2 Buildings Below Street Level and Water Table 3

[cap4000]

I am thinking about raising a 3/4 acre existing parking lot up about 3 feet. This would require all the entrances to have steps down into the 2 buildings that were built on piles around 50 fifty years ago. The entire 2 acre property and roadways flood around 30 inches deep in heavy rainstorms. Would a large perforated ADS pipe detention system under the parking lot be feasible to store some of the water and then discharge it to daylight as the tide lowers. Any tips on how to resolve this matter would be appreciated.

 

[RWF7437]

Why raise the parking lot ? Isn't it the buildings that need to be protected from flood damage ? Are you solving the right problem here ?

I don't know about costs but money spent solving the wrong problem is money washed away ( pun intended ).

good luck

 

[cap4000]

The Elevations are as follows. Main Street El. 9.0, Parking Lot and Building Slab El. 7.0, 100 year High Water Mark El.9.50. As far as raising the interior building slabs it would be a tennant-landlord nightmare/lawsuit to do so.The raised parking lot idea would keep the water out from coming in off the street level which floods from the Hackensack River.

 

[RWF7437]

Thus raising even more questions:

"The Elevations are as follows. Main Street El. 9.0, Parking Lot and Building Slab El. 7.0, 100 year High Water Mark El.9.50."

Are these elevations above mean sea level ?
By "high water mark do you mean the Base Flood Elevation (BFE)as established by a detailed flood study of the Hackensack River adopted by FEMA?
Why does the subject of your original post mention the "water table" ?
Are you aware that the "100 year" flood level has nothing to do with 100 years ?
Are you aware that the "100 Year flood level" is usually considered only accurate to within plus or minus 1 foot ?
Are you certain that flood waters cannot inundate your buildings via some other indirect route even if you raise the parking lot ?
Is flooding caused by high flows in the river, high tides, or some combination of both ?
During flood conditions, what is the velocity of flow near your site?
If your buildings are not residential, have you considered flood proofing them ?
Are they now, or can they be insured through the National Flood Insurance Program (NFIP)?
What local, City or County, tegulations apply to your property?

Answer these questions for yourself and your client. The answers may suggest possible alternatives to "raising" ( filling?) the parking lot.

Good luck

 

[cap4000]

Yes the elevations are related to sea level. Mean average tide at Sandy Hook is Zero here in NJ. Over the past 10 years the owner was hit with 2 - 100 year storms. About 8 inches of rain in one day. Then 2 more inches of rain the following day. I am still looking for alternatives to solve this problem thus this posting. Normal water table is about two foot lower than the slab.

 

[RWF7437]

http://msc.fema.gov/webapp/wcs/stor...eView?storeId=10001&catalogId=10001&langId=-1

 

[RWF7437]

http://hdsc.nws.noaa.gov/hdsc/pfds/index.html

 

[RWF7437]

" Mean average tide at Sandy Hook is Zero here in NJ."

Isn't the average tide always zero ? Isn't that the definition of Mean Sea Level ? Or are you using some other datum such as MLLW ?

 

[cap4000]

My general understanding is that there are various points in the oceans all around the world and thats where the Elevation Zero Datum is generated from. The mean tides are all averaged together. In NJ they use Atlantic City and Sandy Hook as bench mark locations.

 

[RWF7437]

In the United States there are numerous benchmarks ( physical survey monuments ) near the ocean shores on all coasts and across the country. These have been field surveyed and tied together to create a network of known elevations. Taken together, these monuments and their elevations are called the National Geodetic Vertical Datum as adjusted in 1929 (NGVD 1929). Their elevations are expressed in feet above Mean Sea Level at the time of the last adjustment. Mean Sea level was the average elevation of the tide gages used in setting the monuments, i.e. the midpoint between high and low tides over the period of record available at that time.

In New Jersey, I believe from a quick Google search, that the tide range is about 1.5 meters or about 5 feet.

I am not a licensed Surveyor and I live in Salem, Oregon far away from your project. I have worked with flood plains and various datums for 47 years, however. If you have questions about how these elevations are established any good Surveying textbook should answer them better than I can. Also, you could post them in the Surveying Forum on this website.

Back to your original question:
Please visit the websites provided in previous posts. FEMA has a wealth of very detailed information (FREE!) and the NOAA website will give you very good weather data.

Also, get the maps AND the Flood Study for the Hackensack River. Without it you cannot make any informed decision or even compile a list of practical alternatives.

Good luck

 

[cap4000]

Thanks for the surveying definition. For now I am listening to the owner of the two buildings who has gone many big time storms over the last 30 years. Some of the water marks are still on the buildings. This past April we had 10 inches of rain in only 36 hours. I will take a look at the FEMA sites this week. Nice Job.

 

[cvg]

based on what you have posted, it seems that the flooding issue is caused more by tidal action, river backwater and possibly storm surge than it is by rainfall runoff. The underground retention system may be able to store the rainfall that hits the parking lot but how will you keep ponding resulting from the river rising from going around your parking lot and inundating your buildings? You would have to build a ring levee completely around the buildings and then provide 3 feet of freeboard (or more) to meet FEMA standards.

 

[onyeuko]

Why don't you build a defence wall between the property and the offending adjacent side to the property (as to ward-off the overland flow or inflow/infiltation particularily from the street which floods from the Hackensack River.). Then calculate the resultant run-off for the enclosed area(the property and all other possible overland, infiltration-inflow to the area of concern), then design a detaintion tank which you can hide underneath the parking lot as to be release when the condition permits? The option of raising the 3/4 acre existing parking lot up about 3 feet, to me seems a costly approach that will not resolve the problem in the long run.
You can give it a go with SWMM5 or Stormnet. Both have great GUI and are easy to learn.
Good luck
Teddy

 

[RWF7437]

http://www.usace.army.mil/publications/eng-manuals/em1110-2-1607/toc.htm