Micro-Pools and WQCV Outlets at EDBs and Safety Racks,
Ben
technical and public safety issues need to be better addressed by
practitioners, designers, and the technical reviewers of developer
projects. The three issues that are in need of most attention since
they keep coming us over and over are as follows:
1. Need for a micro-pool in extended detention basins to minimize
outlet clogging and to help control mosquito populations.
2. Need for consistent design and construction approach for trash
racks at water quality detention outlets.
3. Need for safety/trash/debris racks at all detention basin outlets
and at pipe entrances to storm sewers and culverts.
Let me try to clarify why these issues need to be brought to
everyone's attention
1. Need for a micro-pool in Extended Detention Basins.
There is a major misconception that a micro-pool is the culprit in
increasing mosquito populations in a community. A micro-pool can and
will produce mosquitoes, but the size and minimum depth (2.5 feet, but
deeper is better) recommended in the USDCM were designed to reduce
this problem as much as possible. When built to achieve at least this
minimum depth, predator species can establish themselves in the pool,
helping to control mosquito larva populations.
I have seen time and time again that not having a micro-pool has
resulted in conditions that can be perfect for breding mosquitoes at
extended detention basins. Whenever a micro-pool is not used as a
part of the trash rack/outlet system, sediment and small trash tend to
clog the lowest perforations very quickly after the facility goes on-
line, creating shallow standing water over large portions of the
basin's bottom and/or boggy bottoms that are extremely difficult and
expensive to maintain. The shallow standing waters that are often
created provide the perfect habitat for mosquito breeding.
My recommendation to all of you is to resist requests by your clients,
project sponsors and developers to eliminate and not use micro-pools.
Try educating them about this issue first, and if that does not work
and they do not want to use an extended detention basin with a micro-
pool, suggest alternate BMPs. Alternate BMPs for smaller catchments
(having less than 10 acres of impervious surfaces) are properly-
designed sand-filters and for very small sites with (see Volume 3) are
porous landscape detention and porous pavement detention. When these
alternate BMPs are designed using the maximum depth for the Water
Quality Capture Volume, they will provide dry conditions within a
relatively short period of time after the storm ends. Mosquito larvae
need five days to hatch and all of these alternate BMPs will drain out
in much less than five days. Regardless of the BMP used, proper
maintenance will be needed to help control mosquito breeding.
One other item of note is that underground facilities with standing
water have been shown by field tests in California to also be prolific
mosquito breeders. In addition, there have been cases reported that
underground tanks have gone anaerobic resulting in odors of
decomposing raw sewage unless they are cleaned on a frequent basis.
Controlling mosquito populations is tricky. Most "knee-jerk"
reactions to address this problem, such as the elimination of micro-
pools or going underground, can create greater problems and extensive
long-term maintenance needs that the MS4 permit holder will be
responsible for regardless who owns the facility.
2. Trash racks at water quality outlets.
We have been seeing many types of trash racks being installed in front
of water quality outlet control orifice plates and risers. Many of
them do not follow the recommended details in Volume 3 of the Urban
Storm Drainage Criteria Manual.
The details that are shown in the latest revision of the 1999 edition
of Volume 3 are based on experience and water quality in mind. The
use of stainless steel well-screen fabric or the aluminum grating for
larger outlets; their sizing requirements; and the orientation of the
racks that also extends below the micro-pool's water surface, all of
which are recommended in the Manual, were carefully chosen by UDFCD
for their physical properties; hydraulic function; and to minimize and
facilitate maintenance. This design has proven itself in the field at
the District's Grant Ranch testing facility and at other locations.
Alternate designs have been considered and were rejected because of
foreseeable maintenance difficulties, greater potential for clogging,
or for water quality reasons. For example, use of zinc-coated wire
fabric or expanded diamond slits introduce zinc into the BMP's
effluent, a constituent of concern when trying to meet water quality
standards set by the State of Colorado. In addition, designs using
such metal fabric and other types of screens are very susceptible to
being clogged by small trash and debris and are also very difficult to
clean out once partially or fully clogged. Is it not the intent to
reduce future maintenance needs as much as possible when we build or
rehabilitate BMPs? I recommend that you become fully familiar with
the recommendations in Volume 3.
3. Need for safety/trash/debris racks at all detention basin
outlets.
A safety/trash/debris rack at the inlet to the outlet structure or
headwall of any stormwater detention facility and at the upstream pipe
entrance end of storm sewers and long culverts is needed for public
safety as well as to keep trash from plunging the outlet. There
should be no exceptions. This public safety need is intended to
reduce liability resulting from any person being trapped against any
such outlet or sucked into it. Long-standing professional literature
record recommends this and there are is a record of legal cases
demonstrating liability when these recommendations are not
followed.
To reduce chances of clogging at the trash/safety rack, I recommend
they be oversized whenever possible and recommend that the guidance
given in the Manual for their sizing be used as the absolute minimum
requirement. This is especially the case when the tributary
watersheds are relatively large. There is one major gulch on the
south side of Denver that is a great example of how the application of
the minimum-sizing guidance can still result in major plugging
problems during larger runoff events. Larger watersheds often come
with a strong potential for delivering much debris during all levels
of flooding. To help understand when and how to use safety/trash
rack at culvert and undergournd pipe entrances I suggest you review
the policy for the use of racks clearly articulated in Section 8.0 of
the Culverts chapter in Volume 2 of the Manual.
I hope the above will help many understand why some things are
recommended in the Manual as they are. Thus, before ignoring it's
recommendations or waiving them during the review process, think of
the long-term maintenance and public safety consequences and potential
for liability.
ZN
1. Once species get a foot hold in the micro-pool how is maintenance
solved? Do we simply eliminate (dredge) those species without causing
additional government intrusion?
2. The reason there are so many variances in the trash rack is that
many contractors have no idea where to purchase the "product". Maybe
UDFCD can provide more specific product data or manufacturers who are
approved by UDFCD to further assist continuity with these types of
structures.
These are actual comments that I have heard/experienced behind closed
doors and maybe this is a first step to helping both sides.....
Thanks...
Zdenko
b...@uwllc.org
It seems that since the original recommendation in Volume 3 was made, the
S.S. #93 VEE Wire filter fabric by US Filter is no longer available since
U.S. Filter does not seem to be offering it in its sales information. On
the other hand,
JOHNSONR S P E C I A LT Y S C R E E N S (A Weatherford Company) offers
stainless steel VEE wire filters that are identical to the ones recommended
in Volume 3. It appears this information need to be updated in Volume 3 and
the source for this material more clearly presented in the manual and
possibly other means on the UDFCD web site.
For now, you can contact North and South America sales for Johnson Specialty
Screens at: sa...@johnsonscreens.com; or 800-VEE-WIRE/651-636-3900
It is incumbent on the design engineer to specify clearly on the plans and
specifications what products to use, where they are available and how they
should be designed.
It is incumbent on the contractor to meet those specifications and to
provide shop drawings to the construction engineer and/or the local
jurisdiction's inspector to show compliance with the plans and
specifications.
It is incumbent on the local jurisdiction (and possible it's inspectors) to
insure the approved plans and specifications were followed before the
facility is accepted as completed.
Ben Urbonas
Urban Watersheds, LLC
845 S Cove Way
Denver, CO 80209-4813
Tel. 303-765-0155 (FAX also)
Cell 303-728-4449
ZN
Zdenko...
>
> - Show quoted text -
Chandan Das
Dear members,
I could not locate the CUHPSWMM converter in the UDFCD website. Would
appreciate your help.
Thanks,
Chandan Das, Ph.D., P.E.
Water Resource Manager
J.F.Sato & Associates
5878 South Rapp Street
Littleton, Colorado 80120
Tel: 303.797.1200
Fax: 303.797.1187
-----Original Message-----
Ken MacKenzie
Chandan, download the CUHP_2005_v1.3.3.zip file from http://www.udfcd.org/downloads/down_software.htm
Unzip the file and run CUHP Powertools 1.3.3.msi. Be sure to select the 'custom' or 'complete' installation option - right now the typical installation doesn't install the converter (we're working on that).
-Ken MacKenzie, UDFCD
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Chandan Das
Ken,
Thank you very much. Did not realize that the converter was packed within the zip file
CUHP_2005_v1.3.3.zip
Regards,
Chandan Das, Ph.D., P.E.
Water Resource Manager
J.F.Sato & Associates
5878 South Rapp Street
Littleton, Colorado 80120
Tel: 303.797.1200
Fax: 303.797.1187
From:
udfcd-...@googlegroups.com [mailto:udfcd-...@googlegroups.com] On Behalf Of Ken MacKenzie
Sent: Monday, February 08, 2010
1:25 PM
To: udfcd-...@googlegroups.com
Subject: RE: [UDFCD-support]
CUHPSWMM Converter
Chandan, download the file from http://www.udfcd.org/downloads/down_software.htm