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ST0301-CS031201EROSION CONTROL Brus .h ng up on erosion control Vegetation and structural stability combine to make soil bioengineering a natural solution to erosion and flooding. By Robbin Sotir Thirty years ago, when residents of Wilmington, N.C., pur- chased homes in Long Leaf Hills, their back yards bordered a p~c- turesque creek that could be crossed easily by jumping. By 1989, drainage problems and erosion had stretched the creek's banks, creating a channel that was 40 feet wide and 10 feet deep at some points. Today, Wilmington officials are using soil bioengmeenng to stabili:e the creek banks, control flooding and restore the natural habitat of Long Leaf Creek. The process is based upon traditional engineering practices and incorporates vegetation (instead of only concrete and riprap) to create the structural systems for stabilization. The environmental emphasis of soil bioengineering, as well as its perma- nence and aesthetic value, are among the benefits cited by city officials who have adopted the discipline. From North Carolina to Oregon and Alas- 18 February 1998 ka, it has been used successfully in an assortment of applications for stream- bank erosion control. VITAL SOLUTION The Long Leaf drainage project encompasses nearly 2,000 lineal feet with a drainage area of 780 acres, says David Mayes, engineer and project director for the city of Wilmington. The drainage area contains residem tial, office, institutional and commer- cial properties. Approximately 25 homes line the Long Leaf segment the creek. At the upstream end of the project, the creek flows adjacent to a state- maintained right-of-way that includes a twin 6-foot-by-6-foot, concrete box culvert. The culvert drains to an 84- inch pipe positioned beneath another road that crosses the creek. "That causes a hydraulic problem," Mayes explains. "Water backs up the creek and causes locali:ed flood- rog.' He notes that downed trees and debri~ also bh<k water t]*,~, adding tint. pr<,pc~x, dw creek ha- aot been 5{,]xinu tl~c dr Hnage problems ~arched hciplc~sl~ as their property -I,,~ ]~ ,h,,,,[~cd; ~tabili:in~ thc banks ct,stbrc, ind thc erosi~m had ~one as ih¸ -in,ix <lmlx .oil. ami x'crtic:tl Using soil bioengineering, natural habitat and water quality have been restored at a popular fishing spot on Alaska's Kenai River. .}~:~-c mid more lalxJ than the home- Thcrct;,~c, thc c:tx ', ,d~.oices for sta- bd~zin< [}~c ~rcck t,.& ~cre reduced t-pq,m~ ,md ,od bioengineering. Pip- ~u[xcrt-, thc Cltk x.,ulJ bo required by .tho[her ~ock <itc (iCl, tf the city Fcbruarx 1998 19 installed concrete m one natural set- ting, it ~vou[d have to remove con- crete from another setting). Soil bioengineering ~vould cost nearly $550,000 less than piping, Mayes says. Additionally, its ecolog~- cai elements sold city officials and citizens. AT ITS ROOT Soil bioengineering incorporates living plant materials to restore and protect streambanks while creating a natural habitat and a natural filtration system for water. It comprises a vanety of techniques that are employed in concert according to the needs of a given site. Some of them include: · vegetated geogrids (live branch cuttings placed in layers with geotex- tile fabric wrapped around soil lifts); · live siltation construction (live branch cuttings placed in trenches at an angle from the bank to offer imme- diate overhang, trap sediment and protect the toe against erosion); · live fascines (sausage-like bundles of live branch cuttings that are placed in shallow trenches, partly covered with soil and staked in place); · brushmattresses (combinations of live cuttings and live fascines installed to provide cover and protection < streambanks); and · live cribwalls (woody cuttin,~ inserted into a log or timber frame work). The woody vegetation used in so bioengineering can !'~ave a significar effect on habitat benefits such as pn viding overhanging cover and shad for streams, nesting and foraging site tbr birds, and cover and food for sore land animals. Willow and dogwoo, are used most commonly because o their excellent rooting ability. When construction begins on th Long Leaf project next fall, the cree~ The ColOrado Department of Transportation r Wh~ included in lbuilt theGoddatd Avenue bypass to improve ~c as well as articulating flow in Trinidad. A 3 a~r~-foot srormwater detentii3fi were selected as suitable m~terials for the ouffalL ~ basin was incorporated to control erosion and capture sediment. Using four culverts, the detention basin intercepts stormwater runoff from the Goddard Avenue drainage system and from watershedilocated north and w~t fifthe interchange. In additio~:t~'~r~ducing sediraent, 'the basin W~ designed tc~ improve'ih~'(vater quality'o~'run0ff ~'t~o a picketwire ditch irrigation canal and to reduc~ the'fre- quency and severity of damhge to the ditch resulting from excessive runoff. The basin was constructed by excavauon and an earthen dam embankment comprised of silty and sand clay was placed in 2-foot lifts. The dam has a top width of 12 feet, a crest lengthSof 150 feet and 4:1 slopes. The crest elevation is 8.5 feet a,bove the bottom of the basin.. Flexible lining that co~d:~:cc~mm6date predicted:ski- tlement and provide spi[l:~a¥ protection for wate-n'loyes~ Pre-cast concrete blocks were used to line the Goddard Avenue stormwater basin in Trinidad, Colo. The general contractor for the Colo.-based Kiewit-Western, opted to install materials, supplied by American Excelsior Texas. A five-man crew hand-laced the 6,inch cast concrete blocks at a rote of 2 ~A thin layer of roadbase 1 spillway geotextile was then placed l°°Sel-~ "shingled" with upstream stops overlapp strips. The blocks were installed from the: the upstream termination trench and then' from the initial point of installation. ( the blocks were backfilled with Soil to tile from ultraviolet radiation and to etation. A native grass seed mix was tmprove aesthetics and increase hydraulic stabilit~ tionally, trees and shrubben/were planted basin and on its slopes to enhance appearance. 20 Februarv 1998 AMERICAN CITY & COUNTY Drainage problems and erosion plague a segment of Long Leaf Creek in Wilmington, N.C. This fall, the city will begin clearing fallen trees and debris from the site, and banks will be reinforced using native vegetation. banks will be cleared, and geotextile · ~bric will be installed to reconstruct nd reintorc¢ the banks. "They start building up the bank of the creek using soil and geotextile fabrics, ant] then they install live cuttings within the bank of the creek," explains Mayes, adding that native vegetauon will be selected locally and installed by the contractor. Vegetated geogrids wd[ be used .tabi[ize the sandy banks, provide overhanging cover for aquanc habitat and aesthetic benefits to the home- oxvners. The upper areas of the banks will be planted, and, where space allows, the banks will be cut back and installed with brushmattresses. Stability will be restored to thc Long Leaf creek bank immediately which Mayes estimates wdl move at a rate of [5 to t6 feet per day. "Once it is instal[ed, it is stable; it will survive a gully washer," he says. "But the stabil- try and the strength of the creek bank will [increase] as the plants grow. "We will end up with a streambank that's stable and alive; it will have plants growing out of it," Mayes explains. "A natural creek system allows for habitat, which a piped, closed system does not. Also, plants along the bank will have the ability to absorb some of the water, and, as a result, they will absorb some of the pollutants." OF FISH AND FISHERMEN As they did in Wilmington, offi- cials in Portland, Ore., and in Soldot- ha, Alaska, have used soil bioengi- neering primarily because of its eco- North American Green C350 Puts Vegetation on Par with 24 Inch Riprap "Hard armor" erosion protection ',uch as riprap and concrete, is hard to install, hard on budgets, hard on the environment, and hard to look at. "Soft armor/' like our C350 permanent ~ turf reinforcement mat works'~sOI! Armor VS.i 'l Armor ,, dh, egetation to pro~'ideCltallnel Lining Analysis comparable protection with ~1~,~1 ~((~') much less strain on your resources. So why would you e~er want to make things hard on yourselt'? \\'hat's more. C350 costs much le:,~, and can be installed ;~ ithout heavy eqmpment and in a fraction of the time it take', to la5 riprap or pour concrete NOHIH AMERICAN GREEN' [4649 High,aa) 4I North E~ansville IN 47711 Vegetation Alone 3.70~ C350 Reinforced Veg.8.00~ 24" Riprap 8.003 FOR MORE INFORMATION: Toll-free 1-800-772-2040 In Canada 1-800-448-2040 or contact .,,our local distribuloL VISIT US AT http://www, nagreen.com %?,/ Circle No. 20 on Reader %r~ice Card AMERICAN CITY ~. COUNTY February 1998 21 When the Oregon Department of Transportation straightened a segment of Johnson Creek in Portland (above}, it used live siltation constructions, brushmattress and Iow vegetated geogrids to control erosion and improve the area's ecology (below), h~cal bcuefit,. While *,ne c~t¥ to rotore water qualit5 and pre~crxe ~rounds for local sport fishermen T~a~sportnr~on (ODOT) plmncd relocate illld ~horten a 1,000-h~ot. shaped ,cgmcnt of Johnson change. During thc pr,<c~s, thc J,dqus,~n Crock ch,utc d~c nt dqhrs ,4 thc rcI,,~.~rcd hnbit it values and because of the local iislqerIcs," says Eric Machorro, water- Enxirolm/ental Services. "There ~s an pr, wide more habitat." pertmt, the state a~reed to use sod the streambnnk~. Live siltation con- ~truct~ms were installed along the chmnel margin on thc reside of the [i~ql ,Hid rlptrian hifl-itat. Additional- [% outside banks were protecte with low vegetated geogrid installed above a rock toe tha extended to the ordinary high water elevation. The vegetation "grew ver quickly; after the first year, the had a nice brush layer there, Machorro says. "It takes a couple ~ years for the vegetation to mature so you can't just walk away; bu~ once the vegetation is establishec it becomes self-perpetuating an~ self-maintaining." In addition to being aesthetical ly pleasing, the vegetation ha helped restore water quality am insect species along the affecte~ area of the creek, Machorro says All those factors have combined t~ support the local salmon popula tion. "a major component of wate- quality is temperature, especially i you're trying to manage a strean for coldwater fisheries," h~ explains. "The simple act of shad lng has a profound impact on th, quality of water supporting tht fisheries." The insects, he adds, art a natural food source for the fish. Preserving Portland's fisherie, was a major concern in remedia tion of Johnson Creek, but in Sol dotna, a town of 4,000 people, offi cials were forced to look at the link between fishing and erosion from another perspective. Soldotna Creek is a tributary to Kenai River and the convergence of the tw~ water bodies provides a popular spot for sport fishing. Years o~ heavy foot traffic, as well as seasonal flooding and outflow of ice, had caused enormous damage to the river- bank. A stretch of bank measuring 650 lineal feet and starting at the mouth of the river was particularly degraded, says Soldoma Mayor Ken Lancaster. "Being a very popular fishing hole, the area took an extreme amount of dam- age to the banks," he says. "Also, the ice goes out in the spring, when the water comes up underneath the ice and forces it to go downriver [causing additional trauma to the banks]." From 199i ro 1995, when remedia- tion of the area was complete, "we'd lost in excess of 20 feet of bank in that 24 Februar~ 19OS AMERICAN CITY g~. COUNTY area," Lancaster says. Without action, the riverbank would have continued to erode "to the point that we may or may not have been able to save it," he notes. The Kenai River solution com- prised a variety of soil bioengineering techniques. Overhanging cover was provided with live siltation construc- tions and live cribwalls. In wet areas, native sod rolls were used to stabilize the bank line and to re-establish vege- tation. To create additional cover for fish, large rocks were placed randomly in shallow water in front of the live cribwalls, and small rootwads were anchored in deeper waters. At the upstream end of the project, the city installed a fishing platform and trails that weave through the newly vegetated riverbank. It also added three staircases leading into the river to prevent foot traffic from degrading the site again. The prolect took three summer sea- sons to complete, Lancaster says, adding that ice prohibited continuous work. However, each portion of the system stabilized immediately upon installation. "We have a tremendous growing season in summer, and the vegetation took root and bloomed immediately," Lancaster notes. "In fact, we've had to trim it back every year because the plants just keep growing and growing. It's really been phenomenal how it's worked." In 1995, following completion of the soil bioengineering project, Kenai River flooded, raising the water nearly 20 feet above the project line. Lan- caster reports that wooden poles anchoring the walkways had to be replaced with metal, but otherwise, the system withstood the flood waters. THE LONG-TERM RESULTS Aside from monitoring early plant growth and periodically trimming mature vegetation, soil bioengineering requires little maintenance, according to those who have used it. In Lo~ Leaf, Mayes anticipates that, in adc tion to trimming the growth, the ci~ will be responsible for removal downed trees as well as sediment debris. "Soil bioengineering is, in m~ mind, a permanent stabilization of the creek bank with only the possibility ci some minor spot maintenance," h* says. Lancaster agrees. "I'm of the opin ion that [this solution] is permanent,' he says. "I anticipate that my kids mac grandkids can fish [on Kenai River] long after I'm gone." Because soil bioengineering is a, new discipline for many cities and counties, and because it is labor- intensive, the initial costs are high. Mochorro says. "l think it's importmal to remember it's a newer technolog3,. so it's more expensive at first. But ii does more than just stabilize the bank: it provides habitat and aesthetic val- ues. I believe over the long term it's cheaper because you're not having te 26 February 1998 AMERICAN CITY ~t. COUNTY Severe degradation marked the spot where Soldotna Creek meets Kenai River, A variety of planting methods, including live siltation constructions, live cribwalls and native sod roJJs, were used to stabilize and vegetate the banks. The restored site is pictured on pages 18 and 19. go back and pour more riprap or repair Mayes says that, although cost was a consideration when planning a solu- tion ~or Long Leaf, residents of the community were swayed by the aes- thetic value of soil bioengineering. "We held two public meetings in which we invited our team of consul- rants, city staff and residents that o~ned property along the creek," Mayes says. "The residents were over- ~thehnmgly in fwor ot soil bioengi- neering. They liked the fact that there was a creek behind their houses, and they wanted to see something [that would preserve the natural aspectsl. "They understand that, when you have a construction project, there's going to be some 'pain and suffering.' There's equipment behind your house, trees are being removed, there's noise back there, and there are people back there," Ma,/es adds. "But they also understand that they will have an aes- thetically pleasing, natural creek s,/s- tern that's stable and wi[[ not erode their property anymore. I was pleas- andy aware that, with some minor exceptions, the citizens really ate this solution up." Mayes believes his city's experience represents of a trend in erosion con- trol. "Engineers can size a pipe, they can size a channel, and they can line it with something that will not erode hke riprap or concrete. But engi- neers need to be looking for more comprehensive sokttions. "Soil bioengineering is one of them," he adds. "I'm anxious to see this project work because i think it could work in a lot more places than just Wilmington." '& Robbm Sotir is president of Robbin B. Sotir & Associates. a soil bioengineering /inn based in Marietta, Ga. 28 Februar,, 1998 CirdeNo. 1g0nReaderSeniceCard AMERICAN CIT'~ ~ COUNTY