CASE STUDY: HYDROLOGY AND FLUVIAL GEOMORPHOLOGY

Question 1

Where is the case study?

Answer 1

Wairau Valley, Auckland, New Zealand.

Question 2

When is the case study?

Answer 2

The late 1960s.

Question 3

What are the features of the drainage basin?

Answer 3

• The drainage basin is approximately 12 square km, comprising low hills (less than 100m in altitude) and a marshy flood plain.
• The presence of clay makes much of the soil impermeable, causing minimal infiltration. Most water is transferred as overland flow.
• The lower course is also dominated by clay topsoil which is a significant factor in creating surface storage of water in the form of swamp/marsh land.
• The rainfall in Auckland is 1200-1500mm per ear. This, along with the impermeable surfaces, causes intermittent channels such as rills to form as water drains overland.
• The short distance between the watershed and the mouth creates a relatively steep and fast flow.
• Erosion on the outsides of bends is potentially high. Surface runoff during storms contributes a lot of suspended load.

Question 4

Describe the human modifications.

Answer 4

• In 1950, about 90% of the catchment of the Wairau Stream was forest and farmland.
• In 1959 the Harbour Bridge opened, leading to dramatic conversion of the land in the area to housing and industrial use.
• By the late 1960s the industrial area in the lower Wairau Valley was flooding regularly due to increased runoff as land covered by houses and roads created impermeable surfaces - preventing infiltration.
• Urban land use was 70% by 1975 and 90% by 2000.
• Between 1963 and 1971, the mean lag-time of the rise of floods decreased from 7 hours to 5.5 hours. The faster transfer of water through the system reduced flood duration from 70 hours to 33 hours. However, faster water flows meant more erosion and transportation of sediment through the system occurred.

Question 5

Describe the responses and flood control measures.

Answer 5

In the 1970s millions of dollars were used to:
- Widen the river bed - so that it could carry more water.
- Straighten the channel - so water could move through it faster.
- Concreting the channel - so it wouldn’t erode and transport sediments more efficiently.
In the 1980s, as more of the catchment was converted to housing, more measures were introduced:
- Gates and ponds were built - so that when it rained the gates could close and the ponds fill up with water. By doing this they could stagger the entry of water into the main channel i.e. allowing water to pass through before allowing water from the side channels to feed in.
- Overflow areas were created e.g. sports fields - so that in big rainfall events water could be diverted over these instead of flowing down the main channel.

Question 6

What has been the result of these measures?

Answer 6

The measures have been successful (especially the ponds) and water flow into the main channel can now be regulated. Flooding of industrial areas has not taken place for several years now.