Historical Streamflow Discharge Trends Considering Climate Change and Application

About the report:

Bridges and large culverts are designed using historical streamflow data. However, historical streamflow data are static and do not account for changes in precipitation, temperature, and watershed characteristics such as the percentage covered by impervious surface. This report details efforts to determine if streamflow peak discharges in Virginia have changed or remained constant for roughly the past century (1916–2015), the extent to which changes are related to physiographic factors and watershed characteristics, and efforts to develop a watershed index that identifies watersheds that may be particularly vulnerable to climate change.

The study finds that changes in precipitation may have raised peak flow discharges. For the 1941–2015 period, most streamflows increased for stream gages classified as minimally altered. This classification is most suitable for determining the effects of climate change. However, most flow increases were not statistically significant, and different results are obtained when examining a shorter period (1966–2015). However, the study does find that precipitation has not remained constant but rather increased. An increase in streamflow did not always accompany increasing trends in precipitation. For 1941 to 2015, 66.4% of stream gages had an increasing trend in precipitation but a decreasing trend in streamflow. A closer examination of the stream gage categories revealed that for minimally altered stream gages, an increase in streamflow most commonly accompanied an increase in precipitation. For regulated stream gages, a decreasing trend in streamflow usually occurred alongside an increase in precipitation. Different trend patterns between stream gage patterns could be because of variations in normalized dam storage, land use, or watershed area. For the period from 1966 to 2015, 60% of stream gages had an increasing trend in both streamflow and precipitation. In this period, most minimally altered stream gages had a nonsignificant decreasing trend alongside an increase in streamflow. Furthermore, most regulated stream gages had a decreasing trend in streamflow alongside an increase in precipitation.

The researcher developed a watershed index to identify areas of Virginia that could be particularly vulnerable to climate change. This index identified areas of Virginia with limited stream gage coverage, stream gages with shorter data records, or stream gages with significant or nonsignificant increasing trends in peak flow discharges. Most of these watersheds are in Virginia’s southeastern, western, and northern regions. A recommended next step, as described in the Implementation section of this report, is to conduct an analysis to determine locations that may benefit from additional stream gages. Research from other states suggests that, for a culvert where peak flow discharge data from stream gages are available, a potential cost savings of between $74,000 and $91,000 can be realized during the lifetime of the culvert based on better design and losses avoided.

Supplemental materials can be found at https://library.vdot.virginia.gov/vtrc/supplements.

_{Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.}

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