Flood Improvement and LID Modeling Using XP-SWMM

Flood Improvement and LID Modeling Using XP‐SWMM Andrew Juan1, Nick Fang2, and Philip Bedient3 Rice University August, 2013 Introduction First developed in 1971, the EPA's Storm Water Management Model (SWMM) has been widely used in urban areas for planning, analysis, and design related to stormwater runoff, combined sewers, sanitary sewers, and other drainage systems. XP‐SWMM, published by XP Solutions Inc., uses the EPA SWMM model as the primary runoff hydrograph simulator for runoff quantity and quality. The major advantage of XP‐SWMM over other software packages is its ability to combine a 1D river hydraulic model with a 2D rainfall‐runoff model to generate floodplain maps. In addition, XP‐SWMM is approved by FEMA for the hydrologic and hydraulic applications detailed in this study. There are three separate modules present in XP‐SWMM: runoff, sanitary, and hydraulic. The runoff module includes subcatchment areas for hydrologic modeling. The sanitary module allows for pollutant loading and water quality analyses, and the hydraulic module is capable of simulating the hydraulics of channels and storm sewers. Most of the work for Rice University’s flood improvement project was accomplished in XP‐SWMM’s hydraulic module, while the LID modeling was conducted using the runoff module. Harris Gully Watershed Rice University is located within the Harris Gully watershed, which covers a drainage area of approximately 5 square miles, consisting mostly of fully‐developed residential and light commercial land 1
Graduate student, Civil and Environmental Engineering Department, Rice University, PH (713) 348‐4221, Email: Andrew.juan@rice.edu 2
Project Manager, Civil and Environmental Engineering Department, Rice University, PH (713) 348‐2398, Email: nickfang@rice.edu 3
Professor, Civil and Environmental Engineering Department, Rice University, PH (713) 348‐4953, Email: bedient@rice.edu 1 ome to Herm
mann Park and
d the Texas M
Medical Centeer, the largestt medical cen
nter in use. The aarea is also ho
the world
d. A number o
of severe sto
orms have impacted the H
Harris Gully w
watershed, th
he most notable of which beiing Tropical SStorm Allison in 2001. TS A
Allison causeed severe floo
oding problem
ms throughou
ut the entire waatershed, resu
ulting in rougghly $5 billion in damage s. Inundatio
on was especially severe aat the downstream portions o
of Harris Gully, where Rice
e University aand the Texass Medical Cen
nter are locateed. Allison and as part of the TTropical Storm
m Allison Recoovery Projectt (TSARP), a number of draainage After TS A
improvem
ment efforts w
were conductted to mitigatte future floooding problem
ms in the watershed (see FFigure 1). Locate
ed at Kirby Dr., McGregorr Dr. (now caalled Cambriddge Dr.), and Hermann Drr., these structural improvem
ments were designed d
to divert d
stormw
water away from the priimary Harris Gully culverrt and distribute
e it along Brrays Bayou. These imprrovements annd other chaanges within
n the Harris Gully watershed
d have been incorporated into this stud
dy. Riice TMC Figure 1: Th
he Harris Gullyy Watershed with drainagge improvemeents depicted in blue.
2 Rice University Flood Improvement Project The main purpose of the project was to represent Rice University’s existing storm water network using a 1D/2D hydraulic model (XP‐SWMM) and to evaluate the effects of a proposed drainage alternative on Rice’s 100yr floodplain, focusing specifically on the area adjacent to Alumni Drive. The pipe network of Rice University was represented as a system of links and nodes in XP‐SWMM. In the proposed alternative, new 48” pipes would be connected to existing 48” sewers under the Center Quadrangle. The pipes would then be run along Alumni Drive, underneath the intramural fields and stadium parking lots, and into Harris Gully. A comparison between the existing and proposed alternatives is depicted in Figure 2. 3 Figures 2(a) and (b): Existing (top) and proposed (bottom) storm water network on Rice Campus. Changes to the existing network are highlighted in blue in 2b. To create the base XP‐SWMM model, Tropical Storm Erin (August 16, 2007) was used. The primary reason for the selection of this storm was the availability of high water marks at several locations throughout the Rice campus. These records later proved to be instrumental in model calibration. The rainfall data for this event were obtained from a HCOEM rain gauge at Holcombe (OEM400) that recorded a total rainfall depth of 6.2 in. The rainfall data are shown in Figure 3 below. 4 Figure 3: Cumulative Rainfall from
m HCOEM Gauuges. Land use was categorrized within XP‐SWMM X
ass either HiFrriction or LoFFriction. HiFriiction represented highly imp
pervious areaas, mainly the
e buildings on Rice campuus. The rest o
of the Rice—tthe areas of lower imperviou
usness—was categorized as LoFrictio
on. Figure 4 shows Rice’s land use characteristiics as representted in XP‐SWM
MM. 5 WMM. Figure 4. Land use chara
acteristics of Rice Universiity as represeented by XP‐SW
After runn
ning the base
e model with TS Erin, the ssimulated watter surface ellevations from
m XP‐SWMM were compared
d with obserrved high waater marks at a various loocations. The simulated inundation depths generally corresponded well with th
he observed h
high water m
marks along Allumni Drive. FFigure 5 show
ws the on between tthe simulated
d and observe
ed water surfaace elevation
ns at Alumni D
Drive for TS Erin. compariso
Figure 5: Comparison between XP‐SSWMM modeeled high watter marks andd those observved during TSS Erin. 6 ormances of e
existing and p
proposed connditions of th
he Rice camp
pus, the 100yyr24hr To compaare the perfo
design sto
orm was simu
ulated. The prroposed alterrnative was foound to be su
uccessful in reeducing the o
overall inundatio
on levels alongg Alumni Dr. for the desiggn storm. In ggeneral, approximately 0.5
5‐1ft of inund
dation reduction
n was observe
ed as illustratted in Figuress 6 and 7. Thi s flooding red
duction woulld be beneficial for several off the existingg buildings ad
djacent to Alu
umni Drive, eespecially tho
ose with baseement laboratories and/or vaaluable equip
pment and re
esearch mate
erial, such ass the Keith‐W
Weiss Geological and Anderson Biological Laboratoriess as well as th
he Space Scien
nce and Tech nology Buildiing. Figure 6: 1
100‐yr flood llevels from Exxisting (left) aand Proposedd (right) Condiitions. 7 Figures 7((a) and (b): Stage Comparrisons at Seleccted Nodes foor 100yr24hr SStorm. The exxisting stormwater network p
performance is symbolized
d by the orange curve. Perrformance wiith the propo
osed improvem
ments is depicted by the blue curve. 8 LID Mode
eling In additio
on to increassing drainage capacities through the iinstallation o
of new pipes,, another po
ossible strategy ffor flood mitigation on Ricce’s campus iis the implem
mentation of Low Impact D
Developmentt (LID) practices. Since the laate 1990s, LID has emergged as an altternative to ttraditional urrban develop
pment strategiess. However, u
unlike most trraditional methodologies, LID features are designed
d to manage sstorm runoff as close as posssible to its oriigin. This is acccomplished through the use of distrib
buted technologies ns, with the eend goal of reeducing the negative impaacts of such as grreen roofs, biio‐swales, and rain garden
urbanization on natu
ural resources by main
ntaining pre‐‐development hydrologicc conditions.. The es and ease o
of use of XP‐SSWMM make it quite suitaable for quanttifying the po
otential benefits of capabilitie
LID adopttion. In this p
preliminary sttudy, a hypotthetical greenn roof scenarrio was simullated in the rrunoff module o
of XP‐SWMM.. Analysis was focused on
n the four buiildings that d
drained to no
ode 335 on A
Alumni Drive. The
e green roofs,, as modeled in XP‐SWMM
M, are shown in Figure 8 beelow. Figure 8: Green rooff modeling ussing XP‐SWM
MM. Comparisons were maade between the base sce
enario (tradittional concreete roofs) and
d the LID sceenario (green roofs) by usingg the Soil Conservation Se
ervice (SCS) CCurve Number (CN) meth
hod to dictatte the 9 oof type. The
e SCS CN metthod is comm
monly used in
n many hydro
ologic hydrologic performancce of each ro
o simulate watershed w
run
noff. The me
ethod appliess an empiriccal approach to determine the models to
appropriaate volume off stormwater runoff for a ggiven area byy characterizing its hydrolo
ogic responsee with a Curve N
Number or CN, a parameter that represe
ents the com bined impactts of soil typee, vegetative ccover, and moisture conditio
ons on runofff generation. Figure 9 prresents somee preliminaryy results from
m the scenarios using Green Roofs and traaditional hydrrologic modeeling schemess in XP‐SWMM
M. particular stud
dy, the green roofs in the LID scenario were represeented by SCS Curve Number 86, For this p
while the
e concrete ro
oofs were rep
presented byy SCS Curve Number 77. Comparisonss were also made between two different runoff metthods: the SW
WMM runoff f method and
d the kinemaatic wave meethod. After simulating several design storms (1in rainfall, 2yr‐3hr, 10yr‐12hr), initial results indicated thaat the ofs were able
e to reduce peak p
flows ass well as totaal runoff volu
ume by apprroximately 20
0%. In green roo
addition, it was obserrved differen
nt routing me
ethods resultted in discrepancies in th
he magnitudee and timing of peak flows. SSpecifically, it was found that the SWM
MM runoff m
method resultted in delayed and d peak flows when compaared to the kinematic k
wa ve method. TThese resultss are by no m
means decreased
conclusive
e, indicating tthat more stu
udies need to be conducteed to validate these findinggs. Figure 9: Preeliminary Ressults from Greeen Roofs vs. Traditional M
Models in XP‐SSWMM. 10 Conclusions and Work in Progress Overall, XP‐SWMM was found to be instrumental in simulating and depicting the drainage conditions for the Rice flood improvement project; and based on the preliminary results, it also has potential for application in the design and evaluation of different LID features. Currently, Rice is in the process of constructing several new buildings throughout campus, and it is likely that this activity will impact existing hydrologic conditions. Therefore, it is paramount that more studies are conducted to further improve our understanding of drainage performance at Rice. Future analyses may include performing a more detailed hydrology/hydraulics study in the areas beyond Alumni Drive as well as the evaluation of other LID features, including porous pavements, swales, and infiltration strips. 11