**1. Introduction**

Urbanization and climate change e ffect the water balance in our cities, resulting in challenges such as flooding, droughts, and heat stress. The implementation of Sustainable Drainage Systems (SuDS) or small-scale Nature-Based Solutions (NBS) can help to restore the water balance by capturing, retaining, treating, and infiltrating stormwater that runs o ff roofs and impermeable surfaces and potentially into the subsurface [1–5]. This will contribute to minimizing flooding, restoring groundwater levels, increasing soil moisture to alleviate drought impacts, and lowering temperatures by evapotranspiration to mitigate heat stress [2,6–11].

As Wakode et al. [12] point out, the urban water cycle is di fferent from that in non-urban areas, where urbanization can influence natural groundwater recharge due to the restriction of infiltration by impermeable surfaces. Even though leakage from water-wastewater infrastructure is known to recharge the groundwater in cities [13,14] this was not substantial enough to recharge and stabilize

the groundwater levels under the UNESCO World Heritage site Bryggen Wharf in Bergen, Western Norway (Figure 1) [15–18]. Therefore, the connected infiltration system at Bryggen was intentionally built for that purpose (Figure 1).

**Figure 1.** The Medieval city in Bergen is located along the shore of Vågen bay.

The sustainable infiltration and drainage system that has been implemented within the premises of the Bryggen is the largest in Norway (Figure 3). It was built with the purpose of raising and stabilizing the groundwater level and increasing the soil moisture in the cultural heritage layers in the ground below Bryggen [19–22]. The infiltration system has proven its effectiveness for raising the groundwater level to desired levels for preservation [16,22]. However, the infiltration system has not been full-scale tested for its infiltration capacity and interaction with the groundwater below. Such testing of SuDS is commonly executed with small-scale infiltrometer tests [23–25] and further upscaled in modeling tools. However, small-scale testing, such as Modified Phillipe–Dunne (MPD) infiltration tests, does not give a picture of the overfall infiltration capacity of the SuDS. Therefore, a full-scale methodology was first implemented for testing impermeable pavements [26,27] and further used for other infiltration systems such as rain gardens, swales etc. [28–30].

A full-scale infiltration capacity test at Bryggen will contribute to the understanding of the urban water cycle, by quantifying the hydraulic conductivity and infiltration capacity of SuDS, the connectivity to groundwater levels, and thus the overall effectiveness of this system in a larger hydrological and hydrogeological context. SuDS in cold climates require higher infiltration capacity than warm climates to maintain functionality below 0 ◦C [31,32]. Bryggen is a unique site due to its 35 boreholes with continuous measurements of groundwater level, soil moisture, oxygen content, and other parameters that are essential for the in situ preservation of the cultural heritages below surface. The subsurface data were continuously collected from 2007, gradually expanding with additional boreholes until 2015 and will continue monitoring onwards [19,21].

At present, only a limited part of the catchment area is connected to the infiltration systems (Figure 2). The municipality inquires if the capacity of the rain gardens is acceptable to connect the entire catchment area with stormwater from both roofs and streets to the system. Therefore, the rain gardens were assessed to determine if they work as designed, and if the infiltration capacity and the effectiveness is satisfactory to preserve the cultural layers and thereby enlarge the connected runoff area. The implementation of SuDS at locations where the infiltration of water is a challenge, such as on cultural layers, is a challenge for urban planners, water authorities, and other stakeholders in municipalities. This paper will describe the full-scale infiltration method [26,28] used for testing the hydraulic infiltration capacity of the rain gardens at Bryggen and the response on the groundwater level measured in several monitoring wells.

**Figure 2.** The catchment area and locations of boreholes.

#### *1.1. Study Area*

The infiltration system at Bryggen Wharf is located in the Medieval City center of Bergen, the largest city on the west coast and second largest city in Norway (Figure 1). The average temperatures are 23.8 ◦C in summer and −4.7 ◦C in winter, giving an annual average temperature of 8.6 ◦C. During 61 years of data collected, only 17 winters had temperatures below 0 ◦C [33]. Bergen is one of the wettest places in Europe, with an annual precipitation of 2250 mm/year [33]. The topography is steep hillsides covered with forest vegetation on scares soil, down to flat laying formerly shorelines with thicker natural sediments and anthropogenic layers (Figure 1). The relief goes from 320 m above sea level to 1 m a.s.l. over a distance of 1 km. These natural conditions make surface runoff water abundant.

Bryggen is a Hanseatic Wharf where several of the buildings originate from 1702 [34]. The Medieval city, located along the shore of Vågen bay, is to a large degree built on anthropogenic waste including remains from city fires and industrial and household waste. These have accumulated into abundant anthropogenic cultural heritage layers rich on organic content that locally are more than 10 m thick [19,21]. The reduction of soil moisture or lowering of the groundwater level will introduce oxygen into the organic matter. This will accelerate the oxidation and disintegration of the organic material causing collapse and compaction of the organic layers in the subsurface [17,20]. Due to the slow decay causing damage to the Wharf, the Bryggen project was initiated in 2010 [17,20,21]. The abovementioned processes will further cause subsidence of the ground and damage on buildings and infrastructure [18,35].

The ground beneath Bryggen is characterized by a steeply sloping mountain side, with depth to bedrock from 2 to 12 m. The layers consist of up to 10 m of organic, anthropogenic deposit as described above, on top of beach sand and moraine of ca. 2 m thickness. The recharge of the groundwater is primarily by runoff from the uphill catchment area [16,36]. A 3D hydrogeological model of Bryggen and its subsurface has been made to understand the groundwater movement, hydrogeological characteristics of the subsurface layers and processes linked to water, or the lack of water [16,17,20,36].

A monitoring system was established in 2001, which was expanded in 2010 with a total of 35 monitoring wells [37]. Initially, this network of monitoring wells was placed to understand the complex flow system in the area and to identify the causes of the local groundwater levels and observed, increased subsidence rates [16,18,29,35]. An automated groundwater-monitoring system was installed in the wells, for high frequency of measurements. Some boreholes are dedicated to measuring parameters for archaeological purposes [17] while other boreholes are continuously monitored for groundwater levels [21], using equipment such as Schlumberger Micro diver DI 501 [38]. During the Bryggen project a strong link between the level and stability of the groundwater and the decay of the cultural layers was established [16,17,20,22,39]. Therefore, an infiltration system was installed in the ground in and around the Wharf to infiltrate as much surface water as possible into the ground (Figure 3) [18,29,40]. All measures at Bryggen, including monitoring wells and SuDS, were directed towards raising and stabilizing groundwater levels. The long-term goal for the area was to elevate groundwater levels to about 1 m below the surface [16].

**Figure 3.** Top: Sketch of the infiltration system at Bryggen (Drawing: Multiconsult AS). Bottom: Cross section of rain garden, swale, and permeable pavement (modified from de Beer & Boogaard [40]).

#### *1.2. The Rain Garden at Bryggen*

The rain gardens in Bryggen are a bioretention system that allows runo ff to temporarily pond in a shallow planted depression before filtering through vegetation, roots, and underlying soils for infiltration [2,3,41]. The rain gardens have the following primary functions: infiltration, storage, and purification. The catchment area, indicted in Figure 2, is upstream of the main street "Øvregaten" (Figure 2), which is salted during winter to reduce icing and tra ffic incidence. Plants in the rain gardens are not salt tolerant [29]. To avoid excess salt from the winter salting, water from the watershed is collected in a manhole on the other side of the street and brought in a pipeline underneath the main street to a manhole connected to the rain gardens, as indicated in Figure 3 and with blue points in Figure 2. The infiltration system has two inflow points from the catchment area: into the rain gardens and into the tank, as indicated in Figure 3. Figure 2 shows the current connected area for surface water (blue line) and the total upstream catchment area (red line) for the rain gardens and infiltration system.
