There is a sequencing habit on many sites in the region: get the structure up, get it roofed, and deal with the external works at the end when the money is clearer. On a flat site with light rainfall you can sometimes get away with it. On a sloping site in Uganda or South Sudan, it is how projects lose months.
Where the water comes from
A site does not only receive the rain that falls on it. It receives everything shed by the ground uphill of it. On a 1-hectare catchment above your plot, an hour of heavy rain delivers a volume that will not politely stop at your boundary.
The first thing we do on a sloping site is walk the catchment above it and work out where that water currently goes. Frequently it goes exactly where the client wants to put the building, because that is the natural low line — which is also why the ground there is flat and looked attractive.
What goes wrong when drainage comes last
Excavations fill. A pad excavation open through a storm becomes a pond. The sides slump, the bearing stratum you carefully exposed turns to slurry, and you are excavating it again. On expansive clay, saturating the formation before you pour is worse than a delay — you have changed the soil you designed for.
Access dies. Concrete trucks and material deliveries need a haul route that survives rain. Without cut-off drains and a culverted crossing, the haul route becomes the drainage channel, and then it becomes impassable. We have watched sites lose three weeks to a road they could have culverted in two days.
Backfill washes out. Newly placed backfill around foundations has no vegetation and no compaction history. Concentrated runoff carves channels through it, undermines the apron, and leaves voids you cannot see until the slab cracks.
Late retrofits are more expensive. Digging a drainage line beside a finished building means working by hand next to a live foundation, and often means breaking up paving that was laid three weeks earlier.
Our sequence
- Survey and catchment assessment. Levels across the plot and for a reasonable distance uphill, plus a note of where neighbours discharge.
- Cut-off drains at the high boundary. An interceptor drain along the uphill edge takes the catchment flow around the works rather than through them. This is the highest-value item on the whole external-works budget and it goes in first.
- Culverts at crossing points. Precast concrete box culverts or pipe culverts wherever the haul route crosses a flow line, bedded on compacted granular material with proper headwalls. Undersized culverts are a false economy — we size for the catchment, not for the pipe that happens to be in the yard.
- Site levels and temporary falls. The working platform is graded so that water leaves it. Ponding on a working area is not a cosmetic issue; it is a compaction issue.
- Then substructure. With the water under control, excavations stay open safely and formations stay intact.
- Permanent drainage and aprons as the superstructure rises, tied into the temporary system that is already working.
Sizing, briefly
We size drainage on a rational method calculation using local rainfall intensity for a 1-in-10-year event on general works, and 1-in-25 where a failure would flood a building or cut the only access. Runoff coefficients matter: a compacted murram haul road sheds almost as much as concrete does, so a site gets progressively less absorbent as construction proceeds. Designing for the greenfield condition is a common and costly mistake.
What the client sees
Money spent on drainage in month one looks like money not spent on the building. It is a difficult conversation, and we have it on every sloping site, because the alternative conversation — explaining a flooded excavation in month four — is much harder.
OK Estate Construction and Supply Limited handles site works, drainage and civil infrastructure alongside building construction in Uganda and South Sudan. Contact us to discuss your site.