Rainwater catchment and conveyance

By Water Research Commission

A key component of any rainwater harvesting system is collecting rainwater from a catchment surface and conveying it to a tank for storage and future use.

Rainwater harvesting (RWH) systems most often utilise the roof of a house or building for collecting rainwater. It is possible to collect rainwater from other surfaces such as lawns and parking lots, but these catchments are not addressed in this manual due to concerns surrounding the quality of rainwater collected from these surfaces.

This article focuses exclusively on the collection of run-off from roof surfaces, or roof catchments. Once rainwater has been collected from the catchment surface, it must be conveyed to the storage tank by means of a ‘conveyance network.’

The most common method of conveying rainwater is through the use of gravity flow, whereby rainwater is transported to the storage tank without the use of pumps or other means of assistance.

Applicable standards and guidelines

The applicable standards found in the South African National Standards are listed in Table 4.1.

Table 4.1


Alternatively, it may sometimes not be feasible or beneficial to collect rainwater from the entire catchment area due to rainwater quality concerns, location/placement of rainwater storage tank, or for other reasons. These and other issues are discussed further in the design and installation guidelines.

Catchment area

Rooftops

The size of the catchment area or roof determines how much rainwater can be harvested. The area is based on the ‘footprint’ of the roof (Figure 4.1), which is basically the sum of the surface area of the building and the surface area of the roof’s overhang.

Differences in roof shapes do not change a building’s catchment area. Nevertheless, the slope of the roof determines how quickly water will run-off during and after a rainfall event. A steep roof will shed run-off quickly and more easily clean the roof of contamination.

On a roof with gentle slopes, run-off moves more slowly, raising the potential for contamination to remain on the catchment surface

Figure 4.1 Roof footprint

There is a vast array of roof shapes (Figure 4.2) and they tend to vary greatly from region to region. The main factors which influence the shape of roofs are the climate, the materials available for roof structure, and the outer covering. Roof shapes vary from almost flat to steeply pitched. They can be arched or domed; a single flat sheet or a complex arrangement of slopes, gables and hips; or truncated.

Figure 4.2 Various roof shapes

Theoretically, for every square metre of roof catchment area, one litre of rainwater can be captured per millimetre of rainfall (Figure 4.3).

Figure 4.3 Theoretical volume

Equations

The volume of water collected is directly proportional to the catchment area (Figure 4.3). Hence, the larger the catchment area, the greater the quantities of rainwater collected per millimetre of rainfall.

Impervious areas

Rainwater that falls on impervious surfaces such as sidewalks, driveways, parking lots, and streets, can also be channelled to an underground tank.

  1. a) The curve number method

The method was developed by the United States Department of Agriculture Soil Conservation Service (now Natural Resources Conservation Service), from an empirical analysis of run-off from small catchments and hillslope plots (NRCS, 1986).

The major disadvantages of the method are sensitivity of the method to Curve Number (CN) values, fixing the initial abstraction ratio, and lack of clear guidance on how to vary Antecedent Moisture Conditions.

The method is used widely and is accepted in numerous hydrologic studies and models such as the SWAT model (Neitsch et al., 2002).

4.3 4.6

  1. b) Run-off coefficient method

The run-off coefficient (C) is a dimensionless coefficient relating to the amount of run-off to the amount of precipitation received. It is a larger value for areas with low infiltration and high run-off (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land).

4.7 4.8

Catchment material

The roofing material is the outermost layer on the roof of a building, sometimes self-supporting, but generally supported by an underlying structure. A building's roofing material provides shelter from the natural elements.

The outer layer of a roof shows great variation dependent upon availability of material, and the nature of the supporting structure. Those types of roofing material which are commercially available range from natural products such as thatch and slate, to commercially produced products such as tiles and polycarbonate sheeting.

The most common roofing material can be categorised as:

  • Thatch — roofing made of dry vegetation such as straw, water reed, sedge (Cladium mariscus), rushes, or heather. This type of roof is common in both urban and rural South Africa.
  • Shingle or roofing slate — the generic term for a roofing material that is in many overlapping sections, regardless of the nature of the material, are made of various materials such as wood, slate, flagstone, fibre cement (in the past, the fibre in the cement material was asbestos which has been banned for health reason), metal, plastic, and composite material such as asphalt shingles. Ceramic roof tiles, which still dominate in Europe and some parts of Asia, are still usually called tiles.
  • Membrane roofing — a type of roofing system for buildings and tanks. It is used on flat or nearly flat roofs to prevent leaks and move water off the roof. Membrane roofs are most commonly made from synthetic rubber, thermoplastic (PVC or similar material), or modified bitumen. Membrane roofs are most commonly used in commercial application, though they are becoming increasingly more common in residential applications.
  • Metal roofing — a type of roofing system made from metal pieces or tiles. Metals used for roofing are: lead; tin and aluminium; copper; galvanised steel; blend zinc, aluminium and silicon-coated steel; stainless steel, and so on.
  • Concrete or fibre cement, usually reinforced with fibres of some sort. Concrete tiles are made from sand of various grading and cement fibre cement is a composite building and construction material, used mainly in roofing and also façade products because of its strength and durability.
  • Structural concrete roofing are usually used for flat roof constructions of large buildings. The three main categories of structure concrete roof are: precast/prestressed, cast-in-place, and shell.

The type of catchment material used by an RWH affects the proportion of rainfall collected during a rainfall event, defined as the ‘collection efficiency’ from the roof catchment, and the quality of harvested rainwater. In South Africa, most houses are mostly covered with ceramic tiles, concrete tiles, corrugated iron, and thatch. Structural concrete mostly cover administrative buildings. 

  1. a) Run-off coefficient

Theoretically, one litre of run-off can be collected from each millimetre of rainfall falling on a one m2 surface area. In reality, some losses occur following contact with the catchment surface. These losses vary depending on the type of catchment material and the geometry of the roof and should be considered when estimating the amount of rainwater that can be collected and utilised by the RWH system.

These losses can be characterised by:

  • an initial loss factor (in mm of rainfall) due to the absorbency of the catchment material, and continuous losses (in percentage of rainfall) from wind and leaks;
  • the run-off coefficient — a dimensionless value that estimates the portion of rainfall that becomes run-off, also taking into account losses due to spillage, leakage, catchment surface wetting, and evaporation (Singh, 1992).

4.9

  1. b) Rainwater quality

The quality of rainwater run-off from a catchment surface can be affected in two ways. Dirt and debris can collect on the roof surface from direct atmospheric deposition, from overhanging foliage or bird and rodent droppings. Alternatively, the roof material itself can contribute both particulate matter and dissolved chemicals to run-off water.

Conveyance system

Rainfall collected by the catchment is transferred to the rainwater storage tank through the conveyance network (Figure 4.4).

Figure 4.4

Rooftop rainwater harvesting conveyance systems consist of two basic elements:

  • gutter(s) that collect water from the roof; and
  • downspout(s) that channel the water collected by the gutters into the storage tank.

Design and installation of catchment area and conveyance network

Selection of the catchment area

  • Only roof surfaces are recommended;
  • Ensure that your rooftop is suitable for RWH. Corrugated iron or metal rooftops are best for harvesting rainwater, followed by tiled and concrete rooftops. Thatched roofs, green roofs or any roof painted with paint-releasing chemicals are not suitable for practical rainwater harvesting; and
  • Sections of the roof with overhanging foliage or trim where possible should be avoided as much as possible.

To maximise the volume of rainwater collected by the RWH system

  • The catchment surface should be as large as possible;
  • When a roof catchment material can be selected and installed in conjunction with the RWH system, material with run-off coefficient close to 1, such as steel, should be selected to convey rainwater using appropriately sized and sloped components, including gutters, downspouts, and/or conveyance drainage piping; and
  • where possible, combine the surface area of several multiple roof catchments and connect them to a central rainwater storage tank.

Gutters and downspouts

  1. Gutter and downspout materials

For gutter and downspout materials, aluminium or galvanised steel are recommended. Copper, wood, vinyl, and plastic gutter and downspout materials are not recommended.

  1. Gutter slope
  • Slope gutters in the direction of the location of the rainwater storage tank.
  • Ensure a minimum slope of 0.5–2% (the greater the slope, the better) is maintained throughout the gutter length.
  1. Gutter size (SANS 10400-R)
  • Any valley or gutter shall have a cross-sectional area of not less than that given in Table 4.2, for the rainfall region in question.

Table 4.2 Roof valley

Such requirements in respect of any downpipe shall be deemed to be satisfied where the internal cross-sectional area of such downpipe is not less than 100mm2 per 1m2 of roof plan area served by such downpipe, provided that such internal cross-sectional area is not less than 4 400mm2.

  • Areas experiencing intense storm events need wider gutters than areas with less intense rain events. The gutters should be sized so that they adequately move rainwater run-off from a 100-year storm event. A 100-year storm event has a 1% chance of occurring every year and produces rainfall with great intensities.
  1. Location and spacing of downspouts
  • Ultimately, the locations of downspouts depend on the configuration, architectural features, and appearance of the building. Whenever applicable, locate them near the storage tank.
  • Each downspout should drain a maximum of 15m of gutter. However, the very design of the building might prevent the installation of more gutters.
  • Avoid locations where water must flow around a corner to reach a downspout.
  1. Downspout size
  • In general, downspouts come in standard profiles and sizes. These should be suitable for most typical residential roof drainage areas and gutter lengths.
  • The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. Adding additional downspouts will require gutter and downspout volume resulting in smaller gutter/downspouts.

Catchment area

Determine catchment area using Equation 4.2.

If sections of one roof catchment or multiple catchment surfaces are used, the catchment area can be determined by summing the multiple smaller areas.

Plan the layout of the conveyance network

  1. Determine the location of the tank (refer to Chapter 5. Rainwater storage and tank sizing for guidance).
  2. Route downspout(s) and/or conveyance drainage piping into the tank.

Tank connection

Rainwater conveyance drainage piping should enter the tank at a height no lower than the overflow drainage piping, or, ideally, 50mm above the bottom of the overflow drainage pipes entering the tank.

Maintenance of catchment network

  1. The catchment surface should be inspected twice a year:
  • Identify any sources of contamination, including accumulated dirt and debris, presence of overhanging tree branches or other foliage, and/or signs of animal activity (for example bird droppings); and
  • If contaminants are present, these should be removed by cleaning the catchment surface by hosing or sweeping and, if applicable, trimming overhanging tree branches/foliage.
  1. The gutters and downspouts should be inspected twice a year
  • Gutters must be maintained regularly to remove leaves and other debris to keep them from clogging. Gutters that are filled with debris can overflow and soak the foundation, and damage the roof structure. Effective gutter guards that keep debris out but allow water to enter are a good alternative to regular cleaning. Gutter protection devices include: strainers, snap-in metal and plastic gutter guards, filtered gutter guards, stainless steel gutter guards, hinged gutter guards, plastic and metal total gutter covers, and gutter brushes.
  • Regardless of the gutter guard protection used, all gutter systems should be examined for cleaning and repaired twice every year.
  • Another option is to use a closed gutter to ensure that debris and leaves do not enter the gutter. The continuous hanger is a way to protect gutters from clogging and damage while reducing required cleaning to a minimum.
  • Repair and/or replace damaged components to ensure proper rainwater flow and to prevent entry of birds, rodents, or insects into the RWH system.

In the next edition, Plumbing Africa will look at rainwater storage tanks.

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