Hydrology: Discharge, Hydrographs, Floods, and Sediment Transport

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Hydrology Discharge, Hydrographs, Floods, and
Sediment Transport

• Unit 1 Module 4, Lecture 2

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Objectives

• Students will be able to
• interpret hydrographs.
• explain the effects of urbanization on storm
  hydrographs.
• describe factors that influence overland flow of
  water.
• describe patterns of deposition based on particle
  size in a stream bed.
• explain how sediment load is related to discharge.

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Hydrology

• Exhibits wide variation across watersheds
• Related to
• Precipitation
• Geology (including topography)
• Landuse land cover
• Hydrology is one of the primary factors
  influencing the physical and biological
  characteristics of streams

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Discharge

• Q - Volume of water passing a point per unit
  time.
• QVA or WDV

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Discharge and hydrographs
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Annual hydrographs
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Storm hydrograph
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Effects of urbanization on a storm hydrograph
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Flow paths
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Overland flow

• Occurs when soils are
• saturated (after snow melt or heavy rain)
• have low permeability (e.g., clay)
• Strongly influenced by landform and land use
• Coarse textured glacial deposits are highly
  permeable bedrock ancient lake bottoms are
  impermeable or have low permeability
• Impervious surfaces (parking lots, structures)
  create impervious surfaces
• Forest harvest practices and agricultural
  practices affect stream flow

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Overland flow and development
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Flooding

• We tend to have a biased view of floods as
  unpredictable, disastrous events, while in
  reality they are predictable and necessary
  occurrences.
• How does impervious surface affect the likelihood
  of flooding?

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Flood probabilities

• Recurrence interval
• 1 in 100 year flood
• probability of 0.01 or 1
• Also referred to as the recurrence interval
• Defined as P 1 / T, where
• P Flood probability
• T Recurrence interval

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Human influence on floods

• Rapid runoff into channelized streams increase
  flood frequency and enhance downstream peaks in
  flood hydrographs.

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Sediment transport
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Sediment transport
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The transportation of materials
Category Category Dia (mm) Wentworth scale
Boulder Cobble Boulder Cobble gt 256 lt -8
Large Small 128 - 256 64 - 128 -7 -6
Pebble Large Small 32 - 64 16 - 32 -5 -4
Gravel Coarse Medium Small 8 - 16 4 - 8 2 - 4 -3 -2 -1
Sand Very Coarse Coarse Medium Fine Very Fine 1 - 2 0.5 - 1 0.25 - 0.5 0.125 - 0.25 0.063 - 0.125 0 1 2 3 4
Silt lt 0.063 gt 5
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Erosion
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Erosion in action
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Deposition

• Deposition resulting in multiple channels

• Deposition within a single channel

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Sediment deposition and current velocity
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Sediment load increases with discharge
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Variation in sediment transport
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Landuse/seasonal affects on sediment delivery to
the Minnesota River

• 2001 Minnesota River at St. Peter, MN daily
  average flow and tss samples (draft)

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Longitudinal profile of a river

• As discharge increases
• Channel Width increases
• Channel Depth increases
• Mean velocity is stable
• Bed Material size decreases
• Slope decreases
• Sediment storage increases