From disaster to sustainability: breaking the cycle of floods in Houston, Texas

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INTRODUCTION
Houston is located in Southeast Texas near Galveston Bay and the Gulf of Mexico.It is the seat of Harris County and the principal city of the Houston metropolitan area.The city has a total area of approximately 601 square miles, and Greater Houston has a total area of 10,062 square miles, according to the United States Census Bureau (U.S. Census, 2019).
The city of Houston grew around the confluence of White Oak Bayou and Buffalo Bayou, which served as a natural turning basin.The development spread east and west along Buffalo Bayou, spurring at the turn of the century with the construction of the Houston Ship Channel (Sipes and Zeve, 2012), and continued to grow through the twentieth century with the discovery of petroleum in the region.Oil refineries proliferated along the Ship Channel placing Houston in the center of new oil and oilfield equipment development, and in the 1920s the city saw its first growth spur.In the 1970s, the Arab Oil Embargo caused the demand for Texas oil to escalate, attracting thousands of workers and their families (Sipes and Zeve, 2012).As it is typical for a growing city, the strongest early growth in Houston was in the core municipality and then gradually shifted to the nearby suburbs.Houston is suburban, and is often characterized as a sprawling urban area, although it has a higher than average urban density if compared to other cities with the same characteristics (Wendell, 2019).
Flooding has always been endemic to Houston.From an environmental viewpoint, it is essential to understand that most of the Houston metropolitan area is nested inside a large bioregion of the Texas Gulf Coastal Plain, consisting of low elevations and nearly level to gently sloped soils that are poorly drained to moderately permeable.Downtown Houston stands about fifty feet above sea level and the entire city is relatively flat, with slopes ranging from zero to eight percent throughout the region, which makes stormwater drainage especially difficult.From an urban planning viewpoint, moreover, the vigorous growth experienced by the region in the last decades, associated with the unique ways the city has chosen to approach development regulation may have intensified the floodprone characteristics of the place.The connection between the absence of zoning regulations and past flood events has been object of great debate, exacerbated after Hurricane Harvey's destructive flooding has impacted, directly or indirectly, most residents of Houston in August of 2017.Presumably, flood as result of a storm of that magnitude would have been catastrophic, despite the existence of zoning.Nonetheless, intensive building development has deforested and sealed areas that should otherwise be absorbing stormwater, and caused imbalance and transformation in their natural function as storage and conveyance system for water, nutrients and sediments (Kiedrzyńska et al., 2015).Harris County has classified 22 major watersheds that each drain into 22 majorwaterways.Buffalo Bayou watershed encompasses most of the study area and for this reason is particularly relevant to the research.This watershed is primarily located in west-central Harris County with a small portion crossing into Fort Bend County.Rainfall within the 102 square miles of the Buffalo Bayou watershed drains to the watershed's primary waterway, Buffalo Bayou -the longest and largest of the county's bayous, travelling 53 miles before meeting San Jacinto River at Galveston Bay.The Bayou is navigable as far as downtown Houston.Just east of downtown Houston near the Turning Basin, Buffalo Bayou becomes the Houston Ship Channel (Harris County Flood Control District, 2019a).
Few watersheds have garnered as much attention after Hurricane Harvey as the Addicks, Barker and Buffalo Bayou watersheds.Many neighborhoods upstream of the Addicks and Barker reservoirs flooded, and the releases from the dams placed sections of the area underwater for weeks.In many ways, however, this watershed is a success story.
Large reservoirs at the upper end of watersheds can be highly effective, and the 75-year history of Addicks and Barker Reservoirs is an example.Until Hurricane Harvey, few homes flooded in the Buffalo Bayou watershed while neighborhoods along other bayous flooded repeatedly.A gradual reduction in the reservoirs' capacity (due to silt and vegetation) has reduced their effectiveness, but Buffalo Bayou still has significantly better flood protection than other watersheds (Greater Houston Flood Mitigation Consortium, 2019).Addicks and Barker reservoirs were built in the 1940s when Congress authorized the United States Army Corps of Engineers (USACE) to construct them as part of the Buffalo Bayou and Tributaries Project (BBTP).The reservoirs are located approximately seventeen miles from downtown Houston, and were designed to collect excessive amounts of rainfall to be released into Buffalo Bayou at a controlled rate.They fulfill their purpose by impounding water during heavy rainfalls and flood events, but do not store water yearround as it is typical of lake-forming reservoirs (U.S. Army Corps of Engineers, 2019).
Under normal conditions, both dams have two conduits set to pass the normal flow of water.When the reservoirs are empty and one inch of rain falls below the dams, the gates are closed until it is safe to make releases.If releases are being made, the gates will be closed if 0.5 inch of rain falls below the reservoirs or flooding is predicted.Normal regulating procedures specify that releases from the two reservoirs, in addition to the uncontrolled runoff downstream, should not exceed 2,000 cubic-feet per second (cfs) as measured at the Piney Point Road gauging station.Addicks Dam has a maximum discharge capacity of 7,852 cfs and Barker Dam 8,734 cfs (U.S. Army Corps of Engineers, 2019).Attempts to improve drainage and manage stormwater have lead over time to alterations of the flows of nearly all of the Harris County bayous.Buffalo Bayou's natural functions have been significantly altered by land development in the watershed, the construction of Addicks and Barker reservoirs, and the deepening and widening of the bayou from State Highway 6 to roughly Beltway 8/Rummel Creek.Despite these modifications, Buffalo Bayou is a stressed natural stream in an urban environment (U.S. Army Corps of Engineers, 2019).
Conventional city design has allowed society to thrive and grow in a range of challenging and often uninhabitable environments.However, as global populations increase and shift from rural to urban areas, the scale of urbanized regions is causing a stress surge on the natural environment and its supporting systems.The natural water cycle is disrupted in urban areas because buildings, concrete and other sealed surfaces prevent water from soaking into the ground.As a result, natural water flows are altered, and excess stormwater is created, which is traditionally directed into local waterways via drainage networks.This disruption to the natural water cycle causes many problems, including pollution of waterways, bays and oceans; waterway erosion and degradation; urban flooding; decreased soil moisture and reduced groundwater levels (Water Sensitive Cities, 2018).
The core questions this research is proposing to explore include: How can flooding volume reduction be achieved in the selected and similar areas?Would  integration of green infrastructure with existing flood control management and water resources significantly mitigate the flooding problem?How can the existing built environment be reconciled with its surroundings to create a cohesive landscape?The questions at issue were comprehensively examined with the use of Geodesign processes, which allow for the integration of sustainable landscape design with multiple supporting disciplines and new-generation spatial information/digital technologies (Huang et al., 2019).

Phase 1. Defining a pilot study area
A section of the Buffalo Bayou watershed containing 57 Census Block Groups and a surface area of approximately 16.9 square miles was outlined as a "pilot study area" for the design model being proposed.An initial approach considered the area adequate due to its proximity to Addicks and Barker reservoirs, Terry Hershey Park, Buffalo Bayou and to the Energy Corridor, the second-largest employment center in the region (Energy Corridor, 2018).Most importantly, the selected area was one of the most impacted during Hurricane Harvey.The underlying idea was to create a "prototype" based on one of the most challenging sections of the study area that could be replicated, with the appropriate adjustments, in comparable areas throughout the region.
The pilot study area (Figures 3 and 4) is highly developed -approximately 90% -and densely populated, and is a blend of residential land uses, intermixed with office, commercial, public/institutional and industrial development, as Houston does not mandate the separation of different land use categories.entire study area from west to east, and Terry Hershey Park runs along the Bayou for approximately six miles.
High urbanization and concrete coverage, and extreme occupation of the 100-year and 500-year floodplains are characteristics of this area, which had one of the highest estimated number of affected units in the region during Hurricane Harvey -approximately 19,84 -according to FEMA data from December 2017, and one of the highest number of applications for individual assistance (www.houstonrecovers.org/numbers/).The combined 1% (100-year) and 0.2% (500-year) floodplains (Figure 6) cover an estimated area of 2,420 acres within the study area, approximately 22% .Harris County Flood Control District (HCFCD) estimates tha t there are 184,546 structures inside the 1% (100-year) FEMA floodplain and 230,838 structures inside the 0.2% (500-year) FEMA floodplain within Harris County's 22 watersheds, as of June 2018 (Harris County Flood Control District, 2017).

Phase 2. Identifying priority areas within the pilot
Based upon FEMA data post-Harvey (Harris County Flood Control District, 2017), the pilot study area was then subdivided into three categories A, B and C.These categories represent the number of structures damaged during the hurricane for each of the 57 Census Block Groups (CBG), and will assist in identifying particularly sensitive areas that should be prioritized.Category A includes areas with a number of damaged structures between 500 and 1600 per CBG.Category B, between 300 and 499 per CBG.Category C, between 1 and 299 per CBG.Table 1 and Figure 7 show the results of this method.Category A areas shown in Figure 7 had the highest number    as result of an overwhelmed drainage network.

Phase 3. Exploring flood volume reduction options
Low Impact Development (LID), Sponge Cities, Eco-Boulevards, Water Squares and Green Alleys were concepts taken into consideration.LID is a comprehensive land planning and engineering design approach with the goal of maintaining the pre-development hydrologic regime in a watershed without solely using conventional development and detention basin techniques to satisfy drainage and flood mitigation requirements.Acceptable LID practices include bioretention, rainwater harvesting, green roofs, permeable pavement and underground stormwater storage, among others (Harris County Flood Control District, 2018a).The Sponge City concept was launched by the Chinese Central Government in selected pilot cities across China in 2014, primarily as a response to increasing flood impacts.The Sponge City approach aims to enhance infiltration, evapotranspiration, and capture and reuse of stormwater in the urban environment.Some of the anticipated benefits of this concept are the reduction of economic losses due to flooding and enhancement of the livability of cities (Zevenbergen and Pathirana, 2018).
The Eco-Boulevard concept is presented in various forms.One architecture firm in Chicago, Urban Lab, has partnered with the City of Chicago to cultivate the Eco-Boulevard project and transform streets into ecoboulevards that could treat the city's wastewater and stormwater naturally.Their approach is to transform existing roadways, sidewalks and parks (the public-way) into a holistic, distributed, passive bio-system for recycling Chicago's water and return all treated water to the Great Lakes, closing Chicago's water loop (Felsen, 2014).Since 2006, more than one hundred Green Alleys and Eco-Boulevards have been built in Chicago as stretches of permeable pavement and bioswales that allow storm waters to filter through and drain into the ground below, where microbes remove bacteria and pollutants before the water is allowed to run into nearby Lake Michigan."The Green Alleys create porosity and permeability so that the water can move in the ways that it moves in the hydrological cycle", according to Jay Homack, a senior landscape architect who designed most of the infrastructure (Hanley, 2017).
Water Squares combine water storage with the improvement of the quality of urban public space.They are generally used in inner-city areas with little room for water buffers and where high groundwater levels make infiltration difficult.Water Squares can be understood as a dual-strategy that makes the money invested in water storage facilities visible and enjoyable.They also generate opportunities to create environmental quality and identity to central spaces in neighborhoods.Most of the time, a water square will be dry and used as a recreational space (Atelier GroenBlauw, 2019; De Urbanisten, 2019).

Phase 4. Calculating detention volume for a 1% probability, 24-h storm
The Watershed Modeling method involves the use of the HCFCD hydrologic methodology developed to identify flood damage reduction and mitigation projects for drainage areas greater than 640 acres in Harris County (Harris County Flood Control District, 2009).Depths of direct runoff for the 24-h, 1% probability rainfall event selected for this model are provided in the HCFCD Policy Criteria and Procedure Manual (PCPM) for each rainfall region (Harris County Flood Control District, 2018b).Buffalo Bayou is classified as Region 2, and the estimated direct runoff values for three impervious conditions are shown on Table 2.
The approximate acreage of impervious surface within the pilot study area was calculated by multiplying the Percent Impervious Cover Per Land Use values (Harris County Flood Control District, 2018b) from the HCFCD Policy Criteria and Procedure Manual by the total acreage of each land use type obtained in Geoplanner for ArcGIS (Table 3).

Summary of acquired data:
Total rainfall = 13.2 inches Duration of storm = 24 h Probability = 1% Contributing area in watershed = 10,809 acres Total estimated impervious surface area = 4,063 acres Runoff Depth = 12.9 inches = 1.075 feet* *Although generalized relationship between percent land development and percent impervious cover differs between various land uses, in the best interest of this study all land uses within the pilot area were considered high-density or 85% impervious, with a runoff depth of 12.9 inches on a 100-year, 24-hour storm event.This classification was deemed appropriate due to the predicted growth in the region for the next decades, to the hydrologic soil classification throughout the area (Group D -poorly drained), and to the evident need for a drainage  system that can endure storms of greater magnitude.
Based on this information, the total detention volume needed for a 100-year, 24-h storm event in the study area should be 88,492 cubic feet (VT= [43,560 × (0.50 × impervious area)]) or 4,368 acre-feet (impervious area 4,063 acres x direct runoff 1.075 feet).

Phase 5. Detecting potential sites for the implementation of flood mitigation strategies
To have quantitative evidence of the pilot area's capability to accommodate the elected strategies, the Land Use map generated in Geoplanner for ArcGIS software -with land use data from the City of Houston GIS portal (Figure 4)was examined, and vacant and undeveloped areas were outlined for each parcel of land within the pilot area.The objective was to obtain a final acreage of potential stormwater detention/storage sites, publicly or privately owned, per land use type (Table 4).

RESULTS
Assuming the area's existing drainage system is adequate and provides the detention volume required for a 100year, 24-h storm event -as specified in the HCFCD-PCPMthis study aimed to identify opportunities to reduce riverine flooding along Buffalo Bayou as well as to identify potential locations for mitigation of secondary local drainage, during unforeseen storm events.Embedded in this purpose is the development of urban design concepts that propose to enhance quality of life of area residents while reducing flood threat.

Proposed flood mitigation strategies
In April, 2018 FEMA amended the longstanding 100-year floodplain standard for new projects such that most new buildings will now have to be two feet freeboard.Freeboard is a factor usually expressed in feet above a flood level for purposes of building above a floodplain.Today, building codes in many jurisdictions require construction to be 1-foot freeboard.The City Public Works Department produced an analysis showing that the projected water level in a 500-year floodplain plus a 2-feet elevation would have spared 84% of homes in city floodplains during Hurricane Harvey (Harris County Flood Control District, 2019b).Considering these results, the stormwater detention volume was recalculated to reflect a runoff depth of 24 inches.

Summary of the adjusted data:
Total rainfall = 24.

Along Buffalo Bayou
Allocation of stormwater storage cisterns in the 279  acres of public land along the Buffalo Bayou: These are underground units to be positioned along Buffalo Bayou at intervals to be determined contingent to technical feasibility.Each unit is to be connected to the bayou through an inlet-outlet pipe at an angle to be determined.During a storm event, if the Bayou reaches a critical level, the water pressure will cause the connection to the unit to open, redirecting part of the flow of water from the bayou into that unit.Once the unit is full, the water pressure inside the unit will cause the connection to the bayou to shut.These units will also be collecting rain from detention basins located above them, and a layer of engineered soil between the basin and the unit will promote the filtration of the water before it enters the unit.Eighteen cisterns along Buffalo Bayou's 6.4 linear miles between State Highway 6 and Beltway 8 (Figure 11), each with a 51.65 acre-feet capacity (2,250,000f 3 ), would provide 930 acre-feet of stormwater detention volume in the  pilot study area.

Beyond Buffalo Bayou
Approximately 205 acres* of private and public vacant land were identified with Geoplanner for ArcGIS as potential stormwater detention development areas (Table 4).
Those sites consist of vacant lots and segments of undeveloped land from school, commercial, office, church, industrial and residential uses.Many of those areas have the potential to being converted into flood mitigation sites through the implementation of localized storm drainage enhancements, such as the use of LID methods and, when feasible, the incorporation of underground stormwater storage units.Contingent upon the neighboring land uses, community enhancements like parks, splash pads, sport courts or water squares/circles could also be incorporated.An initial approach identified fifty possible sites where underground stormwater storage units could be implemented (Figure 13).That number of units would provide 2,582 acre-feet of detention volume.*205 acres = 484 acres total potential stormwater retention sites minus the 279 acres of public land along Buffalo Bayou.

Underground stormwater detention vaults
Approximately 116 acres of parking lot area were identified with Geoplanner for ArcGIS as potentially suitable sites for the construction of underground storage vaults within the study area.Subsurface stormwater storage provides minimal stormwater quality benefit, but can be successfully used for reduction of stormwater runoff flow.
It is also a good option for high density or urban areas with limited permeable area.From the initial 3,758 acrefeet of detention volume needed, 3,512 acre-feet can potentially be reached through the implementation of underground stormwater storage units along Buffalo Bayou and at the undeveloped sites shown in Figure 13.The remaining 245 acre-feet of detention volume can clearly be obtained with the construction of these underground storage vaults throughout the area.

Eco-Boulevards and Green Alleys
Figure 15 shows the conceptual design of a Green Alley connecting Nottingham Elementary School to Buffalo Bayou and Dairy Ashford.The school is encroached on a highly populated area and surrounded by Nottingham Forest -an eight-hundred home subdivision -two   converted into a splash park/water circle over that unit.In addition to improving stormwater drainage in areas very susceptible to flooding, eco-boulevards can provide multiple ecological and social benefits, promoting connectivity and a healthy quality of life.channel modifications, regional stormwater detention basins, major repairs to flood-damaged drainage structure, voluntary buyouts of flood-prone properties, wetland mitigation and property acquisition for preserving natural floodplains, amongst others.High in the priority list are construction-ready projects with funding partners such as USACE and FEMA.From the 237 projects, 22 are designed for the improvement of the Addicks and Barker Reservoirs, and will be evaluating the reservoirs' effectiveness and operation, de-silting channels upstream to restore channel conveyance capacity, performing right-of-way acquisitions, constructing wetland mitigation banks and rehabilitating channels inside the reservoirs (Harris County Flood Control District, 2018c).Also as part of the 2018 flood bond program, HCFCD is set to receive a federal grant to study the feasibility of constructing deep underground tunnels to move stormwater to the Houston Ship Channel without overburdening the area's bayous.Engineers envision a system in which tunnels at least twenty feet wide and one hundred and fifty feet deep use gravity to move water from upstream bayous to the Ship Channel, in some cases a distance of thirty miles.The region's flatness, soft soil and high water table present a significant obstacle, but similar low-lying cities such as Washington D.C. have been able to use this method of flood control (Zach Despart, 2019).

DISCUSSION
Current HCFCD proposed bond projects that include the pilot study area consist of drainage network repairs, construction of detention basins along Buffalo Bayou, and studies to be performed by USACE, such as the C-40 Study of Addicks and Barker Reservoirs and The Buffalo Bayou and Tributaries Resilience Study, to address flooding downstream and upstream of the reservoirs.
The present research was intended to examine potential ways to reach efficient flood control while indorsing sustainability.For instance, the reclamation of stormwater -a natural asset too important to be discarded -with the use of underground storage; the sustainabilization of urban areas via integration of LID methods; and the enhancement of community areas to promote a healthier lifestyle, with the implementation of green alleys, water squares and eco-boulevards.

Conclusions
-The methodology applied in this study has proved to expedite the research process.Outlining priority areas based on the number of structures damaged by Hurricane Harvey helped detect sources of flooding that could be irradiating the problem to surrounding areas and needed special attention.
-Some walkability challenges found in the study area included poor or deteriorated sidewalks, paths that deadend or connect to nowhere, and lack of safer street design standards.Developing attractive, comfortable and safe built environments -in which people could engage in outdoor activities without having to drive to the destination -and incorporating to that concept flood mitigation strategies, is a possibility substantiated in this study.Approximately 5.28 miles of potential Green Alleys and Eco-Boulevards were identified in the pilot study area, with the capability of connecting neighborhoods and schools to infrastructure services, parks and to Buffalo Bayou trails.In addition to the social and economic benefits discussed, the use of LID practices such as bioswales and permeable pavements in the concept would substantially improve the areas' resilience to flooding.
-3,758 acre-feet (163,698,480 cubic feet) of stormwater detention volume were deemed to be required in order to provide flood protection to the 10,809-acre study area in a model 24-h, 1% probability storm with a total rainfall amount of 24.5 inches.The proposed underground storage units can potentially provide the total detention volume and be incorporated to most priority areas.The underground storage vaults in parking lots and the LID tools integrated to green alleys and eco-boulevards would work in parallel to ensure absorbency to other impervious areas.
Institute for Urban Research for providing instrumental data and feedback, without which this report would not have been possible.

Figure 1 .
Figure 1.Map of Texas and Harris County.

Figure 2 .
Figure 2. Location of Buffalo Bayou Watershed in Harris County.2.Materials and Methods The majority of commercial and office structures are distributed along Eldridge Parkway, Dairy Ashford Road, Memorial Drive and Interstate 10.The area has eleven elementary schools, two middle schools, three high schools and ten private schools.Most of the undeveloped land is composed of parks and open space for a total of 1,019 acres, 209 of which belong to Lakeside Country Club, a private golf course, and approximately 500 to Terry Hershey Park, a public pedestrian-friendly greenbelt.Buffalo Bayou crosses the

Figure 3 .
Figure 3. Outline of pilot study area for the proposed design model.

Figure 4 :
Figure 4: Land use map of the pilot study area, generated in Geoplanner for ArcGis with GIS data from the City of Houston.

Figure 5 .
Figure 5. Current land use types and percentages within the pilot study area.

Figure 6 .
Figure 6.Floodplain coverage in the pilot study area.

Figure 7 :
Figure 7: Priority areas in the pilot study area.

Figure 9 .
Figure 9. Green Alley.San Francisco Public Utilities Commission.
5 inches Duration of storm = 24 h Probability = 1% Contributing area in watershed = 10,809 acres Total estimated impervious surface area = 4,063 acres Runoff depth = 24 inches = 2 feet VT= (impervious area= 4,063 acres x direct runoff= 2 feet) = 8,126 acre-feet Subtracting 4,638 acre-feet of provided detention volume from the 8,126 acre-feet new required detention volume, the goal is to provide the remaining 3,758 acre-feet of stormwater detention possibilities.

Figure 10 .
Figure 10.Schematic of the proposed underground stormwater storage unit.

Figure 11 .
Figure 11.Potential sites for underground stormwater storage units along Buffalo Bayou.

Figure 12 .
Figure 12.Schematic of stormwater storage unit under proposed recreation area.

Figure 13 .
Figure 13.Vacant/undeveloped sites for potential stormwater detention and storage beyond Buffalo Bayou.

Figure 14 .
Figure 14.Bioswales carry parking lot stormwater runoff to underground storage vault.

Figure 15 .
Figure 15.3D rendering of conceptual green alley produced with Esri City Engine.

Figure 16 .
Figure 16.Potential locations of Eco-Boulevards and Green Alleys.

Figure 17 .
Figure 17.Pilot study area showing location of potential flood mitigation projects.Rendered in Esri C Engine.

Table 1 :
Number of damaged structures/acre for priority areas categories A, B and C.

Table 2 :
Direct runoff values for Buffalo Bayou watershed according to HCFCD-PCPM(Harris County Flood Control District, 2018b).

Table 3 :
Total impervious area per land use type.

Table 4 :
Total acreage of potential stormwater detention and storage per land use type.

stormwater detention areas Land use type Undeveloped School Office Public Industrial Church Commercial Residential Total
Figure 8. Water Square in Benthemplein, Rotterdam, NL.