PURPOSE OF SURFACE WATER STORAGE

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PURPOSE OFSURFACE WATER STORAGE

• STORE A DESIGN VOLUME OF WATER
• CREATE A DESIGN LAKE SURFACE AREA
• CREATE A UNIQUE HABITAT FOR FISH AND / OR WILDLIFE

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SURFACE WATER STORAGE OBJECTIVES

• PROVIDE A DEPENDABLE SOURCE OF WATER FOR THE
  PURPOSE
• PROVIDE AN ECONOMICAL SOURCE OF WATER FOR THE
  PURPOSE

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Top-of-Dam
What kinds of storage must be considered
in planning a reservoir?
Principal Spillway
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WHAT FACTORS MUST BE CONSIDERED IN STORAGE VOLUME?

• USEFUL STORAGE VOLUME
• SEDIMENT
• LOSSES TO EVAPORATION FROM THE SURFACE
• LOSSES TO SEEPAGE
• FLOOD HANDLING VOLUME

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Top-of-Dam
Freeboard Storage
Large Flood Storage
Emergency Spillway
Small Flood Storage
Evaporation and Seepage Storage
Useful Storage
sediment storage
Principal Spillway
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WHAT FACTORS AFFECT THE AVAILABLE WATER SUPPLY?
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WHAT FACTORS AFFECT THE AVAILABLE WATER SUPPLY?

• PRECIPITATION ON THE WATERSHED
• EVAPORATION ON THE WATERSHED
• INFILTRATION ON THE WATERSHED
• WATERSHED AREA

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WATERSHED WATER BALANCE
THIS APPLIES TO THE WATERSHED AREA
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RESERVOIR WATER BALANCE
THIS APPLIES TO THE RESERVOIR SURFACE AREA
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OVERALL WATER STORAGE BALANCE
HOW WILL THIS BE AFFECTED BY WATERSHED SIZE?
HOW WILL THIS BE AFFECTED BY RESERVOIR SURFACE
AREA?
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Pond Storage CalculationProblem Solving Pairs

• Compare two small ponds
• Both hold 10 ac-ft of water
• Pond A has an average depth of 8 ft
• Pond B has an average depth of 4 ft
• Both lose 6-inches of water per year to
  evaporation and seepage
• How much water volume does each pond lose?

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EFFECT OF WATERSHED SIZE ON STORAGE

• WATERSHED TOO SMALL
• NOT ENOUGH RUNOFF
• WATERSHED TOO BIG
• TOO MUCH RUNOFF
• EXPENSIVE SPILLWAY
• TOO MUCH SEDIMENT
• 3X Minimum Size

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HOW WILL RESERVOIR TOPOGRAPHY AFFECT STORAGE?

• SLOPE OF THE UPSTREAM VALLEY
• DEPTH OF THE VALLEY AT THE DAM SITE

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Reservoir TopographyToo Flat

• Too much surface area
• Too much shallow water
• High water loss to seepage
• High water loss to evaporation

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Reservoir TopographyToo Steep

• Less water storage for given depth
• More earth work for given depth
• High cost per unit of stored water

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Evaluating a Proposed Site for an earthen
reservoir

• FOUNDATION AND FILL MATERIAL
• ROCK
• GRAVEL
• SAND
• SILT
• CLAY
• ORGANIC MATTER

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Evaluating a Proposed Site for an earthen
reservoir

• FOUNDATION AND FILL MATERIAL
• ROCK good structure
• GRAVEL good structure
• SAND good structure
• SILT easily eroded
• CLAY good water sealing
• ORGANIC MATTER easily decayed

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EARTH DAM CROSS-SECTION
W
2
1
H
1
Core Trench
1
10 Ft. min
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EARTH DAM CROSS-SECTIONZONED FILL
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Drop Inlet Pipe Spillway
h
ANTI-SEEP COLLARS
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Anti-seep collars being installed on
a principal spillway pipe.
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Principal Spillway Inlet
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Emergency Spillway
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FLOOD ROUTING
Inflow - Outflow Change in Storage
Peak Inflow
FLOW
Inflow
RATE
Peak Outflow
Outflow
TIME