Text by Mark Voorendt
Hamburg is a major trade and harbour city in the north of Germany, situated about 110 kilometres from the mouth of the river Elbe. The city was flooded in 1962, after which additional flood protection measures have been carried out. The North Sea flood of 1962 was a natural disaster affecting mainly the coastal regions of Germany and in particular the city of Hamburg in the night from 16 to 17 February 1962. In total, the homes of about 60 000 people were destroyed, and the death toll amounted to 315 in Hamburg. The flood was caused by a low-pressure system approaching the German Bight (Deutsche Bucht), coming from the southern Polar Sea. A storm with a wind force of 9 Beaufort and peak wind speeds of 200 km/h pushed water into the German Bight, leading to a water surge the dikes could not withstand. The water reached a level of NN + 5,70 m, which was 0,46 m higher than the highest water level up to then (registered in 1825). Breaches along the coast and the rivers Elbe and Weser led to widespread flooding of huge areas. Especially dikes that had not been heightened after the storm surge of 1952 were heavily damaged while most sea dikes withstood the surge (Wikipedia, 2012). More than 60 breaches occurred in the dikes with a total length of about 1.5 kilometres. The flooded area amounted 12 500 hectares, about 1/6th of the total area of Hamburg (Landesbetrieb SBG, 2012).
Emergency plans were implemented later and dikes were shortened and strengthened, leaving some river arms and bays detached from the sea. The design water retaining height was raised up to NN + 6,70 m and many dikes were reinforced, also in horizontal direction with more gentle slopes. In January 1976 a storm surge exceeded the one of 1962, leading to a water level of NN + 6,45 m. The reinforced dikes, however, were sufficiently high and stable to withstand this water, but there was much damage in the less protected harbour area. The erection of a storm surge barrier in the Elbe mouth (near Brokdorf ) was studied, but could not be agreed upon by the various Bundesländer. In the mid 90s, a flood protection construction programme was started to raise the retaining height with about one metre (to NN + 7,30 at St. Pauli, about 2 kmWest of Hamburg).
The calculation method was more sophisticated now, taking local hydraulic conditions like wave run-up into account per dike section. This led to varying retaining heights of NN + 7,50mup to NN + 9,25m(Landesbetrieb SBG, 2012).
In 2000 a start was made with a project of the redevelopment of an old harbour area in between the Speicherstadt and the Elbe. This new area, the HafenCity, is intended for work, living, retail-trade, recreation, gastronomy and culture2. The HafenCity area, however, is located outside the area protected by dikes (see figure 2.28). Proximity to large expanses of water, namely, is what gives the area much of its charm; dikes would have deprived it of the many exciting sight lines down to the water. It appeared also to be troublesome to start constructing the buildings before the 126 hectare area would have been surrounded by dikes (Website HafenCity, 2012). By elevating the buildings on plinths made of mounds, HafenCity is connected with the existing city.
All new buildings stand on artificial bases eight meters above sea level - safe for the most extreme flooding. On the sides exposed to wind, such as the southern sides of Strandkai and Überseequartier, the external perimeter lies at NN + 8,03 to NN + 8,60 meters. It is the responsibility of the private developers of buildings to put these artificial compacted bases in place, so their number is growing as the number of buildings increases. This has dispensed with any need for premature financing of flood-protection measures years - or even decades - ahead of the sale and deployment of the sites concerned.
The interior of flood-secure plinths provides ample space for underground car garages, which means that almost all stationary traffic can be accommodated. The mounds solution therefore also makes a significant contribution to reducing the volume of private transport in the new part of town. No additional sites for above-ground parking blocks will be needed as a result, which also contributes to the effective use of ground surfaces as a resource. Roads and bridges are also being built above the flood-line at least 7.5meters above sea level.
A broad strip up to 15 meters wide along the edges of the restored historic quays is down at the existing 4,00 to 5,50 meter level of the HafenCity area and provides 10,5 kilometers of waterside walks. This considerably adds up to public urban space right next to the water. In the western part of HafenCity many of these squares and promenades are already in constant use. Thus the mounds solution also has the side-effect of allowing a new topography to take shape - which affects the character and quality of the district (Website HafenCity, 2012). The HafenCity can continue to function virtually without restriction even during a flood and despite its ’island’ situation. In cases of high water, a few underground parking garage entrances along Am Sandtorkai and Brooktorkai (directly opposite the Speicherstadt) do have to close their flood gates. This is because, if the roadways passing directly adjacent to the historic warehouses had been retrospectively elevated, the identity and function of the whole Speicherstadt ensemble would have been affected. Planners of the listed Speicherstadt had worked on the assumption that their area could be flooded in cases of extreme high water (Website HafenCity, 2012). Moreover, windows should be able to withstand high water pressures and steel bulkheads have to prevent eventual damage on the glass windows by floating debris. It is not allowed to live on these ground floors, so they are used for car parks, restaurants and offices. The apartment blocks have different access levels in order to cope with varying water levels around the blocks. There are also escape routes on different heights to guarantee a safe evacuation if needed (Stalenberg, 2010).
The new multifunctional quay walls along the ’Baumwall’ and ’Vorsetzen’, just West of the Hafencity, contain a parking level, public toilets on street level and three places designated for building constructions. These structures are planned to include a restaurant and a kiosk. Due to securing flood protection during the construction period, demolition and construction of the new flood protection barrier must only take place under cover of temporary flood protection. See figures 2.29 to 2.34 for some examples.