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CVEN9625 Fundamentals of Water Engineering

HYDROLOGICAL PROCESSES AND MEASUREMENT 1

Rain producing mechanisms

Rainfall is the form of precipitation which most Australian hydrologists are primarily concerned with. Analysis of rainfall is required for: • •

Review of historical data, and Estimation of design conditions.

Typical reasons for reviewing historical data include: • •

The determination and analysis of recorded flood events Development of drought management plans

• •

Determination of catchment yield relationships, and Assessing the implications of alternative catchment management plans.

In a similar manner, design rainfall conditions are required for the estimation of design flood events, the design of urban drainage systems, and the determination of irrigation water requirements.

1.1

Definition and forms of precipitation

Precipitation is the deposition of liquid from the atmosphere to the ground surface. It may occur as either a liquid or a solid. Dew, fog, cloud or frost contribute to precipitation when saturated air comes in contact with a cooled surface or is intercepted by vegetation. The relatively small amounts of dew make measurement difficult and special gauges are required. It could be argued that as dew generally evaporates again each morning, then there is no need to take notice of it. However, dew fall can be an important source of water in upland regions where dew ponds are constructed The pond area cools rapidly at night by ex radiation (Rb) and the water in the pond is replenished by condensation from dew. There is an area in Southern Oman where all the highland vegetation is supported by dew fall as rain fall hardly ever occurs. It is even possible to construct dew fall wells. An excavation in rock (cave) with a narrow throat to the atmosphere will cool rapidly at night and moisture in the excavation will condense and collect in pools. The water is protected from evaporation the next day as the temperature in the cave remains far below the air temperature on the surface. The process will continue for as long as the moisture content of the air (humidity) remains high enough. The precipitation forms of significance in surface water hydrology are: •

drizzle

•

rain

• •

snow sleet

•

hail

Drizzle Drizzle has a drop size of less than 0.5mm. As the drops of drizzle are small they are readily carried by the wind.

Rain The largest drops that can exist without breaking up are approximately 5mm diameter (Wiesner 1970). Rain is classified into light, moderate and heavy depending upon drop size and intensity.

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CVEN9625 Fundamentals of Water Engineering

Snow Snow crystals occur when water vapour condenses by sublimation to form ice crystals. This only happens when the dew point of the air occurs below 0°C . The low density of snow flakes means that they can be carried for long distances in the air and are strongly affected by wind and turbulence. It is therefore very difficult to accurately measure the amount of snow.

Sleet Sleet forms when rain drops freeze. This can occur when there is significantly cooler air at low elevations and the rain drops, initially formed by condensation at temperatures above 0°C , freeze before hitting the surface. Most years there are significant ice storms in the US where a meteorological situation is maintained for several days where rain/sleet falls. If the ground surface is very cold, due to a previous high pressure area accompanied by significant ex radiation, there can be major build up of ice on the ground and on the branches of trees of utilities. These eventually fail (break off) as a result of the weight of ice.

Hail Hail forms in cumulonimbus storm cells as the result of rain drops being carried up and down through several cycles in the storm cell. Water condenses and is frozen first as sleet, it then has further layers of water, which freeze in turn as the drop moves about the storm cell. Finally it falls to the ground as the updrafts in the storm cell are no longer able to support the hail’s weight.

1.2

Rain producing mechanisms

There are a number of rain producing mechanisms: •

Convective rainfall

• •

Orographic rainfall Frontal rainfall

•

Convergent rainfall

Figure 1 Rainfall producing mechanisms (Glendale Community College GPH111 Supplemental Lecture Notes)

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CVEN9625 Fundamentals of Water Engineering

Convective rainfall Convective lifting occurs when the air mass is heated from below and rises through the cooler air by convection. The usual result of convective lifting is a thunderstorm, which is the most efficient process of rainfall generation. The structure of a storm cell is complex and changes as the storm evolves.

Figure 2 Thunderstorm Structure (Bureau of Meteorology)

Orographic rainfall Orographic lifting occurs when air is forced to rise over a mountain range which acts as a barrier to movement of the air mass. As a result, the windward side of a mountain range is usually a region of high precipitation. An example of an area with a significant orographic effect is the Illawarra Escarpment. Fohn winds occur on the downwind side.

Frontal rainfall This form of lifting occurs when a warm air mass rises over a cooler air mass. The boundary between the air masses is referred to as a frontal surface.

Convergent When an area of low pressure forms then denser air will flow into this area and create an upward draft. This has a similar effect to convection whereby the air rises and then is unstable and rainfall forms.

2 2.1

Precipitation measurement Rain gauges

The amount of precipitation is usually measured by a rain gauge. The simplest rain gauge is a daily read gauge (usually read at the same time each day - typically 09:00 in Australia). The gauge consists of an accurately machined funnel which empties into a calibrated flask. A manual reading of the water level in the flask gives a measure of rainfall in the previous 24 hours. An alternative type of gauge is the pluviometer; pluviometers are continuously recording gauges. The tipping bucket rain gauge (or pluviometer) shown in Figure 4.4 records the time at which a bucket of a given size (usually 0.2 or 0.5mm) has been filled by rainfall. Shown in Figure 4.5 is a typical chart output from a pluviograph.

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2.2

Radar measurement

Radar (Radio Detecting and Ranging) and satellite measurement (remote sensing) of precipitation is being used with advances in electronics increasing the viability of this form of measurement. Radar is commonly used for obtaining a visual image of rainfall producing cells (typically thunderstorm cells) and for tracking their movement. Radar is now essential for: •

short-term warning systems - particularly for severe thunderstorms and tornadoes,

•

providing precise information on the movement of hurricanes, and

•

tracking balloon-borne targets which are used to measure the upper atmosphere.

Radar is microwave electromagnetic energy which is produced, typically in pulses, and radiated out from a source. The microwave energy is reflected back from surfaces which have a contrasting dielectric permittivity, and the reflected energy is sensed back at the source area. A knowledge of the radar frequency and the time to receive the reflected energy can be used to estimate the distance to the reflector. The ability of a radar to detect and measure precipitation depends on a number of factors: • •

the characteristics of the radar; the atmospheric conditions between the radar source and the precipitation;

• •

the distance from the radar to the precipitation; and the nature of the precipitation.

The detection of smaller objects requires higher frequency EM energy. Ten millimeter radiation is required for the detection of cloud droplets, but 30mm to 100mm is required for precipitation. The strength of the return beam is a function of the frequency and the distance to the reflector. A typical weather radar will have a limited range of typically 200 km to 300 km. The distance will depend upon the location of the radar and the topography in the area. The strength of the radar echo also depends upon the size, concentration, shape and state (ice or liquid) of the precipitation particles. The strength of the echoed from relatively small ice particles containing no liquid water is approximately one fifth of that from an equivalent mass of liquid water drops. If ice particles fall into warmer air and melt on the outside, the strength of the echoed will increase markedly.

2.3

Snowfall measurement

The measurement of snow fall in a standard rain gauge is subject to considerable error due to turbulence around the gauge and the tendency for snow to accumulate in drifts. The snow that is caught in the gauge is melted and the water equivalent reported. A water content of 10% of the snow depth provides a rough approximation to the water equivalent, although the density of freshly fallen snow may be much less than this. In the northern hemisphere and in mountainous regions, the accumulation of snow on the ground is an important hydrological parameter. In the Snowy Mountains, the accumulation of snow which melts in the spring is the primary source of replenishment for the reservoirs which supply irrigation water to the agricultural areas in the Murray Basin. The same process is common in many other areas of the world and an accurate assessment of snow depth before the spring melt is used by hydrologists to assist with management of water allocation for the following irrigation season. A thick accumulation of snow can also mean a high flood potential, particularly if the snow is caused to melt rapidly by warm rainfall. Melting snow also efficiently recharges soil moisture and underlying aquifers. The slow release of water means that a minimum of water is lost to runoff. Snow surveys are made periodically through the winter to measure the thickness and density of the accumulated snow. A thin-walled tune with a sharp leading edge is pushed through the snow pack to the ground surface. The tube is then withdrawn and weighed. The weight of the empty tube is subtracted to give

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the mass of the accumulated snow. A snow survey requires measurements along a traverse which are repeated at regular intervals throughout the winter. The Bureau of Meteorology reports a regular snow depth at several locations throughout the Snowy Mountains using automated equipment. A gauge at Spencers Creek, out from Charlotts Pass, provides a standard monitoring point for the Snowy River Catchment. The snow pack survey, when combined with an estimate of the extent of snow cover, is used to assess the water content which is held in storage in the mountains. Melting of the snow pack can only begin when the snow temperature has risen to 0°C. Initial melt water clings to snow granules and is held by surface tension, so that at least 2% to 8% of the snow pack must melt before runoff begins. Lack of access to many mountainous areas means that a measurement of snow depth is not possible. Remote sensing methods are increasingly in use to give an estimate of snow mass. Airborne or space borne radar platforms provide a good estimate. This method makes use of the contrast in dielectric properties between bare ground, water, and snow.

2.4

Minimum gauge densities

The World Meteorological Organisation (WMO) have set minimum densities for rain gauge networks if they are to provide a satisfactory basis for hydrology. These are shown in Table 3.1 after Wiesner (1970). Optimum networks are extensive and should allow accurate prediction of average and extreme precipitation in all areas covered by the network. As rainfall is a relatively easy parameter to measure and extensive long term records exist, much work has been carried out on the statistical analysis of rainfall data and the methods of extrapolation available to cover variation in space and time. Table 1 Recommended gauge densities (WMO)

3

Errors in rainfall

Rainfall forms the main component of hydrological analyses. Accurate measurement of rainfall is crucial for the accuracy of the water balance and subsequent derivation of the catchment outflows. However, significant uncertainties exist in the measurement of rainfall. Some of the common errors associated with rainfall are: 1.

2.

Wind induced errors – strong winds can result in significant reduction of the area of the rainfall trapped by the raingauge. The World Meteorological Organisation states this error can be as high as 30% of the total rainfall during high wind conditions. Tipping bucket errors – The tip of the tipping bucket, while fast, still takes a finite amount of time over which the rainfall is not accounted for. This error is something that can assume significant proportions when the rainfall rate is high. For intensities greater than 200mm/hour, these errors have been reported to exceed 15% of the total rainfall.

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3.

4.

4

Spatial averaging errors – Raingauges require users to assume that the measurements obtained are representative over a larger region than the immediate vicinity of the gauge. Identifying the optimal number of raingauges depends on the type of rainfall received by the catchment. One can expect to need a large number of gauges to adequately capture the rainfall from a short-lived localized convective storm burst, and relatively fewer gauges for a more widespread frontal event. Obstructions/changes in location – Since gauges stay operational for many years at a stretch, it is likely that the area sampled by the gauge change in time because of new construction or obstructions. Such type of errors cause systematic change in rainfall, and are detected using double mass curves.

Double mass curves

In assessing rainfall data, particularly rainfall data over a number of years, it is necessary to check the consistency of the data. This need arises from changes which are rarely, if ever, published but which will influence and amend the records obtained from the gauge. Typical changes which may occur include: •

Changes in the gauge location;

• •

Changes in instrumentation; and Changes in observational procedure.

The usual test for consistency of data records is the Double Mass Curve analysis. This procedure can also be used to test the consistency of other parameters such as evaporation with equal validity. The basis of a double mass analysis is the comparison of a record at one gauge with concurrent records at one or more adjacent gauges. Typically accumulated annual rainfalls are considered, although seasonal, or monthly, accumulated rainfalls may also be considered. The procedure for a double mass analysis is the plotting of the accumulated rainfall at the gauge being tested against the data from surrounding stations; this procedure is shown in Figure 3-3. Where a change of slope occurs, as illustrated in Figure 3-3, a change in the gauge records has occurred. It is not possible to ascertain from this analysis what caused the change in the record; this can only be obtained by investigating in detail the documentation of the gauge and visiting the current gauge location. Records prior to the change can be adjusted so that the total record is then consistent. This adjustment is modification of the records prior to the change by the ratio of the slopes of the two segments. The resultant double mass curve for the modified records should then be a straight line with no defined changes of gradient.

Figure 3 Double mass curve comparing point rainfall to areal averaged rainfall from Silo Datadrill (Ladson, 2008 Figure 2.8)

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5

Streamflow

We need to know how much flow is in rivers and streams to answer the following questions: • • •

Is a flood likely? And if so how big will the flood be? How should water be shared between the agriculture, industry, urban communities and the environment? How much water has been "lost" from the rainfall?

Streamflow gauges help us to answer these questions by measuring the flow (volume per unit time) in a river or stream. In coming weeks we will use records of streamflow to help assess flood risk (Week 6 – Flood Frequency Analysis) as well as considering the interactions between groundwater and surface water (Weeks 10 – 12). To be able to do any of these calculations we need to know how streamflow is measured.

5.1

Streamflow measurements

It is difficult to measure flow in a river due to: • •

The irregular shape of natural rivers. Well defined channels can be more easily analyzed using the principles taught in CVEN3502. Fast moving waters which lead to safety concerns

•

Changes in the shape of the stream over time.

Typical features of a channel are shown in the figure below.

Figure 4 Generalised river channel cross section

There a number of ways of measuring streamflow (which is also called discharge). These are listed here and discussed below. •

Direct measurement – calculating the change in volume per unit time

• •

Measurement at dedicated structures (weirs or gates) Mannings equation

• •

Velocity –area method Stage-discharge measurements using rating curves

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Direct measurement This method involves measuring the time it takes to fill a container of known volume. Alternatively the change in mass of a container can be recorded and this can be converted to volume using the density of water. This method is not practical for streams and rivers but can be used for small scale problems such as household flows. It may also be appropriate for small springs or very small streams. The upper limit for this method is around 50 L/s.

Flow measurement structures This method relies on the standard equations of hydraulics (covered in CVEN2501) through structures such as weirs or sluice gates. Because the shape of the structure is known, then the relationship of the height and flow is known exactly. There are many different designs for hydraulic structures and the choices are based on: •

Rating curve relationships

•

Capacity/cost – installing a weir involves significant cost due to the modifications required in the channel bed.

• •

Available head difference – the hydraulic structure is likely to raise upstream river levels and therefore may lead to flooding Sediment load

•

Fish migration

The hydraulic structures are based on forcing the flow through critical depth, because this is where the stagedischarge relationshi...