Land & Watersheds

Water Resources Engineering · Puerto Rico

Hybrid Urban-Fluvial Hydraulics

Hydrologic & Hydraulic Study Report

Urban Stormwater & River Flooding; [Project Name & Location]

Prepared for

[Client / Owner]

[Company / Municipality]

Prepared by

David Vagnetti, PE

Land & Watersheds

Date · Project No.

[Month YYYY]

No. [LW-0000] · Rev. [0]

GEOHEC-HMS / RAS 2D GEOSTORM · SWMM SIGNED & SEALED

Land & Watersheds

Hybrid Urban-Fluvial H&H · Report

Land & Watersheds · [Project No.] [Project Name]

·Contents

Table of contents.

1 · Introduction[p]

1.1 Project description & location[p]

1.2 Scope & purpose[p]

1.3 Methodology & data sources[p]

1.4 Authorization & study area[p]

2 · Catchment & basin characterization[p]

3 · Hydrology[p]

3.1 Rainfall, NOAA Atlas 14[p]

3.2 Urban runoff & HEC-HMS hydrographs[p]

4 · Hydraulics[p]

4.1 Storm-sewer & detention (SWMM)[p]

4.2 River 2D hydraulics & interaction[p]

5 · Results & flood mapping[p]

6 · Observations & conclusions[p]

Certification[p]

List of Tables

Table 2-1 Catchment & basin parameters
Table 3-1 NOAA Atlas 14 rainfall depths
Table 3-2 On-site & river peak discharge
Table 4-1 Detention routing & tailwater

List of Figures

Fig 1 Effective FIRM & BFE
Fig 2 NOAA Atlas 14 PF estimates
Fig 3 Catchment & storm-sewer plan
Fig 4 HEC-RAS 2D floodplain
Fig 5 Flood-depth map

Appendices

A, NOAA Atlas 14 / FIS data

D, GEOHEC-RAS 2D report

B, GIS basin parameters

E, Detention & tailwater calcs

C, GeoStorm & HEC-HMS report

F, Figures & model results

01Introduction

Introduction.

1.1 · Project description & location

[Client / Owner] has requested Land & Watersheds to perform a combined urban-stormwater and riverine Hydrologic & Hydraulic (H&H) study for [Project Name] in [Barrio, Municipality], Puerto Rico.

The developed site sits low in the watershed adjacent to [River / Quebrada], partly within [FEMA Zone AE / A]. The purpose of this study is to quantify on-site runoff and detention while modeling the river's Base Flood Elevations and tailwater influence on the site, and to demonstrate that the development is protected and does not worsen flooding.

Figure 0 · Project vicinity; replace with site aerial / location map.

1.2 · Scope & purpose

Characterize the urban catchment and the contributing river basin (area, imperviousness, CN, Tc, routing).
Model on-site pre/post runoff and detention in GeoStorm (SWMM) and river hydrographs in GEOHEC-HMS.
Build a 2D GEOHEC-RAS model of the river and floodplain; establish the Base Flood Elevations and the outfall tailwater.
Re-check detention against the river tailwater and demonstrate no adverse upstream / downstream impact.

1.3 · Methodology & data sources

Component	Method / Source
Hydrologic method	NRCS UH, GeoStorm (SWMM) & GEOHEC-HMS
Rainfall	NOAA Atlas 14, Vol. III (PR & USVI)
Urban hydraulics	Storm sewer & detention routing (SWMM)
River hydraulics	GEOHEC-RAS 2D · FEMA BFE
Modeling software	GeoStorm, GEOHEC-HMS & GEOHEC-RAS 2D (CivilGEO)

1.4 · Authorization & study area

This study was authorized by [Client / Owner] on [date]. The study area comprises the project parcel and the immediately contributing and receiving drainage, as shown on the effective Flood Insurance Rate Map (FIRM).

Figure 1 · Effective FIRM & BFEDrop FEMA FIRMette / firm panel for the site

Figure 1 · Effective Flood Insurance Rate Map.

Figure 2 · NOAA Atlas 14Drop PF-estimate table / PFDS screenshot

Figure 2 · NOAA Atlas 14 point-frequency estimates.

02Catchment & Basin

Catchment & basin characterization (GIS).

The on-site urban catchment and the contributing river basin were delineated from LiDAR topography. The catchment was subdivided into [00] sub-areas for the storm-sewer / detention model; the river basin was characterized for GEOHEC-HMS with CN, lag / Tc, and routing.

Table 2-1 · Catchment & basin parameters
Sub-catchment	Area (ac)	% Imp.	CN	Tc (min)
S-1	[0.0]	[00]	[00]	[0.0]
S-2	[0.0]	[00]	[00]	[0.0]
S-3	[0.0]	[00]	[00]	[0.0]
…	–	–	–	–

Figure 3 · Catchment & storm-sewer planDrop storm-sewer plan & basin map

Figure 3 · Catchment, storm-sewer & basin overview.

03Hydrology

Hydrology.

3.1 · Rainfall, NOAA Atlas 14

Design rainfall depths were obtained from NOAA Atlas 14, Volume III, for the site coordinates [lat, long], partial-duration series, for the 24-hour duration.

Table 3-1 · NOAA Atlas 14 rainfall depths (24-hr)
Return period	2-yr	10-yr	25-yr	50-yr	100-yr
Depth (in)	[0.0]	[0.0]	[0.0]	[0.0]	[0.0]

3.2 · Urban runoff & HEC-HMS hydrographs

On-site runoff and detention were modeled in GeoStorm (EPA-SWMM); the contributing river basin was modeled in GEOHEC-HMS. Peak discharges for the site outfall and the river are summarized below.

Table 3-2 · On-site & river peak discharge (cfs)
Storm	On-site post (cfs)	River Q (cfs)	Combined
2-yr	[0.0]	[0.0]	[0.0]
10-yr	[0.0]	[0.0]	[0.0]
25-yr	[0.0]	[0.0]	[0.0]
100-yr	[0.0]	[0.0]	[0.0]

04Hydraulics

Hydraulics.

4.1 · Storm-sewer & detention (SWMM)

The on-site storm-sewer network and detention system were analyzed in GeoStorm under the [10-yr] and [100-yr] storms, with the outfall controlled by the river tailwater (Section 4.2).

4.2 · River 2D hydraulics & interaction

A 2D GEOHEC-RAS model of [River / Quebrada] established the Base Flood Elevations and the tailwater at the outfall. The detention design was re-routed against this tailwater so post-development peaks do not exceed pre-development peaks under the combined condition.

Table 4-1 · Detention routing vs. river tailwater
Stage (ft)	Storage (ac-ft)	Tailwater (ft)
[28.0]	[0.00]	[0.0]
[29.0]	[0.00]	[0.0]
[30.0]	[0.00]	[0.0]

Figure 4 · HEC-RAS 2D floodplainDrop 2D floodplain / BFE map or detention detail

Figure 4 · HEC-RAS 2D floodplain & site interaction.

05Results & Flood Mapping

Results & flood mapping.

The 2D GEOHEC-RAS model produced maximum flow depths and velocities for the [100-yr] event over the developed area. The maps below show the river floodplain and the protected condition at finished-floor elevation [00.0 ft].

Figure 5 · Max flow depthDrop 2D depth raster

Figure 5 · Maximum flow depth.

Figure 6 · Max velocityDrop 2D velocity raster

Figure 6 · Maximum velocity.

06Observations & Conclusions

Observations & conclusions.

On-site post-development peaks, routed through detention against the river tailwater, do not exceed pre-development peaks.
Computed Base Flood Elevations [are consistent with / refine] the effective FIRM; the floodplain over the site is mapped in Figure 5.
Recommended finished-floor elevation is [00.0 ft], providing [0.0 ft] of freeboard above the Base Flood Elevation.
[Add project-specific findings, recommendations, and any residual risk or maintenance notes.]

Note: all designs presented herein are schematic and not for construction. Final design is subject to survey verification and permitting.

·Certification

Engineer’s certification.

I hereby certify that this Hydrologic & Hydraulic Study was prepared by me or under my direct supervision, and that I am a duly licensed Professional Engineer under the laws of the Commonwealth of Puerto Rico.

P.E. SEAL
&
SIGNATURE

David Vagnetti, PE

Civil Site & Water Resources Engineer · License No. [#####]

Date: [Month DD, YYYY]

Site Photographs

Photo 1

Photo 2

Photo 3

DRAFT