High Early Discharge (HED) Strucutures 2

[dcnnng]

How do you define HED? Can you help to give me examples and sketches/drawings. Any refernce to read up this topics? What are the advantages using HED and when should we use them?
Your advice shall be very much appreciated

 

[psmart]

"HED" is not a common term. Where did you encounter it? Presumably that source can provide more detail. A google search suggests that the primary usage is in Australia:

http://www.google.com/search?hl=en&safe=off&q="High+Early+Discharge"structure

One benefit of "high early discharge" would be to make optimum use of detention storage by passing the early flow, thus leaving more of the storage available to handle the peak flow, later in the storm. Vortex values and similar devices are sometimes used for this purpose.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

an off-line or "peak scalping" detention basin may also fit the description of "high early discharge". Smaller flows bypass the basin while higher flows (at the peak of the hydrograph) overtop a weir or other structure and are stored in the basin. One advantage of this type of structure is that it maintains more frequent flows in ephemeral streams which is desirable from an environmental standpoint.

 

[jartgo]

But if you have high early flows (as defined above, I have never heard that term before), then you're dumping the "dirtiest water" (or so they say) into the streams, which is generally not desirable from an environmental standpoint.

Sometimes it feels like we can't win from an environmental standpoint.

 

[dcnnng]

Yes,as pointed out by Peter Smart that this term is being used in Australia and has been quoted in their guidlines and manual but I can't find its definition. Most Australia Council's Guidlines or Manual quote for the PSD and SSR for outlet with HED or without HED respectively. By virtue of its name, I agree with Peter's intepretation, I think that this is for dry detention pond. Any more help!!!

 

[TerryScan]

The term seems to be self defining. When it is applied? is the more pertinent question.

While we do not use the term HED here, its use would apply to water Quantity control rather Quality control (someone pointed out that water quality would suffer...but it could be achieved elsewhere in the overall stormwater management).

Here, we have regulations for quantity contol. An HED type strategy would apply only if one showed that normal quantity control would aggrevate downstream flooding. This would usuallly happen near the bottom of a watershed, where detaining the site discharge peak can make the peak overlap the peak coming from the upper watershed. In this situation it can be advantageous to push the site peak out early so that the peaks aren't additive.

Like Peter, I did a google search of the term and noticed it is used in manuals in AU. The manuals seemed be concerned with outlet control structures (primary discharge orifices in particular) being submerged by tailwater.

 

[cvg]

jthompson

you're dumping the "dirtiest water"(or so they say) into the streams, which is generally not desirable from an environmental standpoint.

yes, that is true. However with ephemeral desert streams (what we call dry washes or arroyos), if you divert them into a detention basin you may dry up the stream completely and that causes the vegetation to die. The permitting agencies generally consider this to be more of an environmental impact than to allow the dirty water to flow in the stream. So generally, low flows are required to be maintained in the stream.

 

[dcnnng]

Contribution from Practising Engineers from Australia on this subject will be very much appreciated. Thank you!