The key decisions are do you want a swale or a deeper open channel and do you really want to keep the gullies? Swales are normally less than 600 mm deep. However it is not unusual to have a channel (or ditch) in a SuDS design, which
is deeper than a traditional swale, but that may have safety implications when located immediately next to a highway – but usually this is only a consideration during those periods when the water is deep and this timeframe is usually a few hours at most. Depths to invert can be controlled to some extent by using check dams and stepping the invert of the channel.
In the first instance what I’d look to do is lose the road gullies, which are not the most effective of devices (even though we have a rather large amount of them scattered around!) and drain the highway into the open structure. Doing this should allow the invert level of the swale/ditch to be no more than 600mm below road surface level and may also allow the flow to run over a filter strip along the road edge thus enhancing treatment. The filter strip has the added advantage of keeping the slope down into the swale at least a metre away from the road edge. The inlets and sediment/trash control ancillaries are crucial components, but if you are going to go for a SuDS approach, why not maximise all the benefits of doing so? And there will be no more gullies to empty along that stretch of highway.