4.1. Precipitation Events and Time for Regeneration of the LECA-Based Roof Storage Capacity
The event durations and precipitation depths varied considerably. Of the selected highest-intensity events, the shortest lasted 8 min in July, and the longest lasted 122 h in October. This presents a widespread range; therefore, the median value of 8.2 h might be more representative. In terms of total precipitation depths, the events ranged from 0.5 mm to 85.4 mm. The total duration of precipitation events and dry periods were determined. Assuming the 6 h ADWP, precipitation occurred 22% of the time during these eleven months. This leaves 78% of the time (dry period) for the regeneration of the roof storage capacity, considered as time between events.
The precipitation at Høvringen (11-months dataset) was compared to the Risvollan stations, located 82 m a.s.l, at an areal distance of 7 km, due to the unavailability of Intensity–Duration–Frequency (IDF) curves at Høvringen. For the observation period, there was 8% more precipitation recorded at Høvring than at Risvollan. The higher elevation of Risvollan makes this difference expected. Comparing the observed events at Høvringen with the IDF curves from the Risvollan station, one can see that the all the events fall below a 2-year return period, with the exception of one from August 19, which lasted more than 1 day (Figure 2
). The figure also shows a zoom to a 30 min resolution, where eventual rapid storms can be found. However, they registered low precipitation depths.
The mean event intensities ranged from 0.1 mm/h to 25.6 mm/h. Particularly, the maximum mean intensity from July 26 that lasted 8 min exceeded the rest of the values; however, it is still below a 2-year return period event.
4.2. Retention Performance
The total rainfall-runoff rate can be observed between 23 January 2017 and 30 November 2017. During this period, the precipitation gauge measured 937.6 mm precipitation. The runoff depths during the examined period were 912.1 mm and 852.4 mm for the black and LECA-based roofs, respectively (Figure 3
, Table 1
). This indicated a discrepancy of 59.7 mm (black vs. LECA-based roof) and 85.2 mm (precipitation gauge vs. LECA-based roof), which is the evaporated volume. Overall, the difference between the precipitation and runoffs was a 3% volume reduction by the black roof, and 9% for the LECA-based roof.
The seasonal variations are shown in Table 1
. The 11 months were divided into four groups: November, January, February, and March represent the winter period; April, May, and June represent the spring period; July and August represent the summer period; and September and October represent the fall period. The winter period confirmed zero evaporation during the cold climate condition. The minor negative difference between the black roof and the precipitation gauge can be attributed to signal noise and measuring uncertainties. The snowfall measurements are most likely the most significant here. Evaporation in the spring season exceeded values from the summer season (Table 1
). This could be explained by an alteration of rainfall patterns in the summer season, when higher intensity rainfalls occur, resulting in decreased retention.
The runoff coefficients calculated from total precipitation and runoff records were 0.97 for the black roof and 0.91 for the LECA-based roof. The normalized daily retention estimated from total precipitation and runoff measurements was 0.27 mm per day. This reflects the fact that the climate in the Trondheim region is relatively cold and wet. The detention in the expanded clay aggregate (LECA), followed by a subsequent slow drainage of the system, was much greater than the evaporation loss rate.
4.3. Detention Performance
The detention capacity of the system was evaluated using peak flow delay and peak flow reduction. Evaluating the LECA-based roof using a wide range of performance indicators for all events indicated that the performance was mainly influenced by duration, intensity, moisture content, and ADWP of the individual events. At the same time, the detention indicators were highly sensitive to the chosen subsets of the rainfall dataset which was used in the calculations. Therefore, the eight events with the highest 5-min peak intensity were selected for detailed examination. Figure 4
illustrates the eight largest events, ranging in duration from 8 min to almost 3 days, with depths of 3.2 mm to 59.6 mm (Table 2
). Additionally, the largest snowmelt (event 24) and rain-on-snow event (event 27) were included to show the different types of events observed on the roofs. For event 24 (the pure snowmelt event), there was a negative lag time delay between the black and the LECA-based roof. This can be explained by the observed temperatures in the LECA-based roof, which were more stable compared to the black roof. The black roof was typically colder than the LECA-based roof, meaning that higher net radiation was needed to initiate snowmelt. Though this was the largest snowmelt event, it was a relatively small compared to the other events in Table 2
and Table 3
, making the peak flow reduction less relevant. Event 27 (the rain-on-snow event) is very difficult to evaluate without knowing the mass of snow on the roofs at the onset of rain. It is not possible to compare peak lag times, as it is possible that the snowmelt initiated prior to the precipitation runoff. A complete mass balance would be needed of the initial snowfall until it was completely melted again. Due to these constraints, these two events were excluded from the comparisons in Table 2
and Table 3
Two alternative precipitation intensities were presented. The mean intensity indicates the ratio between the depth and duration, whereas the peak 5-min intensity is the peak intensity measured over 5 min. Event 45 turned to snow at a later stage, followed by a snowmelt in the end, and therefore the event type was classified as mixed. However, this event was used in the final evaluation, since the peak 5-min intensity and corresponding runoff occurred during “rainy” conditions. Event 80 was quite untypical in terms of mean intensity, which was more than ten times higher than the remaining events. With respect to the return period, event 89 was the only one that fell between the 2-year and 5-year return period (Figure 2
summarizes responses of both black and LECA-based roofs for the eight events. Here, individual events were characterized by comparing the maximum (peak) values registered from the roofs. Overall, the peak delay totaled 1 h and 15 min in median and 7 h and 23 min in mean. A long dry period before the events naturally led to the freeing of the storage capacity. However, this does not necessarily mean ideal conditions for delaying peak runoffs, as can be seen for event 95. On the contrary, the short dry period led to a sufficient delay for event 45. A focus on maximum values was not always the best solution when evaluating the runoff delays. One can see very long delays for the long-duration events 62 and 89. Event 62 even experienced two heavier rainfalls, which obviously led to two responses. Because of this, two alternative solutions of peak delays were suggested (Figure 5
and Figure 6
In terms of peak reduction (Table 3
), the roof demonstrated a high efficiency, with a reduction rate of 80% to 97%, irrespective of the length of the antecedent dry period or the degree of previous saturation. The latter indicates the extent to which the voids in the expanded clay aggregates are filled with water. The saturation measurements ranged between 31% and 61% for the period after which the sensors were installed. In addition to the degree of saturation, the initial runoff may also be used as a performance corrector or predictor. Higher initial runoffs correlate to higher degrees of saturation in the media from previous events.
There were large differences in the runoff duration between the examined roofs. The median runoff duration of the LECA-based roof (31.5 h) lasted 2.2 times longer than of the black roof (14.5 h). Considering average values, the mean runoff duration of the LECA-based roof (42.5 h) lasted 1.6 times longer than of the black roof (26.5 h).
serves to recognize centroid delays of individual runoffs. The steep rises in cumulative runoffs associated with the black roof response after intense rainfalls may be considered a potential cause of rapid floods. One can see that the LECA-based roof transformed these rises to either flat (effective detention performance) or—in extreme cases—gradual runoffs. The extreme cases may be seen in events 45, 62, and 89. Additionally, Figure 5
shows the largest snowmelt and rain-on-snow events; however, these were different types of events, shown only for comparison. Further calculations include only the eight largest events, as outlined in the methods.
Considering shorter ADWP between events would result in a larger number of individual events. For instance, event 62 needed a 3 h ADWP to separate the event into two parts. Events 89 and 92, sharing a similar pattern, needed shorter breaks within the rainfall dataset (e.g., approximately 5–10 min), as there was continuous rainfall during this whole event. Using shorter breaks in the rainfall dataset would underestimate the total performance. For instance, the peak delay would change dramatically to 1 min (median) and 28 min (mean). Considering the centroids as the representative values, the peak delay counted 6 h (median) and 5 h and 51 min (mean).
presents a comparison of the alternative methods for runoff delays. The centroid delays were influenced by the short event 80, with small rainfall depth lasting only 8 min; this obviously led to a shortening of the centroid delay as well. During events 28 and 95, the quick responses can be clearly seen by the LECA-based roof runoffs, compared to the black roof peak runoff at the very beginning. This was probably caused by water collecting directly in the outlet (0.25 m2
). Therefore, those peak runoffs should not be considered as real responses of the LECA-based roof. Overall, the peak reduction presents 95% in median and 92% in mean.