Avrinningskoefficienten – dess relation till regnintensitet och bidragskoefficienten utifrån fallstudier i södra Sverige

Sammanfattning: In order to increase the knowledge regarding how the runoff coefficient varies with increased rainfall intensity, a literature study and calculations in Excel were conducted. The background to how the tabled values of the runoff coefficient have been determined was found to be a book based on an assembly of answers to a survey collected from different companies in USA. The book is called Design and construction of sanitary and storm sewers and is written by the American Society of Civil Engineers (ASCE) and Water Pollution Control Federation (WPCF) in the 1960’s. The study involves seven catchment areas in Sweden, four of which were smaller than 20 hectares, one a tad larger than 20 hectares and two much larger areas. The rational method was utilized in order to study how the runoff coefficients varies with increased rainfall intensity, both mean and peak intensity. Since this method requires the area to be less than 20 hectares, only the five smallest areas were used in this part of the study. For the three largest areas, the study regarded how the contribution coefficient varied with increasing rain volume instead. Where the contribution coefficient was estimated as the percentile of a rain event that generates surface runoff. The resulting graphs over how the runoff coefficient varies with increasing rain intensity showed no significant trend. This does not support the theory where the runoff coefficient increases with increasing rainfall intensity. One must therefore wonder if the utilized method isinadequate. The rational method is simplified as it assumes that the whole catchment area is active during every rain event which is not necessarily the case. Furthermore, the rainfall intensity is assumed constant over the whole area and rain event which rarely is the case in reality. Apart from this the rain gauges, used to collect the data in this project, only collects data in the exact spot it is located and they were not always close to the flow measurement for the area in question. Based on this, the rain data does not necessarily correspond with the actual rain event in the area. Besides this the flow could have been affected by the surrounding environment, leading to a registered flow lower than it should be. There was no clear trend in the graphs visualizing how the contribution coefficient varies with the rain volume. When studying the relationship between the total flow and the rain volume a positively linear trend was found. The weighted runoff coefficient was calculated for the seven catchment areas. These values were compared with the graphs displaying how the runoff coefficient varies with the rainfall intensity for that same area. It was noted that a few of the values in the graphs were higher than the corresponding value of the weighted runoff coefficient which implies that the values of the tabled runoff coefficients may be too low. Since no origin of the runoff coefficient has been found one possibility is that it is the same as the contribution coefficient. One way to estimate the value of the coefficient is to perform laboratory tests in large scale were different types of surfaces and angles are being exposed to artificial rains. It would be preferable if different types of surfaces obtained interval values were the higher number would represent extreme rainfall events. In future studies it is also of importance to evaluate how the rainfall frequency and duration will affect the surface.