Development of an Integrated Watershed/River Model for Flood Management: Assessment of a Record Breaking Event in March 2010 in the Pawtuxet River, RI. Soroush Kouhi1,*, M. Reza Hashemi1,**, Rozita Kian1, Stephanie Steele1, Malcolm Spaulding1, Chris Damon2, and James Boyd3 1 Department of Ocean Engineering; Graduate School of Oceanography, University of 2 Environmental Data Center, University of Rhode Island 3 Rhode Island Coastal Resources Management Council * [email protected], ** [email protected]

Introduction Model Uncertainty Analysis – Continued Effect of Debris on Flooding A record-breaking flood event (about 500-yr return period) that occurred in March 2010 in Rhode Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) and River Analysis Figure 8(left) shows the timeseries of the flow discharge(computed by HEC-HMS) corresponding Debris such as tree limbs and accumulations of trash may contribute significantly to blockage Island (Figure 1), initiated several studies to understand and develop mitigation strategies to System (HEC-RAS) are implemented to calculate runoff and river flooding in this study, re- to various precipitation datasets. Figure 8(right) shows the 90% confidence interval for the flood of flow under bridges. In particular, in extreme storms and weather emergencies, tree trunks or address flooding impacts along the Pawtuxet River. We have developed a spatially distributed spectively. Figure 4 represents the steps applied in the modeling. Spatial distributed data are discharge. Note that the observed discharge is within this uncertainty interval. branches may be broken off into rivers, without chance for removal. During the site visits, we hydrological/hydraulic modeling system for the entire watershed and river using the state of the pre-processed in ArcGIS and HEC-GeoHMS to produce the drainage network model, acting as an observed a lot of broken trees and wooden debris in the river stream or floodplains, one example art GIS-based numerical models, and the most recent watershed and river data. input to the rain-runoff model in HEC-HMS. The HEC-HMS model also takes distributed basin of which is shown in Figure 13(left). Figure 13(right) shows the Bridge. The data, meteorological data, soil/landuse data, baseflow, and modeling control specifications to pier has a width of 7.5 ft. Lagasse et al. 2010[5] suggests the average width of debris to be 15 compute the timeseries of discharge. From the HEC-HMS model, timeseries of flow at upstream times a pier width and the height of the debris to be 0.33–0.5 of the water depth; based on this of rivers are used as input into the river hydraulic model (HEC-RAS), along with river cross assumption, a block of debris with the dimension of 80 ft width and 6 ft height was simulated. sections, channel geometry, river structures, and roughness. Finally, from the hydraulic model, the water elevations are computed. These elevations are superimposed in DEM to compute flood maps in HEC-GeoRAS.

Figure 1: Warwick Mall [1] (left) , Warwick Sewer Treatment Facility [2] (right) during March 2010 Flood. Figure 7: Daily precipitation data based on various data sources (left), and 90% confidence intervals (right) for mid-March to early April, 2010. See Table 2 for sources of precipitation data. Objectives Figure 13: A broken tree (debris) in the North Branch Pawtuxet River (left), and Pawtuxet Village Bridge (Right). • Developing a web/GIS-Based watershed/river model for the Pawtuxet watershed to predict Figure 14 shows the water depth after adding debris to the piers of the Pawtuxet Village bridge flooding along the river flood plains. for a 100-yr event. As it can be seen, debris can significantly increase the flooding extent of an • Assessment of the watershed issues using the developed model: impact of the Scituate Reser- event, and should be modeled for more accurate flood risk assessments. voir on flooding, effect of historical dams on flooding, dam removals, debris, and levee heights. Figure 4: Flowchart of the distributed hydrologic and hydraulic modeling system for flood maping [4]. • Paving the way for a real-time forecasting system for this river, and other rivers in RI. Study Area Simulation Methods The Pawtuxet River is located on the western side of in RI. The geographical The methods used in the rainfall-runoff calculations in HEC-HMS were SCS for surface runoff focus area in this study is the Pawtuxet River watershed, encompassing the Main, North, and calculation, monthly constant method for the baseflow calculation, SCS unit hydrograph for South Branches Pawtuxet River. Inside the watershed are two major reservoirs: the Scituate subbasin flow routing, and lag time method for reach routing calculations. Selections are mostly and the Flat River Reservoirs, as well as several structures (historical dams and bridges) along based on the similar studies in the United States and other countries. Other choices may also Figure 8: Computed HEC-HMS flow discharge based on various daily precipitation data (left), and the confidence the river (Figure 2). lead to similar results by proper calibration, but it is better to use the most appropriate method interval of the flow discharge prediction (right). Table 1 shows an overview of the drainage areas of subbasins in this watershed. Figure 3 shows based on a watershed characteristics. Infiltration or runoff rates are predicted from the empirical the peak discharges for 10-yr, 50-yr, 100-yr, and 500-yr return periods in the Main, North, and loss rate parameter Curve Number (CN). CN is based on the area’s soil type, land use, hydrologic The Effect of on Flooding South branches of the Pawtuxet River [11]. The drainage area ratio is higher for the North condition, and depth of high water table. The runoff equation is expressed as The Scituate Reservoir is located in the middle of the northern part of the Pawtuxet River Branch than for the South Branch. It is interesting to note that the ratio of flow peaks are very 2 watershed. The Scituate Reservoir provides over 60% of the Rhode Island drinking water. The (P − 0.2S) close to the ratio of drainage areas. P = , (1) specifications of the spillway in the Scituate Reservoir are summarized in Table 3. Figure 9 e P + 0.8S shows the relationship between the discharge and water elevation for the spillway. The Scituate where Pe is excess rainfall (runoff), P is precipitation, and S is potential maximum soil moisture which in SI units is Reservoir has the capability to retain runoff during high flow periods as it has a very large reservoir (150 MCM). Figure 10 plots the modeled inflow and outflow to the ogee spillway in the 25400 − 254CN S = . (2) Scituate Reservoir when the reservoir is at full capacity (left), and also when the reservoir water CN elevation is 4 ft below the crest elevation (right). During March 28 – April 4, 2010 the reservoir Figure 14: Effect of debris on the 100-yr flooding extent at the Pawtuxet Village bridge. where 30 <= CN <= 100. The soil type data, land cover and resulting CN map of the Pawtuxet was almost full, but if the Scituate reservoir was just 4 ft below the spillway crest elevation, River watershed are shown in Figure 5. the peak flood discharge would decrease 60%. Adding this flood capacity could be potentially Online GIS-Based Tool to Access Flooding Maps: achieved by draining the reservoir before a rainstorm, or adding controlled gates to the spillway STORMTOOLS [8]. STORMTOOLS is a comprehensive mapping tool for Rhode Island that was originally designed Table 3: The Scituate Reservoir spillway details. for coastal flooding. It provides the ability for homeowners and municipalities to understand Spillway Type Ogee about the risk of their properties encountering floods and storms. It includes sea level rise, coastal Abutments 2 erosion, and coastal flood inundation. The general public has open access to STORMTOOLS Abutment Type Concrete and can view different flood scenarios for coastal areas within Rhode Island. The integration of Vertical Datum/Unit NAVD1988/ft Pawtuxet River flooding maps in STORMTOOLS is an outcome of and one of the contributions Apporach Loss 0.00 Crest Elevation 286.12 of this study (Figure 15). The vision is to integrate inland and coastal flooding in this tool for Crest Length 440.00 all rivers of Rhode Island [9]. Apron Elevation 267.12 Apron Length 200.00 Design Head 8.00 Spillway Coefficient (C) 3.35 (ft1/2) Figure 9: Outflow from ogee spillway

Figure 15: Flood maps publicly available in ArcGIS online.

Conclusions and Recommendations

Figure 5: CN map of the Pawtuxet River watershed (right), Land cover of the Pawtuxet River watershed in 2011(top • The integrated Pawtuxet River watershed and river model developed here can predict and left), and soil type data (bottom left). Figure 10: Effect of Scituate Reservoir on flooding when the reservoir if full (left), and when the reservoir is partially help mitigate flooding damages in future flooding situations. The model can generate the Figure 2: The Pawtuxet River watershed divided into 9 subbasins. The area upstream of the Scituate Reservoir filled (right; water level 4 ft below the spillway crest). flooding maps for any event. outlined and shaded in green (A), area upstream of the Flat River Reservoir outlined and shaded in pink (B), area Calibration and Validation The models and the methodology provide a framework for the real-time flood forecasting of downstream of both reservoirs outlined in orange (C). See Table 1 for more details. Effect of Historical Dams on Flooding • The parameters utilized in watershed modeling such as curve number (CN), time lag, initial inland flooding in state of RI. Table 1: Overview of the Pawtuxet River watershed. abstraction, and baseflow are used in calibration to obtain a best fit of model to the observed The Pawtuxet River watershed became a center of textile manufacturing in the 19th and 20th • Our analysis shows that the model is sensitive to uncertainty in the input precipitation data District Area Area data. Figure 6 shows the validation of the model for March 2010 flood with a 5% of error in the centuries, with many dams created along the river: four in the Main Branch, six in the North more than other parameters; ensemble approach for precipitation gives a more accurate and 2 (mi ) Ratio peak values. Branch, and ten in the South Branch of the river. Figure 11(left) shows the location of the reliable range of model predictions. Total Watershed 229.2 100% four dams in the Main branch of Pawtuxet River. During the 2010 flooding events a portion • The Scituate reservoir elevation can be potentially controlled to mitigate flooding. For in- Upstream of the the highways and the Warwick Mall were inundated. We investigated the effect of removing 90.8 40% stance, a 4 ft capacity in reservoir reduces the peak discharge by more than 60% in a 500-yr Scituate reservoir (A) Pontiac Dam (See Figure 11(right)) downstream of this critical area. The width and the height event, for the North Branch. Using this reservoir is the most effective way to control flood in Upstream or the of the dam is about 100 ft and 8 ft, respectively. It seems that the Pontiac historical dam has a 62.6 27% the Pawtuxet River. As this reservoir is designed for drinking water supply, further research Flat river reservoir (B) large influence on the flooding around the Warwick Mall, which was flooded during the March is necessary to examine this option. Downstream of 2010 event. Figure 12 shows the reduced flooded area in Warwick Mall and upstream area after 75.8 33% • For flood mitigation in the area, we found that removal of the Pontiac Dam in the main branch the reservoirs (C) removing Pontiac Dam in the model for a 100-yr flood event. Results also show that the impact of the Pawtuxet River would decrease flood area extent by 31.8%, 26.1%, and 5.4% in a 50, Upstream of the of this dam for larger events (e.g., 500-yr) is less. 201.0 92% 100, and 500 year flood, respectively. Nevertheless, the Scituate reservoir can mitigate flood Cranston gauge Figure 3: Peak discharges for 10-yr, 50-yr, 100-yr, and 500-yr everywhere and is potentially more effective for flood risk management. Downstream of the 28.2 8% return periods in the North, South, and Main Branches of the Cranston gauge • The presence of debris significantly increases flooding extent. We recommend that the effects Pawtuxet River [11]. of debris be considered in flood risk assessments. The model can be maintained and updated as a real time flood forecasting tool for the river. Data • • With the developed method, inland and coastal floodings can be simulated at once (e.g., Several meteorological, land based, and hydrological data as well as information about river and hurricane inland/coastal flood risk assesment). watershed structures were used in this study. Table 2 shows the sources of the data that we have • For future, we recommend the following: collected. • Study the consequences of possible catastrophic historical dam failures on flood risk and water quality during major floods. Table 2: Sources of the data used for development of the model • Simulating the impacts of climate change on the watershed/river flooding risk. Figure 11: Location of the historical dams in the Main branch of Pawtuxet River: 1- Natick Pond Dam, 2- Pontiac • Installing a stream gauge downstream of the Scituate Reservoir. Data Source Dam 3- Pontiac Downstream Dam 4- Pawtuxet Falls Dam (left), and Pontiac dam during a flood event[6] (right). • Installing a meteorological station for precipitation and temperature measurements in the watershed upstream Meteorological data areas. • Effective management of the Scituate Reservoir for flood control. NOAA’s National Centers for Examining more advanced rainfall runoff methods which will be available in new release of HEC-HMS such Observed Precipitation https://www.ncdc.noaa.gov Figure 6: Model validation at USGS streamgauge in Cranston gauge for 2010 flood: hydrographs for validated • Environmental Information (NCEI) as precipitation dependent Clark unit hydrograph. Climate Forecast System model and observed data (up), and hourly precipitation (bottom). • Develop a real time forecasting model: HEC-HMS, HEC-RAS, and data sources can be integrated in HEC- CFSR Precipitation https://globalweather.tamu.edu/ Reanalysis (CFSR) RTS for real time forecasting [10]. European Centre for Medium-Range http://www.ecmwf.int/en/ Uncertainty Analysis ECMWF Precipitation Acknowledgements Weather Forecasts forecasts/datasets Lack of observed hourly/daily precipitation data in many events in past/future can lead to This study was carried out under a cooperative agreement with the Rhode Island Coastal Resources Management Council with funding from the U.S. Department of Housing and NWS Precipitation National Weather Sevice (NWS) https://radar.weather.gov large uncertainties. For the HEC-HMS model, hourly data were available from one observation Urban Development Community Development Block Grant administered by the Rhode Island Office of Housing and Community Development. Watershed data gauge in Pawtuxet River Watershed, located at the T.F. Green Airport in Warwick. Other DEM Rhode Island Geographical References http://rigis.org supplementary data to fill the gaps are the CFSR, ECMWF, NWS precipitation datasets (Table (1 meter resolution) Information System (RIGIS) 2). Figure 7(left) shows the uncertainty in the precipitation data. Referring to Equation 3, we [1] National Weather Service, NOAA. Website: www.weather.gov, 2016. Landuse/cover (LULC) RIGIS http://rigis.org [2] Warwick Sewer Authority. Website: www.warwicksewerauthority.com, 2016. used the four precipitation datasets to find the confidence interval for the precipitation (Figure [3] Gardner Bent. Data for the Pawtuxet River HEC-RAS Model, Personal Communication, 2015. United States Department of 7(right)). The upper and lower limits for precipitation were used in HEC-HMS model to find a [4] MR Knebl, Z-L Yang, K Hutchison, and DR Maidment. Regional scale flood modeling using NEXRAD rainfall, GIS, and HEC-HMS/RAS: a case study for the san antonio river Soil Types http://www.nrcs.usda.gov basin summer 2002 storm event. Journal of Environmental Management, 75(4):325–336, 2005. Agriculture (USDA) probabilistic distribution of flood discharge timeseries. [5] PF Lagasse, PE Clopper, LW Zevenbergen, WJ Spitz, and LG Girard. Effects of debris on bridge pier scour, NCHRP Rep. 653. Transp. Res. Board, Washington, D. C, 2010. Reservoir data Providence Water Supply Personal Communication[7] [6] Alisa Richardson. Poniac historical diversion dam data, Personal Communication, 2016.  s   s  River Data X¯ − tα √ < µ < X¯ + tα √ , (3) [7] Steve Soito. Scituate Reservoir data, personal communication, 2016. 2 n 2 n [8] Alvaro Sordo-Ward, Luis Garrote, M Dolores Bejarano, and Luis G Castillo. Extreme flood abatement in large dams with gate-controlled spillways. Journal of Hydrology, United States Geological Survey 498:113–123, 2013. Observed Discharge https://waterdata.usgs.gov/ri ¯ [9] Malcolm L. Spaulding, Annette Grilli, Chris Damon, Tresa Crean, Grover Fugate, Bryan A. Oakley, and Peter Stempel. STORMTOOLS: Coastal Environmental Risk Index (USGS) where µ is the population mean, X is the sample mean, s is the sample standard deviation, n is (CERI). Marine Science and Engineering, 2016. the sample size, and tα is computed based on the Student t-distribution, degree of freedom and [10] Ayushi Vyas and John Siby. A real-time decision support system for river basin management. In MATEC Web of Conferences, volume 57. EDP Sciences, 2016. River Geometry/ 2 USGS Personal Communication[3] [11] Phillip J Zarriello, Elizabeth A Ahearn, and Sara B Levin. Magnitude of flood flows for selected annual exceedance probabilities in Rhode Island through 2010. Technical report, Structures data percentage of error, where the degree of freedom is n − 1. Figure 12: Effect of Pantiac dam removal on flooding during a 100-yr event. Reduced flood areas shown in yellow. US Geological Survey, 2012.