Map Modernization Management Support Best Practices Project - FEMA State of Idaho Idaho Department of Water Resources Boise, Idaho November 2008

Introduction FEMA funded as part of the Map Modernization effort Research associate from Center of Ecohydraulics Research (University of Idaho – Boise) Application of new technologies - Green LiDAR + 1D + 2D floodplain hydraulic modeling.

Project Area: Lower Boise River From Ross Dickinson’s work “Historic Floods and Channels on the Lower Boise River” Glenwood Bridge

Why the Lower Boise River Area? Important changes in the floodplain land use since the last FIRM, such as, increasing population, high developments rates…. Control water discharge from the upstream reservoir system Availability of new data collected by cutting edge remote sensing technologies (Green LiDAR) Interest in the project results from multiple agencies and local governments.

From Ross Dickinson’s work “Historic Floods and Channels on the Lower Boise River” First 2-Dimensional Modeling Area: Eagle Island MMMS Modeling Component

First 2-Dimensional Modeling Area Modeling results in the Eagle Island could be significantly improved by a floodplain 2- dimensional hydraulic model. Wide spread changes to topography from gravel pits, fills, roads, houses, bridges, etc. Historically flooded area

Historic Boise River Channels at Eagle Island From Ross Dickinson’s work “Historic Floods and Channels on the Lower Boise River” MMMS Modeling Component

Historic Boise River Channels at Eagle Island Channel evolution pre and post discharge control (Lucky Peak last dam completed in 1955). The 1867 channel location derived from the first public land survey in Idaho last largest flood before the reservoir was completed (approximately cfs) narrower and confine channel.

Boise River Channels at Eagle Island From Ross Dickinson’s work “Historic Floods and Channels on the Lower Boise River” MMMS Modeling Component North Channel South Channel

Boise River Channels at Eagle Island Wider alluvial channel Little development Mostly agricultural lands No gravel pits Most flow on the North channel Compare downstream end of Island between 1939 and 2004

From Ross Dickinson’s work “Historic Floods and Channels on the Lower Boise River” Boise River Channels at Eagle Island MMMS Modeling Component

Boise River Channels at Eagle Island Mayor changes in the floodplain  Houses  Buildings  Gravel pits  Parks  Roads  Bridges Channel is constrained (it does not have enough room to move freely).

Modeling Efforts in the selected area: Current DFIRM 2003 Floodway 100 Year Flood 500 Year Flood MMMS Modeling Component To download the ADA County DFIRM visit

Current DFIRM 2003 (Digital Flood Insurance Rate Map) Map and study adopted in 2003, based on data collected through Changes in the floodplain since the DFIRM was created. Remember “The DFIRM is a actuarial tool for determining flood insurance rates”. Parcel data, in yellow, has been included in the map from County data current information.

Shawkat Ali, UI PhD Modeling Efforts in the selected area: Shawkat Ali 2004 MMMS Modeling Component FEMA PROJECT IMPACT, City of Boise A hydraulic modeling study for the Boise River and its Floodplain Modeling Systems Used : DHI - MIKE Models Modeled Flows: 10-yr flood: 7,200 cfs 50-yr flood: 11,000 cfs 100-yr flood: 16,600 cfs 500-yr flood: 34,800 cfs

First dynamic flood graphic display. Based on the 1998 USGS surveyed cross sections. Calculations performed on a 1-dimensional model. The model was used to study the consequences of debris blocking a bridge. The superior graphic tool raised awareness of flood risk not depicted on DFIRM. Modeling Efforts in the selected area: Shawkat Ali 2004 Shawkat Ali (2004) PhD Dissertation is available at the Center of Ecohydraulics Research at the University of Idaho - Boise

Line-Area Coupling Q < Q bankfullQ > Q bankfullQ ~ Q bankfull MMMS Modeling Component MIKE FLOOD MIKE FLOOD: 1-Dimensional + 2-Dimensional Hydraulic Model, DHI Water and Environment

1-dimensional tools do not represent floodplain flows that have a strong 2-dimensinal component. MIKE Flood couples 1-dimensional (MIKE 11) in the channel with 2-dimensional (MIKE 21) in the floodplain. MIKE 11 predicts the main channel water levels that are transferred to the floodplain at high flood events (above bankfull) and simulated by MIKE 21. Thus, a more realistic prediction of out of channel flow is obtain by coupling these two tools. MIKE FLOOD MIKE FLOOD: Brief explanation MIKE FLOOD is developed by DHI Water & Environment

Downstream Upstream Boundary Conditions River network and main channel cross sections (USGS 1998) MMMS Modeling Component: Eagle Island Model Setup MMMS Modeling Component Floodplain (Red LiDAR 2004) 2006 Hydrograph at Glenwood Bridge Flow Direction

The USGS gaging station located at the Glenwood Bridge provides the data for the model’s upstream boundary condition (Hydrograph). The Eagle Island Floodplain topography is represented by Red LiDAR. The Boise River cross sections at Eagle Island are represented by USGS 1998 surveyed data. Downstream boundary condition is set far away from the Eagle Island area so results are not significantly sensitive to its value. MMMS Modeling Component: Eagle Island Model Setup

MMMS Modeling Component Water Depth (m) Surface Elevation (m) Preliminary Model Results NAD 1983 Idaho TM

Preliminary Model Results MMMS Modeling Component

Water Depth  10 year event results in shallow yet widespread floodplain flow.  10 year event breach the river bank in several areas and the river flow into exiting excavated ponds.  10 year event captured the 1939 south channel toward the downstream end of the island. Surface Water Elevation  10 year event excided the mapped 100 year base flood elevation (BSF) in some areas. Preliminary Model Results

Apply Green LiDAR data Model the 100 and 500 year flood events Compare results from the modeled areas with the current DFIRMs Working with the local governments to select other critical areas to be modeled Public Outreach  Education  Scenario analysis. Next Steps Boise, November Flood