Pluvial flooding
As described elaborately in Sections 1.1 and 1.2, the KNMI projects substantial increases in average precipitation and temperature for the period 2070-2100. In ad- dition, most scenarios show that especially precipitation and temperature extremes are likely to be amplified as a result of climate change. Predictions for sea level rise and increases in river discharge are not considered here as the scope is limited to pluvial flooding only.
The choice for a certain precipitation event depends on the norm employed and what level of water hindrance is deemed acceptable. It is important to make a distinc- tion between mild water hindrance where rainwater is temporarily stored on streets and excess ponding occurs, and large-scale inundation resulting from extreme pre- cipitation causing lasting damage to property and impeding public life. The former is naturally accepted to occur more often than the latter. RIONED has attempted to compose norms regarding pluvial floods in Dutch neighborhoods (RIONED, 2006). Their guideline advises to use two separate precipitation events to assess whether a neighborhood is able to cope with extreme precipitation. An event with a return period of 2 years is recommended to assess the sewage system’s capacity. This event should not cause large-scale problems and no excess water should be stored on the streets. The typical precipitation event used for this purpose under the cur- rent climate is dubbed “bui 08” (“Shower 08”). Its intensity varies within one hour, but the total amount of precipitation is 19.8 mm. Additionally a precipitation event with a return period of 100 years should be forced to gauge vulnerable locations and estimate large-scale damages. Here the choice is adopted to use precipitation events with return periods of 2 and 100 years as climate forcing. This ensures an assessment for both relatively frequent hindrance due to rainwater stored on streets and large-scale flooding.
In their recent work aimed at enhancing civil participation in climate adaptation endeavors, Kluck et al. (2017) pay much attention to climate adaptation criteria and precipitation events that can be used to assess climate adaptiveness. They acknowledge that there is no single standard precipitation event which can be used
across Dutch cities as each event is different and cities respond differently to ex- treme precipitation. However, they do underline the notion that precipitation events are defined by their return periods, duration and intensity. For the purpose of sim- ulation urban pluvial flooding, Kluck et al. (2017) propose the use of an event that lasts one hour. Water travels about 360 m (at 0.1 m/s) during that period so that it can reach surface water. The forcings here therefore have a duration of one hour.
There is no standardization as to the type of forcing of precipitation events in stormwater modeling and STOWA does not have estimates of typical precipitation events in 2085 with a return period of 2 years (only for 10, 50 and 250 years). Current practices to take into account climate change in stormwater modeling vary greatly from applying a single factor to the current precipitation event to formulating own rules. The KNMI found that the total amount of precipitation in 2014 can be scaled by factors of 1.61, 1.65 and 1.67 respectively for the return periods 10, 50 and 250 years to obtain the precipitation amount projected for 2085 (Klein Tank et al., 2015). STOWA (2018) has worked with several other organizations to estimate various precipitation characteristics for the long term. The amount of precipitation (in mm) for several precipitation events (defined by a return period and precipitation duration) is shown in Table 2.2.
Here it was decided to apply the current Shower 08 (current return period 2 years) scaled by a factor 1.65, in line with the scaling of other precipitation events. This yields a precipitation event with a 2085 return period of 2 years and a total amount of 33 mm. To obtain a precipitation event with a return period of 100 years and a duration of 1 hour, linear interpolation was applied to the figures shown in Ta- ble 2.2, which yields 69 mm in 1 hour. These two precipitation events are proposed as a generic model forcing.
Table 2.2: Precipitation amounts (in mm) for several event durations at different re- turn periods as projected for the year 2085. Bold values were used as model forcing. Adapted from STOWA (2018)
Return period Precipitation duration (minutes) (years) 5 10 15 30 60 120 2 33 10 17 24 29 37 44 50 50 25 35 42 53 63 71 100 69 250 - 48 58 73 86 94
2.5. GENERIC FORCING 33
Heat stress
As described in Section 2.4.2, UCAM does not work with a configurable forcing. Rather, the 2006 heat wave in the Netherlands is a fixed forcing and its traits cannot be modified. While limiting, the model currently does not support the configuration of a custom forcing event.
Groundwater
The KNMI reports current values and projections for the 10-day precipitation sum with a return period of 10 years. As groundwater dynamics are slower than surface water dynamics relevant for pluvial flooding, this was considered a better indicator than hourly or daily precipitation amounts for high groundwater levels. Current val- ues and future projections of this forcing are provided in the KNMI climate change scenarios. In the reference climate this 10-day sum was 89 mm. For 2085 this amount is predicted to increase by up to 25% (111 mm) under the WH scenario
(most extreme). It was thus decided to set the generic forcing at 0.01 m d-1(111 mm
over 10 days yields 0.01 m per day). Seepage was disregarded here as its intensity is strongly location-dependent and even in extreme cases (approximately 0.2 m y-1
is considered extreme seepage) makes up a small portion of the forcing.
The generic forcing for drought-induced soil subsidence is also based on the WH
scenario. The KNMI’s primary indicator for drought is the highest precipitation deficit with a return period of 10 years. In the reference climate, this deficit is 230 mm. It is forecast to increase by up to 50% under the WHscenario (most extreme). However,
the precipitation deficit is not a direct local indicator for drought as actual ground- water levels are also influenced by groundwater level management. The generic forcing is therefore not a fixed value, but rather the local decrease in groundwater level during the extremely dry year 2018 (return period 30 years (KNMI, 2019)) in- creased by 50%. The local lowest groundwater level in 2018 can be derived from monitoring wells in the vicinity of the study area. The same report mentions that the return period of such an extreme drought event may be 10 years by 2085 under the WHscenario.
Naturally droughts with other return periods may occur as well, either more or less severe. It is difficult to estimate the occurrence and severity of droughts in the next decades, but based on the 2018 drought, the forcing here is set at ten events where the groundwater level is reduced by 50% for a duration of 30 days.