The flood that didn’t come

The period from August 14/15 to 19 2015 brought some exceptionally intense rainfall for parts of Germany. For parts of Saxony, rainfall sums were comparable to the June-2013 Flood.

Let’s make use of weather radar data and draw a first rough comparison (keep in mind that radar data might be erratic to a certain extend and the analyses have been carried out for the purpose of a relative comparison only):

Comparison of catchment-specific rainfall sums for the May/June-2013 precipitation and the recent August-2015 event.

Comparison of catchment-specific rainfall sums for the May/June-2013 precipitation and the recent August-2015 event. Radar data provided by German Weather Service (RADOLAN-RW product). Abbreviations indicate Saxon catchments, i.g., ZM-O3 means parts of the Zwickauer Mulde above an elevation of 300 m, and so forth..

However, a closer look on the regional precip. distribution reveals partly tremendous differences in the compared rain fields.

However, a closer look on the regional precipitation distribution reveals partly tremendous differences in the compared rainfields (left: 2013; right: 2014; difference plot in the center). Radar data provided by German Weather Service (RADOLAN-RW product).

Thus, the recent rainfall event was a major heavy/continual rain event. However, hydrological consequences were totally different; whereas by end on May 2013, soils were extensively saturated (or even over-saturated), in 2015 we were starting from an extensive drought situation with very low initial soil moisture.

Comparison of pre-event modeled usable field capacities showing the tremendous differences in the antecedent wetness conditions of the two considered events.

Comparison of pre-event modeled usable field capacities showing the tremendous differences in the antecedent wetness conditions of the two considered events. Imagery provided by German Weather Service.

This is directly reflected in a comparison of the reached August-2013 flood levels with the recent hydrological situation with no alert level reached at none of the >100 Saxon flood warning gauges.

Number of gauges with reached/exceeded alert levels (AL) in Saxony during the considered events:
2013: (lowest) AL1: 17 — AL2: 32 — AL3: 31 — (highest) AL4: 36
2015: zero (total: 105)

 

2015 post-event field capacity distribution showing the wetting of soils as a consequence of the preceding rainfall event.

2015 post-event field capacity distribution showing the wetting of soils as a consequence of the preceding rainfall event. Image provided by German Weather Service.

Last question: where did the water go? Well, event-specific runoff coefficients were very moderate for the recent heavy/continual rain event (around 20 per cent for larger catchments) and there was almost no impact on the generally falling groundwater levels (which is a consequence of the ongoing drought). Conclusion: a large amount of water went to the soils, which is reflected in the post-event increase of soil moisture/field capacity.

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Low Flow Situation Elbe // Update

Falling water levels in Germay (see information of the Federal Institute of Hydrology). Situation worst for the Elbe River.

It was quite obvious already during the last months: the Elbe might enter an extensive low flow situation, which now has become reality… Already since fall of 2014, precipitation (including snow accumulation) in the upper (here i.e., the Czech parts) catchment was behind normals.

During the last couple of days, water levels were further dropping:

3-hour mean water level observations for Elbe/Dresden. Stronger fluctuations as well as longer periods of constant flow are a result of dam operation in the Czech part of the catchment.

3-hour mean water level observations for Elbe/Dresden. Stronger fluctuations, as well as longer periods of constant flow, are a result of dam operation in the Czech part of the catchment.

Comparing the current low flow situation with other extreme events gives the follwing image:

Comparison of historic low flow situations with the current one (for gauge Dresden/Elbe). Imagery courtesy of BfG.

Comparison of historic low flow situations with the current one (for gauge Dresden/Elbe). Imagery courtesy of BfG.

What is quite delicate regarding the shown comparison, is the fact that the dam-system at the Vlatva River was in operation on from 1959; this means, that the situation would be even worse compared to the pre-1959 events, considering a current flow increase by 20 to 25 m³/s due to dam release in the Vlatva, Ohre, and Elbe catchment.

In other words, when talking of currently some 65 85 m³/s at gauge Dresden, without upstream control, flow would be virtually some 40 to 50 60 to 70 m³/s (which is important to be considered for any low flow frequency analysis). Furthermore, there is no silver lining yet to be seen… At least for the next couple of days, there is no significant precipitation forecasted for the upper catchment.

N.B.: Intitially given figures for low flow discharges had to be altered in the meantime due to operational flow measurements carried out by WSV required an adaption of the gauge’s rating curve.