IMPACT OF SEDIMENT MANAGEMENT ON RESERVOIR LIFE

As shown in Figure, the Initial Reservoir Storage is the sum of Future Storage Requirements, Active Storage, and Dead Storage. The line designated as Today is the point at which sediment mitigation is implemented.

The Reservoir Capacity line indicates the steady decline of the reservoir’s storage capacity as sediment accumulates over time. This storage loss begins the moment water is stored. The rate of capacity decline (slope of the line) depends on local conditions. In reality, the decline of reservoir capacity is an irregular process with the greatest sediment accumulation taking place during annual spring runoff and during rainfall events. Both of these change over the years too. The rate of capacity decline is estimated as a straight line from the time the reservoir is built until today, and projected into the future.

The Required Storage Capacity line indicates the minimum storage necessary for the water system, of which the reservoir is a part, to function. Notice the intersection of the Reservoir Capacity line and the Required Storage Capacity line. This is the time when the reservoir capacity can no longer meet the requirements for which the reservoir was built. This occurs long before the reservoir is completely filled with sediment.

As time passes beyond this point, reservoir uses can become more and more restricted. The restrictions can pass unnoticed due to the normal variations in the water supply and reservoir usage. With sufficient data, this point can be estimated to plan for the future and help determine the urgency for sediment mitigation actions. This typically varies from when 15 to 40 percent of the reservoir storage is lost.

Case 1 – shows the reservoir capacity over time if no action is taken and sediment is allowed to accumulate with no mitigation.

Case 2 – shows the reservoir capacity if one, or more, sediment mitigation strategies are implemented, resulting in a slower rate of sediment accumulation in the reservoir. Notice the resulting extension of the reservoir’s useful life.

Case 3 – shows the reservoir capacity after removing a certain volume of sediment from the reservoirand simultaneously enacting one, or more, sediment mitigation strategies that results in slowing the rate of sediment accumulation. Notice the greater extension of the reservoir’s useful life than Case 2.

Case 4 – shows the reservoir capacity after enacting one, or more, sediment mitigation strategies that result in stopping sediment accumulation in the reservoir completely. Depending on local conditions, this may or may not be possible. When it is possible, the Reservoir Capacity and Required Storage Capacity lines never intersect and the reservoir’s useful life is potentially extended indefinitely. One scenario to achieve this is a combination of bypassing sediment during spring runoff and hydrosuction removal of previously accumulated sediment. Hydrosuction or another single method alone could also accomplish this.

Precipitation

Water falling in solid or liquid form e.g. rain, snow, and hail..

Uses of Precipitation Data

• Runoff estimation analysis
• Groundwater recharge analysis
• Water balance studies of catchments
• Flood analysis for design of hydraulic structures
• Real-time flood forecasting
• low flow studies

Mechanism Producing Precipitation

Three mechanisms are needed for formation of precipitation.
1. Lifting and Cooling – Lifting of air mass to higher altitudes causes cooling of air.
2. Condensation – conversion of water vapor into liquid droplets.
3. Droplet Formation – Growth of droplets is required if the liquid water present in a cloud is to reach ground against the lifting mechanism of air.

Types of Precipitation

Depending upon the way in which the air is lifted and cooled so as to cause precipitation, we have three types of precipitation, as given below:

• Cyclonic Precipitation
• Convective Precipitation
• Orographic Precipitation

Cyclonic Precipitation:

Cyclonic precipitation is caused by lifting of an air mass due to the pressure difference. Cyclonic precipitation may be either frontal or non-frontal cyclonic precipitation.

1. Frontal precipitation:

It results from the lifting of warm and moist air on one side of a frontal surface over colder, denser air on the other side. A front may be warm front or cold front depending upon whether there is active or passive accent of warm air mass over cold air mass.2.

2. Non-frontal precipitation:

If low pressure occurs in an area (called cyclone), air will flow horizontally from the surrounding area (high pressure), causing the air in the low-pressure area to lift. When the lifted warm-air cools down at higher attitude, non-frontal cyclonic precipitation will occur.

In the case of a cold front, a colder, denser air mass lifts the warm, moist air ahead of it. As the air rises, it cools and its moisture condenses to produce clouds and precipitation. Due to the steep slope of a cold front, forceful rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms.

In the case of a warm front, the warm, less dense air rises up and over the colder air ahead of the front. Again, the air cools as it rises and its moisture condenses to produce clouds and precipitation. Warm fronts have a gentler slope and generally move more slowly than cold fronts, so the rising motion along warm fronts is much more gradual. Precipitation that develops in advance of a surface warm front is typically steady and more widespread than precipitation associated with a cold front. Warm front precipitation is generally light to moderate.

Convective Precipitation

Convective precipitation is caused by natural rising of warmer, lighter air in colder, denser surroundings. Generally, this kind of precipitation occurs in tropics, where on a hot day, the ground surface gets heated unequally, causing the warmer air to lift up as the colder air comes to take its place. The vertical air currents develop tremendous velocities. Convective precipitation occurs in the form of showers of high intensity and short duration.

Orographic Precipitation

Orographic precipitation is caused by air masses which strike some natural topographic barriers like mountains, and cannot move forward and hence rise up, causing condensation and precipitation. All the precipitation we have in Himalayan region is because of this nature. It is rich in moisture because of their long travel over oceans.

Hydrology

What we study in Hydrology?

In hydrology we study Hydrologic cycle, its processes, water balance, precipitation types, estimation of precipitation, and analysis of precipitation data. We also study infiltration phenomena, solution of the Richard’s equation and approximate infiltration models.
Methods of measurement of stream flow, stage discharge relation, unit hydrograph theory, Transposition of Hydrograph, Synthesis of hydrograph from basin characteristics, stream flow routing, flood frequency analysis and attenuation of flood flows are also studied in Hydrology.

Definition of hydrology:

“The study of water in all its forms (rain, snow and water on the earth’s surface), and from its origins to all its destinations on the earth is called hydrology.”

Scope of Hydrology:

1. Water is one the most valuable natural resources essential for human and animal life, industry and agriculture.
2. It is also used for Power generation, navigation and fisheries.
3. Tremendous importance is given to the hydrology all over the world in the development and management of water resources for irrigation, water supply, flood control, water-logging and salinity control, Hydro power and navigation.

Engineering Hydrology:

It uses hydrologic principles in the solution of engineering problems arising from human exploitation of water resources of the earth. The engineering hydrologist, or water resources engineer, is involved in the planning, analysis, design, construction and operation of projects for the control, utilization and management of water resources.
Hydrologic calculations are estimates because mostly the empirical and approximate methods are used to describe various hydrological processes.

Uses of Engineering Hydrology:

Engineering Hydrology Helps in the following ways:

1. Hydrology is used to find out maximum probable flood at proposed sites e.g. Dams.
2. The variation of water production from catchments can be calculated and described by hydrology.
3. Engineering hydrology enables us to find out the relationship between a catchment’s surface water and groundwater resources
4. The expected flood flows over a spillway, at a highway Culvert, or in an urban storm drainage system can be known by this very subject.
5. It helps us to know the required reservoir capacity to assure adequate water for irrigation or municipal water supply in droughts condition.
6. It tells us what hydrologic hardware (e.g. rain gauges, stream gauges etc) and software (computer models) are needed for real-time flood forecasting

Hydrological cycle:

1. The hydrologic cycle describes the continuous re–circulating transport of the waters of the earth, linking atmosphere, land and oceans.
2. Water evaporates from the ocean surface, driven by energy from the Sun, and joins the atmosphere, moving inland as clouds. Once inland, atmospheric conditions act to condense and precipitate water onto the land surface, where, driven by gravitational forces, it returns to the ocean through river and streams.
3. The process is quite complex, containing many sub-cycles.
4. Engineering Hydrology takes a quantitative view of the hydrologic cycle.
5. The quantification of the hydrologic cycle which is an open system can be represented by a mass balance equation, where inputs minus outputs are equal to the change in storage.
6. It is a basic Hydrologic Principle or equation that may be applied either on global or regional scale

I – O = ΔS

The water holding elements of the hydrological cycle are:

1. Atmosphere
2. Vegetation
3. Snow packs
4. Land surface
5. Soil
6. Streams, lakes and rivers
7. Aquifers
8. Oceans