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Breakwater Innovation to Manage River Flooding

durban pier
Dolosse arrayed at a breakwater at Durban, South Africa (public domain)

Published Oct 8, 2021 3:26 PM by Harry Valentine

Engineers of the Mayan Empire built rock dams far upstream along the tributaries of rivers to reduce downstream flooding, as a sudden deluge could destroy a rock dam built further downstream. An innovation developed in 1963 in South Africa and first deployed in 1964 at a breakwater has proven capable of repeatedly withstanding forces imposed by severe ocean waves. The dolos is a mass production structure that could be deployed upstream along tributaries of rivers that are prone to seasonal flooding.

Introduction

Rainfall and precipitation usually occurs in watershed areas where water forms into streams that eventually combine with other streams. The layout is comparable to going down from the highest point of a tree, where small branches connect to fewer larger branches and eventually to the tree trunk. Beginning over 1,000-years ago, engineers of the Mayan Empire of South America had learned that by restricting water flow along small streams at higher elevation, they could minimize seasonal flooding downstream along rivers that flowed through Mayan agricultural land.

Seasonal flooding occurs along many rivers internationally including right up to the present day. Flooding can cause massive destruction of property and infrastructure. Seasonal flooding has a times restricted barges from sailing along navigable inland waterways. While building concrete dams along rivers involves massive expense, the mass production of a structure used at breakwaters offers the possibility of being able to build a series of dams along tributaries of rivers at lower cost.

Ocean Precedent

Over a period of several centuries, efforts were undertaken to build breakwaters using rocks. During an earlier period, the lack of suitable technology restricted the size of rocks and boulders that could be moved to build a breakwater. The development of high-pressure steam engines allowed for the development of technology capable of moving much larger rocks and boulders to be used in breakwater construction. It was not uncommon for builders to install temporary railway lines to move rocks and boulders to construct more robust breakwaters capable of withstanding more severe wave action.

Several strong breakwaters built from large boulders and rocks were in place by the mid-19th century, including at Table Bay at Cape Town, South Africa where seasonal wave action was severe. However, extreme wave action has shifted breakwaters built from boulders and rocks, until the specially designed dolos (concrete “jacks”) appeared. The breakwater at the Port of East London, South Africa was the first to include an array of dolosse that could maintain structural integrity during severe wave conditions. Breakwaters built or reinforced with an array of dolosse exist worldwide and are capable of repeatedly enduring severe sea storms.

Dolosse Dams on Rivers

The dolos is a geometric structure made from concrete, using a mold. It is possible to include small rocks and stones in dolos production which can be done onsite, next to a river with raw materials brought in by rail, truck or other form of transportation. Construction equipment capable of moving heavy weight would be used to place each dolos across each chosen location of river, including with sections of concrete sewer pipe placed on the river bed to allow some water flow rate to occur, while an array of dolosse extends in an arc between river banks.

Ocean precedent suggests that dolosse dams would have greater probability of remaining in place compared to dams built from rocks and boulders, though at some locations, dams of large rocks and boulders built using construction equipment would work. While each dolos used to build an ocean breakwater can weigh up to 80-tons, much smaller dolosse would likely be sufficient along river tributaries where seasonal flooding of valleys could occur without causing any significant property damage in the immediate surrounding area. The main objective would be to reduce severe flooding downstream where greater property damage would occur.

Navigable inland waterways also offer important economic benefit to regions through which they flow and carry commercial maritime traffic. Depending on winter precipitation, high spring time water levels along navigable waterways such as the Lower Mississippi River and Upper St. Lawrence River have restricted navigation. Flooding along these rivers has also contributed to shoreline property damage. The flood control method of the engineers of the Mayan Empire, combined with modern dolos technology, offers a possibility of reducing excess flooding along the lower elevations of major rivers.

Dam Building Animal

The North American beaver lives along quiet streams with low water flow rate and builds dams across such streams, using sticks, logs and mud. While such dams usually causing flooding upstream, the flooding becomes problematic when stream flow through a populated area or when upstream flooding causing the beaver dam to break. Installing low-height dolosse dams at strategic locations along tributaries could encourage beavers to further build on such dams and prevent water from flowing downstream. Including pipes each with an upstream gate could allow for periodic and controlled short-burst releases of water by river caretakers.

At strategic upstream locations where beaver dams already exist, there may be scope to install an array of dolosse across the stream either upstream or downstream of the beaver dam, to reduce catastrophic flooding in the event of a breach of the beaver dam. The natural instinct of the animal would be to repair a breach of its own dam to stop water from flowing. Developing strategies by which to cooperate with the natural dam builder might offer a possible long-term solution to reducing flooding along the lower elevations of rivers.

Conclusions

Seasonal flooding occurs downstream along many rivers internationally and usually causes considerable property damage. Engineers of the Mayan Empire devised a method of building rock dams at regular intervals upstream along tributary stream of main rivers that proved effective at reducing destructive flooding downstream. The mass-production dolos structure from South Africa has proven effective at strengthening ocean coastal breakwaters that are subject to regular and powerful wave actions. There is potential to apply the dolos method upstream along river tributaries to strengthen future rock dams, to reduce seasonal flooding downstream at the lower elevations.

Repeated seasonal flooding causes repeated property damage at considerable cost. It is possible that over the long-term future, the cost of installing dolosse strengthened rock dams along tributaries and streams at higher elevation, would result is less costly repeated property damage at the lower elevations where flooding occurs.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.