Module 3 - Ecological Modelling

Question 1

What is SWMM?

Answer 1

Storm Water Management Model

SWMM is a distributed, dynamic rainfall-runoff simulation model used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas

Question 2

What is SWMM used for?

Answer 2

1. Design and sizing of an urban drainage system for redevelopment, including detention basins
2. Flood plain mapping of natural channel systems
3. Estimation of non-point source (NPS) pollutant loadings for stormwater management planning
4. Evaluating BMPs against receiving environment goals or discharge requirements

Question 3

Limitations of SWMM

Answer 3

Not applicable to large-scale, non-urban watersheds
Not applicable to forested areas or irrigated cropland
Cannot be used with highly aggregated (e.g. daily) rainfall data
It’s an analysis tool, not an automated design tool
Need third party software to interface directly with GIS

Question 4

Hydrological Modelling Features of SWMM

Answer 4

Spatially and time varying rainfall
 Evaporation of standing surface water
 Snow accumulation and melting
 Interception from depression storage
 Infiltration into soil layers
 Percolation into shallow groundwater
 Interflow between GW & channels
 Nonlinear routing of overland flow

Question 5

3 Model compartments within SWMM

Answer 5

1. Atmospheric compartment
   Precipitation inputs via rain gauge objects
2. Land surface compartment
   Subcatchment objects
   Input: Receive precipitation from Atmospheric compartment
   Output: Flows (both as infiltration and surface runoff)
3. Transport compartment
   System of nodes and links; represent channels and pipes
   Input: Surface runoff from Land Surface compartment, artificial inflows
   Output: Flows

Question 6

What are the 4 infiltration methods of SMMW

Answer 6

Horton’s method; Applicability: rainfall intensity > infiltration capacity

Modified Horton’s equation; Rate of decay is stopped so that cumulative infiltration < rainfall
depth, used where initial infiltration rate > initial rain intensity

Curve Number method; Empirical parameter, based on soil type, land use, hydrologic
conditions etc.

Green-Ampt method

Question 7

Types of Flow routing methods within SWMM

Answer 7

Steady Flow Routing
◦ Actually just sums instantaneous subcatchment runoff for all subcatchments upstream of the selected channel
Kinematic Wave
◦ Uniform, unsteady flow
◦ No backwater, no surcharge, tree branch systems only unless flow splits are input
Dynamic Wave
◦ Non-uniform, unsteady flow
◦ Backwater, surcharge, looped or parallel sewers, street routing of flooded sewer manholes

Question 8

what are the limitations of using percent removal to represent a treatment system?

Answer 8

Can apply different removal rates for each pollutant

Can use literature to select a representative percent removal

Question 9

Conveyance of pollutants

Answer 9

Conservation of mass

Advective transport: Pollutant mass moves along conduit (e.g. pipe, channel) at same speed as the water

Longitudinal dispersion: Some of the pollutant mass in a given parcel of water mixes with pollutant mass in the adjacent parcel of water

Reaction gains/losses: Reactions can increase or decrease the concentration of a pollutant in a given parcel of water