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

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.

ISOHYTES HYDROGRAPH

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

THE HYDROLOGIC CYCLE 2: GROUNDWATER

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.

The Groundwater System

The Groundwater System Gaining Streams Losing Streams

The Water Table Mirrors Surface Topography

Groundwater Flows From High Pressure To Low Pressure Areas

Usually, this means Down Gradient

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.

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

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

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.

Porosity Permeability A) Sediments Soil 55% by volume Clay 45 Sand 35 Gravel 20 B) Rocks Shale 5% Limestone 5 Sandstone 15 Granite 1 Fractured Limestone 20 Low Permeability Clay Shale Solid Limestone Sandstone Fractured Limestone Gravels & Sands High

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

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.

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?

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