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Projects
Floodplain Services Our staff includes two professional floodplain managers, and many years of hydrologic and watershed management experience. Please see the following project examples for more details on previous Polaris Group Projects.
Digital Flood Insurance Rate Maps (DFIRMS)
Project Tasks
Project Description Numerous developments along Spring Creek in Northfield, Minnesota changed the floodplain and required the city to update its outdated Flood Insurance Rate Maps (FIRMS). The previous maps showed the approximate floodplain (Unnumbered "A" Zone) and were innaccurate. The outdated maps incorrectly located over 40 properties in the floodplain and did not provide flood elevations. Polaris Group performed a detailed hydrologic study of the 6,500 acre Spring Creek watershed. A detailed hydraulic model of over 2 miles of the stream was developed. Digital Flood Insurance Rate Maps (DFIRMS) were prepared to show the 1 and 0.2 percent chance floodplain, the newly defined floodway, property locations, and other features. Polaris Group also worked with City staff to hold an informational meeting for the City Council and affected residents. Floodplain storage and measures to reduce flooding were also designed. Flood Elevation Determinations
Project Tasks
Project Description A family in southern Minnesota wanted to construct a home on their property near a large river. Because of the home's location, survey of the river study area was difficult and the area had no detailed topographic information. Polaris Group was hired to determine the flood profile for this reach so the home's lowest floor elevation could be established. Polaris Group used field survey and Digital Elevation Model (DEM) data to prepare a 3-D surface of the floodplain. This 3-D surface was used to develop the hydraulic model (HEC-GEORAS) cross-sections and the GIS floodplain maps. The automated floodplain modeling and mapping helped keep the costs for the study within the family's budget while providing more accurate results than conventional methods. Polaris Group also performed a risk assessment for assisting the family in setting their lowest floor elevation. Based on this assessment, the family determined that elevating the home approximately 3 feet above the 1 percent chance flood provided the best balance between flood protection and construction costs. Letters of Map Change (LOMC)
Project Tasks
Project Description The U.S. Highway 63 reconstruction project consisted of construction of two new interchanges, reconstruction of over 4 miles of arterial street and construction of seven new bridges or culverts. Polaris Group, Inc was employed to provide floodplain modeling and permitting. The project time line was compressed since the scheduled construction was rescheduled two years earlier than originally planned. The project was further complicated by the large number of stakeholders including over 40 adjacent landholders. Over two miles of floodplain were re-mapped. Road crossings were designed to prevent increases in flood elevation and prevent road overtopping. Channelization and levee features were modeled and designed to reduce flooding and reduce the size of bridges. The design addressed regional stormwater ponding needs. Existing development adjacent to the roadways was included in the revised mapping. Polaris Group Inc was able to meet the tight schedule and obtain the CLOMR in time for the scheduled construction. Channel and Floodplain Modifications
Project Tasks
Project Description A cooling water intake for a power plant along the Minnesota River has experienced the continued deposition of river sediments in front of the intake. The problem relates to the natural migration of the river. In order to stop the movement of the river away from the power plant, three rock spur dikes were installed along the opposite bank. These spur dikes were designed to provide a long-term benefit by re-directing the flow back toward the power plant thus insuring the intake is kept clear of sediment.
River Crossing Studies
Project Tasks
Project Description A county highway department planned to replace a bridge over a stream. The bridge was designed several years prior to its planned construction and subsequent rule changes required the bridge to have a larger capacity than initially designed. A consultant reviewed the design and determined that a larger bridge was needed to meet the new rules. The delay to re-design, re-permit, and re-bid the project would have extended the schedule beyond the construction deadline established for spending the project funds.
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Polaris
Group designed an additional river training structure to create scouring velocities
in front of the intake. A 2-dimensional hydrodynamic model was developed from
the river bathymetry to test the effectiveness of the design. In addition,
the existing HEC-2 model was updated to determine the effect of the modifications
on the upstream flood stage. Polaris Group aided the client in obtaining the
required permits for the work. Construction drawings and specifications were
developed. To insure the structures were installed correctly, Polaris Group
assisted with the construction quality control.
The
County hired Polaris Group to perform an independent review of the bridge
capacity. This review identified measures that could be implemented to meet
the capacity requirements without increasing the size of the bridge. These
measures consisted of modifications to the roadway profile and grading portions
of the stream upstream and downstream of the bridge. Polaris Group also aided
the client in obtaining the required permits for the work and designed streambank
protection measures to protect areas of instability.