Assessment of Downspout Disconnection by Modeling Infiltration Potential in Urban Areas
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Climate change, which leads to more intense precipitation, combined with an increase of impervious surface, due to an increase of urbanization, has resulted in an increasing amount of stormwater entering the sewer system. This increase will lead to heavier loads on the existing sewer system, which in most cases already are overloaded, resulting in more frequent surface flooding causing damage to buildings and infrastructure. To improve this situation, there has lately been an increase in focusing on a sustainable stormwater management, which is based on Sustainable urban drainage systems, SUDS. One measure, which is considered used by multiple municipalities, is downspout disconnection. This measure is rarely used to day, and there is therefore little information about the effect of this measure. Usually the downspouts are connected to the sewer system and the surface water generated from the roof is therefore directly entering the sewer system. The runoff from the roof is usually generated very fast, and gives thus a great contribution to the amount of stormwater in the sewer system. This can easily be reduced by disconnecting the downspouts and lead the water to an appropriate infiltration surface. The effect of this measure is closely connected to the soils ability to infiltrate water, and it is therefore difficult to predict the reduction of stormwater without conducting field investigations. To be able to investigate the effect of downspout disconnection, and as a tool for making it easier to see if an area is suited for this measure or not, it has therefore ben established a MATLAB- model, in this thesis, which computes the amount of infiltrated water and the amount of generated runoff at each site. The MATLAB- model calculates with both using the Green-Ampt infiltration model and the Philip infiltration model, and was used to calculate the amount of infiltration and runoff at eleven sites in Oslo and four sites in Trondheim. In Oslo, the results showed a variation in the amount of infiltrated water. The best conditions for downspout disconnection was seen at site B28, L34, R44, R5 and S75. At these sites it is suggested that the soil is capable to infiltrate the amount of generated stormwater without further measures. The sites in Trondheim showed all great conditions for infiltration, but because of the size of the infiltration area and the location, was site 3 and 4 preferred. To be able to see how different soil parameters affects the amount of infiltration, some simulations where done by changing some of the soil parameters. These simulations showed that the amount of infiltration is dependent on the soils saturated hydraulic conductivity, the initial soil moisture content and the wetting front capillary suction head. Because of this dependence, it is recommended that these parameters are measured at the site, and it is important that especially the Ksar- value is measured at multiple points at the site in order to obtain a representative value. By comparing the soil types at the sites and the ratio between the size of the roof area and the size of the infiltration area, the following is suggested. If the soil at the site is sandy and the infiltration area is one to twice as big as the roof area, it can be assumed that the infiltration capacity is good enough to infiltrate the amount of generated stormwater. Silty soil covers however a rather large range of Ksat- values depending on the percentage of silt in the soil, and the Ksat- value of the site should therefore be measured. Dependent on the soils ability to infiltrate water, it may be enough that the infiltration area is one to twice as big as the roof area, but this should be considered especially if the soils saturated hydraulic conductivity is of the lower value. The same yield also for a soil with a very high clay content. Whether this soil has a large enough infiltration capacity or not is dependent on the percentage of silt and sand fraction in the soil. The calculations done in this thesis shows also that for a site with a very high clay content, and thus a low Ksat- value, it is recommended to use an infiltration area three times or more, as big as the size of the roof area. The establishment of this model will help to develop downspout disconnection as a measure for stormwater managing, especially in residential areas. The results obtained in this thesis, shows that by disconnecting the downspouts from the sewer system, the reduction of amount of stormwater can be significant, where the soil properties are adequate. Downspout disconnection is therefore evaluated as a measure that should be considered in areas where stormwater management is a challenge.