Urban retention basin in developing city: from theoretical effectiveness to practical feasibility

Abstract

Urban retention basin has been introduced as one of the promising measures to address increased storm water flow in urbanized areas. This man-made water space may not just help existing storm water drainage systems regulating extreme events but also improve urban landscape and micro-climate conditions. Can Tho city, a largest city in Mekong delta is facing with flooding throughout the year. While tide is the typical reason causing flooding, extreme rainfall, upstream floods are other reasons. Moreover, when these phenomenon happen at the same time, traditional flood protection measure, for example: dike, tidal sluice can only applicable for river flood or sewer system is used for draining rain water are not effective enough. Therefore retention pond is introduced as a measure for Can Tho city to overcome this flooding problem. However, implementation of retention pond is not an easy task. Technical feasibility, cost/benefit features, stakeholders’ consensus as well as institutional arrangement must be fully assessed. In this study, the Xang Thoi pond, an urban upgraded project is used as a case study. Roles of the pond in flood reduction, consensus, as well as technical barriers are analyzed. The results showed that the upgraded urban project of the Xang Thoi pond has significantly contributed amenities to urban dwellers. However, to improve its functions (e.g. reduce flood peak) as well as to maintain it in a sustainable manner, it requires considerable efforts from different stakeholders. It also showed the retention basin could be a possible option in the current situation to adapt with urban flooding problems in Can Tho.

Full text

13th International Conference on Urban Drainage, Sarawak, Malaysia, 712 September 2014
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Urban Retention Basin in a Developing City: From Theoretical
Effectiveness to Practical Feasibility
Nguyen Hong QUAN1*, Ho Long PHI2, Pham Gia TRAN3, Assela PATHIRANA4,
Mohanasundar RADHAKRISHNAN4, Chau Nguyen Xuan QUANG2
1Institute for Environment and Resources (IER) – Viet Nam National University Ho Chi Minh
City (VNU HCMC), Ho Chi Minh City, Vietnam
2Center of Water Management and Climate Change (WACC)—VNU HCMC, Ho Chi Minh
City, Vietnam
3University of Social Science and Humanities – VNU HCMC, Ho Chi Minh City, Vietnam
4UNESCO-IHE Institute for Water Education, Delft, The Netherlands
*Corresponding author
Email: hongquanmt@yahoo.com
ABSTRACT
Urban retention basins have been introduced as one of the promising measures to address
increased stormwater flow in urbanized areas. These man-made water spaces may not only
help existing stormwater drainage systems to regulate extreme events but also improve urban
landscape and micro-climate conditions. Can Tho city, the largest city in Mekong Delta, faces
flooding throughout the year. While tide is the major cause of flooding, extreme rainfall and
upstream floods also cause flooding. Moreover, when these phenomena happen
simultaneously, traditional flood protection measures, for example, dike and tidal sluice, can
only be applied when river flood or sewer system used for draining rainwater are not
effective enough when heavy rain occurs. Therefore, retention ponds have been introduced in
Can Tho city to overcome this flooding problem. However, implementation of retention
ponds is not an easy task. Technical feasibility, cost–benefit features, stakeholders’ consensus
and institutional arrangement must be fully assessed. In this study, the Xang Thoi pond, an
urban upgraded project, is used as a case study. The benefits of the pond in flood reduction,
consensus as well as technical barriers are analysed. The results showed that the upgraded
urban project of the Xang Thoi pond has significantly contributed amenities to urban
dwellers. However, to improve its functions (e.g. reduce flood peak) as well as to maintain it
in a sustainable manner, considerable efforts from different stakeholders are required. The
project also showed that the retention basin could be a possible option in the current situation
to adapt with urban flooding problems in Can Tho.
KEYWORDS
Can Tho city, climate change adaptation, urban flood management, retention basin, Xang
Thoi pond
INTRODUCTION
Can Tho city, a largest city in Mekong delta is facing with flooding throughout the year. The
flooding is not only come from the city itself but also from outside. Urbanization processes
induced the increasing runoff, existing storm sewer design flows often exceed its capacity are
the main reasons causing pluvial flood. While high-tide, up-stream are external factors make
city getting flood more often. Moreover, when these phenomenon happen at the same time,
traditional flood protection measure, for example: dike, tidal sluice can only applicable for
river flood or sewer system is used for draining rain water are not effective enough. Figure 1

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shows that by introducing the retention basin into the system, it could be possible to reduce
flooding issues in Can Tho city.
Figure 1: Role of retention basin in urban flooding.
Retention basinswere introduced for stormwater management including e.g. flood reduction,
water quality enhancement(Nascimento et al., 1999). In addition, it also considered as one of
suitable measures that is very well adapted to climate change and sea – level – rise
issues(Birkmann et al., 2010). However, implementation the basin is not always an easy task
since various existing constrains including, e.g., space requirements, water pollution. For
example, spaces for retention basin are very limited in urbanized areas. It normally requires
certain relocation and resettlement for people living around these areas. These activities will
lead to another issue that are very complex i.e. public consensus. Therefore, there is still a
need to find a proper and sustainable way in implementing retention basin. It would require a
consideration on different aspects e.g. law, economic, social and environmental aspects,
among many others(Barbosa et al., 2012)
The Xang Thoi pond was upgraded as one of the main components within the Vietnam Urban
Upgrading Project (VUUP) supported by World Bank to upgrade low-income communities.
The VUUP aims to upgrade low-income communities in four cities, namely Ho Chi Minh,
Hai Phong, Nam Dinh, and Can Tho. The VUUP will provide basic infrastructure and
services improvements to lowincome communities already identified in the cities.(VUUP
CT-SPMU, 2003). Basically, the VUUP aim at (1) to alleviate poverty in urban areas by
improving the living and environmental conditions of the urban poor; (2) to promote the
participatory planning methods for urban upgrading to meet the people's demand; (3) to use
multi-sector approach with communities' consultation in implementation process of
upgrading programs.In addition, the most important principle of the project is to active
community participation in all stages of preparation, design and implementation processes
(VUUP CT-SPMU, 2003).The construction started in 2006 and accomplished by August
2009.It has been reported that in the past, the ponds, canals and arroyos have become
shallower and storage capacities reduced due to sedimentation and solid waste disposal
(VUUP CT-SPMU, 2003). Due to the upgrading project such as dredging the pond, control of

13th International Conference on Urban Drainage, Sarawak, Malaysia, 712 September 2014
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solid waste disposal, the pond currently has more room to store water. Though the Xang Thoi
pond has significantly contributed amenities to urban dwellers
1
(see Figure 2before and after
project), it is still needed to improve its functions in flood reduction (by collecting more rain
water), as well as to enhance water quality in the pond. The pond is used as a case study to
illustrate the effectiveness in flood reduction as well as constrains in implementing this
solution in practice. Results of this study are not only served for other on-going upgrading
projects in Vietnam but also can be shared with other cities in the developing world.
Figure 2: The Xang Thoi pond and its surrounding before and after the construction. (a): the
pond was covered by Ipomoea aquatic; (b): the pond after construction; (c) the canal was
occupied by slums; (d): the canal after construction; (e,f): Top-view of the pondpond and its
surrounding before and after the construction.
1
See: https://www.youtube.com/watch?v=udGoPn_tW98

13th International Conference on Urban Drainage, Sarawak, Malaysia, 712 September 2014
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METHODS
Approach
In this study, the Xang Thoi Pond which was upgraded recently by the urban upgrading
project funded by World bank is used as case study. Firstly, the role of flooding reduction of
the pond is analyzed using a simple hydrodynamic model. Given an existing problem of the
pond is water pollution, past and current monitoring data are collected to examine water
pollution level of the pond. An important aspect of the upgrading project is social
consensus/acceptance. This is assessed based on an extensive interview campaign to local
citizens, officers. Finally, based on the findings, a learning lesson is presented for improving
the current situation as well as for implementing/applying in other cases.
Modeling
A simplified EPA-SWMM model (http://www.epa.gov/athens/wwqtsc/html/swmm.html) was
set-up for Xang Thoi pond and its surroundings to assess the role of reservoir in reducing the
flood in the vicinity of the reservoir. The design rainstorm of 5 hrs with 10 year return period
was used as rainfall input. The tidal variations are incorporated into the model by assigning
the water level time series as downstream boundary condition at the model outfalls.
SWMM model without Reservoir
SWMM model with reservoir
Design rainfall
Design tide
Figure 3.SWMM Model for center of Can Tho city, design rainfall and tide parameters
The model was used to make a preliminary assessment of the effectiveness of the pond as
detention storage for flood control. Four scenarios had been constructed to simulate the
flooding around Xang Thoi pond. The first scenario “Current State” does not include Xang
Thoi pond with the surroundings areas draining directly into the river. The second scenario
does not include the pond and comprises retrofitting of selected drains and enhancement of

13th International Conference on Urban Drainage, Sarawak, Malaysia, 712 September 2014
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pumping capacity based on the results from first scenario. The third scenario considers Xang
Thoi pond as a reservoir, whereas the fourth scenario comprises the pond as reservoir as well
the retrofitting of drains and enhancement of pumping capacity. Xang Thoi pond was
represented in the model as a reservoir with an average depth of 2.5 m , a total area of 4.5 ha
and drains into the river through an outfall with flap gates.
Monitoring
Water quality sampling: Water samples from the sewage to the pond, inside the pond and
after the pond (discharge to the river). Monitoring parameters include basic water quality like
pH, COD, BOD5, Nitrat (NO3--N), Amonium (NH4+- N), Phosphat (PO43-), Total P, Total N,
Total suspended solid, E.Coli, and Coliform. In addition, past data are also collected.
Social survey
The survey consists of structured questionnaires with the sample of 90 households aging from
26 to 77 years old and 10 representatives of local authorities and head of neighborhood. All
respondents have resided in the study site at least 10 years until the December of 2013 – The
time of survey. This sample criterion is set up to ensure only the qualified respondents or the
respondents who experienced the process of construction of pond to participate the data
gathering. The main topics of structured questionnaire include feeling of people about
retention pond, impact of the pond on the environment and living of people, participation of
people in the process of construction of pond and maintenance and recommendation.
Cost – Benefits analysis
Since this aspect was analyzed in the upgrading project report, it is not considered in detail
here. Instead of that, some limitations in the routine CBA are mentioned, for example, it
should include tangible and intangible, trade – off analysis.
RESULTS AND DISCUSSIONS
Modelling
The flooding around Xang Thoi pondwas simulated for four scenariosdescribed in section
“Methodology” and major results are presented as follows. In ”current” scenario (without
reservoir) flooding was noticed in 17 locations around the pond and the total volume of flood
was found to be 18800m3, where as in case of second scenario (without reservoir and
retrofitting of drains) flooding was noticed in 15 locations and the total volume of flooding
reduced to 16500m3. By including Xang Thoi reservoir in the model in the third scenario
(reservoir and no retrofit) the flooding was restricted to 12 locations and the total volume of
flooding was reduced to 17300m3. In the fourth scenario comprising the reservoir, retrofitting
of drains and enhanced pumping capacities, the flooding was limited to 10 locations and the
total flood volume was found to be 15000m3 (Table 1).
Table 1.Flooding scenarios around Xang Thoi pond
Scenarios
Number of
locations flooded
Volume of flood
(m3)
1. Current State
17
18800
2. Retrofitting of drains + enhanced pumping +
No Storage
15
16500
3. Xang Thoi pondas storage
12
17300
4. Xang Thoi pond as storage + retrofitting of
drains and enhanced pumping
10
15000

13th International Conference on Urban Drainage, Sarawak, Malaysia, 712 September 2014
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39.6
42.88
37
38
39
40
41
42
43
44
Scenario 3 - Storage
Scenario 4 - Storage + retrofit
Voulume (1000 m3)
Reservoir Utilisation
(35%)
(38%)
Figure 4.Utilization of reservoirs at different scenarios
From the scenarios it could be observed that the reservoir functions as detention storage and
could prevent flooding to a limited extent. Also from the results it was observed that the
reservoir capacity was not fully utilised. The maximum utilisation of reservoir was 38% of
the total volume of the reservoir (Error! Reference source not found.). The total volume of
the reservoir is 112500 m3. This provides scope for utilising the remaining capacity as
detention storage by further retrofitting the drains, removing pipeline bottle necks and
enhancing the slope of drains.
Monitoring
The Xang Thoi Pond is connected with Hau River though a Xang Thoi canal and Cai Khe
river. The pond and the canal are subjects to upgrade in the VUUP project. They both locate
in a poor community with many social issues including water pollution, slums. The pond was
unofficially used for vegetating i.e. Ipomoea aquatic, while the canal is occupied by slums. In
the past, due to the many years of accumulation solid waste and organic waste the water and
sediment quality of the Xang Thoi Pond and Canal has degraded and it not possible to use
these water for domestic purpose any more(VUUP CT-SPMU, 2003).After finishing the
construction, the areas become one of the favoured spot in Can Tho city. Water quality in the
pond is very much concerned by local citizens especially during dry season (see section
“social consensus”. Domestic wastewater is directly discharge to the pond since the sewer
collection project is not yet finished. Figure 5shows monitoring data of Xang Thoi pond in
2013, where ammoniumparameters exceed up to 4 to 8 times and the dissolve oxygen (DO)
was not enough comparing to the Vietnamese standard (QCVN 08/2008 B1, B2) for surface
water. It seems that the water quality of Xang Thoi pond was not improved, still highly
polluted before and after the construction.

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Figure 5: Water quality of the Xang Thoi pond (CEM: 12 samples of monthly data in 2013
from Can Tho Center Environmental Monitoring – CEM; ProACC: 3 samples in October
2013 from the ProACC – Urban project; Past_data: 6 samples in September 2003 (VUUP
CT-SPMU, 2003)).
Social consensus
The contributions of Xang Thoi retention pond on local environment has been assessed,
according to the respondents, the pond (1) makes the beautiful landscape; (2) makes the
weather equable; (3) reduces inundation and (4) assists the drainage of residential areas
(100.0%, 100.0%, 76.0% and 94.0%, respectively). As for the living of people, nearly all
respondents (98.0%) realize that the pondimproves liveable environment, e.g. place of
physical exercise and entertainment. Also in these positive changes, the cost of house and
land in the areas located along the pond has increased likely twice (86.0%). There are no
differences in recognition of the positive contributions of the retention pond among the local
authority and the community. However, there are still some constrains caused by the
retention pond, including polluted water causing bad smell (especially the dry season),
inundation (especially the rainy season). The still water contributes to spreading of
mosquitoes, health – risk related to children was also identified by respondents. In which, bad
smell and inundation are considered the main constraints (83.0% and 70.0%, respectively)
though it is not really concerned at this moment. Objectively, when comparing the positive
and negative impacts of Xang Thoi retention pond towards community, most of respondent
conclude that its positive impacts are larger than negative ones in both aspects i.e. local
environment and people. As the result, most of respondents state that they wholly support
theXang Thoi retention pond as part of the upgrading project. They also support the idea of
spreading out this pattern of retention pond to other areas (87.0%).
Cost – benefit analysis for decision making
Conventionally, decisions are made based on Cost-Benefit analysis (CBA) or (limited) Multi-
Criteria Analysis (MCA)(Ho, 2013). This approach likely countson tangible properties. The
intangible characters are not easy to quantify and thus is not provided either in cost or
benefits. The cost can be ecological degradation whilst the benefit can be landscape,
safety security. This benefit would be very important in term of mid-, long – term
perspective. Boyer and Polasky (2004) provided a very good review on valuing urban
wetland for both current and future societies. In addition, the conventional might not able to
address the trade – off among different options. In short, though the CBAcould be a very
important aspect in decision making processes for project implementation, it should be fully
addressed (i.e. both tangible and intangible) and placed in a trade – off framework.

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Steps toward for implementing retention ponds
Further improvement Xang Thoi pond
It has been observed that the typical issue in the Xang Thoi pond is environmental pollution.
The pollution comes from 2 sources: (1) domestic wastewater which is not connected to the
wastewater treatment yet; (2) illegal solid waste disposal. The first one can be done in the
near future since it is allocated in the wastewater treatment project funded by the German
KfW Bank
2
. Management of solid waste is not an easy task. This should require a broad
participation. Table 2below shows possible stakeholders in Can Tho should be involved in
solid waste management to prevent illegal disposal causing blocking sewer system in
surrounding of Xang Thoi pond.
Table 2: Task descriptions for different institutionsto preventillegal solid waste disposal.
ID
Institutions
Descriptions
1
Division of Urban Management
(district level)
Knowledge about waste & cleaning behavior by
the citizen
2
Urban sanitation company
Solid waste collection and treatment
3
Dept. of Natural Resources and
Environment
Joins provincial/district solid waste management
program for Can Tho
4
Ward’s People Committee
Encourage people to change behavior
5
Dept. of Information &
Communication
Public awareness building
6
Department of Health
Supports citizen in health issues
7
Union of Youth & Union of
Women
Pollution behavior programs and education for
citizens, e.g. Cleaning Days
8
Dept. of Education and Training
Educating programs for children & students
9
Division of environment and
resources (District level)
Observe how people waste & produce
(Sources: Modified from Clement(2014))
Figure 6: Floating Treatment wetland (FTW)in practice (Sources: Winston et al., 2013 (left),
and http://www.floatingislandinternational.com (right))
Together with pollution sources prevention i.e. reduce wastewater, solid waste disposed to
the pond, improving the pond water quality is also important. Ecological treatment for pond
2
http://canthotv.vn/tin-tuc/day-nhanh-tien-do-du-an-xu-ly-nuoc-thai/

13th International Conference on Urban Drainage, Sarawak, Malaysia, 712 September 2014
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restoration has been applied worldwide. This technique is not only improving water quality
but also can be an attractive spot in urban landscape (Faulwetter et al., 2011; Winston et al.,
2013). Floating treatment wetland (FTW) with aesthetic design could be the most suitable
technique to apply in Xang Thoi pond (Figure 6).
Lesson learnt for other projects
Up-grading projects are very important for developing countries. In addition, this can bring
more opportunities than just only sanitation improvement (Satterthwaite et al., 2007) e.g.
sustainable urban design. Recently, Can Tho city has been funded by World Bank for
upgrading Bun Xang pond(appr. 18 ha). Local leaders found that lessons learnt from this
Xang Thoi pond project are very crucial and will be considered seriously
3
. Based on this
study, a few recommended points can be as follows:
- Community participation in all stages of preparation, design and implementation
processes.
- Synchronizing with other project e.g. Wastewater treatment (collecting and pumping
wastewater) in time and space.
- Environmental, ecological sound planning and management
- New laws/legislation, incentives for house, urban design to adapt with climate change
based on Low Impact Development (LID) techniques e.g. green roof, detention
facility. This should be done together with guideline preparations.
CONCLUSIONS
Urban retention basin has been introduced as one of the promising measures to adapt with
negative impacts of climate change for urbanized areas. In this study we analyses the
possibility of implementing this facility Can Tho city, a largest city in Mekong delta. Since
the city facing with various reasons causing flooding such as extreme rainfall, up-stream
floods, high tide, retention pond could be a “soft” measure for Can Tho city to overcome this
flooding problem. However, implementation of retention pond is not an easy task. Technical
feasibility, cost/benefit features, stakeholders ’consensus as well as institutional arrangement
must be carefully considered. Based on analysis of the Xang Thoi pond, an urban up-graded
project these aspects are fully assessed by on different methods such as monitoring,
modeling, public consultations. The results showed that the upgraded urban project of the
Xang Thoi pond has significantly contributed amenities to urban dwellers. However, to
improve its functions (e.g. reduce flood peak) as well as to maintain it in a sustainable
manner (e.g. FTW, LID), it requires considerable efforts from different stakeholders. It is
hoped that lesson learnt from this study can be applied for other projects.
ACKNOWLEDGEMENT
PRoACC (Post-doctoral Programme on Climate Change Adaptation in the Mekong River
Basin) programme by the Netherlands Ministry of Development Cooperation (DGIS) through
the UNESCO-IHE Partnership Research Fund. It was carried our jointly with UNESCO-IHE
and Center of Water Management and Climate Change (WACC), Viet Nam National
University – Ho Chi Minh City.
3
http://baocantho.com.hvn/?mod=detnews&catid=72&id=136130

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