A retaining wall has to be very stable, which means the construction and installation of the wall have to be done to exacting standards. But those aren't the only factors that go into ensuring the wall stays in place. You have to have very good drainage around the wall if you don't want simple rainwater runoff to destroy the structure, which it can do in a few ways. Drainage throughout the yard needs to be in place and working as soon as that wall is constructed.

Soaked Soil Behind the Wall Weighs a Lot

When water from rain, an irrigation system, or anything else, soaks into the soil held back by the retaining wall, the weight pressing on the wall increases a lot. Hopefully, the wall was engineered to hold back that much pressure. However, even if it was, the soggy soil can press so much against the wall that, when combined with soggy soil at the base of the wall, there is a risk that the wall will start to fail. However, if you have good drainage that allows excess water to flow away from the wall, and for excess moisture in the soil to drain away, then there isn't as much pressure, and the wall is more stable.

Erosion Beneath the Wall Destabilises the Wall

Speaking of soil beneath the wall, if water rushes past the wall as it runs off, it can run off with the topsoil, too. That erodes part of what was keeping the wall stable, as being installed in the ground is another strategy for ensuring the wall stays upright. Drainage has to lead runoff away from the wall and away from the house, not alongside it. And keep in mind that the drainage from the soil behind the wall needs to lead away from the wall, too, and not just dump the water down at the base of the wall.

Poor or No Drainage Drowns Plants

If you have any plants in the soil behind the wall (as many retaining walls also serve as terraces in which people plant flowers and such), poorly draining soil or soil that won't drain lead to plant roots drowning. Too much moisture is not good for plants and prevents oxygen in the soil from reaching the roots. When soil is oversaturated, there's no room for oxygen, so the roots can't take any in. You end up with root rot other fungal problems, and a rather gross-looking wall.

Retaining wall planning should involve plans for drains, ditches, and other ways of directing water away from the wall. The design of the wall itself can also assist with drainage, and the installers can give you more details.