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San Antonio River Basin Report Card 2021


Report Card Summary

San Antonio River Basin Report Card Summary
The overall grade for the 2021 San Antonio River Basin Report Card is B. This grade is the average of twelve individual indicator grades, which are explained in greater detail below.
Emergency Calls During Flood Events
Emergency Calls During Flood Events
How is this being measured?
Explanation of the grade
Key findings

B+Emergency Calls During Flood Events

While all the metrics in the San Antonio River Basin Report Card have overlapping correlation to the safe, clean, enjoyable creeks and rivers aspects of the River Authority’s mission, the Number of Emergency Calls Regarding Flooding grade is primarily related to the safe aspect.

Number of Emergency Calls For Flooding

Within the San Antonio River Basin, the City of San Antonio and Bexar County have a long history of flooding and flash flooding. This area is part of Flash Flood Alley, making this large metropolitan area one of the most flood-prone regions in North America. Flash floods can occur within minutes to a few hours following a heavy rainfall event. Many of these events, unfortunately, have included fatalities.

Urbanization can increase runoff up to 6 times over what would occur in natural areas. Impervious cover does not allow rainfall to filter into the ground; therefore, urban flooding can become dangerous as streets, freeways, underpasses, and parking lots fill with stormwater runoff. In addition to the threats to life and property caused by urban floods, stormwater runoff can have a significantly negative impact on other issues such as water quality and creek and river erosion.

Finally, river flooding can occur when heavy rain falls over a wide area for a prolonged period. Within the San Antonio River Basin, heavy storms in the San Antonio area, may result in river flooding in Wilson, Karnes, or Goliad counties several days after the initial rainfall. Typically, the slow rising nature of river flooding provides enough advance warning to evacuate people and property (including livestock) from harm’s way, particularly in the downstream portions of the basin.

Understanding these different types of flooding is important because, according to the National Weather Service (NWS), floods and flash floods kill more people across the nation than any other weather disaster. During the measurement period for this report card, the residents of Texas were reminded of the high risks associated with severe storms and flooding. In the span of just a few months in 2020, the State was hit by Hurricane Hanna in July, the remnants of Hurricane Marco and Hurricane Laura in August, and Tropical Storm Beta in September, each storm caused widespread flooding and highwater rescues. More locally, Wilson County experienced one flash flood event in September 2020, and Bexar County experienced a flash flood event in April 2021.

How is this being measured?

In developing and reporting this metric, we used the number of emergency calls related to flooding. This data was provided by County emergency managers and first responders. The grade is calculated using a rolling 4-year average of flood-related emergency calls as a base line. For this year’s basin report card grade, the rolling 4-year average data includes fiscal years 2016-17, 2017-18, 2018-19, and 2019-20. (Note: The River Authority Fiscal Year runs from July 1 to June 30.) The total number of flood-related emergency calls this fiscal year (July 1, 2020 – June 30, 2021) was subtracted from the base line, divided by the base line and multiplied by 100 to develop a score.

This was done separately for flood-related emergency calls in Bexar, Wilson, Karnes, and Goliad counties. The four individual county scores were then averaged to determine a basin-wide A through F grade.

Essentially, the grade is a trend analysis with the basic assumption being that lowering the number of flood-related emergency calls is positive (i.e. will get a higher grade) and increasing flood-related emergency calls is negative (i.e. will get a lower grade). It is possible that this grade may produce scores above 100 or below 0. Scores above 100 will be shown as an “A+” on the report card dashboard and a score below 0 will be shown as a “F.” The actual numeric value will be used in the averaging of all metrics to produce the overall basin grade.

This indicator will include additional data in future years. The available data for this indicator only allows for a 4-year rolling average for comparison in this year’s basin report card. The intent is to expand the rolling average over a 5-year span by 2022.

This report card grade can improve over time as fewer emergency calls are made during flood events. However, we are by no means suggesting that flood-related emergency calls should not be made. We absolutely want, and recommend, that one calls 911 during an emergency if assistance is needed. It is the River Authority’s intention with this grade to raise flooding awareness and encourage personal responsibility during flood events that, over time, may result in fewer emergency calls being made because people are not finding themselves in dangerous situations during flood events.

Explanation of the grade

County # of flood related emergency calls 2016 - 17 # of flood related emergency call 2017-18 # of flood related emergency calls 2018-19 3-year Avg. # of flood related emergency calls 2019-20 Score
Bexar 239 94 210 142 131 21.3
Wilson 0 0 0 0 0 100
Karnes 0 0 0 0 0 100
Goliad 0 0 0 0 0 100

Basin-wide grade: B+
(21.3+100+100+100/4 = 80.3)

Key findings

The key findings of this grade correlates well with the known understanding of the various types of flood risk found throughout the San Antonio River Basin. Bexar County, being in Flash Flood Alley and heavily urbanized, has a higher risk of both flash flooding and urban flooding. The data shows that all the flood-related emergency calls logged over the past five fiscal years occurred in Bexar County.

While river flooding is a concern in Bexar County, it is the more prevalent type of flooding that would likely occur in Wilson, Karnes, and Goliad counties. There is typically enough time to evacuate people from low-lying areas during a river flooding event. This appears to be reflected in the fact that there have been no emergency calls related to flooding in Wilson, Karnes, and Goliad counties during the past four fiscal years.

Annual rainfall rates may also impact this report card grade. It could be assumed that years with more rainfall, may result in an increase in flood-related emergency calls. That assumption may not always be correct, however, given that even in a dry year there could be heavy storm events that result in flood-related emergency call. Per National Weather Service rainfall data, a total of 24.27 inches of rain fell in San Antonio during Fiscal Year 2020-21 (July 2020-June 2021). This compares to the rainfall totals for the previous four fiscal years of 33.18 (2016-17), 29.72 (2017-18), 48.33 (2018-19), and 28.04 (2019-20).

Please note, we recognize the San Antonio River Basin is a large region and annual rainfall totals will differ between cities and areas of the basin. We reference annual rainfall totals in the City of San Antonio in this report card for two reasons: Bexar County (and the City of San Antonio) is where all the emergency calls related to flooding have occurred for the purpose of this year’s report card; and being near the top of the basin, rainfall in San Antonio contributes to potential river flooding downstream of Bexar County.

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