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Method For Determining Runoff

METHOD FOR DETERMINING RUNOFF FOR MINOR HYDRAULIC STRUCTURES

The most common means for determining runoff for minor hydraulic structures is the rational formula:

Q=CIA

where
Q= peak discharge, ft3/s (m3/s)

C =runoff coefficient percentage of rain that appears as direct runoff

I= rainfall intensity, in/h (mm/h)

A= drainage area, acres (m2)

COMPUTING RAINFALL INTENSITY.

Chow lists 24 rainfall-intensity formulas of the form:

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Evaporation And Transpiration

The Meyer equation, developed from Dalton’s law, is one of many evaporation formulas and is popular for making evaporation-rate calculations:

E=C (ew-ea) K

K=1+0.1w

where

E= evaporation rate, in 30-day month

C= empirical coefficient, equal to 15 for small, shallow pools and 11 for large, deep reservoirs

ew=saturation vapor pressure, in (mm), of mercury, corresponding to monthly mean air temperature observed at nearby stations for small bodies of shallow water or corresponding to water temperature instead of air temperature for large bodies of deep water.

ea=actual vapor pressure, in (mm), of mercury, in air based on monthly mean air temperature and relative humidity at nearby stations for small bodies of shallow water or based on information obtained about 30 ft (9.14 m) above the water surface for large bodies of deep water.

w=monthly mean wind velocity, mi/h (km/h), at about 30 ft (9.14 m) aboveground

K =wind factor

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Prediction Of Sediment Delivery Rate

Rate of sediment accumulation in a reservoir can be found by two methods.
One approach depends on historical records of the silting rate for existing reservoirs and is purely empirical. The second general method of calculating the sediment delivery rate involves determining the rate of sediment transport as a function of stream discharge and density of suspended silt.

The quantity of bed load is considered a constant func- tion of the discharge because the sediment supply for the bed-load forces is always available in all but lined channels. An accepted formula for the quantity of sediment trans- ported as bed load is the Schoklitsch formula:

Gb=86.7*S3/2 (Qi – bqo)/ ? Dg

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Weirs

Weir is defined as a barrier over which the water flows in an open channel. The edge or surface over which the water flows is called the crest. The overflowing sheet of water is the nappe.
If the nappe discharges into the air, the weir has free discharge. If the discharge is partly under water, the weir is submerged or drowned.

Types of Weirs.

A weir with a sharp upstream corner or edge such that the water springs clear of the crest is a sharp-crested weir.
All other weirs are classed as weirs not sharp crested. Sharp-crested weirs are classified according to the shape of the weir opening, such as rectangular weirs, triangular or V-notch weirs, trapezoidal weirs, and parabolic weirs. Weirs not sharp crested are classified according to the shape of their cross section, such as broad-crested weirs, triangular weirs, and trapezoidal weirs.

The channel leading up to a weir is the channel of approach. The mean velocity in this channel is the velocity of approach. The depth of water producing the discharge is the head.
Sharp-crested weirs are useful only as a means of meas- uring flowing water. In contrast, weirs not sharp crested are commonly incorporated into hydraulic structures as control or regulation devices, with measurement of flow as their secondary function.

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Hydraulic Jump

The abrupt increase in depth of rapidly flowing water is called hydraulic depth.Flow at the jump changes from a supercritical to a subcritical stage with an accompanying loss of kinetic energy. The change in depth occurs over a finite distance, known as the length of jump. The upstream surface of the jump, known as the roller, is a turbulent mass of water.
The depth before a jump is the initial depth, and the depth after a jump is the sequent depth. The specific energy for the sequent depth is less than that for the initial depth because of the energy dissipation within the jump.

F=[ d22– d12]w/2

where d1 =depth before jump, ft (m)

d2 =depth after jump, ft (m)

w=unit weight of water, lb/ft3 (kg/m3)

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