Assessing flood proof urban developments

6.1 Assessing flood proof urban developments

A flood proof urban development is achieved by implementing different design concepts and technical solutions. But under which conditions is an urban development actually flood proof? This depends on the set objective for the area which in turn depends on the characteristics of an area. From the experience gained with the case study Stadswerven, a simple method for assessing urban designs aiming at flood proofness was developed. The method is illustrated in Figure 6.1.

No

Area characteristics

Set goal

Definition of

criteria on

Yes

Goals were

which goals

set

are assessed

realistically? achievable?

Development of urban design(s)

Assessment of

Goals

design(s)

are not reached

Goals are reached

Figure 6.1

Flow chart for assessing urban designs on flood proof aspects.

For the assessment of urban designs, a goal is set to which the design should conform. It is necessary to choose a realistic goal in accordance to the area characteristics. For an area with an expected water depth of 4 meters, it is not realistic to implement a flood proof design aiming at minimal disturbance of daily life. Next, criteria are defined by which a design can be assessed, for instance a safety level or the presence of safe havens. These criteria should be in accordance to the set goal. If a design does not conform to the set criteria, it will be necessary to question the feasibility of the goal and/or the set of criteria.

Setting flood proof goals When considering flood proof targets, it is seen that in general three target levels can

be distinguished:

1. No casualties,

2. and limited damage,

3. and minimal disruption of daily life. With each level the targets are set higher than the previous level. The first level tackles

the safety aspects. Level 2 incorporates the prevention of damage loss due to flooding. For level 3 the goals are set the highest by only allowing minimum disruption of daily life.

Area specific flood characteristics When defining flood proof targets for an urban development, one is dependant on the characteristics of an area. In an area where the flooding depth reaches 5 meters, it will

be almost impossible to develop the area such that minimum disturbance of daily life is accomplished. The ‘Ruimtelijk Plan Bureau’ (Pols, 2007) classifies an area by two flooding characteristics; the rate at which an area becomes flooded and the maximum flooding depth. This results in four classes of risk zones (Figure 6.2).

rapid

Risk zone 1;

Risk zone 2;

flooding

No urban development

Adapted development

Risk zone 3;

Risk zone 4;

Incorporate evacuation

Traditional development

strategy

Areas possibly suitable as

Slow

refuge zones

Figure 6.2 Risk zones by flooding rate and depth

According to Pols it is advised to implement adjusted building concepts in areas where small flooding depths are expected in combination with rapid flooding which leaves no time to evacuate (risk zone 2). The adjusted building concepts provide a safe shelter for people within the flooded area. It is questioned though by others if it is cost effective to use adjusted building concepts in areas where the probability of flooding is very low. If the expected water depths are small, and the rate of flooding is low as well (risk zone 4), enough time is available to evacuate. Therefore no additional measures are suggested unless the area will be used to shelter people from more seriously struck areas.

When looking at the areas where significant flooding depth are to be expected, it is seen that adjusted building concepts are not applicable (risk zones 1 and 3).

In areas where the flooding rate is low (risk zone 3), the focus will need to be on adequate evacuation planning including adequate evacuation routes. For the remaining areas (risk zone 1) with a rapid flood rate, it is even recommended to prevent urban development. In this cases it is advised though to also take the predictability of a flood (lead time) into consideration. If the lead time is large enough, enough time will be available to evacuate the area.

The risk zones 1 and 3 aim at preventing casualties (level 1). Areas which are of type ‘risk zone 2’, target at preventing damages and even at a minimal disruption of daily life. It should be kept in mind though that the choice to implement adjusted building concepts will also depend on cost effectiveness criteria.

The RPB methodology only considers the water depths and rate at which an area is flooded. This method provides a practical way to classify flooding areas. If required, other characteristics can be taken into account to characterise an area such as the probability of a flood, the predictability of a flood (lead time), water velocities, drainage time etc.

Criteria

A checklist with criteria was developed which can be used to assess a flood proof urban development and aid in testing if the flood proof goals have been reached (Table 6.1). The checklist should be seen as an outline on how to test an urban development. These criteria were developed from the experience with the Stadswerven case study and focus on the water related aspects relevant for areas unprotected by dikes. It provides an onset and can be extended if necessary e.g. with non water related criteria.

Table 6.1 Water related criteria for evaluation of flood proof urban design based on the Stadswerven case.

Target Criterion

Water related level

Explanation

parameter

1 Safety level

Design water levels indicate a

Design water level

desired safety level. A flood proof urban design should be able to provide safety up to this safety level.

Water depths velocities and

Maximum reached

Strong currents and deep water

Water velocities depths are within

do not provide a safe

environment. A design should

safety limits

therefore be tested on these parameters.

Anticipation on

Foreseen river water climate change

Due to climate change, it is

expected that the frequency of

levels

high river discharges will increase. A sea level rise is also foreseen. These aspects result in increased water levels. A flood proof urban design should therefore take climate change high river discharges will increase. A sea level rise is also foreseen. These aspects result in increased water levels. A flood proof urban design should therefore take climate change

Strength buildings

Strong currents, deeper water,

Water depths

waves and duration of flooding

Water velocity

can cause buildings to be

Wave impact

Flood duration Safety river under

damaged up to collapsing.

River velocity average conditions

Not only the safety during a

flood should be taken into consideration, but also the safety during daily life.

Safe havens within People can be taken by surprise Velocity of rising of area

by rapidly rising waters.

the water Water depths and water velocities (in relation to wadability and thus possibility to reach safe havens)

The predictability of The choice for providing Lead time the flood taken into evacuation possibilities or account for the

shelters within an area depends

design

among others on the predictability of a flood.

Measures to reduce In areas where the overland Water velocities velocities in streets velocity of water is moderate to taken into account

high, the water velocities in the streets can increase due to the distance between buildings. Measures can be taken for instance by increasing the width or layout of the street.

Effect on other

General flood areas

Changing water patterns in one

area, can have an impact on the modelling for larger water patterns in another area.

area

This should be taken into consideration.

Insight in probability All technical measures have a Probability of failure of failure of the

Insight in floodwall and/or

possibility of failing. Failing

consequences of anticipation on

could result in an uncontrolled

failure possible failure

flooding situation.

Orientation of

Water depths buildings such that

In areas where the overland

Water velocities forces on buildings

velocity of water is moderate to

high, the forces of the water and

do not reach

water depth can increase due to water depth can increase due to

the orientation of the buildings compared to the flow direction.

Incorporation of

Water depths flood proof

For evacuation, emergency

services as well as use for daily

infrastructure

life activities.

Sedimentation area such that

2 Design of public

If areas are designed to flood

frequently and sedimentation

sedimentation is

levels are high, this can cause

limited

unwanted residues in public areas and houses.

Incorporation of

Water depths flood proof

The electricity network will be

damaged if it comes into contact

electrical services

with water. This can be

and other vital

prevented by locating electricity

infrastructure

outlet points and sockets on a safe level.

3 Design concepts

Water levels and buildings

In flood proof development

Probability of water correspond to water character are implemented into

areas with different flooding

levels levels

a design. Some areas are

Confidence interval

lowered to provide frequent

for the water levels

flooding. Others are elevated to prevent flooding. The ground levels for these areas depend on the water levels and on the confidence interval of the water levels.

Controlled flooding The advantage of flood proof

building, is that flooding can occur in a controlled manner. To what extent do the design concepts provide control over the flooding?

Flood proof sewage Sewage systems can be the system

cause of disease if sewage water spreads through a flooded area. In addition damage can be done to sewage systems during flooding.

It is seen that the aspects for which hydraulic input is needed, are mainly focussed on the safety aspects (target level 1). Other aspects, e.g. fresh water supply under flooding conditions, attractiveness of the design, flexibility to adapt to changing circumstances, cost effectiveness etc. are aspects to be considered by other disciplines.