Comparison of Two Rating Approaches to Monitor Uncontrolled Culverts’ Flows Driven by Small Head Differences

The smallest flows that can be rated using a distributed-type hydraulic rating, which must rely on head- and tailwater stages monitored at local stilling wells, are limited by the combined measurement uncertainty of the head drop. In this study, we show that for low head drops, a rating based on index-velocity (IV) methods provides more accurate flow estimates at uncontrolled culverts than a hydraulic rating. To contrast the accuracies of these two rating approaches to rate flows under very low head drops, we calibrated both types of ratings for an uncontrolled culvert in a constructed wetland using concurrent measurements of water stages in local stilling wells, index-velocities by a Doppler shallow-water meter, and flow rates by an acoustic Doppler flow meter (ADFM). Further, to measure head-drops lower than those that can be estimated based on data from the stage sensors in standard wells, we used a portable pressure-differential sensor with a higher resolution and sampling rate. Using the portable head-drop sensor allowed us to extend the lower limit of the head-drop to which the hydraulic rating can be accurately applied, from 1.51 cm (based on stages from the standard wells) to 0.31 cm. This, in turn, extended the range of uncontrolled full-pipe flows that can be rated by the hydraulic rating at the study site from $\sim 0.45{\mathrm{m}}^3/\mathrm{s}$, based on stages monitored at standard wells, to $\sim 0.2{\mathrm{m}}^3/\mathrm{s}$, based on the high-resolution head-drop sensor. In contrast, we found that the IV rating can rate a flow as low as $0.14{\mathrm{m}}^3/\mathrm{s}$ without the need of stilling wells—about 60% and 30% lower than the hydraulic rating can rate with stages from the stilling wells and the portable sensor, respectively. Moreover, the IV rating could rate flows contributing to $\sim 94\%$ of the total annual volume at the study site accurately, while the volume accurately estimated at these sites by the hydraulic rating using stilling wells is less than half of that.