––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– PROCCEDINGS OF THE 2006 ESRI INTERNATIONAL USER CONFERENCE San Diego, California August 7-11, 2006 Copyright © 2006 ESRI Fractal Generation of Artificial Sewer Networks for Hydrologic Simulations Indrani Ghosh 1 , Ferdi L. Hellweger 2* and Todd G. Fritch 3 Abstract Simulating urban hydrology using actual sewer networks can be tedious and even practically impossible for large and/or old areas, especially when considering high spatial resolution requirements of physically- based models. Therefore, the rapid generation of artificial networks is of considerable interest, especially for city-scale analyses. Fractals are scale-independent, self-similar geometric shapes, and fractal trees are fractals consisting of a network of connecting lines. This paper presents a new public-domain application for generating artificial sewer networks using the dendritic and space-filling ‘Tokunaga’ fractal tree geometry (Artificial Network Generator, ANGel). Options include varying the number of generations, clipping to a drainage basin, irregularization, and expansion (densification) of an existing network. Output consists of line, point and area shapefiles with various attributes (e.g. flow length) and a statistics (e.g. Horton’s bifurcation ratio) file. An application of the program to the Faneuil Brook sub-basin in Boston is presented. Artificial sewer networks generated using the program are compared to the actual network in terms of various hydrologic statistics and travel time distributions. 1. Introduction The urban landscape is a complex environmental system that reflects the dynamic and intense alteration of natural environmental processes by human activity. Urbanization radically changes flow paths and the quantity and quality of waters that flow through the drainage network and into receiving water bodies such as wetlands, rivers, lakes and estuaries. While urban hydrology (considered here as water and associated biogeochemical cycles in the urban environment) can be a highly dynamic and intermittent process, the cumulative effects of its impacts on receiving waters are very significant. It is important to better understand the role of short-term (e.g., spatially distributed intermittent rainfall acting on movable material collected in the atmosphere or on the surface since the last storm), and long-term (e.g., the geometry and density of the storm sewer network, or the specific arrangement of land uses and storage elements) dynamic processes of the urban hydrologic system. Urbanization is a dominant global phenomenon and there is an urgent need to advance the science of urban hydrology. 1 PhD Student, Civil & Environmental Engineering Department, 400 Snell Engineering Center, Northeastern University, Boston, MA 02115. 2 Assistant Professor, Civil & Environmental Engineering Department, 400 Snell Engineering Center, Northeastern University, Boston, MA 02115. 3 Academic Specialist, Director Geographic Information Systems, Department of Earth and Environmental Sciences, 70 Holmes Hall, Northeastern University, Boston, MA 02115. *Corresponding author. Tel.: (617) 373- 3992; fax: (617) 373-4419. E-mail address: ferdi@coe.neu.edu