Trash Rack

Hello SWMMers,

Does anyone have experience designing trash racks for open channels? We have an application for such a device to protect a wetland area from becoming littered with floating debris (cans, plastic containers, etc). The trash rack would be installed upstream of the wetland area on an existing open channel. Drainage area to the proposed trash rack is approximately 3800 acres of urban land use.

It is our goal to install a system that diverts floating debris from the channel to a collection point adjacent to the stream. Periodically, the trash would be removed from the collection area. The system should be completely passive (not motorized) and functional up to 1000 cfs at 3 ft/sec.

I would appreciate any suggestions.
Charles Blum
City of Tallahassee

There is a fair amount of experience in Australia at installations upstream of their gross pollutant traps (detention ponds). These trash racks are on fairly large channels and are designed for ease of maintenance. Perhaps some of our Australian colleagues could provide references.

Wayne Huber

You might try the following company.

Fresh Creek Technologies, Inc.
P.O. Box 1184
West Caldwell, NJ 07007-1184
(201) 808-9020

They have both in-line and floatable platforms using disposable mesh nylon bags. Their floating systems appears that it could be adapted to an open channel arrangement. I have reviewed there literature for a municipal client with CSO's but I have not installed them. I understand that the City of Philadelphia has a pilot test currently underway. They also provide the engineering required to install their products.

Roger Lehman
lehmanr@wcpost2.rfweston.com

Trash Rack Design Requirements

Chucky,

Here are a few pointers for designing a trash rack.

1.The grill should be spaced at approximately 60mm centres, constructed of vertical 10mm galvanized flat steel bars. This spacing has been adopted as it will retain 375ml aluminium Fosters cans (a popular trash type in Australia!).

2.The rack should be designed to operate up to the 1 year ARI flood without overtopping including provision for up to 50% blockage. Additionally it should remain stable up to the 100 year ARI or when fully blocked.

3.Allowance for access to the rear of the trash rack should be made for cleaning purposes.

4.Most importantly the rack should be designed to withstand impact load, drag, and debris loads.

I hope this was helpful.

PS the most common design loads are:

Log impact = 9KN
Drag = 1.6 KN per m^2 of bar area
Debris = 1.2 KN per m^2 of trash rack
(SI units)

Regards
Michael Shaw
Willing & Partners, Sydney
Willings@s054.aone.net.au

Regarding this interesting and important thread:

1. This first caution may not apply as strongly in Florida and Aussie, of course, but gross pollutant traps in cold climates impede ice break-up and cause upstream flood damage, which may be significant in urban areas. So my question is aimed at having anyone with experiences of GPT applications in northern climates, to please share them with us unfortunates. Can they be designed to function in heavy ice conditions?

2. My second relates to a problem that I noticed in the tropics a few months ago: an evidently carefree importation of some stormwater practices that are popular in parts of the world that are happily relatively free of waterborne diseases such as schistosomiasis or bilharzia. It seems to me that distributed surface water retention (such as behind some GPTs) oftentimes provides increased habitat for infected snails in highly populated areas (lots of kids playing near the water), and so I wonder whether the impact of various stormwater management procedures on such water-borne human diseases, reputedly among the most widespread in the world, has been studied, and, if so, whether comparative numbers are published anywhere? Anyone from warm parts or elsewise able to comment, please?

Otherwise I must add that I was most impressed by the removal efficacy of the GPTs that I saw in Aussie.

Bill James, Prof., Sch of Eng'rg, U of Guelph