Rerouting Behavior and Travel-Time Penalties Under Flood- Induced Road Closures
Abstract
When a flood closes a rural road, the standard question is how much longer the trip takes. We argue that on sparse rural networks it is the wrong question, because the network usually does not offer a longer trip. Taking satellite-derived closure sets for the 2022 wet season over two Nigerian river basins and routing 10,438 settlement-pair trips across them, we find that between 47 and 60 percent of hit pair-dates have no path at all, and that among those that do reroute the penalty is small: a detour ratio of 1.003 to 1.100 and a worst realized delay of 4.8 hours against a median closure spell of 24 to 36 days. Absorption never fails on the margin. It fails by severance, and severance is local: under every closure input but the widest, every severed pair is a settlement cut into a pocket holding under 5 percent of network length, not a broken corridor. The links responsible are very few, eight out of 8,334 under the high-precision input. Assessed structurally instead, with no flood in the loop, the same network turns on three trunk links that sever 812 pairs each and that this detector puts under water on two acquisitions out of fifteen, under one closure input out of four, so a hazard-conditional ranking misses the largest exposure in the study area. Severed pairs are predictable in advance from structure alone, at an out-of- fold area under the curve of 0.9997, and the count of cut edges on the baseline path reaches 0.992 unaided, while betweenness centrality is the weakest of six predictors. But the identity of the critical links does not survive the detector: the top eight links under the high-precision and bounding readings of the same imagery share not one member. Structural fragility is robust; which fragile pair a given flood actually cut is a statement about the sensor. A depth-disruption test removes severance entirely under the high-precision input, the one that produces the headline, at all three depths swept, halves it under the bounding input at 300 mm and raises the count of merely delayed pairs by up to a factor of 2.5, so the split between severance and delay moves with the depth mapping as much as with the detector threshold.
Keywords
network vulnerabilityflood disruptionrural accessibilityreroutingredundancyremote sensingNigeria 1
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