Document Type

Thesis

Date of Degree

Fall 2015

Degree Name

MS (Master of Science)

Degree In

Civil and Environmental Engineering

First Advisor

Larry Weber

Second Advisor

Nathan Young

Abstract

In order to reduce flooding, communities often try to control runoff with a storm sewer network, detention basins, low impact developments, and upstream storage to reduce stream overflow. Numerical models can help predict the effect these strategies will have before expensive construction projects are underway. A coupled 1D/2D hydraulic model using XPSWMM was created for the town of Kalona, IA, to test different strategies for flood reduction. XPSWMM utilizes one dimensional and two dimensional St. Venant equations to model flow in streams and pipes, or overland flow on the surface, respectively. The town of Kalona, upstream highlands, and the downstream floodplains were modeled utilizing a 4 meter cell-size unstructured grid. The model was neither calibrated nor validated, but its performance was comparable to a previously built MIKE 11/21 model of the same area when given the same inputs.

The city drains into Salvesen Creek, the Central Drainage Ditch, and the East Drainage Ditch, with Salvesen Creek having the largest drainage area. 14 agricultural detention ponds upstream of the town were modeled to determine their effectiveness in reducing stream overflow, while modifications to the storm sewer network and in situ detention provided relief from local runoff. The detention ponds and modifications were modeled both separately and together and compared to a base model using the 10 year, 25 year, 50 year, 100 year, and 500 year, 3 hour storms.

The different methods were compared using three index points: City Hall, Pleasant View Circle, and in a softball practice area. The upstream agricultural detention ponds provided a peak reduction of 2%, 13%, and 9%, respectively, while the in situ modifications reduced flooding by 0%, 44%, and 18%, respectively, for the 10 year storm. The combined techniques reduced flooding by 2%, 44%, and 20%, respectively. During the 100 year storm, the detention ponds, modifications, and combined techniques reduced peak flood depths by 17%, 24%, and 14%; 2%, 3%, and 22%; and 17%, 55%, and 23%, respectively. This demonstrated that the in situ modifications were more effective during low flood events while ponds were more effective at high flood events. The combined approach was most effective when the two methods complemented each other. Future work might determine areas throughout the town where reduced flow and in situ modifications together would be most effective and design approaches to maximize flood reduction. Additional features to be modeled include pumps to increase capacity in the storm sewer network, levees, and supplementary drainage channels.

Public Abstract

A hydraulic model of the town of Kalona, IA, was created to test two methods of flood mitigation. These methods included a distributed network of detention ponds to store water and release it at a much lower rate and in situ modifications to the storm sewer network. These scenarios were tested separately and together to determine their individual and combined impact on flooding. Maximum depths were recorded at three points of interest, Pleasant View Circle, The Softball Practice Area, and City Hall.

Flood and flood reduction maps were created to help decision makers decide which projects should be approved. For storms that have a 10% probability in a given year of occurring, maximum flood depths were reduced by 44% and 20% at Pleasant View Circle and an area south of the softball fields, respectively, using both the upstream detention ponds and in situ modifications. This reduction increases to 55% and 23% for Pleasant View Circle and the area south of the softball fields, respectively, as well as 17% at City Hall for a storm with a 1% probability of occurring in a given year. Only the detention ponds reduced flooding at City Hall, while the drainage network modifications were more effective at the softball practice area and Pleasant View Circle. The flood reduction methods were more effective when combined than either alone. Future work should focus on finding additional potential modifications or areas where the two methods together provide further flood reduction, as well as modeling other flood control measures such as levees, pumps, or additional channels.

Keywords

publicabstract, 2 dimensional, Flooding, hydraulic modeling, Kalona, one dimensional, XPswmm

Pages

xvii, 200

Bibliography

114-117

Copyright

Copyright 2015 David Koser

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