developing in a floodplain

[Sparticus17]

I am looking for general feedback, links or documentation which will give me a better insight on the possible economical solutions to overcome certain problems with developing in a floodplain.

I am working on the preliminary design of 25.1 ha of land which 17.8 ha will be developed. The development will consist of mainly medium density townhouses and two high rise apartments (the 17.8ha development will be composed of 1100 units). This development is fronting on rivers to the south and east. Currently the 25.1 ha of land is below the 1:100 year flood level.

Fill from offsite will be hauled in at $12/m3 which will be used to raise the development area (17.8 ha) at on average of 2 meters (the apartments will all be built on piles since clay fill will not be compacted). The excess land beyond the 17.8ha will be used an environmental reserve (park) where a stormwater pond will be present.

Due to the cost of fill, cost saving measures through innovative engineering will be required. The main floor of the apartments must be above the 1:100 year flood level. We have already stated to the architects that a walkup to the front door of the townhouses and apartments would save on fill.

They have now taken our ideas further and proposed underground parades which are below the 1:25 year flood level. The have used a diking approach where the surrounding local and minor collector roads are elevated and the lots recessed. This means overland major flow would be achieved on the surrounding roads but no overland flow would occur in the townhouse or apartment lots. There is no depression storage located in the onsite parking lots. The proposed minor system is currently designed to meet the standards of conveying a 1:5 year storm.

I am in the information gathering stage. If anyone has any experience with this type of situation or can point me to relevant documentation it would be very much appreciated.

Thank you

 

[queque]

I assume that you have been given tentative approval by the County Floodplain Administrator?

 

[RWF7437]

For the U.S. there is a great deal of useful information at

http://www.fema.gov

For other parts of the world perhaps others can direct you ?

good luck

 

[Sparticus17]

We do have approval to develop in the floodplain. The city has seen the conceptual subdivision plan and the engineering brief and approved it. The developer has changed the lot layout a few times since the city has last seen it but the building envelope area has not changed. This subdivision is in Alberta, Canada.

 

[queque]

It seems that a skecth would be helpful. Does this forum allow diagramming?

 

[flamby]

Uncompacted fill with not stand the flodding anyway so why do you want to fill it? I don't know but guess you can try putting some dikes/ embankments against flooding and keep the ground low?

Isn't this an option?


Ciao.

 

[Sparticus17]

The fill will be compacted to 98% SPD on all roadways. Underneath the apartments will not be compacted to 98% SPD instead the apartments will be built on piles. The 7.3 ha remaining environmental reserve area (25.1 ha total owned lot area - 17.8 ha developable area) will be undisturbed leaving the ground at existing low elevations. There is an existing dense mature vegetation currently present in this area. There will be no property located within the environmental reserve area so flooding is of no concern in this area.

The 1:100 year flood is at an elevation of 250.0m. Apartment occupancy level will be designed at 250.5m.
The minor and major collector roads surrounding the apartments will be at an elevation of 248.0m. The onsite surface parking for the apartments will be at 248.0m but the developer is proposing possibly lower at 247.0m. My opinion 247.0m that would be too many stairs to reach the first floor of the apartment at 250.5m. These apartments do not have basement suits due to the flooding concerns.

There has been diking studies already completed in the area which explored: cost, design, and risk of flooding, potential property damage, and etc. It was decided based on the study that dikes were too expensive to build along the river. The flood plain is approximately 10.5 miles long and 0.9 miles wide on each side of the river centreline.

Any comments are very much appreciated.
Thanks

 

[flamby]

Your fill requirements are huge and costly. Try to fill as liitle area as poosible. Also try to see whether water tanks etc. can be located underground. Check if there there is possibility of creating a lake in the landscape, This will provide you cheap fill material from within your area.

And lakes are impossibly lovely...

Ciao.