Characterisation of flow regimes in previous conditions

6.3.1. Rationale for the characterisation of the previous conditions

Since re-naturalisation is a return to a previous situation it is necessary to characterise the flow regime in the previous conditions, before the urbanisation. This is a very hard work because it will be very difficult to access to all relevant data from the past. The data bases may exist but survey work is impossible.

The difficulties increase as the time of urbanisation is longer. For the old urban areas this may correspond to centuries. There is no way to retrieve the previous conditions at this far back.

The characterisation of flow regimes under previous conditions shall involve always some historical analysis, mainly of the urban evolution, where the evolution of the river reach characteristics may also be included. The historical analysis when related with short term periods (say less then 50 years) will be relatively simple, considering the possibility to interview the living persons with enough memory to track the past changes. For larger time periods really historical investigation shall be used.

The main objective of the study of the past urban evolution is to gather the elements to quantify the hydrologic and hydraulic parameters to be used in the characterisation of flow regimes in previous conditions. However, in some sites it should be admitted it is not possible to collect enough data to quantify with a minimum precision the past situation.

For the previous situation, or indeed, for any case, another rationale may be adopted: an ideal previous condition. This ideal previous condition shall be considered a good alternative. It may prevent a significant amount of work to track the historical evolution from the previous conditions to present one and may also be based on sound scientific knowledge about the natural conditions appropriate for that particular river basin.

The choice of the ideal previous condition may transform the concept of re-naturalisation flow regimes in the concept of the recreation of (equivalent to natural) flow regimes. The natural flow condition is considered the good one to support the previous existing living conditions. The creation of an ideal (equivalent to natural) flow regime to guaranty good living conditions (similar to the previous ones, even in the case they were not known) are as valid as to find out the natural previous conditions. Consequently, we may adopt always the term re-naturalisation, even in the case we did not find a real previous conditions, but only a theoretical approximation to that.

In WP2 questionnaire there are three questions on this subject:

III.e) major changes occurring within area (hydrology, ecology, water quality, urban area, etc) III.f) historical development of area

III.g) environmental development of area.

6.3.2. Characterisation of previous hydrological regime

For most cases the precipitation regime may be considered unchanged. It is considered independent from the urbanisation. Consequently the study of precipitation regime is valid for the present and past situations.

However, there are cases, particularly in large towns, where the presence of urban areas has induced measurable modifications to be taken into consideration. This will be directly considered if long term data bases are used, where these modifications are directly accessed in the study.

There is always more significant change in land use than in precipitation regime. The more drastic modifications are encountered always in the river basin cover. With the knowledge of the time evolution of the urban occupation a reference situation may be chosen. It may correspond to a situation where no urban occupation was present, or alternatively a light occupation existed. The choice is pure arbitrarily, is only a reference.

Knowing all necessary parameters for the precipitation regime and river basin land use it will be possible to characterise the previous condition in parallel to the characterisation done for the present condition, section 6.2.2. The same space and time elements shall be adopted in both conditions, previous and present, in order to compare the results. The comparison will provide the relevant modifications in terms of river discharges.

In simple cases a direct comparison of the past and present characteristics is straightforward. For more complex situations, some additional work shall be considered. Examples are the consideration of different previous conditions (more than one past, considering different past references), strong modification of river basin morphology, of river networks, presence of large hydraulic structures, and so on. For those cases it should be necessary to do more elaborated comparisons.

6.3.3. Characterisation of hydraulics in previous river channels

The modifications encountered in the river channels situated in urban areas may vary from insignificant to large deviations. Similarly to the hydrological modification analysis the knowledge of the time evolution of the urban occupation is the basis for the characterisation of the previous conditions, and a reference situation shall be found. It may correspond to a situation where no urban occupation was present, or alternatively a light occupation existed. The choice is as previously referred to, pure arbitrarily.

In general, there are not enough data to characterise in detail the previous conditions of the river channel, preventing a fine computation of the water levels for previous conditions. The exceptions may be found when old drawings or surveys exist. In the cases the changes are minor in the geometry of the river channel the relevant water level modifications are dependent only from the modifications of hydrological conditions. As referred to in 3.1 the general trend is the increase of the discharges for the same precipitation. Consequently, in previous conditions, even for the same river channel geometry, in previous conditions the water levels would be lower for the same precipitation events.

In simple cases, when there are light river channel modifications, the direct comparison of the past and present characteristics is straightforward. More complex comparisons arise when strong modifications in river network, or in the geometry of river channels. Some times, when the modifications are deep, there is not possible to compare for some parts of the river network. In these cases the differences may be only identified upstream or downstream the modified zone. Examples are the presence of large hydraulic structures, the strong modification on the river networks, the cover of the river channel, the deviation of the river channel, and so on.

For alluvial rivers there is an alternative to consider the previous conditions, the use of the theory of regime, as indicated in 2.2.5. Knowing the hydrological regime and the characteristics of the alluvium present in the site, even in the case this is covered by urban structures, the theory leads to the definition of appropriate previous geomorphology of the river channel.