Technical Memo – Extreme Rainfall Analysis

FINAL TECHNICAL MEMORANDUM

EXTREME RAINFALL ANALYSIS

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EXECUTIVE SUMMARY

EXTREME RAINFALL ANALYSIS

This report documents an extreme rainfall analysis conducted as part of the Fargo-Moorhead Metro Area

Diversion Project (FM Diversion). The analysis was completed to determine the magnitude of flooding

that could be expected from an extreme rainfall event in the Red River Valley, and to identify how the FM

Diversion would operate under these extreme conditions. In addition to the extreme rainfall events

analyzed, this study evaluates the potential effects of the precipitation that fell during the 2009 spring

flood event on the peak flood stage at USGS Gage 05054000, Red River at Fargo.

Several previously developed modeling tools were utilized and expanded upon for this analysis.

Hydrologic Engineering Center – Hydrologic Modeling System (HEC-HMS) models were previously

developed for watersheds within the Red River Basin upstream from the City of Halstad, MN for the

“Fargo-Moorhead Metro Basin-Wide Modeling Approach” report, December, 2011. This study is also

referred to as the Phase 1 Basin-Wide Modeling Effort.

To generate the extreme rainfall inflow hydrographs for the unsteady HEC-RAS model, Next-Generation

Radar (NEXRAD) data was obtained from the National Weather Service (NWS) and the National Oceanic

and Atmospheric Administration (NOAA). The NEXRAD data was obtained for three historic storms from

within the Upper Midwest. The storms include:

a. 2004 Southern Minnesota and Northern Iowa – Southern Minnesota and Northern Iowa

received heavy rainfall on September 14 and 15, 2004 with totals exceeding 12 inches over a

span of approximately 30 hours.

b. 2007 Southeast Minnesota – A wide area of Northern Iowa and Southern Minnesota

received over 6 inches of rainfall with areas of Southeastern Minnesota receiving over 15

inches throughout a five day period.

c. 2008 Northern Iowa – Several rounds of intense rain fell over Iowa, Wisconsin and

Southeastern Minnesota. The hardest hit areas received nearly 10 inches of rainfall.

The rainfall data was manipulated using several Geographic Information System (GIS) tools. It was then

transferred to various locations within the upper portion of the Red River Valley. Several storm

orientations were evaluated in order to identify the most significant resulting peak flow on the Red River

at Fargo.

Flows from the NEXRAD rainfall data were inserted into the HEC-HMS models. Resulting hydrographs

from the HEC-HMS models were extracted and placed in the Hydrologic Engineering Center – River

Analysis System (HEC-RAS) model used for the FM Diversion Project. The unsteady HEC-RAS model was

executed for existing conditions to determine the maximum peak flow, and the FM Diversion model to

identify how the project would operate for the extreme flows. The current FM Diversion model is VE-13

Option A (VE13A) with Bundles, which includes passing more water through the Flood Damage Reduction

Area (FDRA) and operable gates on the diversion inlet.

It was found that an extreme rainfall event distributed upstream of the project area, such as those

evaluated, is not likely to produce discharges greater than that of a 0.2-percent chance event on the Red

River. In addition, the VE13A project operation was set to produce resulting downstream impacts that

were equal to or less than the impacts identified for the 1-percent chance event from the Integrated Final

Feasibility Report and the Final Environmental Impact Statement (FR/FEIS) dated July 2011. The resulting

upstream staging area elevations, stage through town, and downstream impacts for the 2004, 2007, and

2008 events are reasonably comparable to the current FM Diversion design events.

Although the exact FM Diversion operation implemented for this analysis could be modified to produce

more or less downstream impacts resulting in different upstream staging elevations, the analysis

concludes that the proposed project is capable of effectively operating during the extreme rainfall events

evaluated in this analysis.

PRECIPITATION DURING THE SPRING 2009 FLOOD EVENT

In addition to the extreme rainfall events analyzed, this study evaluates the potential effects of the

precipitation that fell during the 2009 spring flood event on the peak flood stage at USGS Gage 05054000,

Red River at Fargo if the precipitation fell as rain. Much of the precipitation fell as snow because of the

cold temperatures leading up to the flood crest. The objective of this analysis was to determine what the

additional peak discharge and resulting stage would be at Fargo if the snowfall precipitation fell as rain

instead.

Two snowfall events occurred during the 2009 spring flood. The first one produced an average of 0.5

inches of Snow Water Equivalent (SWE) over much of the basin, while the second one contributed up to

2.5 inches of SWE. The spatially distributed SWE data was placed into the HEC-HMS and the resulting

hydrographs inserted in the HEC-RAS models in a similar fashion as in the extreme rainfall analysis.

The increase in discharge from the 0.5” water equivalent rainfall on March 25 was approximately 500 cfs

at the flood crest (March 28). The rainfall equivalent from the event occurring between March 30 and

April 1 was more heavily centered on Wahpeton and the southern portion of the Valley. This produced an

increase in discharge of approximately 8,000 cfs centered on April 8th. However, since this event

occurred after the flood crest, the additional discharge would not have increased the overall crest of the

flood.

If the rainfall would have aligned with the crest at Fargo, it is anticipated that the increase in discharge

would have been less than 8,000 cfs because the River’s discharge-elevation relationship varies and a

higher stage would introduce additional natural floodplain storage and higher tailwater. This also makes

it difficult to predict an actual stage increase as a result of the rainfall.

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