The impact of greening measures for private gardens on flood prevention

: Leeuwarden as a case study

Abstract

Private gardens are an important part of the urban landscape. A significant part of these gardens is paved, indicating a substantial potential for greening. Due to the changing climate, private gardens can play a crucial role in climate adaptation within urban areas. Therefore, municipalities committed to the National Delta Programme 2023 (DP 2023) are implementing measures to encourage residents to green their gardens. The question is to what extent the greening of private gardens can contribute to preventing flooding in urban areas.
This study examined the city of Leeuwarden as a case study to investigate the measures implemented by the municipality to encourage residents to green their gardens, as well as the effects of these measures on water infiltration, runoff, and their impact on urban flood prevention. A mixed-methods approach was used, combining policy analysis with practical experiments.
The policy analysis was conducted through a desk study focusing on garden greening measures, types of policies, levels of governance, and involved stakeholders. To gain further insights into the policies, interviews were conducted with municipal professionals to verify, contrast, or gain new insights beyond the information in the policy documents. Using the Open University’s Lime Survey tool, a survey was conducted to understand the motivations and experiences of private garden owners who recently received a subsidy for garden greening. The survey results were analyzed using a garden greening behavior model. Soil samples were collected, and infiltration tests were performed with a double-ring infiltrometer to determine the infiltration rate of the soil in three private gardens. Mapping of the private gardens was carried out using various public GIS maps and the opportunity map from the organization Steenbreek. This involved determining the number and total area of private gardens in Leeuwarden, as well as the average percentage of paved and unpaved areas in these gardens. The Horton equation combined with the Antecedent Precipitation Index (API) method was used to calculate infiltration rates and runoff. Additionally, the infiltration capacity and runoff of all gardens in Leeuwarden were also calculated.
Four rainfall pattern scenarios have been developed, correlated with the KNMI'23 climate scenarios “high emissions (H) and “wet” (n), to calculate the infiltration rate and runoff. The chosen scenarios include short, intense rain showers primarily occurring in summer (scenario Hn-su); moderate rainfall typical of spring and autumn (scenario Hn-s/a); prolonged rainfall mainly observed in winter (scenario Hn-wi); and extreme rain events (scenario Hn-ex), which are expected to become more frequent due to climate change. These precipitation intensities reflect both past patterns and those expected in the future.
Measures to promote the greening of private gardens are included in various policy documents. However, the municipality of Leeuwarden does not have a specific policy focused on the greening of private gardens. Policy analysis shows that the policy to encourage residents to green their gardens does not prevent more paving from being installed in gardens. It is estimated that net paving in gardens will increase. Additionally, subsidies are not a primary motivation for garden greening. Instead, factors such as the attractiveness of green gardens, contributing to a better living environment, concerns about climate change, water drainage, and biodiversity influence residents' decisions to green their gardens. Furthermore, it emerged that residents are willing to collaborate with the municipality to establish a neighborhood initiative for garden greening. However, they emphasized that the municipality should take the initiative, handle the organization, and guide the process, as it requires significant time and effort from the residents. Approximately 20 % of the city of Leeuwarden consists of private gardens. Of the total area of gardens, 58 % are paved and 42% are green, indicating a greening potential towards 30% of the private gardens.
For experimental measurements of three private garden plots, infiltration and runoff were determined. Different soil types were identified in the gardens, such as clay and permeable topsoil. Infiltration varies due to soil type and water retention, measured by drying time after infiltration. Cambuursterpad and Aert van der Neerstraat show better infiltration due to higher initial infiltration rates and permeable topsoil. In contrast, De Gronzen has lower infiltration because of the clay layer, which reduces absorption. As a result, the garden at Cambuursterpad has an average runoff percentage of 60.85% in scenario Hn-su, 14.6% in scenario Hn-s/a, 20% in scenario Hn-wi, and 87.06% in scenario Hn-wi. The garden at Aert van der
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Neerstraat has an average runoff percentage of 63.5% in scenario Hn-su, 0% in scenario Hn-s/a, 0% in scenario Hn-wi, and 87% in scenario Hn-wi. The garden at De Gronzen has an average runoff percentage of 95.5% in scenario Hn-su, 26.7% in scenario Hn-s/a, 20% in scenario Hn-wi, and 97.7% in scenario Hn-wi. It is concluded that scenarios Hn-s/a and Hn-wi reduce the pressure on the sewer system and runoff into water bodies. This shows that the greening of private gardens has a positive effect during prolonged and less intense rainfall, particularly in terms of water infiltration and groundwater replenishment. Greening gardens is therefore beneficial in addressing the increasing drought caused by climate change. Storing more water in the soil creates an important buffer that can be used during periods of water scarcity. However, during heavy rainfall, infiltration in the green areas of these gardens is limited, particularly in the garden of De Gronzen, which leads to runoff and increases the risk of flooding, especially when combined with paved surfaces. This means that the greening of private urban gardens will only contribute to flood prevention to a limited extent. Paved areas consistently cause excessive water runoff.
By following the Steenbreek greening guidelines, 26% more rainwater is infiltrated in scenario Hn-s/a and 3%, while scenarios Hn-su and Hn-ex show an additional 3% to 8% infiltration. In scenarios Hn-su and Hn-ex, runoff in the unpaved area increases by 22% and 27%, respectively, and in scenarios Hn-s/a and Hn-wi by 4% and 5%. Meanwhile, runoff from the paved area is reduced by 30% across all scenarios.
It is concluded that the balance between paved and unpaved surfaces in urban private gardens is crucial for water management. Large paved areas prevent infiltration and increase rainwater runoff. Green, unpaved areas promote infiltration and water retention, reducing runoff. Soil type plays an important role: permeable topsoil allows quick infiltration, while clay soil absorbs water slowly, which can lead to flooding during heavy rainfall.
The following limitations were identified during the analysis of garden plot numbers and the geo-distribution in connection with soil type and its water retention curve: potential bias in survey responses, the double-ring infiltrometer test, limitations in measuring runoff, and calculating the greening potential. Additionally, further research is suggested on: alternatives to garden greening, such as the creation of green ditches (wadi’s), research into varying soil conditions and addressing water runoff issues, and improving infiltration capacity.

Date of Award	1 Feb 2025
Original language	English
Supervisor	J de Kraker (Examiner) & Angelique Lansu (Co-assessor)