Natural flood protection: streamlining the planning of flood detention in natural landscapes for the reduction of urban flooding.

A number of cost-effective and environmentally friendly flood reduction measures can provide detention of runoff from natural landscapes upstream of urban areas, with multiple added benefits. This study presents a methodology for assessing the needs for and feasibility of natural flood detention facilities. The candidate catchments for natural flood detention facilities were identified by GIS analysis and further assessed using data from maps and field inspections. Results for two case catchments show that a suitable topography and nature and biodiversity are key feasibility criteria for natural flood detention facilities. The study concluded that it is possible to streamline the process of selecting the location and type of natural flood detention facilities. Map analyses, field inspections and interdisciplinary collaboration are all important when planning natural flood detention facilities. As a result of the study, the City of Oslo will construct several natural flood detention facilities upstream of the city to gain practical experience with such facilities. While it is not expected that natural flood detention will solve all flooding problems in urban areas, it is expected that natural flood detention can positively contribute to future resilient stormwater management and the implementation of the EU Strategy on Adaptation to Climate Change.

Pesticide retention in an experimental wetland treating non-point source pollution from agriculture runoff.

Pesticide losses to the environment are unwanted due to possible environmental and health hazards. An experimental wetland is established to study the efficiency with respect to retention of sediments, nutrients and pesticides. Pesticides were applied on an arable soil plot in the watershed. Statistical analyses were carried out on three selected pesticides: propachlor, metalaxyl and chlorfenvinfoss. All pesticides were found in the experimental wetland, with peak concenttrations shortly after spraying. In 2003 pesticide retention varied from 11% to 42% and in 2004 retention varied from 19% to 56%. Comparing eight different wetland filters, we found that L6 and L8, with flagstones and straw, respectively, had a higher total pesticide retention than a standard Norwegian wetland (L4). When the compounds were treated separately, however, the picture was different. Statistical analyses showed that the treatments were signficantly different from zero in six of the wetlands for remowal of propachlor, for removal of metalaxyl none were significantly different, and for removal of chlorfenvinphos four treatments were significantly different. For the three compounds none of the relative treatments were significantly different from L4. Chemical properties of the pesticides could explain some of the behaviour in the watershed and in the wetland.

Can constructed wetlands reduce the diffuse phosphorus loads to eutrophic water in cold temperate regions?

Construction of wetlands is a possible supplement to best management practices (BMP) at the field level to mitigate phosphorus (P) pollution from agricultural areas. In this paper, annual results from 17 intensively studied wetlands in the cold temperate or boreal climatic zone are reported and analyzed. Surface areas varied from 0.007 to 8.7% of the catchment area. The average total phosphorus (TP) retention varied from 1 to 88%, and the dissolved reactive phosphorus (DRP) retention from -19 to 89%. Retention varied substantially from site to site, indicating the existence of site-specific factors in the catchment and wetlands that influenced the P removal. Factors important for P retention in wetlands were evaluated through multiple statistical analyses by dividing P into two fractions: particulate phosphorus (PP) and DRP. Both relative (%) PP and DRP retention increased with wetland surface area. However, PP retention was not as sensitive as DRP in terms of wetland size and retention: specific PP retention (gram P retention per m(2) and year) decreased as wetland area (A(w)) increased, suggesting the existence of a site-specific optimal wetland to catchment area (A(c)) ratio. Particulate P retention decreased with increasing DRP to TP ratio, while the opposite was found for DRP. Dissolved reactive P retention was higher in new than in old wetlands, while increasing age did not influence PP retention negatively. Effective BMP in the catchment is important to keep the P loss low, because the outlet concentration of P from wetlands is often positively correlated to the input concentration. However, wetlands act as the last buffer in a catchment, since the retention often increases as the P concentration in streams increases.

The effect of the redox-potential on the retention of phosphorus in a small constructed wetland.

Building wetlands in small arable streams is a popular supplement to best management practice on arable fields. Particle bound phosphorus settles in the small constructed wetlands (CWs), receiving agricultural diffuse pollution. The sorption behavior of phosphorus is, however, redox-sensitive, and bound phosphorus may be remobilized in periods with low redox potential. This paper investigates changes in the redox potential in the free water of wetland Berg (Norway) during a three-year period, and how these redox changes affect the total phosphorus (TP) and total reactive phosphorus (TRP) retention. Despite eutrophic conditions in the wetland, the redox potential was never negative, and usually higher than 400 mV, indicating aerobic conditions. The relative retention was 440% and 43% for TP and TRP, respectively. The specific retention was 100 g TP and 43 g TRPm(-2) yr(-1). Loss of phosphorus was only observed during less than 19% of the total period of time. The net loss was less than 5% of the specific retention. The high positive redox potential probably conserves the redox-sensitive phosphorus in the wetland sediment as long as water fows through the CW.

Pesticide retention in the watershed and in a small constructed wetland treating diffuse pollution.

Loss of pesticides is likely from watersheds where pesticides are used. The herbicides propachlor, linuron and metamitron, and the fungicides propiconazole, fenpropimorph and metribuzin and metalaxyl, were applied on an arable soil plot. A mass balance study showed that approximately 96% of the applied pesticides disappeared within the watershed. Three pesticides remained as residuals in the soil profile one year after the application. The 4% of the pesticides that were lost from the watershed gave peak concentrations, appearing immediately after spraying, reaching levels that can be hazardous to aquatic life. The constructed wetland situated in the first-order stream generally managed to lower the peak concentrations significantly. For the summer season, retention varied from 12 to 67% the first year. The second year, we observed both loss and retention. Increasing the wetland surface from 0.2% to 0.4% of the watershed area increased the average retention with 21% units the first year and 9% units the second year. Chemical properties of the pesticides could explain some of the behaviour in the watershed and in the wetland.

Screening the retention of thirteen pesticides in a small constructed wetland.

When pesticides are used in arable watersheds, residues are usually found in the recipients. However, small constructed wetlands (CWs) in first and second order streams can reduce the loss of pesticides, since water purification processes are stimulated. This paper presents the results of adding 13 pesticides to a CW in Norway. The relative retention increased between 0 and 67% for the pesticides fluroxypyr, bentazone, dicamba, mecoprop, propiconazole, MCPA, dichlorprop, linuron, fenpropimorph, metalaxyl, metribuzin, metamitron and propachlor. In many cases, the CW reduced the peak concentrations to values regarded as non-toxic for aquatic life, even though the wetland covered less than 0.4% of the watershed surface area, and the average hydraulic load often was above 0.8 m d(-1). Possible retention factors were adsorption to soil particles and organic matter, sedimentation of particles, low or high redox-potential, and biodegradation of nitrogen-rich pesticides. However, the retention processes are complex, and are not fully understood.

Clay particle retention in small constructed wetlands.

Constructed wetlands (CWs) can be used to mitigate non-point source pollution from arable fields. Previous investigations have shown that the relative soil particle retention in small CWs increases when hydraulic load increases. This paper investigates why this phenomenon occurs, even though common retention models predict the opposite, by studying clay and silt particle retention in two Norwegian CWs. Retention was measured with water flow proportional sampling systems in the inlet and outlet of the wetlands, and the texture of the suspended solids was analyzed. The surface area of the CWs was small compared to the watershed area (approximately 0.07%), giving high average hydraulic loads (1.1 and 2.0 md(-1)). One of the watersheds included only old arable land, whereas the other included areas with disturbed topsoil after artificial land leveling. Clay particle retention was 57% for the CW in the first watershed, and 22% for the CW in the disturbed watershed. The different behavior of the wetlands could be due to differences in aggregate size and stability of the particles entering the wetlands. Results showed that increased hydraulic loads did affect CW retention negatively. However, as runoff increased, soil particles/aggregates with higher sedimentation velocities entered the CWs (e.g., the clay particles behaved as silt particles). Hence, clay particle settling velocity is not constant as assumed in many prediction models. The net result was increased retention.

Design considerations for increased sedimentation in small wetlands treating agricultural runoff.

Some suggestions to increase the sedimentation of non-point source pollution in small surface flow wetlands are presented. The recommendations are based on results from seven Norwegian constructed wetlands (CWs) after 3-7 years of investigation, and a literature review. The wetlands were located in first and second order streams. Surface areas were 265-900 m2, corresponding to 0.03-0.4% of the watershed. Each CW had a volume proportional composite sampler in the inlet and outlet, in addition to sedimentation plates. The mean annual retention of soil particles, organic particles and phosphorus was 45-75%, 43-67% and 20-44%, respectively. Results showed that erosion and transportation processes in arable watersheds influenced the retention. Sedimentation was the most important retention process, and increased with runoff, because the input of larger aggregates increased. Retention of nitrogen did not follow the same pattern, and was only 3-15%. Making CWs shallow (0-0.5 m) can optimize sedimentation. The hydraulic efficiency can be increased by aquatic vegetation, large stones in the inlet, baffles and water-permeable, low dams. Vegetation makes it possible to utilize the positive effect of a short particle settling distance, by hindering resuspension of sediments under storm runoff conditions. As a result, the phosphorus retention in shallow CWs was twice that of deeper ponds.

The influence of vegetation on sedimentation and resuspension of soil particles in small constructed wetlands.

When initiatives to mitigate soil erosion are insufficient or fail, constructed surface flow wetlands (CWs) could be a final buffer to reduce pollution before reaching recipients. The objective of this study was to determine the influence of CW vegetation on the retention of soil particles from arable land. Retention was measured with water flow-proportional sampling systems in the inlet and outlet, sedimentation traps, and sedimentation plates in four small CWs over a period of 5 yr. The surface area of the CWs was 265 to 900 m2, and the average hydraulic loads were 1.2 to 3.4 m d(-1). Watershed areas were 0.5 to 1.5 km2. Annual soil particle retention was 30 to 80% or 14 to 121 kg m(-2). Results show that macrophytes stimulate sediment retention by mitigating resuspension of CW sediment. Five years after construction, resuspension had decreased approximately 40% and was negligible. As vegetation cover increases, the influence of macrophytes on soil particle retention reaches a level where other factors, such as hydraulic load and sediment load, were more important. Macrophytes increased the hydraulic efficiency by reducing short-circuit or preferential flow. However, vegetation did not have any influence on the clay concentration in the sediment. Hence, a possible stimulation of particle flocculation was not detected. Vegetation makes it possible to use the positive effect of a short particle settling distance in shallow ponds by hindering resuspension.