unsteady flow simulations and inundation mapping

Hosted by

Black & Veatch Corporation

GEI Consultants, Inc.

Kleinfelder, Inc.

MWH Americas, Inc.

Parsons Water and Infrastructure Inc.

URS Corporation

21st Century Dam Design —

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011

On the CoverArtist's rendition of San Vicente Dam after completion of the dam raise project to increase local storage and provide

a more flexible conveyance system for use during emergencies such as earthquakes that could curtail the region’s

imported water supplies. The existing 220-foot-high dam, owned by the City of San Diego, will be raised by 117

feet to increase reservoir storage capacity by 152,000 acre-feet. The project will be the tallest dam raise in the

United States and tallest roller compacted concrete dam raise in the world.

The information contained in this publication regarding commercial projects or firms may not be used for

advertising or promotional purposes and may not be construed as an endorsement of any product or

from by the United States Society on Dams. USSD accepts no responsibility for the statements made

or the opinions expressed in this publication.

Copyright © 2011 U.S. Society on Dams

Printed in the United States of America

Library of Congress Control Number: 2011924673

ISBN 978-1-884575-52-5

U.S. Society on Dams

1616 Seventeenth Street, #483

Denver, CO 80202

Telephone: 303-628-5430

Fax: 303-628-5431

E-mail: [email protected]

Internet: www.ussdams.org

U.S. Society on Dams

Vision

To be the nation's leading organization of professionals dedicated to advancing the role of dams

for the benefit of society.

Mission — USSD is dedicated to:

• Advancing the knowledge of dam engineering, construction, planning, operation,

performance, rehabilitation, decommissioning, maintenance, security and safety;

• Fostering dam technology for socially, environmentally and financially sustainable water

resources systems;

• Providing public awareness of the role of dams in the management of the nation's water

resources;

• Enhancing practices to meet current and future challenges on dams; and

• Representing the United States as an active member of the International Commission on

Large Dams (ICOLD).

Unsteady Flow Simulations 407

UNSTEADY FLOW SIMULATIONS AND INUNDATION MAPPING FOR THE MISSOURI RIVER MAIN-STEM DAM SYSTEM

Thomas Gorman1 Curtis Miller2

Lowell Blankers3 Laurel Hamilton4

Neil Vohl5 Megan Splattstoesser6

ABSTRACT

The U.S. Army Corps of Engineers, Northwest Division, operates six dams and reservoirs on the main-stem of the upper Missouri River. The total system gross storage is about 73.4 million acre feet. For purposes of evaluating multiple dam safety related scenarios which included dam failures, hydraulic analyses were developed for the system. These analyses were conducted by the Corps’ Modeling, Mapping and Consequences (MMC) Production Center for the Critical Infrastructure Protection and Resilience (CIPR) program. Unsteady flow hydraulic analyses were accomplished using the HEC-RAS computer program. It was desired to model the entire reach of the Missouri River from each dam to the river mouth. From the most upstream dam, Fort Peck Dam in Montana, to the confluence with the Mississippi River near St. Louis is a stream distance of approximately 1770 miles. Geometric data from previously developed HEC-RAS and HEC-2 hydraulic models was combined and extended with geometric data extracted from 10-meter digital elevation models (10m DEM) using the HEC-GeoRAS program. The hydraulic models included dam outlet works operations, storage areas, downstream levee systems and tributary inflows. An important part of the analyses was the use of HEC-GeoRAS as an automated method to rapidly develop inundation mapping for the different scenarios. The results of the study are being compiled by the MMC into map atlas end products, or Emergency Action Plans (EAP) maps, providing inundation maps for selected scenarios displaying flood arrival times for the flood peaking at various downstream locations. 1 Hydraulic Engineer, U.S. Army Corps of Engineers, Omaha District, Omaha, NE, 68102, [email protected] 2 Hydraulic Engineer, U.S. Army Corps of Engineers, Omaha District, Omaha, NE, 68102, [email protected] 3 Hydraulic Engineer, U.S. Army Corps of Engineers, Omaha District, Omaha, NE 68102, [email protected] 4 Hydraulic Engineer, U.S. Army Corps of Engineers, Omaha District, Omaha, NE, 68102, [email protected] 5 Hydraulic Engineer, U.S. Army Corps of Engineers, Omaha District, Omaha, NE, 68102, [email protected] 6 GIS Specialist, U.S. Army Corps of Engineers, Omaha District, Omaha, NE, 68102, [email protected]

408 21st Century Dam Design — Advances and Adaptations

MISSOURI RIVER MAIN STEM DAM SYSTEM

The Missouri River Main Stem reservoir system is comprised of six projects in Montana, North Dakota, South Dakota and Nebraska which include Fort Peck, Garrison, Oahe, Big Bend, Fort Randall and Gavins Point Dams. Several of these reservoirs are among the largest in the United States. Figure 1 shows the location of the projects.

Figure 1. Location Map

These projects were constructed by the Corps of Engineers for flood control, navigation, irrigation, hydroelectric power, water supply, water quality, recreation and fish and wildlife. The projects are operated as a hydrologic and electric power generation integrated system to achieve the multipurpose benefits for which they were authorized. Table 1 provides pertinent information about the projects.


Table 1. Data for Missouri River Main Stem Reservoirs Fort

Peck Garrison Oahe Big

Bend Fort

Randall Gavins Point

River Mile (1960)

1771.5 1389.9 1072.3 987.4 880.0 811.1

Drainage Area (sq. Mile)

57,500 181,400 243,490 249,330

263,480 279,480

Gross Storage (1,000 acre feet)

18,688 23,821 23,137 1,859

5,418 470

Flood Control Storage (1,000

acre feet)

3,692 5,711 4,303 177

2,294 149

THE CRITICAL INFRASTRUCTURE PROTECTION AND RESILIENCE PROGRAM

The U.S. Army Corps of Engineers (USACE) Office of Homeland Security (OHS) has tasked the Mapping, Modeling and Consequences Production Center (MMC) to develop and execute a dam break flood inundation mapping initiative on USACE water resources projects (dams). This effort supports National critical infrastructure protection responsibilities led by USACE under the USACE Critical Infrastructure Protection and Resilience Program (CIPR).

The CIPR vision is to achieve a more secure and more resilient USACE civil works infrastructure by enhancing its protection in order to prevent, deter, or mitigate the effects of manmade attacks and improve preparedness, response, and rapid recovery in the event of an attack, natural disaster, and other emergencies. A major initiative within this effort involves the development of updated dam break flood inundation mapping and estimating populations at risk and economic impacts.

Dam break inundation mapping is intended to be a planning and a response tool to determine the effects of a dam break scenario. Therefore, inundation maps are expected to be utilized by a wide range of end-users. CIPR data is intended to provide stakeholders with information needed to make informed decisions regarding this particular study as well as to document the data and assumptions behind the models and mapping.

The elevation datum developed used in all CIPR analyses and reports are to the North American Vertical Datum of 1988 (NAVD), unless otherwise specified.


HYDRAULIC MODELING

Base Data Hydraulic modeling and analyses for the CIPR Program are based on using existing data wherever possible. There was much existing data for the Missouri River main stem system that was used in the analyses.

Ground surface data and aerial photography of the entire study area was required for model and map development. United States Geological Survey 10 meter Digital Elevation Models (DEM’s) were used for cross section development and inundation mapping development.

Additional data used in modeling was obtained from the Operation and Maintenance Manuals for each dam, as well as the existing Emergency System Operating Plans (ESOP) for the Missouri River Main Stem Reservoir System (USACE, 1992). Cross sections extracted from the USGS 10 meter DEMs were used to extend cross sections used for modeling large spillway releases described in the ESOP reports. The ESOP cross sections were used as a base because they were more accurate than those extracted from the USGS 10 meter DEM data.

Hydraulic Model Set-Up

For the CIPR analysis an unsteady flow model was set up for each of the six dams, from the dam location to the Missouri River mouth. Initial model development was in reaches from dam to dam and two reaches from the downstream dam (Gavins Point) to the mouth due to concerns about hydraulic model execution time. When it was determined that continuous models would execute in a reasonable time, they were set up for each dam. The model from Fort Peck Dam to the mouth was the most extensive modeling effort and will be described in the following paragraphs.

The unsteady hydraulic modeling for the Missouri River was conducted using the HEC-RAS software, Version 4.0.0, (USACE, 2008). The analyses also made use of the Geographic Information Systems (GIS) programs, ArcMap, Version 9.3 (ESRI, 2008) and HEC-GeoRAS, Version 4.2.93 (USACE, 2009), which were used for development and analysis of geo-referenced, unsteady hydraulic models. The tie to GIS allowed for efficient model development and utilization of results for estimation of consequences.

The modeling effort began with collection of geospatial and other miscellaneous data used to lay out the model, extract initial model geometry, estimate dimensions of modeled structures, and estimate hydraulic variables of the system. HEC-GeoRAS was then used to set-up the initial hydraulic model geometry which was exported for further development in HEC-RAS. Development of the hydraulic model was finalized within HEC-RAS and dam failure conditions were modeled given the flow conditions calculated in previous hydrograph development efforts. After model simulation, model results were imported back into the GIS environment where HEC-GeoRAS was utilized to develop flood inundation boundaries and depth grids. These products were used to produce consequence estimates.


During the CIPR study process, HEC-RAS hydraulic analyses were initially developed for the reaches from Fort Peck Dam to Garrison Dam, from Garrison Dam to Oahe Dam, from Oahe Dam to Fort Randall Dam, from Fort Randall Dam to Gavins Point Dam, from Gavins Point Dam to Kansas City and Kansas City to the Missouri River mouth. These model reaches followed the extents of the DEMs that were provided. These models were used as the basis to develop a continuous HEC-RAS model from Fort Peck Dam to the mouth.

The initial model layout from Fort Peck Dam to Gavins Point Dam at River Mile 811.1 near Yankton, South Dakota was based on the stream center line for the reach, provided by the Corps’ Kansas City District. The model cross sections locations were those developed for the ESOP study which utilized the HEC-2 hydraulic model. The ESOP cross sections were digitized and incorporated into GIS as cut lines and were then extended to the limits of the DEM to ensure high ground was captured. These extended cut lines were processed using HEC-GeoRAS with the USGS 10-meter DEMs to obtain terrain data at each cross section. The geo-referenced river reach and cross sections were imported into HEC-RAS.

The ESOP HEC-2 models were imported into HEC-RAS. The ESOP cross sections used the National Geodetic Vertical Datum of 1929 (NGVD29). These elevations were adjusted in HEC-RAS based on the datum shift between NGVD29 and NAVD88 at regular points along the river developed by the Corps’ Kansas City District.

The original ESOP cross sections included channel bathymetry, whereas the cross sections extracted from the USGS 10-meter DEMs did not. Therefore, the channel geometry of the adjusted ESOP cross sections and the overbank terrain of the extracted cut lines were merged in HEC-RAS to create an inclusive cross section. Downstream channel and overbank reach lengths were obtained from the ESOP model, and therefore were not created in GeoRAS. All bridges identified in the original ESOP model were removed to facilitate model stability. Ineffective flow areas were incorporated from the ESOP model and additional locations were included manually in HEC-RAS, where necessary. The height of some ineffective flow areas were adjusted to promote model stability.

For the Missouri River from Gavins Point Dam to the mouth, the initial model layout was based on HEC-RAS cross sections from the Missouri River Floodway Study, MRFWS, (USACE, 2007). The geometry in the MRFWS models was based on topographic and hydrographic survey information developed for the Upper Mississippi River System Flow Frequency Study (USACE, 2004).

For these analyses, the cross sections in the hydraulic model were reduced from those in the MRFWS and all bridges were removed from the hydraulic model. Selected MRFWS cross sections were extended to include the entire Missouri River valley from bluff to bluff. The MRFWS cross sections were adjusted based on the datum shift between NGVD29 and the NAVD88. The adjusted MRFWS cross sections and those extracted from the USGS 10 meter Digital Elevation Models using the GeoRAS software were merged to create an inclusive cross section.


Manning’s “n” coefficients were utilized to simulate roughness in the model. The roughness coefficients used in the ESOP modeling were employed upstream from Gavins Point Dam. The MRFWS roughness values used downstream of Gavins Point Dam had been calibrated for a Missouri River 1-percent-annual-chance (100-year) flood event, which would be considerably less than for the larger dam failure events.

Channel roughness values of 0.02 to 0.03 and overbank roughness values ranging from 0.03 to 0.07 were used for the majority of the reach. Contraction and Expansion coefficients are not used in unsteady HEC-RAS computations.

The HEC-RAS model from Fort Peck Dam to the Missouri River mouth included almost 900 original cross sections. The original cross sections were all based on terrain data (surveys or DEM). During the course of the unsteady flow analyses, a limited number of interpolated cross sections were added at selected locations for model stability.

Numerous large and small tributary streams enter the Missouri River in the study reach. The effect of backwater up the smaller streams was included in the HEC-RAS model by extending cross sections out parallel to the tributary stream alignments. The cross section area up the tributaries were made ineffective for flow, but allowed HEC-RAS to account for storage up the tributaries. Larger tributaries were modeled as off-stream storage areas. The elevation-storage relationship for each storage area was developed using GeoRAS. Flow into and out of the storage areas was controlled using simulated lateral structures extracted from the terrain using GeoRAS and added to the hydraulic models. For the Missouri River from the mouth to Fort Peck Dam, approximately 80 storage areas were used. An addition storage area was included for Fort Peck Lake.

The six Corps of Engineers dams on the Missouri River main stem were modeled as HEC-RAS inline structures. Spillway gates and outlet works at the dams were included in the inline structures and adjusted to pass the initial flows at these locations for the start of the simulations. The relationships between the gate opening, reservoir elevation and gate discharge for the spillways at each of the downstream dams were input into the HEC-RAS model from the spillway rating curves and outlet works ratings curves for each dam. In the reach from Fort Peck Dam to the Missouri River mouth, there are no other dams.

Levees were located at several locations along the Missouri River at and downstream from Omaha, Nebraska and Council Bluffs, Iowa. Urban levee systems were located at Omaha/Council Bluffs; St. Joseph, Missouri; Kansas City and near St. Louis. Selected levees were included in the HEC-RAS hydraulic models using the HEC-RAS levee option. The levee top elevations were set from ground point elevations shown in the MRFWS cross sections. Federal levees and levees protecting urban areas were modeled. In the State of Missouri there were a large number of locally constructed levees of varying capacity that protect mostly cropland which were not included in the hydraulic model.

The HEC-RAS levee options make levees effective or ineffective based on water surface elevations computed at each levee location for each time step in the unsteady flow


simulation. This resulted in some levees that were ineffective at the peak becoming effective after the crest of the hydrograph passed and stages fell below the levee crests. This would not occur for an actual dam failure situation because the levee systems would be partially or totally destroyed when overtopped by such large flows. This situation should not have had an effect on the consequence computations, which were based on the peak stages that were attained before the failed levees became effective again.

The starting water surface elevations at the downstream cross section of the HEC-RAS model was set at normal depth. The upstream boundary condition for the upper HEC-RAS model in each simulation was based on stage and flow conditions at the upstream reservoir for the selected scenario.

Hydrologic Modeling Approach

The hydraulic analyses were simulated for three hydrologic loading conditions. These loading conditions were selected for the CIPR Program to cover a wide range of loadings. CIPR has assigned standard nomenclature to these conditions.

Unsteady flow analyses were conducted with water surface elevations in the reservoir at the normal high pool (10% exceedance by duration), top of active storage (full flood control pool) and the maximum high pool (the spillway design flood peak). The normal high pool failure represents a “sunny day” failure situation. The top of active storage condition is the maximum normal hydrologic loading. The maximum high condition represents the maximum pool condition that is reasonably possible and was simulated with the Spillway Design Flood (SDF) inflow hydrograph combined with assumed gate operations at the dam. The SDF hydrographs were obtained from design documents for the specific dam under consideration.

Loading conditions in the analyses were designated as follow:

• Normal High Pool = NH • Top of Active Storage = TAS • Maximum High Pool = MH

For the normal high pool simulation, inflow at the upstream dam was set to the 50% duration flow based on the Reservoir Control Center Technical Report F-99 (USACE, 1999). Outflow was assumed equal to inflow. All inflows below the upstream dam were assumed to be equal to the 50% by duration inflow based upon incremental flow between each dam or each gaging station, with flows proportioned by drainage area of major drainage basins and distributed equally by river mile for all remaining ungaged flows. All downstream reservoirs were assumed to be at the normal high pool elevation, with outflow equal to inflow.

For the top of active storage simulations, the initial outflow at the upstream dam was set to the 0.01 exceedance frequency release, with inflow assumed equal to outflow. All inflows below the dam were assumed equal to the 20% by duration 7-day incremental


flow between dams and stream gages; flows were proportioned by drainage area for major drainage basins, with the remaining incremental flow distributed equally throughout the corresponding reach. 7-day incremental inflows were assumed downstream, as sustained high inflows are necessary to reach top of flood control pool at the project. The starting pools at of the downstream dams were each assumed equal to the 0.10 exceedance frequency pools. Outflow at each downstream reservoir was assumed equal to inflow. The inflow hydrograph for the spillway design flood at the upstream dam was factored down until the maximum pool elevation attained at the upstream reservoir for a non-failure simulation was at the pre-determined top of active storage elevation. For the maximum high pool simulations, starting pool elevations, initial inflows and outflows for the upstream dam and reservoir were from design documents. Downstream inflows were derived by factoring the inflow hydrograph by an appropriate runoff reduction factor based on size of downstream drainage areas as determined by bounding the dam’s incremental drainage with an ellipse and downstream areas with concentric ellipses, until the ellipses encompassed areas greater than 1,000,000 square miles. For all areas downstream of the largest ellipse, inflows were assumed equal to the 20% by duration 7-day incremental flow between dams and stream gages; flows were proportioned by drainage area for major drainage basins, with the remaining incremental flow distributed equally throughout the corresponding reach. Additionally, the factored downstream hydrographs were set equal to the 20% by duration 7-day incremental flow when the factored hydrograph value was less than that value. The starting pools for the downstream reservoirs were the same as for the top of active storage simulation. Dam Failure Breach Parameters

For the CIPR analyses, detailed geotechnical studies were not conducted to determine the breach parameters for the unsteady flow simulations run with dam failure. The dam breach parameters (breach bottom width, side slopes and time to fully develop the breach) were determined based on several sources. This included the breach characteristic used in the previous ESOP analysis and on some of the various regression equations that have been developed for estimating breach parameters for earthen dams. The final selection of the breach characteristics involved considerable engineering judgment. During the unsteady flow simulations, breach parameters for some dams needed to be adjusted for model stability.

The parameters of the CIPR program specified that the upstream dam failure mechanism would be by piping with a sinusoidal breach progression. The potential for cascading dam failure was considered. For the dams located downstream of the initial failure location, failure was allowed to occur from overtopping if any downstream dam embankment was overtopped by more than one foot.

For the normal high pool scenarios, the failure of the upstream dam in the hydraulic model was started 24 hours after the start of the simulation. For the top of active storage and maximum high pool scenarios the upstream dam failure was initiated when the reservoir rose to the selected elevation for failure.


Emergency Gate Operations

In order to effectively model the Missouri River Main Stem system during an emergency situation, it was necessary to model the outlet and spillway gate operations of all downstream dams as a system. Of particular interest are those scenarios in which a dam failure would not initiate a downstream failure, but would instead necessitate the need for large releases, as these releases themselves may cause serious downstream damages. No “blueprint” for system operation during a dam failure emergency currently exists; therefore it was necessary to utilize existing means that could be adapted to determine what gate operations may be necessary during such an emergency.

The Corps’ Northwestern Division Missouri River Reservoir Control Center (RCC) utilizes a three-week forecast model to determine release schedules, primarily for hydropower production. The model utilizes current pool and inflow conditions, as well as forecast flows for the next three weeks. This model underwent minor modifications (e.g., extending rating curves for various gate operations to top of dam, along with overtopping rating curves), and then inputs from each load case were loaded into the model to determine a three-week release schedule. However, the model utilizes a daily time step; in order to improve the coarseness of the release schedule, the model outputs were input to a spreadsheet to interpolate release and pool levels to an hourly time step. Manual lookups were then utilized to determine the necessary gate openings to pass the required flow at the computed pool, and then these gate openings were interpolated to hourly values.

These gate openings were copied into the appropriate table in the unsteady flow editor and the corresponding HEC-RAS plan was computed. The hourly releases and pool levels from the HEC-RAS output were compared to the previously computed RCC releases and pool levels and adjustments were made to gate openings, as needed, to ensure all pools and releases remained within certain bounds. It was then determined that releases and pool levels at all downstream dams matched the RCC values within reason. Gate openings were adjusted to the RCC calculated releases and pool levels for the fail and no-fail simulation for each loading condition with the exception of certain of the Maximum High Pool dam failure scenarios. When the latter scenario resulted in significant overtopping of downstream dams, it was assumed that gate openings would not be adjusted after the start of overtopping and failure of those dams.

Unsteady Flow Computations and Results

The unsteady flow modeling starting time for the various scenarios was set at the far future date of February 1, 2099 to indicate that the modeling was for theoretical flood scenarios and was not a re-creation of historical flood events. For modeling flood events from the most upstream dams, a total simulation time of up to 60 days was used to allow the flood waves to travel down the entire length of the Missouri River to the Mississippi River confluence area.

The computation time step for the unsteady flow simulations varied according to which dam and reservoir was used as the upstream boundary condition. The longest time step


that would allow the computations to run through the entire simulation without becoming unstable was used. The selected time steps varied from five seconds to five minutes.

As was mentioned in a previous section, in some instances the initially selected dam breach parameters had to be adjusted for model stability. At some river reaches with steep slopes and irregular bed profiles, the pilot channel option of unsteady HEC-RAS was used for model stability mostly for lower flows during the receding limb of the hydrographs.

Table 2 is a summary of the magnitude of the peak flood discharges at dam locations for the various failure scenarios. The peak flows shown are for the given dam as the upstream dam in the system for the failure scenarios.

Table 2. Peak Flows at Dams Fort Peck

Dam Garrison

Dam Oahe Dam Big Bend

Dam Fort

Randall Dam

Gavins Point Dam

Scenario Missouri Peak Flow at Dam, cubic feet per second NH-Fail 4,341,000 3,098,000 4,372,000 1,154,000 4,587,000 185,000TAS-Fail 7,811,000 6,099,000 8,123,000 1,276,000 5,925,000 277,000MH-Fail 9,203,000 8,4940,000 18,992,000 1,879,000 8,768,000 1,005,000

Some of the dam failure simulations produced very large flows with multiple cascading failures producing flows far in excess of those shown in Table 2. The HEC-RAS unsteady flow simulations were able to generate hydrographs at all downstream cross section location so that the attenuation of the flood waves could be followed and arrival times for the flood peaks determined.

Inundated Area Mapping

One of the main products of the CIPR program is inundation mapping for the various scenarios. The areas inundated for the maximum water surface elevations were developed for the various loading conditions (failure and non-failure).

The flood boundaries for the maximum flood elevations were delineated using HEC-GeoRAS. That program simultaneously developed grid maps of the maximum flood depth. Because of the large size of the original terrain DEMs and the limitations on the size of DEMs that could be processed by GeoRAS, the original terrain DEMs were subdivided into smaller terrain tiles. For the entire reach from Fort Peck Dam downstream to the mouth of the Missouri River, there were 33 DEM tiles.

In addition to dividing the study reach into smaller terrain tiles, it was found to be necessary to edit the HEC-RAS export file (.sdf ) which is a text file created by HEC-RAS and used by GeoRAS as input for the inundated area delineation. This editing was done to reduce the extents of the raster files created in the GeoRAS inundation mapping


process to reduce the time required for delineation from many hours to a few minutes per tile.

The original HEC-RAS export files for each loading condition were copied and edited with a text editor to create a smaller export file specific to each terrain tile. For each tile, the file was edited to include only the cross sections and storage areas located on that tile. The coordinate points for the stream centerline and bounding polygon were edited so that these features extended just beyond the upstream and downstream extents of the terrain tiles. The extent of the coordinates of the stream centerline and bounding polygon at these locations were determined with ArcMap and recorded before the editing process. Following the editing process, the export files for each tile were used in the normal fashion as input for the GeoRAS inundation mapping process.

Once the RASexport file was imported into GeoRAS, additional adjustments were made during the GeoRAS post-RAS processing step to ensure complete floodplain delineation. Some cross-sections were manually extended to ensure that the Water Surface TIN extended far enough into the floodplain in order to create complete inundation maps. Similarly, the bounding polygon was adjusted to capture the affected floodplain fully.

The original inundation area delineations created by GeoRAS for each tile were checked for accuracy and edited where necessary. Where levees were in place and effective, any flooding developed by GeoRAS on the landward side of the levees was edited out and the flood boundaries followed the levees.

Consequence Analysis

Consequence assessment was performed using HEC-FIA, Version 2.1 Beta (USACE, 2009) a stand-alone, GIS enabled model for estimating flood impacts. The software generates economic and population data for the study area using census blocks and computes urban and agricultural flood damages based on the input event. For this analysis, FEMA’s HAZUS database was used to develop a structure and population inventory. Agricultural damages were not computed for the CIPR study. Loss of life computations are also performed based on warning time and arrival time of the flood as well as depth of water at each structure. Computed results are based on the accuracy of the data sets available at the time of the model run.

Impact areas in HEC-FIA are used to define common input parameters for groups of structures as well as for aggregating consequence results. County boundaries obtained from the HAZUS data set were used to define impact areas. The USGS 10 meter DEMs were used as terrain data in the HEC-FIA simulations. Inundation depth grids developed using HEC-GeoRAS were used for inundation data in the HEC-FIA simulations.

As was mentioned previously, it was necessary to develop the inundation mapping and depth grids from Fort Peck Dam to the mouth as 33 separate tiles. Because of this situation, 33 FIA models were required for each scenario.

The structure inventory was used in HEC-FIA to estimate direct economic damages for a single flood event. For life loss estimates, population must be supplied with the structure


inventory. Because the HEC-FIA analysis for the Fort Peck Dam failure involved seven states, the FEMA HAZUS databases from those states were used to create a combined data base that allowed FIA analyses to be done for reaches that included more than one state.

Life loss parameters were defined in HEC-FIA using suggested values from the HEC-FIA Quick Start Guide (USACE, 2009) per guidance from the MMC. Evacuation time was computed by HEC-FIA using a hazard zone boundary and a nominal evacuation velocity. The hazard zone boundary was developed using the inundation depth grids by selecting all areas with less than 2 feet of depth (assuming that if an individual can reach a location where the maximum depth is 2 feet or less, they will be safe). The nominal evacuation velocity was specified as 10 miles per hour per MMC guidance. The “Emergency Broadcast System, sirens, and auto-dial telephones” warning option was used with the default coefficients. Mobilization was specified as average and the default “Additional Parameters” were used. The warning issuance time was set to one hour after initiation of the dam breach for dam failure scenarios. Warning issuance time for non-failure scenarios was set to 24 hours prior to the dam breach of the corresponding failure scenario.

The HEC-FIA output provides the number of structures flooded according to various categories and the total momentary damages. The Life Loss program included with FIA provides the total life loss. The damages and life loss are most useful for assessing the order of magnitude of the flood impact rather than being considered the exact impact.

SUMMARY

Using readily available software and existing terrain and hydraulic data, a very large system of dams and reservoirs on the Missouri River was successfully modeled for various dam safety scenarios. These analyses will supplement or replace previous dam safety studies that were not able to provide the Missouri River Main Stem Dam System with a continuous model. The use of unsteady flow modeling also allowed the arrival times and attenuation of the flood waves to be determined at downstream locations of interest. The GIS-based system allowed inundation mapping to be rapidly developed for the various stream reaches and scenarios.

REFERENCES

Environmental Systems Research Institute; ArcMap, Version 9.3; Redlands, California; 2008 Federal Emergency Management Agency; HAZUS, Missouri, Kansas, Nebraska, Iowa, South Dakota, North Dakota, Montana; 2008 U.S. Army Corps of Engineers; Hydrologic Engineering Center; HEC-RAS, Version 4.0.0; Davis, California; March, 2008 U.S. Army Corps of Engineers; Hydrologic Engineering Center; HEC-GeoRAS, Version


4.2.93; Davis, California; September, 2009 U.S. Army Corps of Engineers; Hydrologic Engineering Center; HEC-FIA, Version 2.1 Beta; Davis California; September, 2009 U.S. Army Corps of Engineers; Hydrologic Engineering Center; Rapid Consequence Assessment Using HEC-FIA, Quick Start Guide, Version 2.1 Beta; Davis California; September, 2009 U.S. Army Corps of Engineers, Kansas City District and Omaha District; Missouri River Floodway Study; November, 2007. U.S. Army Corps of Engineers, Missouri River Region Reservoir Control Center; Missouri River Main Stem Reservoirs, Hydrologic Statistics, RCC Technical Report F-99; Omaha, Nebraska; February 1999. U.S. Army Corps of Engineers, Omaha District; Emergency System Operating Plan; Missouri River Main Stem Reservoir System; Big Bend Dam to Fort Randall Dam; Revised July 1992 U.S. Army Corps of Engineers, Omaha District; Emergency System Operating Plan; Missouri River Main Stem Reservoir System; Fort Peck Dam to Garrison Dam; Revised July 1992 U.S. Army Corps of Engineers, Omaha District; Emergency System Operating Plan; Missouri River Main Stem Reservoir System; Fort Randall Dam to Gavins Point Dam, Revised July 1992 U.S. Army Corps of Engineers, Omaha District; Emergency System Operating Plan; Missouri River Main Stem Reservoir System; Garrison Dam to Oahe Dam; Revised July 1992 U.S. Army Corps of Engineers, Omaha District; Emergency System Operating Plan; Missouri River Main Stem Reservoir System; Gavins Point Dam to Rulo, Nebraska; Revised July 1992 U.S. Army Corps of Engineers, Rock Island District; Upper Mississippi River System Flow Frequency Study; January, 2004

unsteady flow simulations and inundation mapping

Documents