system

water consumption: the recorded amount of water taken out of a system

water supply > water consumption (usually)

Per Capita water usage

common:

1. low lift pump: bring water from rier to treatment plant

2. high lift pump: go through conduit and go to reservoir

3. distribution system: water reaches It by gravity

benefit: don’t need power to send water to people

4. treatment plant: never sends water directly to the city because it cant keep up with the variance in demand of the city

5. service reservoir: holds water to deal with city variance

uncommon:

1. service reservoir, treatment plant, distribution center: risk here with power (variable demand and power issues)

common trends in Per capita use

1. smaller the city the more variable the demand

2. the shorter the period of flow, the wider the variation is from average

(the hourly peak flow Is much higher than the daily peak)

(time period increase, average increases)

supply has to cope with the pattern of this demand and always satisfy this demand and more (incase of worst case scenario)

water supply tanks

when there are widely varying demands on a water supply distribution system, a distribution reservoir or service tank is provided to accommodate the higher-than-averae demands from an operating storage.

Water distribution system components

water tanks supply water to meet demands

pipes increase energy of the water system

meters measure flow

valve control flow or pressure (but decrease energy)

largest portion of a water distribution system is the pipes

Hydraulic grade line

low demand periods:

• all consumption met by pumping station, tank filled with excess pumped water

High demand periods:

• pumping station and tank both flowing to meet consumption needs

• divert water to decrease head loss from pipes (pipes have friction which causes head loss)

energy and hydraulic grade lines

pressure causes flow even against gravity

(need to know flow and pressure)