Stormwater drainage on a plot: gullies, channels, discharge

Stormwater drainage on a plot: what it collects

Stormwater drainage on a plot collects rainwater from the roof, terraces, parking and paths and leads it where it bothers no one. In Batumi downpours can be so heavy that cubic metres of water come off the roof of an average house in an hour. Discharged straight onto the apron and lawn, it washes out path sub-bases, gets under the foundation and stands in puddles. Stormwater drainage deals with it before it soaks into the ground, while drainage, described on the plot drainage page, collects what does get into the soil.

What the system consists of

• gullies with silt traps under each downpipe;
• drainage channels with gratings at the drive, gate, thresholds and on hardstanding;
• pipes at a constant fall to the discharge point;
• inspection chambers at bends and junctions;
• a discharge point: city network, roadside ditch, stream or soakaway.

The house gutters are designed together with the stormwater system so that each downpipe goes into its own gully, more on the gutters page. Channels at the drive and parking are laid before paving, as covered on the paving and paths page.

Where to discharge the water

This is the key question, and we answer it before the design. If there is a storm sewer in the street, the connection is agreed with whoever operates it. Rainwater must not go into the foul sewer: the water utility does not allow it, and in a downpour the network overflows. If there is no storm sewer, water is led to a roadside ditch or stream, and on sandy plots to a soakaway. On clay a soakaway hardly works, and we look further for a discharge. The foul sewer is described on the sewerage page.

Rainwater for irrigation

Part of the roof water can be collected in an underground tank and used to water the garden. Batumi has dry weeks in summer, and a store of rainwater takes some of the load off the mains or a well. The tank has an overflow to the stormwater system so excess water goes on in a downpour, and its pump is connected to the irrigation, more on the irrigation page.

Sizing and maintenance

Pipe and channel sizes are calculated from the roof and hardstanding areas and the intensity of local downpours, so they come out larger than in dry regions. The fall is kept constant along the whole length, without sags where silt collects. Pipes under the drive are laid deeper or encased in concrete so vehicles do not crush them. Once a year, before the autumn rains, silt traps and chambers are cleaned and the channel gratings cleared of leaves. Before backfilling we run water through each system from a hose and watch it reach the discharge point: sags and reverse falls show up at once, not in the first downpour.

How much stormwater drainage costs

Stormwater drainage in PVC pipe laid to falls in a trench costs from 125 ₾ per linear metre. A drainage channel with grating on a concrete bed from 160 ₾ per linear metre, a point gully with silt trap from 450 ₾, connecting a downpipe to the stormwater system from 350 ₾. An inspection chamber from 750 ₾, a soakaway or infiltration crates from 2 220 ₾, an underground rainwater tank of 3–5 m³ from 4 500 ₾. The final cost is set after a site survey and an agreed estimate.