MAINTAINING CONTINUOUS FLOW IMPROVES EFFICIENCY AND ACCURACY OF HALF-BARREL SEEPAGE-METER MEASUREMENTS

Half-barrel seepage meters are commonly used to quantify seepage in lakes, wetlands and streams. This simple device consists of a chamber placed over a portion of a sediment bed; all water that flows across the sediment-water interface covered by the seepage cylinder is routed to or from (depending on whether seepage is upward or downward) a seepage-measurement bag. The standard measurement procedure requires that flow through the seepage meter, as well as flow across the portion of the sediment-water interface covered by the meter, be stopped during bag attachment and removal. Time-integrated seepage-bag measurements might, therefore, be reduced relative to undisturbed flow because each seepage-meter measurement starts out at zero flow. The effect of accelerating flow during the beginning of a seepage-meter measurement should be inversely proportional to measurement duration and directly proportional to seepage rate. To test for potential reduction in measured seepage, we attached a y-valve fitting that allowed continuous flow through the seepage meter. Seepage was routed either to or from a seepage bag or through a second submerged “exhaust” port when a seepage bag was not actively measuring seepage. Tests were performed by placing seepage meters in a 1.5-m-diameter seepage tank filled with homogenous, isotropic, medium sand. Traditional seepage measurements were compared with uninterrupted seepage routed either to a seepage bag or to the open exhaust port. Tests of both upward and downward flow to the meter were performed at 6.5, 21, and 26 cm/d with periods of bag attachments of 2, 5, 10, and 20 minutes. Five measurements were made for each combination of seepage rate and measurement duration for a total of 60 measurements. When all data are summarized, uninterrupted y-valve-measured seepage rates were 14 to 26 percent larger with a smaller standard deviation for data collected with uninterrupted flow. Bias caused by interrupting flow was greater for shorter measurement durations and faster seepage rates. We conclude that maintaining constant flow through a seepage meter improves flow measurement efficiency and reduces uncertainty.