engleski

Problems with Sedimentation in Wetland BMPs During Highway Construction and Use

This paper analyses the economical and ecological problems associated with surface soil erosion on construction sites, as well as, resulting transport of sediment and its deposition in water bodies – natural streams, reservoirs, wetlands, lagoons, etc. Investigation was focused on the influence of water forces as the main erosion actuator on the watershed, which involves the action of raindrops and surface runoff. Impacts of highway construction and use in Istra region (Croatia) on nearby lagoon BMP are analysed in this paper. Special interest on selected area is given from the aspect of hilly terrain configuration and flysch geological characteristics, which altogether result with increased erosion potential. The selected segment of highway is characterized by implementation of two runoff treatment structures – separator (grit and grease chamber) and treatment lagoon. In accordance with applied design, lagoon is shaped as extended detention pond, which represents the main treatment structure and most of sediments settle therein. Separator manages to detain only 30% of washed material since the transported sediment contain large portion of small-sized particles, which easily reach the lagoon. Such design with separator placed in front of lagoon describes common practice in Croatia, although in general it's not a prescribed standard, so both cases were analyzed. The purpose of the study was to mathematically express the soil loss on selected area due to generation of erosion processes and further transport of sediments. Altogether, three investigations were carried out concerning the erosion BMPs implementation. The first one presumes no protection while the second and third one include the use of erosion control measures (BMPs) on disturbed area. Erosion rate was estimated first and the influence of sedimentation in runoff treatment structures was investigated afterwards. Concerning the main purpose of the study, the segment area with cut slopes only, in total of 4, 0 hectares, has been analyzed. Among dozens of available erosion prediction models, widely accepted RUSLE (Revised Universal Soil Loss Equation) method was used to calculate the sediment quantities generated on observed watershed. Rainfall erosivity index value of 80, 0 N/h was determined by using the generated isoerodent map for Republic of Croatia, which is based on Fournier method and data collected in period 1965-1984. Soil erodibility factor was taken with the value of 0, 55 which is calculated by using the nomograph (Wischmeier et al., 1971). Slope length and steepness factor value of 4, 6 was determined by using the specially designed table (Renart et al., 1997). Considering irregular characteristics of cut slopes along the selected segment of highway, mean slope length and gradient values (L=8, 0m ; S=100%) were taken to simplify the overall analysis. Two erosion BMP's were selected as the appropriate measures at selected area – geonet made of natural biodegradable materials such as straw and/or coir fibres and protective polymers net filled with soil material and additional mixture of fertilizers and seed for permanent greening. Since no field investigations were carried out on selected location, cover and management factor values were estimated based on literature data with the values of 0, 05 regarding the first BMP and 0, 1 regarding the second one. For the case that implicates no erosion control measures the cover and management factor value equals 1, 0. Support practice factor is generally irrelevant at construction sites and was taken into consideration with value of P=1. Calculated values were encountered in numerical model USEPA SWMM-5 to simulate the overall sediment transport and its accumulation in the lagoon. Lagoon treatment efficiency was calculated for each of the six scenarios that were analyzed. The calculation results show moderate reduction in lagoon treatment efficiency disregarding the implementation of erosion control measures at disturbed cut slope areas. Designing the runoff treatment system without separator it is certain that all soil particles that were washed out reach the lagoon and settle down in it. Therein, the lagoon losses more of its effective capacity, which results with additional reductions in its overall efficiency. Simulation results were used as input parameters in estimation of additional expenses caused by necessitating storage capacity enlargement and frequent and costly maintenance. These expenses were subsequently compared with direct expenses of erosion and sediment protection measures at construction site. The results emphasize the importance of erosion control measures implementation not only during construction phase but the highway use as well. Taking the climate characteristics of the selected area into consideration, which are described with average R factor values (analyzing the Croatian territory only), it could be realized the importance of the erosion BMP's implementation at sites with bigger portion of high intensity rainfall. The results also suggest a need to divert the collected runoff besides the treatment structures just for the initial period of highway use until complete vegetation establishment on disturbed areas, even in the case of erosion BMP's implementation.

erosion; sediment buildup; wetland BMP; RUSLE

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