3.4 Drainage network analysis

Drainage network analysis is crucial for understanding how water moves through a watershed. It examines the interconnected channels that drain water to an outlet point, including streams, tributaries, and the main stem. This analysis helps predict how quickly water will flow and where it might pool.

Stream classification, drainage patterns, and network metrics like drainage density and bifurcation ratio are key components. These factors influence a watershed's hydrological response, affecting the speed and intensity of runoff during rain events. Understanding these elements helps in flood prediction and water resource management.

Drainage network components

Hierarchical system of interconnected channels

Top images from around the web for Hierarchical system of interconnected channels

• 

  Stream System View original

  Is this image relevant?

• 

  Using QGIS to conduct watershed analysis and 3D modeling - CUOSGwiki View original

  Is this image relevant?

• 

  HESS - Hydrogeological conceptual model of andesitic watersheds revealed by high-resolution ... View original

  Is this image relevant?

• 

  Stream System View original

  Is this image relevant?

• 

  Using QGIS to conduct watershed analysis and 3D modeling - CUOSGwiki View original

  Is this image relevant?

1 of 3

Top images from around the web for Hierarchical system of interconnected channels

• 

  Stream System View original

  Is this image relevant?

• 

  Using QGIS to conduct watershed analysis and 3D modeling - CUOSGwiki View original

  Is this image relevant?

• 

  HESS - Hydrogeological conceptual model of andesitic watersheds revealed by high-resolution ... View original

  Is this image relevant?

• 

  Stream System View original

  Is this image relevant?

• 

  Using QGIS to conduct watershed analysis and 3D modeling - CUOSGwiki View original

  Is this image relevant?

1 of 3

• A drainage network is a hierarchical system of interconnected channels that drain surface and subsurface water from a watershed to an outlet point
• The main components of a drainage network include streams, tributaries, and the main stem or trunk stream
• Streams are the individual channels that carry water within the drainage network
• Tributaries are smaller streams that feed into larger streams or the main stem
• The main stem or trunk stream is the primary channel that carries water from the tributaries to the outlet point

Stream classification and drainage patterns

• Stream order is a classification system that assigns a numerical value to each stream segment based on its position within the drainage network hierarchy
  • The Strahler method assigns an order of 1 to headwater streams, and when two streams of the same order join, the downstream segment is assigned an order one higher
  • The Shreve method assigns an order of 1 to each headwater stream, and when two streams join, the downstream segment is assigned an order equal to the sum of the orders of the two contributing streams
• Drainage patterns describe the spatial arrangement of streams within a watershed and can be classified as dendritic, trellis, rectangular, radial, or parallel
  • Dendritic patterns (tree-like) are the most common and develop in areas with homogeneous rock types and gentle slopes
  • Trellis patterns form in areas with alternating resistant and less resistant rock layers, creating parallel streams with short tributaries
  • Rectangular patterns develop in areas with jointed or faulted bedrock, resulting in streams that follow the joints or faults
  • Radial patterns form around a central high point, such as a volcano or dome, with streams radiating outward
  • Parallel patterns occur in areas with steep, uniform slopes, such as coastal plains or tilted plateaus

Drainage density and bifurcation ratio

• Drainage density is the total length of streams per unit area of a watershed and reflects the degree of landscape dissection and the efficiency of the drainage network
  • Higher drainage densities indicate a more dissected landscape with a greater total length of streams per unit area
  • Lower drainage densities suggest a less dissected landscape with fewer streams per unit area
• The bifurcation ratio is the ratio of the number of streams in one order to the number of streams in the next higher order and provides insight into the branching characteristics of the drainage network
  • A higher bifurcation ratio indicates a more branched network with a greater number of lower-order streams feeding into higher-order streams
  • A lower bifurcation ratio suggests a less branched network with fewer lower-order streams feeding into higher-order streams

Drainage network metrics

Deriving stream network data using GIS

• Stream network data can be derived from digital elevation models (DEMs) using GIS tools such as flow direction, flow accumulation, and stream definition
  • Flow direction tools determine the direction of water flow from each cell in a DEM to its steepest downslope neighbor
  • Flow accumulation tools calculate the number of cells that drain into each cell, representing the cumulative drainage area
  • Stream definition tools identify cells with flow accumulation values above a specified threshold as stream cells, creating a stream network
• The resulting stream network can be further processed to assign stream orders, calculate stream lengths, and delineate watersheds

Calculating drainage density and bifurcation ratio

• Drainage density is calculated by dividing the total length of streams within a watershed by the watershed area
  • GIS tools can be used to measure stream lengths and calculate watershed area
  • Example: If a watershed has a total stream length of 100 km and an area of 50 km², the drainage density would be 2 km/km²
• The bifurcation ratio is calculated by dividing the number of streams in one order by the number of streams in the next higher order
  • This can be done manually or using GIS tools to count the number of streams in each order
  • Example: If a watershed has 50 first-order streams, 10 second-order streams, and 2 third-order streams, the bifurcation ratio between first and second-order streams would be 5 (50/10), and between second and third-order streams would be 5 (10/2)

Drainage network and watershed response

Influence of drainage density on hydrological response

• Drainage density influences the speed and magnitude of runoff response to precipitation events
  • Higher drainage densities generally result in faster and more intense hydrological responses due to efficient water conveyance
  • Lower drainage densities tend to have slower and less intense responses, as water takes longer to reach the stream network
• Example: A watershed with a high drainage density (4 km/km²) will likely have a faster and more pronounced peak in streamflow following a rainfall event compared to a watershed with a low drainage density (1 km/km²)

Impact of bifurcation ratio and stream order on runoff

• The bifurcation ratio affects the timing and magnitude of peak flows
  • Higher bifurcation ratios indicate a more branched network, which can lead to faster concentration of runoff and higher peak flows
  • Lower bifurcation ratios suggest a less branched network, which may result in slower concentration of runoff and lower peak flows
• Stream order influences the timing and magnitude of runoff contribution from different parts of the watershed
  • Higher-order streams have larger contributing areas and tend to have more sustained baseflow compared to lower-order streams
  • Lower-order streams respond more quickly to precipitation events and contribute to the initial rise in streamflow
• Example: A watershed with a high bifurcation ratio (5) and a large number of lower-order streams may experience a rapid increase in streamflow during a rainfall event, while a watershed with a low bifurcation ratio (3) and fewer lower-order streams may have a more gradual increase in streamflow

Role of drainage patterns in runoff distribution

• Drainage patterns can influence the spatial distribution of runoff and the timing of peak flows
  • Dendritic patterns tend to have more evenly distributed runoff, as tributaries contribute flow from various parts of the watershed
  • Trellis patterns may have more concentrated runoff along the main stem, as parallel tributaries quickly deliver water to the main channel
  • Rectangular patterns can result in rapid runoff concentration along the jointed or faulted segments, leading to faster peak flows
  • Radial patterns may have more evenly distributed runoff, as streams radiate outward from the central high point
  • Parallel patterns can lead to rapid runoff concentration, as steep, uniform slopes promote fast water delivery to the main channel
• Example: A watershed with a dendritic drainage pattern may have a more gradual and prolonged hydrograph, while a watershed with a parallel drainage pattern may have a sharper and shorter hydrograph peak

Horton's laws for stream networks

Law of stream numbers

• Horton's law of stream numbers states that the number of streams of each order forms an inverse geometric sequence with order number
  • This means that the number of streams decreases with increasing stream order in a predictable manner
  • The number of streams in each order can be related to the bifurcation ratio, as $N_u = R_b^{k-u}$, where $N_u$ is the number of streams of order $u$, $R_b$ is the bifurcation ratio, and $k$ is the highest stream order
• Example: If a watershed has a bifurcation ratio of 4 and a highest stream order of 3, the expected number of streams in each order would be: 64 first-order streams, 16 second-order streams, and 4 third-order streams

Law of stream lengths

• Horton's law of stream lengths states that the average length of streams of each order increases geometrically with stream order
  • This implies that higher-order streams are typically longer than lower-order streams
  • The average stream length in each order can be related to the stream length ratio, as $\bar{L}_u = R_L^{u-1} \bar{L}_1$, where $\bar{L}_u$ is the average length of streams of order $u$, $R_L$ is the stream length ratio, and $\bar{L}_1$ is the average length of first-order streams
• Example: If a watershed has a stream length ratio of 2 and an average first-order stream length of 1 km, the expected average stream lengths for each order would be: 1 km for first-order streams, 2 km for second-order streams, and 4 km for third-order streams

Law of drainage areas

• Horton's law of drainage areas states that the average drainage area of basins of each order increases geometrically with stream order
  • This means that higher-order streams drain larger areas than lower-order streams
  • The average drainage area in each order can be related to the area ratio, as $\bar{A}_u = R_A^{u-1} \bar{A}_1$, where $\bar{A}_u$ is the average drainage area of basins of order $u$, $R_A$ is the area ratio, and $\bar{A}_1$ is the average drainage area of first-order basins
• Example: If a watershed has an area ratio of 5 and an average first-order basin area of 1 km², the expected average basin areas for each order would be: 1 km² for first-order basins, 5 km² for second-order basins, and 25 km² for third-order basins

Application of Horton's laws in watershed analysis

• These laws provide a quantitative framework for understanding the hierarchical structure and organization of drainage networks
  • They can be used to compare and contrast different watersheds based on their network characteristics
  • Deviations from the expected geometric relationships may indicate anomalies in the drainage network or underlying geology
• Horton's laws can be applied to predict hydrological behavior based on network characteristics
  • For example, watersheds with higher bifurcation ratios and stream length ratios may be expected to have faster and more intense hydrological responses
  • Watersheds with lower bifurcation ratios and stream length ratios may have slower and more attenuated responses
• Example: When comparing two watersheds, if Watershed A has a higher bifurcation ratio (5) and stream length ratio (3) than Watershed B (bifurcation ratio of 3 and stream length ratio of 2), Watershed A would be expected to have a faster and more pronounced hydrological response to precipitation events