CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
11
this flow. This may lead to situations where the flow is less
than the design flow and consequently turbines are producing
less power than expected. The fact that many micro-hydro
schemes in Nepal report generation way below the installed
capacity is strong evidence of this. It is imperative to
understand whether the flow was measured in a drier than
average year or in an average year, because of the influence
that this has on selecting the design flow. To be able to
produce a design flow as accurately as possible, a prediction
study must be undertaken.
Most potential micro-hydropower sites are located on
ungauged catchments where site specific hydrologic data is
lacking. To estimate yield from ungauged catchments, two
techniques are currently available to predict flow. These are
known as the WECS/DHM and the MIP methods, and are
presented in the subsequent discussions to predict flow in
ungauged catchments in Nepal.
However, at a regional training workshop on low flow
measurement and analysis organised by ICIMOD in April 1999
in Kathmandu, Nepal, it was reported that both the WECS/
DHM and MIP methods for estimating yield from ungauged
catchments had major drawbacks, and use of these methods
had to be exercised with extra caution. It was recommended
that the WECS/DHM studies be reviewed, and estimation of
the parameters be updated from time to time. In this regard,
DHM is now collaborating with WECS to review the previous
studies, and improve and update the parameters by using
more stations with longer records of data. It was stressed that
with Nepal facing big problems in estimating the design of
low flow for a variety of applications including micro-
hydropower, a reliable method was urgently required. In this
context, the Institute of Hydrology, U.K. is undertaking a
project titled “Regional Flow Regimes Estimation for small-
scale Hydropower Assessment (REFRESHA)” in collaboration
with ICIMOD and DHM from 1999, which aims to provide a
reliable method for estimating the hydrological regime at
ungauged sites in the Himalayan region of the country.
REFRESHA is scheduled to be ready in about two years time.
WECS/DHM method
Department of Hydrology and Meteorology (DHM)
method
The Water and Energy Commission Secretariat (WECS) and
(DHM) (Ref. 4) method is based on a series of regression
equations that are derived from analysis of all the hydrological
records from Nepal. The findings of this regression analysis
have been used to produce equations for predicting different
hydrologic parameters such as the low flows, flood flows and
flow duration curves. It is beyond the scope of this book to
explain in detail the WECS/DHM method. Readers are advised
to consult the reference, details of which are provided in Chapter
11. Appendix A describes this method with an example.
Medium Irrigation Project method (MIP)
The MIP method presents a technique for estimating the
distribution of monthly flows throughout a year for ungauged
locations. The MIP methodology uses a database consisting
of DHM spot measurements. The occasional wading gaugings
conducted by DHM include only low flow and these flows do
not represent the natural conditions since they are the residual
flows remaining after abstraction for different purposes like
irrigation. MIP presents non-dimensional hydrographs of
mean monthly flows for seven different physiographic regions.
These hydrographs present monthly flows as a ratio of the
flow in April (assumed lowest annual flow). For application to
ungauged sites, it is necessary to obtain a low flow discharge
estimate by gauging at a particular site. The measured flow is
then used with the regional non-dimensional hydrograph to
synthesise an annual hydrograph for the site. Appendix A
describes this method with an example.
Comparisons of the WECS/DHM and the MIP
approaches
WECS/DHM : Delineation of drainage basins and elevation
contours are often distorted on the available maps; also
regressions were derived on the basis of observed flows for
catchments ranging in size from 4 up to 54,100 km2.
Therefore, for flows in smaller catchments the results would
prove to be unreliable.
MIP : The MIP method approach based on wading measurements
taken on an intermittent basis cannot be expected to give a
good estimation of total flow in the monsoon months. It can,
however, give a reasonable approximation of the divertable
flows in these months. In the wet season, MIP would be
expected to underestimate WECS figures which should more
accurately represent total flow. In the dry season MIP and WECS
should both provide total flow estimates. The MIP procedure,
which explicitly advocates the use of local data to adjust the
regional hydrograph, should give reasonably accurate estimates
through the dry season months that are critical in assessing
micro-hydro projects.
Note that neither the WECS/DHM nor the MIP methods were
derived from data for high altitude snow-fed catchments. For
such catchments, more weight should be given to the results
of site measurements.
It must be emphasised that one can get a feel for annual
floods by measuring flood levels at site. Silt and debris
deposited along the river banks or level just below the
vegetation growth are indications of flood depths. By
measuring flood depth, width and average gradient of the
river at the intake area, it is possible to calculate the flood
flow using Manning’s equation, described in 4.3.2. It is beyond
the scope of this book to describe the different methods of
river gauging: please see Ref. 1 or Ref. 2 for guidance.