Prepared in cooperation with the Rhode Island Emergency Management Agency

and the U.S. Army Corps of Engineers

Flood-Inundation Maps for the Pawtuxet River in 

West Warwick, Warwick, and Cranston, Rhode Island

INTERSTATE I-95

STATE ROUTE 2

STATE ROUTE 5

INTERSTATE I-295

STATE ROUTE 113

INTERSTATE I-295

STATE ROUTE 2

Washington Secondary Bike Path

Warwick Mall

West

Warwick 

Wastewater

Treatment 

Facilty

Pawtuxet River

Pawtuxet River

Meshanticut River

Scientific Investigations Report 2018–5043

U.S. Department of the Interior

U.S. Geological Survey
Cover. Flood-inundation map for a reach of the Pawtuxet River in Warwick, Rhode Island, corresponding to a stage of 21.0 feet, gage 

datum, approximately depicting the March 31, 2010, peak flood stage of 20.79 feet at U.S. Geological Survey Pawtuxet River at Cranston, 

Rhode Island, streamgage 01116500. 
Flood-Inundation Maps for the Pawtuxet 

River in West Warwick, Warwick, and 

Cranston, Rhode Island

By Gardner C. Bent and Pamela J. Lombard

Prepared in cooperation with the Rhode Island Emergency Management Agency

and the U.S. Army Corps of Engineers

Scientific Investigations Report 2018–5043

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Bent, G.C., and Lombard, P.J., 2018, Flood-inundation maps for the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island: U.S. Geological Survey Scientific Investigations Report 2018–5043, 16 p.,

https://doi.org/10.3133/sir20185043.

ISSN 2328-0328 (online)
iii

Acknowledgments

The authors thank the Rhode Island Water Resources Board for funding the operation and 

maintenance of the U.S. Geological Survey (USGS) streamgage at Pawtuxet River at Cranston, 

Rhode Island, 01116500, used for this study. Special thanks are given to the National Weather 

Service for providing forecasted river stages for the streamgage and their continued support of 

the USGS flood-inundation mapping program. The authors thank the Federal Emergency Management Agency for providing the hydraulic model used in the 2015 effective Flood Insurance 

Studies for Kent and Providence Counties, Rhode Island, and the U.S. Army Corps of Engineers 

personnel for surveying the former Pawtuxet Falls dam area in April 2016 and entering surveyed 

data into the hydraulic model. 

The authors also thank USGS field personnel for surveying hydraulic structures and cross sections on the Pawtuxet River during 2011 and August 2016 and USGS personnel for flood map 

preparation.

v

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................4

Study Area Description........................................................................................................................4

Previous Studies ...................................................................................................................................5

Creation of Flood-Inundation-Map Library................................................................................................5

Computation of Water-Surface Profiles............................................................................................6

Hydrology ......................................................................................................................................6

Topographic and Bathymetric Data..........................................................................................6

Hydraulic Model...........................................................................................................................7

Development of Water-Surface Profiles..................................................................................9

Flood-Inundation Maps........................................................................................................................9

Flood-Inundation Map Delivery...............................................................................................13

Disclaimer For Flood-Inundation Maps..................................................................................13

Uncertainties and Limitations Regarding Use of Flood-Inundation Maps .......................13

Summary........................................................................................................................................................14

References Cited..........................................................................................................................................14

Figures

1. Map showing location of the Pawtuxet River study reach in West Warwick,

Warwick, and Cranston, Rhode Island, and U.S. Geological Survey Pawtuxet River

at Cranston streamgage 01116500 .............................................................................................2

2. Graphs showing cumulative precipitation, stage, and streamflow data for the

Pawtuxet River, Rhode Island, during the floods of February 23, 2010, through April

5, 2010: A, cumulative daily precipitation at the National Weather Service

climatological station at Providence T.F. Green Airport, Warwick, Rhode Island, B,

stage, and C, streamflow at U.S. Geological Survey Pawtuxet River at Cranston,

Rhode Island, streamgage 01116500. .......................................................................................3

3. Flood-inundation map for a reach of the Pawtuxet River in Warwick, Rhode Island,

corresponding to a stage of 21.0 feet, gage datum, approximately depicting the

March 31, 2010, peak flood stage of 20.79 feet at U.S. Geological Survey Pawtuxet

River at Cranston, Rhode Island, streamgage 01116500 ......................................................12

Tables

1. Description of U.S. Geological Survey Pawtuxet River at Cranston, Rhode Island,

streamgage 01116500...................................................................................................................4

2. Peak-discharge estimates for selected annual exceedance probabilities at the U.S.

Geological Survey Pawtuxet River at Cranston, Rhode Island, streamgage

01116500, using annual peak discharges for water years 1940–2015..................................5

3. Estimated discharges for selected locations used in the hydraulic model of the 

Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island, and the 

corresponding discharges, stages, and water-surface elevations at U.S. Geological 

Survey Pawtuxet River at Cranston, Rhode Island, streamgage 01116500.........................7
vi

4. Description of bridge crossings, dams, and a levee in the hydraulic model of the

Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island.........................8

5. Differences between 15 simulated water-surface elevations and stage elevations

at 1-foot intervals from 8.0 to 22.0 feet for U.S. Geological Survey Pawtuxet River at

Cranston, Rhode Island, streamgage 01116500 .......................................................................9

6. Differences between the hydraulic model simulated water-surface elevations for

the March 31, 2010, floodflows and the surveyed high-water-mark elevations on the

Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island.......................10

7. Stage, discharge, and approximate annual exceedance probability at U.S. Geological 

Survey Pawtuxet River at Cranston, Rhode Island, streamgage 01116500 for 

profiles mapped on the Pawtuxet River in West Warwick, Warwick, and Cranston, 

Rhode Island................................................................................................................................11
vii

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Datum

Vertical coordinate information is referenced to either stage (the height above an arbitrary 

datum established at a streamgage) or to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Abbreviations

AEP annual exceedance probability

AHPS Advanced Hydrologic Prediction Service

DEM digital elevation model

DGPS differential global positioning system

FEMA Federal Emergency Management Agency

FIS Flood Insurance Study

GIS geographic information system

HWM high-water mark

lidar light detection and ranging

NGS National Geodetic Survey

NWIS National Water Information System

NWS National Weather Service

RTK real-time kinematic

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey

WWTF wastewater treatment facility

Flood-Inundation Maps for the Pawtuxet River in West 

Warwick, Warwick, and Cranston, Rhode Island

By Gardner C. Bent and Pamela J. Lombard

Abstract

A series of 15 digital flood-inundation maps was developed for a 10.2-mile reach of the Pawtuxet River in the 

municipalities of West Warwick, Warwick, and Cranston, 

Rhode Island, by the U.S. Geological Survey (USGS), in 

cooperation with the Rhode Island Emergency Management 

Agency and the U.S. Army Corps of Engineers. The coverage of the maps extends downstream from Natick Pond dam 

near State Route 33/Providence Street bridge in West Warwick 

to the mouth of the river at Pawtuxet Cove (Broad Street 

bridge) on the border between Cranston and Warwick, R.I. A 

one-dimensional step-backwater hydraulic model created and 

calibrated for the Federal Emergency Management Agency 

Flood Insurance Studies for Kent and Providence Counties in 

2015 was updated for this study. The updated hydraulic model 

reflects the removal of the Pawtuxet Falls dam during 2011 

and the raised elevation of a levee surrounding the Warwick 

Sewer Authority wastewater treatment facility during 2014–

17. The hydraulic model was calibrated by using the current 

(2018) stage-discharge relation at the USGS Pawtuxet River at 

Cranston, Rhode Island, streamgage (01116500) and documented high-water marks from the March 31, 2010, flood, 

which had a peak flow greater than the estimated 0.2-percent 

annual exceedance probability floodflow.

The hydraulic model was used to compute water-surface 

profiles for 15 flood stages at 1-foot (ft) intervals referenced 

to the USGS Pawtuxet River at Cranston, Rhode Island, 

streamgage (01116500) and ranging from 8.0 ft (15.2 ft, North 

American Vertical Datum of 1988), which is the National 

Weather Service Advanced Hydrologic Prediction Service 

flood category “action stage,” to 22.0 ft (29.2 ft, North 

American Vertical Datum of 1988), which is the maximum 

stage of the stage-discharge relation at the streamgage and 

exceeds the National Weather Service Advanced Hydrologic 

Prediction Service flood category “major flood stage” of 

13.0 ft. The simulated water-surface profiles were combined 

with a geographic information system digital elevation model 

derived from light detection and ranging (lidar) data with a 

1.0-ft vertical accuracy to create flood-inundation maps. The 

flood-inundation maps depict estimates of the areal extent 

and depth of flooding corresponding to 15 selected flood 

stages at the streamgage. The flood-inundation maps depict 

only riverine flooding and do not depict any tidal backwater 

or coastal storm surge that might occur in the lower part of 

the river reach. The flood-inundation maps can be accessed 

through the USGS Flood Inundation Mapping Science website 

at https://water.usgs.gov/osw/flood_inundation. Near-real-time 

stages and discharges at the Pawtuxet River streamgage can be 

obtained from the USGS National Water Information System 

at https://waterdata.usgs.gov/. The National Weather Service 

Advanced Hydrologic Prediction Service provides flood forecasts of stage for this site (CRAR1) at https:/water.weather.

gov/ahps/. 

The availability of flood-inundation maps referenced to 

current and forecasted water levels at the USGS Pawtuxet 

River at Cranston, Rhode Island, streamgage (01116500) can 

provide emergency management personnel and residents with 

information that is critical for flood response activities, such as 

evacuations and road closures, and postflood recovery efforts. 

The flood-inundation maps are nonregulatory but provide Federal, State, and local agencies and the public with estimates of 

the potential extent of flooding during flood events.

Introduction

During late February through early April 2010, the State 

of Rhode Island, particularly the Pawtuxet River Basin, was 

affected by major flooding. This flooding was characterized 

as the worst in 200 years. Damages were estimated to be 

upward of $100 million, and the entire State was included in 

the President’s emergency declaration (Rhode Island severe 

storms and flooding EM–3311) on March 30, 2010 (Zarriello 

and others, 2014). Flooding along the Pawtuxet River affected 

critical infrastructure, including U.S. Interstate I–95; Cranston, Warwick Sewer Authority, and West Warwick wastewater 

treatment facilities (WWTFs); and the Warwick Mall (Brown 

University, 2011) (fig. 1). Numerous businesses and homes 

along the Pawtuxet River also were flooded. 

Four precipitation events occurred from February 23 

through March 30, 2010, totaling 20.36 inches (fig. 2A) at the 

Providence T.F. Green Airport, Warwick, Rhode Island (fig. 

1; data from Global Historical Climatology Network-Daily 
2 Flood-Inundation Maps for the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island

Base from Rhode Island Geographic

Information System (RIGIS)

Lambert conformal conic projection

North American Datum of 1983

EXPLANATION

Study reach associated with U.S. Geological Survey

streamgage 01116500

Streamgage and identifier 01116500 

INTERSTATE I-295

INTERSTATE I-95

STATE ROUTE 2

U.S. ROUTE 1 

STATE ROU TE

5

STATE

ROUTE 37

U S. . ROUTE 1A / STATE ROUTE 11 7

Warwick

Cranston

West

Warwick

Providence Johnston

East

Providence

Pawtuxet River

Pawtuxet River

Poca sset River

South B ranchPawtuxet River

Meshanti cut Brook

Narragansett Bay

North Branch 

Pawtuxet River

01116500 

Natick

Pond dam

Cranston

wastewater

treatment

facility

West Warwick

wastewater

treatment

facility

Warwick

Sewer Authority

wastewater

treatment facility

and levee

Pawtuxet

Cove

Former

Pawtuxet

Falls dam

Broad

Street

bridge

Providence

T.F. Green

Airport

Pontiac

Mill dam

STATE

ROUTE 33/

PROVIDENCE

STREET

BRIDGE

PROVIDENCE

COUNTY

KENT

COUNTY

71°28' 71°24'

41°48'

41°44'

1

1 2 MILES

2 KILOMETERS

0

0

RHODE

ISLAND

Study area

MASSACHUSETTS

CONNECTICUT

ATLANTIC OCEAN

Figure 1. Location of the Pawtuxet River study reach in West Warwick, Warwick, and Cranston, Rhode Island, and U.S. 

Geological Survey Pawtuxet River at Cranston streamgage 01116500.
Introduction 3

02/23 03/02 03/09 03/16 03/23 03/30 04/06

2010

100

1,000

10,000

100,000

Streamflow (discharge), in cubic feet per second

C

Missing

data

Missing

data

0

5

10

15

20

25

Stage (gage height), in feet 

B

0

5

10

15

20

25

Cumulative daily precipitation, in inches

A

Missing

data

Missing

data

Action stage 8.0 ft

Minor flood stage 9.0 ft

Moderate flood stage 11.0 ft

Major flood stage 13.0 ft

EXPLANATION

NWS AHPS flood categories

Figure 2. Cumulative precipitation, stage, and streamflow data 

for the Pawtuxet River, Rhode Island, during the floods of February 

23, 2010, through April 5, 2010: A, cumulative daily precipitation at 

the National Weather Service climatological station at Providence 

T.F. Green Airport, Warwick, Rhode Island, B, stage, and C, 

streamflow at U.S. Geological Survey Pawtuxet River at Cranston, 

Rhode Island, streamgage 01116500. NWS, National Weather 

Service; AHPS, Advanced Hydrologic Prediction Service; ft, foot.

[GHCND] station number USW00014765; National Oceanic 

and Atmospheric Administration, 2010). Precipitation from 

each of the four events totaled from about 3.2 to 8.9 inches, 

resulting in a series of four floods. The final flood culminated 

with a new peak stage and flow of record at the U.S. Geological Survey (USGS) streamgage on the Pawtuxet River at Cranston, R.I., (01116500; hereafter referred to as the Pawtuxet 

River streamgage) on March 31, 2010 (figs. 2B and C) (U.S. 

Geological Survey, 2016). The new peak of record has a stage 

of 20.79 feet (ft) and a discharge of 14,900 cubic feet per second (ft3

/s) (table 1). The stage was more than 6 ft higher and 

the discharge about 2.7 times greater than the previous peak of 

record (the previous peak stage was 14.50 ft and discharge was 

5,440 ft3/s on June 7, 1982, for water years1 1940–2009). The 

March 31, 2010 peak flow of 14,900 ft3

/s was greater than the 

estimated 0.2-percent annual exceedance probability floodflow 

(table 7 of Zarriello and others, 2012).

Prior to this study, emergency responders in the municipalities of West Warwick, Warwick, and Cranston relied on 

four information sources to make decisions on how to best 

alert the public and mitigate damages during floods. Two of 

these sources are the Kent County and Providence County 

Federal Emergency Management Agency (FEMA) Flood 

Insurance Studies (FISs; Federal Emergency Management 

Agency, 2015a and 2015b, respectively). The third source 

of information is the Pawtuxet River streamgage, just downstream from the westbound State Route (Rt.) 37 bridge (fig. 

1), which provides stage, discharge, and annual peak flow data 

from December 1939 to the present (2018) (U.S. Geological 

Survey, 2016). The fourth source of flood-related information 

is the National Weather Service (NWS) Advanced Hydrologic 

Prediction Service (AHPS), which issues flood forecasts of 

stage for the Pawtuxet River streamgage (CRAR1; National 

Weather Service, 2016). 

Although the current stage at a USGS streamgage is 

particularly useful for residents in the immediate vicinity of a 

streamgage, it is of limited use to residents farther upstream 

or downstream because the water-surface elevation is not 

constant along the entire stream reach. Knowledge of a water 

level at a streamgage is difficult to translate into depth and 

areal extent of flooding at points distant from the streamgage. 

One way to address these informational gaps is to produce 

a library of flood-inundation maps that are referenced to 

the stages recorded at a USGS streamgage. By referring to 

the appropriate map, emergency responders can discern the 

severity of flooding (depth of water and areal extent), identify 

roads that are or will soon be flooded, and make plans for 

notification or evacuation of residents in harm’s way for some 

distance upstream and downstream from the streamgage. In 

addition, the capability to visualize the potential extent of 

flooding has been shown to motivate residents to take precautions and heed warnings that they previously might have 

disregarded. In 2016–17, the USGS, in cooperation with the 

1

A water year is the 12-month period from October 1 through September 30 

of the following year and is designated by the calendar year in which it ends. 
4 Flood-Inundation Maps for the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island

Table 1. Description of U.S. Geological Survey Pawtuxet River at Cranston, Rhode Island, streamgage 01116500.

[Streamgage location is shown in figure 1. USGS, U.S. Geological Survey; NAD 83, North American Datum of 1983; NAVD 88, North American Vertical 

Datum of 1988; mi2, square mile; ft, foot; ft3/s, cubic foot per second]

Site description

USGS streamgage name Pawtuxet River at Cranston, Rhode Island

USGS streamgage number 01116500

Drainage area 200 mi2

Latitude (decimal degrees, NAD 83) 41.75093

Longitude (decimal degrees, NAD 83) -71.44506

Period of peak-flow record, in water years1 1940 to present

Maximum stage (gage datum [elevation, above NAVD 88]); date 20.79 ft [27.99 ft]; March 31, 2010

Maximum discharge; date 14,900 ft3

/s; March 31, 2010

1

A water year is the 12-month period from October 1 of one year through September 30 of the following year and is designated by the calendar year in 

which it ends.

Rhode Island Emergency Management Agency and the U.S. 

Army Corps of Engineers (USACE), conducted a project to 

produce a library of flood-inundation maps for the Pawtuxet 

River in West Warwick, Warwick, and Cranston, R.I., using a 

hydraulic model. 

Purpose and Scope

This report describes the development of a hydraulic model and creation of a series of 16 flood-inundation 

maps for a 10.2-mile (mi) reach of the Pawtuxet River 

downstream from the Natick Pond dam to Pawtuxet Cove 

(its mouth at Narragansett Bay; fig. 1) in West Warwick, 

Warwick, and Cranston, R.I. The maps, in 1-ft increments, 

cover a range in stage from 8.0 to 22.0 ft (gage datum) at 

the Pawtuxet River streamgage. The maps begin at the NWS 

AHPS flood category “action stage” of 8.0 ft and go to the 

maximum stage of the stage-discharge relation of 22.0 ft, 

which exceeds the maximum recorded peak stage (20.79 ft 

on March 31, 2010). The flood-inundation maps depict only 

riverine flooding; they do not depict any tidal backwater 

or coastal storm surge that might occur in the lower part of 

the river reach. The flood-inundation maps were developed 

for display on the USGS Flood Inundation Mapper website 

(https://wimcloud.usgs.gov/apps/FIM/

FloodInundationMapper.html).

Study Area Description

The Pawtuxet River Basin covers an area of about 

232 square miles (mi2) in central Rhode Island, and the 

Pawtuxet River discharges into Pawtuxet Cove at the northern 

end of Narragansett Bay just south of the city of Providence 

(fig. 1). The Pawtuxet River Basin lies within three counties—Providence, Kent, and a small part of Washington. The 

Pawtuxet River begins at the confluence of the North Branch 

Pawtuxet River and the South Branch Pawtuxet River and 

flows generally northeasterly until it reaches Pawtuxet Cove in 

Narragansett Bay. The Pawtuxet River is a low-gradient river 

that flows about 11.2 mi through urbanized areas of Rhode 

Island with commercial and industrial land uses and mediumto high-density residential housing. The Pawtuxet River flows 

through the town of West Warwick, and the cities of Warwick 

and Cranston, R.I., which in 2010 had populations of 29,191; 

82, 672; and 80,387, respectively (U.S. Census Bureau, 2016). 

The study reach begins downstream from the Natick 

Pond dam, which is located about 1.0 mi downstream from 

the confluence of the North Branch Pawtuxet River and the 

South Branch Pawtuxet River (fig. 1). The 10.2-mi study 

reach has several tributaries; the largest ones are Meshanticut 

Brook (13.6-mi2

 drainage area) and Pocasset River (20.6-mi2

drainage area). The Pawtuxet River streamgage (01116500) is 

4.8 mi upstream from the end of the study reach (downstream 

from the Broad Street bridge) and 5.4 mi downstream from the 

beginning of the study reach (downstream from Natick Pond 

dam) (fig. 1). The streamgage is associated with a drainage 

area of 200 mi2. The study reach is crossed by 13 road bridges, 

4 railroad bridges (2 active, 1 abandoned, and 1 bike path), 

and 2 dams. The Natick Pond dam and the State Rt. 33/Providence Street bridge are upstream from (outside of) the study 

reach. The Pawtuxet Falls dam just upstream from the Broad 

Street bridge near the mouth of the river was removed in 2011, 

and a levee around the Warwick Sewer Authority WWTF was 
Creation of Flood-Inundation-Map Library 5

raised during 2014–17 to be higher than the 0.2-percent annual 

exceedance probability flood (fig. 1). 

The water-surface elevations of the lower part of the 

Pawtuxet River study reach from about U.S. Rt. 1A/State 

Rt. 117/Warwick Avenue to Pawtuxet Cove (fig. 1) could be 

affected by tidal backwater from high tides in Narragansett 

Bay. However, from several field observations, it appears that 

when flows are greater than about 1,000 ft3/s (stage of about 

6.83 ft at gage datum) at the Pawtuxet River streamgage, there 

is little to no effect of tidal backwater at normal (non-storm 

affected) high tide upstream from the former Pawtuxet Falls 

dam (removed in 2011), which was about 100 ft upstream 

from the Broad Street bridge (Andrew Massey, U.S. Geological Survey, oral commun., 2016). The FEMA FIS for Kent 

County and the FIS for Providence County (Federal Emergency Management Agency, 2015a and 2015b, respectively) 

show tidal backwater for the 1-percent annual exceedance 

probability (AEP) water-surface elevation extending upstream 

from Pawtuxet Cove about 800 ft (about 440 ft upstream from 

the Broad St. bridge), but with the Pawtuxet River dam present. A study by the Rhode Island Coastal Resources Management Council (2017) depicted tidal surges from the hurricanes 

of 1938 and 1954 extending up the Pawtuxet River to just 

downstream from the Warwick Sewer Authority WWTF and to 

about the Pawtuxet River streamgage, respectively. The study 

also depicted the tidal surge of Hurricane Bob (1991) extending up the Pawtuxet River to about 5,400 ft upstream from the 

U.S. Rt. 1A/State Rt. 117/Warwick Avenue bridge. The effects 

of tidal backwater and storm surge on water-surface elevations 

were beyond the scope of this project, so the flood-inundation 

maps associated with this project depict only riverine flooding 

and do not account for (nor depict) any inundation that could 

result from possible tidal backwater or coastal storm surge in 

the lower part of the river reach.

Previous Studies

Several studies were conducted in the Pawtuxet River 

Basin by the USGS after the late February through early April 

floods in Rhode Island. Fifty high-water marks were flagged 

and surveyed along the main stem of the Pawtuxet River 

by the USGS and the USACE following the floods (tables 

1–1 and 1–2 of Zarriello and Bent, 2011). The magnitudes 

of the floodflows for selected AEPs at the Pawtuxet River 

streamgage were calculated by using annual peak flow data for 

water years 1940 through 2010 (table 7 of Zarriello and others, 

2012) but were updated for this study with data from water 

years 1940 to 2015 (table 2) using the methods described in 

England and others (2018). The USGS developed a hydraulic 

model for the Pawtuxet River in 2013 (Zarriello and others, 

2014) as part of a study to compare simulated flood elevations 

to those observed in 2010 (Zarriello and Bent, 2011). The 

model was calibrated with selected high-water marks from the 

March 31, 2010, flood (Zarriello and Bent, 2011). The hydraulic model was then used by USGS for the FEMA FISs for 

Kent and Providence Counties in 2015 (Federal Emergency 

Management Agency, 2015a and 2015b, respectively).

Three studies by Kleinschmidt Energy and Water 

Resources Consultants (2005), Milone & MacBroom, Inc. 

(2008), and EA Engineering, Science, and Technology, Inc. 

(2010) evaluated the restoration of native ecology, fisheries, 

and wetlands of the lower Pawtuxet River (fig. 1). These three 

studies were considered as part of the removal of the Pawtuxet 

Falls dam in August 2011.

Creation of Flood-Inundation-Map 

Library

The USGS has standardized the procedures for creating 

flood-inundation maps for flood-prone communities so that 

the process followed and products produced are similar across 

studies. Tasks specific to development of the flood maps for 

the Pawtuxet River include (1) modification of the Pawtuxet 

River hydraulic model that was used for the FEMA 2015 

FISs for Kent and Providence Counties (FEMA, 2015a and 

2015b, respectively), (2) verification of energy-loss factors 

(roughness coefficients) in the stream channel and flood plain, 

(3) computation of water-surface profiles with the USACE 

HEC–RAS version 5.0.3 computer program (U.S. Army 

Table 2. Peak-discharge estimates for selected annual exceedance probabilities at the U.S. Geological Survey Pawtuxet River at 

Cranston, Rhode Island, streamgage 01116500, using annual peak discharges for water years 1940–2015.

[USGS, U.S. Geological Survey; mi2, square mile; ft3/s, cubic foot per second; %, percent; RI, Rhode Island]

USGS streamgage 

name and number

Drainage 

area,

in mi2

Estimated peak discharge, 

in ft3/s, for annual exceedance probabilities Peak flow, 

March 31, 2010, 

flood, 

in ft3 50% 20% 10% 4% 2% 1% 0.5% 0.2% /s

Pawtuxet River 

at Cranston, RI 

(01116500)

200 1,850 2,820 3,700 5,110 6,440 8,040 9,990 13,200 14,900
6 Flood-Inundation Maps for the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island

Corps of Engineers, 2016) for the selected 1-ft stage intervals 

at the Pawtuxet River streamgage, (4) production of floodinundation maps corresponding to each modeled flood profile 

using the USACE HEC–GeoRAS computer program (U.S. 

Army Corps of Engineers, 2009) and a geographic information system (GIS), and (5) preparation of the maps as shapefile 

polygons that depict the areal extent of flood inundation and 

as depth grids that provide the depth of floodwaters for display 

on a USGS flood-inundation mapping application.

Computation of Water-Surface Profiles

The water-surface profiles used to produce the 15 floodinundation maps in this study were computed by the HEC–

RAS version 5.0.3 software (U.S. Army Corps of Engineers, 

2016). HEC–RAS is a one-dimensional step-backwater 

model for simulation of water-surface, and it includes 

options for computing steady-state (gradually varied) or 

unsteady-state flow. 

Hydrology

The study reach includes the Pawtuxet River streamgage, 

which has been in operation since December 1939 (fig. 1; 

table 1). The streamgage is 4.8 mi upstream from the mouth 

of the river and 0.1 mi downstream from the State Route 37 

westbound bridge crossing. The river stage is measured every 

15 minutes, transmitted hourly via satellite, and is available 

on the USGS National Water Information System (NWIS) 

website (U.S. Geological Survey, 2016). River stage data from 

this streamgage are referenced to a local datum but can be 

converted to water-surface elevations referenced to the North 

American Vertical Datum of 1988 (NAVD 88) by adding 

7.20 ft. Continuous records of streamflow are computed from 

a stage-discharge relation (rating curve) developed through 

concurrent stage and streamflow measurements collected since 

December 1939. The current (2018) rating curve, 43.1, was 

used for this study. 

The discharges input into the model simulations (table 

3) are from the current (2018) stage-discharge relation (rating 

curve 43.1) and correspond to the target stages. Stage-discharge relation rating curve 43.1 is the same as rating curve 43 

from a stage of 3.20 to 22.00 ft; rating curve 43 has been in 

effect from February 23, 2010 to July 31, 2016. The upper 

end of the rating curve is based on high-flow measurements 

(U.S. Geological Survey, 2016) made during the late February 

through early April 2010 floods. 

Two tributaries—Meshanticut Brook and Pocasset 

River—join the Pawtuxet River within the 10.2-mile study 

reach (fig. 1). The streamgage-derived discharges were 

adjusted as necessary to account for tributary inflows (table 3). 

The adjustments were estimated by applying a drainage-area 

ratio method (Johnstone and Cross, 1949) to the discharges 

at each of the 1-ft stage intervals from 8.0 to 22.0 ft and the 

March 31, 2010, floodflow at the Pawtuxet River streamgage. 

The flows were transferred upstream and downstream by using 

the following equation:

 

Q Q DA

DA u g

u

g

e

= 















 , (1)

where

Qu is the streamflow at an ungaged 

location, in cubic feet per second;

Qg is the streamflow at a gaged location, in cubic 

feet per second;

DAu is the drainage area at an ungaged location, in 

square miles;

DAg is the drainage area at a gaged location, in 

square miles; and

e is the exponent of the drainage-area-only 

regional regression equations (table 16 

of Zarriello and others, 2012) for the 

appropriate AEP.

The 50- to 0.2-percent AEP floodflows at the Pawtuxet 

River streamgage (table 2) that were closest to the discharge 

value for each of the 1-ft stage intervals from 8.0 to 22.0 ft and 

the March 31, 2010, floodflow were matched to determine the 

associated exponent, as shown in Zarriello and others (2012, 

table 16) for equation 1. For example, the discharge for a stage 

of 21.0 ft at the streamgage and in the reach downstream from 

the confluence with Meshanticut Brook, 15,300 ft3/s (table 

3), is closest to 0.2-percent AEP flow of 13,200 ft3/s (table 2), 

which relates to an exponent of 0.76 for the 0.2-percent AEP 

in Zarriello and others (2012, table 16). 

Topographic and Bathymetric Data

All topographic data used in this study are referenced 

vertically to NAVD 88 and horizontally to the North American Datum of 1983 (NAD 83). Cross-section elevation data 

were obtained for the Pawtuxet River corridor area from a 

digital elevation model (DEM) that was derived from the light 

detection and ranging (lidar) data collected for the northeastern United States (National Oceanic and Atmospheric 

Administration, 2013a). The lidar data were collected as 

a part of the American Recovery and Reinvestment Act of 

2009, coordinated and contracted through the USGS National 

Geospatial Technical Operations Center. The lidar data were 

collected from December 2010 through December 2011 and 

processed during 2012–13. The original lidar data have a 

vertical accuracy of 0.49 ft for the bare-earth terrain landcover category and were collected on a 3.28-ft ground sample 

distance (National Oceanic and Atmospheric Administration, 

2013b). By these criteria, the lidar data support the production 

of 2-ft contours of land surface (Snyder and others, 2014). The 

final DEM was resampled to a 6.5-ft grid-cell size to decrease 

the GIS processing time. By using HEC–GeoRAS, a set of 

procedures, tools, and utilities for processing geospatial data in 

Esri ArcGIS (Esri, 2016), elevation data were extracted from 
Creation of Flood-Inundation-Map Library 7

Table 3. Estimated discharges for selected locations used in the hydraulic model of the Pawtuxet River in West Warwick, 

Warwick, and Cranston, Rhode Island, and the corresponding discharges, stages, and water-surface elevations at U.S. 

Geological Survey Pawtuxet River at Cranston, Rhode Island, streamgage 01116500.

[Row shaded gray is the stage and discharge of the March 31, 2010, floodflow. USGS, U.S. Geological Survey; RI, Rhode Island; ft, foot; NAVD 88, 

North American Vertical Datum of 1988; ft3/s, cubic foot per second]

USGS Pawtuxet River at Cranston, RI, streamgage

(01116500)

Estimated discharge at indicated location 

used in hydraulic model, in ft3

/s

Stage of watersurface profile, 

in ft referenced to 

the gage datum

Water-surface 

elevation, in ft above 

NAVD 88

1

Discharge, in 

ft3

/s

At upstream end 

of study reach

Downstream from 

confluence with 

Meshanticut Brook

Downstream from 

confluence with 

Pocasset River

8.00 15.20 1,360 1,260 1,360 1,520

9.00 16.20 1,710 1,580 1,710 1,910

10.00 17.20 2,150 1,980 2,150 2,410

11.00 18.20 2,660 2,460 2,660 2,980

12.00 19.20 3,300 3,050 3,300 3,690

13.00 20.20 4,050 3,740 4,050 4,520

14.00 21.20 4,850 4,490 4,850 5,410

15.00 22.20 5,900 5,460 5,900 6,570

16.00 23.20 7,000 6,480 7,000 7,800

17.00 24.20 8,300 7,690 8,300 9,230

18.00 25.20 9,824 9,100 9,820 10,900

19.00 26.20 11,500 10,700 11,500 12,800

20.00 27.20 13,340 12,300 13,300 14,800

21.00 28.20 15,330 14,200 15,300 17,000

22.00 29.20 17,500 16,200 17,500 19,400

20.79 27.99 14,900 13,800 14,900 16,600

1

Discharge is based on the current (2018) stage-discharge rating curve number 43.1.

the DEM for 184 cross sections and subsequently input to the 

HEC–RAS model. Because lidar data cannot provide ground 

elevations below a stream’s water surface for the model, channel cross sections were surveyed by USGS field crews during 

2011 (Zarriello and others, 2014) and 2016. Additional cross 

section data were provided by USACE during 2016 (Leanna 

Martin, U.S. Army Corps of Engineers, written commun., 

2016), Kleinschmidt Energy and Water Resources Consultants 

(2005), Milone & MacBroom, Inc. (2008), and EA Engineering, Science, and Technology, Inc. (2010). A differential global 

positioning system (DGPS) with real-time kinematic (RTK) 

technology was used to derive horizontal locations and the elevation of the water surface at each surveyed cross section and 

hydraulic structure (bridges and dams) during 2011 and 2016. 

The USGS DGPS with RTK technology field surveys were 

checked periodically against known elevations at National 

Geodetic Survey (NGS) benchmark sites in Kent County and 

Providence County, Rhode Island, during 2011 (Zarriello and 

Bent, 2011) and 2016 (Andrew Massey, U.S. Geological Survey, written commun., 2016). Generally, differences between 

the USGS and NGS benchmark elevations were less than plus 

or minus (±) 0.10 ft.

Where possible, DEM-generated cross sections were 

made to coincide with the locations of the within-channel 

field-surveyed cross sections. In these cases, within-channel 

data were directly merged with the DEM data. For all other 

cross sections, the within-channel data were estimated by 

interpolation from the closest field-surveyed cross section. 
8 Flood-Inundation Maps for the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island

Hydraulic Model

The hydraulic model for this study was developed by 

using the HEC–RAS version 5.0.3 software (U.S. Army Corps 

of Engineers, 2016) and is based on the model used by the 

USGS for the FEMA FISs for Kent and Providence Counties in 2015 (Federal Emergency Management Agency, 2015a 

and 2015b, respectively). The hydraulic model reflects the 

removal of the Pawtuxet Falls dam during 2011 and the raised 

elevation of a levee surrounding the Warwick Sewer Authority WWTF during 2014–17. Seventeen bridge structures, 2 

dams, and 1 levee have the potential to affect water-surface 

elevations during floods along the stream. The bridge structures include 13 road crossings (3 of the 10 crossings—State 

Route 2, Interstate I-295, and State Route 37—have 2 separate 

structures each) and 4 railroad bridges (2 active, 1 abandoned, 

and 1 converted to a bike path) (fig. 1, not all structures are 

shown; table 4). The two dams are the Pontiac Mill dam and 

an unnamed low-head dam/pipe, about 400 ft downstream 

from the State Rt. 5/Lambert Lind Highway bridge and about 

145 ft downstream from the Pontiac Mill dam, respectively. 

The unnamed low-head dam/pipe (about 2 ft high) does not 

affect floodflows and is visible only at low flows. The bridge, 

dam, and levee geometry data were obtained by USGS field 

crews during 2011 (Zarriello and others, 2014) and 2016.

Hydraulic analyses require the estimation of energy 

losses that result from frictional resistance exerted by a channel on flow. These energy losses are quantified by the Manning’s roughness coefficient (n-value). Initial (precalibration) 

n-values were selected on the basis of field observations and 

high-resolution aerial photographs and refined during highwater-mark calibration. Commercial and industrial land uses, 

and medium- to high-density residential housing areas are 

present along the riparian corridor of the Pawtuxet River, 

along with sections that are primarily wooded with fairly 

dense vegetation. The channel n-values range from 0.030 to 

Table 4. Description of bridge crossings, dams, and a levee in the hydraulic model of the Pawtuxet River in West Warwick, 

Warwick, and Cranston, Rhode Island.

[ft, foot; Rt., route; U.S., United States]

River station, 

in ft1 Structure Name Municipality

51,189.89 Bridge East Avenue West Warwick and Warwick

49,524.45 Bridge Washington Secondary bike path West Warwick and Warwick

48,136.76 Bridge Interstate I-295 and State Rt. 2 exit and on ramp Warwick

47,440.87 Bridge State Rt. 2/Bald Hill Road (southbound) Warwick

47,360.42 Bridge State Rt. 2/Bald Hill Road (northbound) Warwick

45,806.50 Bridge Interstate I-295 (southbound) Warwick

45,652.12 Bridge Interstate I-295 (northbound) Warwick

40,645.33 Bridge State Rt. 5/Lambert Lind Highway/Greenwich Avenue Warwick

40,261.48 Dam Pontiac Mill dam Warwick

40,117.76 Dam/pipe Low-head dam/pipe at Pontiac Mill Warwick

30,673.00 to 32,199.00 Levee Warwick Sewer Authority wastewater treatment facility Warwick/Cranston

25,987.96 Bridge State Rt. 37/Lincoln Avenue Freeway (eastbound) Warwick/Cranston

25,701.04 Bridge State Rt. 37/Lincoln Avenue Freeway (westbound) Warwick/Cranston

16,908.42 Bridge Interstate I-95 Warwick/Cranston

15,888.97 Bridge Amtrak railroad (upstream bridge) Warwick/Cranston

15,888.97 Bridge Amtrak railroad (downstream bridge) Warwick/Cranston

14,405.42 Bridge U.S. Rt. 1/Elmwood Avenue Warwick/Cranston

7,645.32 Bridge Abandoned railroad Warwick/Cranston

6,063.79 Bridge U.S. Rt. 1A/State Rt. 117/Warwick Avenue Warwick/Cranston

328.3247 Bridge Broad Street Warwick/Cranston

1

“River station” references the distance upstream to the downstream side of the structure from the most downstream point (starting point) in the 

hydraulic model.
Creation of Flood-Inundation-Map Library 9

0.045 because the channel is mainly sand and gravel with 

some cobbles interspersed. The n-values for the overbanks of 

the riparian corridor (flood plain) range from 0.050 to 0.100 

depending on the openness of the section. Bankfull channeltop widths typically are between about 100 and 200 ft wide. 

As part of the calibration process, the initial n-values were 

varied by flow and adjusted until the differences between simulated and observed water-surface elevations at the streamgage 

and at high-water marks were minimized.

The HEC–RAS analysis was done by using the steadystate flow computation option. Steady-state flow data consist 

of flow regime, boundary conditions, and peak flows that 

produced water-surface elevations at the streamgage cross 

section that match target water-surface elevations. These 

target elevations coincide with even 1-ft increments of stage, 

referenced to the gage datum. Subcritical (tranquil) flow 

regime was assumed for the simulations. Normal depth, based 

on an estimated average streambed slope of 0.00373 ft/ft, was 

used as the downstream boundary condition for the reach. 

The HEC–RAS model was calibrated to the stage-discharge 

relation (rating curve 43.1) at the Pawtuxet River streamgage 

and to 27 of the 50 flagged and surveyed HWMs for the water 

surface of the period of record peak flood of March 31, 2010 

(Zarriello and Bent, 2011). Only 27 of the 50 HWMs were 

used for the model calibration because several locations had 

two or more that were similar in elevation.

Differences between the 15 stages of the stage-discharge 

relation (rating curve 43.1; NAVD 88 stages 15.2 to 29.2 ft) 

and the simulated water-surface elevations at the Pawtuxet 

River streamgage ranged from −0.68 to 0.80 ft (table 5). The 

average and median differences between the 15 stages and 

simulated water-surface elevations were −0.10 and −0.11 ft, 

respectively. Absolute differences were 0.5 ft or less for 10 of 

the 15 stages and between 0.5 and less than 0.8 ft for the other 

5 stages. 

Differences between surveyed and simulated watersurface elevations of 27 selected HWMs in the study reach for 

the March 31, 2010, flood ranged from −1.99 to 1.78 ft (table 

6). The average and median differences between surveyed and 

simulated water-surface elevations for the 27 HWMs were 

−0.12 and 0.07 ft, respectively. Absolute differences were 

0.5 ft or less for 13 of the 27 HWMs, between 0.5 and 1.0 ft 

for 7 of the 27 HWMs, and between 1.0 and 2.0 ft for 7 of the 

27 HWMs. Field crews can sometimes collect a HWM that is 

lower than the peak water-surface elevation of a flood because 

more than one line of marks can be left by debris, seeds, and 

mud as floodwaters recede (Feaster and Koenig, 2017). This 

is likely the case with HWMs USGS–123 and USGS–125 

because both have simulated water-surface elevations that 

are almost 2 ft higher than the surveyed HWM elevations. 

Overall, the results demonstrate that the model is capable of 

simulating accurate water levels over a wide range of flows in 

the Pawtuxet River study reach. 

Table 5. Differences between 15 simulated water-surface 

elevations and stage elevations at 1-foot intervals from 8.0 to 

22.0 feet for U.S. Geological Survey Pawtuxet River at Cranston, 

Rhode Island, streamgage 01116500.

[ft, foot; NAVD 88, North American Vertical Datum of 1988]

Stage,

in ft referenced 

to the gage 

datum

Stage 

elevation, 

in ft above 

NAVD 88

Simulated 

water-surface 

elevation, 

in ft above 

NAVD 88

Difference in 

elevation, 

in ft

8.00 15.20 15.78 0.58

9.00 16.20 16.43 0.23

10.00 17.20 17.24 0.04

11.00 18.20 18.12 -0.08

12.00 19.20 19.20 0.00

13.00 20.20 20.31 0.11

14.00 21.20 21.10 -0.10

15.00 22.20 21.93 -0.27

16.00 23.20 22.73 -0.47

17.00 24.20 23.52 -0.68

18.00 25.20 24.56 -0.64

19.00 26.20 25.65 -0.55

20.00 27.20 26.73 -0.47

21.00 28.20 28.05 -0.15

22.00 29.20 30.00 0.80

Development of Water-Surface Profiles

The calibrated hydraulic model was used to simulate 

water-surface profiles for 15 stages at 1-ft intervals between 

8.0 and 22.0 ft as referenced to the local datum of the Pawtuxet River at Cranston streamgage (01116500; table 7). 

These stages correspond to elevations of 15.2 ft and 29.2 ft, 

NAVD 88, respectively. Discharges corresponding to the 

various stages were obtained from the current (2018) stagedischarge relation (rating curve 43.1) for the Pawtuxet River 

at Cranston streamgage (table 3). The mapped stages were 

selected because the NWS AHPS’s flood category “action 

stage” is 8.0 ft (National Weather Service, 2016) and because 

the stage of 22.0 ft is higher than the peak of the March 31, 

2010, flood and is the upper limit of the current stage-discharge relation. The stage of 22.0 ft is 9.0 ft higher than the 

NWS AHPS’s flood category “major flood stage” of 13.0 ft 

(National Weather Service, 2016). 
10 Flood-Inundation Maps for the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island

Table 6. Differences between the hydraulic model simulated water-surface elevations for the March 31, 2010, floodflows and the 

surveyed high-water-mark elevations on the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island.

[HWM, high-water mark; NAVD 88, North American Vertical Datum of 1988; USGS, U.S. Geological Survey; ACE, U.S. Army Corps of Engineers]

River station, in feet1 HWM 

identification number2 HWM rating2,3

Water-surface elevation, in feet above 

NAVD 88 Difference in 

elevation, 

Simulated HWM2 in feet

51,171.25 USGS-118 Good 41.23 41.79 -0.56

51,025.27 USGS-117 Good 41.27 41.20 0.07

49,743.73 USGS-152 Not noted 41.01 41.07 -0.06

48,008.50 USGS-134 Good 40.01 39.67 0.34

47,554.93 USGS-129 Good 39.94 39.15 0.79

47,335.34 USGS-128 Good 38.02 39.11 -1.09

47,107.87 USGS-141 Excellent 37.91 37.63 0.28

45,950.85 USGS-125 Good 36.55 38.41 -1.86

45,634.68 USGS-127 Good 36.28 37.47 -1.19

45,566.75 USGS-139 Excellent 36.15 36.67 -0.52

45,514.13 USGS-140 Excellent 36.05 36.71 -0.66

40,711.79 USGS-124 Fair 33.81 33.65 0.16

40,242.96 USGS-123 Good 32.25 34.24 -1.99

32,037.30 USGS-151 Excellent 31.32 31.56 -0.24

31,385.16 USGS-147 Excellent 31.04 31.61 -0.57

26,037.60 USGS-121 Good 28.93 28.76 0.17

25,588.75 USGS-120 Good 27.82 28.42 -0.60

25,331.27 USGS-119 Good 27.84 27.73 0.11

23,621.69 USGS-144 Good 26.99 27.45 -0.46

19,070.17 ACE-94 Not noted 26.60 26.14 0.46

16,148.96 ACE-85 Not noted 24.74 24.30 0.44

15,709.02 ACE-86 Not noted 23.13 21.82 1.31

14,471.37 ACE-88 Not noted 22.11 21.93 0.18

14,388.10 ACE-87 Not noted 21.92 22.81 -0.89

10,275.31 ACE-90 Not noted 21.72 20.57 1.15

9,068.13 ACE-92 Not noted 21.60 19.82 1.78

6,042.07 ACE-91 Not noted 16.94 16.84 0.10

1

“River station” references the distance upstream to the downstream side of the structure from the most downstream point (starting point) in the

hydraulic model.

2

HWM information from Zarriello and Bent (2011).

3

Rating of HWMs from Koenig and others (2016, table 2).
Creation of Flood-Inundation-Map Library 11

Table 7. Stage, discharge, and approximate annual exceedance probability at U.S. Geological Survey Pawtuxet 

River at Cranston, Rhode Island, streamgage 01116500 for profiles mapped on the Pawtuxet River in West Warwick, 

Warwick, and Cranston, Rhode Island.

[ft, foot; NAVD 88, North American Vertical Datum of 1988; ft3/s, cubic foot per second; NA, not applicable]

Grid identification1 Stage, in ft referenced 

to gage datum

Stage elevation, 

in ft above NAVD 88 Discharge, in ft3

/s

Closest floodflow’s annual 

exceedance probability, 

in percent2

PawtuxetRI_01 8.00 15.20 1,360 NA

PawtuxetRI_02 9.00 16.20 1,710 50

PawtuxetRI_03 10.00 17.20 2,150 NA

PawtuxetRI_04 11.00 18.20 2,660 20

PawtuxetRI_05 12.00 19.20 3,300 NA

PawtuxetRI_06 13.00 20.20 4,050 10

PawtuxetRI_07 14.00 21.20 4,850 4

PawtuxetRI_08 15.00 22.20 5,900 2

PawtuxetRI_09 16.00 23.20 7,000 NA

PawtuxetRI_10 17.00 24.20 8,300 1

PawtuxetRI_11 18.00 25.20 9,824 0.5

PawtuxetRI_12 19.00 26.20 11,500 NA

PawtuxetRI_13 20.00 27.20 13,340 0.2

3

PawtuxetRI_14 21.00 28.20 15,330 NA

PawtuxetRI_15 22.00 29.20 17,500 NA

1

Grid identification is the shapefile and depth map file names for each of the 16 river stages for the Pawtuxet River study reach provided 

in Bent and Lombard (2018).

2

See table 2 for the estimated peak discharges for the selected annual exceedance probabilities.

3

PawtuxetRI_14 map has the stage value closest to the March 31, 2010, peak flow stage and thus most closely approximates that flood.

Flood-Inundation Maps

Flood-inundation maps were created in a GIS for the 

15 water-surface profiles by combining the profiles and DEM 

data. The maps depict the flood-inundation extent (flood-plain 

boundaries) of flood stages from 8.0 to 22.0 ft (gage datum) 

at the Pawtuxet River streamgage. The flood map corresponding to the simulated water-surface profile at a stage of 21.0 ft 

is presented in figure 3; the stage of this map is the closest to 

the March 31, 2010, flood stage of 20.79 ft. The water-surface 

elevations of the lower part of the Pawtuxet River study reach 

(the most downstream approximately 6,000 ft of reach) from 

about U.S. Rt. 1A/State Rt. 117/Warwick Avenue to Pawtuxet 

Cove (about 330 ft downstream from the Broad Street bridge; 

fig. 1) could be affected by tidal backwater from high tides in 

Narragansett Bay. However, several field observations appear 

to show that for flows greater about 1,000 ft3/s (stage of about 

6.83 ft at gage datum) at the Pawtuxet River streamgage, there 

is little to no effect of tidal backwater at normal (non-storm 

affected) high tide upstream from the former Pawtuxet Falls 

dam, which was located about 100 ft upstream from the Broad 

Street bridge (Andrew Massey, U.S. Geological Survey, oral 

commun., 2016). The flood-inundation maps depict only 

riverine flooding and do not depict any tidal backwater or 

coastal storm surge that might occur in the lower part of the 

river reach.

The DEM data were derived from the same lidar data 

described previously in the section “Topographic and Bathymetric Data” and have an estimated vertical accuracy of ±1 ft. 

Estimated flood-inundation boundaries for each simulated 

profile were developed with HEC–GeoRAS software (U.S. 

Army Corps of Engineers, 2009), which enables the preparation of geometric data for import into HEC–RAS software and 

processes simulation results exported from HEC–RAS (U.S. 

Army Corps of Engineers, 2016). Shapefile polygons and 

depth grids of the inundated areas for each profile were modified, as required, in the ArcMap application of ArcGIS (Esri, 
12 Flood-Inundation Maps for the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island

INTERSTATE I-95

STATE ROUTE 2

STATE ROUTE 5

INTERSTATE I-295

STATE ROUTE 113

INTERSTATE I-295

STATE ROUTE 2

Washington Secondary Bike Path

Warwick Mall

West

Warwick 

Wastewater

Treatment 

Facilty

Base from Rhode Island Geographic Information System (RIGIS) 

orthophotographs, 2014

Lambert conformal conic projection

North American Datum of 1983

EXPLANATION

Inundation area

Pawtuxet River

Pawtuxet River

Mesh

anticut River

Direction of flow

71°29' 71°28'30"

41°43'30"

41°43'

250 500 METERS

2,000 FEET

0

0 250 500 1,000

Figure 3. Flood-inundation map for a reach of the Pawtuxet River in Warwick, Rhode Island, corresponding to a stage of 21.0 

feet, gage datum, approximately depicting the March 31, 2010, peak flood stage of 20.79 feet at U.S. Geological Survey Pawtuxet

River at Cranston, Rhode Island, streamgage 01116500. Gage datum of 21.0 feet is 28.2 feet, North American Vertical Datum of 1988. 

(Location of streamgage 01116500 is shown in figure 1.) 
Creation of Flood-Inundation-Map Library 13

2016) to ensure a hydraulically reasonable transition of the 

flood boundaries between modeled cross sections. 

The flood-inundation areas are overlaid on high-resolution, geospatially referenced aerial photographs of the study 

area (fig. 3). Any inundated areas that were detached from 

the main channel were examined to identify subsurface connections with the main river, such as through culverts under 

roadways. Where such connections existed, the mapped 

inundated areas were retained in their respective flood maps; 

otherwise, the erroneously delineated parts of the flood extent 

were deleted. Bridge surfaces are shown as noninundated 

up to the lowest flood stage that either intersects the lowest 

structural chord of the bridge or completely inundates one or 

both approaches to the bridge. Where the lowest flood stage 

either intersects the lowest structural chord of the bridge or 

completely inundates one or both approaches to the bridge, 

the bridge surface is depicted as being inundated. A shaded 

building should not be interpreted to mean that the structure 

is completely submerged but rather that bare-earth surfaces 

in the vicinity of the building are inundated, and the water 

depth (as indicated in the mapping application by hovering 

the cursor over the inundated area) near the building would 

be an estimate of the water level inside the structure, unless 

flood-proofing measures had been implemented. Estimates of 

water depth can be obtained from the depth-grid data that are 

included with the presentation of the flood maps on an interactive USGS mapping application described in the “Flood-Inundation Map Delivery” section.

Flood-Inundation Map Delivery

The USGS Flood Inundation Mapping Science website 

at (https://water.usgs.gov/osw/flood_inundation) makes USGS 

flood-inundation study information available to the public 

through a mapping application that presents map libraries and 

provides detailed information on flood extents and depths for 

modeled sites in the United States. The mapping application 

helps users produce customized flood-inundation maps from 

the map library through a print-on-demand feature that allows 

the user to zoom to the area of interest, choose the desired 

stage, and print only that part of the map (for example, fig. 3). 

The flood-inundation maps are displayed in sufficient enough 

detail that preparations for flooding and decisions for emergency response can be made efficiently. 

The mapping application provides a link to the USGS 

NWIS website (U.S. Geological Survey, 2016), which presents 

the current (real-time) stage and streamflow at the Pawtuxet 

River streamgage (01116500) to which the inundation maps 

are referenced. A second link connects to the NWS AHPS 

website for the streamgage (CRAR1; National Weather Service, 2016) so that the user can obtain applicable information 

on the forecasted stage at the streamgage.

All GIS files presented on the USGS Flood Inundation 

Mapping Science website and metadata associated with each 

of the files are available as a USGS data release (Bent and 

Lombard, 2018).

Disclaimer For Flood-Inundation Maps

The flood-inundation maps should not be used for 

navigation, regulatory, permitting, or other legal purposes. 

The USGS provides these maps “as-is” for a quick reference, 

emergency planning tool but assumes no legal liability or 

responsibility resulting from the use of this information.

Uncertainties and Limitations Regarding Use of 

Flood-Inundation Maps

Although the flood-inundation maps represent the boundaries of inundated areas with a distinct line, some uncertainty 

is associated with these maps. There are uncertainties associated with the hydrology, the model, the observed water 

surfaces, and the mapping. The flood boundaries shown were 

estimated on the basis of flood stages and streamflows at the 

Pawtuxet River streamgage. There are errors associated with 

the stage-discharge rating curves used to estimate flow at the 

streamgages because the rating curve is smoothed through the 

streamflow measurements and the concurrent stage. Estimates 

of flow were computed for areas upstream and downstream 

from the streamgage by using the estimates of flows at the 

streamgage and adjusting them for the change in drainage 

area from the streamgage to the new location. Meteorological 

factors, such as the timing and distribution of precipitation, 

may cause actual streamflows along the modeled reach to vary 

from those assumed during a flood, which may lead to variations in the water-surface elevations and inundation boundaries shown on the maps.

Water-surface elevations along the stream reaches were 

estimated by using steady-state hydraulic modeling, assuming unobstructed flow, and using streamflows and hydrologic 

conditions anticipated at the streamgage. The hydraulic model 

reflects the land-cover characteristics and any bridge, dam, 

levee, or other hydraulic structure present as of September 2017. The HEC–RAS model is a one-dimensional hydraulic model and, as such, cannot always capture everything that 

occurs during a flood. Additional areas may be flooded as 

a result of unanticipated conditions, such as changes in the 

streambed elevation or roughness, backwater into major tributaries along a main-stem river, or backwater from localized 

debris. HEC–RAS models are more accurate when they are 

calibrated to flows from streamgages and to HWMs collected 

after flooding events. The HWMs collected in the field are 

from actual events and are given a rating from poor (±0.4 ft of 

perceived difference between the field-identified elevation and 

the actual water-surface elevation during the event) to excellent (±0.05 ft) (table 2 of Koenig and others, 2016) at the time 

they are collected (table 6). Ratings of the HWMs often reflect 

the quality of the HWM itself and do not always take into 

account when the mark occurred during a storm; some very 

clear HWMs can occur on the recession of a flood, when the 

stage holds steady for a time. Thus, the models are as good as 

the data to which they are calibrated.
14 Flood-Inundation Maps for the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island

The accuracy of the floodwater extent portrayed on these 

maps will vary with the accuracy of the DEM used to simulate 

the land surface. Thus, the mapping of the flood boundaries 

and the depths of the inundated areas on the maps have some 

uncertainty. Additionally, the flood-inundation maps depict 

only riverine flooding and do not depict any tidal backwater 

or coastal storm surge that might occur in the lower part of the 

river reach during a flood.

If this series of flood-inundation maps will be used in 

conjunction with NWS river forecasts, the user should be 

aware of additional uncertainties that may be inherent in, 

or factored into, NWS forecast procedures. The NWS uses 

forecast models to estimate the quantity and timing of water 

flowing through selected stream reaches in the United States. 

These forecast models (1) estimate the amount of runoff generated by precipitation and snowmelt, (2) simulate the movement of floodwater as it proceeds downstream, and (3) predict 

the flow and stage (and water-surface elevation) for the stream 

at a given location (NWS AHPS forecast point; National 

Weather Service, undated) throughout the forecast period (typically every 6 hours and 3 to 5 days out in many locations). 

Summary

A series of 15 digital flood-inundation maps was developed by the U.S. Geological Survey (USGS), in cooperation 

with the Rhode Island Emergency Management Agency and 

the U.S. Army Corps of Engineers, for the Pawtuxet River 

in the municipalities of Cranston, Warwick, and West Warwick, Rhode Island. The maps cover a reach about 10.2 miles 

long downstream from Natick Pond dam (near State Rt. 33/

Providence Street), West Warwick, to the mouth of the river 

at Pawtuxet Cove (downstream from Broad Street bridge) at 

Cranston and Warwick. The maps were developed by using 

U.S. Army Corps of Engineers Hydrologic Engineering Center’s River Analysis System (HEC–RAS) and HEC–GeoRAS 

software to compute water-surface profiles and to delineate 

estimated flood-inundation areas and depths of flooding for 

selected stream stages. 

Flood profiles were computed for this reach of the Pawtuxet River by means of a one-dimensional step-backwater 

HEC–RAS hydraulic model created and calibrated as part of 

an update to Federal Emergency Management Agency Flood 

Insurance Studies for Kent and Providence Counties (Federal 

Emergency Management Agency, 2015a and 2015b). The 

model was updated to reflect the removal of the Pawtuxet 

Falls dam during August 2011 and the raised elevation of a 

levee surrounding the Warwick Sewer Authority wastewater treatment facility during 2014–17. The hydraulic model 

was calibrated to the current (2018) stage-discharge relation 

(rating curve 43.1) at the USGS Pawtuxet River at Cranston, 

R.I., streamgage (01116500) and to the peak water-surface 

elevations (high-water marks) from the March 31, 2010, flood 

along the 10.2-mile reach. The March 31, 2010, peak flow at 

the Pawtuxet River streamgage was greater than the estimated 

0.2-percent annual exceedance probability floodflow. 

The hydraulic model was used to simulate 15 water-surface profiles for flood stages at 1-foot (ft) intervals referenced 

to the gage datum and ranging from 8.0 ft (15.2 ft, North 

American Vertical Datum of 1988), which is the National 

Weather Service (NWS) Advanced Hydrologic Prediction 

Service’s (AHPS) flood category “action stage,” to 22.0 ft 

(29.2 ft, North American Vertical Datum of 1988), which 

exceeds the maximum recorded stage (20.79 ft, March 31, 

2010) and the NWS AHPS’s flood category “major flood 

stage” (13.0 ft). The 15 simulated water-surface profiles do 

not account for any effect from tidal backwater or coastal 

storm surge in the lower part of the river reach that might 

occur during a flood. The water-surface profiles depict riverine flooding only, such as the late February through early 

April 2010 flooding.

The 15 water-surface profiles were then combined with a 

geographic information system digital elevation model derived 

from light detection and ranging (lidar) data to delineate estimated flood-inundation areas as shapefile polygons and depth 

grids for each profile. These flood-inundation polygons were 

overlaid on high-resolution, georeferenced aerial photographs 

of the study area. The flood maps are available through a 

mapping application that can be accessed on the USGS Flood 

Inundation Mapping Science website (http://water.usgs.gov/

osw/flood_inundation). Within this mapping application, users 

can click within the flood-inundation areas for a general indication of depth of water at any point on the maps. These maps, 

in conjunction with the real-time stage data for the USGS 

Pawtuxet River at Cranston, R.I., streamgage (01116500) 

from the National Water Information System and forecasted 

stage data from the NWS AHPS (CRAR1), will help to guide 

the general public in taking individual safety precautions and 

will provide emergency management personnel with a tool to 

efficiently manage emergency flood operations and postflood 

recovery efforts. The flood-inundation maps are nonregulatory but provide Federal, State, and local agencies and the 

public with estimates of the potential extent of flooding during 

flood events.

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Survey data release, https://doi.org/10.5066/F78C9V6B.
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For more information about this report, contact:

Director, New England Water Science Center

U.S. Geological Survey 

10 Bearfoot Road 

Northborough, MA 01532

dc_nweng@usgs.gov

or visit our website at

https://newengland.water.usgs.gov

Publishing support provided by the Pembroke Publishing 

Service Center
Bent and Lombard—Flood-Inundation Maps for the Pawtuxet River in West Warwick, Warwick, and Cranston, Rhode Island—SIR 2018–5043

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20185043