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re: Back flooding and built up neighborhoods
Posted on 1/21/20 at 9:47 am to CarRamrod
Posted on 1/21/20 at 9:47 am to CarRamrod
quote:
It is about the release of water. that graph someone posted earlier, if you can understand it, kinda explains it. As it rains all water is routed to these catch basins. then usually there is a weir of some sort that releases the water at a certain volume based on how much water is in the basin. so these basins rise and fall and hold the runoff so it doesnt all go down the outfall at once.
This does the same thing as what the neighborhood designs of the 70s,80s and 90s did. Except their catch basins are big culverts under the roads so you cant see them.
Here ya go...
0
Posted on 1/21/20 at 9:48 am to CarRamrod
Walmart did a large off-site mitigation pit along Bayou Fountain and also has some built in storage in the parking lot bioswales and a small detention area along the back of the property.
Posted on 1/21/20 at 9:50 am to CarRamrod
quote:
i dont really remember but they piping under the parking lot could be sufficient in holding the runoff.
Maybe so. Maybe the parking lot is a big reservoir? I don’t know.
Posted on 1/21/20 at 9:51 am to BottomlandBrew
is it that pit between the condo place and The Lake neighborhood?
Posted on 1/21/20 at 9:51 am to Bedhog
For Dr. Wilson's senior project we actually did research on this topic in accordance with Ascension Parish.
Per parish ordinances, developers are required to build 1' above the base flood elevation that is set by fema. They are also required to have proper mitigation, most often times detention ponds, to ensure there is no runoff from the property. However, they jsut have to calculate the mitigation for a 10 or 20 year storm. The 2016 was a 50 year storm. So then there lies water displacement caused by the added fill. When calculated against the floodplain there is practically zero effect.
The next problem is diverting the flow, this is wehre it gets tricky to analyze. We only had 3 months of class and limited programming capabiulites and unless some serious funds and time were developed to this, even the most knowledgable of civil engineers can only make assumptions. I'm pretty sure HNTB was conducting a master study on this, but im sure those results we will never see.
Per parish ordinances, developers are required to build 1' above the base flood elevation that is set by fema. They are also required to have proper mitigation, most often times detention ponds, to ensure there is no runoff from the property. However, they jsut have to calculate the mitigation for a 10 or 20 year storm. The 2016 was a 50 year storm. So then there lies water displacement caused by the added fill. When calculated against the floodplain there is practically zero effect.
The next problem is diverting the flow, this is wehre it gets tricky to analyze. We only had 3 months of class and limited programming capabiulites and unless some serious funds and time were developed to this, even the most knowledgable of civil engineers can only make assumptions. I'm pretty sure HNTB was conducting a master study on this, but im sure those results we will never see.
This post was edited on 1/21/20 at 9:57 am
Posted on 1/21/20 at 9:52 am to statman34
quote:
There is actual proof of houses never flooding in their 40 years of existence and all of a sudden flooding when a higher development goes up.
For example?
Posted on 1/21/20 at 9:53 am to CarRamrod
I believe so. It's hard to remember which pit it was over there as there were a few and it's been more than a decade since I worked on the site plans.
Posted on 1/21/20 at 9:55 am to BottomlandBrew
quote:Do you remember if they drove pilings into the ground for foundation support like they did for the sherwin williams and the racetrac?
BottomlandBrew
Posted on 1/21/20 at 9:56 am to Bedhog
I can't speak to that. I'm just a dumb landscape architect and don't know shite about foundations.
Posted on 1/21/20 at 9:57 am to BottomlandBrew
quote:
Walmart did a large off-site mitigation pit along Bayou Fountain and also has some built in storage in the parking lot bioswales and a small detention area along the back of the property.
Thxs for sharing that.
Posted on 1/21/20 at 9:57 am to Bedhog
quote:
Do you remember if they drove pilings into the ground for foundation support like they did for the sherwin williams and the racetrac?
They didn’t.
Posted on 1/21/20 at 9:59 am to dirtytigers
quote:
Per parish ordinances, developers are required to build 1' above the base flood elevation that is set by fema. They are also required to have proper mitigation, most often times detention ponds, to ensure no water is added to the floodplain. So then there lies water displacement caused by the added fill. When calculated against the floodplain there is practically zero effect.
This, exactly, is the answer.
quote:
The next problem is diverting the flow, this is wehre it gets tricky to analyze. We only had 3 months of class and limited programming capabiulites and unless some serious funds and time were developed to this, even the most knowledgable of civil engineers can only make assumptions. I'm pretty sure HNTB was conducting a master study on this, but im sure those results we will never see.
There's different software you can use, notably HEC-RAS and HEC-HMS to model this, though you need a lot of data to get good results.
Point being unless you went to school to study this stuff you have no idea how they arrived at their outcomes or why they did any of this (detention ponds, fill, etc). Its easy for the lay person to see flooding in any particular year and assume the engineers are full of BS. This thread is a great example of that.
This post was edited on 1/21/20 at 10:03 am
Posted on 1/21/20 at 10:00 am to doubleb
quote:
For example?
I cited houses on Confederate and Petersberg in Shenandoah, June 2019. There were at least 4 possibly more that had water in their houses. Two of the homeowners had been there over 30 years and had never had any water close to their house before, even in 2016. The Rose Garden subdivision had gone up since then directly behind Confederate.
Posted on 1/21/20 at 10:01 am to statman34
quote:
I cited houses on Confederate and Petersberg in Shenandoah, June 2019. There were at least 4 possibly more that had water in their houses. Two of the homeowners had been there over 30 years and had never had any water close to their house before, even in 2016. The Rose Garden subdivision had gone up since then directly behind Confederate.
Was it flash flooding?
Or was it backwater flooding?
This post was edited on 1/21/20 at 10:03 am
Posted on 1/21/20 at 10:02 am to CarRamrod
quote:
So it is the politicians or the new developments? because you seem to be shifting blame here.
Developers do what politicians allow them to do. Nothing more. They are not going to go out of their way when they don't have to. The metro council is supposed to protect our interests so developers have to do the right thing.
Posted on 1/21/20 at 10:02 am to statman34
quote:
Two of the homeowners had been there over 30 years and had never had any water close to their house before
Have they ever had this rainfall event in their 30 years there?
Posted on 1/21/20 at 10:04 am to Ignignot
quote:
our neighborhood was bout 1.5 feet above the floodline and EVERYTHING around us flooded, i'm sure those homeowners probably hated keystone after that
I don't buy that b/c the developers don't 'BRING' in dirt to build up. Every development in the last 20 yrs has been required to dig out the dirt in the same area. They have formulas for it to not change water displacement. Keystone has ELEVEN ponds in that development.
I know this isn't perfect and is a big area of debate in Ascension Parish. But it's not like the developers are simply bringing in dirt and building houses higher up and leaving everyone else to flood.
This post was edited on 1/21/20 at 10:06 am
Posted on 1/21/20 at 10:06 am to doubleb
quote:
Was it flash flooding?
Or was it backwater flooding?
I am not sure about that. General Forrest had issues with flash flooding. I would assume it was flash flooding in this situation because the water had no where to go but I never heard what the city officials officially said the cause was.
Posted on 1/21/20 at 10:10 am to statman34
quote:
I am not sure about that. General Forrest had issues with flash flooding. I would assume it was flash flooding in this situation because the water had no where to go but I never heard what the city officials officially said the cause was.
If the torrential rains could drain off the lots streets fast enough it could easily be caused by the adjacent development. The newer development could have blocked the natural drainage flow or overtaxed the storm sewers.
If it was backwater flooding then it probably wasn’t due to the new development.
Posted on 1/21/20 at 10:12 am to Bedhog
The areas are not brining in extra dirt. They are digging ponds to create the storage in the flood zone, and raising the land with this dirt. No subdivision in EBR, ascension, Livingston, tangipahoa, etc.. is allowed to take away volume from the flood zone. The dirt is coming from the ponds. Homes are required to be 1’ above the 100 year flood zone. The problem for the homes that flooded in 2016 is a majority of them were built below the flood zone before there was a ordinance covering this. The 2016 flood is something that has between a 1:500 and 1:1000 chance of happening in a year. Theoretically it shouldn’t happen again in this area in anyone’s lifetime, although it technically is possible.
The issue for the older homes has nothing to do with the new subdivision but the fact they were not properly built above the flood zone. Many of these older homes were built before Baton Rouge had a storm water ordinance with some homes being 3-4’ below the flood zone. The older subdivisions were built without retention ponds also. The problem for flooding isn’t the newer subdivision that are above base flood and have detention ponds but the older subdivisions that don’t have ponds to hold back water and are in the flood zone already. The new subdivisions have no impact on the flood zone.
The issue for the older homes has nothing to do with the new subdivision but the fact they were not properly built above the flood zone. Many of these older homes were built before Baton Rouge had a storm water ordinance with some homes being 3-4’ below the flood zone. The older subdivisions were built without retention ponds also. The problem for flooding isn’t the newer subdivision that are above base flood and have detention ponds but the older subdivisions that don’t have ponds to hold back water and are in the flood zone already. The new subdivisions have no impact on the flood zone.
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