Fload and coastal protection

The Aabach dam, which is operated by the water board of the same name as a drinking water dam with a flood protection function, has a shaft overflow as a spillway. Current measurements show that the flood discharges at the dam can be safely discharged in design cases 1, 2 and 3 according to DIN 19700-11 from a hydromechanical and structural point of view. Even the discharge of larger events, such as the maximum area precipitation and probably the highest flood inflow peak of 177m³/s, is still possible with appropriate peak reduction by lake retention without causing an overflow of the dam. DIN 19700-11 tolerates minor damage to the waterways from design case 2 onwards.

If material from said damage or deadwood clogs the shaft inlet or the outlet cross-section and the remaining risk of the runoff resulting from MGN with high initial water levels occurs, a crown jam or an overflow of the dam is possible. As a preventive measure, it is recommended that measures be taken to avoid the entry of floating debris into the HWE. THP was therefore commissioned to carry out a study to increase the safety of the HWE in order to analyse ways of reducing the risk of flooding and possible retention measures. To reduce the risk of clogging, measures both in the catchment area (e.g. wildwood barriers) and technical measures in the reservoir (constructions at the HWE, floating cable barriers near the HWE or at the inlets) were analysed, evaluated and illustrated by cost estimates and principle sketches.

The Karpke forebay is operated via a bypass line to increase the reservoir's supply and is defined as a ‘small dam’ in class 2. A secondary purpose of the Karpke is to discharge rainwater runoff into the tailwater. The barrier structure is an earth dam with a sheet pile wall installed in the centre as a seal. An embrasure serves as a spillway. In this area, the upper edge of the sheet pile wall is lower and fitted with a concrete head beam. The spillway ridge on the air side is secured by stone packages and leads into a reinforced channel. The barrier structure of the ‘Große Aa’ forebay dam is a zone dam with a sealing core and is classified as a class 2 dam. Damage-free discharge in the event of flooding takes place via the spillway, which consists of an intake tower, spillway tunnel, outlet structure and stilling basin. There are no facilities for seepage water monitoring or for measuring the groundwater level in either system.

Client: Aabach water board

Processing period: 2022 - 2023

Main data for the preliminary basin:
Year of construction: 1979
Reservoir capacity: 12,000 m³
Basin dimensions: Length 160 m, width 50 m, height: 4.4 m above foundation base

Main data forebay dam:
Year of construction: 1982
Storage capacity: 120,000 m³
Crown dimensions: length 140 m, width 6.4 m, height 9.5 m

Main dam data:
Year of construction: 1975-1979
Reservoir capacity: 1,350,000 m³
Crown dimensions: Length 450 m, width 15 m, height 40 m

Services provided:
Inspection of documents
Site visit with structural condition inspection and coordination meetings
Inspection and assessment with regard to compliance with the generally recognised rules of technology
Examination of possible solutions in the catchment area, pre-basin, pre-dam, main dam storage area
Examination of possible solutions at the drop shaft Variant evaluation, cost estimate, principle sketches
Final report