The operation and maintenance of a crest-stage gaging station

Abstract

The purpose of this manual is to familiarize field personnel with the procedures involved in operating crest-stage gaging stations. Crest-stage gages are used to determine the elevation of a peak stage occurring at a specific location in a stream. A crest-stage gage consists of a length of 14-inch diameter pipe installed in a vertical position in the stream channel (figure 1). The pipe is vented and equipped with an intake system that allows water to enter and leave the pipe as the stream rises and recedes. An equal length of aluminum "T" channel (staff) is inserted in the pipe and made to rest on a positive stop called a datum pin. When granulated cork is placed in the gage, the cork will float on the rising stage, then adhere to the staff after the stage recedes. The peak stage is then determined by removing the staff and measuring the distance between the cork line and the lower end of the staff. The primary purpose of a crest-stage gaging station is to collect hydrologic data for flood-frequency analyses. Collection of data for such analyses involves operating a network of stations for a period of years until sufficient peak flow data are obtained to accurately define flood-frequency curves. Because carelessness in collecting data at the gage site might result in the loss of an entire year's flood event, the importance of proper operation and maintenance of these gages can not be overemphasized. Peak discharges are related to the basin characteristics and major changes in these characteristics must be evaluated. For€st fires, timber harvest, or a new road location that alters the natural drainage of the basin, may effect the intensity and timing of the runoff of the basin. Rigid datum controls must be maintained at the gage site throughout the period of record. Physical changes of the site resulting from flood flows or manmade alterations must be evaluated. If a drainage structure such as a culvert is part of the site features, free-flow conditions must be maintained or obstructions carefully documented.