Flood plains are made by a meander eroding sideways as it travels downstream. When a river breaks its banks and floods, it leaves behind layers of silt. These gradually build up to create the floor of the flood plain. Floodplains generally contain unconsolidated sediments, often extending below the bed of the stream. These are accumulations of sand, gravel, loam, silt, and clay.
Geologically ancient floodplains are often represented in the landscape by fluvial terraces. These are old floodplains that remain relatively high above the present floodplain and indicate former courses of a stream. A flood is an overflow of water that submerges land that is usually dry. It can be caused by heavy rain, snow melt water or collapse of a natural or made dam. Floods can be a natural process of river plains and deltas each year which is essential for farming in many areas of the world as waters bring nutrients rich in river silt and deposits that create very fertile alluvial soils. Many ancient communities relied on annual flooding of the flood plain valley on rivers such as Nile and Euphrates.
Floods can be localised event affecting a small area of land or a very large disaster that can have a impact on entire islands or river basins. After flood water receded land can be contaminated with debris, sewage leaving residents without clean water and power.
Flood banks, levee dykes and stop banks can occur naturally but are often built to regulate the water levels in low lying or coastal areas.
The Whanganui River The 290-km Whanganui River is the second longest in the North Island, after the Waikato. Rising on the north-west flank of Mt Tongariro, it reaches the Tasman Sea at Whanganui. For most of its course, the Whanganui River course is deeply entrenched in sandstone and mudstone, and the runoff makes it very muddy.
Uplifted sedimentary sandstone and mudstone form the underlying geology, with terraces of volcanic deposits dropped or washed downstream from eruptions over the last 2,000 years. Ingrown meanders define the river valley, with sloping sides having greater lateral erosion than the gorges upstream. Land to the west of the river is characterised by its steep, V-sided valleys with sharp ridges of a fairly uniform height and a deeply entrenched drainage pattern. Some exposed escarpments remain prominent due to their contrasting light colour and exposed rock due to the ongoing erosion of their soft base material.
The southern part of this area includes the Atene “meander” or oxbow, where the Whanganui River once flowed in a near circle round the hill Puketapu and now has cut through to create a shortcut to the sea.
Colin Ogle Scientist wrote that before people arrived in what is now Wanganui there was a shifting mosaic of land and water. The Whanganui River and its tributaries flooded periodically changing their courses, sometimes flowing against the base of St John's Hill and Aramoho and other times towards Wanganui East. Silt was deposited either to become vegetated or remobilised in later floods. There is an obvious past course of the Whanganui , roughly from where SH4 meets Kaimatira Rd then down what is now the Matara Stream to Kowhai Park. Seemingly flat ,the floodplain would have had many minor ridges and terraces that the land lifted above all but the largest floods. The terraces offered safe building sites in 19th and early 20th centenary ie above Anzac parade. Almost the only visible record is the swamp forest of Gordon Park Scenic reserve. Floods on a wide vegetated flood plain have lower water levels and slower flows than would otherwise occur
2015 Whanganui River flood 21st June
The city was cut off after the biggest flood on record swept through the city closing all bridges except for Cobham bridge and closing state highway 3 north and south of the city as well as SH4 . About 100 families were evacuated mostly on the eastern banks of the Whanganui River . The river breached its bank around midnight spilling water into the CBD flooding roads businesses and homes. Other rivers in the area flooded as well isolating homes, farms and districts. The community and councils all pulled together to clean up the silt, repair roads and bridges. Kowhai Park was flooded and contaminated with sewage and the flood water was behind the flood banks so could not drain away. Water was pushed up the streams at high tide causing flooding behind the stopbanks. Local streams away from the river also burst their banks flooding houses around the city.
Go to coordinates S 39 56.120 E175 03.404
1. The Whanganui river carries a large amount of silt especially after heavy rain . At the given coordinates collect a container of water (about a litre) and place in a glass bottle. (There are steps down to the water)
*how long did the water take to settle?
*What percentage of the water cleared and what percent was silt?
Optional post a photo of you or your GPS by the river showing the flood damage and the repairs to the embankment,