942_FloodriskmanagementforLaMojana

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Flood risk management for La Mojana


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1206824-000

© Deltares, 2012

Erik Mosselman

Matthijs Kok

Hans Leenen

Meindert Van


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1206824-000-ZWS-0017, Version 3.1, 17 August 2012, final

Flood risk management for La Mojana i

Contents

1 Introduction 1

1.1 Background 1

1.2 Objectives 1

1.3 Project team 2

1.4 Activities 3

2 Review of reports by Universidad Nacional de Colombia 5

2.1 General considerations 5

2.2 Hydrology 5

2.2.1

Data, methodology and results 6

2.2.2 Comments and observations 6

2.2.3 Recommendations 9

2.3 Hydraulic modelling 9

2.4 Geo-engineering and dike stability 11

2.5 Risk analysis 14

3 Recommendations from a Dutch perspective 15

3.1 Risk-based approach 15

3.2 Integrated approaches 18

3.2.1 Consideration of the complete hydrodynamic system 18

3.2.2

Inclusion of fluvial morphodynamics 19

3.2.3

Integrated management approach on basin scale 19

3.3 Governance 19

3.4 Hydraulic engineering sector 20

4 Recommendations for short-term actions 23

5 Conclusions and recommendations 25

5.1 Conclusions 25

5.2 Recommendations 25

5.3 Proposed actions 27

5.4 Translation of proposed actions into potential projects 27

6 References 29


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Flood risk management for La Mojana 1 of 29

1 Introduction

1.1 Background

La Mojana is a lowland area of 5600 km2 in the provinces of Bolívar, Sucre, Córdoba and

Antioquia. As an inner delta of the Río Cauca, it has a topography that for about half of its

area consists of a network of lakes, swamps and canals. The area is sparsely populated:

440,000 inhabitants. Homesteads and towns are often located on ridges of abandoned

alluvial river beds. Poverty is considerable, also when compared to other areas of Colombia.

The area suffers from frequent flooding, apparently related to the occurrence of La Niña. The

original (pre-Hispanic) population intervened already in the hydrology of the area. Morerecently dikes have been constructed along the rivers, with varying degrees of success.

La Mojana was hit hard in 2010 when extensive flooding occurred all over the country. This

was the rationale for the Government of Colombia to designate La Mojana as one of the

priority areas to improve the safety against flooding. The government announced an

extensive investment programme, focusing on the improvement of water safety, the

construction of infrastructure, the promotion of cattle farming, reforestation and socio-

economic development.

The Dutch have a long-standing history and expertise in water management and safety

against flooding, both in the Netherlands and abroad, leading to the adoption of the Dutch

words “dique” and “pólder” into the Spanish language. One of the most recent examples isthat Dutch engineers were invited to review flood risk management practices in the area of

New Orleans (USA) after the Katrina flood disaster in 2005.

It is against this background that the President of Colombia requested the Netherlands to

assist in looking for “Dutch proof” solutions to water safety in a number of areas in Colombia,

including La Mojana. In 2011 and 2012, Universidad Nacional de Colombia carried out a

study to define strategies for improving water safety and socio-economic conditions in La

Mojana. Deltares, in association with HKV and DHV, was commissioned to validate the water

safety parts of this study.

1.2 Objectives

The validation has the following objectives:

1 To validate the Universidad Nacional studies by reviewing the technical and scientific

quality of the following reports: Universidad Nacional (2011, Summary), Universidad

Nacional (2011, Vol. 1), Universidad Nacional (2011, Vol. 2), Chapters 1-3 of

Universidad Nacional (2011, Vol. 3), Chapters 1-2 and Section 3.1 of Universidad

Nacional (2011, Vol. 4) and Section 5.3, Chapter 6 and Annexes 5.3 and 5.4 of

Universidad Nacional (2011, Vol. 5);

2 To express thoughts on the approach to flood risk management in the Universidad

Nacional reports from a Dutch perspective;

3 To provide recommendations for immediate actions on a short term.

Strictly speaking, the third objective is outside the scope of the assignment. The team

included it out of a firm conviction that such recommendations were needed urgently.


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The team was asked to answer the following questions in particular:

• What is the quality of the hydraulic modelling?

• What is the quality of the data used and the data analysis?

• Do the conclusions follow logically from the analysis?

• What is your opinion on the set of measures proposed?

• What would be your recommendations for flood risk management in La Mojana, and do

these recommendations comply with the recommendations of the Universidad Nacional

de Colombia?

• What are your thoughts about a “Dutch proof” approach to water safety in La Mojana?

An important aspect of flooding in La Mojana is pollution of inundated areas with mercury

originating from legal and illegal mining activities. This aspect has been addressed byUniversidad Nacional but was beyond the scope of the present assignment.

1.3 Project team

The project team had the following composition:

• Erik Mosselman (Deltares): team leader and senior specialist in river engineering and

hydraulic modelling;

• Matthijs Kok (HKV Consultants): senior expert in risk-based approaches to water safety;

• Hans Leenen (DHV / Royal HaskoningDHV): senior expert in hydrology;• Meindert Van (Deltares): senior expert in geo-engineering and stability of flood

defences.

Juliana Lopez de la Cruz (Deltares) supported the team with the analysis of reports in the

Spanish language, as well as with her own expertise in risk analysis and stability of flood

defences.

The work was carried out in close collaboration with the Fondo de Adaptación, the Royal

Dutch Embassy in Bogotá and the Dutch Ministry of Infrastructure and Environment.


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Flood risk management for La Mojana 3 of 29

Figure 1.1. Validation mission team with Colombian counterparts and Royal Dutch Embassy representative.

1.4 Activities

The complete team visited Colombia from 23 to 27 June 2012, immediately after the

assignment on 19 June. The team members reviewed and discussed the reports. They visited

La Mojana by helicopter on 25 June, including a meeting with local citizens at Nechí.

They discussed the studies with experts of Universidad Nacional in face-to-face meetings as

well as by telephone and Skype. A draft report and a draft presentation were delivered on 26

July.

Erik Mosselman and Hans Leenen carried out a second mission to Colombia from 30 July to

4 August 2012, together with Renske Peters, Director Water of the Dutch Ministry of

Infrastructure and Environment. The team’s findings were tuned with the Fondo de

Adaptación, the Dutch Ministry and the Royal Dutch Embassy. On August 1, the findings

were presented in the Presidential Palace to the Secretary-General of the Presidency of the

Republic, the Minister of Environment and Sustainable Development, the Minister of

Transport, and high officials of the government and the Fondo de Adaptación.


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2 Review of reports by Universidad Nacional de Colombia

2.1 General considerations

The reports by Universidad Nacional show that the studies have been carried out by capable

experts with a thorough knowledge of the area. The studies are multidisciplinary: they

address technical, economical, environmental and social aspects. The reports represent a

rich source of information and are a good basis for any further study.

A key element in the solution of Universidad Nacional is a dike with weirs or spillways along

the left bank of the Río Cauca, for controlled diversion of part of the water from the riverthrough water courses of La Mojana. The spillways would be opened one by one from

upstream to downstream, without possibilities to close them. Water courses forming the main

drainage arteries of La Mojana would be dredged and cleared to improve their discharge

conveyance capacity. The dike should be located sufficiently far from the river to safeguard it

from undermining by river bank erosion.

The team comprehends the university’s frustration that the dike has not been implemented

according to their original design and guidelines. This explains at least partly the failure of the

system in 2010.

The team would nonetheless like to raise the following general questions and comments:

1 Would it not be desirable to have gates to close the spillways in order to have more

control on the flooding of La Mojana?

2 The controlled flooding is intended for the range of discharges between exceedance

probabilities of 1/25 per year and 1/100 per year. No flooding occurs at lower

discharges. Flooding can no longer be controlled at higher discharges. The

corresponding water levels are not well known for the following reasons: (1) discharge

statistics are available from a relatively short period and are subject to interannual

variations; (2) the stage-discharge relation (providing the relation between water levels

and discharges) is in practice variable due to changes in river bedforms, vegetation,

etc.; (3) the Cauca river is reported to be subject to morphological change, affecting

water levels. How certain can we be that dike crests and spillways have the intended

elevations?3 Which solutions are proposed for discharges higher than the one with an exceedance

probability of 1/100 per year? An inhabitant of La Mojana living 75 years would have a

probability of 53% to experience such conditions at least once in his or her life.

2.2 Hydrology

The hydrological parts of the Universidad Nacional studies have been reviewed on the basis

of Universidad Nacional (2011, Summary), Universidad Nacional (2011, Vol. 2), Universidad

Nacional (2011, Vol. 3), Universidad Nacional (2011, Vol. 4) and a discussion with Prof. Lilian

Posada.


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2.2.1 Data, methodology and results

Relevant data, methodology and results of the hydrological study are presented in Chapter

4.1 of Universidad Nacional (2011, Vol. 2) (Evaluación dique marginal) and in Annex 1-2:

(Estudio Hidroclimatológico, Región de La Mojana). The team concludes that Universidad

Nacional has carried out a thorough study, using an impressive amount of available data of

river discharges and water levels, recalibration of discharge rating curves and different types

of meteorological data, such as precipitation, temperature and evaporation. All data have

been obtained from the Instituto de Hidrología, Meteorología y Estudios Ambientales

(IDEAM).

The study focuses on river discharge characteristics and water level characteristics in a

number of relevant hydrometric stations along the Río Cauca near the La Mojana region.

Particular areas of interest in the study are trends, extreme events and the effects of El Niñoand La Niña. The study has used a number of standard statistical techniques to extract

important information from the available data.

The main results of the hydrological studies are summarized as follows:

1. The study shows a strong indication of a positive trend in river discharges;

2. Effects of El Niño and La Niña are clearly reflected in the monthly discharges of the Río

Cauca;

3. The study has produced a table of estimated return periods for extreme river discharges

at a number of relevant hydrometric stations.

The team discussed hydrological issues with Prof. Lilian Posada of Universidad Nacional. It

finds no reason to question the data or the methodologies used by Universidad Nacional, nor

does it find any reason to doubt the results. In some respects the available data may be

elaborated further to enhance understanding of the water system and the frequencies of

extreme events to assist in developing a safety strategy from a technical point of view.

Existing legislation in Colombia does not provide tangible norms for dike construction, so it is

understandable that the studies by Universidad Nacional do not extrapolate the data further

than what is usually required by the authorities. The comments and observations below take

into account the Dutch approach to flood risk management.

2.2.2 Comments and observations

Probability of extreme events

Although Universidad Nacional provides estimates for return periods of river discharges, it is

unfortunate that Tables 9 and 10 of Universidad Nacional (2011, Vol. 2) stop at a return

period of 100 years. Prof. Lilian Posada explained the team that the existing legislation in

Colombia does not provide strict norms for dike safety, i.e. norms in terms of probability of

failure levels, but that Colombian authorities generally require a safety level with a return

period of 100 years (exceedance probability 1% per year). This safety level implies a 53%

probability of flooding during a lifetime of 75 years, a high probability that would be regarded

as a very low safety level in the Netherlands. The dikes along the main Dutch rivers are

designed at safety levels with return periods of 1250 years (exceedance probability 1/1250

per year), and along smaller rivers at safety levels with return periods of 300 years(exceedance probability 1/300 per year).


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The probability of the 2010 flood discharge on the Río Cauca can be estimated by

extrapolating the data further. This provides a better understanding of the flood that tookplace in 2010. The team used data of the hydrometric station of La Coquera near Caucasia

(codigo 2624702 of the IDEAM data), which has a good data set of about 40 years. This

station can be considered representative for an evaluation of the probabilities of Río Cauca

discharges that are relevant to La Mojana. Figure 2.1 shows how a statistical analysis of the

discharges at La Coquera provides an estimate of the return period of the 2010 discharge. A

Gumbel and Lognormal 3 probability distribution provide the best fit (based on KS-statistic

criteria). The discharge at La Coquera reached a value of nearly 6000 m3/s (5958 m

3/s),

which corresponds to a return period of about 200-300 years when extrapolating the

probability curves.

Figure 2.1. La Coquera, return period estimates for discharge [m3/s] on Cauca river, including year 2010.

Local residents in Nechí informed the team that the flood discharge of 2010 overtopped the

existing dikes. It is thus clear that the existing dike has a safety level below a 200 years return

period, most likely in the order of 100 years or even less. The next highest discharge of

4478 m3/s occurred back in 1973.

Within a period of 40 years of available data, a 1/200-per -year flood discharge has a

probability of occurrence of about 20%. The occurrence of the 2010 flood is hence not as

extreme as it may have seemed at first sight. The extreme events of measured daily

discharges are depicted in Figure 2.2.

Universidad Nacional design and water safety

The design by Universidad Nacional includes a number of spillways (‘diques fusibles’,

‘vertederos’) from the Río Cauca to receiving channels in La Mojana. If we interpret thisdesign correctly, these spillways will overtop with a frequency of 1/25 per year. The dikes ave

been designed with some permeability to allow a certain continuous flow of water into La

Mojana. We have doubts about the logic of this design.


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It would mean that it is unavoidable that La Mojana will be inundated with an average

frequency of once every 25 years. The extent and depth of inundation will depend on themagnitude of the flood on the Río Cauca. For instance, a flood with the same magnitude as in

2010 would inundate large areas of La Mojana again. If the objective of the spillways and dike

permeability is to have some water flow through La Mojana at regular intervals and at the

same time to provide an acceptable safety level, it would make more sense to have a solid

dike construction with a number of gates or spillways that can be completely closed when the

river level becomes too high. In the Netherlands there are examples of this principle. This

principle is the opposite of the design by Universidad Nacional.

Potential effects of climate change

Probabilities of occurrence from the statistical analysis do not provide the complete picture.

The statistics may change in time, either by cyclic variations, for instance on a decadal scaleor a systematic trend of climate change. Universidad Nacional shows a strong indication that

discharges of the Río Cauca are increasing. In addition, research by the Climate Prediction

Center (CPC) shows a long-term trend in the Oceanic Niño Index (ONI) with rising sea

temperatures. This results in a stronger influence of La Niña with higher precipitation in the

Colombian winter and higher extreme discharges in Río Cauca. The extreme events in the

Rio Cauca discharges as depicted in Figure 2.2 have a strong correlation with the yearly

minima in the ONI as depicted in Figure 2.3 (correlation coefficient 0.43). We conclude that

the statistical characteristics of the Rio Cauca discharge are changing. This means that the

return period of the extreme event in 2010 may be shorter than follows from the record of

historic data. The actual probability of occurrence may be higher.

Figure 2.2. La Coquera, discharge extreme events per year.


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Figure 2.3. Minima of ONI per year.

2.2.3 Recommendations

1. A dike safety standard of 1/100 per year is relatively low. It is recommended to review thisstandard by developing a spatial vision or plan and by carrying out a societal cost-benefit

analysis. The resulting standards should be documented in the form of a ‘decreto’ and

become an integral part of Colombian water management practices, to be respected by

all regional and local authorities. Note that this dike safety standard is not intended as the

safety standard for all areas of La Mojana.

2. The team recommends further research into the changes of statistical characteristics of

discharges of the Río Cauca, in view of the long-term changing of the El Niño and La

Niña phenomena. This will help in providing a better grip on a safety strategy for the

future.

3. The team recommends an integral hydrodynamic evaluation of the La Mojana inundations

by including the hydrodynamics of the Río Magdalena, the upstream catchment of the Río

Cauca and the impact of all infrastructural works along the rivers upstream.4. The team recommends revising the design of the spillways (‘diques fusibles’, ‘vertederos’)

into a design with a higher safety level than 1/25 per year, allowing water inflow at low

water levels of the Río Cauca, but being able to prevent inflows at higher levels.

2.3 Hydraulic modelling

The hydraulic modelling in the Universidad Nacional studies has been reviewed on the basis

of Camacho & Lees (1999), Universidad Nacional (2011, Summary), Universidad Nacional

(2011, Vol. 1), Universidad Nacional (2011, Vol. 3) and a discussion with Prof. Luis Alejandro

Camacho.

The hydraulic modelling is based on the 1D Saint-Venant equations for rivers and water

courses and on a multilinear discrete lag-cascade (MDLC) model of channel routing


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(Camacho & Lees, 1999) for swamps and lakes. The major obstacle for translating model

results into inundation maps is the lack of digital elevation data.One of the functions of models is that they integrate knowledge in a systematic way. The

hydraulic model of Universidad Nacional is without doubt the best available integrator of

present knowledge on the hydraulic functioning of La Mojana.

The Terms of Reference require only hydraulic modelling, but Universidad Nacional (2011,

Vol. 1) puts much emphasis on water quality modelling, which was not a part of the

assignment. The water quality modelling is not addressed in this validation.

Universidad Nacional (2011, Vol. 1) motivates the selection of the modelling approach from a

series of shortcomings in other models, such as MIKE11 and WASP. However, the report

does not present an objective comparison between the merits and demerits of the different

models, for instance by means of a multi-criteria analysis. A central argument in themotivation is that the MDLC model, unlike MIKE11 (p.10) and WASP (p.11), can be calibrated

in an objective way. This refers to two aspects. The first aspect is that Universidad Nacional

has access to the code of MDLC, which allows implementation of the objective calibration

procedure of the university. The second aspect refers to the objective calibration procedure

itself, which seeks to offer a solution to the problem of equifinality, i.e. the problem that

different combinations of calibration parameter values can produce the same results.

It is essentially impossible to decide which combination of parameter values corresponds to

the real situation. The objective procedure of Universidad Nacional is based on searching an

optimum combination by carrying out thousands of computations in a monte-carlo approach

using GLUE. The resulting combination is not necessarily closest to the real values in thefield, but is reproducible, as other modellers would arrive at the same result by following this

procedure. This is why Universidad Nacional calls the method ‘objective’. Other models are

considered less suited for this approach because the corresponding longer computation times

complicate the execution of thousands of simulations for calibration. The team appreciates

the elegance of the method, but questions its usefulness, as it does not bring model results

closer to reality.

Another justification for using the MDLC model is that more sophisticated models require

more input, for which no data are available. An example is the input of wind data in MIKE21,

which are not available for La Mojana. The team would like to put forward, however, that not

all input options need to be used. MIKE21 computations could be carried out easily by

assuming all wind speeds to be equal to zero.

Data are available for a limited number of locations in La Mojana only. That is why data for

other locations are generated by interpolation, in order to use more data in the calibration of

the model. The team would not opt for this approach, because it essentially means that data

from one model are compared with data from another model.

Universidad Nacional (2011, Vol. 1) considers the hydraulic model suitable for the present

phase of pre-design and evaluation. The implication is that more sophisticated models might

become appropriate in next phases. Professor Camacho confirmed this view during the

discussion with the team.

Camacho & Lees (1999) state their MDLC model to be valid only where no hydraulicstructures or physical channel changes occur at the downstream location, or where no


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backwater effects due to tides, tributaries or critical control sections affect the hydraulic

response at the boundary.Such conditions seem relevant, however, for La Mojana. Professor Camacho explained in the

discussion with the team that this shortcoming has now been solved by using backwater

results from HEC-RAS computations.

The model of Universidad Nacional can deal with the transport of fine suspended sediments.

It cannot deal, however, with the sediment transport that is relevant for fluvial

morphodynamics. It contains neither transport equations for sand and gravel nor the Exner

sediment balance equation for erosion and sedimentation.

Universidad Nacional (2011, Vol. 3) reviews the feasibility of conveying water discharges up

to 1200 m3/s by a canal parallel to the road between San Marcos and Achí. The report

concludes that this canal is not feasible, because its low hydraulic gradient would necessitatevery large cross-sections. This is a plausible conclusion.

Manning coefficients are erroneously claimed to be dimensionless (‘adm’ on p.19).

Recommendations:

1. Use the existing model as long as no other models are available;

2. Collect digital elevation data to improve model results and to allow the preparation of

inundation maps on the basis of model results;

3. Develop a more physics-based mathematical model for next phases requiring more detail

and precision, provided that digital elevation data are available;4. Develop an overall hydrodynamic mathematical model of the complete river system on a

catchment scale.

2.4 Geo-engineering and dike stabi lity

The parts of the Universidad Nacional studies on geo-engineering and dike stability have

been reviewed on the basis of Universidad Nacional (2011, Summary), Universidad Nacional

(2011, Vol. 2) and a Skype conference on 3 July 2012. More background information was

probably available for the authors of those reports. The team is aware of not having all the

background information such as geological studies or laboratory testing reports. The

comments in this review hence should not be understood as undervaluing the quality of the

reports, but as remarks, questions and suggestions based on having this report informationonly.

A dike built in La Mojana with material available and according to the specifications reported

will need inspection and maintenance, because it will deteriorate in time. Erosion processes

and vegetation will gradually change the outside profile and biologic and weathering

processes will gradually change the impermeability of the core. Steep inner or outer slopes up

to 1:1, which have been proposed in some designs, will erode relatively quickly (in a period of

some years). It is advised to have slopes not steeper than 1:3 (vertical:horizontal). Even then,

retrofitting each 1 or 2 decades could probably be necessary. Such foreseeable relatively

large-scale upgrading has to be considered in spatial planning and in rules for allowing

adjacent building.


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A detailed study of the soil layers (up to 20 metre depth) is necessary for a good design of the

dike. The structure and soil parameters of permeable and impermeable layers under the dikeshould be known.

Current and future designs should incorporate safety factors to account for uncertainties in

soil parameters as well as variation in subsoil layer sequences and thicknesses in order to

assure the reliability of the dike structure. The permeable subsoil layers will have water

pressures that increase when the river level increases. This can cause backward erosion

(piping) and uplift stability failure of the dike. Therefore, the dike body should preferably be

constructed with a relatively wide base. If a wide base is not possible, the dike should have a

properly designed, built and maintained filter construction.

In the chapters on geotechnics, Universidad Nacional (2011, Vol. 2) studies the subsoil of the

dike base only for the first few metres of the first layer under the dike body. It is known fromgeology that subsoil layers in areas with a meandering river vary considerably in location,

type and thickness. Clay, silt, sand, gravel and maybe peat layers can be expected to be

present under the dike. Note: In the Skype conference on July 3rd it was explained that the

subsoil information is not available at the current dike location because the dike has not been

built at the originally designed location. More information on the subsoil layers would have

been available at the original location.

Due to the lack of information on the subsoil layers where the dike was built, Universidad

Nacional (2011, Vol. 2) models the material under the dike only as one type of material called

‘Base Dique’. This layer has a relatively high cohesion and high friction angle in Sections 1 to

3 and low values in Section 4. These properties match with clay or silt layers. In case of sandand gravel layers, the cohesion would be close to zero and the friction angle around 30 to 35

degrees. Nonetheless, it is expected that sand and gravel layers will be present at varying

depths under the dike base. These layers will be much more permeable. The hydraulic head

in the river will also partly be present in these layers, its value depending on the permeability

of the layer and the hydraulic boundary conditions.

Due to an increasing hydraulic head in the dike subsoil sand and gravel layers during a flood

event, mechanisms such as backward erosion (also called ‘piping’) could occur and lead to

breaching of the dike. For the reported typical sizes of the dikes, backward erosion is

expected to be a dominant failure mechanism and should be studied in the assessments of

what happened during the 2010 flood. It should also be incorporated in the design

calculations for upgrading the dike. In addition, it is advised to study at which locations sandboils due to backward erosion have been observed in the field during high water levels on the

river. During the team’s field visit to La Mojana, two persons reported that sand boils had

been observed in this area during the high waters on the river, which confirms the possible

danger of this mechanism.

Considering the failure mechanism related to macro-stability, the hydraulic head in the sand

and gravel layers will have a significant effect too because it reduces the shear strength in the

layer just above the permeable layer. Increased pore pressures will reduce shear strength.

This is also a time dependent process. The longer the river water level is high, the larger the

area of shear strength reduction in and under the dike. Dominant slip planes can and often

will become partially horizontal with sliding along the interface between dike base and

underlying permeable layer. Bishop-type of circular slip plane calculations will not capture thismechanism and predictions with a circular slip plane are unsafe compared with the partially

horizontal slip planes. Other design calculations are needed in case of these hydraulic heads


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in sand or gravel layers, i.e. uplift stability calculations (Van et al, 2005) or Spencer-type of

slip-plane calculations that include the time dependent shear strength reduction.Finite-element computations could capture this mechanism, but are not advised, because

effective stresses around zero due to the uplift condition will often cause severe numerical

problems, making it difficult to obtain stable and reproducible numerical results.

Some detailed comments on the report by Universidad Nacional (2011, Vol. 2) are given

below.

Chapter 5 (from Section 5.3):

• Page 134, Section 5.3.2: The crossing drains (alcantarillas) are constructed to regulate

the water in- and outflow through the dike. However, maintenance is necessary to

prevent vegetation to block the closing mechanism of this structure (foto 58). Such astructure should also have been designed to create a measure against seepage and

concentrated leakage erosion due to settlement of the concrete structure or shrinkage

or erosion of the dike at the interface with the structure.

• Page 137, Figure 76: How is the protection against erosion of the overflow dike

designed, since flow around the corners will increase erosion?

• Page 139, Figure 77: How is backwards erosion under the dike prevented? How to

maintain this section? For example, is vegetation allowed or does the dike have to be

maintained free of vegetation?

Chapter 6:

• In general, this chapter is not clearly structured in:

– What is the original design?

– What is the situation as built before the flood?

– What is the assessment of the failures after the flood?

– What is the cross-section of the new design (only comments on height are given,

but no cross-sections of the newly proposed design).

• The breaches have been assessed in the field, but no calculations have been reported

in order to explain numerically what happened in 2010. This hindcasting is needed for a

good design for retrofitting or rebuilding.

• Crossing structures in dikes (i.e. drains and other pipelines) and transitions of such a

structure to the dike soil body is a weak point in the dike that should be avoided.

Concentrated pipe leakage, that initiated one of the 2010 breaches, should be avoided.If a crossing structure is nonetheless needed, this structure and its interface to the dike

body need careful design as well as a high level of inspection and quality control during

and after construction.

• Results of the tests of Table 25 have not been reported in the report and have not be

reviewed. In general the quality of the field work (i.e. sample disturbance or drying) and

the testing laboratory (i.e. climate controlled) has a large influence on the results.

• The soil base layers that are more than 2 metres below the dike have not been

modelled in the stability analyses in Figures 102, 104, 106 and 108. However, slip

planes could cross these layers (in Figure 104 and 108, the slip circle reaches this

boundary and deeper circles could be less safe). Also in these analyses, a sand or

gravel layer below the dike base layer could be present, which is in connection with the

river and will have a high water pressure that reduces shear strength. This should beaccounted for. Horizontal slip planes can be present with a significantly lower stability

factor. The phreatic line in the dike body is modelled as a line along the interface


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between ‘dique actual’ and ‘contradique proyectado’. This is only the case if the

‘contradique proyectado’ is much more permeable than the ‘dique actual’.This should also continue to be so during the design lifetime and may not be lost due to

vegetation or other blockage processes. In practice therefore a drain would be needed,

but better is to model a higher, more realistic phreatic line during design floods in the

‘contradique proyectado’ layer. This will reduce the calcutated stability factor. Maybe

also micro-stability of the soil layer should be checked if the phreatic line is so high that

water would seep out of the slope. Sections in Tramo 2 (page 172) have been designed

with steep slopes up to 1:1. These slopes are not stable and will erode in time. Only due

to cohesion in the material this design would be possible, but drying and wetting will

crack the material and reduce cohesion. Slopes are advised to be built not steeper than

1:3 (vertical:horizontal).

Recommendations:

1. It is necessary to add more geological information on the subsequent layers under the

dike base, with their types, parameters and thicknesses. Especially the permeable layers

should be known, because of their increased pressure head during a flood event.

2. Failure mechanisms due to piping or backward erosion should be checked explicitly in the

design and in the assessment of the dike, since these are expected to be dominant for

dike failure.

3. The influence of reduced shear strength due to partial-uplift effects as a result of an

increasing hydraulic head in the sand or gravel layer under the dike base should be

incorporated in the design and the assessment of the dike.

2.5 Risk analysis

The risk analysis in the Universidad Nacional studies has been reviewed on the basis of

Universidad Nacional (2011, Summary) and Universidad Nacional (2011, Vol. 5).

The university limited its study to the evaluation and mapping of vulnerability indices. No

complete risk analysis has been carried out, despite valuable elements for such an analysis.

The study does not consider any relation between hydraulic parameters and economic

damage. It does not address the issues of acceptable risk and the corresponding safety

standard.

This lead to the following main recommendation:

1. Carry out a proper risk analysis study in close interaction with the development of a

spatial vision or plan.


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3 Recommendations from a Dutch perspective

3.1 Risk-based approach

Flood risk management in the Dutch perspective is based on annual flood probabilities and

the consequences of a flood. The simplest way of presenting this approach is: ‘the higher the

consequences, the lower the flood probability to be achieved’ (Figure 3.1). The concept of

‘acceptable risk’ is used in answering the question how much we should invest in risk

reduction and how much flood risk the society wants to accept (as is often done with any type

of risk!). A societal cost-benefit analysis (which includes all costs and all benefits) is needed

to assess the amount of investments, as well as the type of measures that can be justified(protection works, river training, spatial planning, type of buildings in the flooded area, crisis

management, inspection during high river floods in order to repair works, etc).

Figure 3.1. Protection level in terms of an acceptable probability of flooding, as a result of evaluating the damage in

case of flooding (low in left panel, high in r ight panel) and investments to avoid this damage. A high potential

damage (right panel) implies an optimum at a high safety level, and hence a low acceptable probability of

flooding, i.e. a high acceptable return period.

The two most important potential consequences of a flood in the Netherlands are the

economic damage (which includes business losses) and ‘loss of life’. The hydraulic

parameters that determine the consequences of a flood are water depth, water velocity and

rate of water level rise. These hydraulic consequences are computed using a hydraulic model

for each possible flood event, called a flood scenario (which might be extreme or veryextreme). In case of flood defences, one or more breaches are schematized in the model. Of

course, an infinite number of flood scenarios is possible. To make the approach feasible in

practice, a representative sample of scenarios is selected. An example of a flood scenario for

a Dutch polder is given in Figure 3.2. For each flood scenario, the economic damage and the

loss of life can be determined (using land use damage and mortality functions). The next step

is to assess the corresponding flooding probabilities. These probabilities are determined by

the hydraulic load (water levels, waves), but also by the strength of the flood defences.

Often, also in the Netherlands, safety standards are expressed as ‘exceedance probabilities’

of design water levels. The strength of the flood defence is not explicitly included in the safety

standard. From a risk perspective it is to be preferred to include the strength of the flood

defence in the safety standard. A stronger dike has a lower probability of failure than arelatively weak dike.


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Moreover, a flood defence may not only fail because of overtopping, but also by other failure

mechanisms, such as for example lack of stability and piping. However, a safety standard interms of an exceedance frequency per year of the design water level (or the design river

discharge) is more practical. The strength of the dike is subsequently designed in such a way

that the dike does not fail at water levels below the design water level, for all possible failure

mechanisms. If the water level is above the design water level, the dike might fail and breach.

Figure 3.2. Example of a computed flood scenario in the Netherlands (Texel island, worst-case flood). The colours

represent the water depth (dark blue is deeper than light blue, white represents no flooding, and yellow

represents sand dunes. The maximum water depth is 4 m.

Important questions in flood risk management are: What is the actual flood risk? Is thatacceptable? What societal values need to be protected? These questions are about

protecting the actual economic values and about economic development of the area. It is also

possible to differentiate with respect to land use: areas which have to be protected, areas

where flooding does not cause much damage, and nature areas where flooding is needed to

maintain the ecological values. Spatial planning is thus a key element in flood risk

management. The development of a spatial vision or plan is challenging, because it involves

local interests and it requires a national strategy.

La Mojana is a rural area, with people living in relatively small towns. The current flood risk

policy is to protect the area with a flood defence along one side of the Cauca river, with a

number of spillways for controlled flooding of the area. This strategy does make sense, but its

implementation can be concluded to be not fully successful yet.


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A more complete risk-based approach would involve a spatial vision or plan for the economic

development of the area, a map with possible water depths (for several flood scenarios) anda plan of spatial differentiation of safety standards for different compartments of the area. This

leads to the following recommendations:

• Develop a spatial vision or plan for the different uses of land in the area (agriculture,

nature, housing, etc). This is a matter of defining which level of protection (probability of

flooding) will be given to which areas within La Mojana and involves decisions on where

to build valuable properties and where to allow continued frequent flooding, for instance

for environmental reasons;

• Perform a societal cost-benefit analysis with respect to investments to reduce flooding

risk, which is a combination of reducing the probability of flooding and reducing the

adverse consequences of flooding.

• Invest in reducing the damage in case of flooding by organizing crisis management,especially if the acceptable probability of flooding from the societal cost-benefit analysis

is high (on the order of 1/100 per year rather than 1/1000 per year).

It is furthermore recommended to address also the adverse consequences of flood protection

measures. The very presence of flood defences attracts occupation and economic

development. This may be desirable, but can imply the need to increase the safety standard

accordingly. Flood defences may be used for roads and houses, making later dike

reinforcement costly or unfeasible. A practical recommendation in this light is to construct the

road on a shoulder of the dike and to make the crest of the dike suitable for light traffic only

(Figure 3.3).

Figure 3.3. Dike cross-section with crest and shoulder.

Another important issue is crisis management. One of its elements is the inspection of the

flood defences during a high river discharge. In the Netherlands, water boards areresponsible for these inspections (which are carried out every 4 or 8 hours at high-water

conditions). Groups of volunteers (‘dike army’) support the water boards in this activity. Each

group of volunteers has its own dike segment for inspection. They report small damages in

and around the dike, and subsequently these small damages can be repaired without

resulting in breaches. Furthermore, communication about expected levels of high water is

important because of the evacuation of people and cattle. This leads to the following main

recommendation from the Dutch perspective:

• Prepare a plan for dike inspections and possible repair actions in periods of high river

floods;

• Prepare plans for crisis management (early warning, evacuation, rescue, food

distribution, medical assistance).


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3.2 Integrated approaches

An integrated approach to flood risk management is strongly recommended. This implies in

particular: (1) consideration of the complete hydrodynamic system, (2) inclusion of fluvial

morphodynamics, and (3) an integrated management approach on basin scale.

3.2.1 Consideration of the complete hydrodynamic system

The Universidad Nacional studies consider the area of La Mojana only. However, the

inundations of La Mojana cannot be assessed in isolation, but should be treated in an

integrated assessment of the whole water system (Figure 3.4). The hydrodynamics of the Río

Cauca and the La Mojana region depend not only on water levels and discharges in the Río

Cauca and drainage in La Mojana, but also on conditions of the Río Magdalena at the

downstream boundary and on upstream conditions in the Río Cauca and other rivers flowingto La Mojana. The downstream conditions on the Río Magdalena affect the water levels on

the Río Cauca and the drainage possibilities from La Mojana through backwater effects.

Infrastructural works further upstream along the Río Cauca may have an impact on the risk

assessment downstream. For instance, dike reinforcement works upstream could increase

the inundation risk at La Mojana when the resulting faster flood wave propagation reduces the

attenuation of flood waves.

Another question to be answered is whether storage of flood water in La Mojana is needed to

mitigate flooding risks on the Río Magdalena further downstream.

An analysis of the complete hydrodynamic system may also reveal alternative solutions forreducing flooding risks in La Mojana. Possible examples might be dike relocation on the Río

Cauca, the use of existing reservoirs on the Río Magdalena and modification of the Brazo de

Loba.

Figure 3.4. Hydrodynamic relations between La Mojana and upstream and downstream river reaches (map adapted

from Universidad Nacional map).


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3.2.2 Inclusion of fluvial morphodynamics

Bank erosion related to the natural meandering behaviour of the rivers appears to be a

mechanism of dike failure and a consideration in tracing an appropriate dike alignment. It is

one of the signs that fluvial morphodynamics is an important aspect of the river system.

However, fluvial morphodynamics has received little attention in the studies by Universidad

Nacional. Relevant questions are:

• What are the expected rates of bank erosion along the Río Cauca? This may be derived

from a multi-temporal bank-line analysis of satellite images or aerial photographs,

building upon information in Universidad Nacional (2011, Vol. 2);

• What is the evolution of longitudinal bed profiles of the main rivers in the area? In the

absence of detailed bed topography surveys, this evolution may be inferred from the

time development of stage-discharge relationships (rating curves) at hydrometricstations (‘specific-gauge analysis’).

• Is there any evidence for the hypothesis on page 33 of Universidad Nacional (2011,

Vol. 3) that instability of the Cauca-Magdalena confluence can be ascribed to increased

sediment loads as a result of mining activities?

• Does sedimentation in La Mojana contribute to flooding? Sedimentation could also be

an effect rather than a cause of flooding.

Morphodynamic studies for La Mojana might benefit from the current hydraulic,

sedimentological and morphological studies by Universidad del Norte for the Río Cauca

between Colorado and Achí (INVIAS, convenio 1069-2011).

3.2.3 Integrated management approach on basin scale

An integrated approach implies a management of the system on basin scale. The team

understands that INVIAS was responsible for the dike construction along La Mojana. At the

same time CORPOMOJANA is involved as environmental organisation. A number of

authorities and organisations seem to be responsible for the dikes along the Río Cauca, but it

is unclear how responsibilities and tasks are divided. Making a serious effort in enhancing

dike safety will require careful consideration by the Colombian authorities of institutional

arrangements of responsibilities and maintenance, to develop legislation, and to enforce such

legislation. The mere construction of dikes is not sufficient. The organization that will hold

responsibility for dike inspection and maintenance on a long-term time scale of decades,

needs to be in place. This is a long-term effort, but if such effort is not initiated, dikes willcontinue to fail, leading to high expenditures on the long run.

In the light of basin-scale management, the creation of a national modelling centre is most

welcome.

3.3 Governance

Water boards arose in the Netherlands about 800 years ago to co-operate on dike building

and safety against flooding. They boast to be the oldest democratic institutions of the country

and are still in effect today. They are responsible for dike inspection, maintenance and

repairs. They could serve as an example for similar organizations in Colombia.


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Major hydraulic interventions to reduce flooding risk in the Netherlands in the twentieth

century surpassed the scale of local water boards and were imposed in a technocraticmanner. Increasing resistance among the population in the 1970s and 1980s subsequently

led to the development of methods to involve all stakeholders in decision making. A key

example is the development of the Planning Kit for the Room for the River programme. This

may serve as an inspiration for a similar approach to finding solutions for La Mojana.

For the Room for the River programme in the Netherlands, all stakeholders had been invited

to propose interventions to reduce flood levels and to improve ecological conditions by giving

more space to the river. This resulted in 693 proposed local measures. Subsequently, the

effects of each measure were assessed by calculating the costs, by evaluating the effects on

nature, landscape and cultural heritage, by calculating the number of houses to be

demolished and, last but not least, by computing the effects on water levels at the design

discharge using a two-dimensional hydraulic model. The results were used to construct aneasy-to-use and interactive tool or “Planning Kit” to compose and analyze integrated

strategies. The user could choose combinations of interventions on a map of the river on the

computer screen, and visualise the resulting lowering of flood levels, the corresponding costs

and the effects on nature, landscape, cultural heritage and houses. By interactively using this

tool, even non-specialist stakeholders discovered which interventions were effective and

feasible, and which ones were not, by taking the perspective of an engineer who has to solve

the problem. In this way, suitable solutions were not imposed by government officials or

technocrats, but discovered by the people themselves. This greatly enhanced the acceptance

of the final solution. The tool turned out to be very popular. It was really used by a broad

section of the stakeholders (including mayors, etc), not only by people with an affinity to

computers or technology. Moreover, the tool made the available information accessible to allparties in the same way. It thus proved to be a truly democratic instrument.

The existing tool cannot be applied to La Mojana straight away. However, it may serve as an

inspiration for following a similar philosophy and approach. Both specialists and non-

specialists will be able to propose interventions. If the number of proposed interventions is

sufficiently high, we could develop a Planning Kit fully dedicated to La Mojana. Its

specifications will be developed in close collaboration with the stakeholders.

3.4 Hydraulic engineering sector

The team did not see any prominent role for engineering consultancy firms in studies for the

reduction of flooding risk in La Mojana. Fondo de Adaptación explained the team to preferuniversities because the corresponding tender procedures are easier. The team therefore

recommends to review these procedures, as engineering consultancy firms can play a

valuable distinct role that is complementary to the role of universities. Consultancy firms are

better equipped for practical advice to bridge the gap between study and implementation.

The competition between universities on the engineering consultancy market hampers

national academic collaboration. For instance, the reports by Universidad Nacional, dated 23

April 2012, do not mention the mathematical modelling of the Río Cauca between Colorado

and Achí by Universidad del Norte, which started 1 September 2011. Academic collaboration

might be improved by providing incentives for collaboration in scientific research funding and

by organizing annual national workshops on La Mojana studies.


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Skilled contractors are important because the safety of a dike depends not only on good

design but also on the quality of how the structure is built. Two items are very important inquality control:

1. The compaction of the dike body. Especially, the impermeable clay core should be

compacted (statically) very well in small layers (not thicker than 20 cm). The other dike

body parts should also be well compacted, which may be achieved dynamically.

2. The clay material should have a precise water content during construction. Too dry will

not be good for compaction and lead to more permeability. Too wet will result in later

cracks in the core due to shrinkage.

Contractors with experience in building reservoir dams have often experience with these two

items.


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4 Recommendations for short-term actions

Although strictly speaking outside the scope of the assignment, the project team provided the

following recommendations for short-term actions during its first mission to Colombia:

1 Timely closure of breaches, in view of (1) reduction of inundation extent and duration,

(2) favourable working conditions for repairs and recovery, (3) connection of Nechí to

national road network (improving also the accessibility of other areas), and (4)

restoration of original flood defence system based on a dike along the left bank of the

Cauca;

2 Closure works and sufficiently far from the river, in accordance with therecommendations by Universidad Nacional. The scour hole in the dike breach is

probably deep. The new dike should hence be constructed at least at some distance

from the main scour hole, in an area with shallower flow channels and lower flow

velocities (this produced the semicircular dike alignments that are still visible on maps of

the Netherlands). Channel closure will stop the flow. Careful attention should be given to

backward erosion and heave mechanisms during and after closure. The width (in the

flow direction) of the base of the closure dike should therefore be relatively large (18

times the water elevation difference between both sides in the Netherlands, in case of a

clay dike on a sand layer). A properly designed filter construction can also prevent

backward erosion during closure. In general, the dike width should also be sufficient to

prevent undermining due to backward erosion or heave at rising river water levels;

3 The immediate works should be completed before the rainy season. Immediate action isrequired if the works are to be executed this year (which is recommended):

3.1 Strong leadership and on-site supervision, possibly by an international consultancy firm,

in close collaboration with the Fondo de Adaptación;

3.2 Immediate study of safe distance for new alignment of the dike and closure works based

on recent trends in river bank erosion (at least 100 m away from the river at Nuevo

Mundo, possibly more). A multi-temporal bank-line analysis of satellite images or aerial

photographs may support estimates of future bank erosion;

3.3 Immediate start of design and tender documents;

3.4 Immediate start of tracing the new alignment of the dike and the closure works, carrying

out topographic and bathymetric surveys along the alignment, identifying sources of

construction materials, planning the logistics, and stock piling material;

3.5 Preparation of a contingency plan for actions in case the works are not completedbefore the next rainy season;

4 Contracting out part of the work to well-equipped contractors (e.g. barges with stones),

but letting another part of the work for small local contractors and labour. This creates

not only local employment, but also local ownership of the dike;

5 Acceptance of paying indemnification to land owners if the dike is located further away

from the river;

6 The team does not recommend bank stabilization or river training as a part of short-term

action. Adequate bank stabilization or river training along the Río Cauca would require

implementation over a considerable length and would require special techniques such

as the application of launching and falling aprons.

Subsequently, the Fondo de Adaptación has already given immediate follow-up by

contracting a local expert team, by fielding a mission of this team to Nuevo Mundo and by

starting the development of a workplan.


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The short-term actions recommended here are a part of the solutions on a medium term anda long term. Closure of the breaches is a prerequisite for creating conditions to implement

works for long-term sustainability. The recommendation to abstain from bank stabilization

implies that locally periodic dike relocation may be necessary due to bank erosion. This

should be seen as normal maintenance of the system. It is recommended to construct the

new dike section well before the abandonment of the old dike section. This new dike would

still allow use of the land thus excised.


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5 Conclusions and recommendations

5.1 Conclusions

The studies by Universidad Nacional are impressive and provide a good basis for finding

solutions to water safety in La Mojana. Nonetheless, improvements are possible, as proposed

in this report. The most important finding from a Dutch perspective is the absence of a

complete risk-based approach, despite valuable elements for such an approach in

Universidad Nacional (2011, Vol. 2). Another important observation is that the hydrodynamics

of La Mojana have been analyzed without considering the relations with upstream and

downstream river reaches that on the one hand might offer solutions to reduce flooding risk inLa Mojana and on the other hand might experience adverse effects of interventions in La

Mojana.

The hydraulic MDLC model of Universidad Nacional is the best available integrator of present

knowledge on the hydraulic functioning of La Mojana. It is a good model in the present

phases of pre-design and evaluation, and the best that can be expected in the absence of

detailed data on terrain elevations. Next phases will require collection of digital elevation data

as well as a transition towards more physics-based models.

Conclusions generally follow logically from the analyses in the Universidad Nacional reports.

An example is the conclusion that a canal parallel to the road between San Marcos and Achí

is not feasible. Another example is the recommendation to dredge and clear the watercourses that form the main drainage arteries of La Mojana. The selection of the MDLC model

does not follow logically from the analysis in the report, but can nonetheless be supported for

the initial phases of pre-design and evaluation. The safety levels of the dike along the Río

Cauca and its spillways are not supported by an underlying analysis.

5.2 Recommendations

A complete risk-based approach is recommended in order to develop rational safety

standards and an appropriate spatial vision or plan for La Mojana. It is recommended to

involve all local stakeholders actively in the process, using inspiration from Dutch experiences

with the Planning Kit at the start of the Room for the River programme. The solutionsproposed by Universidad Nacional will form the starting points, but alternatives will arise.

These alternatives may enhance the plan of Universidad Nacional, or at least enhance the

public acceptance of this plan if the alternatives turn out to be less suitable. This

recommendation can be elaborated through the following elements:

• Develop a spatial vision or plan for the different uses of land in the area, considering

different alternatives;

• Develop land use damage and mortality functions in relation to hydraulic parameters;

• Identify representative flooding scenarios using a hydraulic model;

• Define different investment options to reduce flooding risks;

• Perform a societal cost-benefit analysis with respect to investments to reduce floodingrisk;

• Evaluate the effects of different spatial visions or plans and flooding risk reduction

measures for representative flooding scenarios;


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• Present the results of the evaluation in an accessible way to all stakeholders for

decision making on measures to be implemented, inspired by the Planning Kit approachin the Netherlands;

• Select the final plan in a participatory way.

A more integrated approach is recommended, based on consideration of the complete

hydrodynamic system, inclusion of fluvial morphodynamics and an integrated approach to

management on a basin scale.

The review of the studies on hydrology leads to the following recommendations:

• Document the dike safety standard resulting from the complete risk-based analysis in

the form of a ‘decreto’ so that it becomes an integral part of Colombian water

management practices, to be respected by all regional and local authorities;• Carry out further research into the changes of statistical characteristics of discharges of

the Río Cauca, in view of the long-term changing of the El Niño and La Niña

phenomena. This will help in providing a better grip on a safety strategy for the future;

• Carry out an integral hydrodynamic evaluation of the La Mojana inundations by

including the hydrodynamics of the Río Magdalena, the upstream catchment of the Río

Cauca and the impact of all infrastructural works along the rivers upstream;

• Revise the design of the weirs or spillways (‘diques fusibles’, ‘vertederos’) in the dike

along the Río Cauca into a design with a higher safety level than 1/25 per year, allowing

water inflow at low river water levels, but being able to prevent inflows at higher levels.

The review of the studies on hydraulic modelling results in the following recommendations:

• Use the existing model of Universidad Nacional as long as no other models are

available, as it is the best available integrator of knowledge on the hydraulic functioning

of La Mojana;

• Collect digital elevation data to improve model results and to allow the preparation of

inundation maps on the basis of model results;

• Develop a more physics-based mathematical model for next phases requiring more

detail and precision, provided that digital elevation data are available;

• Develop an overall hydrodynamic mathematical model of the complete river system on a

river basin scale.

The review of the studies on geo-engineering and dike stability gives rise to the followingrecommendations:

• Obtain more geological information on the subsequent layers under the dike base, with

their types, parameters and thicknesses. Especially the permeable layers should be

known, because of their increased pressure head during a flood event;

• Explicitly check failure mechanisms due to piping or backward erosion in the design and

in the assessment of the dike, since these are expected to be dominant for dike failure;

• Incorporate the influence of reduced shear strength due to partial-uplift effects in the

design and the assessment of the dike. These partial-uplift effects are a result of an

increasing hydraulic head in the sand or gravel layer under the dike base.

Finally, short-term actions to close the breaches in the dike along the Río Cauca arerecommended in accordance with Chapter 4.


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5.3 Proposed actions

Table 5.1 proposes seven concrete actions to implement the recommendations, along with

first products that could be delivered in 2013 as intermediate steps and possible contributions

from the Netherlands.

Table 5.1. Concrete actions to implement the recommendations.

5.4 Translation of proposed actions into potential projects

During the second mission to Colombia, part of the proposed actions to implement the

recommendations have been elaborated further into five concrete projects that could be

prepared for tendering on a short term. These potential projects are presented in Tables 5.2

to 5.6. The action numbers in these tables refer to the actions in Table 5.1.

Table 5.2. Potential project on dike safety standards.

Project nr: 1 Title: Dike safety standards

Contents: Definition of dike safety standards for dikes in Colombia, for different classes of hinterland

(economic damage, population). These standards regard dikes, not areas in Colombia.

Required expertise: risk analysis expert, dike design expert, economist

Results: Input for dike design in Acti on 6. First exercises for follow-up of Action 4 (for discussion and

introduction of risk-based concepts in Colombia)


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Table 5.3. Potential project with risk analysis at basin scale.Project nr: 2 Title: River analysis at basin scale

Contents: Analysis of whole river system ( Action 3). Quick set-up of 1D hydrodynamic model ( Action

5) of main rivers (Cauca, Nechí, San Jorge, Brazo de Loba, Magdalena, La Mojana) using available

data. Computations for analysis of effects of different parts of the system and effects of measures

Required expertise: hydrodynamic modeller

Results: Insight into effects from interventions upstream and downstream on flood water levels in La

Mojana. Insight into effects from interventions in La Mojana on flooding risk downstream. Design

conditions for dike. Effects of alternatives on flooding risk.

Table 5.4. Potential project for preliminary inundation maps and stakeholder involvement.

Project nr: 3 Title: Preliminary inundation maps and stakeholder involvement

Contents: Collection of digital elevation data (SLR, lidar) ( Action 5). Preparation of preliminary

inundation maps using digital elevation data and existing model of Universidad Nacional. Invitation of

stakeholders to give vision on land use development and possible measures.

Required expertise: hydrodynamic modeller, people’s participation expert for organizing and guiding

consultation, good contacts with citizens in La Mojana

Results: First inundation maps and consultation results

Table 5.5. Potential project on functional dike design.

Project nr: 4 Title: Functional dike design

Contents: Sharp formulation of specifications for dike design including safety standards ( Action 6).Required expertise: hydraulic engineer, geotechnical engineer (dike design expert)

Results: Basis for actual design or basis for tendering of turnkey project for the full lifecycle of design,

financing, implementation and maintenance (under Dutch lead with Colombian partners)

Table 5.6. Potential project on institutional framework.

Project nr: 5 Title: Institutional framework

Contents: Development of advice for reinforcing the existing institutional framework ( Ac tion 7), in close

consultation with Colombian partners

Required expertise: governance expert, possibly representatives from institutions in other countries

Results: Recommendations to reinforce the existing institutional framework


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6 References

Camacho, L.A. & M.J. Lees (1999), Multilinear discrete lag-cascade model for channel

routing. Journal of Hydrology, Vol. 226, pp.30-47.

Universidad Nacional (2011, Summary), Estudios, analisis y recomendaciones para el

ordenamiento ambiental y el desarollo territorial de La Mojana; Resumen ejecutivo.

Informe final, Bogotá, 23 April 2012.

Universidad Nacional (2011, Vol. 1), Estudios, analisis y recomendaciones para el

ordenamiento ambiental y el desarollo territorial de La Mojana; Volumen 1:Modelación Matemática. Final report, Author L.A. Camacho, Bogotá, 23 April 2012.


ordenamiento ambiental y el desarollo territorial de La Mojana; Volumen 2: Actividad

A - Evaluación Dique Marginal. Final report, Author L. Posada, Bogotá, 23 April 2012.



B - Viabilidad Canal Paralelo. Final report, Author L.A. Camacho, Bogotá, 23 April

2012.

Universidad Nacional (2011, Vol. 4), Estudios, analisis y recomendaciones para elordenamiento ambiental y el desarollo territorial de La Mojana; Volumen 4: Actividad

C - Priorización de Caños. Final report, Author L. Posada, Bogotá, 23 April 2012.



D - Diagnóstico Prácticas e Instrumentos Gestión. Final report, Author V. Botero,

Bogotá, 23 April 2012.

Van, M.A., A.R. Koelewijn & F.B.J. Barends (2005), Uplift phenomenon: model and validation.

International Journal of Geomechanics – Special issue on Soft Clay Engineering and

Soft Clay Improvement, No. 5, pp. 98-106, June 2005.

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