Increased Floodplain Storage Volume
Increased Floodplain Storage Volume
(OP)
I am trying to convince the City Engineer that excavating areas within the floodplain to increase the storage volume of the floodplain is more beneficial for our project and for the upstream and downstream property owners than providing the typical detention pond. The increased runoff (100yr) created by developing this site is less than 2% of the existing flow rate of the stream running through the property. This small increase in the runoff rate does not increase the water surface elevations (as calculated by HEC-RAS) more than 0.10 feet at any cross section (before any excavation). With excavation the water surface elevations can be lowered to existing levels or even below existing levels. The City Engineer is not convinced this is the best method because it does not decrease (or even hold constant) the volume of water entering the stream. Any comments???





RE: Increased Floodplain Storage Volume
RE: Increased Floodplain Storage Volume
Really you should be providing detention for the smaller, more frequent storm events. A 100-year storm is going to be erosive to the stream channel regardless, but a 2-year storm event may not be currently, but if the development peak is not detained it could become so.
RE: Increased Floodplain Storage Volume
You need to use the Tc angle. If you analyze the entire watershed, you may be actually increasing the peak. Show the Eng that, then the liability is on him/her. In my duties as a designer, I have requested waivers using this approach, and recieved approval of it. In my duties as a Municipal reviewer, I approved waivers using this approach.
Engineering is the practice of the art of science - Steve
RE: Increased Floodplain Storage Volume
RE: Increased Floodplain Storage Volume
RE: Increased Floodplain Storage Volume
RE: Increased Floodplain Storage Volume
Given: 3 identical sequential cross sections.
Take the middle cross section and add 5% area to it.
How much did one increase the conveyance in the system?
Also, when one excavates in the floodpain to provide "additional storage". If this pocket fills early in the flood event...did it do ANYTHING for conveyance at the peak flow?
RE: Increased Floodplain Storage Volume
If one encroaches on a floodplain with fill (from one side of the floodplain), I can see practical mitigation by excavating the oposite side of the floodplain the same amount (maintaining the conveyance of the channel).
RE: Increased Floodplain Storage Volume
I worry about the long term viability of excavation of a flood plain. When you are cutting it so fine that you are balancing cuts to provide storage what happens when you get sediment or debris.
I have also seen and been asked to alter a roughness for a channel. That will move capacity up, but what happens if the town has trouble performing maintenance.
I remember someone getting shot down when trying to buy land to widen the stream on the other side only for the reviewer to ask how that helps the water get through the bridge (this was purely for capacity).