An underflow dam unit provides a barrier to pollutants floating in runoff water which otherwise finds its way into storm drains and streams. The underflow dam unit includes a dam disposed between limiting sidewalls, and a baffle spanning between the sidewalls above the dam. The baffle's lower limit extends below the height of the dam, forming a weir channel between the baffle and the dam. Floating pollutants become trapped against the baffle while hydraulic pressure allows subsurface stream water to flow through the weir channel and over the dam. In one embodiment, a plurality of portable emergency dam units may be bolted together and installed through a temporary dirt levee built across a flowing stream or ditch to capture pollutants spilled upstream. In another embodiment, the conventional curb-level inlet to a storm sewer catch basin is replaced by a surface grate which drops runoff water into a chamber buried adjacent the catch basin. The chamber includes a dam disposed beneath another opening leading into the catch basin. A baffle disposed over the dam forms a weir channel, and the baffle may be adjustable for peak flow rates. Means to suppress churning of water pooling in the chamber by incoming runoff may be provided below the inlet grate, and access means to the chamber interior allows for siphoning off trapped pollutants and for adjusting the baffles.
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