Three mechanisms, and why the distinction matters
Separating them is the first useful step on any Hull site.
- Tidal risk from the Humber. The one people picture, and the one the defences exist for. It is a defended risk rather than an absent one, and the residual case matters for anything that has to keep operating.
- Fluvial risk from the River Hull and the drains feeding it, which behaves differently again and is influenced by what the pumps and the tidal cycle are doing at the time.
- Surface water flooding. This is the mechanism that caused the city's worst recent event in June 2007, it is a function of how flat the ground is and how much of the drainage is pumped rather than gravity-fed, and it is the one that gets planned for least.
Why flat, pumped ground makes drainage design harder
On a site with fall, drainage is largely a geometry problem. On Hull's ground it is a capacity and outfall problem, because there is very little fall to work with and the receiving system may be constrained or pumped.
That has consequences a designer has to accept rather than wish away: shallower gradients with less margin for construction error, attenuation that has to be genuinely sized rather than nominal, and outfall agreements that need establishing early because they can determine whether a layout works at all.
It is also why a flood risk assessment and a drainage strategy should be written by the same people. The levels, the attenuation and the outfall are one design, and two consultants producing two documents that do not quite agree is a reliable way to generate a planning query and lose a month.
Floor levels: the decision everything else follows
On a flood-exposed site the finished floor level is the single most consequential structural and planning decision, and it is made early whether or not anybody realises they are making it.
Raising floor levels reduces flood exposure and costs something in every other direction: access and level thresholds, ramp lengths, the relationship to the street and to neighbouring buildings, drainage falls to the outfall, and on soft ground the additional fill loading the compressible material underneath - which brings its own settlement question.
That last point is the specifically Hull version of the problem. Raising a level with imported fill on soft alluvium is not free structurally; it is a load. It belongs in the foundation design conversation rather than being treated as a landscaping decision.
Flood-resilient construction, honestly assessed
Two different strategies get run together under the word resilience, and they suit different situations.
Resistance means keeping water out - barriers, sealed thresholds, non-return valves on drainage, and construction detailed to limit ingress. It works up to a depth and a duration, and beyond that it can make things worse, because water held out is also water pushing against a structure that may not be designed for the pressure.
Recoverability means accepting water in and designing so the building dries and returns to use quickly - solid floors rather than suspended timber, closed-cell insulation, services and sockets raised, sacrificial finishes below a defined level, and materials chosen to be washed down rather than replaced.
For most properties in a genuinely exposed location, recoverability is the more realistic strategy. It is also the less marketed one, because there is less to sell.
After a flood: what is structural and what is not
Most flood damage is not structural, and it is worth knowing that early because the drying and reinstatement industry moves quickly.
What is worth an engineer's eye: any building where water stood against a wall to significant depth and then dropped rapidly, timber floors and joist ends that were submerged, walls that show new cracking after the event, and any structure where the ground around it was scoured or where a drain or culvert failed during the flood. Washout beneath a foundation is a real mechanism and it does not announce itself while everything is wet.
A structural survey and report is a fixed £580 + VAT with the report inside 10 working days of the inspection, guaranteed. If you are dealing with an insurer, tell us at the enquiry stage - it is worth knowing where an independent report sits in that process before it is commissioned.
And the ground underneath all of it
None of this can be separated from what Hull is built on. Soft alluvial clays and silts many metres thick, over glacial deposits with Chalk at depth, close to sea level and with a high water table.
Water and soft ground interact: groundwater affects excavation and what can be built below ground, raised levels load compressible material, and a leaking or surcharged drain washing fines out from beneath a foundation is both a flood consequence and a settlement cause. On a Hull site they are one problem, and we would rather see a trial hole than infer anything from a map.
Frequently asked questions
What kind of flooding actually affects Hull?
Three kinds, and they are different problems. Tidal risk from the Humber, which is defended; fluvial risk from the River Hull and its drains; and surface water flooding, which caused the city's worst recent event in June 2007 and is a function of flat ground and pumped drainage. The third is the one people plan for least.
Is raising the floor level always the answer?
It reduces flood exposure and it costs something in every other direction - access and thresholds, drainage falls to a constrained outfall, and on soft ground the additional fill is itself a load on compressible material. On Hull's ground that last point is a foundation question, not a landscaping one.
Should I try to keep water out or plan to dry out?
For most properties in a genuinely exposed location, planning to dry out and recover quickly is the more realistic strategy - solid floors, raised services, sacrificial finishes and washable materials. Resistance works up to a depth and duration, and beyond that water held out is water pushing against a structure that may not be designed for the pressure.
My house flooded. Is the structure damaged?
Usually not, and most flood damage is not structural. What is worth an engineer's eye is where water stood deep against a wall and dropped quickly, where timber floors and joist ends were submerged, where new cracking has appeared since, and where the ground was scoured or a drain failed - washout beneath a foundation does not announce itself while everything is still wet.
Do you prepare flood risk assessments and drainage strategies together?
Yes, and on this ground they should be. The levels, the attenuation and the outfall are one design, and on flat pumped ground with a constrained outfall there is very little margin. Splitting them between two consultants is a reliable way to generate a planning query.