“My site has a river running through it, I can direct my surface water into it can’t I?”

Surface water drainage can present a material risk to a developer when considering a site acquisition – getting rid of considerable and unpredictable volumes of water by a means approved by the relevant authorities is often a challenge that requires careful consideration.

Whilst foul water can often be pumped, surface water volumes cannot be reliably estimated and pumping systems are difficult to obtain consent for, costly and ultimately may not be able to cope with exceptional rainfall.

For many sites, a surface water drainage solution is to deliver the surface water into an existing ditch, stream or river running through the land. Current practice has been to design and obtain relevant statutory consents for drainage schemes which mimic greenfield runoff rates and then discharge surface water into the relevant channel. But is this a legitimate approach? Case law indicates that:

• A natural watercourse can exist even if there are occasional periods where it dries up, but a dry channel which is only filled during temporary flooding is not a watercourse (and therefore does not give rise to riparian rights);
• Just because a natural watercourse has been culverted at some stage does not stop it from being a natural watercourse;
• A riparian owner can carry out reasonable drainage operations on its land, but not in a way that increases the water flow by artificial means or affects the water quality; and
• Surface water drainage in accordance with an approved scheme should amount to reasonable drainage operations.

So if you want to rely on riparian rights to drain your development you need to:

• Own a natural watercourse running through or next to your site;
• Design your drainage in a manner which does not increase the flow or affect the quality of water in the watercourse; and
• Obtain the necessary technical consents for your proposed drainage operations.

While it seems it would take a particularly keen downstream objector to mount a serious effort to injunct a consented drainage scheme, that risk remains and of course the problems encountered with site drainage often stem from a developer not owning an adjacent river or stream, not knowing whether it is a natural watercourse or being unsure whether it can carry out physical headwall works in the bank itself.