Assessment of the influence of the hydroaccumulation layer on the initial moment of rainwater runoff from the green roof structure

. The article deals with the assessment of the influence of the hydro-accumulation layer on the initial moment of rainwater runoff from green roof. The authors deal with a comparison of two basic types of green roof compositions, where they assess the initial moment of rainwater runoff following the use of a hydroaccumulation layer.


Introduction
Together with colleagues, we verified and compared the efficiency of the hydroaccumulation layer in the composition of the green roof.The goal was to find out how much influence the hydroaccumulation layer has on the runoff conditions of the roof.We were led to the idea by the reconstruction of the green roof on Bratislavská Street (fig.1), which had to be carried out precisely because of the absence of this layer and the negative effects caused by this absence.These were mainly visual shortcomings, when no greenery took hold.This led us to create two models on which we performed measurements using simulated rain.The original goal of the measurement was to determine whether the hydroaccumulation layer will significantly change the rate of rainwater runoff from the roof.However, this was not confirmed.We obtained but two additional values from the experiment.We will deal with these values in the following lines.

Preparation and implementation of the test roof
For the purposes of the experiment, we assembled two identical experimental roofs in the AdMaS research area.The existing roof structure made of wooden beams, OSB boards and waterproofing made of modified asphalt strips was used as the basic building element of the experiment.OSB boards were used for the formwork of the model green roofs, which were stabilized with lumber struts.The material was also used for the composition of the green roof.Geotextiles, retention plates Aquadesk Retex and substrate, as we can see in Fig. 2. Lumber, garden hoses, spray valves and an adjustable water flow meter were used on the structure to simulate rain.The measurement itself took place on an electric scale and plastic tubs were used to catch the water.The construction for rain simulation consisted of a simple grid made of lumber with a square plan of 1.2 m in length, as we can see in Fig. 4. A garden hose was placed on the grid, in which spray valves were placed, so that the surfaces of the test roofs were contained by a uniform intensity of the test rain.An adjustable flow meter was placed at the inlet end of the hose.In the last row, an electric scale was prepared.This was placed under a gutter, from which rainwater was led during the experiment into a tub, which was placed on the measuring device.The display of the scale, which we see in Fig. 5, was placed next to a stopwatch and a watch, in order to monitor the temporal continuity of the experiment.

The course of the experiment
The experiment was started by placing the rain simulation structures on the formwork and starting the water flow.The rate of simulated rain was based on 27 l/m²/ for 15 minutes.For an area of 1.44 m², the flow rate was therefore 2.59 l/min, as we can see in Fig. 6.The total amount of rainwater was 38.88 L for each roof tested.In Fig. 6 shows the moment of the beginning of the experiment, when both roofs were subsequently exposed to fifteen minutes of rain.The moment we started the simulated rain, the measurement of the volume of water in relation to the elapsed time was started.The value of runoff rainwater was recorded continuously.

Original data
As we mentioned at the beginning, the primary measurement was to confirm or disprove the thesis that the hydro-accumulation layer will significantly reduce the rate of rainwater runoff.However, the experiment did not confirm this thesis.The flow rate did not differ for most of the time, a significant difference occurred only between 11 and 13 minutes.

Data including the initial flow moment
However, while working with the measured data, we observed and recorded another data.
From the graph below, it can be read that the initial moment of roof runoff without a hydroaccumulation layer is at 2:25.The initial moment of runoff of the roof with hydroaccumulation layer is 4:50 after the start of the simulated rain.For clarity of the experiment, we converted the measured weight of rainwater to volume in the ratio 1kg = 1 l.

Results
We can see a significant difference compared to the first interpretation of the data right at the beginning of the graph.In this case, the measuring time was started at the same moment when the 15 minutes of artificial rain started.In contrast to graph 1 (fig.8), when the measurement was started only at the moment when the rainwater started to flow away.In graph number 2 (fig.9), we can therefore observe the values of the initial moment of runoff, when the rainwater began to flow from roof no. 1 at 2:25 and at roof no. 2 at 4:50.The difference in the amount of rainwater runoff increases for almost the entire 15 minutes of simulated rain.At minute 13, the increase slows down slightly, but still reaches a difference of 8.2 l.After 15 minutes, the difference starts to decrease, and after less than 35 minutes, it stops at 1.9 l.At this time, the experiment is finished because no more water flows out and retroactively, no detention is observed either.

Conclusion
Compared to the original intention of the measurement, the real differences between the roof without the hydroaccumulation layer and the roof with the hydroaccumulation layer were highlighted due to the initial moments of the outflows.In the original measurement, where we examined only the speed of water outflow, the biggest difference was 2.44 l in 13 minutes.
After taking into account the delay caused by the initial flow moment, the largest difference on 1.44 m2 of experimental roof is 8.2 l between 13 and 15 minutes.This interpretation of the data offers a more realistic model of possible behavior than the originally intended interpretation.A more pronounced difference in the speed of rainwater runoff confirms the ability of the hydroaccumulation material to slow down the initial runoff of rainwater and thereby more evenly load the use of rainwater drainage.From the measurements and working with the data, it was also found that none of the tested roofs had any detention, i.e. a condition where even the smallest amount of water would run off after a long time (hours) after the end of the rain.
Working with data and measured values, we came to the conclusion that even if the hydroaccumulation layer does not significantly change the course of rainwater runoff, it does significantly change the initial moment of runoff.This can, for example, result in a more even distribution to the storm sewer in a development where there is partial use of green roofs.

Fig. 2 .
Fig. 2. Constructing of experimental roof.On the already prepared roof, we assembled formwork from OSB boards and lumber, which we arranged in two identical squares with an area of 1.44 m².We placed a geotextile on the waterproofing asphalt layer in the formwork.Next, we placed a substrate on model roof No. 1. (Tab. 1) After the geotextile, a hydro-accumulation layer of retention plates was placed on sample roof No. 2 and a substrate was placed on top of them.(Tab.2) The resulting state is shown in Fig.3.

Fig. 9 .
Fig. 9. Graph presenting results including the initial flow moment.
Fig. 3. Completed experimental roof with final layer of substrate.