- STORM DRAINAGE DESIGN AND CONSTRUCTION
Design: The dimensions will depend on:
- Rainfall characteristic: Obtainable from the South African weather service records.
- (We work on the basis of a 1 in 10 year probability of a storm occurring which is heavier than the drain is made to carry).
- Area of catchment (from which the water is drained)
- Soil type of catchment
- Vegetation of catchment
- Slope of catchment
- Shape of catchment (a short squat catchment has a shorter gathering time than a long and narrow catchment)
- Channel material (i.e. soil or rock)
- Gradient of storm drain:
- (This is really part of the design, but usually kept at 1:150. Sometimes it is necessary to steepen the drain (because of obstacles) but then it may be necessary to pave the channel.
- When the above four factors have been determined, tables are consulted to give the exact dimensions of the storm drain required in a particular situation. This involves the use of six tables.
- The design of storm drains is usually completed with the aid of aerial photography.
- As a rough guide the following figures have been extracted from the tables (and metricated), for a catchment of medium vegetation cover, medium slope medium permeability (short and squat in shape).
For a storm drain of gradient 1 in 150, in a sandy clay soil:
Table 1: dimensions of storm water drains
|Area Drained (ha)||Width of Drain||Depth of Drain|
|4||2 meters||0.4 meters|
|10||2.6 meters||0.6 meters|
|20||3 meters||0.9 meters|
|40||3.5 meters||1.1 meters|
|100||8 meters||1.2 meters|
For a storm drain of gradient 1 in 100, the required size for 100 hectares drained would be 5 meters wide and 1,2 meters deep BUT the velocity of flow would be too great in most soils. Therefore the drain would have to be made wider ‐ which would make construction very expensive.
RULES OF THUMB:
- Never drain over 130 hectares with a single storm drain.
- If constructed in sand the drain depth should not exceed 0.4 meters
- If constructed in a clay loam the depth can be up to 1.2 meters
- Relation of size of bank to depth of channel should be
- D = (2 x d) + 0.3 (see diagram on left)
DIAGRAMS TO SHOW THREE DIFFERENT LAYOUT POSSIBILITIES
PEGGING OF STORM DRAINS
Study aerial photos before pegging the drain line. Always peg the drain starting from the discharge (outlet) end, and pegging uphill, preferably at a gradient of 1 in 150. The outlet is the most vulnerable part of a storm drain and a very stable (and preferably level) section of the waterway (or stream or vlei) must be chosen for this outlet point. Pegs’ are placed at say, 20 meter intervals to mark the channel position. (This line could be marked by means of a plough, which is a more permanent marker than pegs).
Extra pegs should be placed to indicate places which might require special digging or building up although this should be avoided.
CONSTRUCTION OF STORM DRAINS
Storm Drains should be pegged and constructed before the contour ridges within the land are built (but usually after the preparation of waterways). This is to ensure that no storm water can run onto the land when the contour ridges are made as this would result in the top contour (and probably all the others) breaking.
In Central Africa, storm drains are usually dug by hand unless special trench digging equipment is available. The soil from the channel is removed and placed on the downhill side to form a bank. The bank should be rounded off and consolidated. In wet areas the bank can serve as a useful raised road. (In some countries the channel is used as a road too.)
The digging of a storm drain should be started at the outlet end so that any rainwater collected by the uncompleted drain is safely discharged outside of the land.
Shape of the channel is important. A rectangular shaped channel can carry more water than a dish‐ shaped channel of the same depth and width (see diagram below). Thus a channel built with a plough and grader (which is dished) should be made slightly wider than a rectangular section if the two are to carry the same flow.
Figure 1: Two different shapes of a storm water drain
2. PEGGING OF MECHANICAL CONSERVATION WORK
Design of conservation works in arable lands has been dealt with in previous lectures.
Proper design on a map or aerial photograph is carried out before pegging of works in the land. Very often the original plan will have to be modified while pegging in the field. This is because of obstructions (e.g. anthills and depressions) which do not show up on the photograph.
The principles of pegging storm drains and contour ridges are:
- To peg from the discharge end upwards;
- To peg the storm drain first;
- To peg the top contours in the land next.
In the case of parallel contour systems, the key contours are pegged first, and the intermediate ones filled in afterwards.
After pegging, the gradient of each contour should be checked (possibly by measuring the contour intervals at the ends of the land).
- The distance between pegs may vary from 10 meters to 30 meters depending on the slope of the land.
- “Straightening out” of the lines of pegs may be done to some extent. However, pegs should not be moved more than 3 meters on a 2% land slope; 2 meters on a 3% land slope; 1 meter on a 5% land slope (See Table 3).
- If pegs are moved more than the above distances, then hand labor must be used to dig out the high spots, or build up the low spots, so caused.
- Positions which may need special attention (i.e. with hand labor) should be marked by means of double pegs.
- The line of pegs should be replaced as soon as possible by a scratch‐mark made by tractor and plough; this will serve as a more permanent mark.
- A second line of pegs (or scratch‐mark) is usually placed 3 meters below the original pegs ‐
to mark the line for the return trip by the tractor and contour constructing implement.
3. GULLEY CONTROL
Cause 1: Very Erodible Soils e.g. mopani “brackish” soils. Control by stocking very lightly, fencing off, avoiding dam building etc. Treatment with gypsum may improve plant cover.
Cause 2: Concentration of flow e.g. in inadequate water ways, at contour outlets, road drains. Control by correct design and proper construction.
Cause 3: Lack of vegetal cover e.g. on dam spillways storm drains etc, use fertilizer liberally (and lime and manure if needed), plant suitable species of grass, cut regularly to induce creeping.
Cause 4: Waterfall Action ‐ the cut‐back effect, which is a main cause of gullying. Probably the most important consideration in gulley control.
The basic principle is to protect the land against the impact of falling water on channel bed, and on back wall.
Always start at the head of a gulley in attempting to control its spread and to repair the damage. Stabilization involved slowing down the flow and establishing good vegetation cover. Examples of possible methods are given hereunder:
Figure 2: Shows examples of methods (1 to 6) one can use to control and prevent erosion in gulley’s.
4. CONSTRUCTION OF WATERWAYS
Natural waterways (depressions in the land) may not require widening or reshaping, but their dimensions should be carefully checked. Avoid destroying stable vegetation cover unnecessarily, but narrow sections may have to be artificially shaped and replanted.
The ideal shape for a grass waterway is that of a very shallow dish. A too deep dish will result in a concentration of water and possible gulleying. (Some farmers prefer to maintain a flat base to their waterways.)
Contour outlets should be staggered where they enter the waterway.
The steps in construction of a waterway using a plough or one way disc are as follows:
- Plough along the edges of the water way area, turning at the ends, and travelling in an anti‐ clockwise direction.
- Move inwards, until the whole waterway area has been “cast”. A furrow will be left in the centre.
- Repeat this process, starting one cut inside the original line of pegs, normally, after 2 repetitions, it will be necessary to allow the soil to settle before continuing.
- Continue with the plough disc or grader blade until the desired shape is obtained.
- Sufficient compost, manure and fertilizer should be incorporated, especially where subsoil is exposed, to encourage grass growth.
Establishment of grass cover is the most essential part. Plant runner grass at close spacing (0,5m x 0,5m) and it should be watered if possible. Fertilize with single Superphosphate and apply nitrogen top dressing.
Paspalum notatum is a good grass for wetter areas (although rather slow at first).
Digitaria Swazilandensis (Swazi finger grass) is suitable in most areas. In drier areas, Eragrostis curvula (weeping love grass) and Chloris gayana (Rhodes grass) have been used successfully. As a temporary measure to obtain a quick cover, Sudan grass or wheat has been used. Panicum coloreaum (Bushman’s Mine Panicum) is also used but may require water.
Avoid star grass or other species of grass which will be a problem in the lands adjacent to waterways.
5. MAINTENANCE OF CONSERVATION WORK
Storm drains should be checked and cleaned out periodically, and the bank built up at low places (e.g. path crossings).
Contours must be maintained mechanically at least every two years but preferably every year as part of the ploughing operation. If contours are ploughed over, their gradient should be checked with a dumpy level (or they should be repegged) before they are rebuilt. Discharge points should be cleaned out every year.
Waterways must not be driven along as roads (although roads can cross them). Fertilize regularly to maintain grass cover. Fence and graze, in a controlled manner to maintain vigor. (They can serve as a useful fertilized pasture.)
Evaluation of how well your conservation system is functioning can best be achieved by visiting the lands during a heavy storm and observing water flowing. After a storm, the extent of wash and silt deposition in contours, drains and waterways can also be a guide to future modifications to your system.
6. CONSTRUCTION OF FARM ROADS
The following are the various categories of roads and the bodies responsible for them:
- Main Roads
- District Roads
- Branch Roads ‐ Department of Transport
- Public Roads
- Some Farm Roads
- Farm Roads ‐ The farm owner
- Communal Areas – Local Municipalities (or Community Development), except for main roads linking two Road Councils which are the responsibility of the Department of Transport.
REQUIREMENTS OF A FARM ROAD
- Provide all weather access to key points on the farm.
- Provide quick and safe access without damaging vehicles.
- Give minimum trouble and expense in maintenance.
- Cause minimum inconvenience to farming operations.
- Be well drained without causing erosion
Farm roads must therefore be well planned, constructed and maintained.
SITING OF FARM ROADS
The number and position of roads on a farm will depend on the farming operations, and on the topography.
There will be some main permanent farm roads in use throughout the year. There will be minor feeder roads leading from the main ones to lands, dips, etc. that will be (used only occasionally). But both types must be passable; to bog down at any point is equally bad whenever or wherever it occurs.
There are three main principles in deciding where to site any road. These are:
- Along a watershed (or crest), or along a secondary crest (or ridge) leading off a main watershed into a valley.
- Straight down a slope and not diagonally across a slope.
- If the road has to cross a slope, construct a storm drain above the road.
The above principles will apply to nearly all road siting exercises. Construction and maintenance of roads is much simplified by adopting these principles.
Position of suitable stream crossings may affect road siting.
Vlei or stream crossings may be necessary to avoid excessive road distances. One has to weigh up the cost of the additional distances, against the cost of the stream crossing (and its maintenance). One stream crossing may cost as much as 5 km of road construction. Here long term planning becomes important. It is often possible to co‐ordinate road siting with water conservation works e.g.: taking a road across a dam wall or combining a weir and road crossing but only if these works are especially designed for this purpose.
Drifts are a common cause of erosion and expense. Careful stone lining is needed, and the stone be firmly keyed in. Protect with concrete if necessary.
FARM ROAD DIMENSIONS AND SHAPE
Will vary with purpose and traffic type and intensity. The smallest farm roads will have a 3 meter crown, while larger roads might be 6 meters in width. Beware of bogging two‐way traffic in the road drains because of too narrow design.
Remember that the total width required for a road (crown and side drains) is about twice the crown width.
Is of great importance that a road is raised above general ground level will shed water quickly, and is easily built and maintained.
Extra soil to raise the road can be most cheaply taken from either side of the road which provides added drainage (main drains).
Figure 3: The diagram below shows the ideal shape of road and main drains. Note that side drains
are dish shaped.
Height above the surrounding land is usually 15 to 30 cm, and is maintained (as necessary) by means of a grader or tractor‐mounted blade.
The side drains (or main drains) are automatically completed with the building of the road. If the road has been sited according to the correct principles, the drains will carry little water. The drains have gently‐sloping sides, and cause no hindrance to tractors, mowers or other machinery.
CROSS‐SECTION OF A FARM ROAD
It is necessary to cut off the flow in the side drains at intervals by means of miter drains which spill the water into the veld or safely into a waterway. Where a road passes through a cultivated land, the miter drains will correspond with the contour ridges (which must be strengthened to take this extra water)
The spacing of miter drains will depend on the road location. As a rough guide miter drains can be placed at 1 metre vertical intervals. However, if erosion is evident in either side of drain or miters, extra drains should be cut.
Miter drains should also be dish shaped (not V‐shaped) and at a gradient of about 1: 50 if possible. They should be side‐spilling where slopes are steep.
Where it is necessary to depart from the crest, more water will collect in the upper side drain and needs to be diverted across (or under) the road at Bolsters or inverts (dips) or culverts can be used for this purpose. Culverts are expensive and must be carefully designed (see table on page 10).
Bolster material should not be taken from the road surface, and should preferably be gravel.
Grassing of drains result in less erosion and less maintenance. Mowing encourages spread of grass.
FARM ROAD CONSTRUCTION
Peg centre line of road.
Peg extreme outer edge of side drains. (Total width 2 x required road width). Use a plough, one way disc, plough and blade, or grader to construct the road.
The implement takes its first cut just to the left of the centre line of pegs, throwing soil inwards, and returns on the other side of the centre line of pegs, doing the same. Repeat the process one width further out, and so on until three‐quarters of total width is covered. Thereafter the process is continued (to outer pegs) at a reducing depth of implement.
The process is then repeated, starting near the centre line again and again, until the desired road width and height is attained, and the side drains are the correct shape. The height of road is regulated largely by the depth of cut.
Some farmers prefer, with a one‐way disc to begin at the outside and to work inwards. A disc harrow or a land plane helps in final road leveling.
Where heavy motor graders are used, the system is the same as that employed for ploughs, but with fewer trips.
The best time to build roads is at the end of the rainy season when there is moisture left in the soil.
Do not reduce the Soil to a powder. Leave it to consolidate (preferably with rain) and return later to complete operations.
FARM ROAD MAINTENANCE
Raised roads built on a crest need a minimum of maintenance, especially if verges are covered with couch grass. Avoid removing all grass cover in drains during maintenance. A well‐built road will require maintenance only once every few years.
Maintenance includes grading the road up, clearing drains out, strengthening bolsters, filling in holes, etc. with gravel.
Teach drivers to use whole road surface evenly and to avoid the appearance of two worn strips which will increase maintenance problems. Ruts must be filled in as soon as possible after they have been noticed.
A gravel surface is not usually necessary where a farm road can be properly raised and drained. Where the soil is very slippery, or where the road cannot be raised, or there is a lot of traffic, gravel may be needed in parts for all‐weather access (e.g. across a vlei).
Suitability of gravel for road surfacing can best be determined by placing a sample on a busy stretch of road and observing the amount of wear. Some forms of laterite make excellent road surfacing material.
COST OF FARM ROAD CONSTRUCTION
Where roads can be made by simply ploughing (i.e. no blasting) and drainage there is no problem, will give the lowest cost per kilometer. Culverts are an added expense, and carting gravel adds greatly to the cost. Soil type and amount of bush clearing will affect the cost.
Money spent on making good roads is easily repaid by ready access to crops when needed, wear and tear saved on equipment, and time and labor saved in travel and repair.
As a guide, road gradients on a farm should not exceed 1 in 20 to ensure that loaded vehicles can pass in wet weather. Some Eastern Districts farm roads are as steep as 1 in 12 but these are often impassable in wet weather. (Land Cruizer tracks are often steeper but these are a serious erosion hazard). The steepest tar roads are about 1 in 6.
An Abney Level (or Dumpy level) is useful for siting roads in steep country.
- Trees, if planted, should be 5 metres from the outside of the drains.
- Use only pneumatic tyred vehicles.
- Keep sleighs off the roads.
- Cattle, if they constantly use a road, can cause rapid wear.
- “Good roads are the sign of a good farmer”.
Table 2: Culvert size according to aspect and size of lands
|Diameter of Culvert||Catchment Area Drained|
|(Meters)||Steep (ha)||Rolling (Hectares)||Flat Country (ha)|
The above figures are intended as a guide only and proper tables or formulae should be consulted for accurate design. The figures are based on:
- Water Velocity of 3 meters/second.
- No upstream head.
- No submerged outlet (i.e. free or raised).
- Maximum rainfall intensity of 100 mm per hour.