Perhitungan Debit Banjir Rencana

options(OutDec = ".")
library(dplyr); library(tidyr); library(ggplot2)

# ── Parameter DAS ─────────────────────────────────────────────
A     <- 50      # luas DAS (km²)
L     <- 10      # panjang sungai utama (km)
alpha <- 2.0    # parameter resesi Nakayasu
C_run <- 0.65   # koefisien limpasan DAS
L_kir <- 5000   # panjang saluran untuk Kirpich (meter)
S_kir <- 0.010  # kemiringan untuk Kirpich (m/m)

# ── Curah hujan rencana dari analisa frekuensi (Gumbel) ───────
T_values <- c(2, 5, 10, 25, 50, 100)
R_T      <- c(96.4, 118.7, 133.4, 151.9, 165.6, 179.2)  # mm

# ── Waktu konsentrasi Kirpich ─────────────────────────────────
tc_min <- 0.0195 * L_kir^0.77 * S_kir^(-0.385)
tc_jam <- tc_min / 60
cat("tc Kirpich =", round(tc_jam, 2), "jam\n")

# ── Intensitas Mononobe pada t = tc ───────────────────────────
mononobe <- function(R24, t_jam) (R24 / 24) * (24 / t_jam)^(2/3)

# ── Debit puncak metode rasional ─────────────────────────────
I_tc  <- sapply(R_T, mononobe, t_jam = tc_jam)  # intensitas (mm/jam)
Q_ras <- 0.278 * C_run * I_tc * A              # debit (m³/s)

hasil_rasional <- data.frame(
  T      = T_values,
  R_T_mm = R_T,
  I_mmjam = round(I_tc, 1),
  Q_m3s   = round(Q_ras, 1)
)
print(hasil_rasional)

# ── Parameter Nakayasu ────────────────────────────────────────
tg  <- if(L < 15) 0.21 * L^0.7 else 0.40 + 0.058 * L
tr  <- tg                     # simplifikasi: tr = tg
tp  <- tg + 0.8 * tr
T03 <- alpha * tg
Qp  <- A / (3.6 * (0.3 * tp + T03))  # m³/s per mm

cat("tg =", round(tg, 2), "jam | tp =", round(tp,2),
    "jam | T03 =", round(T03,2), "jam | Qp_unit =", round(Qp,3), "m3/s/mm\n")

# ── Hidrograf satuan (dt = 0.1 jam) ──────────────────────────
nakayasu_unit <- function(t) {
  if      (t <= tp)             Qp * (t / tp)^2.4
  else if (t <= tp + T03)       Qp * 0.3^((t - tp) / T03)
  else if (t <= tp + 2.5*T03)   Qp * 0.3^((t - tp + 0.5*T03) / (1.5*T03))
  else                           Qp * 0.3^((t - tp + 1.5*T03) / (2  *T03))
}

t_seq      <- seq(0, 24, by = 0.1)
unit_hydro <- sapply(t_seq, nakayasu_unit)

# ── Hujan efektif & hidrograf banjir per kala ulang ──────────
R_eff   <- C_run * R_T               # hujan efektif (mm)
Qpeak_N <- Qp * R_eff               # debit puncak Nakayasu (m³/s)

# ── Buat data frame hidrograf semua kala ulang ────────────────
df_hydro <- lapply(seq_along(T_values), function(i) {
  data.frame(
    t    = t_seq,
    Q    = unit_hydro * R_eff[i],
    T_yr = paste0("T", T_values[i])
  )
}) |> bind_rows()

# ── Plot hidrograf semua kala ulang ───────────────────────────
cols <- c(T2="#64748b", T5="#2563eb", T10="#0284c7",
         T25="#d97706", T50="#dc2626", T100="#7c3aed")

ggplot(df_hydro, aes(x = t, y = Q, color = T_yr)) +
  geom_line(linewidth = 0.9) +
  scale_color_manual(values = cols) +
  geom_vline(xintercept = tp, linetype = "dashed", color = "gray50", linewidth = 0.5) +
  annotate("text", x = tp + 0.2, y = max(df_hydro$Q) * 0.95,
           label = paste0("tp = ", round(tp, 2), " jam"), size = 3, hjust = 0) +
  labs(
    title    = "Hidrograf Banjir Rencana — HSS Nakayasu",
    subtitle = paste0("A=", A, " km² | L=", L, " km | α=", alpha, " | C=", C_run),
    x = "Waktu (jam)", y = "Debit Q (m³/s)", color = "Kala Ulang",
    caption  = "Metode: HSS Nakayasu | SNI 2415:2016"
  ) +
  theme_minimal(base_size = 11) +
  theme(plot.title = element_text(face = "bold"))

ggsave("hidrograf_banjir_rencana.png", width = 36, height = 22, units = "cm", dpi = 300)