1. Living with Streams in Flood
Dayton, OH March 25, 1913

2. A Recipe for Disaster
A century of land clearance, drainage and development.
Major city in a flood plain at confluence of four rivers.
Large Meander downstream = bottleneck
Stream channel gets smaller downstream.
Heavy late Spring snowfall to saturate the ground followed by heavy Spring rains.

4. ISOHYTES HYDROGRAPH

5. Physical Flood Prevention
Build Flood-control dams on all major tributaries.
Raise levees and flood walls
Strengthen levees against erosion due to faster stream flow.
Straighten and deepen stream channels through major cities

6. THE HYDROLOGIC CYCLE 2:
GROUNDWATER

7. The Hydrologic Cycle - Fresh Water Storage
Reservoir % of Total Fresh Water
Glaciers (Frozen) 76%
Groundwater 22%
Rivers & Lakes < 2%
Groundwater – largest liquid fresh water storage reservoir.

8. The Groundwater System

9. The Groundwater System
Gaining Streams
Losing Streams

11. The Water Table Mirrors Surface Topography

12. Groundwater Flows From High Pressure To Low Pressure Areas

13. Usually, this means Down Gradient

14. So, To Understand Groundwater Flow We Need To Know:
Depth of the water table.
Gradient (slope) of the water
table.
3) Characteristics of the rocks
that the groundwater is
traveling through.

15. Depth and Gradient: Gradient determines the direction of groundwater flow.

16. Gradient = (h1- h2)/L
Where: h1and h2 are the elevations of the water table at two locations, and L is the horizontal distance between them h1 h2

17. Rock Characteristics: Porosity – How much water the rock will hold.
Permeability – The ability of a rock to transmit water. Depends on size and connectivity of pores.

18. Porosity Permeability
A) Sediments
Soil % by volume
Clay
Sand
Gravel
B) Rocks
Shale %
Limestone 5
Sandstone 15
Granite
Fractured
Limestone 20
Low
Permeability
Clay
Shale
Solid Limestone
Sandstone
Fractured Limestone
Gravels & Sands
High

19. The first person to study these characteristics of the groundwater system was a geologist named Henri Darcy.
Darcy noted that the velocity of groundwater flow was proportional to the gradient of the water table.
1) V α (h1- h2)/L but we need to account for permeability
V = k (h1- h2)/L where k is a permeability variable, and
Q (discharge) = Ak (h1- h2)/L where A is a measure
of cross sectional area

20. Q (discharge) = Ak (h1- h2)/L = AV
Darcy’ Law
Q (discharge) = Ak (h1- h2)/L = AV
Pore
Remember, when calculating cross – sectional area the water is only flowing through the open pores (unlike a river channel). So
A (area) = width X depth X porosity.

21. Problem
What is the gradient of the water table whose elevation is 500’ at well 1, 300’ at well 2, and the wells are 2000’ apart?
If the aquifer has a width of 40 feet, and a depth of 40 feet, a porosity of 5%, and a k = 0.01 ft/day, what is the groundwater velocity and discharge?

22. 1) Gradient = (h1- h2)/L =
500’ – 300’/2000’ = 0.1
2) Velocity = k (h1- h2)/L =
0.01ft/day x 0.1 = .001ft/day
3) Q = Ak (h1- h2)/L =
40’ x 40’ x .05 x .001 =
.08 ft3/day