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Design of Highway Surface Drainage System

Surface drainage system is most important in Highway engineering. A pavement without proper drainage facilities will not serve for long time. The water or rainfall on road should be collected by side drains which carries the drain water to nearest stream or any water course.

So, prior to the construction of road, the designer should leave required space for providing proper drainage facilities as well as the pavement should also be constructed with minimum camber.

Surface Drainage System Design_engineersdaily.com

Fig 1: Surface drainage system on a highway

Design of Surface Drainage System for Highway

The design of surface drainage system carried by two types of analysis:

Hydrologic analysis
Hydraulic analysis

Hydrologic Analysis of Drainage for Highway

Whenever there is a rainfall, some of the rain water infiltrated into the ground and stored as ground water and some of the portion may evaporate into the atmosphere. Other than these losses, the water left on the surface is called as run off.
The method of estimating the run off is called hydrologic analysis. To estimate the maximum quantity of water expected to reach the drainage system is the main objective of hydrologic analysis. For this, one need to know the factors affecting run off and they are:

Rate of rain fall
Moisture condition
Soil type
Ground cover presence
Topography

Other than the above factors, rain fall intensity, occurrence of storms in that area are to be studied from the old records. Hence, maximum run off can be estimated to build safe surface drainage system. The run off can be calculated by below formula:

Q = C i A_d

Where;
Q = run off (m³/sec)
C = run off coefficient
i = intensity of rain fall (mm/sec)
A_d = area of drainage (m²)

Pavement Drainage Design_engineersdaily.com

Fig 2: Pavement Drainage Design

Run off coefficient “C” is the ratio of run off to the rate of rainfall. So, it is not same for all types of surfaces. It varies for different types of surfaces and its values for different surfaces are as follows:

Type of Surface	Coefficient of run off
Pervious soil surface	0.05 – 0.30
Soil covered with turf	0.30 – 0.55
Impervious soil	0.40 – 0.65
Gravel & WBM roads	0.35 – 0.70
Bituminous & C.C roads	0.80 – 0.90

Fig 3: Types of Surfaces & their Coefficients

If the drainage area contains different surfaces in it then run off coefficient is calculated as:

C = (A₁C₁+A₂C₂+A₃C₃) / (A₁+A₂+A₃ )

Where C₁, C₂, C₃are run off coefficients for different surfaces and A₁, A₂, A₃are their respective areas.
In the Next stage, Intensity of rainfall “i” is to be calculated. To find this, first we need to know the time taken by water to reach drainage inlet from the drainage area. This can be found out from the below graph. This is called as inlet time.

Fig 4: Rainfall Time-period

Now we need to calculate the time required for water to travel from inlet of drainage to the outlet which is called as travel time This is calculated from the velocity allowed in the drainage line and generally it is kept at 0.3 – 1.5 m/sec.After that both times (inlet time and travel time) are added which finally gives us the time of concentration. From this total duration, read the rain fall intensity from the below graph by assuming frequency of rainfall occurrence (say for 5 years, 10 years etc.)

Fig 5: Rainfall Intensity

Lastly area of drainage is calculated by studying on the topographical maps of that region. Hence, the design value of run off “Q” is obtained finally.

Hydrologic Analysis of Drainage for Highway_engineersdaily.com

Fig 6: Hydrologic Analysis of Drainage for Highway

Hydraulic Analysis of Highway Drains

Now comes the second stage hydraulic analysis, in which the dimensions of drainage channels or culverts are designed based on “Q” obtained in the above stage of analysis. Now we have discharge which is designed run off “Q”.
If we know the allowable velocity “V” in the channel, then the area of channel can be calculated from below formula:

Q = A.V

But the allowable velocity is not same for all types of channels. If the channel is lined, then the allowable velocity can be kept at normal. But if the channel is unlined it may cause severe damage to the channel in the form of silting or scouring.
So, the allowable velocity for different cases of unlined materials is as follows:

Soil type	Allowable velocity (m/sec)
Sand or silt	0.30 – 0.50
Loam	0.60 – 0.90
Clay	0.90 – 1.50
Gravel	1.20 – 1.50
Soil with grass	1.50 – 1.80

Fig 7: Soil Types & their Velocities

Hydraulic Analysis of Highway Drains_engineersdaily.com

Fig 8: Hydraulic Analysis of Highway Drains

Now we can find out the area of channel in m2. Next, the longitudinal slope of channel “S” is to be calculated by Manning’s formula:

V=1∕n R⅔S½

Where;
V = Allowable velocity (m/sec)
N = Manning’s roughness coefficient
R = Hydraulic radius (m)
S= Longitudinal slope of channel
In the above formula, we already know the “V” value. Hydraulic radius “R” is the ratio of area of the channel to its wetted perimeter. Now comes, the roughness coefficient which is again varies according to lining material as follows:

Lining material	Manning’s roughness coefficient, n
Ordinary soil	0.02
Soil with grass layer	0.05 – 0.10
Concrete lining	0.013
Rubble lining	0.04

Fig 9: Rough Coefficients

Finally, longitudinal slope “S” is known and all the dimensions of drainage channel are known. Thus, the design of surface drainage system is complete. This method is mostly used for designing side drains of roads.

Analytical Review of Pakistan Flood 2010 by Raza M. Farrukh

9:24 PM Civil Engineering , Hydrology , Irrigation Engineering

Analytic Review of Pakistan Flood 2010 by Raza M. Farrukh

Pakistan observed one of the worst floods in its history during July‐August 2010, which caused wide spread devastation. The floods caused by unprecedented rains in many of the catchments of major and small rivers washed away large number of public and private infrastructure in the northern parts of the country and inundated millions of hectares of land, thousands of human settlements, large network of communication infrastructure, and irrigation and drainage system in the centre and south, resulting in displacements of about 20 million people across the country.

The flood water originating in the northern steeper valleys gushed through Indus, Swat, Kabul rivers and many small hill torrents with extremely high velocities washing away almost everything on its way before spreading over millions of hectares in relatively flatter plains. The high intensity instantaneous flood peaks at various locations on these rivers surpassed historic peaks recorded in the past. Peak discharge recorded in Indus river upstream Tarbela reservoir, Swat river at Amandar and Munda, and Kabul river at Nowshehra was 60‐200 percent higher than the historic maximum.

The flashy floods in Khyber Pakhtunkhwah (KP) Province gave no response time and caused maximum human casualties. In other provinces, the Indus river carrying cumulative flows of all rivers got breached at seven locations, four in Sindh and three in Punjab.Six of these breaches were reportedly caused by high water pressure and one in Punjab was made by the provincial authorities at designated locations in the Right Marginal Bund (RMB) of Jinnah barrage. Water pressure breached Left Guide Bund (LGB) of Jinnah barrage, Left Marginal Bund (LMB) of Taunsa barrage, LMB of Guddu barrage, Tori bund between Guddu and Sukkur barrage on right bank, SM bund downstream Kotri Barrage on left bank, and PB bund downstream Kotri barrage on the right bank. These breaches occurred while flood water was still below their top levels. The damages incurred in Punjab, Sindh, and Balochistan resulted mainly from breaching of these bunds.

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Land Based Hydrological Measurements

12:55 PM Civil Engineering , Hydrology

Hydrological measurements are used to obtain data on hydrological processes. Academic research and practical engineering projects all depend on the hydrological data to calibrate and validate the relevant models.

1. Basic Terms

Hydrological processes vary in time and space. Although they are continuous in time and space, they are usually measured at point samples. The following information is relevant to hydrological measurements.

1.1 Time series

A time series is a sequence of data points, measured typically at successive times, spaced at (often uniform) time intervals. For example, the rainfall measured by a rain gauge at a specific location is a time series.

1.2 Time domain

Time domain refers to the analysis of hydrological time series with respect to time. A time domain graph shows how a hydrological process changes over time. It uses tools such as auto-correlation and cross-correlation analysis.

1.3 Frequency domain

A frequency domain graph shows how much of the time series lies within each given frequency band over a range of frequencies. The frequency tools include spectral analysis and wavelet analysis.

1.4 Spatial data

Spatial data have some form of spatial or geographical reference that enables them to be located in two or three dimensional space (such as remote sensed images). Spatial data are often accessed, manipulated or analyzed through Geographic Information Systems (GIS).

1.5 Spatial time series

It is a collection of time series with spatial or geographical references. For example, the data from a network of rain gauges are a typical spatial time series.

1.6 Aliasing

It is an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled. In such a case, distortions will occur when the signal reconstructed from samples is different from the original continuous signal.

Figure : 1 Signal aliasing

1.7 Nyquist frequency

A perfect reconstruction of a signal is possible when the sampling frequency is greater than twice the maximum frequency of the signal being sampled. For example, if a signal has an upper band limit of 100 Hz, a sampling frequency greater than 200 Hz will avoid aliasing and allow theoretically perfect reconstruction. If a lower sampling rate has to be used, an anti-aliasing filter should be used to prevent aliasing.

2. Land Based Measurements

2.1 Rain gauge

Rainfall is recorded by two types of gauges: a nonrecording gauge is simply a container to store rain water. They are read manually at long time intervals (daily, weekly, etc.). In contrast, recording gauges automatically record the depth of rainfall with a high temporal resolution (available 15 minutes or hourly). A recording gauge has various ways of measuring rainfall intensity (tipping bucket, float, weighing, optical, etc.). The tipping bucket gauge is the most widely used rain gauge by the water industry due to its low cost and high reliability.

A rain gauge is more accurate than other rainfall measurement devices (e.g., weather radar and satellite), but it can only measure rainfall at a specific location, and its quality can be affected by wind, fallen tree leaves, etc.

Figure : 2 A non recording gauge (left) and a recording gauge (tipping bucket)

2.2 Snow pillow

A snow pillow measures the water equivalent of the snow pack based on snow pressure on a plastic pillow.

2.3 Evaporation pan

An evaporation pan is used to hold water during observations for the determination of evaporation at a given location.

Figure :3 A river weir

2.4 Lysimeter

A lysimeter is used to measure evapotranspiration and made with a tank of soil in which vegetation is planted to emulate the surrounding ground cover. The amount of evapotranspiration is measured by water weight balance from the water input and output of the tank.

2.5 River weir/flume

The discharge in a river (small to medium sizes) can be measured by a weir or flume (Figure 3). The water depth upstream of the weir/flume is measured and the discharge can be derived from the energy equation. For large rivers, water levels are measured and discharges are derived from the calibrated stage discharge rating curves.

2.6 Soil moisture sensors

Soil moisture sensors measure water content in soil. There are three commonly used soil moisture sensors: capacitance sensor, tensiometer and neutron probe. All the sensors need to be calibrated for different soil types.

a) A tensiometer provides a direct measure of the tension at which water is held in soil. This instrument comprises a water filled tube which is sealed at one end, with a porous ceramic filter at the other end. When buried in soil, it allows water to flow freely through it, but not air. The suction of the water within the tube provides a direct measure of the suction pressure in the surrounding soil. With the suction pressure and soil moisture content curve, soil moisture can then be derived.

b) A capacitance sensor uses capacitance to measure the soil water content. It is a simple sensor made from two plates and the capacitance between them is measured to derive the soil water content.

c) A neutron moisture meter consists of two main components, a probe and a gauge. The probe is inserted in a hole in the ground and it emits fast neutrons. The emitted neutrons are slowed down and reflected by the water molecules in the surrounding soil. The gauge monitors the flux of the slow neutrons scattered by the soil. The degree of reflection is proportional to the soil moisture content. The operator of a neutron probe needs nuclear safety training.

2.7 Infiltrometer

Infiltrometer is a device used to measure the rate of water infiltration into soil or other porous media. Commonly used infiltrometers are of single ring or double rings. It is easy to use, but the soil structure is usually disturbed.

2.8 Radiation sensors

Solar radiation is shortwave and the earth’s radiation is longwave (infrared) and they are measured by different devices. Pyranometer (also called solarimeter) is used to measure solar radiation on a planar surface. The solar radiation is absorbed by a blackbody thermopile and the temperature difference between the metal in the radiation and the one under the shade represents the solar radiation intensity. A plastic dome is used to block longwave radiation so that only shortwave radiation is measured.

Pyrgeometer is a device that measures infrared radiation. Its working mechanism is similar to pyranometer except its plastic shield blocks shortwave radiation. Net radiometer is used to measure net radiation at the earth's surface (incoming radiation minus outgoing radiation). Two radiation sensors (one upward facing and one downward facing) are needed to derive net radiation. If both net radiations for shortwave and longwave are needed, four sensors will be required.

A sunshine recorder is originally made with a glass sphere filled with water and later on with a solid glass sphere. When the sphere burns, it records a trace on the recorder cards attached to it, the length of which shows the duration of bright sunshine.

2.9 Anemometer

Anemometer is a weather instrument that measures wind speed. The most widely used anemometer consists of three or four cups that spin according to the speed of the wind. Modern ultrasonic anemometers are able to measure wind speed in three dimensions.

2.10 Air Temperature

Thermometers placed in a Stevenson screen are used to measure the ordinary, maximum/minimum air temperatures.

2.11 Hygrometer

Hygrometers are used for measuring relative humidity. Old style hygrometers with wet and dry bulb thermometers are called psychrometer. Evaporation from the wet bulb lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer. The dryer the air, the larger the temperature difference will be. A psychrometer depends on the accuracy of its thermometers. If one or both of the thermometers is off, large errors will occur. Modern electronic hygrometers use the changes in electrical resistance due to temperature condensation, and changes in electrical capacitance to measure humidity changes. Electronic hygrometers are extremely accurate and can continuously and automatically record relative humidity.

2.12 Barometer

A barometer is an instrument used to measure atmospheric pressure. There are various types based on air, water or mercury. Italian physicist Torricelli invented the first mercury barometer with a tube of 1m long filled with mercury. A more widely used barometer is called aneroid barometer which is made of a small, flexible alloy metal cell. Small changes in external air pressure cause the cell to expand or contract so that the attached mechanical levers can amplify the tiny movements of the capsule for visual display.

Figure :4 Weather radar, satellite and rain gauge

2.13 Weather radar

In contrast to a rain gauge that is a ground based measurement, weather radar measures rainfall well above the ground (Figure 4). Weather radars send directional pulses of microwave radiation. Between each pulse, the radar serves as a receiver and listens for return signals from rainfall drops in the air. Return echoes, called reflectivity, are analyzed for their intensities in order to establish the precipitation rate in the scanned volume. Weather radars can cover large areas and are able to observe precipitation over the sea. Several radars can be combined to provide a composite rainfall image (Figure 5). There are several error sources in weather radar measurements. Radar pulses spread out as they move away from the radar station, decreasing resolution at far distances. Radar beams may also suffer from attenuation, shielding, anomalous propagation, brightband, etc.

Figure : 5 A composite weather radar rainfall image

The Hydrologic Cycle

3:21 AM Civil Engineering , Hydrology

The circulation and conservation of earth’s water is called hydrologic cycle

It describes the continuous movement of water above or below the earth surface.Many processes work together to keep Earth's water moving in a cycle.It is a complex combination of following eight processes that make up the hydrologic cycle:

1. Evaporation

2. Condensation

3. Transportation

4. Precipitation

5. Infiltration (percolation),

6. Transpiration

7. Runoff.

8. Evapotranspiration

These occur simultaneously and, except for precipitation, continuously. 75 % of the earth is covered with water. 97.25 % of earth’s water is in the oceans.

Factors affecting the hydrologic cycle

Temperature.

Temperature plays direct affect and caused evaporation.

Infiltration rate.

If infiltration rate is high then hydrologic cycle will slows down.

Utilization of water.

Utilization of water by plants and animals also slows down the hydrologic cycle.

1.Evaporation

“Evaporation is the transfer of water from a liquid to a gas from the surface to the atmosphere”. Evaporation is when the sun heats up water in rivers or lakes or the ocean and turns it into vapour or steam which rises in to the air.

Approximately 80 percent of all evaporation is from the oceans, with the remaining 20 percent coming from inland water. Winds transport the evaporated water around the globe, influencing the humidity of the air throughout the world.

Factors affecting the rate of evaporation

Concentration of substances in air.

If the concentration of other substances (salt crystals, water vapours) increases in the air, the evaporation rate will decrease and the substances decreases the evaporation increases.

Intermolecular forces.

If the inter molecular forces are strong and then rate of evaporation will low due to strong bonding in water molecules and if these forces are weak the evaporation will be high.

Surface area.

If the surface area is large the rate of evaporation will high and if area is small the rate of evaporation will be low.

Temperature affect

If the temperature is high the evaporation will be high and if the temperature is low the rate of evaporation will be low.

2. Condensation (the opposite of Evaporation)

“Condensation is the change of water from its gaseous form (water vapor) into liquid water.”

Water vapour in the air gets cold and changes back into liquid, forming clouds. This is called condensation.

Condensation generally occurs in the atmosphere when warm air rises, cools, and loses its capacity to hold water vapor. As a result, excess water vapor condenses to form cloud droplets. The upward motions that generate clouds can be produced by convection in unstable air, convergence associated with cyclones, lifting of air by fronts, and lifting over elevated topography such as mountains.

3. Transportation

In the hydrologic cycle, “transport is the movement of water through the atmosphere, specifically from over the oceans to over land”. Some of the Earth's moisture transport is visible as clouds, which themselves consist of ice crystals and/or tiny water droplets. Clouds are propelled from one place to another by either the jet stream, surface-based circulations like land and sea breezes, or other mechanisms. However, a typical 1-kilometer thick cloud contains only enough water for a millimeter of rainfall, whereas the amount of moisture in the atmosphere is usually 10 to 50 times greater.

Most water is transported in the form of water vapor, which is actually the third most abundant gas in the atmosphere. Water vapor may be invisible to us, but not to satellites, which are capable of collecting data about the moisture content of the atmosphere.

4. Precipitation

“Water falls to the earth in the form of rain, hail, sleet or snow it is called as presipitation”. Precipitation occurs when so much water has condensed that the air cannot hold it anymore. Precipitation is the primary mechanism for transporting water from the atmosphere to the surface of the Earth. There are several forms of precipitation.

Factors affecting the rate of precipitation

· Precipitation is influenced by geographic location, elevation and aspect. Precipitation increases in areas of high humidity, as well as areas with high elevation.

· Since lifting of air masses is the cause of almost all precipitation, amount and frequency of rain is generally greater on windward side of the mountain. As downslope motion of air results in decease in humidity, thus the opposite sides of barriers usually experience relatively light precipitation. High amount of presipitation is reported at higher elevations.

5. Infiltration

“This refers to water that penetrates into the surface of soil”. Infiltration is controlled by soil texture, vegetation, and soil moisture status etc.

Factors affecting the rate of Infiltration

Soil Texture

Coarse-textured soils with large well-connected pore spaces tend to have higher infiltration rates than fine textured soils. However, coarse-textured soils fill more quickly than fine-textured soils due to a smaller amount of total pore space in a unit volume of soil. Runoff is then generated quicker than one might have with a finer-textured soil.

Soil moisture status
High infiltration rates occur in dry soils, with infiltration slowing as the soil becomes wet.

Soil characteristics:

Some soils, such as clays, absorb less water at a slower rate than sandy soils. Soils absorbing less water result in more runoff overland into streams.

Vegetation
Vegetation also affects infiltration. For instance, infiltration is higher for soils under forest vegetation than bare soils. Tree roots loosen and provide conduits through which water can enter the soil.

Precipitation:

The greatest factor controlling infiltration is the amount and characteristics (intensity, duration, etc.) of precipitation that falls as rain or snow. Precipitation that infiltrates into the ground often seeps into streambeds over an extended period of time, thus a stream will often continue to flow when it hasn't rained for a long time and where there is no direct runoff from recent precipitation.

Land cover:

Some land covers have a great impact on infiltration and rainfall runoff. Vegetation can slow the movement of runoff, allowing more time for it to seep into the ground. Impervious surfaces, such as parking lots, roads, and developments, act as a "fast lane" for rainfall - right into storm drains that drain directly into streams. Agriculture and the tillage of land also changes the infiltration patterns of a landscape. Water that, in natural conditions, infiltrated directly into soil now runs off into streams.

Slope of the land:

Water falling on steeply-sloped land runs off more quickly and infiltrates less than water falling on flat land.

6. Transpiration

“Transpiration is the transfer of water to the atmosphere by plants and vegetation from the leaves and stems of plants”.

It is a process by which plants lose water from their leaves. The water rises in to the air. Plants absorb water through their roots and this water can originate from deep in the soil. (For example, corn plants have roots that are 2.5 meters deep, while some desert plants have roots that extend 20 meters into the ground). Plants pump the water up from the soil to deliver nutrients to their leaves. This pumping is driven by the evaporation of water through small pores called "stomates," which are found on the undersides of leaves. Transpiration accounts for approximately 10 percent of all evaporating water.

Factors affecting the rate of transpiration

Humidity in air.

The transpiration rate decreases with rise in vapors pressure in outer atmosphere and vice versa.
Temperature.

High temperature increase transpiration and also open stomata.

Light.

Light influences transpiration indirectly by increasing the temperature.
Wind.

Wind mostly increases the rate of transpiration but high winds and mechanical shocks causes stomatal closure and reduces the transpiration.

7. Runoff

“It is the movement of water on the surface”.
Rivers, lakes, and streams transport water from land to the oceans. Runoff consists of precipitation that neither evaporates, transpires, nor penetrates the surface to become groundwater. Even the smallest streams are connected to larger rivers that carry billions of gallons of water into oceans worldwide. Too much rainfall can cause excess runoff, or flooding.

Factors affecting the surface runoff

Soil type
Soil moisture
Grade (steepness of ground)
Vegetation
Rate of rain fall.

8. Evapotranspiration

Evapotranspiration is the term used for area where evaporation through water and transpiration through plant’s leaves and/or stems occurs collectively and not measured separately.

About the Author: Kashif Ali Bhatti is a B.S in Geology from University of Sargodha, Pakistan

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