3.18 FLOODPLAINS

3.18.1 Existing Conditions

Floodplains. Portions of the beltway alternatives are located in 100-year floodplains established by the Federal Emergency Management Agency (FEMA). The City of Lincoln and Lancaster County participate in the FEMA National Flood Insurance Program (NFIP) which regulates construction within the 100-year floodplain. A 100-year floodplain is estimated as the limits of the water surface created by a 100-year flood, or an event with a 1 percent annual chance of occurrence.

The concept of a floodway and floodway fringe is used to regulate encroachment (development) within the floodplain. A floodway consists of a channel and portion of the adjacent floodplain area required to convey the 100-year flood discharge with no more than 0.3 m (1 ft) increase in the 100-year flood elevation. Floodway fringe is defined as the area between the limits of the floodway and the 100-year floodplain. In essence, once a floodway is established for an area, theoretically the entire floodway fringe may be developed in most cases.

Some locations along the beltway alternatives are within areas where floodplain boundaries have only been estimated. In other locations regulated floodway and floodway fringe have been established. The boundaries of the 100-year floodplains and floodways are shown on Figure 2.25.

Through participation in the NFIP, the local floodplain ordinances require that floodplain development permits be obtained from the City/County. Any development within designated floodways must include certification from a registered professional engineer that the proposed development will result in no increase of the water surface elevation associated with the 100-year flood event. In some locations, where no floodway has been designated, development may not cause more than 0.3 m (1 ft) of rise in the 100-year flood elevation. These ordinances allow some, but only very minor encroachment.

Currently, the City and the Lower Platte South Natural Resources District (LPSNRD) are evaluating a more stringent floodplain ordinance for the City of Lincoln and its 5 km (3 mi) zoning jurisdiction to protect infrastructure and properties within the floodway fringe. Some concepts being discussed include provisions for no net loss of floodplain storage, and no net rise of base flood elevation within the floodway fringe/floodplain. These provisions might also have the potential for application within the County's jurisdiction to achieve a more uniform countywide floodplain management program.

Salt Creek Hydrologic Study. As part of the Wilderness Park Subarea Study, the US Army Corps of Engineers (Corps) was contracted by the LPSNRD to prepare the Hydrologic Study of Salt Creek at Wilderness Park (1999). This study evaluated various alternatives to determine their effects on the peak flows (discharges) and stages (water surface elevations) on Salt Creek through Wilderness Park. Pertinent recommendations from the study that apply to the beltway study were:

• Channel confinement or shortening within upstream areas should be strongly discouraged.
• Any fill within the Salt Creek floodplain should be compensated for by providing an equal amount of storage elsewhere on the site.
• Analysis of all the alternatives involving bridge removal or additions resulting in findings that there would be no significant reduction in peak discharge, and there would be no system wide impact on flow rates.

Stevens Creek Watershed Plan. LPSNRD prepared the Concept Comprehensive Flood Management Plan for Stevens Creek (1997) study identifying measures that would provide at least a 40 percent reduction of flood damages in the watershed. Several components were identified that would provide a holistic approach to stormwater management in the watershed and meet the flood damage reduction goal. These include:

• Preserving portions of the Stevens Creek mainstem and tributary 100-year floodplain as open space through acquisition and regulation,
• Augmenting culverts at select locations to reduce flooding,
• Erosion healing treatment in the lower channel reach using "soft engineered" solutions,
• Land treatment as a conservation measure with new and retrofit pipe outlet terraces, and establishment of permanent vegetative cover on Class IV lands,
• Watershed management using integrated watershed models, and
• Providing detention storage for flood waters using a system of ten farm pond dams and reservoirs (see Figure 2.25).

The LSPNRD has obtained partial support through state funding to construct the system of farm ponds. Five of the ten sites are currently in final design stage (sites A6-1, A7-1, A9-1, A9-6 and A17-1), and land rights negotiations are underway. Construction is anticipated to begin late in 2001. The remaining five sites are currently scheduled for construction in 2002. Each of the east beltway alternatives may impact one or two of the proposed dam sites.

Pertinent recommendations in the plan include (1) land rights acquisition using conservation easements in the Stevens Creek floodway and flood fringe between the MoPac Trail and the Murdock Trail, (2) continued reliance on existing floodplain regulations for development in the rest of the delineated Stevens Creek mainstem and tributary floodplains, and (3) a public access easement for a trail along Stevens Creek from the MoPac Trail to Salt Creek. The study team has been in contact with LPSNRD and its consultant regarding potential beltway impacts.

FEMA Mitigation Grant Program. The City of Lincoln is looking into establishing conservation easements in the south beltway corridor in the vicinity of Salt Creek for the purposes of flood control. Such easements would be established so as not to preclude construction of the south beltway. The City has applied for a grant through FEMA for this prupose.

3.18.2 Impacts of the Four Finalist Alternatives

Since the beltway project will comply with all local floodplain ordinances, floodplain encroachment will not be significant and will not adversely affect the natural and beneficial floodplain values for any of the beltway alternatives. Review for this evaluation did not include an analysis of project impacts on flood storage or conveyance due to the preliminary design stage of the project.

SM-4 Alternative. SM-4 crosses the floodplain and main stem of Salt Creek between Saltillo Road and Bennet Road near 25^th Street, as well as a tributary of Salt Creek at the US 77 interchange. SM-4 also crosses the Wagon Train tributary of Salt Creek, and one of its tributaries, between Saltillo Road and Bennet Road near 84^th Street. A floodway has not been established in any of these locations.

EC-1 Alternative. EC-1 crosses the floodplains of the main stem of Stevens Creek, two tributaries to Stevens Creek, and an unnamed tributary to Salt Creek. The mainstem crossing of Stevens Creek occurs between Adams Street and Holdrege Street where a floodplain has been delineated, base flood elevations and flood hazard factors have been determined, and a floodway has been established. A floodway has also been established in the vicinity of the tributary crossing location between A Street and Van Dorn Street. No floodways have been established for the tributary crossing location between Fletcher Avenue and Alvo Road, or the Salt Creek tributary crossing.

EC-1 will impact two of the proposed LPSNRD farm ponds. The alignment goes through the Sky Ranch Dam site (Exhibit EC1-4). Moving the dam upstream (to the southwest) away from the beltway would likely reduce the flood control capacity of the structure, and the beltway would obstruct the emergency spillway of the dam--requiring an additional culvert under the beltway capable of conveying the flow from the emergency spillway. It is not practical to shift the beltway alignment at this location due to its proximity to the Sky Ranch Acres subdivision. The alignment also goes through a portion of the A3-2 dam site (Exhibit EC1-2). Moving this dam site upstream (to the west) away from the beltway would likely reduce the flood control capacity of the structure. This shift would also bring the permanent pool closer to 98^th Street and would require that the culvert under 98^th Street be evaluated and possibly replaced. In addition, the beltway could obstruct the emergency spillway of the A3-2 dam--requiring an additional culvert or bridge under the beltway capable of conveying the flow from the emergency spillway. It is not practical to shift the beltway at this location due to its proximity to a housing development at 112^th and Pine Lake Road. A shift would also impact the Pine Lake Road interchange, possibly increasing the required structure lengths and therefore cost. Both dam sites are in Phase 2 of the LPSNRD project and final design has not begun.

EM-1 Alternative. EM-1 crosses the floodplain and mainstem of Stevens Creek in two locations, as well as the floodplains of two tributaries to Stevens Creek, and two unnamed tributaries to Salt Creek. The two mainstem crossings of Stevens Creek occur near Yankee Hill Road and 120^th Streets, and between O and A Streets near the Walton. No floodway has been established in the vicinity of the Yankee Hill Road crossing. However, the floodway is designated in the vicinity of the crossing near Walton. In this location, the floodplain is very flat and broad and is crossed at approximately a 45-degree angle by the proposed route. The tributaries of Stevens Creek which are crossed by EM-1 are located in the lower reaches of the drainage basins; no floodways have been established in these locations between Pine Lake Road and Yankee Hill Road, and between Van Dorn and Pioneers Boulevard. No floodways have been established on the Salt Creek tributaries.

EM-1 will impact one of the proposed LPSNRD farm ponds, A2-1, in the vicinity of the emergency spillway at the west side of the dam (Exhibit EM1-1). The permanent pool also appears to be very close to the westbound-to-northbound ramp of the beltway/N-2 interchange. The spillway for the dam could be relocated on the east side of the dam, thus eliminating the conflict. The impacts relative to the permanent pool can not be accurately determined based on the information available at the present time and should be addressed during final design of both projects. Dam site A2-1 is in Phase 2 of the LPSNRD project and final design has not begun.

EF-1 Alternative. EF-1 crosses floodplains along five tributaries to Stevens Creek; the route does not cross the main stem of Stevens Creek. No floodways have been established in any of these locations.

EF-1 will impact two of the proposed LPSNRD farm ponds. The alignment goes through the west end of the A9-1 dam site (Exhibit EF1-3). It appears that the dam embankment could be tied into the roadway embankment; it is likely that this could cause the beltway embankment to have water from the permanent pool against it. This issue would need to be addressed in the final design of the beltway to provide adequate protection of the roadway embankment. EF-1 is also close to the A-17 dam site, but does not directly impact the dam (Exhibit EF1-4). The emergency spillway for the dam would have to be addressed during design of the beltway bridge to ensure that the spillway flows would be conveyed downstream unimpeded. Both dam sites are in Phase 1 of the LPSNRD project and final design has already begun.

3.18.3 Proposed Mitigation

Floodplain Development. The proposed project will comply with or exceed the current minimum requirements of the City/County floodplain management ordinance. There are no additional mitigation measures above what is already required by the City/County program.

Channel Realignment. According to the US Army Corps of Engineers requirements, if channel straightening is proposed for any of the crossings, it will be necessary to demonstrate that there are no other practicable alternatives.

SM-4 Alternative. It is anticipated that all major crossings in this alternative will allow channels to remain in their existing configurations.

EC-1 Alternative. The Stevens Creek tributary crossing at Adams Street may require minor channel realignment at an existing meander due to encroachment of the channel by the roadway embankment.

The Salt Creek tributary crossing north of Fletcher Avenue cuts across an existing channel meander. There are several options to accommodate the road crossing. The channel can be realigned to provide a relatively straight section of channel at the bridge location, and meanders upstream and downstream of the crossing can be reconfigured to enable the total channel length to remain the same as before the realignment. Other options would be to straighten the channel (with the effect of shortening the channel approximately 210 m (700 ft), or extending the bridge approximately 98 m (320 ft) to span the meander.

EM-1 Alternative. The Stevens Creek tributary crossing south of Pine Lake Road cuts across two existing channel meanders. Again, there are several options. The channel can be aligned to provide a relatively straight section of channel at the bridge location, and the meanders upstream and downstream can be reconfigured to enable the total channel length to remain the same as before the realignment. Other options would be to straighten the channel (with the effect of shortening the channel approximately 73 m (240 ft), or extending the bridge approximately 38 m (125 ft) to span the meanders.

The Stevens Creek tributary crossing between Van Dorn Street and Pioneers Boulevard may require minor channel realignment at two existing channel meanders due to encroachment of the channel by the roadway embankment.

It is not anticipated that other major crossings along EM-1 will require channel reconfiguration.

EF-1 Alternative. The Stevens Creek tributary crossing between Old Cheney Road and Pioneers Boulevard cuts across two existing channel meanders. The channel can be aligned to provide a relatively straight section of channel at the bridge location, and the meanders upstream and downstream can be reconfigured to enable the total channel length to remain the same as before the realignment. Other options include straightening the channel (with the effect of shortening the channel approximately 177 m (580 ft), or extending the bridge approximately 94 m (310 ft) to span the meanders.

The Stevens Creek tributary crossing south of Van Dorn Street cuts across an existing channel meander. In this location, the channel can be aligned to provide a relatively straight section of channel at the bridge location, and the meanders upstream and downstream can be reconfigured to enable the total channel length to remain the same as before the realignment. Other options include straightening the channel (with the effect of shortening the channel approximately 111 m (365 ft), or extending the bridge approximately 61 m (200 ft) to span the meanders.

It is not anticipated that other major crossings along EF-1 will require channel reconfiguration.

Stevens Creek Farm Ponds. All of the east beltway alternatives will require continued coordination with LPSNRD, as well as modifications to the roadway design to reduce conflicts with the proposed farm ponds. EC-1 will require (1) moving the Sky Ranch dam site upstream, (2) installing an additional culvert or bridge under the beltway capable of conveying the flow from the Sky Ranch emergency spillway, (3) moving the A3-2 dam site upstream, and (4) evaluation and possible replacement of the culvert under 98^th Street near the A3-2 site. In addition, the beltway could obstruct the emergency spillway of the dam--requiring an additional culvert or bridge under the beltway capable of conveying the flow from the emergency spillway. The resulting loss of flood control capacity from moving the dams upstream has not been estimated at this time; therefore, mitigation options have not been identified. EM-1 will require (1) the A2-1 spillway to be relocated on the east side of the dam, and (2) adequate protection of the roadway embankment at the same site. EF-1 will require (1) adequate protection of the roadway embankment at the A9-1 site, and (2) accommodation of the emergency spillway flows at the A-17 site.

Stevens Creek Conservation Easements and Trail. For EC-1 and EM-1, proposed development in the Stevens Creek mainstem floodplain between the Murdock Trail and the MoPac Trail would be required to comply with the terms of the conservation easements to be acquired by LPSNRD relative to the Stevens Creek watershed plan; however, these terms will not prohibit construction of a beltway facility.

3.18.4 No Build Alternative

The no build alternative would avoid any direct alteration to the existing floodplain conditions other than that required for construction of the planned roadway network improvements. Even without a beltway project, modifications to the existing floodplains are likely to continue as agricultural land is converted to suburban and urban uses for paved County roads, housing and subdivision developments, golf courses, and commercial and industrial sites. The no build alternative would avoid any conflicts with the Stevens Creek watershed project.

3.18.5 Compliance with Executive Order 11988, Floodplain Management

To meet the project purpose, the south beltway must cross the Salt Creek floodplain because it is impossible to connect US 77 to N-2 without crossing Salt Creek within the defined 200 km² (80 mi²) study area. Based on the defined east study area, only alternatives located east of 134^th Street would be able to avoid any impacts to the Stevens Creek floodplain. Of the three east beltway alternatives, only EF-1 meets this criteria.

Through the Lancaster County participation in the National Flood Insurance Program, it is A local requirement that floodplain development permits be obtained from the City/County. This requires that any developments within designated floodways must include certification from a registered professional engineer that the proposed development will result in no increase along the floodway water surface profiles. The City/County are both sponsors of the project and are aware of floodplain management requirements.