The Manhattan Free Press On Line

Airport Expansion Plan Calls For Millions To Alter Area Drainage

The City of Manhattan has started the process of expanding the Manhattan Airport to the northeast.

They hit a snag when the FAA (Federal Aviation Administration) objected to the plan. (See Feb. 28th Free Press.) Here is what the FAA said: "The proposed runway extension shown on the draft AIP, is not needed for civilian use and, therefore, any costs associated with the proposed runway extension are not eligible for FAA participation."

The AIP is the Airport Improvement Program which is required to get FAA money.

Also in the Feb. 28th Free Press story is a problem FAA is having with two detention ponds constructed by the City.

Now FAA is having problems with a Runway Safety Area Anaysis because the two detention ponds were not shown on the drawing of the Airport (right).

City officials would not return phone calls over the past three days. Here is part of the List of Alternatives that were given to FAA by the City of Manhattan along with the cost of the project. Again the FAA said they will not pay for the changes:

LONG LIST OF ALTERNATIVES

The long list of alternatives includes five main alternatives with different mitigation options to reduce the cost of the alternative andior flooding. The long list of alternatives are to:

1. Enclose the charmel with a 600 foot box culvert perpendicular to RSA.

2. Relocate the channel 100 feet outside of RSA.

3. Relocate the channel around RSA of future runway extension.

4. Relocate the channel around future runway extension.

5. Relocate the channel to Eureka Lake.

The options to mitigate the floodwaters for these alternatives are:

1. Offline Detention.

2. Online Detention.

3. A Detention Basin on Fort Riley.

4. Reconstruction of channel downstream of the RSA.

Each option or combination of options to mitigate the floodwaters could be implemented for the enclosure alternative. A balance between costs, available land for storage, and maintenance will be made.

Alternative 1

The first alternative is to enclose the EVT and is shown in Exhibits 1 through 4 with the different mitigation options. The enclosure would be at the most logical location and alignment to minimize the length of the enclosure and be compatible with the existing alignment of the channel. Preliminary investigation identified that a four-foot high box culvert will be used to enclose the channel. The FAA grading requirements of the RSA restricts the height of the culvert. The preliminary hydraulic analysis showed that backwater from K-18, which is located east of the airport, controls the flow capacity of the enclosure. Therefore, a box culvert higher than four feet \vould not provide additional capacity under desiun conditions. To convey the 100-year flood event at least six 12' X 4' box culverts would be needed. This would not be cost effective and presents maintenance issues.

Mitigating the floodwaters reduces the flow to be conveyed, therefore reducing the size and cost of enclosing the channel. A balance between the cost of mitigating the floodwaters and the cost of the enclosure is made providing the optimum lowest cost for the alternative.

The first option to mitigate the floodwaters is to provide offline detention storage. A cross-sectional view of an offline detention basin is shown in Exhibit 1. Offline detention is basically a levee system keeping the flows within the channel until the capacity of the enclosure is met. As the flow increases the water will overtop the embankments into the storage area keeping the flow going through the enclosure constant. When the floodwaters begin to recede the storage area will drain through a system of pipes into the channel. These pipes will be equipped with check valves to ensure the water does not enter the storage area through the pipes.

An advantage with this option is that the flooding frequency of the storage area can be controlled, allowing the possibility of the land to be farmed. The storage area needed varies from approximately 60 acres with two 12' X 4' box culverts to 15 acres with five 12' X 4' box culverts. This method uses less storage area by storing only the peak of the flood event as shown in Figure 1. The cost for this option ranges from $2.2 to $3.2 million.

The second option to mitigate the floodwaters is to provide online detention. A cross-sectional view of an online detention basin is shown in Exhibit 2. Online detention allows the water to spread out over the entire basin as the water rises behind the enclosure. This requires more storage area since water is being stored throughout the flood event and not just during the peak of the event. Exhibit 2 shows that approximately 65 acres of storage area are needed with three 12' X 4' box culverts. This area will experience flooding during most events eliminating the agricultural use of the land. This option does not require the maintenance of the levees and piping systems. The cost for this option was estimated at $2.8 million.

The third option is to build a detention basin on Fort Riley. The Corps or Engineers(COE) and the Soil Conservation Service (SCS) previously investigated this option in the early 90’s. Exhibit 3 shows the approximate location of the dam and the estimated flooded area during maximum storage for a 25-foot high dam approximately 1550 feet long. This would provide enough storage based on preliminary analysis for the channel to be enclosed with two 12' X 4' box culvert. Coordination with Fort Riley will need to be made to determine the extent of floodable area and maximum time of inundation. The size of the dam will be balanced with the number of box culverts to minimize the total cost and maintenance issues. The conceptual design for this option was with an uncontrolled outlet structure. A controlled outlet structure could be installed to allow Fort Riley to increase the rate of release after storm events, to recover the flooded area quicker if so desired. With a controlled outlet structure an operation manual will need to be put together, detailing when the release of the water can be increased. The cost of this option was estimated at $1.3 million.

Providing detention on Fort Riley would greatly reduce the flooding along the EVT. The dam would also reduce the sediment load of the EVT, and increase the capacity of the channel. This option would minimize land acquisition needed for storage area.

The fourth option is to reconstruct the channel downstream of the RSA. This would improve the hydraulics of the creek so that one - 12' X 10' box culvert would be adequate to enclose the channel at the RSA. This option entails widening and deepening the channel from the RSA to K-iS. This option would reduce flooding along this reach of the EVT. Maintenance would be required to keep the channel from siitiiig in and brush growing up which would reduce the capacity of the channel. The cost of this option would be around $2.8 million.

Alternative 2

The second alternative relocates 1400 feet of channel approximately 100 feet outside the RSA as shown in Exhibit 5. The channel would be designed so that the proposed flood elevations would be at or lower than the existing flood elevations. Some storage would be required to compensate for the filling in of the floodway due to the grading of the RSA. This would either be supplied by regrading the floodway or by providing offline detention. This alternative would not alleviate any flooding problems. The channel would need to be maintained to its design capacity. This alternative would require enclosure of the creek and flood mitigation measures to be built for the future runway extension. This alternative was estimated at an initial cost of $700,000.

Alternative 3

The third alternative, shown in Exhibit 6, involves 3600 feet of new channel to locate the EVT around the RSA of the future runway extension. The channel would be designed so that the proposed flood elevations would be at or lower than the existing flood elevations. Some storage would be required to compensate for the filling in of the floodway due to the grading of the RSA. This would either be supplied by regrading the floodway or by providing offline detention. This alternative would not alleviate any flooding problems. The channel would need to be maintained to its design capacity. This alternative does not require enclosure of the EVT. The approximate cost for the construction of the channel is $3.8 million.

Alternative 4

The fourth alternative, shown in Exhibit 7, involves 2200 feet of new channel to route the EVT around the proposed runway extension at an initial cost of approximately one million dollars. The channel would be designed so that the proposed flood elevations would be at or lower than the existing flood elevations. Some storage would be required to compensate for the filling in of the floodway due to the grading of the RSA. This would either be supplied by regrading the floodway or by providing offline detention. This alternative would not alleviate any flooding problems. The channel would need to be maintained to its design capacity. When the runway is extended, the channel would be enclosed and flood mitigation built. The alignment would be such that the enclosure would not be under the runway, reducing the structural requirements of the enclosure.

Alternative 5

The fifth alternative, shown in Exhibit 8, diverts the EVT to the Eureka Valley Lake to the north. According to the COE reconnaissance report the entire valley drained to Eureka Valley Lake until the EVT was built. This would require the construction of a channel approximately 5300 feet long. Drainage structures that handle the Eureka Valley Lake discharge would need to be analyzed if their capacity needs to be increased. The channel would be designed so that the 100-year flood event would be within the channel. This would cost approximately $3.3 million.