Frequently asked questions on sand dams
Provided by Excellent/ASDF with input from Dabane Trust
Q.
What regions are suited to sand dams?
Anywhere with seasonal rivers containing coarse water-bearing sediments and
where the bed rock or consolidated sub-soil is reasonably close to the surface.
This type of rivers is found in arid, semi-arid and dry sub-humid climatic
regions. From our experience, optimum conditions occur in the transition
between hills and plains where slopes range from 5 to half a percent and in
close proximity of a water scarce population.
Q.
Do the dams not reduce the water available to communities
downstream of the dams?
A.
During the rainy season, a small percentage of the flow is held back.
Research (by TU Delft) on sand dams in the neighbouring district of Kitui in
Kenya estimates that only 2-4% of the flow is retained by a dam and
approximately 98% of the water flows downstream. However, the dams cause
an increase in the level of the water table adjacent to the dam. The raised
stream bed behind the dam is sloped and hence the water held in this
sediment has a similar hydraulic gradient and isn’t flat as with water in an open
surface dam. This water percolates slowly through the sand dam aquifer and
the river banks throughout the year and contributes to an increase in base
flows, so that communities downstream actually receive increased flow during
the dry season when water is most scarce. This also accounts for why when
dams are built in series ephemeral rivers turn into perennial rivers
downstream of the series of dams (Aerts, et al. 2007 Source).
Q.
Where do you get the figure for 2% of flow withheld using a sand dam
from?
A.
Studies by TU Delft, for example Hut et al, found “only about 2% of the
total water coming from the particular catchment of one dam is stored in its
reservoir” (Aerts, et al. 2007. Source).
Q.
How many cubic meters of water are stored in a typical sand dam?
1
A.
Research by de Haas (2006) give a figure of 1-8000m3 per sand dam
(cited in Aerts, et al. 2007).
However, this only accounts for the water stored within the sand deposits. The
actual water available is greater than this when groundwater flow from the
upstream river banks into the sand storage dam is included and replenishment
by the bimodal rains (2 rainy seasons) (Eastern Africa).
Q.
What volume of water is held in the sediments behind the dam?
A:
25-40% typically: source: field tests cited in water from Sand Rivers
(Nissen-Petersen, 2006)
Source: Page 15 Water from Sand Rivers
However, others have found different figures; Stephen Hussey (Dabane Trust,
Zimbabwe) found the porosity of clean (washed) river channel sediment was
10 – 20%, with the 20% being from coarse sand (Hussey, S.W, 2007).
Q.
How do you know the water table rises above and below the dam?
A.
Firstly, our observation of water levels in scoop holes in the raised river
bed and water levels in wells adjacent to the sand dam
Secondly, the study of Kitui sand dams “Hydrology of Sand Storage Dams – A
case study in the Kiindu catchment, Kitui District, Kenya” by Borst, de Haas,
(2006), supports this.
Thirdly, research of Subsurface dams to harvest rainwater also support this, for
example a case study of the Swarnamukhi River basin, Southern India:
“An analysis of hydrographs of piezometers of four subsurface dams,
monitored during October 2001–December 2002, reveals that there is an
2
average rise of 1.44 m in post-monsoon and 1.80 m in the pre-monsoon period
after the subsurface dams were constructed. Further, during the pre-monsoon
month of June, much before construction of subsurface dams in October 2001,
the water level was found fluctuating in the range of 3.1–10 m, in contrast to
the fluctuation ranging from 0.4 to 3.1 m during the period following the
construction of dams” (Raju, Reddy, Munirathnampage, page 1 Source ).
Q.
How does the sand clean the water? How do you know the water is
clean?
The sand acts like a massive slow sand filter, a tried and trusted form of water
purification. The sand filters any suspended solids out of the water and
removes pathogens and bacteria.
Q.
Don’t you have to build sand dams in stages?
A.
That was the view of a consultant Erik Nissen-Petersen (Nissen Petersen,
2011, Source) which has been repeated quite widely, but this contradicts the
experience of both ED and SASOL in Kenya. We will continue to monitor
whether this assumption holds true during field trials in countries with
catchment topography, soil characteristics and rainfall patterns that differ
from S.E. Kenya.
Q.
How do sand dams help in combat the impacts of climate change?
They help water scarce communities cope better with droughts. By holding
back storm flows, they also act as a buffer against the risk of flooding and by
helping restore degraded environments, soils and trees, they help sequester
carbon within the ecosystem and safeguard bio-diversity. Communities with
robust organisations and experience of working and learning together have a
greater capacity to develop their own coping and adaptation strategies, using
sand dams as a tool to help them do so.
Q.
Given the high volume of cement required for their construction, don’t
sand dams have a huge carbon footprint?
Given their durability and low maintenance, sand dams have a low carbon
footprint relative to alternative water technologies especially when considered
3
over their full lifetime. That said, we are constantly looking for ways to reduce
the amount material used and alternative construction methods.
Q. Is sand harvesting a problem?
A. Yes. Riverine sand deposits are a major source of sand for the construction
industry. Some very limited and sustainable sand harvesting from sand dams is
permitted for local construction and block making, however by legally
registering ownership of their sand dam Self Help Groups (SHG)have legal
protection against unauthorised and unsustainable commercial sand
harvesting. Elsewhere unregulated sand harvesting has caused considerable
damage to the environment and water courses leading in 2008 to the Kenyan
Ministry for Environment introducing new regulations. However, enforcement
is hampered by limited capacity or desire of authorities to maintain tax
revenues raised on sand harvesters.
Q. Aren’t you effectively privatising a public resource?
A. The Self Help Group (SHG) manages the resource for public benefit and
prevents people from outside coming to make money out of the water/sand.
They never sell the water and water is freely available provided powered
(petrol or diesel) pumps are not used.
Q. What are the barriers to application in other countries?
Knowledge of the design golden rules and principles of construction, but above
all, community ownership is key to getting right for successful application in
new context.
It is important to identify what are the enablers and barriers to application,
and a detailed PESTLE (Political, Economic, Social, Technical, Legal,
Environmental) analysis is required.
Critical factors include:
 Land and water rights: Where there is a lack of legal and social
structures to safeguard community, SHG and farmers’ rights to water,
trees, land and access, this may represent potential barriers to
replication in other countries.
4
 Local knowledge related to river flow regimes, technical design and
construction knowledge
 Assessment of the river channel - whether the sediment is fine or coarse
and the depth of sand, which indicates the source material for the sand
dam
 Assessment of the catchment area of the proposed sand dam - ground
cover, soil erosion, soil type/types and an estimation of the clay content
which provides an indication of the material that will accumulate in sand
dam basin
 Capacity of local partners to manage grants and implement activities
 Our capacity to support partners and raise and manage funds.
Q.
What are the disadvantages of sand dams?
They usually require significant community labour during their construction
and specialist technical knowledge, these are more barriers to adoption that
can be overcome, rather than disadvantages.
Q.
Why are sand dams not more widely used?
Some of the best technologies never take off, so our challenge is to bridge the
chasm between small-scale application and wider, mass adoption. We also
know that a lack of awareness and the necessary technical knowledge are
major barriers. Finally water investments often overlook small-scale,
appropriate solutions in favour of more top-down, quicker fixes – the success
of sand dams relies on very strong and effective community engagement which
takes time and skill, not just technical ability and money.
References
Aerts, J. Lasage, R., Beets, W/., de Moel, H., Mutiso, G., Mutiso, S., de Vries,
A. 2007. Robustness of Sand Storage Dams under Climate Change. Soil Science
Society of America.
de Haas, B. 2006 “Hydrology of Sand Storage Dams – A case study in the Kiindu
catchment, Kitui District, Kenya” (Master thesis Hydrogeology at Vrije
University with Aqua for All and the Acacia Institute )
5
Hussey, S.W. 2007. “Water From Sand Rivers. ”Water, Engineering and
Development Centre, Loughborough University.
Nissen-Petersen, E. 2011. “Sand dams or silt traps” SamSam Water
Nissen Petersen, E. 2006. “Water from Sand Rivers” RELMA
Raju, N., Reddy, T.V.K, Munirathnam, P. 2005. Subsurface dams to harvest
rainwater— a case study of the Swarnamukhi River basin, Southern India.
Springer-Verlag.
6