CURRIDABAT Reconnecting the Rivers to the Urban Landscape Aerial Image of the Curridabat Cantón

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A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT Reconnecting the Rivers to the Urban Landscape Aerial image of the Curridabat Cantón. Source Bing Maps A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT Reconnecting the Rivers to the Urban Landscape

Author(s) Hans Gehrels Eskedar Gebremedhin Laurene Bouaziz Begoña Arellano Jaimerena A Water Sensitive Strategy for the Sweet City of Curridabat Reconnecting the Rivers to the Urban Landscape

Client. Municipality of Curridabat, Costa Rica Contact. Irene María García Brenes Keywords. Water sensitive cities, urban flooding, bio-diversity, urban rivers, hydraulic modeling

Document control Version 1 Date March 2020 Project n. 11201197 Pages 60 Status. Final Report

Doc. version Author Reviewer Approver 1 J.C. Gehrels D. Meijer L.L.F. Janssen

4 Contents

Executive summary 6

1. Introduction 8 1.1. Sweet City as a development model 9 1.2. Challenges and opportunities in Curridabat’s water management 12 1.3. Approach 14

2. Hydrology and geography of Curridabat 16 2.1. Introduction 16 2.2. River basins 17 2.3. Urbanization and land use 20 2.4. Hydrological catchment modeling 22

3. Hydraulic simulation of urban flooding 26 3.1. Introduction 26 3.2. Data preparation 26 3.3. D-Flow FM simulation results 29 3.4. Potential of enhancing infiltration and drainage 34

4. Conclusions on urban flooding 38

5. Recommendations for flood mitigation 40 5.1. Introduction 40 5.2. Recommendations for specific locations 44 5.3. General recommendations 56

References 59

5 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT Executive summary map in Figure 5.3). Flooding from rainfall generally showed higher maximum water depths and extents compared to fluvial flooding. In addi- The Municipal government of Curridabat (San tion, pluvial flooding caused significant flooded José Province, Costa Rica) aims to rehabilitate areas away from the main river beds. This is very biodiversity and to regenerate the natural riv- likely caused by malfunctioning of the drainage er system within the city. Deltares was invited system, either because of limited drainage ca- to support the city’s development of a master pacity or because of disconnections in the drain- plan on the future water management with a age lines. This interpretation was confirmed in comprehensive analysis of the water system of the field visits carried out during rain hours. Curridabat. Elements of particular importance are the rivers that connect the city with other Simulation of additional green infrastructure (for cities in the same river basin. This is based on flood prevention) and well-connected drainage the city’s vision entitled “Ciudad Dulce” which lines resulted in lower maximum water depths aims to increase biodiversity and quality of life and inundation areas compared to the initial in cities. model, confirming that urban flooding can be mitigated by increasing the amount of green in- A model-based analysis was conducted of the frastructure at the right locations and improving hydrological system of the territory of Curridabat the drainage system in terms of both capacity at two spatial scale levels: at the scale of the and connectedness. This served as a basis for river basins and the scale of the city. Two types exploring the potential of alternatives for the of flood mechanisms were considered: rainfall straightforward drainage of water from the city storm events leading to local flooding due to in- in terms of blue and green infrastructural inter- sufficient drainage capacity or bottlenecks in the ventions directed to delay, retain, and store wa- urban drainage system (pluvial flooding); and ter and hence mitigate urban flooding at specific heavy rainfall in the upstream catchment areas locations. leading to high river discharges and river bank overtopping in the city (fluvial flooding). The results from the identification of measures were then validated with a field inspection A hydrological model was set up for Curridabat’s carried out by Zamora Sauma et al. (2019). This river basins. The distributed hydrological model evaluation has been important to corroborate enabled to determine water quantities at any the flood simulations and to further specify pro- location within the model grid and to quantify posed actions. the water balance of the area. Simulated discharge time series were used as boundary condi- A first recommendation is to improve connections for a hydraulic model of the water system tions and take out bottlenecks of the urban of the city. A hydraulic model was constructed storm water system. Specific locations were to simulate maximum water depths and flood identified with model simulations and described extent for fluvial, pluvial and combined (fluvial in the report. When improving drainage in this and pluvial) flood analysis. The hydraulic model way, it is important to consider the effect this was used to locate areas in Curridabat with high may have on the areas downstream of these probability of flooding and to identify locations bottlenecks. A second recommendation is to in- with flow obstructions (bottlenecks). The model crease infiltration of rainwater and delay runoff. predictions of flooding at 6 locations were val- Supported with additional information from idated with field inspections. The field inspec- the field inspection the report describes specific tions confirmed the predicted flooding at 5 out locations for implementation of retention and of 6 locations. infiltration projects. Thirdly, it is recommended that more space be provided for the river system. The impact of river flooding appeared to be most Although difficult to implement because of ex- significant along the Tiribí River and downstream propriation of strategic lands, specific locations of the confluences of the Puruses and María along the María Aguilar may be suitable for this. Aguilar, and Chagüite and Tiribí rivers (see the A final general recommendation is to invest in

6 FINAL REPORT rainfall and river gauging stations and in capacity building of professionals in Costa Rica in using simulation models such as developed in this study – to further support master-planning towards a water sensitive and biodiverse city.

Open concrete ditch in the city

7 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT 1. Introduction

The Municipal government of Curridabat aims to rehabilitate biodiversity and to regenerate the natural system of multiple rivers within the city. Deltares was invited to support the city’s development of a master plan on the future water management for the Municipality of Curridabat with a joint comprehensive analysis of the water system of Curridabat. Curridabat is interested and inspired by recent work of Deltares on water sensitive urban design. Deltares incorporates the required capabilities with an initial understanding and recognition of Curridabat as a territory for future water-sensitive strategies. The purpose of this study is to contribute to the city’s master plan on the water management of the city of Curridabat. Elements of particular importance in Curridabat are the rivers that connect the city with other cities in the same river basin, the biodiversity, a sustainable water infrastructure, and social cohesion. A master plan for Cur- ridabat should therefore involve a water sensitive approach that is in line with the concept of a ‘Sweet City’ (as introduced in the next section) that is used as a development model in Currid- abat. It should also inspire neighboring cities to join Curridabat in a ‘Sweet River’ approach that extends to the scale of the entire river basin. In this study, a model-based analysis was conducted of the hydrological system of the territory of Curridabat, based on field observation, map- ping of local and open data, and interviews and discussions with local experts and stakeholders. A hydrological analysis at two spatial scales was carried out: at the scale of the river basins and at the scale of the city. This served as a basis for the development of recommendations for infrastructural blue and green interventions to mitigate urban flooding. In the next sections, the vision of the city is introduced, the water-related challenges and opportunities are discussed, and the approach followed is explained.

8 FINAL REPORT

1.1. Sweet City as a development model

The local government in Curridabat has developed a vision entitled “Ciudad Dulce” which aims to increase biodiversity and quality of life in cities. An important element of the vision is to bring back the most important pollinizer, the native bee, as a member of the community (Image 1.1). The objective of “Ciudad Dulce” is to establish nature conservation as an urban ac- tivity to create a more bio-diverse, comfortable, Image 1.1: Bee hotel pollinator station in a park in Cur- clean, calm, colorful and better organized urban ridabat. environment (Image 1.2).

+ + +

Polinizers+ Citizens& Vulnerable groups+

Pets

’

Figure 1.1: Development vision of the Curridabat Cantón. Source: Guia de Plantas Dulces, Municipalidad de Currid- abat

9 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

The vision of a Sweet City differs from tradition- Essential in this vision is a harmonious devel- al urban planning and urban design models. opment that uses biodiversity conservation as According to the Municipality’s vision, “Ciudad its main driver. One of the main actions that is Dulce” can be seen as a platform to visualize the promoted across the city is planting of vegeta- urban development of Curridabat in a 360-de- tion that facilitates pollination, both in public gree angle and manage specific projects in five and private spaces. The vision of Curridabat rec- dimensions, which invigorate both citizen ex- ognizes the strong links and potential synergies perience and the place where they live: biodi- between urban and natural systems, and aims versity, infrastructure, habitat, coexistence and to prevent further aggravation of current (and productivity (Figure 1.1). future) challenges such as urban flooding.

Image 1.2: The vision of “Ciudad Dulce” communicated on throughout the city.

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A major component of the vision for city of Cur- and built space). With new challenges as climate ridabat is to reconnect the rivers flowing through change, urban infrastructure should make cities the city with the surrounding urban landscape. resilient and capable of adaptation while reduc- Exploring possibilities to recover the river system ing their ecological footprint within the region. and the hydrological balance relies on a better The guidelines and principles of the Ciudad understanding of the hydrological system and Dulce development model are consistent with the physical geography of the area. Important the well-known concept of a water sensitive city questions include: how can we accommodate as formulated by Brown et.al. (2008) (Figure 1.2). infrastructure so that rivers serve and enhance their ecological purpose within the territory? Water sensitivity implies an adaptive multifunc- How does a drop of water travel within our city? tional infrastructure and urban design reinforc- ing water sensitive behavior. This requires solu- This foundation –a territorial development vision tions beyond the traditional technical measures oriented towards the integration of nature and in a more integrated approach, in which public urban development and a five-dimension sys- space design and water management are inte- tem of layers– is the basis for the current study grated into a more comprehensive strategy. The on a water sensitive approach and river-oriented concept of water sensitive cities is being applied development at the local and river basin scale. in several cities in the world and it is proving to The present study aims to support future urban be effective in many ways: raising public health development with the restoration, regeneration and awareness, mitigating climate problems, and enhancement of the regional hydrological creating quality spaces and added values to the system and water cycle by carefully interweaving city. landscape with urban systems (infrastructure

Cumulative socio-political drivers

Social Water amenity & Intergenera- supply ac- Public environ- Limits on tional equity cess & se- health Flood mental natural & resilience curity protection protection protection resources to C.C

Water Water Sewered Drained Waterways Water cycle supply sensitive city city city city city city

Supply hy- Separate Drainage Point&dif- Diverse Adaptive draulics sewage channeliza- fuse source sources & multifunc- schemes tion pollution conservation tional in- manage- promoting frast.& ment waterways urban de- protection sign, water sensitive behaviours

Service delivery functions

Figure 1.2: Urban water management transition states (based on Brown et al. 2008), showing the different states that a city goes through before becoming a water sensitive city. While Curridabat has not reached this state yet, it aspires to be resilient to climate change, sensitive to the natural system and adaptive to the changing environment.

11 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

1.2. Challenges and opportunities in • Restricted river beds (informal housing) – Curridabat’s water management causing obstruction of river flow (Image 1.4); • Obstructions (e.g. bridges) – causing ob- During the initial field inspections and work- struction of river flow (Image 1.5). shops the challenges and opportunities of the current water system and its management have Opportunities been investigated and discussed with a variety Flooding is connected to the spatial scale of the of stakeholders. The discussions circled around river basin. There is a general need for: the themes of governance, urban flooding, water quality and urban design. For the present report • Increase of local storage capacity; the identified challenges on urban flooding and • Increase of upstream storage (peak reduc- urban design are directly relevant as a basis for tion); the work that has been carried out. • Robust strengthening of outer river bends; • Widening of the river bed; Flooding • Removal of obstructions (elimination of Challenges bottle necks); • Room for the river (peak reduction). It was assessed that flooding takes place at several locations throughout the city of Curridabat Outcomes of the initial workshops were that in five categories: there are opportunities to create local storage capacity along the river in the city center. • Confluences – resulting from limited local In addition, opportunities were identified for storage capacity and high peak flows; upstream storage in the river catchment of Río • Flat topography / river planes – where María Aguilar. water easily stagnates; • Eroding outside bends in river meanders – with higher risk of landsliding (Image 1.3);

Image 1.3: Vulnerable location downstream along Tiribí River showing an eroding outer river bend

12 FINAL REPORT

Image 1.4: Informal settlement encroaching the river bed along María Aguilar River. Opportunities The rivers are the opportunities. The mere presence of so many rivers offers the possibility to create access points at river crossings: along public spaces, infrastructure and buildings. The initial workshops elaborated on this and formulated four main categories of river-management typologies. They concluded that two “lenses of Image 1.5: Highway 2 bridge crossing María Aguilar River approach” – i.e. the three main points of access and obstructing river flow. (public spaces, infrastructure and buildings) and the four river management typologies that were Urban design identified – serve as guiding structures for com- Challenges prehensive planning and urban design formu- lations for Curridabat. Singapore was considered It was summarized how rivers are considered in an inspirational example for Curridabat (Image the urban design as follows: 1.6 and Figure 1.3). • Rivers are everywhere; • River are disconnected from the public space; • Buildings facing the river use fences and walls; • Steep canyons and dense vegetation make rivers difficult to access; • Linear parks provide access to some river locations; • Rivers access locations can be unattractive Image 1.6: Bishan Park Singapore – connecting the river because of poor water quality. to the urban landscape.

13 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Figure 1.3: Singapore’s Active, Beautiful and Clean Master Plan demonstrates how individual projects contribute to a city-wide strategy of how rivers become part of urban development.

1.3. Approach Hydrological analysis Based on the available digital spatial (GIS) and The approach of this study focuses on a hydro- temporal (climate) information, and some of the logical model analysis of the river basin and a most relevant available literature, we first con- hydrodynamic model for the city of Curridabat. struct a hydrological model at the scale of the In this way, this study contributes to the de- river basin. For this purpose, the Deltares Wflow velopment of the Municipal Masterplan with distributed (grid) rainfall-runoff model is used improved understanding of the water system, (Figure 1.4). This distributed model can be run specific proposals for implementing blue and using available global datasets updated with green infrastructure to mitigate flooding and to local information. This model delivers a water reconnect the river and channel system to the balance for the river basin and provides the urban fabric. boundary conditions for the hydraulic model of Two scale levels are considered in the hydrolog- the city of Curridabat. ical system analysis. First, the scale of the river basin in which also adjacent cities are located. Second, the localized scale of the city of Currida- bat situated along rivers.

14 FINAL REPORT

In Curridabat two types of flood mechanisms are considered. Firstly, rainfall storm events (pluvial flooding) are considered that can lead to local flooding due to insufficient drainage capacity or bottle necks in the urban drainage system. Secondly, heavy rainfall in the upstream catchment areas is considered that lead to high river discharges and river bank overtopping in the city (fluvial flooding).

Recommendations for flood mitigation and river development

The results of the models are the basis for the formulation of recommendations that feed into the master plan that will be published under the auspices of the Municipality of Curridabat. The Figure 1.4: Schematization of the parameters in the dis- master plan will contain an analysis of the cur- tributed rainfall-runoff model Wflow. rent urban water system as well as concepts and ideas on the potential future development of the Hydrodynamic modeling water system that embraces the river and the hydrology of the river basins. After the hydrological analysis at the river basin scale, a hydrodynamic model is constructed The potential of alternatives for the straight- of the city of Curridabat. This model enables to forward drainage of water from the city are simulate the water system and to quantitatively explored in terms of options and interventions evaluate potential options for blue-green infra- directed to delay, retention, storage and re-use structural interventions related to spatial plan- of water. In this way, the master plan may show ning, changes in soil sealing and infiltration of future improvements in the city in terms of wa- water, changes in river waterfronts, etc. ter, sustainability and biodiversity. The model is constructed with the D-FlowFM software developed by Deltares. The model includes 2D urban runoff, open drainage channels and rivers. The model is largely based on public- ly available open data augmented with data and information provided by the city. The results are visualized in a GIS environment.

Difference between a hydrological and hydrodynamic model The hydrological Wflow model calculates a water balance in a spatially distributed grid. The hydrodynamic D-FlowFM model calculates water movement. The 2D model simulates overland flow, which requires a digital terrain model with elevation data. The model requires specific upstream and downstream boundary conditions. Typically these are upstream discharge hydro- graphs and downstream water levels. The boundary conditions can also be based on the Wflow model. Also characteristics of structures like reservoirs, weirs, gates and there operation needs to be specified in the hydrodynamic module.

15 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT 2. Hydrology and tional authority) is located in San José Province, Costa Rica. The county consists of the Districts of geography of Curridabat Curridabat, Sanchez, Granadilla and Tirrases and has an extent of in total 16 km2 at an elevation of 1,200 m above mean sea level (amsl) with a 2.1. Introduction population of ca. 65,000 people. By achieving a better understanding of the hy- The County of Curridabat (i.e. the territory over drological system and the physical geography, which the Municipality has political and jurisdic- it becomes possible to explore opportunities for

Figure 2.1: (left to right) Rivers, forest coverage and population density of Costa Rica

ío R iv e r

0 500m

Figure 2.2: Elevation, main rivers and forest coverage of the Cur- ridabat Cantón

16 FINAL REPORT recovering the river system and the hydrological of Costa Rica and includes the upstream area balance. Section 2.2 of this chapter describes the draining into the city and the downstream part river basins that surround Curridabat. Maps on of the Tiribí and María Aguilar rivers until their urbanization and land use are shown in Section confluence (see Figure 2.3). The total area of the 2.3. Section 2.4 describes the hydrological catch- river basin at the confluence of Tiribí and María ment modeling Aguilar rivers is 230 km2. Elevation ranges from 1060 m amsl at the confluence to 2710 m amsl in 2.2. River basins the North East part of the basin (see Figure 2.4). The two main hydrogeological environments The study area around the city of Curridabat is identified within the basin include volcanic de- located in the central Plateau of the Republic posits of Quaternary age and Pyroclastic forma-

CURRIDABAT

Figure 2.3: Location of Curridabat within the larger Tarcoles basin (above) and close up of Curridabat at the confluence of the Tiribí and Río María Aguilar rivers with a background of Google maps (bottom).

17 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Figure 2.4: Digital elevation model based on SRTM 90m resolution. The dotted line indicates the location of Curridabat.

tions of andesitic character and Plio-Pleistocene stant throughout the year with mean values of age. The volcanic deposits of Quaternary age 2.6 mm/day (950 mm/year) and 20ºC respec- are located in the eastern part of the Tiribí and tively. Jovel et al. (1972) report that precipitation Río María Aguilar basin and include lava flows is partitioned in about 35% evaporation and of basaltic and andesitic character overlain in 65% runoff in the Tarcoles River Basin (which is some areas by layers of volcanic ash of varying a much larger basin further downstream of the thickness. Jovel et al. (1972) report that the re- study area). charge in these formations can take up 50 to The main rivers flowing through the city are the 60% of annual rainfall. The Pyroclastic forma- Río María Aguilar and Río Tiribí (Figure 2.5). Fig- tions are located in the western part and take up ure 2.5 also shows the catchment areas of the 40 to 50% of annual rainfall as recharge to the four main inflow locations into the city and Fig- groundwater reservoir. Their ability to store and ure 2.6 shows the upstream catchment area of transmit water is limited (Jovel et al., 1972). critical points with regular flooding within the Annual rainfall is about 2200 mm/year and is city of Curridabat at the confluence of the main highly seasonal, as nearly 80% of the rainfall rivers. occurs between May and November. Potential evaporation and temperature are rather con-

18 FINAL REPORT

Legend Elevation 2750m

0 2.5km 1000m

Figure 2.5: Main rivers flowing through Curridabat. Also shown are the upstream catchments of the four inflow locations into Curridabat, from North to South, at Quebrada Granadilla, Río María Aguilar, Río Chaguite and Río Tiribí.

Legend Elevation 2750m

0 2.5km 1000m

Figure 2.6: Upstream catchment area of critical points with regular flooding in the city of Curridabat at the confluence of Río María Aguilar and Río Puruses (in the North) and at the confluence between the Río Tiribí and Río Chaguite (in the South).

19 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

2.3. Urbanization and land use As local data availability is restricted, we have assessed different sources of global data to re- trieve the necessary information to run the hy- The city of Curridabat is affected by regular drological model (see next section). Figure 2.9 flooding, mainly in the form of flash floods, due and Figure 2.10 show land use maps retrieved to an increased urbanization and a tropical cli- from two sources (MODIS and GLOBCOV). While mate with high rainfall intensities. Figure 2.7 and forests are mapped relatively similarly in both Figure 2.8 illustrate the increase in urbanization datasets, the mapped urban areas are clearly between 1984 and 2016. different. Based on Landsat and Google images, Many bridges and culverts have a reduced ca- it seems that MODIS more realistically character- pacity because they were designed when Currid- izes the land use of the basin than GLOBCOV. abat was still mostly a coffee plantation. These Physical characteristics on land use and soil structures create backwater effects in the river characteristics (including rooting depth, saturat- and lead to flooding. The increased urbaniza- ed conductivity over the soil profile, soil depth tion and informal settlements near the river bed and porosity) are required to run the model. For further exacerbate the impacts of river flooding. more detailed information on soil characteristics, During the initial field visits, it was reported that the reader is referred to the technical report on just upstream of the confluence of the Río María the hydrological systems analysis (Bouaziz and Aguilar with Río Zopilote, near Plaza Domus, Gehrels, 2018). flooding occurs regularly.

Figure 2.7: Satellite images of Curridabat in 1984 (retrieved from https://earthengine.google.com/timelapse/ on 2018-06-01).

Figure 2.8: Satellite images of Curridabat in 2016 (retrieved from https://earthengine. google.com/timelapse/ on 2018-06-01) showing the increase in urbanization since 1984. 20 FINAL REPORT

Figure 2.9: Land use map according to 500m MODIS based Global Land Cover Climatology map (USGS) (Dataset 1).

Figure 2.10: Land use map according to GLOBCOV (Bontemps et al., 2011) (Dataset 2).

21 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

2.4. Hydrological catchment modeling Model parameters are discussed in detail in the technical report on hydraulic modeling (Bouaziz and Gehrels, 2018). Model set up Model runs To analyze the surface water hydrology, we have set up a hydrological rainfall-runoff model using The hydrological wflow_sbm model was run with the wflow_sbm model code that was developed both datasets (Dataset 1 using maps from Dai at Deltares. The wflow model is a complete- et al., (2013) and Shangguan et al. (2014) and ly distributed (gridded), rainfall-runoff model the MODIS land use map; Dataset 2 using data that calculates the runoff at any given point in from Imhoff et al., (2018 in preparation) and the the model at a given time step, based on soil GLOBCOV land use map) in order to assess poten- parameters and meteorological input data. The tial differences and uncertainty. The model was distributed nature of the model implies that run between 1979 and 2014 at a daily time step the model is run on each grid cell and that wa- and 1979 was used as warming-up of the mod- ter flows from one grid cell to another, either el. Unfortunately, no observed discharge records through the kinematic wave routine or through were available to validate the models. Expert lateral groundwater flow. For a more detailed judgement of local actors is required to validate description of the model code, see: https://oss. the results of both models. deltares.nl/web/wflow/home/-/blogs/welcome- The technical report on the hydrological mod- to-the-wflow-webpage eling describes the results for both models (see The wflow_sbm model was set up with a pixel Bouaziz and Gehrels, 2018). This report only size of approximately 100 x 100 m. To set up the shows a summary of the results for Dataset 1 for model, both static and dynamic data are need- two catchments: ed. Static data define the physical characteristics • Catchment 1: upstream of the confluence of the modelled area. The static data used in the of Río María Aguilar and Río Puruses current model includes a digital elevation model • Catchment 2: upstream of the confluence (DEM), a river network, a land-use map, a set of of Río Tiribí and Río Chaguite parameters defining the properties of different soil types, land-use types and sub-basins. The catchment areas are shown in Figure 2.6 and main catchment characteristics are shown Dynamic data used in the model includes spa- in Table 2.1. Catchment 1 is small and has a high tially distributed meteorological data (precipi- fraction of urban and crop/natural vegetation tation, temperature and potential evaporation) area and almost no forests (based on the MODIS used to force the model. The model uses month- land use map). On the other hand, Catchment 2 ly estimates of Leaf Area Index to take phenology is about 3.5 times larger and mainly consists of into account. Global gridded products are used forested and crop/natural vegetation areas. to set up the model.

Table 2.1 Catchment characteristics Catchment Catchment Río M. Aguilar + Río Puruses Río Tiribí + Río Chaguite (catchment 1) (catchment 2) Area (km2) 17.5 63.9 Fraction forest (%) – MODIS 4 38 Fraction Urban (%) – MODIS 42 16 Fraction Crop / natural vegetation 54 46 (%) – MODIS Gauge location – x -84.0393 -84.0328 Gauge location – y 9.9166 9.9075

22 FINAL REPORT

In the next sections, a summary of the results is oration. The technical report on hydraulic mod- shown in terms of: eling (Bouaziz and Gehrels, 2018) contains more detailed tables with numerical information on • Mean annual water balance water balance components. • Mean monthly water balance • Daily and monthly discharge time series Both models predict higher evaporation from interception rates in the forested catchment Annual water balance – inter-annual variability compared to the urban catchment, which is In Figure 2.11 and Figure 2.12 annual sums of the expected. Recharge rates vary a lot between different components of the water balance are the models. Recharge is higher in the forested shown for both catchments. These figures show catchment compared to the urban catchment as a high inter-annual variability of precipitation expected. and discharge and much less variability in evap-

Figure 2.11: Inter-annual variability of water balance components in the small urban catchment based on model results with Dataset 1 (where P is precipitation, Q is discharge, Ep is potential evaporation, Ea is actual evaporation and Int is interception).

Figure 2.12: Inter-annual variability of water balance components in the large forested catchment based on model results with Dataset 1 (where P is precipitation, Q is discharge, Ep is potential evaporation, Ea is actual evaporation and Int is interception). 23 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Monthly water balance – intra-annual variability Monthly and daily discharge time series

The inter-annual variability of the different wa- Figure 2.15 and Figure 2.16 show mean month- ter balance components is shown at the monthly ly and daily time series of modelled discharges time step in Figure 2.13 and Figure 2.14 for the using both models for the larger forested catch- small urban catchment and the large forested ment. The baseflow predicted by the model catchment. with Dataset 2 is much lower than using Dataset

Figure 2.13: Mean monthly water balance components (mm/month) in the small urban catchment based on model results with Dataset 1 (where P is precipitation, Q is discharge, E is potential evaporation, E is actual evaporation and p a Int is interception).

Figure 2.14: Mean monthly water balance components (mm/month) in the large forested catchment based on model results with Dataset 1 (where P is precipitation, Q is discharge, E is potential evaporation, E is actual evaporation and p a Int is interception).

24 FINAL REPORT

1 (Figure 2.15). Realistic estimates of baseflow in this location can help to improve the model simulations. The model with Dataset 1 predicts higher peak flows than model with Dataset 2 (Figure 2.16). Realistic peak flows should be checked based on expert judgement.

Figure 2.15: Mean monthly runoff (mm/month) for both datasets for the larger forested catchment.

Figure 2.16: Daily modelled runoff for both datasets for the year 2000 for the larger forested catchment.

25 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT 3. Hydraulic simulation of formation on stream structures (embankment heights, bridges), hydro-meteorological bound- urban flooding ary conditions, and design scenarios for 10, 25 and 50-year events. The reader is referred to the technical report on the hydraulic modeling 3.1. Introduction (Gebremedhin, 2018) that provides a detailed description of all processed data sources for set- This chapter describes the modeling of urban ting up the hydraulic model. As an illustration, flooding in Curridabat. First, data preparation is this section highlights two crucial data sources briefly addressed. Model simulation results are on elevation and discharge. shown for various return times and local flood A post-processed DTM that reflects the terrain issues are discussed. Finally, the potential of of the model domain is of key importance for flood mitigation is estimated by increasing infil- setting up the hydrodynamic model. The terrain tration capacity and improving drainage connec- data is used to overlay the ground elevation data tions. over the computation grid cells of the model. This serves as the bathymetry of the model over 3.2. Data preparation which the surface runoff is simulated. In this project, we obtained 109 pieces of DTM/DSM LI- The development of a hydrodynamic model re- DAR data covering Curridabat. We pre-processed quires the preparation of input data and esti- and merged the small pieces into a single DTM. mation of model parameters. This included data Figure 3.1 shows the small pieces (on the left) required for the 2D gridded domain; elevation and the merged (on the right) DTM covering the data (Digital Terrain Model (DTM)), land use, line model domain. shape files describing main river system, in-

Figure 3.1: 109 pieces of LIDAR data (on the left) and pre-processed DTM (on the right) covering the city of Curridabat.

26 FINAL REPORT

Discharge time series from the wflow_sbm model elled discharge time series at boundary location (Chapter 2) were used as input in the hydraulic 4 (Tiribí river) for the year 2014 and for the whole model (see Figure 3.2). Time series were gener- simulation period with the annual peaks. The ated for four inflow boundary locations at the wlfow modelled discharge time series (1979-2014) Quebrada Granadilla, María Aguilar, Chaguite and was used to prepare the design floods for each Tiribí Rivers and two downstream locations at boundary location. Design floods were chosen the María Aguilar and Tiribí Rivers. Daily time se- with similar return periods as those from rainfall ries were extracted from the hydrological model so that both pluvial and fluvial flooding could be ranging from 1979 to 2014 (Gebremedhin, 2018). simulated for various return periods. Figure 3.3 and Figure 3.4 show the daily mod-

Figure 3.2: Flow boundary points in red, river in blue and wflow sub-catchments in green.

27 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Figure 3.3: Daily modelled runoff at boundary 4 (Tiribí River) for the year of 2014.

Figure 3.4: Daily modelled discharge (in blue) and annual maximum discharges (highlighted in red) at boundary location 4 (Tiribí River).

28 FINAL REPORT

3.3. D-Flow FM simulation results the D-Flow FM model was created, the elevation was interpolated on the computational grid cells. The elevation used for the interpolation D-Flow FM Model set-up contained the elevation point which had been D-Flow Flexible Mesh (D-Flow FM) is a Deltares converted from the post-processed DTM. The software package for modeling of river, coast- resulting computation grid along with the inter- al and estuarine areas. The 2D model grid was polated bed level of the hydrodynamic model is cropped to a selected area based on the DTM of shown in Figure 3.5. For further details see Geb- Curridabat, to limit the number of 2D cells. Once remedhin (2018).

Figure 3.5: 2D D-Flow FM computational grid with ground elevation level of Curridabat.

For the design storms and boundary locations that the flood extent is within the floodplain of flows 24hr total simulation time was imposed. the rivers and mostly less than 1m water depth. For detailed analyses, we used the 10T and 50T Looking at the downstream side of the rivers, return times because the difference between however, water depths are higher than 1m. This 10T and 25T appeared to be very small. Model could be for two reasons: (1) the slope of the up- parameters are discussed in more detail in the stream sections of the rivers is (ca. 5%) higher, technical report on hydraulic modeling (Geb- causing faster runoff from upstream with water remedhin, 2018). accumulating in the downstream flatter parts and (2) obstructions (bridges, narrow passages) Simulated fluvial flooding blocking flow in the downstream parts of the Fluvial flooding is purely based on the river flow rivers. Figure 3.7 shows the upstream longitu- entering the city of Curridabat. Figure 3.6 shows dinal profile of the María Aguilar river for a 10T the simulated fluvial flood extent and flood flood event, illustrating the decrease in slope depths of the rivers in Curridabat purely as a re- upstream of the river confluence. sult of river flooding (without any rainfall in the city). With a detail zoomed in at the confluence of Río María Aguilar, for the 10T event. We see

29 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Legend 2D Maximum water depth (m) 0.00 0.2575 0.5050 0.7525 0 500m > 1.000

Figure 3.6: Simulated fluvial flood extent and flood depths of the María Aguilar River (highlighted in red) for the 10T event. Also shown is a detail of the confluence of the María Aguilar River and Puruses River.

Figure 3.7: Longitudinal profile of the upstream section of the María Aguilar river (see Figure 3.6) with simulated maximum water levels for the 10T event.

30 FINAL REPORT

Simulated pluvial flooding tem, causing flooding in areas not influenced by river flow. Field inspection is required to corrob- Figure 3.8 shows simulated pluvial flood extents orate whether this blockage is really occurring or and flood depths (i.e. the impact of the rainfall an artefact in the DTM. runoff generated within the city itself without any river flow) for the 10T event for the entire Simulated compound flooding city. We can observe that pluvial flooding, both Figure 3.10 shows the compound (combined plu- in terms of maximum water depths and flood vial-fluvial flooding) flood extents and depths extents, shows higher values compared to fluvial for the entire city. The map shows the same flooding. distinct inundation hotspots, but with deep- Also shown in Figure 3.8 is a detail of an area er maximum depths and flow pathways. Also where we suspect blockage of the drainage sys-

Legend 2D Maximum water depth (m) 0.0000 0.4875 0.9250 1.3625 0 500m > 1.8000

Figure 3.8: Simulated pluvial flood extents and flood depths for the 10T event within the city of Curridabat, zoomed into an area where blockage of the drainage system is suspected.

31 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

0 500m Legend Pluvial flood Fluvial flood Figure 3.9: Comparison of simulated pluvial and fluvial flood extent for a 10T event.

shown in Figure 3.10 is a detail of the María Agu- Overall, pluvial flooding shows to be dominant, ilar River for the 10T event (above) and 50T event i.e., causes deeper and more wide-spread inun- (below). Greater water depths are seen in the dation, while fluvial inundation is concentrated 50T compared to the 10T event. Figure 3.11 shows mainly around the floodplains of the Tiribí River. the difference in maximum compound flood It is concluded that intense local rainfall gener- water depths for the 10T and 50T events. There ating local overland flow within the city is the is generally a significant difference between the primary cause of flooding. Figure 3.12 shows the two return periods, as can be seen from all three simulated runoff flow paths. The figure illus- figures, with more than 80cm difference along trates that rainfall runoff is generated locally and both María Aguilar and Tiribí rivers. visualizes the intensity and flow direction of local runoff generating urban flooding within the city of Curridabat.

32 FINAL REPORT

Legend 2D Maximum water depth (m) 0.00 0.6325 1.2550 1.8775 0 500m > 2.000

Figure 3.10: Simulated compound flood extents and flood depths of the city of Curridabat, zoomed in at the María Aguilar River for the 10T (upper left) and 50T event (lower left)

Legend 2D Maximum water depth (m) 0.00 0.2075 0.4050 0.6025 0 500m > 0.800

Figure 3.11: Difference in maximum flood depths of the compound simulation between the 10T and 50T events.

33 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Figure 3.12: Flow velocity vector-based runoff showing the source and direction of overland flow.

3.4. Potential of enhancing ered by impermeable surfaces such as pavements infiltration and drainage that increase runoff and reduce infiltration (Im- age 3.1).

Increasing infiltration capacity The original hydraulic model assumes that the area is urbanized with negligible infiltration Urbanized areas in Curridabat are generally cov- capacity. However, OpenStreetMap (OSM) data

Image 3.1: Urban fabric of Curridabat illustrating the omnipresence of impermeable surfaces.

34 FINAL REPORT suggests areas with green infrastructures such as day (5mm/hr) for unpaved areas for all identified parks, forests and small farmland in the city of green infrastructure areas from OSM. Figure 3.14 Curridabat (see Figure 3.13) that were not com- shows the simulated compound flood extent pletely included in the land use data used in the and flood depths for the entire city, with a detail original model (cf. Gebremedhin (2018) for open around the María Aguilar river for the 50T event, data that was used as model input). with increased infiltration capacity on the left and the original model on the right. The results We tested the potential of increasing infiltra- clearly show a significant reduction in flood ex- tion capacity of these areas, to assess how much tent and maximum water depth in the case of runoff can be reduced by increasing the amount increased infiltration capacity. The additional of green infrastructure in the urbanized areas of green infrastructure reduces the overland flow Curridabat, as a measure to reduce urban flood- by allowing more infiltration of rainfall, hence ing. reducing urban flooding. We added an infiltration capacity of 120 mm/

Figure 3.13: Infiltration capacity was increased for the green areas found in OpenStreetMap within the city. (See text for more explanation.)

35 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Legend 2D Maximum water depth (m) 0.00 0.2075 0.4050 0.6025 0 500m > 0.800

Figure 3.14: Simulated compound flood extent and flood depths, zoomed in around the María Aguilar river for the 50T event with increased infiltration capacity (upper left) and original model (lower left).

Improving drainage line connections Figure 3.15 shows the simulated result (for a compound flood 50T event) with improved con- As discussed previously, one of the likely caus- nections applied for the entire city, zoomed into es for local inundation is disconnectedness of an area with seemingly disconnected drainage. drainage lines (see also Figure 3.8). To test the The detailed figure on the right, with discon- impact of improving drainage line connections nected drainage, shows a maximum water depth we connected a number of seemingly discon- of higher than 1.5m. The detailed figure on the nected drainage lines by lowering the DTM to the left, with connected drainage, shows a maxi- local drainage bottom level. Figure 3.16 shows mum water depth of lower than 1m, with also the locations in the city where drainage connec- the flood extent reduced. The residual inundations were restored. tion could be due to lack of drainage capacity.

36 FINAL REPORT

Legend Drainage connected

Figure 3.15: Locations where drainage lines were connected.

Legend 2D Maximum water depth (m) 0.00 0.5375 1.0250 1.5125 0 500m > 2.000 Figure 3.16: Simulated compound 50T flood depth and extent with and without improved drainage connections, zoomed in into an area where drainage was reconnected, with improved drainage (upper left) and original DTM model (lower left). 37 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT 4. Conclusions on urban The hydraulic model was used to locate areas in Curridabat with high probability of flooding flooding and to define locations where flow obstructions should be removed. Design events were derived Catchment model based on a frequency analysis that provides the boundary conditions of the 2D flood modeling A hydrological model was set up for the catch- for the city of Curridabat. Due to a lack of data ment area around the city of Curridabat. Results the model output was not calibrated, and the from this model were used to better understand uncertainty of the model output was not quan- the hydrological functioning of the catchment to tified. As an alternative, field inspections were provide input for the development of a master carried out by Zamora Sauma et al. (2019) to cor- plan for a water sensitive city. roborate the results, which is described in the The distributed hydrological model enables to next chapter. determine water quantities at any location with- The model shows that as a result of the high el- in the model grid and provides insight in the evation differences within the city, runoff from water balance of the area. The low permeability the upstream parts of the city quickly flows of the area in combination with high rainfall downward and accumulates at the downstream intensities during the months May to November sections of the María Aguilar and Tiribí rivers. lead to saturated conditions in which most of For fluvial flooding (purely caused by river flow), the water runs off rapidly towards the river. This the differences in flood extent between the is translated in a high runoff coefficient (of ap- 10T and 50T events are not significant in most proximately 70%). Interception is an important of the city. The impact of river flooding is most component of total actual evaporation. Recharge significant along the Tiribí River (Figure 3.9) and mostly takes place in the upstream forested ar- downstream of the confluences of the Puruses eas and varies considerably between models and and María Aguilar, and Chagüite and Tiribí rivers areas (it varies from 35% to 87% of the runoff in (Figure 4.1). the assessed catchments and models). Pluvial flooding (resulting from purely rainfall) Due to a lack of observations, the model was not generally shows higher maximum water depths calibrated. Differences between the two applied and larger extents compared to fluvial flooding datasets highlighted the high level of uncertain- (Figure 3.9). In addition, pluvial flooding causes ty of model results. It is therefore important that significant flooded areas outside and away from the current models are assessed against expert the main river beds. This is very likely due to knowledge and that future models are calibrated drainage problems, i.e. too little drainage ca- with observed data from gauging stations. pacity or disconnections in the drainage lines. Simulated discharge time series at the four in- Although very likely, it is important to consider flow locations into the city of Curridabat were that the exact locations are uncertain, as the used as upstream and downstream boundary simulated drainage problems may be caused conditions for the hydraulic model. by inconsistencies in drainage connections burned into the DTM. It is therefore vital to check Simulating urban flooding whether the simulated inundations coincide A hydraulic (D-FlowFM) model was constructed with field observations. for simulating maximum water depths and flood The joint or compound event flood map por- extent for fluvial, pluvial and compound (the trayed the characteristics of the combination of combined fluvial and pluvial) flood analysis. both fluvial and pluvial flooding. The maximum

38 FINAL REPORT

flood depths and extents show similar patterns maximum water depths and inundation areas of vulnerable areas but with higher depths. The compared to the initial model, indicating that difference in maximum compound flood depths urban flooding can be mitigated by increasing between the T10 and 50T events are significant. the amount of green infrastructure and improving the drainage system in terms of both capaci- Simulation of additional green infrastructure and ty and connectedness. well-connected drainage lines resulted in lower

Legend Max. water depth (m) (pluvial 50T) 0.05

0 500m > 1.8

Figure 4.1. Simulated fluvial flood extent and depth for a 10T event

39 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT 5. Recommendations for • The locations most vulnerable to urban flooding are situated downstream of the flood mitigation confluences of rivers Puruses and María Agu- ilar, and Chagüite and Tiribí (Figure 5.1) • Other large simulated flood areas away from 5.1. Introduction the main rivers are likely caused by malfunctioning of the drainage system (e.g. Figure Based on the hydrological and hydraulic model- 5.2). This interpretation was confirmed in ing described in the previous chapters, measures the field visits during rain hours, when it for flood reduction were identified and prelim- was observed that most of these locations inary recommendations were formulated. These have failing drainage systems, with frequent preliminary recommendations were then evalu- pluvial flooding as a result. ated in a field inspection carried out by Zamora • Both the effects of a pluvial flood and the Sauma et al. (2019). The team led by Mr. Zamora effects of a combined pluvial/fluvial flood conducted field visits to review the areas for show significant differences between the 10T which recommendations were formulated based and 50T return periods, with larger depths on the hydraulic simulations. Visits were made and extents for a 50T flood (Figure 5.1). The during dry and wet periods. differences between the simulated 10T and the 25T simulation results were found to be This chapter contains the final recommendations very small. based on the main conclusions from the model • Field observations showed that flooding simulations and the subsequent corroboration across Highway 2 is considerable as well. from the field inspection:

0 500m

Legend Max. water depth (m) (pluvial 50T) 0.05

> 1.8

Figure 5.1. Difference in simulated fllood for the 10T event compared to the 50T event for: (left to right) fluvial flood, pluvial flood, and compound pluvial-fluvial flood

40 FINAL REPORT

Figure 5.2. Simulated pluvial flood for a 50T event, zoom-in in a location with suspected drainage problems

Legend Drainage Max. water depth (m) problem (pluvial 50T) Drainge 0.05

> 1.8

41 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

2 3

0 500m

Figure 5.3: Flood map indicating six representative locations within Curridabat that present urban flooding issues according to the results of the hydrodynamic model. 42 FINAL REPORT

43 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

5.2. Recommendations for specific • Large paved areas used as parking space locations • Large open areas covered by grass, used seasonally to host events • Absence of forest Given the extensive territory that this study ad- dresses, six representative locations were chosen Field observations: pluvial flood problems were to elaborate the general recommendations for indeed observed in front of “Centro de Desarrollo specific locations (Figure 5.3). Humano El Hogar” in accordance with the simulated problems shown in Figure 5.4. It was found Site 1. San Gerardo Neighborhood that the storm water drainage system is not able This site covers the areas from southwest of to capture and drain all rainwater from that the Jose María Zeledón neighborhood, the sur- particular area. The probable cause as identified roundings of “Centro de Desarrollo Humano de El from the field visit is that the pluvial sewerage Hogar” and areas from the back side of “Aliss”, works are completely filled with rainwater from “Registro Nacional” and “Auditorio Nacional”. the different zones of Jose María Zeledón without capacity left to drain the rainwater from this Anticipated problem: Urban flooding in low-ly- area. Image 5.1 shows flooding at the location ing areas, likely due to poor infiltration of “Centro de Desarrollo Humano El Hogar” from Site features: May 2019. • Disconnected from María Aguilar River Possible measures: • Combination of low-rise single-family Infiltration basin. Given the existence of a large residential buildings and institutional/cor- open area, it is recommended that infiltration porate buildings (large impermeable roof basins be implemented that can delay rainwater surface) runoff during the rainy season and that can also

0 100m

Figure 5.4: Site 1. Simulated 25T pluvial flood event.

44 FINAL REPORT

Image 5.1: Flood event in front of “Centro de Desarrollo Humano El Hogar” in May 2019. increase infiltration into the unsealed soil. In that contribute to delaying surface runoff before addition, given the intensive rainfall pattern in it reaches the ground. The type of vegetation Curridabat, it is important to consider a connec- will depend on the capacity of the building to tion to the drainage system at this location in support additional weight, but native species are case the capacity of the basin is not enough for preferable. As this measure requires particular the total amount of rainfall. constructive specifications to support the additional weight from the plants and substrates, it Permeable pavement (Image 5.2). Large paved is more viable to implement this in future build- areas used as parking space can be improved by ing projects. using permeable pavements that allow for the infiltration of rainwater on site. This measure can Additional recommendation after site inspec- be complemented with bioswales in the parking tion: to conduct a study of the pluvial sewerage area to retain the rain water that is not infiltrat- system of the entire José María Zeledón area and ed directly (Image 5.3). to consider the potential of storage measures upstream of this area to reduce the discharge to Green roof (Image 5.4). The presence of large Centro de Desarrollo Humano El Hogar. impermeable roof surfaces, for example, on top of the institutional buildings, presents an op- portunity for the implementation of green roofs

Image 5.2. Permeable pavement, by Cell Code US Image 5.3. Bioswale at the River East Center, Portland USA. Credits Alice Webb 45 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

vo has settled very near to the river protection zone, potentially being at risk. In addition, solid waste dumps were observed at this location in the river, which will block river flow at bridges and hence may cause flooding between the blocked locations and the river protection zone of María Aguilar River (site 2) (Figure 5.5). Possible measures: Based on the field visit this site has been discarded as a priority. In case flooding does occur in future, the following measures could be considered: 2.1 Provide more space for the river by relocating the houses built on the riverside. Not only are these houses particularly vulnerable to flooding, but they also interfere with the riparian landscape. By providing more space for the river, its discharge capacity increases, and the natural

Image 5.4. Green roof in Magayon House in Costa Rica. processes of erosion and sedimentation can take Designed by SARCO Architects place, shaping the riparian landscape. 2.2 Improve drainage of the football field. Plu- Site 2. Lower María Aguilar River vial floods in this site affect almost exclusively Anticipated problem: Urban pluvial and fluvial the football field and the constructions on the flooding in low-lying areas, likely due to poor riverside. To decrease the impact of flooding, it infiltration and lack of suitable drainage. is necessary to improve infiltration in the flood prone area. This can be done by increasing the Site features: amount of permeable pavement and by im- • Open area used for recreation (football plementing a drainage line connecting to the field) María Aguilar River. In addition, the elevation • Low-lying area adjacent to María Aguilar of the football field could be increased to fur- River ther lower flood risk. This could, however, have • Predominantly low-rise single-family res- implications for the neighboring properties if idential buildings in the surroundings the measure is not complemented with suitable • Forest coverage predominantly along the drainage. river Additional recommendation after site inspec- • Presence of (informal) housing on the riv- tion: it is recommended that in a future mod- erbank (Image 5.5) eling study the hypothesis be tested whether Field observations: At the time of the site visit, the difference between the simulation and the low river flows were found without any sign of field observation is explained by the convolution flood problems, so it was discarded by the team under the bridge crossing Highway 2. This can be as a priority zone of intervention. However, 1 km done by adjusting the bridge cross section and upstream of this point in the Río María Aguilar, evaluate whether flooding at this site is thereby where Highway 2 crosses the river, river con- reduced. veyance is greatly restricted by the bridge. If in the future, river flow capacity under the bridge would be improved, then this could lead to flood problems at this site 2. In addition, it was observed that a community called Pueblo Nue-

46 FINAL REPORT

0 100m

Figure 5.5. Site 2. Simulated 25T pluvial flood event.

Image 5.5 . Houses next to one of Curridabat’s urban rivers

47 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Image 5.6 : Houses in the river protection zone of María Aguilar River and solid waste from Pueblo Nuevo community dumped directly into the river. 48 FINAL REPORT

Site 3. Lower Tiribí River Possible measures:

Anticipated problem: Urban pluvial flood likely 3.1 Create a drain in areas with lower elevation due to the lack of suitable drainage. and connect to the existing drainage system (but only if the downstream part of the drainage sys- Site features: tem has sufficient capacity). • Combination of densely built area with Increase forest coverage in order to improve in- predominately low-rise single-family filtration in open areas. houses and forest/farmland area • Disconnected from the Tiribí River Provide more space to the river. Taking advan- • Apparent lack of drainage in low-lying tage of the existing open space, soften the river areas meandering and steep slopes to facilitate the restoration of the riparian landscape. Field observations: at Site 3 flooding is observed at the north-east side of the map in Figure 5.6. Small-scale green infrastructure. In private lots, Pluvial overflow; The rainwater flowing from the implement rain gardens and bioswales to con- nearby Highway 2 is conducted through an open tribute to rainwater retention thereby also in- channel and, at the end of the highway, crosses creasing infiltration and filtering of rainwater. In the street through a single drainage pipe result- addition, it is recommended that green roofs be ing in overflow of the open channel (Image 5.7). implemented to further delay runoff by retaining rainwater on site.

0 100m

Figure 5.6: Site 3. Simulated 25T pluvial flood event.

49 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Image 5.7 : Open channel for storm water drainage from Highway 2.

Site 4. Hacienda Vieja & La Lía Neighborhoods Field inspection showed that this is a point where the open drainage channel (with ample In this neighborhood, rainwater from the Flor- capacity), routing the water from the north, encio del Castillo Highway in the north and rain- turns into a pipe (culvert) with apparently lower water from the Hacienda Vieja and La Lía com- drainage capacity at a distance of 500 m before munity is routed through a channel to the Tiribí it enters into the Tiribí River in the south (Figure River in the south. 5.7). Anticipated problem: Urban pluvial flood due to General measures: a lack of storm drainage capacity. Increase drainage capacity in areas that are Site features: prone to floods and connect to existing drainage • Combination of densely built area with system (but only if the downstream part of the predominately low-rise single-family drainage system has sufficient capacity). houses and open areas with grass or forest Increase forest coverage in order to improve coverage infiltration in open areas currently covered by • Free open space but poor infiltration grass. • Lack of connection to drainage system Give more space to the river. Taking advantage Field observations: flooding at this location of the existing open space, soften the river me- was confirmed in the field inspection. Frequent andering and steep slopes to facilitate the resto- flooding was found in front of La Lia School. ration of the riparian landscape

50 FINAL REPORT

Small-scale green infrastructure. In private lots, More specific recommendations after site in- implement rain gardens and bioswales to con- spection: the field crew proposes to add reten- tribute to rainwater retention thereby also in- tion and infiltration infrastructure in Hacienda creasing infiltration and filtering of rainwater. In Vieja Park and a retention / infiltration basin addition, it is recommended that green roofs be near Florencio del Castillo Highway, see Zamora implemented to further delay runoff by retaining Sauma et al. (2019). rainwater on site.

0 100m

Figure 5.7. Site 4. Simulated pluvial flood for a 25T event

Image 5.9: Pluvial flooding in the “Hacienda Vieja” neighborhood. Left: channel draining storm water from the highway and neighborhood; right: flooding at the location where the open channel is routed into a pipe. 51 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Site 5. Tacaco Neighborhood draining the simulated flood zone into María Aguilar River. However, downstream, directly Anticipated problem: Urban pluvial flood likely southwest of this point, flooding does take place due to the lack of drainage connection. along Tacaco Street and in Mariana Park (Figure This flood problem occurs in an area outside 5.10) as a result of limited drainage capacity. the reach of the main river, suggesting that the Possible measures: cause is a storm water drainage problem caused by a blockage as shown in the DTM (Figure 5.9). The field crew recommends increasing the ca- However, field observations will have to corrob- pacity (i.e. the diameter) of the storm water orate whether this blockage is real or an artefact drainage system in Tacaco Street and imple- in the DTM. menting retention and infiltration works in the park area. Field observations: After field inspection of Taca- cos Street, it was found that the actual drainage system is not blocked and capable of adequately

0 100m

Figure 5.8: Site 5: Simulated 25T pluvial flood event.

52 FINAL REPORT

Legend 2D Maximum water depth (m) 0.05 0.4875 0.9250 1.3625 0 500m > 1.8000

Figure 5.9: Simulated pluvial flood extent and flood depths of 10T, zoomed into the Tacacos area. The upper zoom shows a blockage in the drainage system burned into the DTM. The lower zoom shows the simulated flood extent.

Figure 5.10: Pictures and map from pluvial drainage floods in Tacaco Street during rain hours, showing inundation at Parque Mariana (green area) (photo 4) and stormwater overflow along Tacacos Street (red area) (Source: Zamora Sauma et al., 2019). 53 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Site 6. Bridges along Highway Route 2 crossing Río just next to each other cause backwaters that María Aguilar overflow the riverbanks (Image 5.10). This river plain is relatively flat, as a result of which it is This location is just upstream of the confluence easily flooded. of Río María Aguilar and Río Zopilote on Route 2 from “El Cruce de la Galera” to a traffic light in Possible measures: the corner known as “Semaforo de Pepe Figue- Increase river discharge capacity by widening res” (Figure 5.11). the river channel and enlarging the bridge span Anticipated problem: Urban pluvial flooding due width. to the confluence of river and open channel flow Provide more space to the river upstream of the in combination with small bridges restricting the flood zone and create retention areas to reduce flow capacity. peak flow at the location of the bridges Site features: This area around Condominium Recalculate the river system with the hydrau- Mallorca and Plaza Domus is regularly (ca. once lic model to evaluate the mitigating impacts every year) subject to flooding. of these measures. This is also very important Field observations: The confluence of the two because increasing river discharge at this point rivers causes flooding. The two narrow bridges should not lead to more flood problems downstream.

0 100m

Figure 5.11: Bridge along Highway Route 2 over Río María Aguilar (yellow circle) just upstream of the confluence of Río María Aguilar and Río Zopilote in the area around Condominium Mallorca and Plaza Domus. 54 FINAL REPORT

image 5.10: Two bridges along Highway Route 2 crossing Río María Aguilar during dry weather (top) and flood conditions (bottom). 55 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

5.3. General recommendations be mitigated by increasing the infiltration capacity on site, which also reduces urban flooding downstream. Further flood reduction can be The field inspection as carried out by Zamora achieved by delaying surface runoff, which is Sauma et al. (2019) validated the results from the especially relevant when options to improve in- modeling presented in this report with expert filtration are lacking. Zamora Sauma et al. iden- and local knowledge at all 6 locations where tified specific locations in the field surveys for flooding was predicted. This evaluation has been which they formulate proposals for the imple- very important to corroborate the flood simula- mentation of SUDS (sustainable urban drainage tions and to specify proposed actions. The field systems) and proposals for the implementation inspection confirmed the predicted flooding in of retention and infiltration projects in specific 5 out of 6 locations. This is considered as an parks and open spaces. In a next phase of the indication that the model results are generally project, these proposed interventions should be reliable. implemented into the hydraulic model, so that A first recommendation is to invest in rainfall it can be tested to what degree these measures and river gauging stations, so that future mod- mitigate the flood risk. els can be forced with more detailed hydromet A final general recommendation is to provide data and calibrated against time series of obser- more space for the river system. The continued vations. This will greatly reduce uncertainty of urbanization of Curridabat has reduced the river model results. flood plains to a minimum and has destroyed Hydraulic models predicting flooding from most of the riparian urban landscape. This poses rainfall are not commonly used in Costa Rica. a risk to both the biodiversity depending on the It is recommended that capacity building be riparian ecosystem and the urban areas next to increased of professionals in using simulation the rivers, especially downstream, where the models such as developed in this study, as the effects of pluvial and fluvial floods are more current study demonstrates that model results severe. These areas present opportunities to can support decision making on becoming a wa- provide more space for the river by restoring or ter sensitive city. widening floodplains or deepening the river Connect. It is recommended that drainage bed. This creates more room for the restoration connections be improved and bottlenecks be of the riparian landscape while increasing the removed from the urban storm water system. river’s discharge capacity. Although Zamora Sau- Model predictions of flooding outside the fluvial ma et al. confirm that this is a valuable option, flood plains are associated with the drainage they regard this as the most difficult goal to system, either due to a lack of capacity or dis- implement, because it requires expropriation of connections. If site inspection shows no actual strategic lands. However, specific locations along drainage disconnections, then the Digital Terrain the María Aguilar were already targeted for this Model likely needs to be updated. The evalua- purpose. Additional spaces would have to be tion by Zamora Sauma et al. confirms the need identified in a next phase of the project by com- for the Municipality to conduct localized studies bining maps of urban planning and information to continue improving drainage connectivity. on land ownership with additional localized While improving connectivity, the effect on the hydraulic calculations of most effective zones for downstream area should always be considered. river bed widening. Another recommendation is to increase rainwater infiltration and delay surface water runoff. The most important flood risk is pluvial flooding, caused by a combination of extreme rainfall, lack of infiltration capacity and excessive runoff, leading to accumulation of storm water in low-lying areas. Localized pluvial flooding can

56 FINAL REPORT

57 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

58 FINAL REPORT References

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Brown, R., Keath, N. and Wong, T. (2008). Tran- sitioning to Water Sensitive Cities: Historical, Cur- rent and Future Transition States. 11th Int. Conf. Urban Drainage, Scotland, 10 pp.

Dai, Y., W. Shangguan, Q. Duan, B. Liu, S. Fu, G. Niu (2013). Development of a China Dataset of Soil Hydraulic Parameters Using Pedotransfer Functions for Land Surface Modeling. Journal of Hydrometeorology, 14: 869-887.

Gebremedhin, Eskedar T. (2018). Hydrodynamic modeling in the city of Curridabat. Technical Re- port, Deltares, Delft, 45 pp.

Jovel, J.R. and Ahlgren, L.E. (1972). The water resources of the Río Grande de Tárcoles Basin, Costa Rica. Hydrological Sciences Journal, 17(4), pp.405-418.

Shangguan, W., Dai, Y., Duan, Q., Liu, B. and Yuan, H. (2014). A Global Soil Data Set for Earth System Modeling. Journal of Advances in Model- ing Earth Systems, 6: 249-263.

Zamora Sauma S., Saborío J., Mora E., García I., Mora G., Muñoz A. (2019). Review of Recom- mendations from Deltares for a Water Sensitive Curridabat. Technical Report, Municipalidad Cur- ridabat, 35pp.

59 A WATER SENSITIVE STRATEGY FOR THE SWEET CITY OF CURRIDABAT

Final version Edited by Begoña Arellano Jaimerena Delft, Netherlands March 2020