WAVERLY FLOODPLAIN BACKGROUND INFORMATION

Transcription

1 WAVERLY FLOODPLAIN BACKGROUND INFORMATION Who designates the floodplains in Waverly? The Federal Emergency Management Agency (FEMA) is responsible for delineating floodplains. The floodplain FEMA uses for regulatory purposes is based on the 100-year flood event. This is the event that has a one in one hundred, or one percent, chance of occurring every year.. When was the last time the floodplain has been changed? In 2008 Waverly was notified that the floodplain would be remapped which would result in many neighborhoods that were not previously identified as being in the floodplain being designated in the new 100 year floodplain. Actual changes to the FIRM map were made in both 2011 and Why is it in Waverly s best interest to control flooding and reduce the 100 year floodplain? Floodwater will damage our homes and businesses, interrupt transportation and emergency services, damage roads, bridges & utilities, cause injuries and/or loss of life, temporarily close businesses, and in many cases require home and business owner to buy costly flood insurance. Property values and economic development potential can also be adversely impacted if homes and businesses are located in the regulatory floodplain. What areas in Waverly are most susceptible to flooding? The areas in Waverly susceptible to flooding are along Salt Creek both northwest and north of Waverly and along Ash Hollow on the west side of town. Street flooding can occur at about any location if intense heavy rain occurs, although those areas are not necessarily in the FEMA-designated 100 year floodplain. What can be done to reduce flooding and the area impacted by the 100 year flood? The City and NRD hired HWS/Benesch in 2009 to complete a watershed master plan for the City. The plan identified many projects that can reduce flooding. The plan recommended building a dam south of town on Ash Hollow to reduce the flooding and the 100 year floodplain.

2 Dam Frequently Asked Questions November 21, Why do we need a dam? The 2010 Watershed Master Plan identified a dry dam along Ash Hollow to be the best flood reduction alternative to reduce the peak flow rates along Ash Hollow. 2. How often does FEMA update the Waverly floodplain maps? There were almost 10 years between the last two publications of floodplain maps. The previous regulatory Flood Insurance Rate Map (FIRM) was published in September 2001 with the most recent updates being completed in February 2011 and April How many properties will the dam take out of the floodplain and floodway? Based on the best available information, there will be 94 structures that will be taken out of the floodplain. For most structures, the entire property will also be removed from the floodplain. For some structures, portions of the properties will remain in the floodplain. 4. Where would it be built? The proposed dry dam would be constructed along Ash Hollow Creek, south of Interstate 80, just upstream of McKelvie Road. 5. What is the timeline for it being built? The timeline will vary depending on funding partners, permitting, and land acquisition. Below is a tentative timeline, with the assumption that the process were to start in January 2015: Funding Requests, Final Design & Permitting Land Acquisition 2017 Construction 6 Month Duration 6. How much will the dam cost? The Engineers Opinion of Probable Development Costs identified the project costs around $2,785,000 in 2012 dollars. 7. How will the City pay for it? With revenue collected from the city sales tax.

3 8. How much does flood insurance cost? It varies depending on whether the property is residential or non-residential, or a preferred risk policy. The only way to get a correct rating for an individual property is to talk to a qualified insurance agent. 9. Will the City purchase the land or acquire easements? There will be approximately 30 acres of land for the dam embankment and spillway and another 111 acre flood pool area that easements will need to be acquired. 10. How many acres are needed for the dam? Approximately 30 acres for the dam embankment and spillway. 11. How many of the acres will be farmed? All of the 111 acres within the flood pool can be farmed. The 30 acres of the dam embankment and spillway would not be farmed. 12. Are there other options instead of the dam? There were two options identified in the 2010 Watershed Master Plan which included the reduction of peak flows with the construction of a dam or increasing the channel capacity. To increase channel capacity would require widening of the Ash Hollow channel along with various bridges throughout town. The overall cost opinion to increase channel capacity was approximately 2 to 3 times that of the flood reduction project. 13. How were these homes able to be built in the floodplain? The homes that are in the current floodplain were constructed prior to the floodplain remapping. So they were not in the regulatory floodplain at the time of construction. 14. Do the landowners want to sell the land and if not what happens? The City will work with the landowners 15. Who owns the land? The proposed dam site would be on four different parcels. The parcel numbers and property owners are: (Douglas W. Althouse), (Douglas W. Althouse), (Darlene Althouse), and (Clarence A. Althouse Jr. Trustee).

4 16. Could it ever be a wet dam and/or a permanent pool? Per the request of the current land owner, the structure will not be designed to easily accommodate a future wet dam. Why wouldn t you design and construct a wet dam? Per the request of the land owner, the dam will be designed as a dry dam only. 17. Who will maintain the dam and what are the expected costs? The dam will be maintained by the City or another entity through an inter-local agreement. Based on the Nebraska Resource Development Fund Guidelines, it was assumed that the Operation Maintenance and Rehabilitation costs would be 0.75% of Construction Costs or approximate $11,000 annually. 18. Will the land owner(s) be paid for crop loss after a flood? Crop loss due to flooding is taken into account as part of the appraisal process. Land owners would be compensated for any potential reduction in use of their land at the time of land acquisition. How many acres would be under water during the 100 yr. flood? The 100-year flood pool is approximately 117 acres (111 acre easement area as well as 6 acres along the dam embankment). 19. Were other locations for a dam considered? There were a total of four locations looked at for the centerline of the dam with the optimum location selected based on environmental impact, cost of structure, permanent depth of water within the channel banks, flood pool elevation in relation to county roads, and ability to meet flood reduction goals. 20. How long will land/crops be under water during the 100 yr. flood? For a 100-year 24 hour storm event, the dry dam facility would reach peak storage in approximately 24 hours and then draw down over approximately 67 hours. 21. Who will own the DAM? This hasn t been decided yet. It will be determined as part of the Responsibilities package that will be included in the Interlocal Agreement. 22. Was the TR-55 methodology used to determine peak flows, in accordance with FEMA standards and practices? In this case, TR-55 methodology was used in the modeling performed for the watershed for both existing and proposed conditions. It is important to note, TR-55 is only one of dozens of acceptable methodologies for hydrology submittals to FEMA. TR-55 is suitable for this analysis because conservation of volume is achieved with TR-55.

5 When you are routing flow hydrographs through reservoirs conservation of volume is important. 23. Is there a gauge on the watershed? As with the vast majority of watersheds we study, there is no gauge on the watershed. If a gauge was available, and the gauge records extended over a reasonable time period, say 20 years or so, we would prefer to use a gauge. Since we don t have gauge records, we use the TR-55 methodology to compute peak flows for the watershed. We can compare our results to similar watersheds in the region that do have a gauge. 24. Was the dam on watershed time of concentration taken into account? Yes. Detailed modeling was performed to determine the impact of the dam on watershed runoff, including an analysis of the changes in peak flows and timing of runoff. This information is provided in the Preliminary Design Summary Report. Detailed HEC-HMS models were developed to perform the analyses. 25. Was the Stevens Creek overflow accounted for when OA computed runoff for the Ash Hollow watershed? Using the FEMA maps and information, there is no evidence that Steven s Creek overflows would be a factor in determining Ash Hollow runoff. Steven s Creek flows into Salt Creek near 84 th and Cornhusker. The Steven s Creek overflow overtops Cornhusker highway just west of 98 th Street and flows north. There, Steven s Creek overflows either flow overland to the northwest, into Salt Creek, or flow overland to the northeast and overtop I-80 in the sag located west of the Waverly interchange (elevation ~1126 ft, where 104 th Street would intersect the Interstate if it extended that far north). In several locations, including the Novartis plant on the south side of Cornhusker between 98 th and 112 th, high ground prevents flows from continuing northeast towards the Ash Hollow watershed. The high point of Cornhusker Highway near Novartis is at elevation ~1134, which is ~6 ft higher than the road at the overflow crossing to the west. The flood elevation on the north side of Cornhusker and southwest of I-80 is 1126 ft. As you travel northeast from the interchange, the high point of Cornhusker, nearly the auction house is ~1129 ft. Steven s Creek overflows will rejoin the Salt Creek floodplain before reaching the Ash Hollow watershed. 26. What caused the flows to increase so much from 1981 to 2008? When FEMA restudied the watershed in 2008, they computed flows that were 2-3 times higher than the original study in This is not because of any changes to the watershed upstream of Waverly. This is entirely because the original flows FEMA computed were considerably lower than they should have been. The watershed has

6 now been restudied by the FEMA contractor in 2008, Benesch shortly after that, and now Olsson. I believe the results of the three analyses are consistent (though FEMA reported the wrong values in the FIS) and the flows used for the watershed today are accurate and reasonable. 27. Did the September 30 th and October 1 st rain approach a 2 year level of peak intensity (inches per hour of rain)? The rain gauge records for the region clearly indicate that the rainfall event of September 30 th and October 1 st did not even approach a 2-year level of peak intensity (inches per hour of rain). The peak intensity recorded for the Havelock gauge (nearest hourly gauge I ve found so far) is less than 1 inch per hour. The peak intensity in a 100- year design storm is 3.5 inches per hour. We received a lot of rain; but, it was spread out over many hours. We were very fortunate. A shorter, much more intense rainfall is possible and could cause significant flooding throughout the Ash Hollow watershed. The 100-year design storm is a 24-hour storm; but, it is not uniform over the 24-hours. The peak hour has 3.5 inches of rainfall. The peak two hour period has 4.4 inches and the peak three hour period has almost 5 inches. Approximately 70 percent of the total rainfall in a 100-year design event falls within three hours. The rainfall during the peak three hours for the recent storm was less than 2.5 inches, which is between a 2- and a 5-year event. I provided copies of the gauge information last night. 28. How do we know if FEMA will agree if the proposed dam will fix the problem and take all these structures out of the floodplain and if so, can we get it in writing? Several people noted that we should get it in writing from FEMA that this will fix the problem before we do any project. We did get it in writing. We have an approved CLOMR from FEMA (Case No R) that says in writing that the floodplain maps can be changed as we showed them on our proposed conditions map, if we build the dam according to the concept design plans. They also asked what would stop FEMA from coming in and changing the floodplain back again. We have all the detailed models and information necessary to prevent FEMA from doing this. Appealing any future map changes from FEMA is something we can and will do. The CLOMR we obtained from FEMA is based on a more conservative analysis than the original FEMA flood maps and future expansion of the floodplain due to restudy is highly unlikely. 29. If we build detention cells along Ash Hollow would we need a dam? Other locations along Ash Hollow were investigated as locations to place a small detention cell but due to the flat nature of the Ash Hollow watershed north of I-80, there is not enough area to construct a single detention cell or a series of detention cells that would provide a significant reduction in peak flows.

7 30. Does the change from a wet pool design to a dry dam reduce the anticipated construction costs and if so by how much? The cost should be decreased slightly by going from a permanent pool to a dry dam. The permanent pool area will not have to be graded and shaped now since there won t be a permanent pool area. The excavation/embankment would be reduced by approximately 3,000 cubic yards. Also, the principal spillway is now just a straight tube and it will no longer need a multi-stage riser structure on the inlet. Thus the 30 outlet pipe would be approximately 20 shorter. The projected cost savings is approximately $15,000. As for the change from a permanent pool to a dry dam, a new CLOMR would only be required if the change resulted in a significant difference in downstream flows for the 100 year or 500 year event. FEMA only cares about the peak flows and that the dam is designed to meet standards. The flows from the dam (permanent pool or not) are limited to what the 30 principal spillway pipe can discharge. The discharges do not change significantly going from a wet dam to a dry dam. The plans for the dam have changed, but it will not affect our future Letter of Map Revision (LOMR) application. 31. If the dam is constructed will all 94 structures be removed from the floodplain? If the dam is constructed, yes, based on the data we have, the 94 structures will be out of the floodplain (Salt Creek backwater and Ash Hollow floodplain). Based on the best data we have available, there are 94 structures that would be damaged if a 100 year event were to occur today. This project would eliminate those damages in the event of a 100 year storm. From what Olsson Associates has mapped, the Salt Creek backwater does not damage any structure. There are industrial properties without structures that benefit also. There is a large open area north of HWY 6 and west of Canongate that also benefits. There are so many ways to look at the data, but the bottom line is, based on the data available, none of the structures along Ash Hollow would be in a regulatory floodplain if the dam project were constructed. Properties and portions of the properties would be in the floodplain, just not structures. 32. What is a CLOMR? A Conditional Letter of Map Revision (CLOMR) is FEMA s comment on a proposed project that would, upon construction, affect the hydrologic or hydraulic characteristics of a flooding source and thus result in the modification of the existing regulatory floodway, the effective Base Flood Elevations (BFEs), or the Special Flood Hazard Area

8 (SFHA). The letter does not revise an effective NFIP map, it indicates whether the project, if built as proposed, would be recognized by FEMA.