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The State of Iowa, located in the Midwestern United States, has experienced an increased frequency of large floods in recent decades. After extreme flooding in the summer of 2008, the Iowa Flood Center (IFC) was established for advanced research and education specifically related to floods. IFC seeks to improve Iowa’s flood hazard awareness through the development of easily accessible, highquality mapping products. Mapping initiatives consist of two model development approaches: (1) statewide floodplain delineation using onedimensional (1D) models, and (2) urban flood mapping using detailed onedimensional/twodimensional (2D) coupled models. The statewide floodplain project will benefit Iowans through the creation of a comprehensive set of floodplain maps developed under a single consistent methodology. These will be important tools in evaluating flood risk, regulating floodplains, and participating in the National Flood Insurance Program. Detailed urban flood analyses are used to develop inundation map libraries. These map libraries are meant to supplement National Weather Service river stage flood forecasts by providing a visual representation of potential flood extent according to predicted river stage at stream gage locations.
According to the National Weather Service (NWS), flood damages in the US have averaged nearly $8.4B per year since 1990 [
Inflationadjusted yearly estimates of flood damage in the US [
In addition to the effects of climate change, increased development in hazardprone areas could be contributing to the positive trend in flood damages. Typical factors contributing to continued development in flood prone areas include: (1) lack of knowledge, (2) low real estate prices, (3) high demand for housing near larger cities and (4) false sense of security due to flood defense systems [
The US Congress established the National Flood Insurance Program (NFIP) in 1968 to allow purchase of subsidized flood insurance in exchange for communityscale adoption of floodplain management regulations that reduce future flood damages [
The increasing affordability of high resolution Light Detection and Ranging (LiDAR) data have made updating of flood studies and largescale mapping efforts more feasible than ever. The State of North Carolina, in partnership with several state and federal agencies, is nearing completion of a pioneering statewide floodplain mapping project that relied heavily on LiDAR data to produce flood hazard maps [
The state of Iowa is located in the Midwestern US and is bordered by the Missouri River to the west and the Mississippi River to the east, as shown in
The state of Iowa located in the Midwestern United States. Most of Iowa’s major cities are located along rivers.
The Midwestern United States experienced widespread, summerlong flooding during 1993. This event was the result of intense, persistent weather patterns producing widespread precipitation from January to September, with accumulated precipitation approaching 2.3 times the summer average [
Much like the floods of 1993, antecedent conditions were significant factors contributing to the June floods of 2008. Precipitation from December of 2007 to May 2008 was the second wettest period on record from 1895 to 2008 [
State of Iowa rainfall totals for the month of June 2008 [
Downtown Cedar Rapids, Iowa at the peak of the 2008 flood, taken from Buchmiller [
The Iowa Flood Center (IFC) was established by the State of Iowa in response to devastating floods of 2008. IFC serves as a central location for advanced research and education specifically related to floods. Some of IFC’s responsibilities include development of hydrologic models for physicallybased flood frequency estimation and realtime forecasting of floods, including hydraulic models of floodplain inundation. Community outreach activities include development of programs to improve flood monitoring and prediction along Iowa’s major waterways and to support ongoing flood research. IFC is charged with sharing resources and expertise to assist in the development of a workforce knowledgeable regarding flood research, prediction, and mitigation strategies. Webbased inundation maps have been of particular interest among citizens and government officials. IFC inundation mapping initiatives consist of two model development approaches: (1) statewide floodplain delineation using onedimensional (1D) models, and (2) urban flood mapping using detailed onedimensional/ twodimensional (2D) coupled models.
The state of Iowa, like many areas, has never had a comprehensive set of flood inundation maps. Until recently, a general lack of organization, funding, and data prevented development of such a comprehensive set of maps. In 2006, the Iowa Department of Natural Resources (IADNR) initiated a statewide LiDAR survey with foresight that one of the uses would be a statewide floodplain mapping project. The LiDAR data points have an average spacing of 5 feet or less, and have been interpolated to develop 3.28 foot (1 meter) resolution rasterbased digital elevation models (DEMs). These highdensity data are available across the entire state.
Availability of LiDAR data together with the commendable flood recovery efforts of Iowans following the 2008 floods led the U.S. Department of Housing and Urban Development (HUD) to provide a $15M grant to the State of Iowa for modernizing and developing flood maps for the 85 counties declared federal disaster areas. The State of Iowa directed $10M of these funds to IFC and IADNR to execute the statewide floodplain mapping effort. The project began in 2010 and will take approximately five years to complete. The goal of the project is to provide webbased floodplain maps that will aid in planning, guide emergency management, and allow citizens and officials to better understand flood hazards. An additional purpose of the statewide floodplain mapping initiative is to provide FEMAapproved Flood Insurance Rate Maps (FIRMs), which regulate new construction and determine which property owners must purchase flood insurance as part of the NFIP. These maps will be defined by FEMA as “Zone A”, which identify special flood hazard areas for which no base (100year) flood elevations have been provided [
While the scale of the statewide floodplain mapping is sizable, Iowans will reap several benefits from the analyses. Flood analysis and mapping will be completed through a single, consistent methodology, rather than several methodologies unique to different institutions. Counties and cities that would otherwise lack the resources to organize a mapping project will have the opportunity to participate in the NFIP. Many existing floodplain mapping analyses were completed using topographic data that is outdated and of lowquality compared to currently available LiDAR data. Analysis using the latest topographic data will more accurately delineate floodplains.
Datasets are utilized in the analysis of either hydrologic or hydraulic properties of basins and streams. The bulk of data used for developing floodplain maps is derived from LiDAR data using geographic information systems (GIS). IFC, in partnership with IADNR, is currently developing a highly accurate stream centerline dataset for streams draining greater than 24 acres using LiDAR data. Other datasets derived from LiDAR data include drainage area, basin boundaries, crosssection geometry, and DEMs. Peak annual stream flow records are provided by the USGS and United States Army Corps of Engineers (USACE) at approximately 207 locations across the state. Land use data are provided by the United States Department of Agriculture (USDA) in the form of the National Land Cover Dataset (NLCD) [
Stream discharge estimates are calculated to establish hydrologic conditions associated with 50, 20, 10, 4, 2, 1, 0.5, and 0.2percentannualchance flows (2, 5, 10, 25, 50, 100, 200, and 500year return period flows, respectively). The USGS developed methods in 1987 by Lara [
Summary of methods used to estimate annual exceedance discharges.
Site Description  Gage Record (Years)  Drainage Area (Square Miles)  Method  Reference 

ungaged site on an ungaged stream    1–20  1987 methods for return intervals ≤ 100 years, extrapolated 1987 equations > 100 years  Lara 1987 [ 
  20–50  average the 1987 or extrapolated 1987 equations and 2001 methods  Lara 1987 [ 

  >50  2001 singleparameter regression equations  Eash 2001 [ 

gaged site      weighted estimates for gaged sites  Eash 2001 [ 
ungaged site on a gaged stream  <25    regressionweighted estimate for ungaged sites  Eash 2001 [ 
≥25    areaweighted estimate for ungaged sites  Eash 2001 [ 
Annual exceedance discharges for ungaged locations on ungaged streams draining between 1 and 20 square miles are calculated using the regression equations developed by Lara [
Lara (1987) [
Region 1  Region 2  Region 3  Region 4  Region 5 

Hydrologic regions for utilizing annual exceedance regression equations (
Since equations for estimating the 0.5 and 0.2percentannualchance (
Extrapolated 0.5 and 0.2percent regional regression equations using Lara 1987 equations assuming a LogPearson Type III distribution.
Region 1  Region 2  Region 3  Region 4  Region 5 

Eash [
Eash (2001) SingleParameter USGS Regional Regression Equations for the State of Iowa [
Region 1  Region 2  Region 3 

Annual exceedance discharges for sites draining greater than 50 square miles are calculated exclusively using the singleparameter regression equations, shown in
At locations where a stream gage is operated by the USGS or USACE, annual exceedance discharges are estimated by the USGS. A comprehensive analysis of Iowa gages was last conducted by Eash in 2001 [
where: Q_{t(wg)} is the weighted discharge estimate for a gaged site for recurrence interval t, Qt(pg) is the flooddischarge estimate (logPearson Type III) for a gaged site for recurrence interval t, ERL is the effective record length for a gaged site, in years (equivalent to the systematic record length if historical data are not considered; calculated according to Eash [
Due to additional years of record since publication of Eash [
When considering ungaged locations on streams that are gaged, the drainage area ratio between the gaged site and ungaged site is used to determine whether it is appropriate to use gage information in estimating annual exceedance discharges [
where: DAR is the drainage area ratio, defined as the absolute value of the difference between the drainage areas of the gage site (A_{g}) and the drainage area of the ungaged site (A_{u}) divided by the drainage area of the gaged site (A_{g}).
When the drainage area ratio is 0.5 or less, the ungaged location is considered to be on a gaged stream segment; and regression and areabased weighting schemes described in Eash [
When the period of record of the gaged site is less than 25 years, annual exceedance discharges are estimated using the regressionweighted method, shown in Equation (3), as described in Eash [
where: Qt(rw) is the regressionweighted discharge estimate for an ungaged site on a gaged stream for recurrence interval t, Qt(ru) is the regional regression discharge estimate for an ungaged site for recurrence interval t, determined using methods described in previous sections, Qt(wg) is the weighted discharge estimate for a gaged site for recurrence interval t, and Qt(rg) is the regionalregression discharge estimated for a gaged site for recurrence interval t, as listed in
When the period of record of the gaged site is greater than or equal to 25 years, annual exceedance discharges are estimated using the areaweighted method, shown in Equation (4), as described in Eash [
where: Q_{t(aw)} is the areaweighted discharge estimate for an ungaged site on a gaged stream for recurrence interval t, Q_{t(wg)} is the weighted discharge estimate for a gaged site for recurrence interval t, A_{u} is the drainage area of the ungaged site, A_{g} is the drainage area of the gaged site, and x is the mean exponent for 2001 hydrologic regions defined in
Hydraulic modeling is performed with the USACE Hydrologic Engineering Center River Analysis System (HECRAS). HECRAS requires geometric data describing the stream network and boundary conditions describing stream discharges and downstream water surface elevations to complete standard step backwater calculations.
Streams at least 0.5 mile long with a drainage area of 1 square mile of area or greater are modeled with HECRAS. The necessary geometric data required for model development is constructed using the HECGeoRAS extension for ESRI’s ArcGIS. HECGeoRAS allows the user to create HECRAS geometry data within a GIS environment using a DEM and userdefined polylines describing the stream topology. Stream centerlines are taken from the IFC/IADNR centerline dataset. Banklines are manually digitized along the top of bank based on 3.28 foot (1 meter) resolution LiDAR data. Overland flow paths, which define the approximate path taken by the bulk of outofbank flow, are somewhat subjective and are digitized at a coarse scale. Crosssections are used to define the channel and floodplain geometry. Crosssections are placed approximately perpendicular to anticipated flow paths and are typically spaced a maximum of 1,600 feet apart. Spatially varying roughness maps are based on the 2001 NLCD and published Manning’s values in Chow [
Standard step backwater calculations performed by HECRAS are based on the principle of conservation of energy. Simulations require a discharge and water surface elevation at the upstream and downstream boundaries, respectively. Discharges corresponding to 50, 20, 10, 4, 2, 1, 0.5, and 0.2percentannualchance flows are calculated at every crosssection location using methods described in
HECRAS simulation results are exported to ArcGIS and floodplain boundaries are delineated. Simulated water surface profiles are intersected with a 3.28 foot (1 meter) DEM. Due to the high fidelity of the LiDAR data, there is an abundance of inundated regions disconnected from stream channels and nonwetted regions inside the floodplain boundaries. These disconnected and nonwetted regions are removed from the inundation maps, using area as the determining factor for removal. At confluences, the inundation results from separate stream models are merged for discrete flows to remain consistent with assumed boundary conditions. Preliminary mapping results of the 1 and 0.2percentannualchance floods for Poweshiek County, Iowa are available on the IFC website [
Preliminary floodplain mapping result for the 0.2percentannualchance flood available on the Iowa Flood Center website [
A comprehensive set of flood inundation maps developed under a unified methodology will greatly improve Iowa’s management of flood risk. However, the accessibility of these maps along with the supporting data will greatly impact the value of the statewide floodplain mapping effort. A database is currently under development to efficiently store and query the statewide floodplain information so it is viewable and downloadable on the Internet. This is the most efficient method of disseminating flood hazard information to the public while providing resources for future updates. After a quality control and quality assurance review, maps of some areas will be submitted to FEMA to begin the adoption process. The adoption process will include periods for public comment and appeal depending on funding availability and time since the effective map was published. After any necessary revisions, communities will have an additional six months to implement new floodplain regulations before the maps become effective [
During recent flooding events, particularly the floods of 2008, many Iowa communities relied on NWS river stage forecasts to anticipate potential flood levels and to manage flood fighting efforts. However, NWS forecasts of river stage or discharge are made only at stream gage locations.
Inundation maps are a more effective way to communicate flood risk, but the public’s access to this resource is limited. The majority of publiclyavailable inundation maps are FEMAdistributed FIRMs [
IFC is developing libraries of urban inundation maps corresponding to river stage using highresolution hydraulic models. USGS stream gaging stations are located near the center of most large communities in Iowa. Simulation scenarios are based on 0.5 foot river stage increments at the nearest USGS gage. The USGSpublished relationships between river stage and discharge are used to develop flow scenarios.
Depiction of a general 1D model of the river channel coupled with a 2D model of the floodplain.
The presence of hydraulic structures (e.g., levees, weirs, and bridges) requires use of the energy equation to correctly predict head loss, while urban floodplain complexity requires the depthaveraged SaintVenant equations to accurately predict multidirectional flow patterns. Studies have demonstrated that most effective approach to modeling complex floodplains using the least computational effort is through onedimensional (1D) treatment of the main channel and twodimensional treatment of the floodplain [
Many datasets used in the statewide floodplain mapping project are also used in detailed urban flood analyses. Urban flood inundation modeling relies heavily on highresolution LiDAR elevation data to define geometric properties of floodplains. Land use data are provided by the USDA in the form of the NLCD. The river stage to discharge relationship at stream gage locations is provided by the USGS. Structural information describing levees, weirs, and bridges is provided by managing municipalities or federal agencies, or collected by IFC. Bathymetric mapping is performed by IFC using single and multibeam echosounding, georeferenced with a Realtime Kinematic (RTK) Global Navigation Satellite System (GNSS).
Detailed urban flood modeling is performed using MIKE FLOOD, a coupled hydrodynamic modeling software package. MIKE FLOOD models the river channel using MIKE 11, a 1D model, and the floodplain using MIKE 21, a 2D model. Data required for model development include a highresolution DEM of the terrain and river bed, distributed roughness, and asbuilt structural plan sets.
Model development begins with the integration of bathymetric data into the LiDARderived DEM. LiDAR resolution is insufficient to depict flood walls less than three feet wide. Therefore flood wall elevations are inserted into the DEM. Buildings are inserted into the DEM when building footprint data are available. Distributed roughness values are determined based on the NLCD and Manning’s roughness values published by Chow [
A 1D hydrodynamic model of the river channel is developed using DHI’s MIKE 11 GIS ArcGIS extension to create geometric files. Crosssections are digitized at an approximate spacing of 300 feet. Bridges and weirs are digitized from asbuilt plan sets.
A 2D structured computational mesh is created by aggregating the 3.28 foot (1 meter) resolution DEM to a resolution that is less computationally intensive, typically 32.8 to 65.6 feet (10 to 20 meters). The dilution of levee and floodwall elevations during aggregation of the DEM requires manual insertion of unaltered structure elevations using GIS techniques. While this ensures accurate topoflevee elevations are included in the mesh, it may overestimate the elevations at which failure of levees or flood walls occur.
The 1D and 2D models are coupled using MIKE FLOOD such that the river channel is represented by fully dynamic, sectionaveraged solutions to the SaintVenant equations at discrete cross sections and the floodplain is represented by depthaveraged SaintVenant equations at structured grid cells. MIKE FLOOD 1D/2D coupling allows two models to dynamically exchange information about water levels and discharge. Due to the large number of structures within study reaches, lateral coupling of the river channel was selected over other link types due to its ease of development. Lateral links are 1D explicit elements intended to model overtopping of a river bank or levee. A simple weir equation calculates flow through the lateral link. Lateral weir structure elevations are based on a bed level determined by crosssection endpoints and a width determined from the resolution of points defined along the structure [
All hydrodynamic boundary conditions are specified in the 1D model. Flow rate is specified at the upstream 1D boundary and water surface elevation is calculated at the downstream 1D boundary according to a normaldepth rating curve. At the downstream boundary, shown in
Example downstream boundary of coupled 1D/2D model.
Coupled 1D/2D MIKE FLOOD models are calibrated to low flow and high flow conditions by iteratively modifying bed friction resistance in the channel and floodplain until simulated water surface elevation in the channel and floodplain matches measured data. Measured data include high water marks, stream gage data, and low flow water surface profiles.
Simulation scenarios are based on discharges corresponding to 0.5 foot river stage increments at stream gages with NWS forecasts. Several estimated flood quantiles are also simulated. All simulations are allowed to reach steady state to provide a conservative measure of inundation. Lowerresolution maps of simulated water surface elevation are postprocessed in ArcGIS to generate 3.28 foot (1 meter) resolution flood boundaries. Disconnected and notwetted regions interior to the floodplain boundaries are removed based on area. The final inundation maps are formatted for webbased visualization.
Inundation map libraries have been developed for the communities of Des Moines, Cedar Rapids, Charles City, Cedar Falls, Waterloo, and Iowa City, and are hosted on IFC’s Iowa Flood Information System (IFIS) [
Example of the detailed urban flood mapping interface available on the Iowa Flood Information System (IFIS). Reported river stage at the gage location is viewable from within the interface. Inundation maps are viewable by river stage or return period using the slider on the right.
The state of Iowa has experienced increased frequency of large floods in recent decades that have severely damaged homes, businesses and infrastructure. These floods have revealed vulnerabilities and a general lack of flood hazard awareness amongst community officials and citizens. The Iowa Flood Center seeks to improve Iowa’s flood hazard awareness through the development of easily accessible, highquality mapping products. IFC’s mapping initiatives consist of two model development approaches: (1) statewide floodplain delineation using 1D models, and (2) urban flood mapping using detailed 1D/2D coupled models.
Iowa’s statewide floodplain mapping project will benefit Iowans through the creation of a comprehensive set of floodplain maps developed under a single consistent methodology. The availability of LiDAR data and stream gage records has been vital in generating support for the project and performing the analyses. Communities that would otherwise lack the resources to conduct flood studies will have the opportunity to participate in the NFIP. Since model data and georeferenced riskbased inundation maps are developed using a single methodology, products can be hosted in a webbased database for viewing and downloading.
Through the use of coupled 1D/2D hydrodynamic models, the IFC has developed flood inundation map libraries for several Iowa communities. These inundation map libraries are meant to supplement NWS river stage flood forecasts by providing a visual representation of potential inundation extent according to predicted river stage at stream gage locations. Maps are hosted on the IFC’s Iowa Flood Information System [
Implementation of a largescale mapping project requires significant funding, government support, coordination among stakeholders, and access to large quantities of data. The topographic detail and accuracy of Iowa’s statewide LiDAR dataset facilitated the development of highquality mapping products on a large scale. Other countries interested in a similar mapping project could justify the initial investment in a fullscale LiDAR survey based on the wide applicability of the dataset for engineering, risk assessment, and education. Although the rate of urban development is increasing in some areas of Iowa, the majority of the state’s agricultural landscape will remain unchanged in the coming decades. The time interval for updating the entire statewide LiDAR dataset has not been established. Other regions or countries with higher degrees of urban development may require more frequent updates. A stream gage network of sufficient resolution and record is critical for reachscale floodplain mapping. Other states or countries interested in largescale mapping efforts may benefit from completing a pilot study to evaluate data needs and optimize hydrologic and hydraulic methodologies that may be unique to their geographic location and climate.
The projects presented in this paper were supported by the Iowa Flood Center, the Iowa Department of Natural Resources, the State of Iowa, and the US Department of Housing and Urban Development.
Since regression equations for estimating the 200 and 500year discharges were not generated by Lara [
where:
where: z_{t} is the standard normal deviate corresponding to a return period of t, k is γ/6, γ is the skew coefficient of the logarithm of the annual maxima.
Extrapolating the regional flood relations developed by Lara [
The three points used to define the LP3 distribution are the 2year (
Logratios can be developed using the previous equations and the 100year return period as a reference:
Dividing Equation (6) by (7) results in a ratio called f, which only depends on γ:
Rearranging Equation (8) results in:
Equations (2) and (9) can be used to solve for the skew coefficient γ using numerical methods. Once the skew coefficient has been determined, it can be used to estimate a discharge for any return period for a given drainage area. Using
where:
If Equation (10) is solved for
The USGS regression equations developed by Lara [
Developing similar mathematical equations for the 200 and 500year return periods is desirable. To define the two fixed parameters
The regional regression equations developed by Bradley [