Tracing the Cause of Sediment Loads on the Little Lehigh Creek

Abstract

The City of Allentown Water Resources Bureau's (COAWRB) primary source of water is the Little Lehigh Creek. Increasing sediment and turbidity in the creek has been cause for concern for a number of years. The COAWRB commissioned CDM to assess the current sediment loading, identify potential sources, establish potential trends in the loading, and develop an approach to flag significant changes in the loading pattern and rate in the future.Sediment loads can come from a variety of sources, and from almost anywhere within the watershed. Sources include overland stormwater flow (wash load), as well as in-stream sources from bank or bed erosion (erosion load). A definitive answer on the exact source of sediment is difficult to achieve. An effective and affordable approach to answering the questions posed required a variety of assessments, all incomplete in and of themselves, but when taken together, provided enough information to identify the most important source with a reasonable degree of confidence. The assessment techniques that proved most effective included:• stream flow analysis and raw water sampling • a linked database/GIS system that allowed for spatial and temporal data analysis • a stormwater and sediment loading model using SWMM to assess the link between land use and stormwater sediment loading• a Rosgen-based stream assessment of the main stem and tributariesThe study resulted in development of database, GIS, and modeling tools, a report delineating the most likely sources of sediment and their location, and an approach to annual data review designed to allow the COAWRB staff to input data that is routinely collected into the database, and generate comparative statistics and curves that will help flag changes in sediment loading and flow that may be occurring.400