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From:
To:
Date:
Subject:
Suzan,
"Loretta Mokry" <loretta@apaienv.com>
<staylor@ci.coppell.tx. us>
1 /28/2004 1:28:52 AM
Re: Channel
Page 1
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3:1 is about as steep a slope recommended for most vegetation, but it is possible to combine a steeper
slope protected with gabions or other structural material where flows are constricted in conjunction with
more gradual vegetated slopes where flow conveyance is adequate.
Some of the bioengineering recommendations for erosion protection use steeper slopes constructed with
willow wattles (bundles of limbs or small logs) or fascines rather than gabions. However the species of
willow usually used are not typically found in north central Texas. The black willow, which is the dominant
willow species found throughout most of Texas, is too rigid and produces too much resistance to flow.
Most engineers with whom I have worked do not want black willow within a channel. The herbaceous
vegetation that provides the most erosion control would include the spikerushes which produce a thick
root system similar to a sod grass, and several grasses including switchgrass, buffalograss, and other
native tall grasses common in floodplains. Since the City was planning on a mown grass, the buffalogras
would probably be an acceptable choice for the better drained areas combined with spikerush in the wetter
areas. The buffalograss doesn't get more than 6 inches tall. Legumes for nitrogen fixation and
wildflowers for diversity and aesthetics can be added to a seeding mixture. Of all the grasses, switchgrass
is probably the best erosion control species but it is a tall grass with leaves about 4 foot tall and up to 6
foot tall when it produces seed heads. It is not unattractive, but will provide more resistance to flow than
the buffalograss. The information from the hydraulic runs will help determine what species may be
appropriate to use in a natural channel.
I'll bring a copy of a planting plan I prepared for Trammel Crow Residential for an area along North
Mesquite Creek and a small tributary that flow through one of their multi-family complexes in Mesquite.
North Mesquite Creek was already channelized before the project was built but the planting plan was
developed to stabilize the slopes, developed the creek channels as mitigation area, and to provide an
aesthetic amenity for the complex. There was a high pressure gas pipeline crossing the tributary channel
and a concrete mattress was used to provide erosion control over the pipeline before the complex was
built. We used herbaceous planting within the channel and along the water's edge with shrubs along the
slopes and canopy trees along the upper bank. The slopes are mostly 3:1 but mortared rock retaining
walls halfway up the slopes were used to provide the cross section needed for flow conveyance.
I will be in the office Wednesday morning. Give me a call if you have a chance. If I can make it up to
Coppell early enough on Thursday, I plan to walk the channel before the meeting at 2 p.m. to get a first
hand look at the project area.
Loretta
Loretta E. Mokry
Senior Environmental Scientist
Alan Plummer Associates, Inc.
1607 Todd Trail
College Station, Texas 77845
(979) 694-7619 home office
(817) 845-3280 cell
(979) 694-7619 fax (call first)
Through Arlington office
(817) 461-1491
(817) 860-3339 FAX
<loretta@apaienv.com>
Suzan i ayior -rte: Channel Page 2 j
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»> "Suzan Taylor" <staylor@ci.coppell.tx.us> 01/27/04 02:04PM »>
Loretta:
What slopes can we use on the sides if we use different types of
vegetation other than mowed grass. Mowed slopes are 3:1. Can we go
with a 2:1 or a 1:1 depending on the type if vegetation? The channel
will be 78 feet wide if we go with grass and a 3:1 side slope.
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