The Bridge That Wasn't Consulted
A local authority plans a new footbridge over a river to connect a housing estate to a school, cutting a 1.2 km walk to 300 m. The chosen design is a concrete beam bridge on two piers in the riverbed. The river is a spawning route for a protected fish species, and the flood plain has flooded twice in five years. Residents support the bridge; the environment agency has raised concerns; the cost estimate is £1.8 million with a 15% contingency. A cheaper cable-stayed design with a single pier was rejected early because it was unfamiliar to the council's engineers. Construction is scheduled to begin in eight weeks, before the fish spawning season ends.
1. Identify the decision errors in how the design was chosen and explain their consequences.
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Several errors compound:
- Premature narrowing of options. Rejecting the single-pier design because it was unfamiliar is a familiarity bias, not an engineering judgement. It removes the option that best addresses the river constraint before any evaluation.
- Ignoring a known constraint at design stage. Two piers in a spawning river create an environmental impact that will require mitigation or consent conditions, potentially delaying or stopping the project.
- Schedule pressure overriding evidence. Starting before spawning season ends inverts the proper sequence: environmental clearance should precede construction scheduling, not follow it.
- Optimism in cost and contingency. A 15% contingency on a flood-plain bridge with unresolved environmental conditions is likely inadequate; flood risk and mitigation costs are not yet priced.
Consequence: the project may face legal challenge, redesign after mobilisation, or cost escalation - all avoidable had the options been evaluated against the river constraint first.
2. Recommend how to proceed and what evidence would justify a final decision.
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Proceed by pausing construction and re-running option appraisal against explicit criteria: environmental impact on the spawning route, flood resilience, whole-life cost, constructability, and programme.
Required evidence:
- A hydraulic and flood-risk assessment showing how each design performs in a 1-in-100-year flood, including afflux from piers.
- An ecological assessment of spawning timing and habitat, with a construction window agreed with the regulator.
- A costed comparison of the beam and single-pier designs, including mitigation and delay risk, not just capital cost.
- A peer review by an engineer outside the council to counter familiarity bias.
Recommendation: defer construction past the spawning season, commission the single-pier design comparison, and set a contingency based on the flood and environmental assessment. The decision should be justified against published criteria, with the regulator's conditions recorded, so that the chosen design is defensible rather than merely familiar.