Quote:
Originally Posted by anifani821
Steve - I have heard this - that the 100 year rain amounts were being reported . . . and I was wondering . . . does this mean that we truly have experienced something extraordinary . . . or does it mean our waterways and streams are overburdened to a much higher degree than would be estimated for a 25 year rain level? IN other words, do we need to recalculate stress on existing systems and set the criteria differently - b/c changes in topography and construction (more concrete, less friable soil) have affected absorption rates and drainage paths?
I don't know if I am using the right terms or explaining this correctly, but I guess what I am trying to convey is . . . could our estimates for containment be off? We need more drainage fields/culverts . . . ??? Does any of that make sense, LOL???
|
When you design a storm drainage system, you decide on a "design storm" which is typically dictated by local codes. This design storm (e.g. the 10-, 25-, 50-, or 100-year storm) provides an engineering best guesstimate based on historical rainfall amounts for how much rain will fall out of the sky. Period. It's typically reported in an inches per hour unit (like 9.3 in/hr). This data is mixed together with soil type (which dictates permeability) and the average imperviousness of a drainage basin (think of a desert full of dunes, and the "bowls" formed between the dunes are basins--in our context, each basin is relieved of it's storm water either by a pipe system or a channel). This provides an estimated amount of runoff that needs to be handled by storm pipes or a stream/creek/channel.
Long story short, the rainfall estimates used to size systems were not wrong. If anything, the estimates get refined and/or reassured now that we have another significant data point in our rainfall history.
What you're referring to with ditches and channels that spilled out of their banks during our storms has to do with a couple of issues, none of which have to do with the predicted amount of rainfall.
1. The ditches/streams/creeks become undersized when we over-build and provide no means of retention/detention to subdue the amount of runoff we're adding. This is a cumulative effect over years and years of neglecting this issue. Mainly, because you can't see it immediately, and you see it what, maybe once every 5-10 years when we get a big rain and have some flooding? This makes people bump the issue way down on the priority list.
2. As I told you in another post, a healthy stream has a connection to its floodplain. These floodplains are almost always swallowed up and mistakenly deemed as "buildable land" in suburban and urban regions like Charlotte. A stream wants to spill out of its banks when it expereinces high flows. This helps dissipate the energy of the water rushing through it (which avoids damage to the stream banks and features like rocks and trees). Take away this floodplain and the stream doesn't know it...
It is very expensive to design, aquire real estate and/or easements, and reconstruct streams (as well as pipe systems). It's somewhere on the order of $350-$500+ per foot of stream to resotre. There is no average stream length, but you could figure the big arteries in the region are on the order of miles, not feet. So not only is it very difficult finaincially, you also have to deal with folks who don't want to give back that floodplain or just don't want construction on their property period!
The reason other large cities, as some posters have referenced, have massive underground channeling systems is that those cities must have those systems in order to survive...meaning the entire city would drown without it. Flooding is flooding and its bad no matter how you slice it, but Charlotte's flooding is relatively localized and not the problem of everyone...therefore, everyone is not going to want to invest hundreds of millions (which is what it would take to build these grandiose systems that people mention in their posts) and the many years to design, construct, and maintain these systems.
This leaves us with no other option but to be reactive and fix problems as they appear and can be worked into the schedule of fixes.
Please register to participate in our discussions with 2 million other members - it's free and quick! Some forums can only be seen by registered members. After you create your account, you'll be able to customize options and access all our 15,000 new posts/day with fewer ads.
Mecklenburg Flood Zone Interactive Mapping
