## 3 Results
### Reductions in Runoff
An ANCOVA was run to determine if there was a difference between flow level detected in two manholes either side of the SuDS features, after controlling for variation in rainfall during the monitored period. After adjusting for rainfall, there was a statistically significant difference in manhole flow level either side of the SuDS interventions, F(1, \(125\)) = \(400.231\), \(p<0.0005\), partial \(\eta\)2 = \(0.762\). Data collection in manhole 1 (on the adjacent street) was not collected for the full duration of this period: only 9915 points were collected between 18 th September and 22 nd October 2018 (53.8% data completeness). This street did not have SuDS tree pits installed, but did have some SuDS features such as raingardens and permeable paving. The mean flow level in manhole 1 was \(148.2\pm 10.4\) mm for this period. In the second period, between 18 th January and 25 th February, 2019 (Figure 8B) a paired-samples t-test revealed the level in manhole 6, upstream of the SuDS trees, was statistically significantly higher (\(221.2\pm 3.3\) mm) than the level in manhole 7 (\(154.8\pm 20.8\) mm), which was downstream of the SuDS trees, t(\(38\)) = \(20.114\), \(p<0.0005\) (Table 1). The assumption of normality of data was not violated for manhole 6 (\(p>0.05\)) (Figure 9C), but was violated for manhole 7 (\(p\) = \(0.001\)) (Figure 9D), as assessed by a Shapiro-Wilk's test. Whilst the paired samples t-test is robust to violations of normality, a non-parametric alternative (the Wilcoxon signed-rank test) was carried out. The distribution of differences (Figure 10B) was not symmetrical violating one of the Wilcoxon signed-rank assumptions. As such, a related-samples sign test showed that the median flow level in the manhole downstream of the SuDS trees was statistically significantly lower (\(146.1\) mm) than the median flow level in the manhole upstream of these interventions (\(221.6\) mm) during the second period (\(39\) days, \(18\) January-25 February), z = \(-6.085\), \(p\leq 0.0005\) (Table 1). An ANCOVA, which controlled for variation in rainfall during the monitored period, showed there was a statistically significant difference in manhole flow level either side of the SuDS interventions, during the second monitored period F(1, \(75\)) = \(381.315\), \(p<0.0005\), partial \(\eta\)2 = \(0.836\). Whilst these results are promising, they are indicative of a \(25\)-\(30\%\) reduction in flow level between the SuDS features, and not causal, as the flow measurement in manhole 7 (downstream of manhole 6) was not measured prior to the installation of the SuDS interventions. This highlights a need for gathering baseline data ahead of installing interventions to truly evaluate their impact. The gaps in the data also highlight the difficulties in deploying monitoring kit to evaluate \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Time Period** & **20 August–22 October 2018** & **18 January–25 February 2019** \\ \hline  & **Manhole 6** & **Manhole 7** & **Manhole 6** & **Manhole 7** \\ \hline Data completeness (\%) & 98.5 & 99.2 & 96.7 & 71.2 \\ Mean flow level (mm) & \(163.9\pm 15.5\) & \(124\pm 3.8\) & \(221.2\pm 3.3\) & \(154.8\pm 20.8\) \\ Median flow level (mm) & \(159.5\) & \(123.3\) & \(221.6\) & \(146.1\) \\ Paired sample t-test & t(\(63\)) = \(18.125\), \(p<0.0005\) & t(\(38\)) = \(20.114\), \(p<0.0005\) \\ Related samples sign test & z = \(-7.875\), \(p<0.0005\) & z = \(-6.085\), \(p<0.0005\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of the statistical analysis of the differences in flow level (mm) recorded by the ultrasonic sensors upstream (manhole 6) and downstream (manhole 7) of the SuDS tree pits. SuDS interventions in urban retrofit scenarios, due to both the physical environment and the way in which connections can behave unpredictably.