  Just as natural river networks are known to be globally self-similar, recent
research has shown that human-built urban networks, such as road networks, are
also functionally self-similar, and have fractal topology with power-law
node-degree distributions (p(k) = a k). Here we show, for the first time, that
other urban infrastructure networks (sanitary and storm-water sewers), which
sustain flows of critical services for urban citizens, also show scale-free
functional topologies. For roads and drainage networks, we compared functional
topological metrics, derived from high-resolution data (70,000 nodes) for a
large US city providing services to about 900,000 citizens over an area of
about 1,000 km2. For the whole city and for different sized subnets, we also
examined these networks in terms of geospatial co-location (roads and sewers).
Our analyses reveal functional topological homogeneity among all the subnets
within the city, in spite of differences in several urban attributes. The
functional topologies of all subnets of both infrastructure types resemble
power-law distributions, with tails becoming increasingly power-law as the
subnet area increases. Our findings hold implications for assessing the
vulnerability of these critical infrastructure networks to cascading shocks
based on spatial interdependency, and for improved design and maintenance of
urban infrastructure networks.
