Building - Next, look for natural drainage

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Water Runs Downhill

Next, look for natural drainage patterns when investigating potential building sites. If you take a close look at the topographical map of the building sites below, some of the lots have low spots in the middle, probably about where a house would sit. Having water coming downhill toward your home’s foundation from all directions, even with the slightest slope, is not the preferred drainage plan and can lead to lots of costly problems in the future. If this is your situation, it will require designing alternative drainage routes and installing stormwater management controls.

We’ll discuss the importance of managing storm- water onsite. For now you just need to make certain that you are selecting a building site where it will be possible to establish positive water drainage away from your foundation. Verify that the site does not have  flooding or access issues, or is not in a valley shaped like a bowl, making it impossible to manage stormwater drainage, or at the least, very expensive.

To take this concept a little further, though, it’s important to note that water also runs down a roof and down the wall and basement assemblies too. So in designing the home, make sure you establish a drainage pattern that provides a path to get water o  and away from your home in rain and wind-driven storm events.  inking ahead at this early point in the process will pay o  many times over later in money saved on basement and foundation water management and on maintenance to repair water damage to your home.

Your site management plan should include effective temporary (i.e., during construction) erosion controls and stormwater management. You don’t want to lose your valuable topsoil that is ex- posed during the construction of the home. Silt fencing has long been recognized by building codes for meeting temporary erosion control standards, but other more natural materials work as well. Many green builders use straw bales to line the lot.

Laying Drains.

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Ground water (land) drains are laid in trenches at depths of 0.6 and 0.9 m in heavy soils and 0.9 to 1.2 m in light soils. The nominal bore of the pipes is usually 75 and 100 mm for main drains and 65 or 75 mm for branches.

The drain pipes are laid in the bed of the drain trench and surrounded with clinker, gravel or broken pervious rubble which is covered with inverted turf, brushwood or straw to separate the back fill from the pipes and their surround. Excavated material is back- filled into the drain trench up to the natural ground level.

The drain trench bottom may be shaped to take and contain the pipe or finished with a flat bed as illustrated in Fig. 22, depending on the nature of the subsoil and convenience in using a shaping tool. 

Fig. 22 Land drains.

Where drains are laid to collect mainly surface water the trenches are filled with clinker, gravel or broken rubble to drain water either to a drain or without a drain as illustrated in Fig. 23 in the form known as a French drain. Whichever is used will depend on the anticipated volume of water and the economy of dispensing with drainpipes. 

Fig. 23 (A) Surface water drain (B) French drain.

Drains - Natural system, Herring bone system, Grid system, Fan system.

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Natural system.-

This system, which is commonly used for field drains, uses the natural contours of the ground to improve run off of surface ground water to spine drains in natural valleys that fall towards ditches or streams. The drains are laid in irregular patterns to follow the natural contours as illustrated in Fig. 19A.

Fig 19 (A) Natural system. (B) Herring bone system.

Herring bone system.-

In this system, illustrated in Fig. l9B, fairly regular runs of drains connect to spine drains that connect to a ditch or main drain. This system is suited to shallow, mainly one way slopes that fall naturally towards a ditch or main drain and can be laid to a reasonably regular pattern to provide a broad area of drainage.

Grid system.-

This is an alternative to the herring bone system for draining one way slopes where branch drains are fed by short branches that fall towards a ditch or main drain, as illustrated in Fig. 20A. This system may be preferred to the herring bone system, where the run off is moderate, because there are fewer drain connections that may become blocked. 

Fig. 20 (A) Grid system. (B) Fan systems.

Fan system-

A fan shaped layout of short branches, illustrated in Fig. 20B, drains to spine drains that fan towards a soakaway, ditch or drain on narrow sites. A similar system is also used to drain the partially purified outflow from a septic tank, (see Volume 5), to an area of subsoil where further purification will be effected.
On sloping building sites on impervious soil where an existing system of land drains is already laid and where a new system is laid to prevent flooding a moat or cut off system is used around the new building to isolate it from general land drains, as illustrated in Fig. 21.

The moat or cut off system of drains is laid some distance from and around the new building to drain the ground between it and the new building and to carry water from the diverted land drains down the slope of the site. Plainly the moat drains should be clear of paved areas around the house.