The city of San Marcos is in the process of developing a two-dimensional model of how water flows during flood events in the city, which officials hope will allow for better flood mitigation and planning. Laurie Moyer, director of engineering and CIP for the city, said at a floodplain management workshop Nov. 30 that the city’s current flood maps are based on a more simplistic method than what is now available. “A one-dimensional analysis is a very conservative analysis,” Moyer said. “The granularity of what the water is actually doing is not there. That is what our current flood maps are based upon.” The new 2-D model will allow the city to better understand how to mitigate future flood damage and how to plan for developments near flood-prone areas. The results of the 2-D model will be used to fine-tune the traditional 1-D model to ensure it is more in line with how water actually flows in the city. Moyer said city staff has delayed bringing the new 2-D model to council because verifying the results of the model has been time-consuming for city staff and Halff & Associates, the firm contracted by the city to assist in the study of the floodplain. “We haven’t wanted to bring it forward until we can say, ‘We feel really comfortable based upon elevations and properties and things we’ve seen that this is actually reflecting what occurred in 2015,’” she said. Planning for how to manage flood risks in San Marcos is made more difficult by the confluence of the Blanco and San Marcos rivers, located just east of the city. “There is a lot going on,” Moyer said. “We have the San Marcos River, and it runs all the time; it’s spring fed. It doesn’t have a huge watershed, but it does have some streams that come into it and influence it. We have the Blanco River, which has a big watershed. It makes modeling [flooding scenarios] a challenge to do.” Moyer also said I-35 acts as a dam, which can influence the way water flows in parts of the city. Additionally, San Marcos is located in what is commonly referred to as “Flash Flood Alley,” a swath of land stretching from Dallas to near Del Rio. Meteorological factors combined with the thin soil and sloping topography in the region make the area susceptible to flooding, Moyer said. According to city records, there have been 19 floods in San Marcos since 1952. Six floods have occurred in October, giving it the highest frequency of flood events. Council Member Lisa Prewitt said she hoped the city would work with neighboring communities to help plan for future floods. Impervious cover—a surface that water cannot penetrate, such as a paved parking lot—in or near a floodplain will determine the severity of future floods. Impervious cover is of particular concern during flood events. Rainfall that otherwise would have fallen on dirt and been filtered through the ground back into an aquifer is now directed to a stream or river, which can greatly increase the amount of water a river carries, causing flooding events similar to what was seen in May and October. As cities in or near the Blanco River’s watershed—including Kyle, Buda and Wimberley—continue developing, it will be important for each city to know what is being constructed so they understand how it will affect the way water enters the river, Prewitt said. “Otherwise we’re not going to be able to collect the data and add in that impervious cover to what those different floods are going to look like as they move around,” she said. “Where the rain falls on the Blanco and San Marcos [rivers] and how long and how it moves around—we will never be able to statistically show where it’s going to flood and how much. We just know it’s going to [flood]. That’s history.”