For the more than 200,000 citizens who live and work in the greater Fargo-Moorhead Metropolitan Area, the arrival of spring is both welcomed and feared. The warm embrace that thaws the icy landscape often smothers the region with an abundance of moisture. Floods are common, and some are devastating.
Located in the Red River Valley of North Dakota and Minnesota, Fargo is bisected by the Red River of the North (named as such to distinguish it from the Red River that forms a portion of the border between Texas and Oklahoma). The river is approximately 550 miles long, with about 395 miles of that length located within the U.S. Many dynamics impact the surrounding regions and contribute to the annual flooding, making it difficult to prepare for the events. However, the City of Fargo has discovered that continually updating key mapping data and preventive plans is an effective strategy in helping to ensure the safety of its citizens and infrastructure.
The Red River of the North forms the boundary between North Dakota and Minnesota and flows north from its origination at the confluence of the Bois de Sioux and Otter Tail Rivers before eventually draining into the Arctic Watershed. In comparison to much of the other geology found throughout the U.S., the river is still young. The Red River Valley was formerly the floor of a massive, ice-dammed lake--Glacial Lake Agassiz--and is considered one of the flattest expanses of land in the world. In Fargo, the actual valley of the Red River is only a few hundred feet wide. That area of land was exposed only 9,300 years ago, when the Red River began its journey north. As such a young river, it hasn’t had time to create its own significant system.
Underlying the Fargo-Moorhead topography are approximately 30 meters of sediment that are predominantly clays with expansive properties. As the clays absorb moisture from the spring flooding, they expand until they are saturated. These expansive clays make infrastructure engineering and construction very difficult; shifting and cracking of surface streets, curbs and buildings is common.
While the expansive soils play a part in the flooding, the gradient of the Red River plays an even greater role. The river averages 5 inches of drop per mile of length, and in the region of Drayton-Pembina, Canada, the gradient is only 1.5 inches of drop per mile. With so little gradient to the river and soils that, once wet, will not absorb any more water, the area is highly susceptible to flooding on a regular basis.
In past years, the Red River has passed into flood stage at least once per year and has typically seen minor flooding. However, since 1993, several of the floods have been extremely destructive. The flood of 1979 crested at 49 feet. The spring flood of 1997 crested at 54 feet in Grand Forks and was the most severe flood of the river since 1826, with total damages of $3.5 billion. The 2009 Red River flood brought record flood levels to the Fargo-Moorhead area as a result of saturated and frozen ground, and additional rain and snow storms. The Red River crested at 40.84 feet that year. In 2011, the Red River began its flooding in April and eventually crested at 19.59 feet, which was lower than traditional cresting due to flood control measures that had been put in place.
With such continuous flooding and a changing flood plain, it’s important for the City of Fargo to be prepared for future events. Updated mapping records assist the city in maintaining a continuous understanding of the Red, its flood plain, and its potential for flooding in inhabited areas. In addition, the area is experiencing tremendous growth, thus challenging the city to have increased data and knowledge about the flood-prone areas.
Since the mid-1990s, the city has retained Merrick & Company to provide support in regularly updating the existing mapping records. Planning maps are updated on a three-year cycle and provide an increased understanding of the flood plain, updates to topographic studies, and information for improved erosion management. The information is also shared with the US Army Corps of Engineers for their records.
In 2011, the city retained Merrick & Company to provide digital orthophotography, color aerial photography, LiDAR contour and mapping, and planimetric updates. The additional data would provide crucial information about the elevation of permanent flood protection devices that had been installed over the last three years, such as new clay levees, flood walls and lift stations, and it would also help officials determine the best places to tie into the devices with temporary protection in extreme situations. Importantly, the new data would also help the city keep the public well informed.
The overall project timeline (including acquisition) was scheduled for five months beginning in the spring of 2011 and ending in September. The data collection target covered 404.25 square miles in the cities of Fargo, Moorhead, Dilworth, West Fargo, and portions of Cass and Clay Counties.
Many challenges can arise on a project of this sort. The biggest obstacle surrounds the data acquisition. The primary goal is to acquire the data while the leaves are off the trees, while secondary concerns include solar angle, site conditions (i.e., water levels), and weather. However, with a late spring melt, which happened this year, there is a thin line between obtaining the data through leafless trees and flying an area that hasn’t drained yet from the flooding or lost its snow cover.
Ultimately, emerging vegetation won out, and the city instructed Merrick to begin the acquisition to ensure leaf-off imagery knowing that some areas of the project continued to experience flooding and standing water. The LiDAR and imagery acquisition commenced in mid-May 2011 and was completed later that month.
Using its ALS50-II multiple-pulse scanning system co-mounted with its Digital Airborne Camera System (DACS), the Merrick team captured digital color aerial imagery simultaneously with LiDAR in order to rectify new 6-inch pixel resolution color digital orthophotography (404.25 square miles) and develop a new digital terrain model (DTM) and 1-foot contour database for approximately 341 square miles. The LiDAR point cloud was collected at a point density of four points per square meter (4 ppsm), and designed to meet a vertical accuracy of 0.6 foot at 95 percent confidence level (0.3 foot RMSEz). Planimetric features were updated for approximately 291 square miles and included pavement edges and building footprints. Approximately 50 square miles of impervious surface feature updates added sidewalks and driveways. The 4-inch/four-way oblique imagery (acquired by Pictometry International Corp.) covered approximately 62 square miles. Houston Engineering Inc. of Fargo provided the ground control surveying services, which included establishing approximately 80 control points and aerial targets in support of the LiDAR and imagery acquisition.
The digital orthophotos and oblique imagery will be used by the city auditor for tax appraisals (new and revised), while updates to the impervious surfaces will be used by the city’s engineering department to evaluate runoff. The LiDAR data will add a valuable layer of information that will benefit internal operations as well as the general public.
Although the Red River of the North can never truly be tamed, the City of Fargo is doing everything in its power to subdue the river by developing detailed maps that can help keep citizens and infrastructure out of harm’s way.