Water Measurement

Water Measurement

Water flow is measured to assess how much water is available for a supply and to check the quantity of water flowing through a system

Water MeasurementDischarge is defined as the rate of flow or the volume of water that passes through a channel cross section in a specific period of time.

Q = V AMeasured in units of cubic feet per second, (ft3/s or cfs) or its metric equivalent of liters per second (Lps or m3/s)

Example

If the width of the channel is ten feet, the depth is one foot, and the velocity is two feet per second, then

A = 10 ft x 1 ft = 10 ft2

Q = 2 ft/s x 10 ft2 = 20 cfs

Different sources of water require different methods of flow measurement and can be divided as follows: Surface water Springs Wells

Water Measurement

Surface water flow is simply the continuous movement of water in runoff or open channels.The two most common methods of flow measurements of surface water are: Velocity-Area Method Overflow Weir Gauging

Surface Water Sources

Velocity - Area Method

Figure 3.11 The water velocity is found using a flow meter, which looks like an anemometer

Figure 3.12 One cubic foot per second, cfs (or one cubic meter per second, cms) is equivalent to one cubic foot (or meter) of water flowing past a given point in a one-second time interval.

Gauging Weirs Method

Figure 3.13 A weir is an obstruction in an open channel which constricts the flow and causes it to fall over a crest.

Figure 3.14 The hydrograph of a river can look similar to this example after a brief but intense rainfall event.

Stage

Stage is the depth of flow (m or ft) which most commonly measured the elevation of the water surface relative to an arbitrary fixed point.To relate the discharge to the river stage, a Rating Curve is used.

Rating Curve

Figure 3.15 To relate the discharge to the river stage, a Rating Curve is used.

Flood Events

Flood Frequency

Flood Frequency is the likelihood that a large flood will happen.Flood Frequency Analysis uses historical records of peak flows to produce guidance about the expected behavior of future flooding.

Flood Frequency Analysis

Two primary applications of flood frequency analyses are: To predict the possible flood

magnitude over a certain time period

To estimate the frequency with which floods of a certain magnitude may occur.

Return Period

Two primary applications of flood frequency analyses are: To predict the possible flood

magnitude over a certain time period

To estimate the frequency with which floods of a certain magnitude may occur.

Return Period also referred to as the Recurrence Interval is the time period over which it is likely that a particular magnitude flood will occur.100-year floodA flood that can occur on average once every 100 years or 1% chance of occurring in a given year*** 100 years is considered the return period

Return Period

Exceedance Probability or the Probability of Occurrence is the chances over a given time period that a flood will reached or exceed a specific magnitude.100-year floodA flood that can occur on average once every 100 years or 1% chance of occurring in a given year***0.01 considered the exceedance probability

Exceedance Probability

Figure 3.16 Flood damage can be predicted based on the intensity of a storm and the topography of a region.

Extreme Events

• Probable Maximum Precipitation (PMP)• The most extreme rainfall possible• Used for estimating the effects of

extreme weather

• Probable Maximum Flood (PMF)• The most extreme flood possible• Used for estimating maximum extent

of flooding

GIS Mapping

Geographic Information Systems (GIS) is used to organize spatial information. These information are stored in the computer.Its GIS layers include topography, soils, hydrography, vegetation, land use, etc.