| August has already been a very very wet month and reports that just 2" more will tip us over the edge. The ground just can't absorb any more. A slow moving hurricane makes the rainfall flooding threat that much worse. And this will be a long event.
As a drainage engineer, I should explain how we design things here in New Jersey. When we design storage devices to hold water before sending it to the municipal drain pipes, we design for the 100 year event that Governor Christie mentioned. The amount of rainfall in a 100 year event is 8 inches of rainfall. We design in case it all happens in the space of 10 minutes - a worst case. HOWEVER, nearly all the drainage pipes found throughout NJ are designed to carry only the 25 year storm capacity. That is the best case - when the pipes are new and there is no sediment sitting in them and choking off the flow. Meaning, our infrastructure WILL be overwhelmed by this event if the amount of rainfall exceeds roughly 6.7 inches of rain, which a slow moving hurricane can easily do. NOAA reports we may expect 6 to 10 inches, with isolated rainfall up to 15 inches. We are guaranteed trouble from Irene. We have already seen rainfalls greater than this just this year - which has caused widespread flooding in the Pascack Valley and elsewhere.
To make matters worse, the way we improve water quality, in impoundment ponds, is try to maximize the length of time the water sits in the pond so that 24 hours after a storm at least 10% of the stormwater volume is still in the pond. This actually reduces the storage available should we have an extremely large event of long duration - like in this case. It is almost certain we will exceed the 100 year event and most detention ponds and underground storage chambers may be easily overtopped.
A few months ago when we experienced devastating floods in Paterson and Wayne, I wrote about the major reasons why we flood. http://www.bluejersey.com/diar... It was technical and a bit boring, but folks may be more willing to read it now. Our preparation for flooding events shouldn't just happen days before a major hurricane. It shouldn't be a case of the average citizen after the fact demanding that elected officials just "do something!" While we did not cause the hurricane (well we maybe did due to human-induced climate change) we are all responsible in our small ways as citizens for increased runoff and the only real solution will take the efforts of everyone working together. When we pave over our backyards without a thought, when we widen our already humongous driveways, when we put concrete patios absolutely everywhere, when we direct the downspout on our house directly to the driveway that leads to the street, when we connect our sump pump directly to the catch basin, WE are the problem. Whenever we cut down just one tree, we add to runoff. When we clear our yards to build a bigger lawn we add to the runoff.
In the Pascack Valley right now a battle is raging between the residents of the Pascack Valley and the water company. The residents believe that the flooding where they live (which by the way is in a designated flood plain and has been for years) is all caused by the action or the inaction of the water company, when the real reason for the increase in runoff is the thousands of residential homes and commercial parking lots - many of them new, that each contribute more runoff to the whole system than they ever did - without a second thought. Every tree that is cut, every forested acre replaced with a McMansion with a tennis court and a palatial entrance with an airplane runway-sized driveway and entrance courtyard, contributes to the problem.
We cannot solve the problem by demanding the water company empty the reservoir before every rainfall. We ALL need to take responsibility. That includes finding ways of returning the runoff profile to what it was before we built up NJ. It is possible with new techniques called LID or Low Impact Development. By mimicking Mother Nature, we can reduce the amount of runoff BEFORE it ever gets to the stream or creek or river and it becomes the water company's responsibility. The NJDEP has already come up with help for designing LID methods: http://www.nj.gov/dep/stormwat...
LID is less than 20 years old, but the NJDEP has been working with private industry to come up with ways to reduce runoff. The problem is that most elected officials in the state have never heard of these techniques and lack the political will to really encourage their use - because much of the burden of using these new methods lies with the residents and property owners (read - the taxpayers and the voters) in each town. The answer we are finding is not the huge Hoover dam-like solution that many folks have seen in the past and are calling for because it is all they know, but a team effort by all involved stakeholders - ESPECIALLY home owners. We cannot dam or pipe our way out of this problem. The science is pointing to a more DE-centralized approach. We want to stop the rivulets from each property before they combine into a raging river. At the same time, we will recharge our groundwater, which is more effective in a decentralized approach. If we do it right, it's a win-win.
Here is the NJDEP's excellent webpage that we drainage engineers use to design systems in NJ. Everything you ever wanted to know about stormwater but were afraid to ask. http://www.nj.gov/dep/stormwater/
Good luck tonite folks - and stay safe. This is going to be tough, as we lose trees they may make stream flooding even worse as they block watercourses. Get to high ground, safe indoors, and think about what I've said here as you wait it out by candlelight.... |
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