Micro-level water-resources engineering—3

The deccan trap basalt as the most widespread feature of the geology of Maharashtra:

The geological map for India shows a large uniform portion for the deccan plateau. It consists the hard basalt rock, and not soft or sandy alluvial soils. The deccan trap basalt portion goes over Maharashtra, MP, Karnataka, and the adjacent areas from other states. (To my surprise, it seems that the geologists do not include the south Karnataka region in the same deccan trap basalt region.)

As far as regions of water-scarcity go, there is a very wide continuous band in India. Take India’s map, and mark two slanted lines: the top one going across Rajasthan, MP, Orissa, and the bottom one going across Gujarat, Maharashtra, Karnataka, Telangana and AP, even Tamil Nadu. Statistically speaking, the greatest number of the most severe droughts seem to occur in the regions falling in between these two lines.

The area of my interest is in Maharashtra. The worst drought-prone regions of Marathwada, South Maharashtra and parts of Vidarbha and Western Maharashtra all fall in between those two lines.

If Maharashtra is seen at a large, national, scale [say, 1 cm:100 km], the topmost geological layer is comprised mostly of the deccan trap basalt.

The water-seepage characteristics of the deccan trap basalt:

Speaking in general terms, if you take, say, a 10 cm X 10 cm X 10 cm cube of basalt, you will find it to be a hard, impermeable rock. You might therefore conclude that it is not very easily conducive to groundwater seepage.

However, when viewed at a larger scale, even a top-layer of basalt is not uniform either in composition or in shape (i.e. in terms of its surface morphology). First of all, there are inhomogeneities introduced and fissures formed right at the time of formation of these geological layers aeons ago. Then, there are earth-quakes, introducing cracks and fissures. Further, there also are some very slow processes that nevertheless make their effects felt over the geologically long time-scale of tens, even hundreds of thousands of years.

Due to the inhomogeneity of their composition and morphology, the daily thermal expansions and contractions experienced by the surface layers of rocks are inhomogenous. These inhomogeneities lead to thermally induced mechanical stresses. Over the geological time-scale, the repeated thermal stresses result in local fractures, especially near the surface (where the temperature gradients are the greatest). Further, the mechanical effects of erosion due to water flow leads to deposition of sand; it also serves to erode the fissure openings. The chemical action of dissolved minerals and chemicals lead to enlargement of fissures and opening of cavities at surface as well as deeper layers. Even in a nominally hard rock like basalt.

Thus, due to fracturing and weathering at the surface layers, if you consider relatively bigger patches, say those at the scale of, 10 m to a few hundreds of m (or bigger), even a top layer of a nominally hard rock like the basalt, can begin to act like the more permeable alluvial layer.

Since the cracks are highly irregular and elongated, percolation from a surface water body into the deeper underground layers is highly inhomogeneous and anisoptropic.

In the above discussion, we have considered the seepage from the surface layers. As far as the underground flow through aquifers goes, there is a presence of local sub-layering within an overall top layer of basalt. Further, fissuring and cavitation also has occurred deeper underground. Therefore, local underground aquifers are observed to exist even within an overall basalt layer. Such aquifers often are quite directional, and not too criss-cross. Hence, anisotropy (or directionality) to the local underground flow is only to be expected.

As an example of a locally restricted fracturing/fissuring, observe the groundwater falling over the passing trains and buses in the tunnels of the Khandala ghat on the Mumbai–Pune routes. (BTW, in case you have ever wondered whether these fissures/fractures pose risk, don’t worry!  Their presence is already factored in, while designing for tunnels—fracture mechanics, by now, is a fairly well understood technique.)

One notable reference here is by Prof. Deolankar of Uni. of Pune: Deolankar, S. B. (1980) “The deccan basalts of Maharashtra, India—their potential as aquifers,” Ground Water, vol. 18, no. 5, September–October 1980, pp. 434–437 [(.PDF) ^]. Note the comparisons to the basalt layers elsewhere, and the quantitative estimates for parameters such as porosity, yield, transmissivity and specific capacity.

To conclude, (i) a top layer of basalt layer also allows for seepage of water, even if (ii) the effect varies greatly from place to place (due to the inhomogeneity of fracturing) and the flow is directional (due to anisotropy).

Therefore, groundwater seepage, and therefore artificial groundwater recharge work, appears feasible even in the deccan trap region of Maharashtra. However, it is only to be expected that the seepage aspect won’t be as pronounced as in the regions having a sandy alluvial top layer.

The importance of the local geology:

Due to the local inhomogeneity and anisotropy, there also arise certain difficulties or challenges.

The main difficulty is that unless a detailed geological study of the local hydro-geology is carefully conducted, it would be impossible to tell whether any underground water recharge work would at all be feasible in a given village or not.

Artificial groundwater recharge work may lead to very impressive results in some village or a cluster of villages, but it may not at all give economical returns even in some nearby  villages—even if all of them fall under the same governmental administrative unit of a taluka (or even a block). [The same Collector; the same Block Development Officer! … Two results! (LOL!)]

Thus, in Maharashtra (and similar regions), it becomes crucially important to know what kind of local geology there is—the surface geology, as well as the geology and morphology of the underground strata.  The depth to which these features should be known would vary from place to place; it may range from 10 m to even hundreds of meters.

Unfortunately, the geological surveys in the past were conducted only at much grosser scales. The relevant geological data at the micro-level of villages (i.e. covering just 5 km X 5 km areas) are simply not available.

If experts (say GSA) are asked to conduct such surveys at the micro-level for the entire country, it would be a very time-consuming and costly process.

However, realize that what you need for the water-conservation work is not the most elaborate kinds of surveys. You don’t need surveys of the kind that GSA or the mining engineers make. You aren’t really interested in things like detailed rock-compositions, percentages of minerals, etc. Your main interest is things such as: what kind of strata run where underground, what kind of intermediate layers occur in between the layers of hard rocks and at what depths, the depth and the direction at/in which the local fissures and aquifers run, whether a given fissure extends up to surface or not, etc.

Some of this data (concerning the local geological strata) can be gathered simply by observing the traditional wells! Often-times, the wells are either not at all covered with walls, or even if there is a masonry work, it does not extend beyond a certain depth, and so, the underground layers stand adequately exposed at the traditional wells. Other data can be had by observing the exposed surfaces of nallahs, rivers, hill-sides, etc.

And, of course, data about the local underground strata can always be had by drilling observation bore-wells (though it would be a costlier method).

The economic relevance of computational modelling:

In places like Maharashtra, since the groundwater seepage, flow, and water-holding characteristics crucially involve local variations and directionality, 3D computational models should prove to be of definite use.

Use of 3D computational models would not only streamline the collection of data, it would also lead to far more accurate predictions concerning economic feasibility of projects—ahead of spending any money on them.

A case in point, here, is that of a small check-dam built at the initiative of the IIT Bombay alumni. More details can be found at the CTARA Web site. As a measure of the difficulty in making predictions for underground water flow, notice that in spite of certain geological studies (of conductivity measurements etc.) conducted by the IIT Bombay experts prior to building of this check dam, it still has not resulted in any enhanced ground-water seepage downstream. Chances are, if a 3D model were to be built by drilling observation bore-wells, either a significant amount of money could have been saved, or deployed at a more suitable location.

An apparent counter-case in point is that of the success of the Shirpur pattern, at its original location, viz., near Shirpur (where else?). No detailed micro-level 3D computational modelling was conducted for it. Still, it was successful. How come?

The local geology of the Shirpur region as not being representative of the entire state of Maharashtra:

As it so happens, my father, a retired irrigation engineer, had worked in the Shirpur area. (I thus happened to have had a considerable stint of my school education in and around Shirpur.) I had discussed the issue with my father quite a few years ago. From whatever I now recollected, he had mentioned that the local geology there indeed was more conducive to underground seepage. There were sandy soils at the top level, and some hard rock well underneath. Both these factors lead to better seepage characteristics. The strategy of deepening and widening of the nallahs, as followed in the Shirpur pattern, therefore is a good strategy. As to the rest of Maharashtra, the local geological characteristics differed, he had mentioned it.

[I guess we had this conversation some time in 2007 or 2008. I have been having this idea of not getting discouraged if there is no water at a bore-well location, but instead turn the situation on its head and use the out-coming data regarding the underlying geological strata, to build better predictive computer models at a very fine level of granularity. I have been having this idea since at least 2008, and so, our conversation must be that old. As to the appreciation of having to carefully build 3D models, I owe it to my training in materials engineering, in particular, stereology.]

Anyway, in the recent weeks, I therefore checked the local geology for the Shirpur region, consulting some of the references listed in my earlier post in this series. It turns out that the depth to the water level near Shirpur is at roughly 20–30 m bgl (i.e. below ground level); see ref. here: Aquifer Systems of India, Central Ground Water Board, Plate XXVII on page 58 [(34 MB) pdf ^]. Now, this is a region through which Taapi, a major river, flows. As any school-boy in Shirpur would know, the river has enriched the top layers with a rich black soil. What is the official geological nature of this top layer? Turns out that it is “alluvial.” The black soil does not have the best permeability. However, in the Shirpur region, the alluvial deposits also are sandy in nature, esp. as you go below a certain depth (of 1 m to a few meters). Next, check out the distinctive yellow patch of the alluvial region in this map, standing in sharp contrast to the green patch for the basalt layer for the major parts of Maharashtra [(370 kB pdf) ^].

A top layer of alluvial soil, esp. if deeper than 10 m, if it is then also supported underneath by a highly impervious layer (e.g. basalt in Maharashtra), then the approaches that seek to enhance ground-water seepage do make good sense.

In contrast, if there is a top layer of basalt itself, then, in general, it is less conducive to groundwater seepage; it is more conducive to construction of check-dams for water storage (as in contrast to water percolation/seepage), or for the Kolhapur-type weirs for both storage and redistribution, etc.

As an inevitable conclusion, the local geology holds very important implications for selection of effective water conservation strategies.

Naturally, you can’t just go ahead and apply the Shirpur pattern everywhere in Maharashtra.

“Give me the funds for a few Poclains per taalukaa, and I will make everything green,” is a statement therefore strongly reminiscent of “Give me a place to stand and with a lever I will move the whole world.” The point is not that the whole world won’t be moved; the point is the natural difficulty in providing the guy with a place to stand (complete with air to breathe etc.), not to mention the engineering difficulty of supplying him with a strong enough, and long enough, a lever. And, of course, the difficulty of arranging a place to keep the fulcrum of that lever.

Dramatic statements, both!

I will go ahead, stick my neck out, and say that the Shirpur pattern—inasmuch as it incorporates the seepage mechanism as a strategy—is not likely to be the most optimum solution at any places other than in the Tapi and the Purna river regions! Check out the map if you have not done so already [(370 kB pdf) ^]!

The idea of small dams as storage—and not seepage—devices:

Come to think of it, then, with all the due qualifications—i.e., speaking only in general terms, and only for most parts of Maharashtra (not all), and ignoring any fracturing present in the local geology—the idea of small-dams or check-dams as storage devices, rather than as a seepage devices (or as a groundwater recharge devices), has begun to make much better sense to me. …

[… Yes, the famous government-funded Poclains, and the government-funded work to be contracted out to some of the local parties, and the government funds to be timely released only to some of those parties…. The whole she-bang does stand to be applied also here; more on it, later, if at all necessary. …]

…For the time being, here is an exercise for you.

Exercise:

Take a smallish river (or a bigger nallah), say, 50 km (or 10 km) long. Build an enormously simplified geometrical model of the river, by assuming a rectangular pyramid for its water-carrying volume.

Thus, ignore all the bends in the course of the river and instead assume that the river looks like a long, acicular triangle in the plan (i.e. in the top view). Further, assume that the vertical cross-section of the river remains rectangular throughout; it goes on linearly increasing in area from zero at the origin of the river to a certain value at the end of the river.

Assume typical figures for the dimensions of the river/nallah: how about a vertical cross section that is 50 m wide and 2–3 m deep at the mid-length of the river (i.e. 25 km downstream from the origin)? Assume also a suitable slope for the river, so that water does indeed flow downstream: how about a fall in the height of the ground level of, say, 50 to 100 m, over its 50 km length?

Now, if a series of check dams were to be built on this “river” such that they would submerge some 75% of the total river area present in the plan view into water-holding areas, calculate how much total volume of water would be made available. Compare this volume to the storage capacity of a single conventional dam known to you. …

[While making your calculations, realize (i) that the max. height of the dam cannot exceed the depth of the river bed (because only the river area would go under water), (ii) that the bottom of the river slopes down, and therefore (iii) that the depth of river bed below the water surface goes on decreasing as you go upstream from the check dam location, coming to zero at some location upstream. The third factor severely delimits the total volume of water that can be held via the series of check dams.]

To put the water volume in context, assume that the per-capita consumption for daily individual consumption is some 135–150 liters. Using this assumption, determine the size of the town/city whose needs could be met by this series of check dams. (Note, this figure does not include demand for agriculture and industrial usages.)

Then, consult a practising civil engineer and find out the current cost of construction of all these check dams. Compare this cost with that of a single conventional dam.

Think about any advantages the series of check dams may have; consider water distribution, flood control, and sedimentation and maintenance aspects.

Include the costs of canal construction in the conventional approach. Include the costs of lift-irrigation schemes in the check-dams approach.

Include the fact that since check-dams won’t have a great height (say 2–4 m), the evaporation losses (estimated at about 20–30% in the conventional dams) may even lead to this circumstance: all the water in a check dam plain evaporates in the thin air even before the next summer season approaches. Realize here that, as a rule of thumb, evaporation losses over the eight non-monsoon months are as high as about 1.67 m of height loss per square m of the average of top and bottom surface areas in the plan. [To help put this figure in some kind of a context, the average annual rainfall in Maharashtra is about 110 cm—if no rainwater were to be lost to seepage, runoff or evaporation, and if all of it could be collected, a tank with a square meter of bottom surface area would hold a water body 1.1 m tall.]

Include the economics of maintenance and mechanization in the regions where there is no traditional “Rajasthan culture” of water conservation, but instead people expect government to bring them everything wherever they are.

* * * * *   * * * * *   * * * * *

I will come back later with some further notes and observations (including those on software) on this topic of micro-level water-resources engineering. In particular, I want to make a few notings related to the GIS software. However, I belong to those old-fashioned kind of engineers who, in their practical life (as in contrast to their avatars in blogosphere, for instance) always first do a quick back-of-the-envelop calculation before they switch on a computer to do any computational modelling. If you are like me, you should finish the above exercise first, so that the exploration of software is better grounded in reality.

* * * * *   * * * * *   * * * * *

A Song I Like:
(Marathi) “gangaa aali re, angaNi”
Lyrics: G. D. Madgulkar
Music: Datta Davajekar
Singers: Jayawant Kulkarni, Sharad Jambhekar, Govind Powale, H. Vasant, Aparna Mayekar

[Minor updates done right on 2015.04.08 after posting the very first version. Guess I will not make any significant revisions to this post any further. May come back and correct typos and grammatical streamlining; that’s all—no new points.]

[E&OE]

 

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