Understanding Earth Dams, Rockfill Dams, and Spillways
Design of Earth Dams
Earth dams are designed by forming a fill using readily available materials with low permeability. This fill should be constructed at the lowest possible cost. The borrow area for the fill material should be located near the construction site to minimize transportation expenses.
Reducing Leakage in an Earth Dam
Leakage in an earth dam can be minimized through several methods:
- Constructing a very high base.
- Placing an impermeable apron on the upstream perimeter.
- Using a clay core.
- Incorporating a diaphragm made of wood, steel, or concrete.
Drains in an Earth Dam
Drains in an earth dam typically consist of a rock toe, apron, or thick layer of rockfill material. These drains collect seepage water and direct it to a safe dissipation point.
Slope Protection in Earth Dams
Slope protection is achieved by using riprap, concrete, or a 3-foot layer of rock placed by dumping.
Rockfill Dams
Rockfill dams consist of two primary structural components: a waterproof membrane and an embankment that supports the membrane. The dam typically features an upstream section of dry stone or masonry and a downstream section of loose rockfill.
Timber Crib Dams
Timber crib dams are structures formed by bolted wooden cribs filled with rocks or planks. Their shear strength is affected by buoyancy due to the lower specific gravity of the dam. The lifespan of a timber crib dam ranges from 10 to 40 years, depending on weather conditions and the type of wood used.
Cofferdams
Cofferdams are temporary structures used to dewater an area, allowing for construction work. They should be cost-effective and as impermeable as possible. The simplest type of single-wall cofferdam is a cellular structure with sheet piling, sometimes reinforced with earth or rockfill on both sides.
Purpose of a Spillway
Spillways are designed to safely discharge floodwaters and prevent damage to the dam.
Types of Spillways
a. Free Flow Spillway
A free flow spillway is a section of the dam designed to allow water to flow over its crest. They are commonly used in gravity, arch, and buttress dams. The design of spillways for low dams is generally not critical and utilizes simple crest arrangements. For large dams, it’s crucial to minimize the load on or above the spillway crest and ensure smooth water flow with minimal turbulence. If the overflowing water loses contact with the chute surface, a vacuum can form, potentially leading to cavitation.
Discharge is calculated using the equation: Q = CwLh^(3/2), where Q is the discharge in cubic feet, C is a coefficient, L is the crest length in feet, and h is the head on the spillway.
b. Rapid Flow Spillway (Chute Spillway)
In a rapid flow spillway, water flows over the crest into an open channel called a chute or waterfall. The chute is typically constructed with reinforced concrete slabs, 10 to 20 inches thick. This structure is relatively lightweight and well-suited for earth or rockfill dams where topography permits.
c. Shaft Spillway (Morning Glory Spillway)
A shaft spillway directs water into a vertical shaft, which then flows through a horizontal conduit downstream of the dam. This type is often used where space constraints preclude other spillway designs. If topography prevents the use of a chute or side channel, a shaft spillway through the foundation material may be a suitable option. Small shaft spillways can be constructed entirely of metal, concrete pipe, or clay. The vertical shaft of larger structures is typically made of reinforced concrete, while the horizontal conduit is often excavated into rock.
Formula for Calculating Runoff Volume
R = PLG
Where:
- R = Volume of direct runoff
- P = Precipitation
- L = Recharge basin
- G = Increase in groundwater
For small watersheds, the formula can be simplified to:
R = KP
Where:
- K = Runoff coefficient
Infiltration
Infiltration is the process of water movement through the soil surface and into the soil profile.
Factors Affecting Maximum Infiltration Rate
- Shape and condition of soil pore spaces
- Soil permeability
- Rainfall characteristics
Infiltration Rate Calculation
The average infiltration rate or index (W) can be calculated as:
W = (P – R) / T
Where:
- W = Infiltration index
- P = Precipitation
- R = Runoff volume
- T = Duration
Alternatively:
K = (i – W) / i
Where:
- K = Runoff coefficient
- i = Rainfall intensity
Aquifers
Aquifers are geological formations that store and transmit groundwater. Their capacity is measured by porosity.
Measuring Water Porosity
Porosity can be determined by oven-drying an undisturbed soil sample and weighing it. The quantity of water in an aquifer can be obtained by measuring the discharge of flowing water or by gravity methods.
Drought
Drought is defined as a lack of water for a specific purpose, typically characterized by a fixed time period with below-average rainfall.
Limitations of Runoff Curves
The primary limitation of runoff curves is that they do not accurately represent the natural sequence of runoff.
Drought Frequency Analysis
Drought frequency is analyzed using similar methods as flood frequency analysis.