Hydrological Design for Reservoirs and Dams Flashcards
What factor need to be considered when choosing the design return period?
What does a longer design period result in?
Cost and damage!
Longer design period =
Increases initial project cost
Less damage costs if a flood occurs
What is a reservoir and how are they created?
Artificial lake to store water.
Made by creating a dam which once completed river naturally fills reservoir
What are the 3 types of reservoirs for water supply? Describe each
A) Direct Supply Reservoir
Impounding of a gravity inflow and the piping of outflow to supply. Reservoir is built on river and directly supplied.
B) Pumped Reservoir
Reservoir inflow is from pumping. Reservoir is built off river in a nearby valley and filled by pumping from river. Aqueduct to supply is direct from reservoir.
C) Regulating Reservoir
Impounding water (reservoir) is upstream of abstraction point. It regulates flow to the downstream aqueduct and saving costs on this component.
What is the basic design procedure for finding the required reservoir storage and dam height for a water supply project
- Estimate water demand based on population and other factors.
- Select potential dam sites based on a contour map
- Check if river flow is sufficient to meet demand
- Calculate the reservoir size and dam height required
- Select final site based on other factors such as economic, geological and environmental.
What 5 categories is water demand divided into?
Domestic
Trade
Agricultural
Public
Losses
How is abstraction rate calculated?
population x water consumption
What is compensation flow?
Minimum flow to be released from the reservoir to compensate the downstream water demand for people and ecosystems
How is water demand calculated?
Safety Factor x (Abstraction Rate + Compensation Flow)
What is catchment yield?
Portion of precipitation on a catchment which can be collected for use
What is a project requirement when comparing average river flow with water demand?
Average Flow > Water Demand
What is safe yield?
Minimum yield recorded for a given past period.
What is abstraction?
Intended or actual quantity of water withdrawn for use
What is firm yield?
Mean annual rate of release of water through the reservoir which can be guaranteed.
When is a reservoir not required?
When minimum flow of stream is well above the minimum abstraction which must be satisfied by the project.
What is firm yield related to and what is the firm yield limit?
Larger reservoir storage = larger firm yield
Firm yield can never be greater than mean inflow to reservoir
How does flow variability effect a reservoir’s design?
Higher flow variability means reservoir capacity needs to increase so it can redistribute water when there is less inflow.
What is a reservoir used for?
Retaining excess water from periods of high flow for use during periods of low flow
What are an impounding reservoir’s 2 functions?
a) impound water for beneficial use
b) attenuate flood flows
What must be considered when estimating a reservoir’s storage and yield?
a) Reservoir presents a water surface for evaporation causing losses
b) Possibility of large seepage losses
c) People and ecosystems downstream may be entitled to a certain flow (compensation flow)
Water must be added to abstraction to account for these extra uses of water flow when finding reservoir storage.
What are the 3 approaches to estimating the required reservoir storage?
Mass Curve Method
Water Balance Method
Synthetic Minimum Flow Method
What is a mass curve of supply?
What is required to make one?
A curve showing the total (cumulative) volume entering a reservoir site over a certain time period (years).
Flow data records
Describe the method of calculating a reservoir’s storage using the Mass Curve Method
- Tabulate and plot accumulated flow with time (FL)
- Compute mean flow
- Add a demand line (DL)
a) MC gradient > DL gradient = reservoir filling
b) MC gradient < DL gradient = reservoir emptying - Add DL tangent to FL peaks and troughs
- Find largest C - the distance between demand line and trough
- From largest C extend line past FL and find distance from DL to next peak. This is the required storage.
How is the Water Balance Method similar to the Mass Curve Method?
Uses same approach but in a table instead of graph
Describe the method of calculating a reservoir’s storage and spillage using the Water Balance Method
Create a table with the following columns:
No. of Measurement
Year of Measurement
Q (m3/s) of inflow
Demand (m3/s) - generally constant after Year 0
Difference (Q - D)
Accumulated difference with max and to be added to be 15.5m3/s
Spillage (Accumulated - Storage)
Water in Reservoir (Accumulated - Spillage)
