Increased Floodplain Storage Volume 1

[stmpjmpr12]

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???

 

[Maury]

Based on the information provided, I agree with the City Engineer. While your development may not have a significant impact on runoff relative to the total steam drainage area, if additional development occurs, the cumulative effect will be signifincant in a short time. He can't very well allow you to build withour detention, and then make everyone else provide it.

 

[francesca]

What is the time of concentration on the stream? In larger streams, development water has peaked and left before the stream peaks, making your HEC-RAS analysis misleading if the times of concentration are not concurrent. In some cases, detention can delay the peak runoff off a site to coincide with the peak in the stream and make matters worse, though usually not by any significant amount. (E.g. a development detained flood peak of 100 cfs on a 2,000 cfs stream.)

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.

 

[LHA]

Agree with Francesca...sort of.

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

 

[blueoak]

I am not so sure. If you are only 2% of a stream system you cannot get too hung up on timing. Detention should help or be modified later to help. Large drainage master plans help to see where flow should be speeded and where to hold it, but without that it would be hard to guess. Also the city engineer has one shot to get detention before it is covered with houses. If it isn't needed it can become a full time park. Try finding detention in an overbuilt urban area.

 

[Lih]

Just as filling of a floodplain reduces flood volume storage and hence potentially results in higher flood levels, conversely doesn't excavation of the floodplain increase the floodplain storage/potentially decrease flood levels?

 

[francesca]

You would think... but consider a narrower river cross-section and constant flow rate; the velocity increases (Q=VA), increasing velocity head and reducing water surface elevation (locally, anyway); the converse applies to a wider cross-section. Upstream of the former you may get a higher water surface elevation due to the constriction, making modeling of an appropriate study area essential.

 

[TerryScan]

francesca hit a key point here -cross sections. Folks get focused on storage in a floodpain, when CONVEYANCE is usually the issue.
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?

 

[TerryScan]

An additional thought as far as mitigation of fill in a floodplain:
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).

 

[blueoak]

I think that last is called relocation.

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).