Skip to content
flowss
Start freeChecking your session
Bowtie risk templates

Bowtie — Embankment Dam Overtopping

Reservoir safety bowtie for 18 million cubic metres impounded above a valley town, computing a 0.00641/yr overtopping frequency from three flood threats and ranking the annual scour-valve exercise above the spillway design itself.

Bowtie riskScientific PlotsBowtie risk syntax docs
Template previewBowtie risk
Embankment dam overtopping in an extreme floodHAZARD18 million cubic metres impounded above a valley t…THREATSPREVENTIVE BARRIERSRECOVERY BARRIERSCONSEQUENCESSpillway sized to the 1 in10000 year inflow90%Auxiliary fuse plugspillway on the leftabutment70% · mitigationBoom anchorage not inspectedsince the last floodPost-flood inspectionmandated within 7 daysDebris boom and trashscreen upstream of theweir65%Catchment tree managementnear the inlet45%Generator fuel supply contractlapsedAnnual full-strokeexercise of the scourvalve80% · detectionStandby generator for thevalve actuator75%Wave wall anderosion-resistantdownstream face55% · mitigationFlood warning giving 90minutes to evacuate70% · recoveryInterconnector from theneighbouring zone60% · recoverySection 10 inspection withsafety measures40%Reservoir emergency planrehearsed with the LRF35% · recoveryProbable maximumflood exceedsspillway capacity0.002/yr → 6.0e-5/yrSpillway blocked bytrees and debris inthe flood0.02/yr → 0.00385/yrDraw-off valve failsto open for flooddrawdown0.05/yr → 0.0025/yrBreach and inundationof the towndownstreamsev 5 · risk 0.004327Loss of the reservoiras a water supplysev 3 · risk 0.007692Statutory enforcementand rebuild costsev 3 · risk 0.0075Water flowsover theembankment…0.00641/yrtop-event frequencyTop event0.00641/yrinherent 0.072/yrResidual risk0.0195inherent 0.792Risk reduction97.5%threat side 91.1%Barriers113 threats · 3 consequencesDominant threatSpillway blocked by trees and debris in…60.1% of the top eventResidual risk 0.0195 against an inherent 0.792 — the barriers remove 97.5% of it. 2 things are probably not what was meant — every path and every escalation factor was checked.Barrier criticality — residual risk if that one barrier were removed1. Annual full-stroke exercise of the scour valve+0.0305 ×2.62. Standby generator for the valve actuator+0.0228 ×2.23. Debris boom and trash screen upstream of the weir+0.0218 ×2.14. Interconnector from the neighbouring zone+0.0115 ×1.65. Flood warning giving 90 minutes to evacuate+0.0101 ×1.56. Catchment tree management near the inlet+0.009592 ×1.5FindingsConsequence "Loss of the reservoir as a water supply" rests on ONE barrier. Defence in depth is the claim a bowtie is drawn to make, and a single barrier is a single point of failure: the day it fails is the day the consequence…Escalation factor "Generator fuel supply contract lapsed" degrades barrier "Standby generator for the valve actuator" and nothing is holding it off. The 75% credited to that barrier — and every figure computed from it — assumes…Each of the 3 consequences is credited the full top-event frequency of 0.00641/yr — the model here is that one top event produces all of them, so the residual score of 0.0195 adds 3 outcomes to one event. That is right when the…

Make it your own.

title "Embankment dam overtopping in an extreme flood"
hazard "18 million cubic metres impounded above a valley town"
top "Water flows over the embankment crest"
unit "/yr"

threat "Probable maximum flood exceeds spillway capacity" likelihood: 0.002 {
  barrier "Spillway sized to the 1 in 10000 year inflow" effectiveness: 0.9
  barrier "Auxiliary fuse plug spillway on the left abutment" effectiveness: 0.7 type: mitigation
}

threat "Spillway blocked by trees and debris in the flood" likelihood: 0.02 {
  barrier "Debris boom and trash screen upstream of the weir" effectiveness: 0.65 {
    escalation "Boom anchorage not inspected since the last flood" {
      control "Post-flood inspection mandated within 7 days"
    }
  }
  barrier "Catchment tree management near the inlet" effectiveness: 0.45
}

threat "Draw-off valve fails to open for flood drawdown" likelihood: 0.05 {
  barrier "Annual full-stroke exercise of the scour valve" effectiveness: 0.8 type: detection
  barrier "Standby generator for the valve actuator" effectiveness: 0.75 {
    escalation "Generator fuel supply contract lapsed"
  }
}

consequence "Breach and inundation of the town downstream" severity: 5 {
  barrier "Wave wall and erosion-resistant downstream face" effectiveness: 0.55 type: mitigation
  barrier "Flood warning giving 90 minutes to evacuate" effectiveness: 0.7 type: recovery
}

consequence "Loss of the reservoir as a water supply" severity: 3 {
  barrier "Interconnector from the neighbouring zone" effectiveness: 0.6 type: recovery
}

consequence "Statutory enforcement and rebuild cost" severity: 3 {
  barrier "Section 10 inspection with safety measures" effectiveness: 0.4
  barrier "Reservoir emergency plan rehearsed with the LRF" effectiveness: 0.35 type: recovery
}