Land and Water - Taking the DRED out of Dredging.pdf
Dredging is always such a “dreaded”word. The word dredging typicallyinduces images of heavy machinerycrushing the surrounding landscape, roadways that are rutted and muddy, and theodor of organic material that can’t be forgotten soon enough. But dredging doesn’talways require draining the pond or lakeand destroying aquatic environments.There are basically two types of dredgingwhen it comes to removing silt, sedimentand other organic and inorganic accumulation from ponds, lakes, streams and otherwaterways; mechanical and hydraulic.
Hydraulic Dredging Explained
Just as the name implies, hydraulicdredging is the use of water to transport theaccumulated material through a temporarypipeline, using a pumping system to basically “vacuum up the muck”. Sometimeshydraulic dredging is also referred to as“suction dredging”. The mixture of organicand inorganic materials along with wateris referred to as “slurry” and is pumpedthrough the pipeline to a strategically located area for dewatering. Hydraulic dredgescome in all shapes and sizes from largeocean going dredge ships all the way down to small pontoon mounted, hand dredgessuch as the one described in this article.When it comes to hydraulic dredging thereis no “one size fits all” dredge. The size of the dredge, pump system and material agitation method are all project specific andmany times require custom configurations.Dewatering of the slurry can be handled in a variety of ways. When evaluatingfuture projects, hydraulic dredging contractors must always determine what willhappen to the material at the discharge siteand how far the material must be pumped.Often circumstances such as inadequatespace, permitting or aesthetics will dictatethe dewatering method. Geo textile tubes(geo tubes) are often the preferred methodfor dewatering in residential communities,golf courses and city parks. The slurry mixture is pumped into a strategically placedgeo tube where it contains the organic andinorganic dredged materials while releasing the water. The decant water is thenreturned to the lake either by gravity flowor a pumping system. Once the dewatering process is complete; the dried materialcontained inside the geo tube can be repurposed onsite or removed from the property.While mechanical dredging may bebetter suited in some situations for pondand lake dredging projects that will allowall of the water to be drained and aquaticlife relocated hydraulic dredging offers agreener, cleaner alternative to sedimentremoval without the harsh environmentalfootprint left behind by heavy machineryand millions of gallons of displaced water. Yorba Regional Park in Orange CountyCalifornia is one example of the advantagesof hydraulic sediment removal over conventional mechanical dredging methods.
The Project
Yorba Regional Park is a 175 acre dayuse park in Anaheim, California that accommodates well over one million visitorseach year. The park was developed alongside the Santa Ana River with four smalllakes connected by small streams and includes bike paths that connect to the nationally recognized Santa Ana River Trailthat leads some 20 miles to the PacificOcean. A typical day at Yorba Park mayinclude activities like fishing, boating, cycling, volleyball, picnicking, horseshoes,and even horseback riding. Locals love thepark and use it daily for exercise or walking the family pet. The park is plentiful inamenities such as pavilions, picnic tables,restrooms, playgrounds, bicycle and boatrentals, baseball fields and more. It is nosurprise that it is one of the busiest parksin the Orange County park system; therefore, a project of any kind must take into consideration the impact it will have ondaily park use, as well as the environmental footprint it will have.The upper lake (.35 acres) at Yorba Regional Park was designed with acirculation pipe at the bottom/center of the lake that draws water froma well downstream and allows it tooverflow to the connected lower lakesand streams. An onsite survey showedsediment accumulation in the bowl ofthe lake causing the circulation pipe toclog. The lake was originally designedto be five foot deep in the bowl, and thesediment depth in the bowl had reachedup to three feet, leaving only two feet ofwater. The nutrient-rich, shallow water hadbecome the perfect environment for invasive vegetation including algae and charathat may have been taking up as much as50% of the lake bottom. Through a natural process referred to as eutrophication, athick layer of “muck” – the remains of treeleaves, grass clippings and decayed aquaticplants – has quietly settled in the pond overthe years. Decomposing material consumesoxygen, which is also in short supply at thebottom of the pond making it more difficult for fish and aquatic habitats to flourish. And without oxygen, decompositionproduces hydrogen sulfide and a rotten-eggsmell which can cause complaints frompark visitors. The pond muck also containsa high level of nitrogen, phosphorus andother nutrients that allow invasive aquaticplants such as chara to flourish. When sedimentation caused by eutrophication reaches this point, it often leads to a continuouscycle of chemical treatment to control thevegetation.When it became apparent that dredging was needed, the goal of SupervisingPark Ranger, Joanette Willert, was to leave little or no footprint on the surroundingenvironment, while minimizing any disruption to the daily activities of the park.According to Willert, “The communityis vested in Yorba Park,” and always concerned with what happens in and aroundthe park.Another consideration when evaluating dredging methods for this projectwas to be as conservative as possible withthe water resources in Orange County.Draining the lake and using traditionalmechanical dredging equipment was nota viable option.Envirodredge, a company specializing in hydraulic sediment removal, wasbrought in to evaluate the project and offersolutions. There were many factors to consider when planning for this project:• The upper lake had cement walls andcement throughout most of the bottom• Limited access for equipment mobilization• Shallow water depths• The lake was in the center of pedestriantrails, bicycle paths, picnic tables, and playgrounds• Limited space and access for dewatering (to reach the designated geo tube dewatering area, the dredge discharge hoseand decant return water hose must cross aheavily traveled pedestrian and bike path).• The chosen method must be nonintrusive to park patrons and activitiesclosing the park was not an option• The method must be environmentallyfriendly with little impact on the surrounding habitats• Water conservation during the projectmust be a top priority.
The Solution
After careful consideration, a smallpontoon mounted hand dredging systemwas selected. The “hand dredge” system,which could be lifted by 2 men and placedinto the lake, consisted of a 4 inch primarypump for suction and a 1.5 inch secondary pump for hydro agitation. A suction headwith 1.5 inch x 3-inch screen openingswas used to prevent larger objects such assticks, rocks and other debris from entering the suction hose and possibly damaging the pump. A pole was mounted to thesuction head that allowed the operator to control the dredge depth while basically“vacuuming” the lake bottom. However,the chara that had consumed most of thelake bottom, posed a challenge for thesmall dredge. Chara would accumulatequickly around the suction head thereby“clogging the vacuum.” To eliminate this condition, the chara was removed by handwhich provided much better access to thesilt and sediment accumulation.Using a temporary pipeline, the handdredge system pumped the material acrossa heavily traveled pedestrian and bike pathinto a geo tube for dewatering. The decantwater was contained and pumped backacross the cart path to the lake. The noiselevel of the gas powered hand dredge system is also quite diminished in comparisonto larger diesel powered hydraulic dredges,so there is minimal disturbance to park patrons and wildlife alike. During the projectnormal park activities continued as usual.A portable hose bridge was deployed toaccommodate the dredge discharge andreturn water hoses so park visitors and employees could continue using the pedestrian and bike paths. Birthday parties continued uninterrupted, and a volleyball matchwas played next to the geo tube dewateringarea. Even wildlife seemed undisturbed bythe work in progress.The project was completed on schedule and within budget with good reviewsfrom park visitors and park management.Ranger Joanette Willert states, “We arepleased with the results, and how we wereable to complete the entire project with nodisruption to park activities or environment. We had a few curious people stopto inquire about the project, but again, thepeople are vested in their park and whatgoes on here.”It took five days to complete the project during the month of July, one of thebusiest times of the year for Yorba RegionalPark.Now that the nutrient overload hasbeen reduced by removing years of organicsediment accumulation, aquatic vegetationin the pond will be much easier to manage.Aeration along with a professional pondmanagement program will keep this pondhealthy and looking great for park visitorsfor years to come!
