Headworks Design Day 1

Mr. Bharat Raj Pandey is a senior engineer at Hydro Consult Engineering Limited. Today, he shared valuable lessons in our hydropower engineering class, focusing on hydro headworks design.

The initial step involves a topographical survey, requiring a D card from the survey department of Nepal to obtain XYZ coordinates. Control points are established through surveying techniques like triangulation to create a detailed map at a 1:1000 scale.

The importance of a detailed map lies in understanding the depth and width needed for foundation work during hydropower headworks construction. Cross-section work, at least 500 meters upstream and downstream, at 20m intervals and 20 meters from the high flood level, is essential for design. Additionally, L-section of the river for about 1 km upstream and downstream of the headworks is crucial.

Hydrological and sedimentological requirements involve establishing a gauging station to collect discharge data over time. This data aids in river cross-section analysis, hydrological analysis, regression analysis, flow duration curve derivation, frequency analysis, and sediment data collection for the river basin. The sediment data helps in designing of various components of hydropower headworks by considering sediment flow and calculating the time required for sediment filling in the reservoir.

Mr. Bharat Raj Pandey then discussed the calculation of the design flood. Different categories of reservoirs have varying general and minimum standards. For instance, for Category 1, where a breach endangers lives, the general standard is the probable maximum flood (PMF), and the minimum standard is 0.5xPMF or a 10,000-year flood.

Breaching of dams can occur due to factors like glacier load, earthquakes, or slope failure. Geology and geotechnical requirements involve detailed geological surveying, geophysical and geotechnical investigations, including core drilling, rock and soil classification, borehole logs, seismological studies, construction material survey and testing, test pitting, trenching, geological tests, in-situ rock tests, and laboratory tests, water pressure tests, and permeability tests. These tests are essential for a comprehensive understanding of the site.

Here are the key points he covered:

Topographical Survey Requirement:

  • Identification of the topography landscape is crucial.
  • D card from the survey department of Nepal is required to plot data with XYZ coordinates.
  • Control points are established through survey methods like triangulation.
  • Mapping of the headworks is done at a scale of 1:1000 to create a detailed map.
  • Understanding the depth and width for foundation work is essential during hydropower headworks construction.
  • Cross-section work is needed at least 500 meters upstream and downstream, at 20m intervals.
  • Presentation of L-sections of the river at least 1 km upstream and downstream is also necessary.

Hydrological and Sedimentological Requirements:

  • Design discharge for future design work is crucial, requiring monthly and daily design with plotted discharge data.
  • Gauging stations are established to measure discharge data over time.
  • Hydrological data collection is followed by river cross-section analysis.
  • Hydrological analysis, regression analysis, flow duration curve (FDC), monthly flow duration curve, and frequency analysis are performed.
  • Collection of sediment data is necessary to establish sediment flow in the river basin, including data on suspension load and bed load.
  • The collected data helps to design various components of hydropower headwork.

Design Flood and Standards:

  • Calculation of the design flood involves general and minimum standards for different categories.
  • Categories include scenarios where a breach may endanger lives, result in extensive damage, or pose negligible risk.
  • Breaching of dams can occur due to various factors such as glacier load, earthquakes, or sloped failure.

Geology and Geotechnical Requirements:

  • Detailed geological survey and mapping, geophysical investigation, and geotechnical investigation are essential.
  • Core drilling, rock classification, soil classification, borehole logs, seismological studies, and various tests (in-situ rock test, laboratory test, water pressure test, permeability test) are part of the geotechnical requirements.

Criteria for the Selection of Headwork Site:

  • Considerations include topography, foundation, potential hazards, and construction aspects.
  • Topography should have a straight reach, uniform velocity, higher river banks, and sufficient flushing slope.
  • Foundation should ideally be in rocky areas to minimize treatment.
  • Potential hazard considerations involve geological instability and seismic events.
  • Construction considerations include quarry site proximity, cost considerations, and environmental impact.

These comprehensive criteria ensure a thorough understanding and selection of an appropriate headwork site for hydropower projects.

When selecting a site for the headwork, several criteria must be considered:

Topography:

  • Look for a straight reach, preferably in an outer bend, to avoid excessive sediment accumulation during the monsoon season.
  • Ensure uniform velocity of the river flow.
  • River banks should be at a higher level to allow for a sufficient flushing slope.
  • Opt for a narrow site to minimize construction costs.
  • The site should have enough capacity to accommodate the diversion structure or foundation, preferably in a rocky area.
  • Consider a safe passage for Glacial Lake Outburst Floods (GLOF).
  • Allocate space for intake, gravel trap, settling basin, and the intake portal.

Foundation and Site Geology:

  • Prefer a rock foundation for a permeable foundation.
  • Avoid excessive treatment of the foundation.
  • Select a site with homogeneous lithology and stable geology as much as possible.

Potential Hazards:

  • Choose a site with minimal potential hazards in the epistemic and downstream areas.
  • Look for a location with low geological instability and seismic activity.
  • Ensure the site is less vulnerable during the event of a diversion structure failure.

Construction Considerations:

  • The quarry site should be within 20 kilometers of the periphery.
  • Consider the cost of diversion and its protection structures.
  • Ensure sufficient space for facilities such as material storage areas, workshops, batching plants, and camps.
  • Provide easy access for transportation of construction materials.
  • Take environmental adverse consequences into account during the planning process.

Considering these factors in a human-like manner ensures a thoughtful and comprehensive approach to selecting a suitable headwork site for a hydropower project.

In our comprehensive discussion guided by Mr. Bharat Raj Pandey, we delved into the functional requirements and general considerations for effective headworks design.

  1. Withdrawal of the River: For Run-of-River (RoR) projects, the capability to withdraw discharge in both dry and wet seasons is imperative. Additionally, provisions must be made to meet hourly withdrawal requirements for PRoR projects.

  2. Flood Damage: Headworks must ensure safe passage of floods; design floods 1:100 or larger. The structure should withstand floods, including those resulting from Glacial Lake Outburst Floods (GLOF), CLOF, and landslide breaches. Structures should not clog, and attention must be given to the backwater level on the upstream side due to dam-induced water level rise.

  3. Provision for Trashes and Floating Debris: Structures should be designed to prevent clogging with floating debris, timber, logs, and vegetation.

  4. Provision for Sediment Handling: Capacity should not be reduced during the passage of heavy flood loads, boulders, or gravel. Bed control at the intake is crucial, ensuring that the intake remains unblocked with provisions for sediment control.

    Placement of intakes was discussed, emphasizing the need for careful analysis, especially in Nepal’s context where standard guides are lacking. Reference is made to Indian standards, though challenges arise due to the predominant presence of boulders in Himalayan rivers.

  5. Provision of Sediment Control: Implementation of sediment control measures, including excursion of suspended sediment, exculpation of sediment, and less maintenance requirement for penstock and electromechanical equipment. Flushing arrangements should be in place, capable of removing both fine and coarse sediment.

  6. Bed Control at Intake: Ensuring the intake is not blocked with sediment and has a flushing arrangement for the exclusion of sediment, capable of handling both fine and coarse sediment.

General Requirements of Headworks: A holistic exploration of factors affecting headworks design was undertaken. Differences between barrage, weir, and spillway structures were discussed, along with their placement considerations. Factors such as the construction of under sluices, the number of settling basins required for suspended sediment removal, and sediment load estimations from the river were also covered.

Noteworthy was the insight into Nepali hydropower design, where 17 to 25 percent of floor discharge is generally considered for bed load. Examples of projects designed by him, including an underground sedimentation tank designed to address landslide risks, were also shared. Considerations for velocity, forebay design, and fish passage requirements for environmental preservation were emphasized, with specific guidelines for maintaining fish passage and managing scour depth.

The discussion highlighted the need for careful adaptation of design criteria in Nepal, considering its unique geological features and the lack of extensive research in certain areas.

Extra Information

When determining the location of intake for hydropower projects, careful consideration of the river’s geomorphology, flow dynamics, and sediment transport is crucial. The choice between placing the intake on the inner or outer bend of a river involves various factors that impact the efficiency and maintenance of the structure.

In the context of Nepal, where Himalayan rivers are prevalent, the terrain presents specific challenges. Here’s an exploration of the considerations for the location of intakes, drawing examples from the Nepali landscape:

Example from Nepal:

  • In Nepali hydropower projects, the choice between inner and outer bend placement is often influenced by the predominant presence of boulders in Himalayan rivers. It’s important to note that the decision on intake location involves a site-specific analysis, and a detailed understanding of the river’s behavior and sediment transport dynamics is critical. Additionally, while Nepali engineers may refer to international standards, adaptation to the unique conditions of Himalayan rivers is imperative for successful and sustainable hydropower projects.

References