A methodology to reduce uncertainties in the high-flow portion of a rating curve

Authors: Baffaut, Claire; Sadler, Edward; Ghidey, Fessehaie

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2014
Publication Date: 6/30/2014
Citation: Baffaut, C., Sadler, E.J., Ghidey, F. 2014. A methodology to reduce uncertainties in the high-flow portion of a rating curve. Transactions of the ASABE. 57(3):803-813. DOI: 10.13031/trans.57.10383.

Interpretive Summary: Continuous measurement of flow in a stream is done by measuring water stage, or the height of the water above a known point. The stage is then converted into a flow rate using a rating curve that describes the relationship between stage and flow. This curve is developed from actual flow measurements at various stages. During out-of-bank flow conditions, measurement errors increase because of safety concerns and difficulties in measuring flow velocity in the flood plain. With increasing frequency of large rain events associated with climate change, it becomes critical to have an accurate rating curve for out-of-bank flows. We propose a methodology based on physical and mathematical principles to test and develop a rating curve that is accurate for high flows. The methodology was tested using flow data from the Goodwater Experimental Watershed, a 72-km2 watershed in Northeast Missouri. The rating curve developed with this methodology met the necessary physical and mathematical requirements and resulted in flows relative to the drainage area that were comparable to those from nearby stations. Results indicate that flow values that correspond to out-of-bank stage were overestimated by 30% in the original rating curve. This method provides additional and innovative means to verify the accuracy of the high-flow portion of a rating curve. It is of value to hydrologists and flow data users for any site where the rating curve is in question.

Technical Abstract: Flow monitoring at watershed scale relies on the establishment of a rating curve that describes the relationship between stage and flow and is developed from actual flow measurements at various stages. Measurement errors increase with out-of-bank flow conditions because of safety concerns and difficulties in measuring flow velocity in the flood plain. With increasing frequency of large rain events associated with climate change, it becomes critical to have an accurate rating curve for out-of-bank flows. This paper presents a set of techniques that, when used concurrently, can help identify errors in the rating curve. They include commonly used techniques such as comparison of average annual discharge per unit area with those from nearby watersheds having similar characteristics. New techniques are proposed based on the hydrologic budget of a storm event, and the analysis of the change in flow relative to time and stage. The methodology was tested using 10-minute flow data from the Goodwater Creek Experimental Watershed, a 72-km2 watershed in Northeast Missouri. Results showed that the established rating curve overestimated flow values for out-of-bank conditions by 30%. A new rating curve was proposed that satisfied the requirements of water balance and change in flow relative to time and stage. These techniques are useful for retrospective analysis of flow data series and for identifying inconsistencies in the rating curve.