My First SWMM/R Hydrology Homework
Victoria Wanjiku Wanjohi
August 2022
One-Variable-At-a-Time Hydrology Sensitivity Analysis
We start with a ābaseā hydrology ONLY SWMM model with one subcatchment:
[SUBCATCHMENTS]
;; Total Pcnt. Pcnt. Curb
;;Name Raingage Outlet Area Imperv Width Slope Length Pack
;;------------ ------------- -------- ----- ------ ------ ----- ----- --------
Shed1 RG_TD25-102B Outlet1 100 70 4174.206511 1 0
[SUBAREAS]
;;Subcatchment N-Imperv N-Perv S-Imperv S-Perv PctZero RouteTo PctRouted
;;------------ ---------- ------- --------- ------- ------- ------- ----------
Shed1 0.013 0.25 0.05 0.2 25 OUTLET 100
[INFILTRATION]
;;Subcatchment Suction HydCon IMDmax
;;------------ ---------- ---------- ----------
Shed1 6.57 0.1 0.167
[OUTFALLS]
;; Invert Outfall Stage/Table Tide
;;Name Elev. Type Time Series Gate
;;------------ ---------- ---------- ---------------- ----
Outlet1 4 FREE NOWhere our OPTIONS, FILES, EVAPORATION, RAINGAGES sections are setup as follows:
[OPTIONS]
FLOW_UNITS CFS
INFILTRATION GREEN_AMPT
FLOW_ROUTING DYNWAVE
START_DATE 10/01/2005
START_TIME 00:00:00
REPORT_START_DATE 10/01/2005
REPORT_START_TIME 00:00:00
END_DATE 11/01/2005
END_TIME 00:00:00
SWEEP_START 01/01
SWEEP_END 12/31
DRY_DAYS 0
REPORT_STEP 00:05:00
WET_STEP 00:05:00
DRY_STEP 00:05:00
ROUTING_STEP 0:00:15
ALLOW_PONDING NO
INERTIAL_DAMPING NONE
VARIABLE_STEP 0.50
LENGTHENING_STEP 20
MIN_SURFAREA 12.566
NORMAL_FLOW_LIMITED BOTH
SKIP_STEADY_STATE NO
FORCE_MAIN_EQUATION H-W
LINK_OFFSETS DEPTH
MIN_SLOPE 0
[FILES]
USE RAINFALL "IDS_2005_V2_Mod_update2017.rff"
[EVAPORATION]
;;Type Parameters
;;---------- ----------
CONSTANT 0.01
[RAINGAGES]
;; Rain Time Snow Data
;;Name Type Intrvl Catch Source
;;-------------- --------- ------ ------ ----------
; RG Vol/Intensity Time Step Snow catch File/Timeseries RGNumber depth
RG_TD25-102B VOLUME 00:15 1 FILE "IDS_2005_V2_Mod_update2017.rff" RG_TD25-102B INIn later lessons, we will learn how modifying the OPTIONS section modifies our model results. In this lesson, we will modify inputs in the SUBCATCHMENTS, SUBAREAS, and INFILTRATION sections.
Question 1a: How does varying the subcatchment slope impact runoff?
We varied the slopes of our subcatchments from 0.02 to 20%.
[SUBCATCHMENTS]
;; Total Pcnt. Pcnt. Curb
;;Name Raingage Outlet Area Imperv Width Slope Length Pack
;;------- ------------- -------- ----- ------ ------ ----- ----- --------
Shed1 RG_TD25-102B Outlet1 100 70 4174.206511 0.02 0
Shed2 RG_TD25-102B Outlet2 100 70 4174.206511 0.05 0
Shed3 RG_TD25-102B Outlet3 100 70 4174.206511 1 0
Shed4 RG_TD25-102B Outlet4 100 70 4174.206511 2.5 0
Shed5 RG_TD25-102B Outlet5 100 70 4174.206511 5 0
Shed6 RG_TD25-102B Outlet6 100 70 4174.206511 10 0
Shed7 RG_TD25-102B Outlet7 100 70 4174.206511 15 0
Shed8 RG_TD25-102B Outlet8 100 70 4174.206511 20 0
Shed9 RG_TD25-102B Outlet9 100 70 4174.206511 25 0
Answer 1a:
Increasing the percent slope results in an increase in total and peak runoff. As the slope increases the water velocity increases as well resulting in great runoff in the system. Increasing percent slope decreases total infiltration. Greater slopes and increased water velocity reduces the rate at which water is able to infiltrate.
Question 1b: How does varying the percentage of impervious area impact runoff?
We varied the percentage impervious from 20 to 100%.
[SUBCATCHMENTS]
;; Total Pcnt. Pcnt. Curb
;;Name Raingage Outlet Area Imperv Width Slope Length Pack
;;------- ------------- -------- ----- ------ ------ ----- ----- --------
Shed1 RG_TD25-102B Outlet1 100 20 4174.206511 1 0
Shed2 RG_TD25-102B Outlet2 100 30 4174.206511 1 0
Shed3 RG_TD25-102B Outlet3 100 40 4174.206511 1 0
Shed4 RG_TD25-102B Outlet4 100 50 4174.206511 1 0
Shed5 RG_TD25-102B Outlet5 100 60 4174.206511 1 0
Shed6 RG_TD25-102B Outlet6 100 70 4174.206511 1 0
Shed7 RG_TD25-102B Outlet7 100 80 4174.206511 1 0
Shed8 RG_TD25-102B Outlet8 100 90 4174.206511 1 0
Shed9 RG_TD25-102B Outlet9 100 100 4174.206511 1 0 
Answer 1b:
As we increase the percentage of our subcatchment with impervious area, we increase both the total runoff and peak runoff. With a greater percentage of impervious area rain has less area to infiltrate resulting in an increase in total and peak runoff. Meanwhile, our total infiltration decreased with percent impervious. As percent impervious increases, percent pervious decrease and there is less area to infiltrate the runoff.
Question 1c: How does varying the manningās roughness of the pervious area impact runoff?
We varied N-Perv from 0.013 to 0.2.
[SUBAREAS]
;;Subcatchment N-Imperv N-Perv S-Imperv S-Perv PctZero RouteTo PctRouted
;;------------ ---------- ------- --------- ------- ------- ------- ----------
Shed1 0.013 0.013 0.05 0.2 25 OUTLET 100
Shed2 0.013 0.015 0.05 0.2 25 OUTLET 100
Shed3 0.013 0.02 0.05 0.2 25 OUTLET 100
Shed4 0.013 0.04 0.05 0.2 25 OUTLET 100
Shed5 0.013 0.06 0.05 0.2 25 OUTLET 100
Shed6 0.013 0.08 0.05 0.2 25 OUTLET 100
Shed6 0.013 0.1 0.05 0.2 25 OUTLET 100
Shed6 0.013 0.12 0.05 0.2 25 OUTLET 100
Shed6 0.013 0.14 0.05 0.2 25 OUTLET 100
Shed6 0.013 0.16 0.05 0.2 25 OUTLET 100
Shed6 0.013 0.18 0.05 0.2 25 OUTLET 100
Shed6 0.013 0.2 0.05 0.2 25 OUTLET 100
Answer 1c:
Greater manningās roughness results in a decrease in total and peak runoff and an increase in total infiltration. At higher manningās roughness, the water velocity decreases and results in an decrease in runoff while increasing the total infiltration within the system.
Question 1d: How does varying the percent routed from impervious to pervious area in the shed impact runoff?
We changed RoutedTo āOUTLETā to āPERVIOUSā and then varied %ROUTED from 50%, 60%, 70%, 80%, 90%, and 100%.
[SUBAREAS]
;;Subcatchment N-Imperv N-Perv S-Imperv S-Perv PctZero RouteTo PctRouted
;;------------ ---------- ------- --------- ------- ------- ------- ----------
Shed1 0.013 0.25 0.05 0.2 25 PERVIOUS 100
Shed2 0.013 0.25 0.05 0.2 25 PERVIOUS 90
Shed3 0.013 0.25 0.05 0.2 25 PERVIOUS 80
Shed4 0.013 0.25 0.05 0.2 25 PERVIOUS 70
Shed5 0.013 0.25 0.05 0.2 25 PERVIOUS 60
Shed6 0.013 0.25 0.05 0.2 25 PERVIOUS 50
Answer 1d:
Varing percent routed from impervious to pervious area results in a decrease in both total runoff and peak runoff, while the total infiltration increases. Within a more pervious area, rainfall will have greater opportunity for infiltration resulting in runoff reductions.
Question 1e: How does varying the shed width impact runoff?
We varied width from sqrt(Area) *2, sqrt(Area), sqrt(Area)/2, sqrt(Area)/10, to sqrt(Area)/100.
[SUBCATCHMENTS]
;; Total Pcnt. Pcnt. Curb
;;Name Raingage Outlet Area Imperv Width Slope Length Pack
;;------- ------------- -------- ----- ------ ------ ----- ----- --------
Shed1 RG_TD25-102B Outlet1 100 70 4174.206511 1 0
Shed2 RG_TD25-102B Outlet2 100 70 2087.103256 1 0
Shed3 RG_TD25-102B Outlet3 100 70 1043.551628 1 0
Shed4 RG_TD25-102B Outlet4 100 70 208.7103256 1 0
Shed5 RG_TD25-102B Outlet5 100 70 20.87103256 1 0
Answer 1e:
At greater subcatchment width, we see an increase in total runoff and peak runoff and a decrease in total infiltration.
Question 1h: How does changing the amount of depression storage impact runoff in Questions 1a to 1e?
In our ābaseā hydrology setup, the parameter PctZero is set to 25. That means that 25% of the impervious area has depression storage. What is depression storage? Did you make any plots to support your answer?
Answer 1h:
WRITE YOUR ANSWER HERE
Summary
Going back through the plots/answers to Questions 1a thru 1e, the following table is a summary of how INCREASING the following variable will impact the various runoff responses:
For next timeā¦
What happens when we modify MORE than one runoff parameter at a time??? Next time, we will continue exploring hydrology & sensitivity analysis with SWMM. We will circle back to questions 1f and 1g. To prepare for next time, please RE-RUN your models for 1a thru 1e with PctZero = 25 for the entire year (see OPTIONS below).
[OPTIONS]
START_DATE 01/01/2005
START_TIME 00:00:00
REPORT_START_DATE 01/01/2005
REPORT_START_TIME 00:00:00
END_DATE 01/01/2006
END_TIME 00:00:00
SWEEP_START 01/01
SWEEP_END 12/31
DRY_DAYS 0
REPORT_STEP 00:01:00
WET_STEP 00:01:00
DRY_STEP 00:01:00
ROUTING_STEP 0:00:15For next time, we will use the post-processing tools to extract the binary outputs and convert our 1-minute outputs to 15-minute outputs.