Sea Level Rise Effect on Groundwater Rise and Stormwater Retention

Article Sea Level Rise Eﬀect on Groundwater Rise and Stormwater Retention Pond Reliability

Rahman Davtalab 1 , Ali Mirchi 2,*, Rebecca J. Harris 1, Mark X. Troilo 1 and Kaveh Madani 3,4 1 Singhofen and Associates Incorporated, 11723 Orpington St., Orlando, FL 32817, USA; [email protected] (R.D.); [email protected] (R.J.H.); [email protected] (M.X.T.) 2 Department of Biosystems and Agricultural Engineering, Oklahoma State University, 111 Agricultural Hall, Stillwater, OK 74078, USA 3 Centre for Environmental Policy, Imperial College London, London SW7 2AZ, UK; [email protected] 4 Department of Physical Geography, Stockholm University, SE-106 91 Stockholm, Sweden * Correspondence: [email protected]

Received: 9 March 2020; Accepted: 12 April 2020; Published: 15 April 2020

Abstract: The coastal areas of Florida, United States, are exposed to increasing risk of flooding due to sea level rise as well as severe hurricanes. Florida regulations suggest constructing stormwater retention ponds as an option to retain excess runoff generated by the increased impervious area and to protect the environment by reducing pollutants from new developments. Groundwater level rise can significantly lower the soil storage capacity and infiltration at retention ponds, in turn, reducing the pond’s capacity to capture consecutive storms due to longer pond volume recovery time. Partial groundwater inundation can affect retention ponds’ ability to decrease peak flow rates and keep the post-development outflow lower than or equal to pre-development conditions. In this paper, the reliability and performance of a retention pond near Tampa Bay, Florida, was evaluated under sea level rise conditions. An integrated surface water and groundwater model was developed, and the groundwater table was projected for future conditions as a function of sea level rise. The results showed that sea level rise could increase the seasonal high water elevation of the retention pond up to 40 cm by mid-21st century. This increase lowered the reliability of the retention pond by about 45%. The pond failed to recover the designed treatment volume within required 72 h because of the high groundwater table, increasing the risk of pollutant discharge. Furthermore, the peak flow and volume of runoff significantly increased under sea level rise and associated groundwater table rise conditions. The study results suggest that it is imperative to consider future sea level rise conditions in stormwater design in low-lying coastal areas of Florida and around the world to prevent poor pond performance and increased risk of flooding in the future.

Keywords: sea level rise; groundwater; retention pond; reliability; Florida

1. Introduction Global warming leads to ocean thermal expansion and melting of ice sheets in Greenland and Antarctica and consequently causes sea level rise (SLR) [1]. The observed data support an increase of about 1.7 mm/year in the average global sea level rise in the 20th century, which is projected to increase about one more meter by the end of the current century [2]. Local SLR can be much more than these global averages. For instance, the local sea level rise rate is estimated to be about 4.4–8.8 mm/y in Murgia and Salento in Southern Italy [3]. The National Climate Assessment (NCA) reported that sea levels may rise up to 2 m along the United States coastlines by 2100 [4]. Local sea level rise estimates for Acapulco, Mexico, Manila, Philippines, and in Chomklao, Thailand are 7.5 mm/y, 14 mm/y, and 16.5 mm/y, respectively [5–7].

Water 2020, 12, 1129; doi:10.3390/w12041129 www.mdpi.com/journal/water Water 2020, 12, 1129 2 of 18

Most of the studies on climate change and flooding issues have addressed anthropogenic changes of rainfall intensity and frequency as well as land-use changes due to development and urbanization [8–10]. These changes substantially alter the peak flow, volume, and duration of flooding and exacerbate domestic inundation areas [11]. In addition to impacts due to rainfall and land-use changes, there is a significant marine flooding risk to highly-populated low-lying cities in coastal areas, including major economic centers such as London, New York, Shanghai, and Mumbai [12]. Globally, it is estimated that more than 20 million people live below the normal high tide elevation and more than 200 million people are under threat of extreme sea level events caused by storms [12]. The United States National Oceanic and Atmospheric Administration (NOAA) reports that about 3.2 billion people worldwide live and work within 200 km of a coastline [13,14], which can be adversely affected by SLR-related closure of access roads and reduced functionality of other infrastructure. In the United States, the state of Florida ranked 3rd for the number of National Flood Insurance Program (NFIP) claims in the last four decades [15]. The average SLR in Florida for two time horizons, namely 2060 and 2100, are estimated to be about 0.76 m (2.5 feet) and 1.95 m (6.4 feet), respectively [4]. Meanwhile, about 10% of Floridians live in areas with a ground elevation below 1.5 m [16]. Senate Bill (SB) 1094 was passed in 2015 by the Florida legislature to consider the future flood risk from storm surge and sea level rise in certain portions of local government master plans. Nevertheless, stormwater system design in the state is still, generally, based on assumptions of stationary sea level conditions, although the need for accounting for nonstationary future conditions in hydrologic analyses has been recognized [17–19]. The cost increases associated with designs that account for SLR and the combined effects of SLR and rainfall increase are significantly lower when compared to the cost to retrofit a system after construction [20]. Retention ponds are an integral component of stormwater management plans in Florida, as well as many other areas. Retention ponds help protect the infrastructure and ecosystems in coupled natural-human systems. They are typically designed to maintain the predevelopment runoff condition by detaining the outflow or infiltrating the additional runoff generated by impervious areas, and by providing pollutant removal through settling and biological uptake [11,21]. The outlet structure for retention ponds is designed to provide complete storage and percolation of a specified treatment volume to remove pollutants. This study examines how the reliability of stormwater retention ponds in regulating runoff and removing pollutants can be affected by groundwater changes due to SLR. SLR can increase groundwater-related flooding by raising groundwater tables [22–24]. The loss of percolation capability due to reduced infiltration and subsurface storage because of high groundwater tables can have a significant impact on the treatment capacity provided by a retention system. Using a case study from Tampa Bay, Florida, we demonstrate the importance of accounting for SLR-related higher groundwater tables in the design of retention ponds to prevent poor performance in the long run.

2. Study Area and Regulatory Framework The study area covers 49 hectares (122-acres) in the Southwest Florida Water Management District (SWFWMD). The project area is connected to Tampa Bay, which is classified as impaired, and, ultimately, the Gulf of Mexico to the southwest (Figure1). Florida has enacted some of the most restrictive environmental requirements in the United States to protect its water resources and natural habitats. New development is often regulated through the state’s Environment Resource Permit (ERP) requirements. The Florida Department of Environmental Protection (FDEP) has delegated this responsibility to regional water management districts. Two major regulations must be followed to obtain an ERP for a new development [25]. First, stormwater pollutant removal must be provided prior to discharging project runoff off-site, including discharge to public drainage systems or other water bodies, to ensure the new development does not increase pollutant loads. Second, off-site peak discharge for the post-development condition should not exceed pre-development discharge from the property. SWFWMD allows an exemption for Water 2020, 12, 1129 3 of 18

Water 2020, 12, x FOR PEER REVIEW 3 of 18 thenot pre- exceed and post-dischargepre-development requirement discharge from when the property. the receiving SWFWMD point allows is a tidal an waterexemption body, for such the as the Gulfpre- of and Mexico. post-discharge In this situation, requirement there when is no the requirement receiving point onpeak is a tidal rate water of discharge. body, such Since as the the Gulf discharge pointof Mexico. for this In study this situation, has about there 1.5-km is no (~1 requirem mile) distanceent on peak to therate Bayof discharge. and is not Since directly the discharge discharging to it, thepoint peak for ratethis requirementsstudy has about for 1.5-kilometer retention pond (~1 mile) design distance were considered.to the Bay and The is o ffnot-site directly discharge is typicallydischarging estimated to it, the using peak the rate rate requirements of discharge for forretention 25-year, pond 24-h design storm were for considered. open basins The and off-site the volume dischargedischarge for is 100-year,typically estimated 24-h storm using for closedthe rate basins. of discharge for 25-year, 24-h storm for open basins andThere the volume are multiple discharge ways for to 100-year, address 24-h these storm requirements. for closed basins. The treatment volume is determined based There are multiple ways to address these requirements. The treatment volume is determined upon the methodology chosen for the project, which depends on soil and groundwater conditions based upon the methodology chosen for the project, which depends on soil and groundwater at the project site. Two of the most commonly used approaches are retention- and detention-based conditions at the project site. Two of the most commonly used approaches are retention- and methodologies.detention-based Retentionmethodologies. methodology, Retention methodology, which is applied which inis applied this study, in this provides study, provides storage of the treatmentstorage of volume the treatment and recovery volume and of that recovery storage of th throughat storage percolation through percolation or evaporation or evaporation over several over days. Detentionseveral days. provides Detention temporary provides storage temporary of the storag treatmente of the volumetreatment in volume a pond in equipped a pond equipped with a control structurewith a control and bleed-down structure and device bleed-down that slowlydevice that releases slowly the releases treatment the treatment volume volume to surface to surface water over severalwater days. over several Based ondays. the Based ERP on [25 the], the ERP required [25], the treatment required treatment volume forvolume the considered for the considered project is one inchproject (2.54 is cm) one overinch (2.54 the entirecm) over development the entire develo area.pment This area. volume This mustvolume be mu recoveredst be recovered within within 72 h, so the treatment72 h, so volumethe treatment is available volume tois available capture andto ca treatpture runoand fftreatfrom runoff subsequent from subsequent storms. storms. The total The outflow total outflow for existing and post-project conditions are evaluated to illustrate the performance of for existing and post-project conditions are evaluated to illustrate the performance of the pond under the pond under these two conditions. these two conditions.

Figure 1. Location of project area in Southwest Florida. Figure 1. Location of project area in Southwest Florida. 3. Input Data 3. Input Data Primary input data include rainfall, evapotranspiration, soil properties, land-use map, and tide data. Long-termPrimary input rainfall data include data from rainfall, Next evapotranspiration, Generation Weather soil properties, Radar (NEXRAD) land-use map, was and used tide in this studydata. to Long-term perform rainfall a continuous data from simulation Next Generation of the Weather pond’s performanceRadar (NEXRAD) under was SLR. used Calibratedin this study 15-min to perform a continuous simulation of the pond’s performance under SLR. Calibrated 15-min NEXRAD rainfall data are available on a 2-km grid [26]. Atlas 14 rainfall intensity data [27] and the NEXRAD rainfall data are available on a 2-kilometer grid [26]. Atlas 14 rainfall intensity data [27] Soiland Conservation the Soil Conservation Service (SCS) Service Type (SCS) II Type Florida II Flor Modifiedida Modified rainfall rainfall distribution distribution were were used used for for synthetic designsynthetic storm design simulations. storm simulations. Based on Based Atlas on 14,Atlas the 14 25-year,, the 25-year, 24-h 24-h rainfall rainfall for for the the study study area is is about 21about cm. The 21 potentialcm. The potential evapotranspiration evapotranspiration (PET), which(PET), which is a significant is a significant loss in loss continuous in continuous simulations, wassimulations, obtained fromwas obtained United from States United Geologic States Survey Geologic (USGS). Survey The(USGS). PET The data PET were data provided were provided in the same spatialin the grid same as spatial the NEXRAD grid as the data NEXRAD on a daily data on time a daily scale. time scale. TheThe soils soils zone zone mapmap andand related parameters parameters for for modeling modeling infiltration infiltration using using the Green–Ampt the Green–Ampt methodmethod (e.g., (e.g., porosity porosity andand hydraulic conductivity) conductivity) we werere obtained obtained directly directly or based or based on data on from data the from the

National Resources Conservation Service database [28]. The majority of the project site has good soils with high infiltration and high porosity (Figure2a and Table1). The groundwater table, however, is relatively close to the ground surface, especially in the Tampa Bay area. Land-use data available from Water 2020, 12, 1129 4 of 18 the SWFWMD were used along with post development land-use for developing TR-55 [29] impervious area lookup tables (Figure2b). Existing land-use on the project site consists of mixed hardwood conifer and open land. Post development land-use conditions include commercial building space, parking lot, and a new retention pond. The Manning’s roughness values were also determined based on the land-use analysis for use by the surface water model (see Section 4.1). Two different Manning’s n values were applied for Water 2020, 12, x FOR PEER REVIEW 4 of 18 representing shallow flow (i.e., up to 0.914 m (3 ft)) and deep flow (i.e., greater than 0.914 m) conditions toNational capture greaterResources roughness Conservation close Service to the ground database surface [28]. The in shallowmajority flowof the compared project site with has good deep soils waters, whichwith willhigh haveinfiltration smaller and Manning’s high porosity roughness (Figure 2a values. and Table The 1). study The areagroundwater is mostly table, covered however, by dense is tidalrelatively marsh close plants to andthe ground brushes surf closeace, to especially the ground in the surface. Tampa The Bay density area. Land-use of the canopy data available declines from as the elevationthe SWFWMD increases, were typically used reachingalong with about post 0.914 development m above the land-use ground level,for developing which is the TR-55 basis for[29] the depthimpervious threshold area for lookup applying tables deep (Figure Manning’s 2b). Existing roughness land-use coeffi oncient. the project The model site consists generates of verticallymixed variedhardwood Manning’s conifer n values and open up to land. 0.914 mPost above development the ground land-use elevation conditions based on aninclude exponential commercial equation usingbuilding the specified space, parking shallow lot, and and deep a new flow retention Manning’s pond. n values [30]. Deep flow Manning’s roughness coefficientsThe Manning’s are used when roughness the flow values depth were exceeds also determined the 0.914 m based threshold. on the Table land-use2 shows analysis the Manning’s for use roughnessby the surface values water for di modelfferent (see land-use Section types. 4.1). Two different Manning’s n values were applied for representingTide data forshallow a nearby flow Tampa (i.e., up Bay to gage 0.914 (station m (3 ft)) ID 8726520)and deep were flow obtained (i.e., greater from than NOAA 0.914 to setm)the seaconditions level boundary to capture conditions greater forroug thishness study. close The to meanthe ground high water surface elevation in shallow (MHW) flow iscompared 0.6 m (1.98 with feet), abovedeep the waters, mean which lower will low have water smaller elevation Manning’s (MLLW), roughness which isvalues. 0.15 m The based study on area North is mostly American covered Datum ofby 1988 dense (M-NAVD-88). tidal marsh Forplants surface and waterbrushes/groundwater close to the modeling ground surface. and the The SLR density study, groundof the canopy elevation playsdeclines a key as role. the elevation Therefore, increases, a 1.5 m LiDAR-based typically reaching digital about elevation 0.914 m model above (DEM) the ground was used level, in which this study. is the basis for the depth threshold for applying deep Manning’s roughness coefficient. The model The FDEP has data for various hydrogeologic components including Florida Aquifer Units (FAU) and generates vertically varied Manning’s n values up to 0.914 m above the ground elevation based on potentiometric surface, which were also used in this study. The projected SLR for a time horizon of an exponential equation using the specified shallow and deep flow Manning’s n values [30]. Deep 2060 is about 79 cm (2.6 feet), which was determined from NCA [4] to adjust tailwater for the future flow Manning’s roughness coefficients are used when the flow depth exceeds the 0.914 m threshold. conditions simulation. Table 2 shows the Manning’s roughness values for different land-use types.

(b) Soil Map (a) Land-use Map

FigureFigure 2. 2.Natural Natural Resource Resource ConservationConservation Service (NRCS) (NRCS) soil soil map map (a ()a and) and SFWMD SFWMD land-use land-use map map (b). ( b).

Tide data for a nearby Tampa Bay gage (station ID 8726520) were obtained from NOAA to set the sea level boundary conditions for this study. The mean high water elevation (MHW) is 0.6 m (1.98 feet), above the mean lower low water elevation (MLLW), which is 0.15 m based on North American Datum of 1988 (M-NAVD-88). For surface water/groundwater modeling and the SLR study, ground elevation plays a key role. Therefore, a 1.5 meter LiDAR-based digital elevation model (DEM) was used in this study. The FDEP has data for various hydrogeologic components including Florida Aquifer Units (FAU) and potentiometric surface, which were also used in this study. The projected SLR for a time horizon of 2060 is about 79 cm (2.6 feet), which was determined from NCA [4] to adjust tailwater for the future conditions simulation.

Water 2020, 12, 1129 5 of 18

Table 1. Green–Ampt soil parameters from Natural Resource Conservation Service (NRCS) data base.

Soil Zone Kv Sat. (m/d) MC Sat. MC Res. MC Init. MC Field MC Wilt. Pore SI Bubble Press (cm) Arents, nearly level 4.0 0.451 0.018 0.091 0.091 0.035 0.502 2.7 Malabar fine sand 3.2 0.419 0.018 0.091 0.091 0.036 0.506 3.2 Myakka fine sand, 0%–2% slopes 3.4 0.416 0.020 0.099 0.099 0.039 0.552 3.8 Myakka fine sand, frequently flooded 3.4 0.455 0.024 0.121 0.121 0.048 0.549 2.8 Myakka-Urban land complex 3.6 0.407 0.009 0.051 0.051 0.018 0.575 3.7 Pomello-Urban land complex, 0%–5% slopes 6.2 0.435 0.006 0.036 0.036 0.011 0.560 3.2 Urban land 4.0 0.443 0.002 0.009 0.009 0.004 0.505 2.8 Wabasso fine sand, 0%–2% slopes 3.4 0.434 0.036 0.151 0.151 0.072 0.465 3.4 Wabasso-Urban land complex 3.0 0.409 0.014 0.067 0.067 0.027 0.478 3.9 Note: Kv Sat: saturated vertical hydraulic conductivity; MC Sat: saturated moisture content; MC Res: residual moisture content; MC Init.: initial moisture content; MC Field: moisture content at field capacity; MC Wilt: moisture content at wilting point; Pore SI: pore size index; Bubble Press: bubble pressure.

Table 2. Shallow and deep Manning’s roughness coeﬃcient (n) for 2D surface model (based on Chow [31] and TR-55 [29]).

Roughness Zone Shallow Manning’s n Deep Manning’s n Bays and Estuaries 0.060 0.030 Concrete Channel 0.060 0.030 Emergent Aquatic Vegetation 0.300 0.200 Freshwater Marshes 0.300 0.200 Hardwood Conifer Mixed 0.400 0.200 Mangrove Swamps 0.300 0.200 Natural Channel 0.100 0.080 Open Land 0.150 0.040 Paved Road 0.014 0.012 Pine Flatwoods 0.300 0.200 Reservoirs 0.300 0.200 Saltwater Marshes 0.300 0.200 Shrub and Brushland 0.300 0.150 Upland Coniferous Forest 0.400 0.200 Wet Prairies 0.300 0.200 Wetland Forested Mixed 0.400 0.200 Water 2020, 12, 1129 6 of 18 Water 2020, 12, x FOR PEER REVIEW 6 of 18

features in ICPR, such as pond control volumes and channel control volumes, force the creation of 4. Modeling Framework vertices (2D nodes) along their respective polygons during mesh creation. 4.1. HydrologicThe scenario and Hydraulicbuilder uses Model various Structure map andlayers Components and surfaces with the mesh, honeycombs and triangles to calculate the parameters required by each 2D link (triangle sides) and 2D node (vertices) for theAn hydrologic integrated and surface hydraulic water (H&H) and calculations. groundwater During model the was simulation developed process, to evaluate the model the reads eﬀects thatparameters SLR-related like roughness groundwater values, changes impervious may percentage, have on the and performance hydraulic conductivity and reliability from of lookup retention pondtables functions. related to Theeach Interconnected map layer. Climate Channel data and and 1D Pond H&H Routing input data (ICPR) are modelindependent [30], which components includes a fully-integratedof an ICPR model 1D and/2D surfacethey can water be added module to the coupled model withdatabase a 2D after groundwater 2D scenario module building was before applied. Thesimulation. coupled model allows interaction between surﬁcial aquifer systems and surface water bodies. FigureFor surface3 shows water, the the general initial ground structure condition and major was featuresassumed ofto be an dry ICPR and 2D the model. initial elevation ICPR has awas pre-processer the same as for the mesh ground generation elevation. as For well grou as andwater, scenario the builder initial thatwater must table be elevation executed was before theestimated simulation based is on conducted. a digital elevation The model model generates (DEM) and triangular water table meshes information for both from surface the NRCS water soil and groundwaterdatabase. It should calculations be noted and that parameterizes for continuous all simulations, components a “warm-up” for the simulation. run was Severalexecuted of for the a 2D featurescouple of in months ICPR, suchto allow as pondthe initial control water volumes table to andstabilize. channel The controlwarm-up volumes, simulation force was the then creation used of verticesas a “hot (2D start” nodes) for alongthe continuous their respective simulations. polygons The duringaverage mesh of wet creation. season water elevations (June through October) was used as the initial water table for discrete event simulations.

FigureFigure 3.3. GeneralGeneral structure structure and and primary primary features features of the of IC thePR 2D ICPR model. 2D DEM, model. digital DEM, elevation digital model; elevation model;H&H, H&H,hydrologic hydrologic and hydraulic; and hydraulic; DCIA, directly DCIA, directlyconnected connected impervious impervious area; SLR, area; sea SLR,level searise; level SW, rise; SW,surface surface water; water; GW, GW, groundwa groundwater;ter; CV, control CV, control volume. volume.

4.2. TheExcess scenario Rainfall, builderInfiltration, uses and various Groundwater map layers and surfaces with the mesh, honeycombs and triangles to calculate the parameters required by each 2D link (triangle sides) and 2D node (vertices) Both event-based simulations and continuous simulations were considered in this study. The for the hydrologic and hydraulic (H&H) calculations. During the simulation process, the model Green–Ampt method [32] provided in ICPR is a useful rainfall-runoff method that allows water readstracking parameters during hydrologic like roughness simulations values, throughout impervious the percentage,soil column, andincluding hydraulic dry and conductivity wet periods. from lookupThe Green–Ampt tables related method to each is based map on layer. Darcy’s Climate law: data and 1D H&H input data are independent components of an ICPR model and they can be added to the model database after 2D scenario building 𝑞=−𝑘 𝐼 before simulation. , (1)

whereFor 𝑘 surface is the water, vertical the hydraulic initial ground conductivity condition (m/day) was assumed and I is to the be drygradient and thedefined initial as elevation the wassummation the same of as ponding the ground head elevation. (H), saturated For groundwater, wetting front the below initial the water ground table (𝑍 elevation), and the was capillary estimated basedsuction on head a digital (ℎ) as elevation shown in model Figure (DEM) 4. and water table information from the NRCS soil database. It should be noted that for continuous simulations, a “warm-up” run was executed for a couple of months to allow the initial water table to stabilize. The warm-up simulation was then used as a “hot Water 2020, 12, 1129 7 of 18 start” for the continuous simulations. The average of wet season water elevations (June through October) was used as the initial water table for discrete event simulations.

4.2. Excess Rainfall, Inﬁltration, and Groundwater Both event-based simulations and continuous simulations were considered in this study. The Green–Ampt method [32] provided in ICPR is a useful rainfall-runoﬀ method that allows water tracking during hydrologic simulations throughout the soil column, including dry and wet periods. The Green–Ampt method is based on Darcy’s law:

q = kvI, (1) − where kv is the vertical hydraulic conductivity (m/day) and I is the gradient deﬁned as the summation Water 2020, 12, x FOR PEER REVIEW 7 of 18 of ponding head (H), saturated wetting front below the ground (Z f ), and the capillary suction head (hc) as shown in Figure4. 𝐼=𝐻+𝑍 +ℎ /𝑍 I = H + Z f + hc /Z f ,, (2)

Figure 4. Governing factors for vertical water movement in the Green–Ampt method. H, ponding head; Figure 4. Governing factors for vertical water movement in the Green–Ampt method. H, ponding head; Z f , saturated wetting front below the ground; Vw f , velocity of wetting front. 𝑍, saturated wetting front below the ground; 𝑉, velocity of wetting front.

ICPR estimates hc based on moisture content and bubble pressure from soil data (Table1). ICPR estimates hc based on moisture content and bubble pressure from soil data (Table 1). Velocity of the wetting front is estimated using the following equation: Velocity of the wetting front is estimated using the following equation: V = dZ /dt = q/n, (3) w𝑉 f =𝑑𝑍f /𝑑𝑡 = 𝑞/𝑛, (3) where n is thethe ﬁllablefillable porosity.porosity.

Integrating Equation (3) for time 0 to t0 and Z𝑍f betweenbetween 00 toto Z𝑍0yields: yields:

Kvt0 = z0 −ln1+ Z0, (4) (=) () ln 1 + , (4) n(H + hc) (H + hc) − H + hc Equation (4) is used to simulate decay of the infiltration rate. 𝑍 is the maximum amount of waterEquation that can (4)infiltrate is used and to simulateexcess water decay above of the th infiltratione ground represents rate. Z0 is rainfall the maximum excess or amount ponded of waterstorage. that In reality, can infiltrate rainfall and is not excess always water sufficient above to the saturate ground the represents soil column rainfall and evapotranspiration excess or ponded (ET)storage. can Inbecome reality, a significant rainfall is not loss always term. Therefor sufficiente, toICPR saturate uses thea modified soil column Green–Ampt and evapotranspiration method which is explained in the ICPR user manual [30]. Vertical water movement is only conveyed through unsaturated soil zones and horizontal conductivity is applied for water transfer in saturated soil zones. Evapotranspiration is allowed on the surface to the root depth zone which can be defined in the ICPR model. Deep percolation can also be considered, including use of the potentiometric surface data to provide estimates of gradient and leakance, although leakance was not considered in this study (see Section 4.5).

Water 2020, 12, 1129 8 of 18

(ET) can become a significant loss term. Therefore, ICPR uses a modified Green–Ampt method which is explained in the ICPR user manual [30]. Vertical water movement is only conveyed through unsaturated soil zones and horizontal conductivity is applied for water transfer in saturated soil zones. Evapotranspiration is allowed on the surface to the root depth zone which can be defined in the ICPR model. Deep percolation can also be considered, including use of the potentiometric surface data to provide estimates of gradient and leakance, although leakance was not considered in this study (see Section 4.5).

4.3. One-Dimensional Surface Water Components The ICPR 1D surface model has three primary features: nodes, links, and basins. The basin feature represents areas that contribute runoff to nodes and require a number of parameters for excess rainfall and routing calculations, including basin area, impervious and directly connected impervious area (DCIA), time of concentration, etc. Basins are used for loading of 1D nodes. Areas modeled using 2D surfaces do not require basin features (2D surface modeling is explained in the next section). A basin is associated with a node. Nodes represent locations in the model where mass balance is tracked and are used to represent ponds or drainage system junctions; the storage data for a node is typically contained within the basin assigned to that node. Runoff transfers from one node to another node through links which represent water conveyance features such as culverts, channels, overland flow weirs or other types of hydraulic connections. ICPR has three options for flow rate calculations through channels and pipes: the energy equation, the momentum equation, and the diffusive wave equation [30]. The energy method was used for flow calculations in this study:

2 2 α1V α2V Z + 1 = Z + 2 + h + h + h + h + h , (5) 1 2g 2 2g f eddy entrance exit bend where subscripts 1 and 2 represent conditions at nodes 1 and 2, respectively; Z represents elevation (m); 2 α1 and α2 are energy loss coefficients; V represents velocity (m/s); g is gravitational acceleration (m/s ); hf is the friction loss coefficient; heddy is an eddy loss coefficient; hentrance is an entrance loss coefficient; hexit is an exit loss coefficient and hbend is a bend loss coefficient. Equation (5) is solved for velocity to obtain Q as follows:

 1/2      Z1 Z2  Q =  −  , (6)  α α α α α Centrance α C α C   1 2 1 + C 1 2 + 1 + 2 exit + bend bend + ∆xC   2g A2 A2 eddy A2 A2 A2 A2 A2 f  2 − 1 1 − 2 1 2 bend where Q is flow (Velocity Area); A and A are cross sectional areas at node 1 and node 2, respectively; × 1 2 C represents different loss coefficients of eddy, entrance, exit, bend, and friction, and ∆x is the length between nodes in the X-direction. There are other links like weir or orifice which have specific equations for flow rate calculations [33]. The 1D portion of the H&H model in this study includes about 3 km of channels extending from the northeast all the way to the southwestern bay area and about 370 m of cross-drain concrete pipes along the way (Figure1).

4.4. Two-Dimensional Surface Water Components ICPR’s two-dimensional overland flow option can be applied for surface water simulation. In this method a flexible triangular computational mesh is used to simulate overland flow instead of using “traditional” lumped basins and 1D node/link networks. The 2D approach implemented in ICPR treats the triangle vertices of the mesh as 2D nodes and the sides as 2D links. The mesh is constructed semi-automatically using a number of features to “fine tune” the mesh to represent ridge lines and Water 2020, 12, 1129 9 of 18

ﬂowWater ways. 2020, The12, x FOR mesh PEER isnormally REVIEW created with a much higher resolution than would be practical9 of using 18 1D features alone. boundaryICPR automatically stage line with generates the same a time “honeycomb” stage data andwas provided “diamond” for network boundary of conditions polygons duringalong tidal mesh construction.2D areas of Thethe model. honeycombs For continuous are centered simulation, around th eache varied of the tide triangle’s data and vertices varied andprojected are analogous tide data to catchments(i.e., projected or basins per SLR) in a 1Dwere model used (Figure for the5 ).time Hydrologic series boundary parameterization data under of current the honeycombs and future is performedconditions, in essentiallyrespectively. the For same discrete manner events as foror synt 1Dhetic basins. storm The simulations, diamonds are a fixed centered value around of Seasonal the 2D linksHigh and Water are used (SHW) to calculatewas used overlandfor current ﬂow and hydraulicfuture conditions properties. at the time/stage boundary node.

Figure 5. Groundwater/surface water computational meshes for model domain (left) and project siteFigure (right). 5. Groundwater/surface water computational meshes for model domain (left) and project site (right). A series of lookup tables (e.g., rainfall excess and Manning’s roughness) are used to specify parameter4.5. Two-Dimensional values for Groundwater the H&H parameterization. Components DEM and polygon map layers of roughness, imperviousThe two-dimensional areas, soils, and rainfallgroundwater zones component are imported of ICPR into ICPR.is designed The map to simulate layers and horizontal lookup flow tables, throughthrough the the Scenario surficial Build aquifer, tool, arevertical used toleakance generate through the required the confining parameters layer, for H&Hand seepage calculations into in thechannels, mesh area. ponds, and depressions. Darcy’s law is the basis for these simulations. Like 2D surface modeling,ICPR applies the 2D agroundwater special case module of Equation uses a (5),flexib whichle computational only includes triangular hydraulic mesh head (Figure and 5). friction The andtriangles velocity are losses intersected for water with transfer porosity between and conducti the 2Dvity nodes zone andpolygon through map thelaye links.rs to estimate Water may average move betweenporosity the and 2D conductivity. surface and Triangle 1D channels nodes or are 1D inters ponds.ected This with water different transfer surfac ises, carried such as out the atground, vertices alongconfining special layer, 2D features initial knownwater table, as channel and potentiometric control and pond elevation control for volumes. extracting The the overland required flow regionparameters. boundary Deep alone percolation serves asor aleakance “glass wall” is allowed and surfacein ICPR water and the is notpotentiometric permitted surface to pass can through be used to estimate gradients and amounts of leakance. Since leakance is significantly lower than it, however, boundary stage lines along the region or 1D links with connections to a boundary node horizontal and vertical conductivity, and assuming it is constant for existing and future conditions, can be provided to allow water to leave the model domain or otherwise interact with upstream or it was not considered in the current analysis. downstream boundary conditions (i.e., water bodies with known elevations). 4.6.Tide-a Impactff ectedAssessment areas Framework of Tampa Bay serve as the tailwater. Discharge from the downstream end of the system is through a double barrel 91 cm concrete pipe to the bay outfall (Figure5). In addition, a boundaryTo reduce stage model line with run thetimes, same a two-stage time stage assessm data wasent framework provided forwas boundary applied (Figure conditions 6). In alongthe tidalfirst 2D stage, areas the of ICPR the model. 2D surface For continuousand groundwater simulation, model thewas varied used to tide conduct data andcontinuous varied projectedsimulations tide for the entire study area, which contains the Tampa Bay project site. In the second stage, a reduced data (i.e., projected per SLR) were used for the time series boundary data under current and future model domain was prepared only for the project site and a small buffer area around it. The study conditions, respectively. For discrete events or synthetic storm simulations, a fixed value of Seasonal area was simulated under current condition, as well as future SLR-conditions with higher tailwater High Water (SHW) was used for current and future conditions at the time/stage boundary node. elevations. The smaller model domain allowed evaluation of different design options and adaptation scenarios with reasonable computational effort while using consistent boundary conditions from the study area simulation. The project site model boundary conditions provided by the continuous simulation results from the study area model, including current and future SHW levels for discrete

Water 2020, 12, 1129 10 of 18

4.5. Two-Dimensional Groundwater Components The two-dimensional groundwater component of ICPR is designed to simulate horizontal flow through the surficial aquifer, vertical leakance through the confining layer, and seepage into channels, ponds, and depressions. Darcy’s law is the basis for these simulations. Like 2D surface modeling, the 2D groundwater module uses a flexible computational triangular mesh (Figure5). The triangles are intersected with porosity and conductivity zone polygon map layers to estimate average porosity and conductivity. Triangle nodes are intersected with different surfaces, such as the ground, confining layer, initial water table, and potentiometric elevation for extracting the required parameters. Deep percolation or leakance is allowed in ICPR and the potentiometric surface can Water 2020, 12, x FOR PEER REVIEW 10 of 18 be used to estimate gradients and amounts of leakance. Since leakance is significantly lower than horizontalstorm simulations and vertical and conductivity,surface water/groundwater and assuming it time is constant series fordata existing for continuous and future simulations conditions, it(Figure was not 6). considered in the current analysis. While the simulations included multiple years, 2012 was selected as a representative year for 4.6. Impact Assessment Framework current conditions to provide a basis for comparing retention pond performance under different SLR and adaptationTo reduce model scenarios. run times,The study a two-stage area received assessment 113 cm framework rainfall in was 2012, applied which (Figure is close6). to In the the long- first stage,term average the ICPR (125 2D cm). surface Likewise, and groundwater the rainfall for model the 2012 was wet used period to conduct is close continuous to the long-term simulations average. for theTropical entire Storm study Debby area, which dropped contains about the 16.5 Tampa cm of Bay rain project in the site. study In area the second between stage, 23 June a reduced and 25 model June, domain2012. However, was prepared even if only the June for the rainfall project were site excluded and a small from bu fftheer area2012 aroundwet period it. The (i.e., study consider area wasJuly simulatedto Octoberunder only), currentthe wet condition, period rainfall as well in as2012 future remains SLR-conditions close to the with long higher term average tailwater for elevations. the July- TheOctober smaller period model (58.4 domain cm vs. allowed 63.5 cm). evaluation The dry ofpe diriodfferent rainfall design foroptions this year and (November–May) adaptation scenarios was withbelow reasonable the long-term computational average (i.e., effort 60 while percent using of th consistente long-term boundary average). conditions However, from the the focus study of areathis simulation.study was on The the project wet period, site model when boundary retention conditions ponds are providedmost active. by theThus, continuous 2012 was simulation deemed suitable results fromfor baseline the study analysis area model,because including it included current an extrem ande future event SHW to examine levels forthe discreteperformance storm of simulations the system andunder surface a wide water range/groundwater of hydrologic time conditions. series data for continuous simulations (Figure6).

Figure 6. Components of the two-stage impact assessme assessmentnt framework. SHW, seasonal high water.

WhileAn ICPR the land simulations use map included layer was multiple prepared years, to reflect 2012 waspost selecteddevelopment as a representativeconditions. Southwest year for currentFlorida conditionsWater Management to provide District’s a basis for ERP comparing regulatory retention requirements pond performance were used underto design diﬀerent the SLRstormwater and adaptation facility [25]. scenarios. The stormwater The study retention area received pond 113 must cm store rainfall and in percolate 2012, which the is first close 1” to (2.54 the long-termcm) of rainfall average over (125the site cm). as Likewise, required theby SW rainfallFWMD. for theThe 2012total wetarea period of development is close to (pre the long-termand post) average.is 3966 square Tropical meters Storm (or Debby 0.98 acre) dropped and 1” about of rainfall 16.5 cm over of this rain area in the is studyabout area101 cubic between meters 23 June(or 0.082 and 25ac-ft). June, Storage 2012. However,for this volume even if is the to Junebe provided rainfall wereby a excluded364 square from meter the retention 2012 wet periodpond with (i.e., an consider outlet Julyat elevation to October of 1.3 only), M-NAVD-88. the wet period The total rainfall project in 2012area consists remains of close 1619 to m the2 (0.4 long ac) termimpervious average surfaces for the July-October(e.g., building, period parking, (58.4 and cm pavement) vs. 63.5 cm). while The the dry remaining period rainfall 1983 form2 this(0.98 year ac) is (November–May) pervious. was belowPost the development long-term average hydrologic (i.e., 60 impact percent analysis of the long-termconsisted average).of volume However, recovery theassessment focus of and this studylong-term was performance on the wet period, of the when retention retention basinponds under areSLR. most Based active. on ERP Thus, requirements, 2012 was deemed the treatment suitable volume should be recovered within 72 h. A drawdown analysis was conducted using the small project site model for current and future conditions to ensure that the volume recovery requirement is satisfied. The project site model was also used to simulate the 25 year-24 h design storm. A year- long continuous simulation was performed to evaluate performance of the retention pond under representative current and future conditions (i.e., 2012 and 2060). For the annual design storm simulation, the tailwater and groundwater elevation were assumed to be constant and were set to current and future SHW levels as estimated from the study area simulation. For the continuous simulation, the boundary data were varied with time based on results from the study area model.

Water 2020, 12, 1129 11 of 18 for baseline analysis because it included an extreme event to examine the performance of the system under a wide range of hydrologic conditions. An ICPR land use map layer was prepared to reflect post development conditions. Southwest Florida Water Management District’s ERP regulatory requirements were used to design the stormwater facility [25]. The stormwater retention pond must store and percolate the first 100 (2.54 cm) of rainfall over the site as required by SWFWMD. The total area of development (pre and post) is 3966 square meters (or 0.98 acre) and 100 of rainfall over this area is about 101 cubic meters (or 0.082 ac-ft). Storage for this volume is to be provided by a 364 square meter retention pond with an outlet at elevation of 1.3 M-NAVD-88. The total project area consists of 1619 m2 (0.4 ac) impervious surfaces (e.g., building, parking, and pavement) while the remaining 1983 m2 (0.98 ac) is pervious. Post development hydrologic impact analysis consisted of volume recovery assessment and long-term performance of the retention basin under SLR. Based on ERP requirements, the treatment volume should be recovered within 72 h. A drawdown analysis was conducted using the small project site model for current and future conditions to ensure that the volume recovery requirement is satisfied. The project site model was also used to simulate the 25 year-24 h design storm. A year-long continuous simulation was performed to evaluate performance of the retention pond under representative current and future conditions (i.e., 2012 and 2060). For the annual design storm simulation, the tailwater and groundwater elevation were assumed to be constant and were set to current and future SHW levels as estimated from the study area simulation. For the continuous simulation, the boundary data were varied with time based on results from the study area model.

4.7. Reliability of Stormwater Retention Pond The reliability of performance of the retention pond was evaluated using the results of the continuous simulation under current and future conditions. The reliability index defined by Hashimoto et al. [34] has been widely used in the water resources management literature [35–39]. In this study, reliability can be defined as the percentage of time that the retention pond is able to fully capture the storm runoff. In other words, any day with at least one-time outflow (regardless of duration), was defined as system failure or unmet target (Equation (7)). The ERP permitting criteria stipulates retention ponds on project sites discharging stormwater to an Outstanding Florida Water (OFW) or impaired waters, must retain more than 95% of storm events.

No. o f Fi = 0 R = , (7) n

F = Maximum(0, Peak CELV), (8) i i − where Peaki: peak stage for retention pond for day i; CELV: pond control elevation; Fi ﬂood depth above control structure for day i; and n: number of days which is 365 in the current application.

5. Results and Discussion

5.1. Continuous Simulation The ﬁrst set of simulations was completed for the base year of 2012 (current condition) and future year of 2060 with projected tide data as the tailwater. Year 2012 was a normal year including Tropical Storm (TP) Debby, so it was selected intentionally to examine the ﬂexibility of retention pond during the extreme event. It should be noted that Tropical Storm Debby occurred between the 23 and 25 of June 2012 and dropped more than 16.5 cm of rainfall in the study area. The total time for each simulation was 12 h and the total mass balance error was less than 0.5 percent. The hourly groundwater elevation time series was processed inside the ICPR model to generate median groundwater table elevations for current (2012) and future (2060) conditions. Figure7 displays these median groundwater elevations for each pixel of the 2D model of the study area, including Water 2020, 12, x FOR PEER REVIEW 11 of 18

this study, reliability can be defined as the percentage of time that the retention pond is able to fully capture the storm runoff. In other words, any day with at least one-time outflow (regardless of duration), was defined as system failure or unmet target (Equation (7)). The ERP permitting criteria stipulates retention ponds on project sites discharging stormwater to an Outstanding Florida Water (OFW) or impaired waters, must retain more than 95% of storm events. . 𝑅= , (7)

𝐹 =𝑀𝑎𝑥𝑖𝑚𝑢𝑚(0,𝑃𝑒𝑎𝑘 −𝐶𝐸𝐿𝑉), (8)

where 𝑃𝑒𝑎𝑘: peak stage for retention pond for day i; CELV: pond control elevation; Fi flood depth above control structure for day i; and n: number of days which is 365 in the current application.

5. Results and Discussion

5.1. Continuous Simulation The first set of simulations was completed for the base year of 2012 (current condition) and future year of 2060 with projected tide data as the tailwater. Year 2012 was a normal year including Tropical Storm (TP) Debby, so it was selected intentionally to examine the flexibility of retention pond during the extreme event. It should be noted that Tropical Storm Debby occurred between the 23 and 25 of June 2012 and dropped more than 16.5 cm of rainfall in the study area. The total time for each simulation was 12 h and the total mass balance error was less than 0.5 percent. Water 2020, The12, 1129 hourly groundwater elevation time series was processed inside the ICPR model to generate12 of 18 median groundwater table elevations for current (2012) and future (2060) conditions. Figure 7 displays these median groundwater elevations for each pixel of the 2D model of the study area, the projectincluding site. the There project is site. a significant There is a change significan betweent change current between and current future and groundwater future groundwater conditions, especiallyconditions, in the especially bay area (Pointin the bay A in area Figure (Point7), whichA in Figure is most 7), which a ffected is most by the affected tide. Theby the black tide. line The in this figureblack is the line primary in this outflowfigure is paththe primary of the studyoutflow area. path As of shown the study in Figurearea. As8, theshown discrepancies in Figure 8, between the discrepancies between groundwater elevations in current and future conditions can be up to 1 meter. groundwater elevations in current and future conditions can be up to 1 m. Another important result Another important result depicted in these profiles is that the median groundwater elevation in depictedcurrent in theseconditions profiles is below is that the the ground median surface groundwater for the majority elevation of the in flow current path conditionswhile, under is future below the groundconditions, surface forthe thegroundwater majority oftable the is flow elevated path abov while,e the under ground future surface conditions, for a significant the groundwater portion of table is elevatedthe profile above (Figure the ground8). surface for a significant portion of the profile (Figure8).

Water 2020, 12, x FOR PEER REVIEW 12 of 18

Figure 7. Spatial distribution of median groundwater elevations for current (2012) and future (2060) conditions. P denotes project site. B and A show the start and end of the main drainage flow path (black line), respectively.

Figure 9 displays the groundwater elevation at the project site for current and future conditions. The two groundwater elevation time series plots in this graph were used as boundary data (or tailwater) for the project site model (i.e., stage 2 of the impact assessment framework). Figure 9 also shows the average groundwater elevation for the project site during Florida’s wet season, namely the

five months that span from June through October. This average is known as the seasonal high water Figurelevel (SHW), 7. Spatial which distribution is a constraint of median for retention groundwater system elevations design and for analysis current in (2012) Florida. and Based future on (2060) this conditions.figure, the SHW P denotes at the project site site. increased B and A from show 0.8 the M-NAVD start and in end current of the conditions main drainage to 1.2 M-NAVD ﬂow path (black in future line), conditions respectively. in 2060.

FigureFigure 8. Comparison 8. Comparison of ground of ground surface surface elevation elevation and an mediand median current current and and future future groundwater groundwater profiles alongprofiles the primary along the outflow primary path outflow in the path study in the area. stud By andarea. A B representand A represent the start the andstart endand ofend the of primarythe outflowprimary path, outflow respectively. path, respectively.

Water 2020, 12, 1129 13 of 18 for the project site model (i.e., stage 2 of the impact assessment framework). Figure9 also shows the average groundwater elevation for the project site during Florida’s wet season, namely the ﬁve months that span from June through October. This average is known as the seasonal high water level (SHW), which is a constraint for retention system design and analysis in Florida. Based on this ﬁgure, the SHW at the project site increased from 0.8 M-NAVD in current conditions to 1.2 M-NAVD in future conditionsWater 2020, in 12 2060., x FOR PEER REVIEW 13 of 18

FigureFigure 9. 9.Groundwater Groundwater elevation elevation and and SHWSHW atat projectproject locationlocation for for current current and and future future conditions. conditions. 5.2. Retention Pond Performance 5.2. Retention Pond Performance ForFor drawdown drawdown analysis analysis of of the the retention retention pondpond forfor current and and future future conditions, conditions, the the pond’s pond’s initial initial waterwater elevation elevation was was set set to theto the top top of theof the treatment treatment volume volume and and the the boundary boundary data data were were set set to theto the fixed SHWfixed of SHW 0.8 m of and 0.8 1.2m and M-NAVD-88 1.2 M-NAVD-88 for current for current and future and future conditions, conditions, respectively. respectively. The The model model was simulatedwas simulated for 72 hfor without 72 h without any rainfall. any rainfall. Figure Figure 10 shows 10 shows the recoveredthe recovered volume volume for for the the current current and futureand conditions.future conditions. The stormwater The stormw retentionater retention pond recoverspond recovers the treatment the treatment volume volume (101 cubic (101 meters)cubic withinmeters) 26 hwithin in current 26 h in conditions, current conditions, meaning meaning that the that retention the retention pond meets pond themeets required the required water water quality criteria.quality The criteria. storage The recoverystorage recovery under futureunder conditionsfuture conditions indicates indicates that thethat retentionthe retention pond pond will will only recoveronly recover 79 cubic 79 meters cubic ofmeters its required of its required stormwater stormwater capture capture volume volume after 72 after h. In 72 this h. case,In this the case, retention the pondretention does not pond comply does with not SWFWMD’scomply with requirements, SWFWMD’s whichrequirements, are those which used inare the those permitting used in process. the In otherpermitting words, process. although In other the designedwords, although retention the pond designed fully retention meets the pond permitting fully meets requirements the permitting under currentrequirements conditions, under it fails current in important conditions, ways it fails to meetin important the same ways requirements to meet the in same 2060 requirements when impacts in of SLR2060 on when groundwater impacts tableof SLR are on accounted groundwater for table in the are analysis. accounted for in the analysis. Under the 25-year, 24-h design storm (about 21 cm), which is the basis for designing most stormwater facilities in the SWFWMD, the peak discharge flow rate for current and future conditions are 52 L/S and 76 L/S, respectively. Therefore, the projected peak flow increase for future conditions is about 47%. The peak discharged volume for current and future conditions are estimated as 143 and 288 cubic meter, respectively. Therefore, the discharged volume for future condition is more than 100% greater than current condition. The pond’s reliability was also analyzed under peak flows and fixed SHW. A strict rule was used for reliability calculations whereby a day is considered a failure if the pond has an outflow anytime

Water 2020, 12, 1129 14 of 18 during the course of that day. The pond has an outflow anytime the water elevation is higher than the control elevation. Figure 11 illustrates the pond’s water elevation under current and future conditions and the outlet invert elevation 1.3 M-NAVD, which is used as the control elevation. The results showed that under current conditions, the reliability of the retention pond is more than 99%, meaning that the pond caught and contained the runoff from the system over 99% of the time (i.e., 362 days) in 2012. Conversely, results also showed that the reliability of the stormwater pond decreased to 54% in future conditions, which is a significant drop. Of the 168 failure days (i.e., pond outflow > 0) under future condition, the outflow exceeded pre-development runoff during 160 days. These results suggest that a pond that was permittable under current conditions will be much less reliable, even unpermittable, whenWater considering 2020, 12, x FOR SLR PEER in theREVIEW mid-century. 14 of 18

120

Current Condition 100 Future Condition

Volume (Cubic Meters) (Cubic Volume 20

0 0 1020304050607080 Time (hr)

Water 2020, 12Figure, x FORFigure PEER 10. 10.Recovered REVIEW Recovered treatment treatment volumevolume for th thee current current and and future future conditions. conditions. 15 of 18

Under the 25-year, 24-h design storm (about 21 cm), which is the basis for designing most stormwater facilities in the SWFWMD, the peak discharge flow rate for current and future conditions are 52 L/S and 76 L/S, respectively. Therefore, the projected peak flow increase for future conditions is about 47%. The peak discharged volume for current and future conditions are estimated as 143 and 288 cubic meter, respectively. Therefore, the discharged volume for future condition is more than 100% greater than current condition. The pond’s reliability was also analyzed under peak flows and fixed SHW. A strict rule was used for reliability calculations whereby a day is considered a failure if the pond has an outflow anytime during the course of that day. The pond has an outflow anytime the water elevation is higher than the control elevation. Figure 11 illustrates the pond’s water elevation under current and future conditions and the outlet invert elevation 1.3 M-NAVD, which is used as the control elevation. The results showed that under current conditions, the reliability of the retention pond is more than 99%, meaning that the pond caught and contained the runoff from the system over 99% of the time (i.e., 362 days) in 2012. Conversely, results also showed that the reliability of the stormwater pond decreased to 54% in future conditions, which is a significant drop. Of the 168 failure days (i.e., pond outflow > 0) under future condition, the outflow exceeded pre-development runoff during 160 days. These results suggest that a pond that was permittable under current conditions will be much less reliable, even unpermittable, when considering SLR in the mid-century.

FigureFigure 11. 11.Pond Pond water water elevation elevation underunder cu currentrrent and and future future conditions. conditions.

5.3. Adaptation Scenario Several methods can be used to improve the performance of a stormwater system in areas of high groundwater table. Low Impact Development (LID) approaches and raising the site elevation through additional fill are two potential adaptation options. Here, increasing the project site ground elevation by adding fill was evaluated as a possible option. Adding fill helps address the stormwater management system’s deficiencies under future conditions by allowing to build the retention pond on higher ground, thus providing increased storage. In the first attempt, 15 cm of fill was applied, which increased the reliability of the stormwater pond to 89% (Figure 12). Although this is a significant improvement, the pond still fails to meet the ERP permitting criteria. Since Tampa Bay is listed as impaired waters, the increased reliability will be inadequate from a permitting standpoint. Adding 30 cm of fill will increase the reliability of the stormwater pond at the Tampa Bay project site, in future conditions, to 98% which is acceptable based on most permit regulations (Figure 12).

Water 2020, 12, 1129 15 of 18

5.3. Adaptation Scenario Several methods can be used to improve the performance of a stormwater system in areas of high groundwater table. Low Impact Development (LID) approaches and raising the site elevation through additional fill are two potential adaptation options. Here, increasing the project site ground elevation by adding fill was evaluated as a possible option. Adding fill helps address the stormwater management system’s deficiencies under future conditions by allowing to build the retention pond on higher ground, thus providing increased storage. In the first attempt, 15 cm of fill was applied, which increased the reliability of the stormwater pond to 89% (Figure 12). Although this is a significant improvement, the pond still fails to meet the ERP permitting criteria. Since Tampa Bay is listed as impaired waters, the increased reliability will be inadequate from a permitting standpoint. Adding 30 cm of fill will increase the reliability of the stormwater pond at the Tampa Bay project site, in future conditions, toWater 98% 2020 which, 12, x isFOR acceptable PEER REVIEW based on most permit regulations (Figure 12). 16 of 18

Figure 12. Pond water elevation under two different fill scenarios of 15 cm (6 inch) and 30 cm (1 foot) Figure 12. Pond water elevation under two different fill scenarios of 15 cm (6 inch) and 30 cm (1 foot) as adaptation options. as adaptation options. 6. Conclusions 6. Conclusions Florida has adopted stringent requirements for environmental protection and stormwater permitting.Florida Despitehas adopted this stringentstringent policy,requirements the results for ofenvironmental this case study protection in the Tampaand stormwater Bay area showpermitting. that SLR Despite and related this stringent groundwater policy, eff theects results may become of this acase significant study in challenge the Tampa for Bay implementing area show floodthat SLR protection and related measures groundwater in low-lying effects coastal may areas. become Currently, a significant for most challenge of permit for implementing studies, the climate flood dataprotection and boundary measures data in low-lying are assumed coastal to areas. be stationary Currently, with for respectmost of topermit SLR. studies, The simulation the climate results data demonstrateand boundary that data SLR are can assumed substantially to be increase stationary groundwater with respect elevations to SLR. and The decrease simulation inthe results soil storagedemonstrate capacity. that In SLR the presentedcan substant caseially study, increase this change groundwater would double elevations the discharge and decrease volume in releasedthe soil fromstorage stormwater capacity. pondIn the and presented increase case the study, peak flow this bychange about would 50%. double Although the not discharge covered volume in the released from stormwater pond and increase the peak flow by about 50%. Although not covered in the present analysis, stronger and more frequent recent tropical storms and hurricanes would further exacerbate the performance of stormwater retention ponds. The cumulative effects of peak flow increases reduced percolation from stormwater ponds in coastal communities can pose a significant challenge for existing primary drainage systems, which are designed for current conditions (or older). Therefore, more domestic road flooding should be expected toward the mid-century as a result of SLR-related groundwater rise alone. By the same token, under future conditions, there will be more incidents of retention ponds failing to recover treatment volumes within the required time period. These incidences will lead to release of untreated stormwater and could lead to environmental risks in the future unless plausible groundwater table rise scenarios are considered in the design and retrofit of retention ponds.

Author Contributions: Conceptualization, R.D., M.X.T., and A.M.; methodology, R.D. and A.M.; validation, R.D.; formal analysis, R.D.; writing—original draft preparation, R.D.; writing—technical review and editing, R.J.H., M.X.T., A.M. and K.M.; visualization, R.D.; supervision, M.X.T. and A.M. All authors have read and agreed to the published version of the manuscript.

Water 2020, 12, 1129 16 of 18 present analysis, stronger and more frequent recent tropical storms and hurricanes would further exacerbate the performance of stormwater retention ponds. The cumulative effects of peak flow increases reduced percolation from stormwater ponds in coastal communities can pose a significant challenge for existing primary drainage systems, which are designed for current conditions (or older). Therefore, more domestic road flooding should be expected toward the mid-century as a result of SLR-related groundwater rise alone. By the same token, under future conditions, there will be more incidents of retention ponds failing to recover treatment volumes within the required time period. These incidences will lead to release of untreated stormwater and could lead to environmental risks in the future unless plausible groundwater table rise scenarios are considered in the design and retrofit of retention ponds.

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