"Rip Rap" Iles de la Madeleine EarthCache
Hidden : 1/24/2014
Difficulty:
1.5 out of 5
Terrain:
1.5 out of 5

Size: Size: other (other)

Join now to view geocache location details. It's free!

Watch

How Geocaching Works
Looking for a different adventure?

Please note Use of geocaching.com services is subject to the terms and conditions in our disclaimer.

Geocache Description:


Welcome to the small fishing village of the Pointe-aux-Loups, typical of those found along the several hundred kilometre coastline of the Iles de la Madeleine.

This Earthcache will bring you to a shoreline protection system referred to as riprap, rubble, shot rock or rock armour. It is rock or other material used to protect shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour, water or ice erosion.

It is made from a variety of rock types, commonly granite or limestone, and occasionally concrete rubble from building and paving demolition. An interesting concrete form found in several areas around the island is "dolos", shown in Figure 1 below (visit link) It can be used on any waterway or water containment where there is potential for water erosion.

Shoreline Erosion

Shoreline erosion, caused by the repeated pounding of waves against a shoreline, is a major problem on Iles de la Madeleine (visit link) (visit link)

Warmer winters and fiercer storms, rising seawaters and the slow sinking of the islands are responsible for an alarming loss of coastline, and the erosion appears to be accelerating.

Dominant northwest winds blow through the Gulf of St. Lawrence throughout the winter; typically, ice cover in the north is driven south and accumulates along the north side of the island chain. A high concentration of sea ice (30 percent of the water surface or more) obstructs the storm waves that would otherwise batter cliffs and reshape road-bearing stretches of dune. Coastal ice shields the archipelago’s shores from the destructive effects of rainwater and sudden freezes. But according to ongoing studies, there will be no ice formation in the gulf.

The coastline of Iles de la Madeleine is characterized by easily erodible sedimentary rock, composed mostly of sandstones and clay stones. Sandstone is susceptible to gelifraction, or frost shattering. More frequent freezing and thawing cycles are characteristic of progressively mild gulf winters. Water either melts or is rained into cracked and porous sandstone and shale, where it expands and “explodes” the rock as it freezes. Average shoreline erosion rates vary from 10 to 110 cm per year around the perimeter of the islands, though intense storms can destroy up to 10 m along some cliffs. The higher erosion rate on the north and west coast is directly attributable to the high degree of exposure to storm conditions. It's predicted that by 2050 the coastline will recede, on average, 80 metres in areas where the shoreline is low and sandy, and 38 metres in areas with rocky cliffs.

There are four ways waves can break down a shoreline:

1. Hydraulic Action

This occurs when air within the cracks in the shoreline material compress from the force of the water on it; as the air compresses, it loosens the material so that repeated pounding breaks the material free.

2. Attrition

This action occurs when loose material carried with the water collides with the shoreline, grinding and chipping each other, causing the material to become smaller and smaller and allowing the material to be easily carried away.

3. Corrasion

This occurs when material from the shoreline breaks free and then is used to further erode the shoreline again.

[the difference between attrition and corrasion is the source of material –attrition is material from elsewhere while corrasion is local material]

4. Corrosion

This form of erosion occurs not on the wave action but is based on the pH level of the sea water. It occurs when the pH level of the sea is below neutral 7.0. When this occurs, particularly at high pH levels, rock is corroded away. Wave action then takes part by removing the corroded material.

Riprap works by absorbing and deflecting the energy of waves before they reach the defended structure. The size and mass of the riprap material absorbs the impact energy of waves, while the gaps between the rocks trap and slow the flow of water., lessening its ability to erode soil or structures on the coast. Where the shoreline is of a very light and soft material, a fabric is often placed under the rock so that the water can’t carry away the underlying material. The mass of riprap also provides protection against impact damage by ice or debris, which is particularly desirable for bridge supports and pilings.

Riprap is also used to protect the base of old Edwardian and Victorian sea walls, which being vertical, are often undermined. The riprap absorbs the impact of the waves as they shoot up the wall and then fall back down.

To claim this earthcache, you must perform the following:

1. Determine which erosion action would take place here if the riprap wasn’t installed. (check the existing shoreline)

2. What type of rock is placed here to lesson the wave’s energy?

3. Take a picture of yourself and/or GPSr with the riprap in the background (optional)

Do NOT post your answers, encrypted or otherwise, on your log.

Do NOT log as a find until you have forwarded the answers for this EC. Failure to answer required questions through e-mail will result in a log deletion without notice.

Additional Hints (No hints available.)



Advertising with Us