What degree of slope requires terracing on a new herbaceous bed?

ian.hayes

I am renovating my slope based garden.  On one side I plan to add a new herbaceous bed  that will be about 15m long by 2m wide. The slope drops about 2m from top to bottom, with a steeper slope at the top and a more gradual slope towards the bottom.

I'm worried about soil run off by rain, and we get a lot of rain here in the Welsh hills.

Should I consider adding some terracing in the herbaceous bed? 

Fairygirl

Could be worth it - perhaps just one section dividing the steepest section from the shallower one. It largely depends on the length of the sloped areas, but also what you intend planting. Some plants are better than others at holding the ground and preventing slippage.
Have you a photo? That will help 

Buttercupdays

My main border is roughly the same size and slope as yours. Leaving it  would make  access more difficult and plants often don't grow upright. I used the stone I dug up from the garden to make 2 mini stone walls, one quite near the top and the other at the bottom, so that the soil surface is more level. It is much easier to move around for weeding or planting and the plants are displayed better. There is no problem with run-off. My wall was only a rough and ready amateur job, but it has stood the test of time and looks good.

CrankyYankee

I agree with @Buttercupdays .  I have a sloped garden and one side is terraced, which makes it much easier to get around the plants.  The other side I just built beds into the hillside and the footing can be tricky, plus things like to roll away from me. 

Lizzie27

I'd terrace it, probably using lightweight modern sleepers and making a narrow pathway at the foot of the sleepers for easier access to both the top and bottom beds.

Fairygirl

If @ian.hayes can offer more info, that will help. Anything affecting the rest of the site can also be a factor in the decisions made - where the border meets the house or a patio, what's next to the border [lawn/steps?]  other buildings, budget etc..
Terracing of some kind is likely to be the ideal solution, but this is where a photo really helps. 

ian.hayes

The bed is question is to the right of the path

Fairygirl

It looks a lot narrower than 2 metres. However, I don't think there's much to worry about there. You could put a couple of barriers similar to the path edging in at a couple of points. Again - what you plant can have an effect too on any excess water. Good shrubs interspersed with herbaceous plants should be fine 
The ground will need a fair bit of organic matter added to it, in order to give any planting a good start. That will also improve the structure of it