Development of the Stage Flow Relations for a Tropical Watershed

Abstract

The effluent discharge from wastewater treatment plants can permeate through the ground and become a major source of contaminant to ecosystems and water bodies. These toxic contaminants can transport and find a way to the ocean via underground streams harming the aquatic life and promoting algal blooms. This paper presents meteorological and hydrological data collection and analysis to better understand the characteristics of the effluent discharge from the wastewater treatment plant and receiving water in Toguan watershed on the Island of Guam so as to support future regulatory discussions regarding modification of permit conditions and/or water quality standards to achieve compliance. Hydrological data such as rainfall, turbidity and stream level were collected over a period of two years. Crossflow measurements for two stations, one downstream, the other upstream of the effluent discharge point were collected bi-weekly, except at times of very low flow or dangerously high storm runoff in the stream. To better understand the behavior of the Toguan River sub-watersheds, aerial photos were taken from a single-engine Cessna aircraft. By analyzing the collected hydrological data, the relationships among the stream level, volume discharge and rainfall, were developed. The manually collected crossflow data allowed for the calculation of a rating curve for the Toguan River. This effort was somewhat hampered by prolonged drought conditions during roughly half of the period of data collection that severely limited the amount of crossflow measurements taken at high stage heights. The continuous measurement of stream level and rainfall allowed for a more comprehensive analysis of the relationship between the stage height and rainfall. Finally, a complimentary stream stage height short-term prediction scheme was developed using a routing model, and refinements to the stream flow discharge curve using a simple model for open channel flow is also presented. A further benefit of the study was the establishment of a baseline for the hydrologic conditions of the Toguan watershed which can be used to assess changes related to any future improvements to the wastewater treatment plant operations or other significant developments within this watershed.