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close this bookWater Purification, Distribution and Sewage Disposal for Peace Corps Volunteers (Peace Corps, 1984)
close this folderSection 1: Water supply sources
View the documentOverview:
View the documentBackground information
View the documentEvaluation of sources
View the documentFactors influencing the quality of water
View the documentThe quantity of water
View the documentTypes of sources
View the documentDevelopment of water sources
View the documentThe basic requirements of a water supply
View the documentSelection of the source of supply
View the documentLesson plans

Background information

WATER SUPPLY SOURCES AND TREATMENT

WATER SUPPLY SOURCES

TOPOGRAPHIC MAPPING

In planning a water distribution or sewage disposal system, the layout of the community with respect to any water supply sources must be mapped. When no maps of an area are available, crude sketch maps are sufficient. Such maps should indicate the approximate placement of any man-made structures, livestock grazing areas, water supply sources, and disposal systems.

Topographic Contouring

A topographic map is a means of illustrating, through the use of contour lines, the shape of the ground surface. This exercise involves the determination of ground relief (topography) from points whose elevations above sea level are known.

The method is called "contouring from spot elevations". The data might have been obtained by surveying with a plane table and alidade, although modern topographic maps are made much more easily and accurately by the stereoscopic plotting of airphoto information.

Rules and hints in topographic contouring.

1. All points lying on a contour are of the same elevation above (or below) a reference point. However, one contour need not satisfy all the points of equal elevation; eg. adjacent hilltops of similar relief might require separate, closed contours each showing comparable levels. Some contours may be cut by the edges of the map and appear to be discontinuous but if the map be made large enough every contour eventually closes on itself, becoming continuous.

2. With rare exceptions, the contour interval is constant for the map area and is defined as the vertical distance between successive contours. The contour interval is stated as part of the scale of the map so that the vertical dimension of the contoured surface has identity. Ten-foot, twenty-foot, fifty-foot and 100-foot intervals are common. The interval is selected to best show the shape of the surface at the desired horizontal scale without requiring an unnecessary, unreadable number of lines. The relief of the area to be mapped also influences the choice of the contour interval.

3. Contours do not cross. Such a situation would illustrate an impossible ground surface shape. Contours are closely spaced on steep slopes, and distantly spaced on gentle slopes.

4. Closed depression contours are hachured on the lower side. They are used when all points within the line are below the level of the line. Obviously they are only required to show depressions which are completely surrounded by high ground. Gullies and river valleys are not usually closed on the downstream side and therefore are not illustrated by depression contours. A depression contour takes its value from that of the lowest, topographically adjacent regular contour.

5. In contouring gullies and valleys, the contours vee in the upstream direction. Be careful to confine the stream to the lowest part of its valley by passing the stream through the notch of the vee.


Fig. 1 Contour Lines

Contours are broken where numbering is necessary, to improve readability.

6. The use of some degree of "artistic license" is recommended in contouring. Do not attempt to just satisfy the point data. Try to make the trend of a contour reflect the trend of its neighboring contours.


Fig. 2 Symbols for Topographical Maps


Fig. 3 Contouring from spot elevations

1. Draw and label the contours every 100' (300', 400', 500', etc.)

2. Indicate with dashed line a divide; with dotted line an area of future stream capture. Label the highest point A, the lowest point B.

3. Distinguish areas of high and low stream gradient, indicate direction of flow of streams.

General

In making a crude topographic sketch very little attention should be paid to detail. Scale can be determined by pace, and direction by a compass. Periodic sighting of a distant object in a path and determination of its position relative to à present mark will serve for proper orientation.

Determining Pace

Lay out a one hundred foot interval on level ground, an uphill, and a downhill slope. If only a foot ruler is available, this may be used to mark out three or four feet on a stick, and this stick in turn used to measure the 100 feet. Being careful to work normally, the map maker then determines the number of paces over the 100 foot interval for each slope. By division, it is then possible to find a number of feet in an average pace for uphill, level, and downhill slopes.

Taking Bearings with the Compass

A bearing is the compass direction from one point to another. A bearing is always in a unidirectional sense; for example, if the bearing from A to B is N30W, the bearing from B to A can only be S30E. To read accurate hearings, three things must be done; (l) the compass must be leveled, (2) the point sighted must be centered exactly in the sights, and (3) the needle must be brought to rest.

In a sketch map, contours are used only to show the relative differences in elevation and the nature of the topography. It is unnecessary to know the elevation of a given contour or to connect all of them. Contours can be drawn arbitrarily to indicate that a hill is steep, a stream runs in a given direction, the community is uphill from a water source, etc. A realistic sketch map can he drawn by estimating the relief of an area around a point and then proceeding to a point on the periphery of this area. Plane table mapping may prove more adequate when more detailed maps are required.

Map Making Using a Plane Table

A description is given for the construction of serviceable maps using a plane table. Such maps are valuable for irrigation, drainage and village layout plans.

Tools and materials needed are:

Plane Table
Paper
Pencil
Ruler
Pins
Tape measure (optional)
Spirit level (optional)

The first step is to decide on a scale for the map. This is determined by judging the longest distance to be mapped and the size of the map desired. It should be noted that the map does not have to be made on a single sheet of paper but can be spliced together when completed. As an example, if one wanted a map 2 1/2 feet long to portray an area whose mayor distance is l/2 mile, 2640 feet, then a scale of 100 feet to the inch would be convenient.

Paper should be placed on the plane table and the plane table oriented on or near some principal feature of the map, that is, a path, road, creek street, etc. A pin should then be placed vertically in the spot on the finished map where this location is desired. The plane table should be made level - by use of a spirit level, if available. The table should be rotated to a proper orientation, that is, so that the direction will appear on the finished map in the desired way. Now sight along the first pin to another principal feature which is visible from the table location (a bend in the road, a hill or any feature that will tie the map together), moving the second pin into the line of sight. A ruler may be used for this purpose if it has a sighting edge or even a couple of pins stuck into it. Now draw a line in the direction defined by the two pins. Measure the distance to the feature observed either by pacing or with a tape. Scale this distance along the line drawn, starting at the initial pin. Repeat this process for other principal features which may be seen from this location. When this has been done, move the table to one of the points Just plotted, selecting one which will enable you to move over the territory in a convenient fashion. For example, follow a lane or creek or some feature which ties things together. Set up the plane table over this point and reorient the table. Do this by putting pins into the map at the present and previous locations. Next rotate the table so that the pins line up with the previous location. This procedure in fact locates the line joining the two locations on the map in the same direction as the line exists in nature. Again from this new location map in the desired features which can be conveniently sighted.

In this way the entire region to be mapped may be covered in a systematic way. If gaps appear or if more detail is needed, you may go back and set up over some mapped feature, reorient the map by sighting on a second feature, and proceed to map in the detail.

An alternate procedure may he used in mapping features which are not going to be used as plane table locations in the mapping process. This involves drawing a line in the direction of each feature from two plane table locations. The intersection of these two lines corresponding to a single feature locates the feature on the map. As a result this avoids the necessity for measuring distances. Note, however, that it is impossible to avoid measuring the distances between plane table locations.

If a spirit level is available, it is possible to level the plane table accurately, and using a ruler or other sighting device, relative elevations may be plotted on the map. A stick about six or eight feet long should be marked off in inches, and the person holding the stick vertically can, by moving his finger, identify to the person sighting, the distance up from the ground through which the line of sight passes.

ROCK FORMATIONS AND THEIR WATER-BEARING PROPERTIES

The rocks that form the crust of the earth are divided into three classes:

Igneous - rocks which are derived from the hot magma deep in the earth. They include granite and other coarsely crystalline rocks, dense igneous rocks such as occur in dikes and sills, basalt, and other lava rocks, cinders, tuff, and other fragmental volcanic materials.

Sedimentary - rocks which consist of chemical precipitates and of rock fragments deposited by water, ice, or wind. They include deposits of gravel, sand, silt, clay, and the hardened equivalents of these - conglomerate, sandstone, siltstone, shale, limestone, and deposits of gypsum and salt.

Metamorphic - rocks which are derived from both igneous and sedimentary rocks through considerable alteration by heat and pressure at great depths. They include gneiss, schist, quartzite, slate. and marble.


Fig. 4 Plane table mapping

The pores, joints, and crevices of the rocks in the zone of saturation are generally filled with water. Although the openings in these rocks are usually small, the total amount of water that can be stored in the subsurface reservoirs of the rock formations is large. The most productive aquifers* are deposits of clean, coarse sand and gravel: coarse porous sandstones; cavernous limestones; and broken lava rock. Some limestones, however, are very dense and unproductive. Most of the igneous and metamorphic rocks are hard, dense, and of low permeability.

(* A formation, group of formations, or part of a formation that is water bearing.)

They generally yield small quantities of water. Among the most unproductive formations are the silts and clays. The openings in these materials are too small to yield water, and the formations are structurally too incoherent to maintain large openings under pressure. Compact materials near the surface, with open joints similar to crevices in rock, may yield small amounts of water.

EXAMPLES OF MIXED ROCK

Medium sand with fine gravel, gray

Fine gravel

20%


Coarse sand, gray

30%


Medium sand, gray

40%


Fine sand, gray

10%

Medium gravel with coarse sand, brown

Coarse Gravel, brown

20%


Medium gravel, brown

30%


Fine gravel, brown

20%


Coarse sand, brown

20%


Medium sand, brown

10%

Clay with sand and fine gravel, blue

Fine gravel, gray

5%


Coarse sand, gray

5%


Medium sand, gray

10%


Fine sand, blue

20%


Clay, blue

60%

The nomenclature used in consolidated sedimentary rocks is very similar to that used for the unconsolidated rocks. The following names should be applied to the consolidated equivalent of the eight classes of un-consolidated rocks.

Name of unconsolidated rock

Name of consolidated equivalent

1. Boulders

Boulder conglomerate

2. Coarse gravel

Coarse conglomerate

3. Medium gravel

Medium conglomerate

4. Fine gravel

Fine conglomerate

5. Coarse sand

Coarse sandstone

6. Medium sand

Medium sandstone

7. Fine sand

Fine sandstone

8. Clay

Claystone

TABLE 1: CLASSIFICATION OF UNCONSOLIDATED MATERIALS

Classification

Grade Name

Particle(mm)

Dimensions**(in.)
(** American Geological Institute Data Sheet No. 7)

Name to be Applied in Logging Water Wells


Very large boulders

2048 to 4096

80 to 160



Large boulders

1024 to 2048

40 to 80



Medium boulders

512 to 1024

20 to 40

Boulders


Small boulders

256 to 512

10 to 20



Large cobbles

128 to 256

5 to 10


GRAVEL

Small cobbles

64 to 128

2.5 to 5

Coarse Gravel


Very coarse pebbles

32 to 64

1.3 to 2.5



Coarse pebbles

16 to 32

.6 to 1.3

Medium Gravel


Medium pebbles

8 to 16

.3 to .6



Fine pebbles

4 to 8

.16 to .3

Fine Grave


Very fine pebbles

2 to 4

.08 to .16

l


Very coarse sand

1 to 2


Coarse Sand


Coarse sand

.5 to 1



SAND

Medium sand

.25 to .5


Medium sent


Fine sand

.125 to .25




Very fine sand

.062 to .125


Fine sand


Coarse silt

.031 to .062




Medium silt

.016 to .031




Fine silt

.008 to .016




Very fine silt

.004 to .008



CLAY AND SILT

Coarse clay

.002 to .004


Clay


Medium clay

.001 to .002




Fine clay

.0005 to .001




Very fine clay

00024 to .0005




Fig. 5 Nomenclature of unconsolidated rocks

Sedimentary rocks and volcanic rocks are sometimes interbedded with strata of volcanic ejecta. Some of these strata have been deposited by water and others by direct air fall. The terms "pumice" and "cinders" should be used to describe these materials.

Pumice

A very light, excessively cellular volcanic glass. Its color is generally light gray or white and is often so light that it will float on water.

Cinders

Uncemented glassy and vesicular ejecta from a volcanic cone. Generally black or red in color. In logging a well penetrating into strata of cinders, the cinders should be also described by color and degrees of coarse ness.

MISCELLANEOUS TERMS RELATED TO SEDIMENTARY ROCKS:

Caliche

A hard lime deposit generally found in the soil zone in arid regions. It is usually found in layers ranging from a few inches to a few feet in thickness.

Chalk

A soft, white to gray, fine-grained limestone. Often incorrectly used on well logs to describe diatomite.

Diatomite

A soft white to gray, fine-grained rock composed of the siliceous shells of diatoms.

Dirt

A term used by many well drillers to describe the soil zone. The term "soil" with a descriptive adjective as "gravelly soil" or "sandy soil" is recommended for use on well logs.

Gumbo

A term applied by some well drillers to a soft sticky clay. The term "soft sticky clay" is preferred for use on well logs.

Hardpan

A term that has been applied to many hard impermeable rocks including glacial sill, caliche, conglomerate, claystone, and sandstone. The term should never be used on a well log.

Loess

Wind deposited material composed chiefly of silt but may contain subordinate amounts of very fine sand and clay. Loess should generally be reported as clay on a well log.

Mud

A term used by many well drillers to describe soft clay or silt. The term "soft clay" is preferred for use on well logs.

Quicksand

A term often applied to ''running" or "heaving" sand. The terms "finesand, waterbearing" or medium sand, waterbearing" is preferred for use on well logs.

Shale

A laminated claystone. As laminations are not generally discernible in drill cuttings, the term "claystone.' Is preferred for use on well logs.

Slate

A metamorphic rock possessing a very well developed platy cleavage. The term has been used by many well drillers to describe a ''hard claystone". It is recommended that this term not be used to describe sedimentary rocks but restricted to true slates.

POROSITY AND PERMEABILITY

Porosity is essentially the capacity of a rock or sediment to contain water. It can be measured as the total volume of a material that is void space. Permeability is the capacity of a rock or sediment to transmit water. Permeability is measurable as the quantity of water flowing through a given cross-sectional area per unit time. Permeability is directly proportional to grain size. Thus clay, which has a high porosity, has a very low permeability because it is fine grained.

The following figures indicate the porostiy of common soils and rocks:

Sand and gravels of fairly uniform size and moderately compacted

35% - 40%

Well-graded and compacted sands and gravels

25% - 30%

Sandstone

4% - 30%

Chalk

14% - 45%

Granite, schist, and gneiss

0.02% - 2%

Slate and shale

0.5% - 8%

Limestone

0.5% - 17%

Clay

44% - 47%

Topsoils

37% - 65%

Silts may be as high as 80% porosity In general, soils with fine, separate particles, such as clay, to soil, and silt, have a very high porosity. In other words, they have a big volume in which water can be stored.