Delft University of Technology Comparing floodplain evolution in channelized and unchannelized urban watersheds in Houston, Texas Juan, Andrew ; Gori, Avantika; Sebastian, Antonia DOI 10.1111/jfr3.12604 Publication date 2020 Document Version Final published version Published in Journal of Flood Risk Management Citation (APA) Juan, A., Gori, A., & Sebastian, A. (2020). Comparing floodplain evolution in channelized and unchannelized urban watersheds in Houston, Texas. Journal of Flood Risk Management, 13(2), [e12604]. https://doi.org/10.1111/jfr3.12604 Important note To cite this publication, please use the final published version (if applicable). Please check the document version above. Copyright Other than for strictly personal use, it is not permitted to download, forward or distribute the text or part of it, without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license such as Creative Commons. Takedown policy Please contact us and provide details if you believe this document breaches copyrights. We will remove access to the work immediately and investigate your claim. This work is downloaded from Delft University of Technology. For technical reasons the number of authors shown on this cover page is limited to a maximum of 10. Received: 30 October 2017 Revised: 18 December 2019 Accepted: 18 February 2020 DOI: 10.1111/jfr3.12604 ORIGINAL ARTICLE Comparing floodplain evolution in channelized and unchannelized urban watersheds in Houston, Texas Andrew Juan1 | Avantika Gori1 | Antonia Sebastian1,2 1Department of Civil and Environmental Engineering, Rice University, Houston, Abstract Texas, USA In this study, we compare the ability of two riverine flood control approaches: 2Department of Hydraulic Engineering, channelization and stream preservation/setbacks, to alleviate the adverse Faculty of Civil Engineering and impacts of rapid urbanization. To study the effects of structural intervention Geosciences, Delft University of Technology, Delft, The Netherlands and urban development on the evolution of the floodplain, we have chosen two neighboring urban watersheds in Houston, TX: Brays Bayou and Buffalo Correspondence Bayou. While the two watersheds are similar in size, topography, and develop- Andrew Juan, Department of Civil and Environmental Engineering, Rice ment level, they have contrasting riverine flood management approaches. University, 6100 Main Street, Houston, TX Brays Bayou is channelized, whereas Buffalo Bayou remains mostly 77005. ® Email: [email protected] unchannelized. We use the distributed hydrologic model, Vflo , and the hydraulic model, HEC-RAS, to analyze channel hydraulics and floodplain Funding information extent in the two watersheds under the 10- and 100-year rainfall scenarios at the Houston Endowment and the NSF PIRE, Grant/Award Number: OISE- three points in time: 1970s (early development), 2011 (current development), 1545837 and 2040 (future development). We find that, while floodplain extent in both watersheds increases over time, the relative change in floodplain extent for Brays Bayou (channelized) is substantially larger than that for Buffalo Bayou (unchannelized). The results in this study contribute to a better understanding of the long-term performance of two flood mitigation approaches (channeliza- tion and setbacks) on riverine flood risk and provide insight into best manage- ment practices for cities experiencing rapid urban growth. KEYWORDS channelization, distributed hydrologic modeling, flood risk management, floodplain, HEC-RAS, ® land use/land cover change, urbanization, Vflo 1 | INTRODUCTION measures such as flood insurance, more stringent drain- age criteria, and building codes to mitigate flood risk Throughout the 1800s and early 1900s, large-scale flood (Tarlock, 2012). While the integration of structural and control structures (e.g., dams, levees, channels) domi- nonstructural measures should be expected to encourage nated flood control policy and floodplain management in sustainable floodplain management, the payouts from the United States (Birkland, Burby, Conrad, Cortner, & the Federal Emergency Management Agency's (FEMA) Michener, 2003; NRC, 2013). However, since the 1960s, National Flood Insurance Program (NFIP) have increas- U.S. flood policy has also incorporated nonstructural ingly exceeded the revenue generated through insurance This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. Journal of Flood Risk Management published by Chartered Institution of Water and Environmental Management and John Wiley & Sons Ltd. J Flood Risk Management. 2020;e12604. wileyonlinelibrary.com/journal/jfr3 1of19 https://doi.org/10.1111/jfr3.12604 2of19 JUAN ET AL. premiums, resulting in annual deficits since 2005 watershed to severe flooding (compared to Buffalo (Sebastian & Gori, 2018). By the end of 2016, the NFIP's Bayou), raising questions about the effects of the two dif- total debt exceeded $20 billion (FEMA, 2016; ferent riverine management strategies on the evolution of GAO, 2009). risk in the two watersheds. To increase long-term urban resilience to floods, it is In the following section, we discuss one of the most crucial to better understand the performance of different common flood control practices in the United States, floodplain management approaches and their impact on channelization, and describe previous research related the spatial extent and depth of the floodplain at the to its performance in urbanizing watersheds. Addition- watershed scale. For this analysis, we define the flood- ally, we provide a short history of the two watersheds plain as the area inundated due to the overtopping of and discuss flood management practices in the context channel banks from a given design storm, for example, of urban flooding. In Section 3, we present the methods the 10- or the 100-year SCS Type III rainfall. While many and models used in this study. The results and discus- studies have focused on the environmental costs of chan- sion are presented in Section 4. Finally, in Section 5, we nelization and its impacts on watershed hydrology (Little present concluding remarks and recommendations for Inc, 1973; Rose & Peters, 2001; Schoof, 1980), few have future research. explicitly analyzed its contribution to the floodplain rela- tive to long-term urbanization changes (Habete & Ferreira, 2017; Suriya & Mudgal, 2012). Moreover, to our 2 | BACKGROUND knowledge, previous studies have yet to consider how dif- ferent riverine management approaches perform under Channel modifications, collectively referred to as chan- nonstationary land use conditions or their combined nelization, are intended to increase the carrying capacity impact on floodplain evolution. In this paper, we com- of a natural stream. Channelization is especially preva- pare the evolution of the floodplain in two neighboring lent in the south and southeastern United States, where watersheds: Brays Bayou and Buffalo Bayou, located in low topographic relief leads to wide and shallow flood- Houston, Texas, in which contrasting flood management plains. By structurally altering one or more of the strategies have been implemented (Figure 1). Brays hydraulic variables that govern channel flow (e.g., slope, Bayou was fully channelized in the 1960s, whereas the depth, width, roughness), velocities in the channel are main channel in Buffalo Bayou has been left largely in its increased and the height of water decreased. The primary natural state. The watersheds are similar in size and advantage of lowering the depth of water in the channel topography, and development patterns across the two is that it reduces the size (i.e., spatial extent) of the flood- watersheds are comparable. Recent flood events have plain, allowing for economic development and growth in exposed the vulnerability of midstream communities previously flood-prone areas. (located between the upstream [US] and downstream Supporters of channelization have often touted its [DS] gauges shown in Figure 1) in Brays Bayou local flood risk reduction benefits; however, many studies FIGURE 1 Brays Bayou and Buffalo Bayou watersheds, with watch point (gauge) location JUAN ET AL. 3of19 have also shown that channelization can also have rates of repetitive flood losses and flood-related fatalities unintended consequences in other parts of the watershed, in the United States. Furthermore, a large percentage of especially in DS communities (Booth, 1991; Prestegaard damages in Harris County occur outside of the mapped et al., 1994; Rose & Peters, 2001; Shankman & Samson, regulatory floodplain areas (Highfield, Norman, & Brody, 1991). For example, Prestegaard et al. (1994) found that 2013). As of June 30, 2017, the total insured losses that areas DS of a modified channel section of Raccoon River, have been paid out in Harris County exceeded $3 billion Iowa, had experienced higher-magnitude floods com- USD (FEMA, 2017). pared to similarly sized areas US. Likewise, Rose and Buffalo and Brays Bayous are two of the primary Peters (2001) showed that channel improvements drainage basins for the City of Houston (Figure 1). increase the flood wave velocity, thereby decreasing peak Together, they drain nearly 596 km2 (230 mi2), flowing timing DS. Others have focused primarily on the negative eastward before converging near the Houston Ship Chan- effects of channelization on