World Library  
Flag as Inappropriate
Email this Article

Specific storage

Article Id: WHEBN0001863215
Reproduction Date:

Title: Specific storage  
Author: World Heritage Encyclopedia
Language: English
Subject: Aquifer, MODFLOW, Geotechnical engineering, Soil mechanics, Hydraulic conductivity
Collection: Aquifers, Hydrology, Soil Mechanics, Water
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Specific storage

In the field of hydrogeology, "storage properties" are physical properties that characterize the capacity of an aquifer to release groundwater. These properties are Storativity (S), specific storage (Ss) and specific yield (Sy).

They are often determined using some combination of field tests (e.g., aquifer tests) and laboratory tests on aquifer material samples.

Contents

  • Storativity 1
    • Confined 1.1
    • Unconfined 1.2
  • Specific yield 2
  • See also 3
  • References 4

Storativity

Storativity or the storage coefficient is the volume of water released from storage per unit decline in hydraulic head in the aquifer, per unit area of the aquifer. Storativity is a dimensionless quantity, and ranges between 0 and the effective porosity of the aquifer.

S = \frac{dV_w}{dh}\frac{1}{A} = S_s b + S_y \,

Confined

For a confined aquifer or aquitard, storativity is the vertically integrated specific storage value. Therefore, if the aquitard is homogeneous:

S=S_s b \,

Unconfined

For unconfined aquifer storativity is approximately equal to the specific yield (S_y) since the release from specific storage (S_s) is typically orders of magnitude less (S_s b \ll \!\ S_y).

S=S_y \,

The specific storage is the amount of water that a portion of an aquifer releases from storage, per unit mass or volume of aquifer, per unit change in hydraulic head, while remaining fully saturated.

Mass specific storage is the mass of water that an aquifer releases from storage, per mass of aquifer, per unit decline in hydraulic head:

(S_s)_m = \frac{1}{m_a}\frac{dm_w}{dh}

where

(S_s)_m is the mass specific storage ([L−1]);
m_a is the mass of that portion of the aquifer from which the water is released ([M]);
dm_w is the mass of water released from storage ([M]); and
dh is the decline in hydraulic head ([L]).

Volumetric specific storage (or volume specific storage) is the volume of water that an aquifer releases from storage, per volume of aquifer, per unit decline in hydraulic head (Freeze and Cherry, 1979):

S_s = \frac{1}{V_a}\frac{dV_w}{dh} = \frac{1}{V_a}\frac{dV_w}{dp}\frac{dp}{dh}= \frac{1}{V_a}\frac{dV_w}{dp}\gamma_w

where

S_s is the volumetric specific storage ([L−1]);
V_a is the bulk volume of that portion of the aquifer from which the water is released ([L3]);
dV_w is the volume of water released from storage ([L3]);
dp is the decline in pressure(N•m−2 or [ML−1T−2]) ;
dh is the decline in hydraulic head ([L]) and
\gamma_w is the specific weight of water (N•m−3 or [ML−2T−2]).

In hydrogeology, volumetric specific storage is much more commonly encountered than mass specific storage. Consequently, the term specific storage generally refers to volumetric specific storage.

In terms of measurable physical properties, specific storage can be expressed as

S_s = \gamma_w (\beta_p + n \cdot \beta_w)

where

\gamma_w is the specific weight of water (N•m−3 or [ML−2T−2])
n is the porosity of the material (dimensionless ratio between 0 and 1)
\beta_p is the compressibility of the bulk aquifer material (m2N−1 or [LM−1T2]), and
\beta_w is the compressibility of water (m2N−1 or [LM−1T2])

The compressibility terms relate a given change in stress to a change in volume (a strain). These two terms can be defined as:

\beta_p = -\frac{dV_t}{d\sigma_e}\frac{1}{V_t}
\beta_w = -\frac{dV_w}{dp}\frac{1}{V_w}

where

\sigma_e is the effective stress (N/m2 or [MLT−2/L2])

These equations relate a change in total or water volume (V_t or V_w) per change in applied stress (effective stress — \sigma_e or pore pressure — p) per unit volume. The compressibilities (and therefore also Ss) can be estimated from laboratory consolidation tests (in an apparatus called a consolidometer), using the consolidation theory of soil mechanics (developed by Karl Terzaghi).

Specific yield

Values of specific yield, from Johnson (1967)
Material Specific Yield (%)
min avg max
Unconsolidated deposits
Clay 0 2 5
Sandy clay (mud) 3 7 12
Silt 3 18 19
Fine sand 10 21 28
Medium sand 15 26 32
Coarse sand 20 27 35
Gravelly sand 20 25 35
Fine gravel 21 25 35
Medium gravel 13 23 26
Coarse gravel 12 22 26
Consolidated deposits
Fine-grained sandstone   21  
Medium-grained sandstone   27  
Limestone   14  
Schist   26  
Siltstone   12  
Tuff   21  
Other deposits
Dune sand   38  
Loess   18  
Peat   44  
Till, predominantly silt   6  
Till, predominantly sand   16  
Till, predominantly gravel   16  

Specific yield, also known as the drainable porosity, is a ratio, less than or equal to the effective porosity, indicating the volumetric fraction of the bulk aquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity:

S_y = \frac{V_{wd}}{V_T}

where

V_{wd} is the volume of water drained, and
V_T is the total rock or material volume

It is primarily used for unconfined aquifers, since the elastic storage component, S_s, is relatively small and usually has an insignificant contribution. Specific yield can be close to effective porosity, but there are several subtle things which make this value more complicated than it seems. Some water always remains in the formation, even after drainage; it clings to the grains of sand and clay in the formation. Also, the value of specific yield may not be fully realized for a very long time, due to complications caused by unsaturated flow.

See also

References

  • Freeze, R.A. and J.A. Cherry. 1979. Groundwater. Prentice-Hall, Inc. Englewood Cliffs, NJ. 604 p.
  • Johnson, A.I. 1967. Specific yield — compilation of specific yields for various materials. U.S. Geological Survey Water Supply Paper 1662-D. 74 p.
  • Morris, D.A. and A.I. Johnson. 1967. Summary of hydrologic and physical properties of rock and soil materials as analyzed by the Hydrologic Laboratory of the U.S. Geological Survey 1948-1960. U.S. Geological Survey Water Supply Paper 1839-D. 42 p.
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.