Frequently Asked Questions

Floodplain management is a decision-making process that aims to achieve the wise use of the nation's floodplains. "Wise use" means both reduced flood losses and protection of the natural resources and function of floodplains. The goal of floodplain management is to prevent loss of life, reduce property damage, and enhance the natural and beneficial function of the floodplain.

Best Management Practices (BMPs) help local officials determine the best opportunities to manage the potential flood risks and identify key factors to help the community be resilient.

Related: Floodplain and Drainage

• Nonstructural measures are methods and techniques for reducing flood risk, floodplain impacts, and flood damages occurring within floodplains.
• Structural measures such as dams, levees, and floodwalls alter the characteristics of the flood and reduce the probability of flooding in the location of interest. Related: Stormwater Post Construction BMP Guidance Manual

• 2 year event = 50% annual chance of occurrence
• 10 year event = 10% annual chance of occurrence
• 50 year event = 2% annual chance of occurrence
• 100 year event =1% annual chance of occurrence
• 500 year event = .2% annual chance of occurrence

The 1 percent annual chance flood event is the federal, state and local regulatory flood for floodplain mapping and administration purposes.

Large flood events can have devastating, community-wide impacts that go far beyond the impacts to those who were flooded. Economic damages can be a massive burden for a community, particularly one trying to recover from a flood event. Businesses can be disrupted throughout the community because of loss of rail, vehicle, or air services. Loss of power, water supply, or sanitary sewer services can impact the entire community. Loss of tax revenues can lead to budget shortfalls and disruption of future services and infrastructure investment.

Related: Floodplain and Drainage

A floodway is the actively flowing portion of the floodplain that must be kept free from obstructions. In some locations, the active flowing portion of the floodplain is wider and, in some locations, narrower.

A floodplain is the extent of the area that is inundated during a flood event. The floodplain includes shallower areas at the edges, where the flow is not as active.

• Detention cells are typically designed to offset (mitigate) increased runoff rates due to development for the 2-year, 10-year, and 100-year events.
• Bridges and roadway culverts are typically designed based on the level of service (how much traffic) and location. For example, a residential road in the City of Lincoln may be designed so that water doesn't overtop the structure during the 50-year event. A county road with little traffic may be designed to not overtop during a 5-year event.
• Storm drain systems are designed to convey the 5-year, or 10-year event in the infrastructure pipes. Excess flows travel along the curb and gutter of the street. Typically, the bypassed flows from larger events that don't make it in the storm drain system, are required to be contained within the roadway right-of-way.
• If the rainfall amounts associated with the design event for a detention cell, bridge or culvert for a roadway crossing, or storm drain system change, that can have an impact on the cost to replace that infrastructure. Increased precipitation amounts typically lead to increased costs for the construction and maintenance of drainage features.

Stream gages measure how much water is flowing in a stream. The measurement is reported in cubic feet per second (cfs). Stream gage data is available from USGS, USGS Current Water Data for Nebraska.

Related: Floodplain map with interactive flood stages

• Precipitation data is recorded by rain gages and is the measure of how much rainfall occurred. Precipitation data is usually measured and reported on an hourly or daily basis.
• Streamflow data is the measure of how much water is flowing in the stream. Streamflow can be measured on a continuous basis and we typically use the peak annual streamflow (peak streamflow for each year of the stream gage record) to analyze extreme flood events and develop an estimate of the flows for the one percent annual chance, or 100-year, regulatory event.

Generally, the amount of flow in a stream is a result of the precipitation received and the ground surface conditions in the watershed. The more impervious (paved areas, roof tops, etc.) areas there are within a watershed, the greater and quicker the amount of runoff will be generated for a given rainfall event. If soils are saturated (can't soak up any more water), or if the ground is frozen, that can also lead to more runoff.

Increases in atmospheric temperature have a direct influence on precipitation. Increased precipitation leads to increased streamflow volume.

Water quality is different than floodplain management, but some design items offer a benefit to both. Healthy floodplain corridors and minimum buffer stream corridors that include green spaces can help improve water quality along our streams.

Related: Stormwater Runoff Pollution Prevention

A man-made structure, usually an earthen embankment, designed and constructed to contain, control or divert the flow of water to reduce the risk from temporary flooding. Levees are typically built parallel to a water way, to reduce risk on the "landward" side.

The City of Lincoln adopted detention standards city-wide in the late 1990s, a No Adverse Impact (NAI) policy in 2004 for new growth areas and added storm water quality standards in 2015. These policies have been very effective at reducing the adverse impacts of developments. A minimum stream corridor policy was also adapted in 2004, which has significantly increased the amount of natural streams in recent development projects, as well as an associated buffer area for the environment and also helps to protect neighboring properties from flooding and issues with streambank erosion.

Related: Flood Standards

The U.S. Weather Bureau’s Technical Paper No. 40 (TP40), which dates from 1961, was used as the standard for floodplain management for more than 50 years. More up-to-date precipitation data, through December 2012, is contained in the National Oceanic and Atmospheric Administration (NOAA) Atlas 14. This data shows that the 50-, 100-, and 500-year 24-hour point precipitation values have increased since TP40 was published in 1961 and are expected to increase even more in the future. Comparisons of wettest years and other rainfall data nationwide and in the Midwest confirm this increase in rainfall values.

More info: Section 5.2.1 - Salt Creek Floodplain Resiliency Study

Rainfall amount and frequency do not always directly translate into flood frequency. For example, 4.7 inches of rainfall may be a 10 percent annual chance (10-year) event in the spring or summer. However, if the ground is frozen or saturated with water, less of the rain will soak into the ground and more runoff will be generated. The 4.7-inch rainfall event may create a 25-year or 50-year runoff event if water can't infiltrate into the ground. In March 2019, we had a combination of snow, frozen or saturated ground, and rainfall that resulted in a runoff event that was between a 100-year and 500-year event in magnitude. The rain and snow alone were not sufficient to create such a large event.

The City of Lincoln and Lower Platte South NRD have completed many flood control projects, including the Antelope Valley Flood Control Project, the Upper Antelope Creek Flood Reduction Project, the Beal Slough Flood Reduction Project, and the ongoing Deadman's Run Flood Control Project.

Related: Watershed Management Projects

This is a foundational study to examine potential future measures for floodplain management and flood control. New regulations and flood control measures may be part of the recommendations that come forward from this report; but, are not specifically being brought forward for formal approval at this time.

The levee generally provides protection and does not overtop for the approximate two percent annual chance (50-year) flood event. Minimal overtopping of the levees occurred during the May 2015 flood event.

Based on NOAA Atlas 14 precipitation data, all the areas protected by the Salt Creek levees are at increased risk of flooding and flood damages. This includes the areas on the west and north side of downtown Lincoln.

No. A regulatory floodplain map update was not included in the scope. The Salt Creek floodplains were remapped in 2005.

The purpose of study is to develop recommendations to reduce adverse impacts from flooding to life and property, based on current and future flood events. The study includes review of national floodplain best management practices and the potential impacts of a warming climate. Recommended flood control measures will be evaluated using current and future flood events.

Development creates more runoff due to more impervious surfaces, which causes an increase in runoff, quicker stormwater runoff to the stream, and an easier path for pollutants to get to local streams and lakes. Developments are required to maintain the runoff rates from preexisting development conditions and to account for stormwater quality discharges. Detention areas and stormwater quality areas are the areas reserved for holding stormwater, slowing it down to reduce runoff rates, and improve the downstream water quality.

Based on the new precipitation data (NOAA Atlas 14) future detention cells may need to be larger to accommodate the increased runoff that comes with larger rainfall events. The one percent annual chance flood event from Atlas 14 is approximately 10 percent larger than the precipitation used for the 100-year design storm today.

The green space in city parks often provide flood storage as well as outlots in residential and commercial developments. One example of flood storage in a park is Wildemess Park along Salt Creek.

Since the early 1980's, when it was identified that the existing flood control project would not protect Lincoln from the 1 percent annual chance flood, the LPSNRD and the City of Lincoln have been working closely with USACE to address increasing the level of protection and structural integrity of the Salt Creek levee system. The most promising options to restore the level of protection of the levees to the 1 percent annual chance flood and potentially meet technical and economic feasibility have been evaluated as described in Section 4 of the report. Because of its probable high cost (e.g. bridge and utility replacements, taking of private property, and other items), widening the channel to accommodate the 1 percent annual chance flood below the bank elevations has not been evaluated in previous reports (see Section 4).

Past studies of the Salt Creek floodplain conducted by the USACE have looked at numerous options to reduce flood damages and improve the level of protection provided by the levee system. A 1987 USACE study (described in Section 4 of the report) concluded "...that it is not economically feasible to improve the level of flood protection along the entire existing Salt Creek levee and channel project or to its original design level of protection (100-year);..." Other studies have inferred or come to the same conclusion (see Section 4).

The "Sponge Cities" concept is very similar to Low Impact Development (LID). LID commonly incorporates numerous best management practices to achieve environmental and economic benefits. Recommendations in this study for cluster subdivisions regulations, setbacks and riparian preservation, and low impact development regulations support the "Sponge Cities" concepts. In addition to these recommendations, there are many other voluntary practices that can be implemented by the public such as the installation of rain gardens, green roofs, using pervious pavement, and amending soils to increase infiltration, reduce runoff, and improve water quality.

Typically, these practices work well in more frequent rainfall events (i.e. smaller rainfall events that occur more often) and consequently work well for the purposes of stormwater quality. However, the "Sponge Cities" concept is typically less effective for larger rainfall events because the runoff volume greatly exceeds the capacity of the LID components to hold runoff. The intensity of rainfall in larger events can also create more runoff than the LID features can infiltrate. LID will not provide significant flood control benefits unless the LID practices are done very extensively and with significantly more capacity in both existing and developing areas.