190
5
“Hyper” Index (Hydraulic’s Permeability
and Elements of Retention)
A building lot capable of ensuring proper disposal of surface water and rainwater,
avoiding their stagnation and promoting a controlled outflow, contributes to mitigation of floods in urban area and non-urban areas, thus helping to reduce the hydraulic risk. The technical characteristics of building lots, necessary to meet these
conditions, are defined in the municipal Building Town-Planning Regulations
(RUE, binding requirement No. XXVIII.3.14).
The RUE define various principles to manage the hydraulic risk in the territory.
For defining these principles, several data are necessary: the size of the surface of
the building lot analysed, the fraction of the total area deemed to be impermeable,
the hydraulic load class (which makes it possible to differentiate the various basins
as “critical” and “non-critical”) and the type of action (new building or restoration
of areas already urbanised).
The applicable principles require a maximum value to be applied for the outgoing extent from the building lot in the planned scenario; they may be the
following:
• principle of controlled hydraulic increase: it allows a possible specific extent
increase of up to 100 %, 50 % or 30 % of the specific value of the outflow of the
area in question, as it was before the start of the action;
• hydraulic invariance principle: it keeps the specific value of the outflow of the
area in question unaltered, as it was before the start of the action;
• principle of hydraulic attenuation: it requires a specific outflow extent reduction
of at least 30 %, 40 % or 50 % of the specific value of the outflow of the area in
question, as it was before the start of the action.
All the principles are based on the specific value of the outflow of the area in
question unaltered, as it was before the start of the action. This value is usually calculated when the hydraulic sheet of the building lot in question is drafted and
requires the number of square metres of the various yards and roofs to be specified.
These surfaces, associated with specific outflow coefficients and the hydrological
parameters defined in the RUE, make it possible to determine the size of the rainwater drainage network (with a return time of 20 years).
5
“Hyper” Index (Hydraulic’s Permeability
and Elements of Retention)
A building lot capable of ensuring proper disposal of surface water and rainwater,
avoiding their stagnation and promoting a controlled outflow, contributes to mitigation of floods in urban area and non-urban areas, thus helping to reduce the hydraulic risk. The technical characteristics of building lots, necessary to meet these
conditions, are defined in the municipal Building Town-Planning Regulations
(RUE, binding requirement No. XXVIII.3.14).
The RUE define various principles to manage the hydraulic risk in the territory.
For defining these principles, several data are necessary: the size of the surface of
the building lot analysed, the fraction of the total area deemed to be impermeable,
the hydraulic load class (which makes it possible to differentiate the various basins
as “critical” and “non-critical”) and the type of action (new building or restoration
of areas already urbanised).
The applicable principles require a maximum value to be applied for the outgoing extent from the building lot in the planned scenario; they may be the
following:
• principle of controlled hydraulic increase: it allows a possible specific extent
increase of up to 100 %, 50 % or 30 % of the specific value of the outflow of the
area in question, as it was before the start of the action;
• hydraulic invariance principle: it keeps the specific value of the outflow of the
area in question unaltered, as it was before the start of the action;
• principle of hydraulic attenuation: it requires a specific outflow extent reduction
of at least 30 %, 40 % or 50 % of the specific value of the outflow of the area in
question, as it was before the start of the action.
All the principles are based on the specific value of the outflow of the area in
question unaltered, as it was before the start of the action. This value is usually calculated when the hydraulic sheet of the building lot in question is drafted and
requires the number of square metres of the various yards and roofs to be specified.
These surfaces, associated with specific outflow coefficients and the hydrological
parameters defined in the RUE, make it possible to determine the size of the rainwater drainage network (with a return time of 20 years).