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San Antonio River Basin Report Card


Report Card Summary

San Antonio River Basin Report Card Summary
The overall grade for the 2020 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

A-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 area are part of Flash Flood Alley, making this large metropolitan area one of the most flood-prone regions in North America. San Antonio has a long history of flooding and flash flooding, many of these events, unfortunately, have included fatalities. Flash floods can occur within minutes to a few hours following a heavy rainfall event, and they cause most of the fatalities associated with flooding.

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, this urban flooding can become dangerous as streets, freeways, underpasses, and parking lots can fill with stormwater runoff. In addition to the threats to life and property caused by urban floods, even small amounts of 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 rains fall 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 period of measurement for this report card, the residents of Texas were reminded of the high risks associated with severe storms and flooding. Tropical Storm Imelda made landfall near Freeport, Texas on September 17, 2019. Within hours of landfall, the storm was downgraded to a tropical depression. However, stalling between Houston and Lufkin, this Southeast Texas area received between 30 to 44 inches of rain, with the greatest amount recorded at 44.29 inches. As a result, the NWS stated that Imelda was “the 7th wettest tropical cyclone to impact the United States, 5th wettest in the contiguous United States, and the 4th wettest in the state of Texas.” Tragically, the storm also resulted in two fatalities in Jefferson County, Texas. In these separate incidents, individuals were recovered from their vehicles.

How is this being measured?

In developing and reporting this metric, we are using the number emergency calls made related to flooding. This data is being provided to us by county emergency managers. The grade is calculated by using a rolling 3-year average of emergency calls made related to flooding as a base line. For this year’s basin report card grade, the data for the rolling 3-year average comes from fiscal years 2016-17, 2017-18, and 2018-19. (Note: The River Authority Fiscal Year runs from July 1 to June 30.) The total number of emergency calls made related to flooding this fiscal year (July 1, 2019 – June 30, 2020) is subtracted from the base line, then divided by the base line, and multiplied by 100 to develop a score.

This is done separately for emergency calls made related to flooding in Bexar, Wilson, Karnes, and Goliad counties. The four individual county scores are 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 amount of emergency calls made related to flooding is positive (i.e. will get a higher grade) and increasing emergency calls related to flooding is negative (i.e. will get a lower grade). Being a trend analysis, 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 rolling 3-year average for comparison in this year’s basin report card. The intent will be to expand the rolling average into a 5-year span, which will be implemented by the 2022 river basin report card.

This report card grade can improve over time as fewer emergency calls are made during flood events. However, by no means are we suggesting that emergency calls related to flooding should be made. We absolute want and recommend that you call 911 during an emergency if assistance is needed. It is the River Authority’s intention with this grade to raise flooding awareness, and by doing so, encourage personal responsibility during flood events that, overtime, 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 94 210 181 123 32
Wilson 0 0 0 0 0 100
Karnes 0 0 0 0 0 100
Goliad 0 0 0 0 0 100

Basin-wide grade: A-
(32+100+100+100/4 = 83.0)

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 emergency calls related to flooding logged over the past four 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, and 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 also impact this report card grade. Years that have more rainfall may be likely to have more flooding which may result in an increase in emergency calls. Per National Weather Service rainfall data, a total of 28.04 inches of rain fell in San Antonio during Fiscal Year 2019-20 (July 2019-June 2020). This compares to the rainfall totals for the previous three fiscal years of 33.18 (2016-17), 29.72 (2017-18), and 48.33 (2018-19). The data appears to show some degree of correlation between the number of emergency calls made regarding flooding and the annual rainfall totals.

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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