The problematic flooding on tributary streams in these neighborhoods occurs in the downstream part of the tributary watershed. Peak flows at the downstream part of each tributary watershed are a combination of flows contributed from the entire watershed -- both from the "headwaters" (upstream part of the watershed, generally less urbanized) and the local area (more urbanized and, presumably, at times affected by backwater from the high flows in the Pearl River).

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Under natural, pre-development conditions, the peak high flows in these areas would occur tens of hours or days after the beginning of the storm -- the watersheds absorb some of the initial rain, and then when all of the "storage" areas are full, start to send most of the ongoing rain directly into channels that are all feeding to the mainstem. 

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Notably, the peak high flows on the Pearl River occur days after these tributary basins peaked -- the timing is a function of the basin size.  Smaller basins peak within hours, and larger basins peak within days.

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Headwater flooding refers to this still semi-natural part of the peak flow hydrograph -- the peak that arrives at the site tens of hours after the storm arrives and passes through.

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Flash flooding refers to peak flows resulting at a location from intense rainfall in the local area.  For example, there might be several storm drain networks dumping water into the tributary from subdivisions and urbanized sub-basins close to the site. Because these tributary watersheds are smaller, the peak flow may occur within 15 to 30 minutes after the storm.

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You can imagine that the semi-natural "headwater" flooding causes the peak flow at our problematic flooding location to gradually increase to the peak 10s of hours after the storm begins, but there could be other small storm flows rapidly increasing the peak flow at our location -- e.g., the "flash flood" peak might happen at 4:00 pm but the headwater flood peak won't arrive until 10:30 pm. Typically the flash flood peak is larger, higher, and more damaging than the headwater peak.  

 

It seems like there are also obstructions in these tributary creeks -- especially bridges and culverts that are too small (constrictions).  So the worst-case scenario for frequent flooding damage is in a location upstream from a constriction and downstream from an urbanized subwatershed.  That location might be flooded routinely by the (somewhat urbanized upper watershed) headwater flood peak but always gets flooded by the (densely urbanized local area) flash flood peak.