Drainage Engineer Interview Questions UK: 8 Real Questions and What Interviewers Want

Most Drainage Engineer interviews in the UK follow a pattern, and knowing it beats guessing. Below are eight questions from our bank of 12, each with a note on what the interviewer is really after.

Eight Drainage Engineer interview questions, and what the interviewer is thinking

1. How do you design a below-ground drainage system for a housing development, and how do you check it meets adoption standards?

Why they ask: Core drainage design knowledge.

2. What does a drainage engineer do each day, and what qualities does the role need?

Why they ask: Tests understanding of the role.

3. What is the SuDS hierarchy and how does it shape your drainage strategy for a new site?

Why they ask: SuDS is now at the heart of drainage design.

4. How do you calculate the greenfield runoff rate and the attenuation storage volume for a site?

Why they ask: Core hydrology and hydraulic calculation task.

5. What do you check when a development proposes to connect surface water or foul flows to an existing public sewer?

Why they ask: Connection approvals involve key regulatory knowledge.

6. During construction, the contractor discovers the invert level of an existing sewer is 500 millimetres lower than shown on the survey you used for your design. What do you do?

Why they ask: Tests handling of site discrepancies with live design.

7. A planning officer objects to your drainage strategy because it relies on discharge to a watercourse at a rate higher than greenfield. How do you respond?

Why they ask: Tests handling of regulatory challenge.

8. Tell me about a time you worked on a drainage design that had to be revised after a change in site layout.

Why they ask: Design change is routine and tests adaptability.

A sample answer to question 1

How do you design a below-ground drainage system for a housing development, and how do you check it meets adoption standards?

I begin with the site constraints: topography, outfall location and level, existing sewers, ground conditions and any flood risk. I lay out the foul and surface water systems separately, and size foul pipes using the discharge units or Sewers for Adoption 8th edition guidance of 4,000 litres per dwelling per day, with a peak flow factor of six times dry weather flow. I check minimum gradients and velocities, normally 0.75 metres per second self-cleansing, and maximum velocities, and keep depth to invert reasonable. Surface water is modelled in Microdrainage or Causeway Flow using FEH rainfall, with an allowance for climate change and urban creep, and designed so there is no flooding of the system for a 1 in 30 year storm, with exceedance routes for 1 in 100. I follow Sewerage Sector Guidance for adoption, including manhole spacing, pipe materials, easements and cover depths. I then agree details with the water company in a Section 104 application, and check the design with a colleague before submission.

Common mistake to avoid: Ignoring exceedance routes; Not planning for adoption early

Want the rest?

Full model answers, key points and common mistakes for all 12 questions are on the Drainage Engineer interview questions page. The first five are free, no sign-up needed.