Downflow direction, density for Lower East Side Manhattan

In the attached map I used GRASS 6.4.1. to build a hydrology map showing downstream flow from rain showing direction, density and flow using the street elevation file as a basis.  I cannot give an answer why the area around 14th Street on the Lower East Side has such a high density of flow since this area is relatively flat but of low elevation.  This is just an experiment and not a professional assessment.